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1Cell Biology and Chemistry2

The Cell1

 
1.1. ORGANIZATION AND COMPOSITION OF CELLS
 
I. TYPES OF CELLS
Cell is the smallest structural and functional unit of life. The cells are divided into eukaryotic cell and prokaryotic cell based on the presence or absence of a discrete nucleus (Table 1.1.1).
 
Prokaryotic Cell
Definition Prokaryotic cells lack a well-defined nucleus.
Structure Prokaryotic cells are smaller in size and their typical size ranges from 0.4 to 4 μm. Prokaryotic cells are characterized by the absence of nucleus and nucleolus. Only one chromosome is present in prokaryotes. DNA is a circular DNA. Prokaryotes contain plasmids. These cells do not contain cytoskeleton and subcellular organelles. The ribosomes present in prokaryotic cells are 70S ribosomes. 70S ribosome contains 50S large subunit and 30S small subunit. These cells do not have transport systems. Prokaryotic cells are characterized by the presence of a cell wall (Fig. 1.1.1).
Cell division Prokaryotes divide by binary fission.
Examples are bacteria and mycoplasma.
 
Eukaryotic Cell
Definition Eukaryotic cell contains a nucleus.
Structure Eukaryotic cells are larger in size and the typical size ranges from 5 to 50 μm. They are characterized by the presence of nucleus and nucleolus. More than one chromosomes are present. DNA is a linear DNA. Eukaryotic cells contain cytoskeleton and subcellular organelles. The ribosomes present in the eukaryotic cells are 80S ribosomes containing 60S large subunit and 40S small subunit. Eukaryotic cells have transport systems such as endocytosis.
Table 1.1.1   Prokaryotic and eukaryotic cells
Features
Prokaryotic cell
Eukaryotic cell
Nucleus
Absent
Present
Nucleolus
Absent
Present
Subcellular organelles
Absent
Present
Transport system
Absent
Present
Ribosome
70S
80S
Cell wall
Present
Absent
Cell divison
Biniary fission
Mitosis, meiosis
Size
Smaller
Larger
Examples
Bacteria
Animal cell
Mycoplasma
Plant cell, fungi
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FIGURE 1.1.1: Structure of prokaryotic cell
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Cell wall is absent in eukaryotic cells (Fig. 1.1.2).
Exceptions There are some exceptions to the general structural features of eukaryotic cell. Erythrocytes lack mitochondria and nucleus. Plant cells have a thick cell wall.
Cell division occurs by mitosis and meiosis.
Examples are animal cells, plant cells and fungi.
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FIGURE 1.1.2: Structure of eukaryotic cell
 
II. ORGANIZATION OF EUKARYOTIC CELLS
The eukaryotic cell is organized into distinct structural components that are involved in the fine control of cellular functions (Table 1.1.2).
 
Subcellular Organelles
Subcellular organelles are distinct internal membranous structures within the cell. Subcellular organelles include nucleus, Golgi complex, endoplasmic reticulum, mitochondria, lysosomes, and peroxisomes (p. 8).
 
Plasma Membrane
Plasma membrane is a limiting membrane enclosing the cellular contents. It is composed of phospholipid bilayer containing embedded proteins and cholesterol. Carbohydrates are present on the surface as glycoproteins or glycolipids (p. 16).
Table 1.1.2   Organization of eukaryotic cells
Subcellular organelles
Nucleus, mitochondria, peroxisomes, endoplasmic reticulum, lysosomes, golgi complex
Cytoplasm
Includes all the contents of the cell outside the nucleus, excluding cell membrane
Cytosol
Definition: Part of the cytoplasm not containing subcellular organelle.
Contents: Proteins, RNA, nutrients, metabolic products, ions, cytoskeletal proteins, inclusion bodies
Functions: Glycolysis, protein synthesis, antioxidant reactions
Cytoskeleton
Microtubules, intermediate filaments, actin filaments
Cell junctions
Tight junctions, anchoring junctions, gap junctions
 
Cytosol
Cytosol is that part of the cytoplasm that does not contain any subcellular organelle.
Contents The contents present in the cytosol are (1) proteins (enzymes), (2) RNA, (3) nutrients (glucose, fatty acids), (4) metabolic products (urea), (5) ions (potassium), (6) cytoskeletal proteins (microtubules, microfilaments and intermediate filaments), and (7) inclusion bodies (glycogen granules in liver and muscles, triacylglycerol droplets in adipose tissue) (Fig. 1.1.3).
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FIGURE 1.1.3: Cytoskeletal proteins
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Functions Examples for metabolic reactions that are active in the cytosol are: (i) glycolysis, (ii) pentose phosphate pathway, (iii) activation of amino acids, (iv) protein synthesis, and (v) antioxidant reactions.
 
Cytoplasm
Cytoplasm includes all contents of the cell outside the nucleus, excluding the cell membrane. Cytoplasm contains cytosol and subcellular organelles.
 
Cell Junctions
Cell junctions are regions that are involved in the interconnections of neighboring cells. Types of cell junctions are tight junctions, anchoring junctions and gap junctions (p. 31) (Fig. 1.1.4).
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FIGURE 1.1.4: Cell junctions
 
Cytoskeleton
Cytoskeleton consists of proteins that are responsible for structural network, movement of the cell and cellular regulation. Cytoskeletal proteins include microtubules, microfilaments and intermediate filaments (p. 32).
 
III. CHEMICAL COMPONENTS OF CELLS
 
Chemical Components of Cells
The main chemical components of cells are water, minerals and organic molecules (Table 1.1.3).
 
Water
Water is the major component of human body comprising about 60% of cell volume.
 
Minerals
Minerals constitute about 7% of cell volume. Examples of minerals present are calcium, phosphorus, magnesium, sulfur, sodium, potassium, chloride, iron, copper, zinc, iodine, cobalt, fluorine, manganese, selenium, molybdenum and chromium.
Table 1.1.3   Chemical components of cells
Water
Comprises about 60% of cell volume
Minerals
Calcium, phosphorus, magnesium, sulfur, sodium, potassium, chloride, iron, copper, zinc, iodine, cobalt, fluorine, manganese, selenium, molybdenum, chromium.
Organic Molecules
Macromolecules: Polysaccharides, proteins, nucleic acids.
Small Molecules: Monosaccharides, fatty acids, amino acids, nucleotides, vitamins, coenzymes
 
Organic Molecules
Organic molecules constitute about 33% of cell volume. They include large organic molecules as well as small organic molecules.
Large organic molecules The large organic molecules are also called macromolecules. Examples for macromolecules are polysaccharides, proteins and nucleic acids.
Small organic molecules Examples of small organic molecules are monosaccharides, fatty acids, amino acids, nucleotides, coenzymes and vitamins.
 
IV. Macromolecules
Macromolecules are further grouped based on composition or functions into biomolecules, biopolymers and informational macromolecules (Table 1.1.4).
 
Biomolecules
Biomolecules are components of cells and tissues that perform specific functions. Examples are carbohydrates, lipids, proteins, and nucleic acids.
Table 1.1.4   Biomolecules
Macromolecules
Definition
Examples
Biomolecules
Cell components performing specific functions
Carbohydrates, lipids, nucleic acids, proteins
Biopolymers
Molecules containing repeating units of monomers
Polysaccharides, DNA, RNA, proteins
Informational macromolecules
Macromolecules involved in transfer of genetic information
RNA, DNA
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Proteins constitute about 17% of cell volume. Proteins serve as structural, regulatory, contractile and defensive molecules in the body (p. 101).
Lipids constitute about 14% of cell volume. Lipids are sources of energy (p. 70). and major structural components of cells.
Carbohydrates constitute about 1% of cell volume. Carbohydrates are important sources of energy. They play structural and regulatory role (p. 35).
Nucleic acids constitute about 1% of cell volume. DNA is essential for replication and transcription (p. 547). RNAs are involved in the biosynthesis of proteins (p. 543).
 
Biopolymers
Biopolymers are complex molecules composed of repeating units of building blocks. The building blocks are called monomers. Examples for biopolymers are DNA, RNA, proteins and polysaccharides.
DNA is a polydeoxyribonucleotide in which nucleotides are linked through phosphodiester bond.
RNA is a polyribonucleotide in which nucleotides are linked through phosphodiester bond.
Proteins are polyamino acids in which amino acids are linked through peptide bond.
Polysaccharides are made up of more than ten monosaccharide units that are linked through glycosidic linkage.
 
Informational Macromolecules
Informational macromolecules are macromolecules that are involved in the transfer of genetic information to daughter cell or translation of the genetic information to the formation of proteins.
DNA and RNA are informational molecules (Fig. 1.1.5).
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FIGURE 1.1.5: Informational macromolecules
 
V. COLLOIDS (TABLE 1.1.5)
 
Definition
Compounds are classified into crystalloids and colloids based on their ability to diffuse through a semipermeable membrane (membrane with small molecular sieve).
Crystalloids are compounds that diffuse through semi- permeable membrane. They are usually small molecules. Examples are urea and sodium chloride.
Colloids are compounds that cannot diffuse through semi- permeable membrane. Hence, they are retained in the same compartment.
Colloidal state The classification of compounds into crystalloids and colloids was originally proposed by Graham. However, such classification is not absolute since crystalloids may be converted into colloids or colloidal state. Therefore the term colloidal state is more appropriate to describe the colloidal compounds.
 
Classification and Examples
Colloidal system comprises colloidal particles (dispersion phase) and solvent system (dispersion medium). Based on the consistency or solubility of colloidal state, colloids are classified into sol and gel
 
Sol Colloids
Sol colloids are colloids that are dispersed in liquid dispersion medium (solid/liquid colloidal system).
Sol colloids are further classified into lyophilic and lyophobic colloids.
  1. Lyophilic colloids (Emulsoids) (Fig. 1.1.6) have affinity for water. Lyophilic colloids are also called emulsoids. Examples are biomolecules and biological fluids. Proteins, carbohydrates and lipids present inside the cell as well as plasma membrane are colloidal in nature.
    zoom view
    FIGURE 1.1.6: Sol colloids (Emulsoids)
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    Biological fluids such as plasma exist as colloidal system. Albumin in plasma is lyophilic colloid. Casein present in milk exists in colloidal state.
  2. Lyophobic colloids (Suspensoids) do not have affinity for water. Therefore, they are not readily soluble in water. They are also called suspensoids. Examples are globulins.
 
Gel Colloids
Gel colloids refer to colloids in liquid dispersion particles present in solid dispersion system (liquid/solid colloid system). Example is formation of insoluble fibrin polymer during clotting process.
 
Properties and Significance of Colloids
Brownian movement Colloids exhibit Brownian movement. Brownian movement refers to continual and haphazard movement of colloidal particles. It is caused by bombardment of the colloids in the dispersion medium. Brownian movement contributes to the distribution of colloids in the dispersion medium.
Osmotic pressure Colloids exert osmotic (oncotic) pressure. Therefore, they cause movement of water to their compartment. Plasma osmotic pressure is responsible for distribution of water between plasma and intercellular compartment. Plasma osmotic pressure is mainly contributed by albumin. Decrease in plasma albumin levels results in the development of edema. Decreased plasma albumin levels are seen in liver disease, Kwashiorkor and nephrotic syndrome.
Non-permeability (non-diffusible nature) The property of non-permeability of colloids through semi-permeable membrane contributes to phenomenon of dialysis and Donnan membrane equilibrium.
Dialysis Colloids are non-dialyzable molecules. Dialysis refers to process in which small molecular compounds will pass though semi-permeable membrane whereas colloids are retained. The non-dialyzable property of colloids account for (1) separation of colloids from diffusible molecules (2) removal of toxic compounds during dialysis procedures for renal failure, (3) formation of urine and formation of cerebrospinal fluid.
Donnan membrane equilibrium refers to equilibrium attained by diffusible ions in the presence of colloids (non-diffusible ions such as proteinate ions).
Tyndall effect Colloids exert Tyndall effect. Tyndall effect is light scattering property of colloids. Tyndall effect contributes for the detection of colloid particles by electron microscopy.
Charge property Colloids exist as charged molecules. This property accounts for the precipitations of proteins by acids and heavy metals. Charge property also accounts for buffering action of proteins in acid base balance.
Large surface area and adsorption Colloids adsorb compounds because of their charge property. Adsorption is further increased because of large surface area of colloids. This property contributes to more efficient action of enzymes in biological process. Adsorption also contributes to prevention of precipitation of inorganic molecules or lyophobic molecules.
Water of hydration Proteins are kept in solution because of water of hydration. Such colloids are called lyophilic colloids. Removal of water of hydration by alcohol or ammonium sulfate results in precipitation of proteins (Fig. 1.1.7).
Table 1.1.5   Colloids
Definition
Compounds that are non-diffusible and exist in colloidal state
Classification and examples
 Sol
Exists as soluble particles
 Lyophilic
Colloids having affinity for water: Albumin
 Lyophobic
Colloids not having affinity for water: Globulins
 Gel
Exists as insoluble molecules
Properties and significance
Brownian Movement
Dispersion in medium
Osmotic Pressure
Maintenance of plasma osmotic pressure and distribution of water
Non- permeability
Donnan membrane equilibrium Dialysis: formation of urine, hemodialysis, formation of CSF, purification of proteins
Tyndall effect
Light scattering effect: Detection of molecules in electron microscopy
Charge property
Precipitation of proteins
Water of hydration
Precipitation of proteins, isolation of proteins
Adsorption
Enzyme reactions, protection of charged or lyophobic molecules
Emulsification
Bile salts: Digestion and absorption of fats
Sol-gel
Formation of fibrin from fibrinogen
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This property is used for the separation and isolation of proteins (p. 134).
Emulsification Colloids such as bile salts act as micelles (emulsoids) because of their lyophilic nature. They help in the digestion and absorption of triacylglycerols.
Sol-gel conversion The soluble sol form of colloids can be converted into insoluble gel form of colloid either by enzymatic or chemical process. Example is blood clotting. Fibrinogen is present as soluble particle (sol form of colloids) in the circulation. However, during clotting process, it is acted upon by thrombin and factor XIII to form insoluble fibrin polymer (gel form of colloids). This results in the formation of tight clotting plug along with platelets.
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FIGURE 1.1.7: Precipitation of colloids
 
1.2. SUBCELLULAR ORGANELLES
Subcellular organelles are distinct internal membranous structures within the cell. They are endoplasmic reticulum, Golgi complex, mitochondria, lysosomes, peroxisomes and nucleus (Table 1.2.3).
 
I. ISOLATION OF SUBCELLULAR ORGANELLES
 
Homogenization
The initial step during subcellular fractionation is homogenization (Fig. 1.2.1). Homogenization is disruption of the cells using homogenizers. Usually cells and tissues are disrupted by
  1. Suspending in isotonic medium (such as 0.25 molar sucrose buffered to pH 7.4, and
  2. Homogenizing using glass or teflon/glass homogenizers.
    9
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FIGURE 1.2.1: Homogenizatioin
 
Differential Velocity Centrifugation
After homogenization, the subcellular organelles are isolated by differential velocity centrifugation. The separation is based on their size. In differential velocity centrifugation, the homogenate is centrifuged at different speeds to provide pellets that contain different subcellular components. Example is isolation of subcellular organelles of liver cells (Fig. 1.2.2).
Nucleus Centrifugation at 960 g for 10 minutes yields pellets containing mainly nuclei.
Mitochondria, lysosomes and peroxisomes. Centrifugation of 960 g supernatant at 25000g for 10minutes yields pellets containing mitochondria, lysosomes and peroxisomes.
Golgi complex Golgi complex is obtained as a pellet by centrifugation of 25000 g supernatant at 34000 g for 30minutes.
Endoplasmic reticulum Centrifugation of 34000 g supernatant at 105000 g for 100 minutes yields pellets (microsomal fraction) containing rough endoplasmic reticulum and smooth endoplasmic reticulum.
Cytosol 105000 g supernatant contains cytosol containing proteins such as enzymes and translation factors.
Further purification of subcellular organelles is carried out by density gradient centrifugation using compounds such as sucrose. In this technique, the homogenate is layered over a sucrose gradient and centrifugation is carried until the subcellular components reach equilibrium with the density of the surrounding medium.
 
II. IDENTIFICATION OF SUBCELLULAR ORGANELLES
 
Methods
Once isolated, the purity of different organelle preparations are determined by electron microscopy for assessing the morphology or biochemical methods using markers for each of the subcellular organelle (Table 1.2.1).
 
Marker Enzymes
Marker enzymes are enzymes that are found only with one particular subcellular organelle or structural components of cells. The measurement of enzyme activity of marker enzymes is useful in the assessment of purity (or degree of contamination) of the subcellular organelle.
Examples DNA polymerase is a marker enzyme for nucleus. ATP synthetase is a marker enzyme for mitochondria. Acid phosphatase is a marker enzyme for lysosomes. Catalase is a marker enzyme for peroxisomes. Galactosyl transferase is a marker enzyme for Golgi complex. Glucose 6 phosphatase is a marker enzyme for endoplasmic reticulum.
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FIGURE 1.2.2: Isolation of subcellular organelles
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Marker enzymes are also used to identify cell components other than subcellular organelles (Table 1.2.1). The marker enzyme for cytosol is lactate dehydrogenase and plasma membrane is 5'nucleotidase.
Table 1.2.1   Isolation and identification of subcellular organelles
Subcellular Organelle
Isolation
Identification Marker enzymes
Nucleus
960 g × 10 min
DNA polymerase
Mitochondria
25000 g × 30 min
ATP synthetase
Lysosomes
25000 g × 30 min
Acid phosphatase
Peroxisomes
25000 g × 30 min
Catalase
Golgi complex
34000 g × 30 min
Galactosyl transferase
Endoplasmic reticulum
105000 g × 100 min
Glucose-6-phosphatase
 
III. NUCLEUS Structure
Nucleus is the largest subcellular organelle. Nucleus is composed of nuclear membrane, nuclear pore, chromatin, nuclear matrix and nucleolus (Fig. 1.2.3).
Nuclear membrane is a bilayered membrane that is continuous with endoplasmic reticulum.
Nuclear pore is formed by the fusion of outer and inner layer of nuclear membrane.
Chromatin is composed of DNA complexed with histone proteins and nonhistone proteins. Chromosomes consist of DNA packaged into chromatin material (Fig. 1.2.3). Each somatic cell contains 46 chromosomes arranged in 22 pairs of different chromosomes (autosomes) and two sex chromosomes. Males have one x and y sex chromosome females have two X chromosomes.
Nuclear matrix is composed of factors required for DNA replication, DNA repair and transcription.
Nucleolus is a small dense material rich in ribosomal RNA. It has no limiting membrane. Nucleolus may be one or more in number depending on the cell type.
 
Functions
The functions of the nucleus are cell division and regulation of gene expression (Table 1.2.3).
Nuclear membrane compartmentalizes the nucleus.
Nuclear pore allows transport of molecules between the cytoplasm and the nucleus.
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FIGURE 1.2.3: Nucleus
Chromatin DNA is essential for replication (formation of DNA) and transcription (formation of RNA).
Nuclear matrix is involved in replication, DNA repair and transcriptional process in the nucleus.
Nucleolus is involved in the synthesis of rRNA and ribosomes.
 
Clinical Significance
Disorders of DNA include disorders of mitosis, apoptosis, chromosomal disorders and mutations (Table 1.2.5).
Disorders of mitosis Increased and uncontrolled mitosis leads to cancer. Anticancer drugs cause cell cycle arrest and used in the treatment of cancer (p. 612).
Apoptosis or programed cell death is characterized by fragmentation of DNA. Apoptosis is seen during development and carcinogenesis.
Chromosomal disorders Example of chromosomal disorder is Trisomy. Trisomy is a disorder resulting from failure in the separation of sister chromatids (non-junction) during meiosis (p. 630).
Mutations Mutation refers to change in the sequence of DNA. Mutations are underlying mechanisms for many genetic disorders and cancer. Example is sickle cell disease (p. 684).
 
IV. ENDOPLASMIC RETICULUM
 
Definition
Endoplasmic reticulum is a highly convoluted network of membranes. It is continuous with nuclear and plasma membrane.11
 
Types
There are two types of endoplasmic reticulum. They are rough endoplasmic reticulum and smooth endoplasmic reticulum.
 
Rough Endoplasmic Reticulum (RER)
Structure RER is a network of membranes studded with ribosomes (Fig. 1.2.4).
Functions RER is the site of protein synthesis because of attached ribosome. RER predominates in cells that secrete proteins. RER is also the site for post-translational modifications of proteins such as glycosylation.
Ribosomes Ribosomes are made up of proteins and ribosomal RNA. Ribosomes are either attached to RER or float freely in the cytoplasm. In eukaryotes, ribosomes are 80S ribosomes consisting of 60S large subunit and 40S small subunit. Large subunit contains 28S, 5S and 5.8S rRNA. Small subunit contains 18S rRNA. Ribosomes are the sites of protein synthesis. Protein synthesis occurs by the interaction of mRNA, tRNA and ribosomes (Fig. 1.2.5).
 
Smooth Endoplasmic Reticulum (SER)
Structure SER is a network of membranes but not studded with ribosomes (Fig. 1.2.6).
Functions SER is the site of synthesis of phospholipids and cholesterol in many tissues. SER is the site of synthesis of steroid hormones in adrenals, gonads and placenta. SER is the site of glycogen metabolism in liver and muscle. SER is the site of both fatty acid elongation and desaturation. SER is involved in the storage and releases of calcium in cells (sarcoplasmic reticulum of skeletal muscle).
SER is also the site of detoxification of drugs and chemicals in liver and kidney. The enzyme involved in drug detoxification is cytochrome P450. Example for cytochrome P450 catalyzed reaction is hydroxylation of drugs such as barbiturates and phenytoin.
Biomedical importance Alcohol induces cytochrome P450 and thus affects the metabolism of drugs.
 
V. MITOCHONDRIA
 
Structure
Mitochondria are elongated organelle surrounded by two-layered outer and inner membrane (Fig. 1.2.7).
Outer membrane is freely permeable to small organic molecules and larger proteins.
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FIGURE 1.2.4: Rough endoplasmic reticulum
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FIGURE 1.2.5: Rough endoplasmic reticulum
zoom view
FIGURE 1.2.6: Smooth endoplasmic reticulum
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zoom view
FIGURE 1.2.7: Mitochondria
Inner membrane exhibits selective permeability. The inner membrane is highly folded and the foldings are called cristae. It contains the components of electron transport chain and oxidative phosphorylation.
Inter-membrane space is the space between outer membrane and inner membrane. It contains few enzymes. Example is adenylate kinase.
Matrix The large internal compartment is called the matrix. It contains enzymes involved in intermediary metabolism such as enzymes of citric acid cycle and β-oxidation of fatty acids. It also contains DNA and ribosomes (Table 1.2.2).
 
Functions
Inner mitochondrial membrane and mitochondrial matrix are involved in ATP production and metabolism.
Inner mitochondrial membrane is involved in the production of ATP since it contains the components of the electron transport chain and oxidative phosphorylation.
Mitochondrial matrix is the site of metabolic reactions (oxidation of fatty acids, citric acid cycle, and steroid hormone synthesis) and replication of mitochondrial DNA.
 
Clinical Significance
Disorders of mitochondrial function can be caused by inherited and acquired causes. Several inhibitors cause impairment of mitochondrial function.
Inherited disorders Examples for inherited disorders of mitochondrial metabolic functions are (i) fumarase deficiency (mitochondrial encephalomyelopathy) and (ii)multiple acyl CoA dehydrogenase deficiency (organic aciduria or glutaric aciduria type II). Example for inherited disorder of oxidative phosphorylation is LEON disease (p. 632).
Table 1.2.2   Mitochondrial enzymes
Location
Enzyme
Function
Outer membrane
Monoamine oxidase
Catabolism of biogenic amines
Inner membrane
Respiratory chain enzymes (NADH dehydrogenase cytochrome reductase cytochrome oxidase)
Electron transport
ATP Synthase
Oxidative phosphorylation
Matrix
Citrate synthase Fatty acyl CoA dehydrogenase
Oxidation of acetyl CoA β-oxidation of fatty acids
Apoptosis Mitochondrial dysfunction is one of the early events in the cells undergoing apoptosis. Apoptosis is a programed cell death and plays important role in development, differentiation and cancer.
Inhibitors Examples for inhibitors of mitochondrial function are cyanide and carbon monoxide. Cyanide and carbon monoxide inhibit cytochrome oxidase (component of electron transport chain) thus decreasing ATP synthesis. Hence they act as respiratory poisons.
Megamitochondria (larger mitochondria) is seen in viral hepatitis and alcoholic liver disease. Mitochondrial functions are impaired in conditions associated with megamitochondria.
Lack of mitochondria in erythrocytes Erythrocytes lack mitochondria. Therefore, anaerobic glycolysis provides energy requirement of erythrocytes (p. 237).
 
VI. GOLGI COMPLEX
 
Structure
Golgi complex consists of flattened membrane-lined cisternae. It is located near the nucleus (Fig. 1.2.8).
 
Functions
The functions of Golgi complex include (1) Intracellular sorting of proteins, and (2) post-translational modifications of proteins.
Intracellular sorting of proteins Golgi complex has three faces. The cis face is the face where the rough endoplasmic reticulum releases the protein to the Golgi complex. In the medial face, the proteins are modified by glycosylation and related processes. The processed proteins then move into the trans face. From the trans face, the proteins are destined to different locations, e.g. (1) plasma membrane, (2) lysosomes, and (3) storage granules to be used for exocytosis.
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zoom view
FIGURE 1.2.8: Golgi complex
Post-translational modifications of proteins Post-translational modifications of proteins include glycosylation and sulfation of proteins.
 
Clinical Significance
I cell disease (also called inclusion cell disease or mucolipidosis II) is a disorder of Golgi function caused by impairment in the delivery of enzymes destined for lysosomes. The lysosomal enzymes are synthesized by the endoplasmic reticulum but they are secreted extracellularly instead of being delivered to the lysosomes. The condition is characterized by (1) deficiency of lysosomal enzymes and (2) increased accumulation of glycosaminoglycans and glycolipid in connective tissues such as fibroblasts.
 
VII. LYSOSOMES
 
Structure
Lysosomes are spherical membrane-bound structures containing hydrolytic enzymes (Fig. 1.2.9).
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FIGURE 1.2.9: Lysosome
Table 1.2.3   Structure and functions of subcellular organelles
Subcellular organelle
Structure
Functions
Nucleus
Nuclear membrane, nuclear pore, nuclear matrix, chromatin, nucleolus
Replication, transcription, DNA repair, rRNA ribosome synthesis
Endoplasmic reticulum
Rough ER
Studded with ribosomes
Protein synthesis, post-translational modification of proteins, glycogen metabolism, fatty acid synthesis, steroid metabolism, detoxification.
Smooth ER
Not studded with ribosomes
Golgi complex
Flattened membrane lined cisternae
Intracellular sorting of proteins, post-translational modification of proteins
Mitochondria
Outer membrane, inner membrane, inner membrane space, matrix
Oxidative phosphorylation, TCA cycle, fatty acid oxidation, mitochondrial DNA metabolism
Lysosomes
Spherical membrane bound sacs. Contain hydrolytic enzymes in an acidic environment
Intracellular digestion of macromolecules, phagocytosis, receptor mediated endocytosis
Peroxisomes
Membrane bound sac containing enzymes such as catalase and uric acid oxidase
Oxidation of amino acids, oxidation of long chain fatty acids, H2O2 metabolism, bile acid synthesis, plasmalogen synthesis
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Functions
Functions of lysosomes include (1) control of intracellular digestion of carbohydrates, proteins, lipids and nucleic acids, and (2) digestion of phagocytosed foreign particles.
Control of intracellular digestion of carbohydrates, proteins, lipids and nucleic acids Control of intracellular digestion of carbohydrates, proteins, lipids and nucleic acids occurs by lysosomal enzymes. Lysosomal enzymes are hydrolytic enzymes that are active at acidic pH (pH 4–5). Examples are cathepsin, phospholipase, acid maltase and acid phosphatase. Cathepsin (protease) degrades proteins. Phospholipase (lipase) degrades phospholipids. Acid maltase (glycosidase) degrades glycogen. Acid phosphatase (phosphatase) degrades nucleotides.
Digestion of phagocytosed foreign particles Digestion of phagocytosed foreign particles occurs by forming secondary lysosomes. Phagolysosomes are secondary lysosomes formed by the fusion of phagocytosed molecules such as bacteria with the lysosomes (p. 25). Endolysosomes (p. 26) are secondary lysosomes formed by the fusion of endosome with the lysosome. Endosomes are formed by receptor-mediated endocytosis such as uptake of LDL by the LDL receptor.
 
Clinical Significance
Disorders of lysosomes may result from inherited disorders or acquired causes.
Inherited disorders Lysosomal storage disorders are caused by inherited deficiency of lysosomal degradative enzymes. Examples of lysosomal disorders are Gaucher's disease (p. 310), Niemann Pick disease (p. 308) Hunter's syndrome (p. 790), Pompe's disease (p. 269). Gaucher's disease is a lysosomal storage disorder of glucocerbroside degradation due to the deficiency of the enzyme glucocerebrosidase.
Acquired disorders: Inflammatory disorders Release of lysosomal enzymes by leukocytes in response to chemical injury (uric acid), allergens, and microbes results in inflammatory response. Example is phagocytosis of uric acid crystals by macrophages in gouty arthritis. The phagocytosis of uric acid crystals results in Iysosomal damage and release of lysosomal enzymes.
Autolysis Rupture of the lysosomal membrane will result in leakage of lysosomal enzymes, which digest the cell itself. This is known as autolysis. Autolysis occurs after cell injury.
Table 1.2.4   Lysosomal enzyme deficiencies in lysosomal disorders
Disorder
Substrate accumulated
Enzyme deficiency
Gaucher's diasease
Glucocerebroside
Glucocerebrosidase
Pompe's disease
Glycogen
Acid maltase
Hunter's syndrome
Heparan sulfate
Iduronidase
Dermatan sulfate
Table 1.2.5   Clinical aspects of subcellular organelles
Subcellular organelle
Clinical aspects
Nucleus
Disorders of mitosis: Cancer
Apoptosis: Aging, cancer
Chromosomal disorders: Trisomy
Mutations: Sickle cell disease
Smooth ER
Induction of cytochrome P450 by ethanol
Golgi complex
I cell disease due to secretion of enzymes extra-cellularly instead of being delivered to lysosomes
Mitochondria
Inherited disorders (LEON disease); apoptosis (Toxic insult, cancer) megamitochondria (alcoholic liver disease)
Lysosomes
Inherited disorders (Gaucher's disease, Hurler's syndrome, Hunter's syndrome), autolysis inflammation (arthritis).
Peroxisomes
Zellweger's syndrome due to inherited defect in import of peroxisomal proteins into the matrix
 
VIII. PEROXISOMES
 
Structure
Peroxisomes are membrane-bound sacs containing enzymes such as catalase and uric acid oxidase.
 
Functions
The functions of peroxisomes are the following:
Oxidation of amino acids and fatty acids Peroxisomes are involved in degradation of amino acids, and oxidation of long chain fatty acids (C-26 to C− 28).
Hydrogen peroxide metabolism Peroxisomes are sites of metabolism of hydrogen peroxide. Amino oxidase reaction produces hydrogen peroxide. Catalase reaction decomposes hydrogen peroxide to water and oxygen.
Biosynthesis of bile acids and plasmalogens Bile acid synthesis and synthesis of plasmalogens occur in peroxisomes.15
 
Clinical Significance
Zellweger's syndrome (cerebrohepatorenal syndrome) is an inborn error of peroxisomal dysfunction. The basic defect is failure to import peroxisomal proteins into the matrix. The biochemical findings are low tissue plasmalogen level, and accumulation of long chain polyunsaturated fatty acids (C-26 to C-28) in the brain. Clinical features are neurologic and hepatic dysfunction. The victims die within one year after birth.
 
1.3. PLASMA MEMBRANE
 
I. STRUCTURE OF PLASMA MEMBRANE
 
Composition and Organization: Fluid Mosaic Model
Plasma membrane is a limiting membrane required for cellular organization and integrity (Fig. 1.3.1 and Table 1.3.1).
Fluid mosaic model The organization of plasma membrane is explained by the fluid mosaic model that describes plasma membrane as a phospholipid bilayer containing embedded proteins and cholesterol. Carbohydrates are present on the surface as glycoproteins and glycolipids.
 
Lipids
The major lipid components of membrane are phospholipids, glycolipids and cholesterol. Lipids act as permeability barriers. They are essential for the maintenance of fluidity of membranes.
Phospholipids include phosphatidylcholine, phospha- tidylethanolamine, phosphatidylserine, phospha- tidylinositol, plasmalogens and sphingomyelin (Fig. 1.3.2).
zoom view
FIGURE 1.3.1: Fluid mosaic model of biological membranes
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Table 1.3.1   Structure of plasma membrane
Composition and organization
Fluid mosaic model
Phospholipid bilayer with embedded proteins and cholesterol. Carbohydrates are present on the surface as glycoproteins or glycolipids
Lipids
Phospholipids, glycolipids, cholesterol, cholesterol esters
Proteins
Integral proteins-Na-K ATPase
Peripheral proteins-adenyl cyclase
Carbohydrates
Galactose, mannose, glucose, N-acetyl-gluosamine, N-acetyl galactosamine
Membrane fluidity
Factors affecting
Temperature
Chain length of fatty acids
Degree of unsaturation of fatty acids
Cholesterol content
Membrane asymmetry
Lipids
PC: Exterior, PS and PE: Interior
Proteins
C-terminal end on the cytosolic side
Membrane skeleton
Spectrin
Present in erythrocyte membrane
Dystrophin
Present in muscle
Phospholipids have hydrophilic (polar) head and a hydrophobic (non-polar) tail. The polar heads are due to the presence of phosphate and additional groups. The polar head groups are exposed to the aqueous media both outside and inside the cell. The non-polar tails are made up of saturated and unsaturated fatty acids. These non-polar tails are directed away from the water (Fig. 1.3.3).
Glycolipids include cerebrosides (glucocerebroside, galactocerebroside), sulfatide, globosides and ganglio- sides. They are present on the surface of the membrane
Cholesterol may be present either as free cholesterol or as cholesterol ester. Cholesterol is embedded in the hydrophobic tail region of phospholipids.
 
Proteins
Proteins mediate most of the functions of plasma membrane. Examples are receptors, ion channels, enzymes and transport proteins. The proteins present in the plasma membrane are grouped into peripheral proteins and integral proteins based on their location.
Peripheral proteins are associated with surface of the membrane. Examples are acetylcholinesterase on the extracellular side of the membrane and adenyl cyclase on the intracellular side of the membrane.
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FIGURE 1.3.2: Components of phospholipid molecule
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FIGURE 1.3.3: Asymmetry of proteins and lipids
Integral proteins are deeply embedded in the lipid layer.Transmembrane proteins are integral proteins that span the lipid bilayer, exposing the protein to both the extracellular space and the cytoplasm. Examples are transporter proteins such as sodium-potassium ATPase and ion channels such as sodium channel.
Marker enzymes Marker enzymes for plasma membrane are 5′ nucleotidase and Na-K ATPase.
 
Carbohydrates
Carbohydrates are covalently bound to lipids to form glycolipids or proteins to form glycoproteins. They are the minor components of the cell membrane.
Monosaccharide units present are glucose, galactose, mannose, fucose, N-acetyl- glucosamine and N-acetylgalactosamine.
Carbohydrates are present on the surface of the membrane.17
 
Variations in Protein, Lipid and Carbohydrate Contents
The proportions of protein, lipid and carbohydrate components vary from one cell type to another. For example, liver plasma membrane contains about 54% protein, 39% lipid and 7% carbohydrate whereas myelin sheath contains about 20% protein, 75% lipid and 5% carbohydrate.
 
Membrane Asymmetry
Membrane asymmetry refers to different composition of outer and inner layer of membrane in both lipid and protein content.
Asymmetry of lipids The outer membrane is usually rich in sphingomyelin and phosphatidylcholine. The inner membrane is largely composed of phosphatidylserine and phosphatidylethanolamine.
Asymmetry of proteins The C-terminal end of trans membrane proteins is usually found in the cytosolic side of the plasma membrane. The N-terminal end of the transmembrane proteins is found extracellularly.
 
Membrane Fluidity
The membrane fluidity is essential for many functions of the membrane such as permeability and enzyme activity. The factors affecting membrane fluidity are (1) temperature, (2) chain length and degree of unsaturation of the fatty acids, and (3) the presence of cholesterol (Fig. 1.3.4).
Temperature Membrane fluidity is increased by increase in the temperature. Thermal changes cause continuous random motion of the hydrocarbon chain in the center of the bilayer. Plasma membrane, therefore, behaves as a viscous fluid.
Chain length and unsaturation of fatty acids Membrane fluidity is increased by the presence of long chain unsaturated fatty acids. Unsaturated fatty acids with their cis double bonds do not pack easily.
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FIGURE 1.3.4: Factors affecting membrane fluidity
Cholesterol Membrane fluidity is decreased by the presence of cholesterol. The physical rigidity due to steroid ring of cholesterol makes the lipid bilayer stiffer in the tail region. Cholesterol prevents the movement of fatty acyl chains.
 
Membrane Skeleton
Membrane skeleton is a fibrous meshwork of proteins that is present in the plasma membrane and linked to integral membrane proteins. Examples are dystrophin and spectrin.
 
Dystrophin
Structure Dystrophin is a membrane skeletal protein present in skeletal, cardiac and smooth muscle fibers. It is a large protein with 24 spectrin repeats. It has actin binding and calcium binding domains (Fig. 1.3.5).
Functions Dystrophin stabilizes the sarcolemma during deformations that occur during muscle contraction.
 
Spectrin
Structure Spectrin is a meshwork of long thin fibers present under the plasma membrane of erythrocyte membranes. Spectrin fibers are present as tetramers of α spectrin and α spectrin. Spectrin network is attached to actin and peripheral membrane protein called ankyrin. Actin is attached to band 4.1 protein and ankyrin is attached to band 3 protein(Fig. 1.3.6).
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FIGURE 1.3.5: Dystrophin
18
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FIGURE 1.3.6: Spectrin
Functions The spectrin provides structural integrity for erythrocytes. It also provides resilience for erythrocytes against mechanical insults during their travel through capillaries.
 
II. FUNCTIONS OF PLASMA MEMBRANE
Functions of plasma membrane are (i) transport of molecules across the plasma membrane, (ii) cell-cell communication, (iii) transmembrane signaling and cellular regulation, (iv) compartmentalization and (v) membrane modifications for specialized functions (Table 1.3.2).
Table 1.3.2   Functions of plasma membrane
Transport of molecules
Control of cell volume, excitable property, secretion, phagocytosis pinocytosis
Cell-cell communication
Role of glycoproteins
Cell signaling
Growth factor and hormone action
Compartmentalization
Distinct subcellular organelles with specific functions
Modifications
Cell junctions, myelin sheath of neuron, synaptosomes, microvilli
 
Transport of Molecules Across the Plasma Membrane
Control of cell volume Membranes have specific molecular channels and pumps that provide selective permeability. For example, sodium-potassium ATPase pump maintains electrolyte composition (p. 23).
Excitable property The membrane proteins function to regulate permeability for inorganic ions. The generation of transmembrane voltage difference results in the generation of membrane potential. This accounts for the excitability of biological membrane (p. 21).
Secretion Secretion occurs by exocytosis. Example is release of acetylcholine from the nerve terminals (p. 841).
Phagocytosis is engulfing large particles such as bacteria by monocytes and neutrophils (p. 25).
Pinocytosis is the process of engulfing large molecules in suspension by the cell. Example is low density lipoprotein (LDL) receptor mediated endocytosis of low density lipoproteins (p. 26).
 
Cell-cell Communication
Cell to cell communication and cell adhesion are essential for coordinated function in a tissue. These functions are mainly due to the presence of carbohydrates (p. 30).
 
Transmembrane Signaling and Cellular Regulation
Membranes contain enzymes, proteins and receptors that allow the cell to respond to external stimuli. Example is regulation of cell function by epinephrine (p. 822).
 
Compartmentalization
Membranes allow cellular functions to occur in distinct compartments of the cell. For example, nucleus contains almost all the cellular DNA and enzymes for replication and transcription.
 
Membrane Modifications
Membrane modifications permit specialized functions of the plasma membrane. Examples are cell junctions, myelin sheath, synaptosomes and microvilli (Fig. 1.3.7).
Cell junctions permit intracellular transport of molecules and cell to cell adhesion.
Myelin sheath is essential for neural function. Multiple sclerosis is a disorder of demyelination.
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FIGURE 1.3.7: Membrane modifications
19
Synaptosomes containing presynaptic and post synaptic region function in neurotransmission.
Microvilli in small intestinal mucosal cell increase the surface area for digestion and absorption of foods.
 
III. CLINICAL ASPECTS OF PLASMA MEMBRANE
Disorders of membrane structure and function are caused by diverse factors and disease states. Examples include (i) mechanical injury, (ii) hypoxic injury, (iii) mutations of membrane proteins, (iv) autoantibodies to membrane receptors, (v) defective membrane lipid composition, and (vi) disorders of membrane skeleton (Table 1.3.3).
Table 1.3.3   Clinical aspects of plasma membrane
Mechanical injury
Sickle cell disease: Hemoglobin S
Chemical injury
Snake bite: Activation of phospholipases
Hypoxic injury
Ischemia: Activation of phospholipases
Mutations of proteins
Familial hypercholesterolemia: Mutation of LDL receptor
Antibodies to receptors
Myasthenia gravis: Antibodies to acetylcholine receptor
Abnormal lipid components
Gaucher's disease: Accumulation of glucocerebroside
Membrane skeletal disorders
Duchenne muscular dystrophy: Mutated dystrophin
Hereditary spherocytosis: RBC spectrin mutation
 
Mechanical Injury
Sickle cell disease is an example for mechanical injury to the plasma membrane. Erythrocyte membrane damage occurs by precipitates of deoxyhemoglobin S.
 
Chemical Injury
Snake bite Snake venom poison contains phospho- lipases that destroy the membrane by acting on phospholipids.
 
Hypoxic Injury
Ischemia causes hypoxic injury to the plasma membrane. Ischemia can affect the membrane dysfunction, leading to the dysfunction of ion channels and sodium-potassium ATPase.
 
Mutations of Membrane Proteins
Familial hypercholesterolemia is an example for a disorder of mutation in the plasma membrane protein. It is due to mutations in the gene encoding the LDL receptor.
 
Autoantibodies to Membrane Receptors
Myasthenia gravis is an example for membrane receptor disorder. It results from autoantibodies to acetylcholine receptor in skeletal muscle.
 
Defective Membrane Lipid Composition
Gaucher's disease is an example for disorder of altered plasma membrane lipid composition. In Gaucher's disease, deficiency of glucocerebrosidase leads to the accumulation of glucocerebroside that causes membrane dysfunction.
 
Disorders of Membrane Skeleton
Duchenne Muscular dystrophy is an X-linked inherited disorder caused by the absence of dystrophin in skeletal muscle.
Hereditary spherocytosis and hereditary elliptocytosis are caused by inherited defect in spectrin, ankyrin or other band proteins of erythrocyte membranes. The condition is characterized by the presence of small round shaped erythrocytes (spherocytes) or ellipsoidal shaped erythrocytes (elliptocytes). These cells are very fragile and hemolyzed during their passage through splenic capillaries.
 
1.4. PLASMA MEMBRANE TRANSPORT MECHANISMS
Transport mechanisms The different transport mechanisms that occur across the plasma membrane are (i) passive transport, (ii) active transport, (iii) endo- cytosis and (4) exocytosis.
 
I. PASSIVE TRANSPORT
Passive transport occurs in four ways: simple diffusion, facilitated diffusion, osmosis and filtration (Table 1.4.1).
 
Simple Diffusion
 
Definition
Simple diffusion (Fig. 1.4.1) is the movement of particles from the area of higher concentration to an area of lower concentration.
 
Features
Simple diffusion occurs down the concentration gradient. It does not require metabolic energy; therefore it is passive. It is not carrier mediated.
 
Examples
Examples for transport by simple diffusion are (1) transport of lipid soluble molecules, and (2) transport of ions.
Table 1.4.1   Passive transport across cell membrane
Simple diffusion
Occurs down the concentration gradient
Does not require energy
Does not require carrier protein
 Examples
Transport of gases
Transport of ions through ion channels
 Clinical aspects
Disorders: Cystic fibrosis due to mutations in the chloride channel
Sulfonylureas: Cause closure of potassium channels and used for treating diabetes mellitus
Facilitated diffusion
Occurs down the concentration gradient
Does not require energy
Requires carrier protein
 Examples
Transport of fructose, transport of glucose
 Clinical aspects
Disorders: Glucose transporter defect (inherited)
Defect in glucose transporter recruitment (diabetes)
Osmosis
Movement of molecules due to osmotic pressure
 Examples
Movement of water between plasma and cells
 Clinical aspects
Edema caused by hypoalbuminemia
Filtration
Passage of molecules depending on hydrostatic pressure
 Examples
Filtration across capillaries, formation of urine
Clinical aspects
Decreased GFR: Damage to nephrons Edema: Altered volume
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FIGURE 1.4.1: Simple diffusion
Lipid soluble, neutral molecules readily diffuse through the lipid membrane. Examples are diffusion of oxygen and urea.21
Ions Because of their charge property, ions cannot be transported readily across the membrane. However, they can pass through ion channels. Ion channels (or gated pores) are transport systems that open and close on receipt of a signal. Ion channels are created by membrane protein molecules. Ion channels are selectively permeable to specific ions. Ion channels are of two types: voltage gated channels and ligand gated channels.
  • Voltage gated channels Channels that open and close based on the charge across the membrane are called voltage gated channels. Examples are calcium channel, sodium channel, potassium channels and chloride channel.
  • Ligand gated channels. Channels that are opened by chemical signals are called ligand gated channels. Examples are channels opened by acetylcholine or glutamate.
 
Clinical Significance
Disorders of simple diffusion can occur by both inherited diseases and acquired causes. Several drugs are ion channel inhibitors and used in the treatment of various disease states including hypertension and diabetes mellitus.
Cystic fibrosis results from mutations in the chloride channel.
Ion channel modulators as drugs Examples are sulfonylureas. They are used as oral antidiabetic drugs. They cause closure of potassium channel leading to insulin release from the islets of Langerhans in the pancreas.
 
Facilitated Diffusion
 
Definition
Facilitated diffusion (Fig. 1.4.2) is diffusion of molecules down the concentration gradient requiring the presence of a carrier.
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FIGURE 1.4.2: Facilitated diffusion
 
Features
Facilitated diffusion occurs down the concentration gradient. It does not require metabolic energy and therefore passive. It is carrier mediated and therefore exhibits specificity, saturation and competition. It is more rapid than simple diffusion.
 
Mechanism
Facilitated diffusion occurs by ping-pong mechanism. The molecules to be transported bind to the carrier protein, which is in pong configuration. The carrier then undergoes a conformational change to ping form to allow the molecule to pass through to the other side of the membrane. The carrier then regains its original conformation (pong form).
 
Examples
Examples for facilitated diffusion are (1) fructose absorption, and (2) glucose transport.
Fructose absorption from the small intestine and proximal renal tubules occurs by facilitated diffusion.
Glucose transport in adipose tissue and muscle occurs by facilitated diffusion. Insulin increases the glucose transport in adipose tissue and muscle by increasing the recruitment of glucose transporters.
 
Uniport and Antiport (Fig. 1.4.3)
Based on the direction of movement of molecules, facilitated diffusion can occur by uniport or antiport.
Uniport Transport of single type of molecules in one direction is called uniport. Examples are absorption of fructose and transport of glucose by facilitated diffusion.
Antiport Transport of molecules in opposite direction is called antiport. Example is chloride-bicarbonate exchanger (anion transpoter). Chloride-bicarbonate exchanger is involved in secretion of hydrochloric acid in the stomach and chloride shift in erythrocytes.
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FIGURE 1.4.3: Uniport and antiport
22
 
Osmosis
 
Features
Osmosis means the diffusion of water through a semipermeable membrane. Water molecules move from a more dilute solution into a more concentrated solution.
Osmotic pressure is produced when semipermeable membrane (a membrane permeable only to water) separates the two solutions that have different concentrations of particles. Colloid osmotic pressure draws water into the capillaries from the interstitial fluid.
 
Clinical Significance
Decreased formation of urine in hypovolemic conditions Formation of urine is dependent on the balance between hydrostatic pressure and osmotic pressure in the glomerular capillaries in the nephron. Osmotic pressure is exerted by albumin since albumin is not filtered by the glomerulus. Decrease in hydrostatic pressure is seen in hypovolemic shock. It results in impaired glomerular filtration due to unopposed osmotic pressure preventing flow of water out of capillaries.
Edema due to hypoalbuminemia One of the factors determining plasma volume and interstitial volume is plasma osmotic pressure contributed by plasma albumin. In conditions associated with hypoalbuminemia (such as cirrhosis of liver (p. 765) and nephrotic syndrome (p. 724) plasma osmotic pressure is decreased. This results in flow of water to the interstitial compartment. The condition is called edema. Fluid accumulation occurs in peritoneal cavity (ascitis), pleural cavity (pleural effusion) and dependent parts of body (edema of legs).
Isotonic solution: Physiological saline for replacement of fluid loss Physiological saline solution contains 0.9% sodium chloride. It does not cause hemolysis of erythocytes. It is administered to replace fluid loss in conditions such as gastroenteritis.
Hypertonic solution: Administration of mannitol for cerebral edema Hypertonic solution draws water to its compartment from the other compartment such as cellular compartment. Mannitol is a osmotically active compound. It is used in the treatment of cerebral edema.
Hypotonic solution: Osmotic fragility test for hereditary spherocytosis Water flows into the cell such as erythrocytes in hypotonic solution (0.2% sodium chloride). Flow of water into erythocytes leads to lysis of erythrocytes (hemolysis). In hereditary spherocytosis (p. 19) red cell membrane is more fragile. When erythrocytes are placed in hypotonic solution, they become more fragile compared to normal erythrocytes. This test is used to confirm the diagnosis of hereditary spherocytosis.
 
Filtrat
 
Definition
Filtration is the passage of water containing dissolved molecules through a membrane as a result of a mechanical force on one side of the membrane exerted by fluid (or hydrostatic pressure).
 
Features
Pressure gradient or hydrostatic pressure Filtration is a passive process and it is driven by a pressure gradient. It is mainly seen in the capillaries. The pressure gradient pushes the solute containing fluid (filtrate) from the higher pressure area to the low pressure area across the capillaries. The pressure gradient (hydrostatic pressure) is generated by contractions of the heart, gravity and vascular tone in the blood vessels.
Selectivity Filtration is not completely selective. The cells of blood and proteins are left behind in the blood and small molecules and water are pushed across the membranes by the hydrostatic pressure.
 
Examples and Significance
Filtration across the capillaries supply water and nutrients to the tissues.
Formation of urine in the nephron of the kidney is essential for removal of waste products and regulation of body homeostasis.
 
II. ACTIVE TRANSPORT
Active transport occurs in three ways: primary active transport, secondary active transport and group translocation (Table 1.4.2).
 
Primary Active Transport
 
Definition
Primary active transport is transport against concentration gradient in which energy is used directly.
23
Table 1.4.2   Active transport across the plasma membrane
Primary active transport
Transport of molecules against concentration gradient
Energy is used directly
 Examples
Na-K ATPase: Cellular regulation
Ca-ATPase: Muscle contraction
H-K ATPase: Gastric HCl secretion
 Clinical aspects
Digoxin: Na-K ATPase inhibitor for CCF
Omeprazole: H-K ATPase inhibitor for peptic ulcer
Secondary active transport
Against the concentration gradient Requires energy indirectly
 Examples
Symport—Sodium glucose transporter, Antiport—Na-H exchanger
 Clinical aspects
Disorders—Cystinuria, Hartnup disease
Inhibitors—Phlorhizin: renal glycosuria
Group Translocation
Modification of molecules to be transported across the membrane
 Example
Gamma glutamyl cycle
 
Features
Primary active transport occurs against the concentration gradient. It requires energy in the form of ATP and the energy is used directly. It is carrier mediated.
 
Examples
Examples are sodium-potassium ATPase, calcium-ATPase and potassium-hydrogen ATPase.
Sodium-potassium ATPase This is also called sodium pump. This pump moves 3 sodium ions out of the cell and moves 2 potassium ions to inside the cell for each ATP consumed. Thus it maintains the low intracellular sodium and high intracellular potassium (Figs 1.4.4 and 1.4.5).
Calcium-ATPase is present in the sarcoplasmic reticulum or cell membrane. It transports calcium against the concentration gradient
Potassium-hydrogen ATPase is present in the parietal cells of the stomach. It is involved in the secretion of hydrochloric acid. It transports hydrogen ions into the lumen of the stomach against the concentration gradient.
 
Clinical Significance
Inhibitors of sodium pump and potassium hydrogen ATPase are used as drugs. Examples are digoxin and omeprazole.
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FIGURE 1.4.4: Sodium-potassium ATPase
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FIGURE 1.4.5: Mechanism of sodium-potassium transport
Digoxin is a specific inhibitor of sodium-potassium pump. Inhibition of sodium-potassium ATPase results in increase in the intracellular concentration of calcium. Digoxin is used in congestive cardiac failure to increase the myocardial contractility.
Omeprazole is a specific inhibitor of potassium-hydrogen ATPase causing reduced secretion of hydrochloric acid. It is used in the treatment of peptic ulcer.
 
Secondary Active Transport
 
Definition
Secondary active transport is a coupled active transport in which energy is provided indirectly.24
 
Features
In secondary active transport, the transport of two or three molecules are coupled. The transport is coupled to Na-K ATPase, that requires ATP.
 
Types
Secondary active transport occurs by symport and antiport.
 
Symport
In symport (also called co-transport), the molecules move in the same direction. Examples are sodium-glucose symporter and sodium-amino acid symporter (Fig. 1.4.6).
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FIGURE 1.4.6: Symport: Sodium-glucose transporter
Sodium-glucose symporter is present in the small intestine and proximal renal tubules. It is involved in the absorption of glucose.
Sodium-amino acid symporter is present in the small intestine and proximal renal tubules. It is involved in the absorption of amino acids.
 
Antiport
In antiport (also called counter transport), the molecules move in opposite directions. Examples are sodium-hydrogen exchanger and calcium hydrogen exchanger.
Sodium-hydrogen exchanger is present in renal proximal tubule. It is involved in acid-base regulation (Fig. 1.4.7).
Calcium-hydrogen exchanger is present in cardiac muscle. It is involved in maintaining myocardial contractility.
 
Clinical Significance
Disorders Cystinuria and Hartnup disease are inherited disorders caused by mutations in the sodium-amino acid symporter for cystine (cystinuria) and tryptophan (Hartnup disease).
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FIGURE 1.4.7: Antiport: sodium-hydrogen exchanger
Inhibitors Phlorhizin is an inhibitor of sodium-glucose symporter system. It inhibits the coupling of sodium to the sodium-glucose symporter. Hence it blocks the absorption of glucose at the proximal renal tubules causing increased excretion of glucose in urine. The condition is called renal glycosuria.
 
Transport by Group Translocation
 
Definition
Transport by group translocation refers to modification of molecules to be transported across the membrane.
 
γ-Gluatmyl Cycle (Meister cycle)
Example for group translocation is transport of amino acids involving γ glutamyl cycle. The cycle occurs in the nephron and jejunum. Glutathione is involved in this cycle (p. 207).
 
III. EXOCYTOSIS
Transport by vesicle formation or bulk transport. Exocytosis and endocytosis are examples for transport by vesicle formation since the transport processes involve the formation of membrane bound vesicles. Exocytosis and endocytosis are also examples for bulk transport because the transport process involves transport of macromolecules and cells and occurs by engulfing (endocytosis) or bulk release of molecules from vesicles (exocytosis) (Table 1.4.3).
 
 
Definition
Exocytosis is expulsing out of molecules from the cell.
 
Features
Molecules released by exocytosis have different fates:
Peripheral proteins They can attach to the cell surface and become peripheral proteins. Example is antigen.
25
Table 1.4.3   Exocytosis
Features
Expelling molecules from the cell
Examples
Different fates of molecules:
 Peripheral proteins: Antigen
 Part of extracellular matrix: Collagen
 Release to extracellular medium: Release of neurotransmitters
Mechanism
Agonist induced transient change in intracellular calcium
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FIGURE 1.4.8: Exocytosis
Part of extracellular matrix They can become part of extracellular matrix. Example is collagen.
Release to extracellular medium They can be released extracellularly. Examples are release of neurotrans- mitters and insulin.
 
Mechanism
The signal for exocytosis is often a hormone. Hormone binds to the cell surface receptor, induces a local, transient change in cytosolic calcium concentration that triggers exocytosis (Fig. 1.4.8).
 
IV. ENDOCYTOSIS
Endocytosis is the process of engulfing large molecules by the cell. Types of endocytosis are phagocytosis and pinocytosis (Table 1.4.4.)
 
Phagocytosis
 
Definition
Phagocytosis is a type of endocytosis in which large substances such as bacteria are taken within the cell.
Table 1.4.4   Endocytosis
Phagocytosis
Engulfing large substances such as bacteria into the cell
 Features
Cells: Macrophage, neutrophils
Events: Formation of phagosome containing the bacteria (phagolysosome) and hydrolysis of the bacterial contents.
 Clinical aspects
Chediak-Higashi syndrome: Disorder of phagocytosis (Recurrent infections)
Pinocytosis
Engulfing fluid contents into the cell
Fluid phase pinocytosis
Non-selective uptake of solute by formation of small vesicles.
Examples
Occurs in fibroblasts, small intestine, kidney
Receptor mediated endocytosis
Engulfing material through a specific receptor
Binding of molecule (ligand) to the receptor. Formation of endosomes and secondary lysosomes. The molecules are hydrolysed by the lysosomal enzymes.
 Examples
Uptake of iron-transferrin complex by the transferrin receptor, uptake of LDL by the LDL-receptor
 Clinical aspects
Type II hyperlipoproteinemia due to defective LDL-receptor or mutation in the region of apo-B 100.
 
Features
Cells Phagocytosis occurs in specialized cells such as macrophages and polymorphonuclear leukocytes.
Events Phagocytosis involves the ingestion of large particles such as viruses, bacteria, cells or debris. When bacteria contact the outer membrane of macrophages or neutrophils, a small portion of the cellular membrane buds off internally to become a membrane bound vesicle (phagosome). The phagosome containing the bacteria fuses with lysosome within the cell, forming phagolysosome. Within the phagolysosome, the lysosomal enzymes hydrolyze the bacterial contents (killing of bacteria) (Fig. 1.4.9).
 
Clinical Significance
Chediak-Higashi syndrome is an inherited disorder characterized by delayed fusion of the phagolysosome. It is characterized by recurrent infections due to impaired phagocytosis of bacteria by macrophages and neutrophils.
26
zoom view
FIGURE 1.4.9: Phagocytosis
 
Pinocytosis
Pinocytosis is a process in which fluid or fluid contents are taken within the cell. Two types of pinocytosis are (i) fluid phase pinocytosis, and (ii) receptor mediated endocytosis.
 
Fliud Phase Pinocytosis
Fluid phase pinocytosis is a non-selective process. It is characterized by uptake of solute by formation of small vesicles. It is a routine process in cells. It occurs in various cells including fibroblasts and cells involved in absorption such as cells lining the small intestine and renal tubules.
 
Receptor Mediated Endocytosis
Definition Receptor mediated endocytosis is a process of engulfing a material through a specific receptor on the surface of the cells (Fig. 1.4.10).
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FIGURE 1.4.10: Receptor mediated endocytosis
Features The molecule (ligand) binds to the receptor. The ligand-receptor complex invaginates in a region called coated pits and gets internalized in a small vesicle called endosome. The endosome delivers the ligand-receptor complex to lysosomes by fusing with lysosome to form secondary lysosome. The molecules are hydrolysed by the lysosomal enzymes. The products are released to the cytosol.
Examples Examples for receptor mediated endocytosis are uptake of iron-bound transferrin by the transferrin receptor and uptake of low density lipoproteins (LDL) by the LDL-receptor
Uptake of LDL by the LDL-receptor LDL contains protein called apolipoprotein-B that binds to the LDL-receptor in cells including fibroblasts. This is followed by the internalization of LDL-LDL-receptor complex to form endosome. The endosome fuses with lysosomes. The receptor is released and recycled back to the cell surface membrane. The apolipoprotein-B of LDL is degraded and the cholesterol ester is hydrolyzed.
 
Clinical significance
Type II hyperlipoproteinemia is an inherited disorder of receptor mediated endocytosis. The condition is due to defective LDL- receptor or mutation in the region of apo-B 100. This leads to reduced clearance of LDL resulting in elevated levels of plasma LDL and hypercholesterolemia. Increased LDL and hypercholesterolemia are risk factors for coronary heart disease.
 
V. IONOPHORES
Ionophores are compounds transporting small molecular compounds such as ions across the membranes. There are two types of ionophores: carrier ionophores and channel forming ionophores. Examples are antibiotics such as valinomycin and gramicidin A (Fig. 1.4.11).
Valinomycin is carrier ionophore. Carrier ionophore increases the permeability for a particular ion. It binds to the ion and releases the ion on the other side of the membrane
Valinomycin is selective for the transport of potassium. It binds to potassium and transports potassium across the membrane. Valinomycin inhibits the proton gradient across the mitochondrial membrane thus inhibiting oxidative phosphorylation.
27
zoom view
FIGURE 1.4.11: Carrier ionophore (valinomycin) and Channel forming ionophore (gramicidin)
Gramicidin A is a polypeptide. It is a channel forming ionophore. Channel forming ionophore forms a channel in the membrane by creating a space between two adjacent phospholipid molecules.
Gramicidin facilitates the passage of sodium and potassium ions across the membrane. It inhibits the proton gradient across the mitochondrial membrane thus inhibiting oxidative phosphorylation.
 
VI. Donnan Membrane Equilibrium (Table 1.4.5)
 
Definition
Donnan membrane equilibrium refers to unequal distribution of diffusible ions between two compartments due to the presence of non-diffusible ions in one of the compartments.
Diffusible and non-diffusible ions are separated by a membrane which is impermeable to one of the ions (non-diffusible ions) because they are too large to pass through the pores of membrane.
Donnan effect refers to effect of non-diffusible ions causing unequal distribution of diffusible ions between the two compartments. Donnan effect is caused by colloidal nature of non-diffusible ions.
 
Distribution of Diffusible ions during Donnan Membrane Equilibrium
Distribution of diffusible ions in the absence of non diffusible ion Equal distribution of diffusible ions such as K+ and Cl occurs between the two compartments if non diffusible ions are not present in any compartments (Fig. 1.4.12)
Distribution of diffusible ions in the presence of non- diffusible ion in one of the compartments (Donnan effect) If one of the compartments contains non-diffusible ion such sodium proteinate, the events occur during distribution of diffusible ions are as given in Figure 1.4.12:
  1. Diffusible ions having charge opposite to that of proteinate, accumulate in the same compartment.
  2. On the other hand, diffusible ions having same charge of proteinate ion accumulate in compartment other than non-diffusible proteinate ion.
  3. After equilibrium, the distribution of diffusible ions is unequal between the compartments.
  4. The product of diffusible ions is same between the two compartments.
 
Significance of Donnan Membrane Equilibrium
Generation of difference in osmotic pressure The difference in osmotic pressure causes osmotic flow of water to the compartment having non-diffusible ion. In vascular system, accumulation of sodium ions in the plasma due to the presence of albumin causes inflow of water to the vascular compartment from intravascular compartment such as interstitial space. Osmotic pressure contributed by plasma colloids (albumin) also contributes to water transport and formation of urine.
Generation of resting membrane potential Resting membrane potential is due to unequal distribution of Cl and Na+/K+ between intracellular and extracellular compartment.
Membrane hydrolysis Membrane hydrolysis refers to the unequal distribution of H+ or OH of water on one side of the membrane in the presence of non-diffusible ion making one side of the membrane either acidic or alkaline. Membrane hydrolysis has been suggested to be one of the factors for attaining high concentration of H+ ions in gastric juice.
Chloride shift in erythrocytes During buffering action of hemoglobin, HCO3 accumulates in erythrocytes. However, HCO3 diffuses into plasma in exchange for Cl ions to maintain electroneutrality (p. 753).
28
Table 1.4.5   Donnan membrane equilibrium
Definition
Unequal distribution of diffusible ions between two compartments due to the presence of non-diffusible ions.
Factors
Caused by colloidal nature of non diffusible ions
Significance
Generation of difference in osmotic pressure Generation of resting membrane potential
Membrane hydrolysis
Chloride shift in erythocytes
zoom view
FIGURE 1.4.12: Donnan membrane equilibrium
 
1.5. CELL-TO-CELL COMMUNICATION
 
I. CELLULAR RECEPTORS
Cellular receptors are very specific protein molecules on the plasma membrane, cytoplasm or in the nucleus capable of recognizing and binding the extracellular signaling molecules (ligands). Receptors are grouped into cell surface receptors and intracellular receptors.
 
Cell Surface Receptors
The plasma membrane receptors are classified on the basis of their location and function. Three main families of receptors have been identified (Table 1.5.1).
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Table 1.5.1   Cellular receptors
Receptors
Features
Examples
Extracellular receptors
 Ion channel linked
Open or close to allow transport of molecule into the cell
Glutamate receptor
 G-protein coupled
Activate or inactivate plasma membrane bound enzymes through G proteins
Epinephrine, glucagon angiotensin II
 Catalytic
Activation of the receptor results in activation of kinase, or
Insulin receptor with tyrosine kinase activity
Guanyl cyclase activity
ANP receptor with guanyl cyclase activity
Intracellular receptors
Location: Cytoplasm, nucleus, mitochondria Signaling: In the nucleus, the receptor–ligand complex interacts with specific part of DNA, resulting in changes in gene expression patterns.
Steroid hormone receptor, thyroid hormone receptors, Retinoid receptors. Calcitriol receptors
 
Catalytic Receptors
Catalytic receptors (also called enzyme linked receptors) are receptors with enzyme activity. The activation of the receptor results in stimulation of kinase, phosphatase or guanylate cyclase activity.
Receptor protein tyrosine kinases phosphorylate specific protein kinases in the target cell. Example is receptor tyrosine kinase activity of insulin receptor. Through the cascades of highly regulated phosphorylation, elaborate sets of interacting proteins relay signals from the cell surface to the nucleus. This alters the patterns of gene expression and cellular functions (Fig. 1.5.1).
Guanylate cyclase Binding of atrial natriuretic peptide (ANP) results in the activation of membrane associated guanylate cyclase and formation of cyclic GMP.
 
G-protein Coupled Receptors
Features G-protein coupled receptors activate or inactivate plasma membrane bound enzymes or ion channels via trimeric GTP-binding proteins (G-proteins).
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FIGURE 1.5.1: Receptor protein tyrosine kinase (catalytic receptor)
Activation or inactivation of cyclic AMP Some G-protein linked receptors activate or inactivate adenyl cyclase and alter the intracellular concentrations of cyclic AMP. Example is activation of adenyl cyclase by glucagon or epinephrine in the liver.
Activation of phospholipase C G-protein linked rece ptors activate phospholipase C and generate inositol triphosphate (IP3), which increases intracellular calcium levels. Example is activation of phospholipase C by angiotensinII (Fig. 1.5.2).
 
Ion Channel Linked Receptors
Features Following the binding of a specific signal, ion channel linked receptors open or close briefly to allow transport of molecules into the cell (Fig. 1.5.3).
Significance Ion channel linked receptors (also called ligand gated ion channels) are involved in rapid synaptic signaling between electrically excitable cells (neurons). Example is binding of glutamate to glutamate receptors leading to influx of calcium.
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FIGURE 1.5.2: G-protein coupled receptors
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FIGURE 1.5.3: Ion-channel linked receptors
 
B. Intracellular Receptors
Location Intracellular receptors belong to the nuclear hormone receptor superfamily and are located in the cytoplasm or nucleus (Fig 1.5.4).
Ligands include steroid hormones, vitamin D3, retinoid and thyroid hormones. These are small lipophillic molecules.
Signalling. The ligand binds to its intracellular receptor. In the nucleus, the receptorligand complex interacts with specific part of DNA, resulting in changes in gene expression patterns.
 
II. ADHESION MOLECULES
Adhesion molecules (Fig. 1.5.5) are protein molecules that are involved in binding one cell to another cell or binding the cell to extracellular matrix. Examples of cell adhesion molecules include (i) integrins, (ii) cadherins (iii) selectins and (iv) neuronal cell adhesion molecules (Table 1.5.2).
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FIGURE 1.5.4: Intracellular receptors
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FIGURE 1.5.5: Adhesion molecules (N-CAM = Neural cell adhesion molecule)
 
Integrins
Structure Integrins are heterodimers of α and β subunits. The extracellular domains of integrins serve as binding site for collagen and fibronectin.
Biomedical importance Integrins act as cell receptors that function in cell adhesion and intracellular signaling.
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Table 1.5.2   Adhesion molecules
Adhesion molecules
Structure
Functions
Integrins
Heterodimers of α and β subunits. Serve as binding site for collagen and fibronectin.
Leukocyte adhesion. platelet aggregation, tissue repair, immune function and tumor invasion.
Cadherins
Transmembrane glycoproteins
Calcium dependent cell-cell adhesion.
Neural cell adhesion molecules
Immunoglobulin like molecules
Calcium independent. cell-to-cell adhesions
Selectins
Have extracellular domains that are site for oligosaccharides on the surface of the adjacent cell
Blood clotting, inflammation, tumor invasion
They are required for leukocyte adhesion. They regulate various cellular processes such as platelet aggregation, tissue repair, and regulation of immune function. They are also involved in tumor invasion.
 
Cadherins
Structure Cadherins are transmembrane glycoproteins. They occur in most cells.
Functions They mediate calcium dependent cell-cell adhesion. They interact with cadherins of adjacent cell.
 
Neural Cell Adhesion Molecules
Structure Neural cell adhesion molecules are glycoproteins. They are immunoglobulin like molecules.
Functions They are involved in cell to cell, and cell to matrix adhesions. They mediate calcium independent cell-cell adhesion interactions.
 
Selectins
Structure Selectins have extracellular domains that provide binding site for oligosaccharides on the surface of the adjacent cell. The adhesion interaction is dependent on the presence of calcium. Selectins are present on leukocytes and endothelial cells.
Functions Selectins are involved in blood clotting process, inflammatory reactions and tumor invasion.
 
III. CELL JUNCTIONS
Cell junctions are distinct regions in the cell that make extensive contact with other cells and with extracellular matrix. Cell junctions are mainly classified into three groups: tight junctions, anchoring junctions and gap junctions (Fig. 1.5.6 and Table 1.5.3).
 
Tight Junctions (Occluding Junctions)
Occurrence They occur in cells such as transporting epithelial cells lining the digestive tract and nephron.
Functions Tight junctions restrict the leakage of solutes through the space between the two neighboring cells.
 
Anchoring Junctions
Occurrence They are prominent in tissues that regularly undergo mechanical stress such as skin.
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FIGURE 1.5.6: Cell junctions
Table 1.5.3   Cell junctions
Cell junctions
Occurrence
Functions
Tight junctions
Epithelial cells lining the digestive tract and nephron.
Restrict the leakage of solutes between the cells
Anchoring junctions
Tissues having mechanical stress (skin)
Provide firm inter connections of either neighboring cells or cells and basal laminae.
Gap junctions
Cardiac muscle, smooth muscle, epithelial cells
The gap is bridged by connexin. Allow passage of ions or water soluble molecules across the cells.
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Functions Anchoring junctions provide firm interconnections of either neighboring cells or cells and basal laminae. They serve as the anchoring points for cytoskeletal filaments.
Loci for cytoskeletal attachment They consist of two loci for cytoskeletal attachment. Actin filament attachment loci called zonula adherans and intermediate filament attachment loci called desmosome and hemidesmosome.
 
Gap Junctions (Communicating Junctions)
Occurrence They occur in cardiac muscle, smooth muscle, epithelial cells and nervous system.
Structure Gap junctions are regions that separate the two neighboring cells by a uniform narrow gap of 2 to 4 nm. The gap is bridged by non-selective channel protein called connexin.
Functions In epithelial cells they facilitate the passage of ions and water soluble molecules between the cells
 
1.6. CYTOSKELETAL PROTEINS
Cytoskeleton is a network of protein filaments extending throughout the cytoplasm. Cytoskeletal proteins include microtubules, intermediate filaments and actin filaments (Fig. 1.6.1 and Table 1.6.1).
 
I. MICROTUBULES
 
Structure
Microtubules are hollow, polar cylinders formed by the protein tubulin. There are 2 types of tubulin: α tubulin and β tubulin They are the thickest and strongest cytoskeletal proteins. The diameter of microtubules is about 25 nm.
Microtubule is made up of 13 parallel protofilaments of α and β dimers of tubulin that are polymerized in a head to tail array. The polymerization end is a plus (+) end and deploymerization end is a minus (−) end. The protofilaments fold to form a hollow microtubule.
 
Functions
Functions of microtubules include structural support, movement of cellular components and role in mitosis.
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FIGURE 1.6.1: Cytoskeletal proteins
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Table 1.6.1   Structure and functions of cytoskeletal proteins
Cytoskeletal proteins
Structure
Functions
Microtubules
Made up of 13 parallel protofilaments of α nand β dimers of tubulin.
Structural support
Movement of cellular components Role in mitosis
Intermediate filaments
Polymers of mechanically stiff, rod like proteins.
Vimentin: Stabilizes epithelium
Desmin: Links the myofibrils.
Neurofilaments: Provide strength to neurons
Keratins: Play a structural role in skin, hair nails.
Lamins: Maintenance of integrity of nuclear membrane.
Microfilamens
Cytoskeletal proteins made up of actin
Provide consistency to cytoplasm, involved in cell motility, required for muscle contraction, play a role in phagocytosis
Structural support Microtubules are used as structural supports.
Movement of cellular components Microtubules serve as binding sites for motor proteins. Motor proteins convert chemical energy into mechanical energy for movement of cellular components. Examples for motor proteins are mysoin, dynein and kinesin. Myosin is a motor protein that is abundant in muscle. Dynein is involved in ciliary and flagellar movement. In the epithelial cells of respiratory tract, they are responsible for sweeping mucous towards the mouth. In ova and sperm, they cause movement of ova and sperm. Kinesin is a driving force for the movement of vesicles and organelles along microtubules.
Role in mitosis Microtubules are important in mitosis. Microtubules form mitotic spindles that are responsible for the separation of chromosomes during mitosis.
 
Inhibitors
Microtubule inhibitors are used as drugs. Examples are vincristine, vinblastine, colchicine and taxol (Table 1.6.2).
Vinblastine and vincristine inhibit mcirotubule assembly. They are used as anticancer drugs.
Colchicine inhibits micotubule assembly. It inhibits mirotubule dependent phagocytosis of uric acid crystals by leukocytes. Colchicine is used in the treatment of gout.
Taxol inhibits microtubule disassembly. It stabilizes micotubules. Taxol is an anticancer drug.
 
Disorder
Primary ciliary dyskinesia is a primary disorder associated with celiary dysfunction (Table 1.6.3).
Table 1.6.2   Inhibitors of microtubules
Inhibitors
Actions and uses
Vinblastine
Inhibit microtubule assembly
Vincristine
Used as anticancer drugs
Colchicine
Inhibits microtubule assembly.
Used in the treatment of gout.
Taxol
Inhibits microtubule disassembly.
Used as an anticancer drug.
Table 1.6.3   Disorders of microtubules
Disorder
Features
Primary ciliary dyskinesia
Cause: Disorder of dynein
Features: Respiratory infections, male infertility.
The condition results from structural abnormalities of dynein arms of microtubules.
The clinical effects include recurrent upper and lower respiratory tract infections caused by impaired bacterial clearance and male infertility caused by impairment of normal sperm motility.
 
II. INTERMEDIATE FILAMENTS
 
Structure
Intermediate filaments are polymers of mechanically stiff, long rod like proteins. The diameter is about 10 nm.
 
Examples and Functions
They provide mechanical support to the cell (resilience). The intermediate filaments of different kinds of cells show a distinctive protein preference.34
Vimentin is found in many epithelial cells and fibroblasts. It plays a role in stabilizing epithelium.
Desmin is found in muscle cells. It links the myofibrils.
Neurofilaments are found in axons. They provide strength and rigidity to neurons.
Keratins are present in epithelial cells, hairs and nails. They play a structural role in skin, hair and nails.
Lamins A, B and C are intermediate filaments found in the nucleus. They are essential for the maintenance of integrity of nuclear membrane. The phosphorylation of lamins during cell division leads to disintegration of nuclear membrane.
 
III. MICROFILAMENTS (Actin Filaments)
 
Structure
Microfilaments are cytoskeletal proteins made up of actin. Microfilaments are the most abundant intracellular proteins that are concentrated in a narrow band just under the plasma membrane. The diameter is around 7nm. Microfilaments undergo polymerization and depolymerization. In the unpolymerized state, actin is a globular protein called G-actin. In the polymerized state, G-actin has a polarity and polymerizes in a head to tail manner to form a filamentous form called F-actin.
 
Functions
Functions of microfilaments are provision of consistency to cytoplasm, cell motility, muscle contraction and phagocytosis.
Consistency to cytoplasm They provide physical consistency to the cytoplasm.
Cell motility They are required for cell motility.
Muscle contraction The functions of actin depend on a large variety of actin-binding proteins. The most common actin binding protein is myosin in both muscle and nonmuscle cells. The interaction of the actin, myosin and ATP can generate a contractile force for muscle contraction.
Phagocytosis Interaction of actin binding proteins and actin in plasma membrane can cause movements of the cells including phagocytosis.