Biochemistry Pankaja Naik
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1Structure and Function of the Cell and Bio-molecules2

The Cell1

  • ➢ INTRODUCTION
  • ➢ MOLECULAR AND FUNCTIONAL ORGANISATION OF A CELL AND ITS SUBCELLULAR COMPONENTS
  • ➢ CELL MEMBRANE
  • ➢ CYTOSOL
  • ➢ ENDOPLASMIC RETICULUM
  • ➢ GOLGI APPARATUS
  • ➢ LYSOSOMES PEROXISOMES
  • ➢ MITOCHONDRIA
  • ➢ NUCLEUS
  • ➢ CYTOSKELETON
  • ➢ MEMBRANE TRANSPORT
  • ➢ CELL FRACTIONATION
  • ➢ REVIEW OF THE CONTENTS
  • ➢ REVIEW QUESTIONS
 
Introduction
Cell means a small room or chamber, cells are the structural and functional units of all living organisms. The major parts of a cell are the nucleus and the cytoplasm.
The electron microscope allowed classification of cells into two major groups, prokaryotes and eukaryotes based on the presence and absence of the true nucleus.
  • Eukaryotes have nucleus which is covered by nuclear membrane. (Greek: Eue = true, karyon = nucleus). Animals, plants and fungi belong to the eukaryotes
  • Eukaryotic cells are much larger than prokaryotes
  • Unlike prokaryotes, eukaryotes have a variety of other membrane bound organelles (subcellular elements) in their cytoplasm, including:
    • mitochondria
    • lysosomes
    • endoplasmic reticulum and
    • golgi complexes
  • Prokaryotes have no typical nucleus. (Greek: Pro = before). Bacteria and blue green algae belong to the prokaryotes.
Most cells of higher plants have a cell wall outside the plasma membrane, which serves as a rigid protective shell.
 
Molecular and Functional Organisation of a Cell and its Subcellular Components
A cell has three major components:
  • Cell membrane
  • Cytoplasm with its organelles and
  • Nucleus.
A typical cell as seen by the light microscope is shown in Figure 1.1. The following sections describe the structure (Figure 1.2) and functions (Table 1.1) of the components of eukaryotic cells in more details.
 
CELL MEMBRANE (PLASMA MEMBRANE)
  • The cell is enveloped by a thin membrane called cell membrane or plasma membrane.
  • Cell membrane separate cells from their external environment and divide the interior of the cell into compartments. They are 75 to 90 Å thick.
  • The fluid outside the cell membrane is called extra-cellular fluid (ECF), while that inside the cell, covered by the cell membrane is called intracellular fluid (ICF).
 
STRUCTURE AND CHEMICAL COMPOSITION
  • Cell membranes mainly consist of lipids, proteins and smaller proportion of carbohydrates that are linked to lipids and proteins. The ratio of protein to lipid can vary widely from 1:4 to 4:1
  • The chemical composition of cell membranes varies widely as shown in Table 1.2
  • The basic organisation of biologic membranes is illustrated in Figure 1.3
  • Electron microscopy has revealed the cell membrane as an organised structure consisting of a lipid bilayer primarily of phospholipids and penetrated protein molecules forming a mosaic like pattern (see Figure 1.6)
  • Membranes are asymmetric. The two faces of a membrane are different. The outer and inner surfaces of all known biological membranes have different components and different enzymatic activities
    4
    Table 1.1   Biochemical functions of subcellular organelles of the eukaryotic cell
    Subcellular organelles
    Function
    Plasma membrane
    Transport of molecules in and out of cell, receptors for hormones and neurotransmitters
    Lysosome
    Intracellular digestion of macromolecules and hydrolysis of nucleic acid, protein, glycosaminoglycans, glycolipids, sphingolipids
    Golgi apparatus
    Post-transcriptional modification and sorting of proteins and export of proteins
    Rough endoplasmic reticulum
    Biosynthesis of protein and secretion
    Nucleus
    Storage of DNA, replication and repair of DNA, transcription and post-transcriptional processing
    Peroxisomes
    Metabolism of hydrogen peroxide and oxidation of long chain fatty acids
    Nucleolus
    Synthesis of r-RNA and formation of ribosomes
    Mitochondrion
    ATP synthesis, site for tricarboxylic acid cycle, fatty acid oxidation, oxidative phosphorylation, part of urea cycle and part of haem synthesis
    Smooth endoplasmic reticulum
    Biosynthesis of steroid hormones and phospholipids, metabolism of foreign compounds (cytochrome P45o detoxification)
    Cytosol
    Site for glycolysis, pentose phosphate pathway, part of gluconeogenesis, urea cycle and haem synthesis, purine and pyrimidine nucleotide synthesis
    zoom view
    Figure 1.1: Structure of cell, seen in light microscope
    zoom view
    Figure 1.2: A diagrammatic representation of a typical eukaryotic cell
  • Membranes are fluid structures. The unsaturated fatty acids bound to phospholipids contribute to the fluid state of the membrane. At body temperature lipids are in a fluid state and this fluidity of the membrane is essential for the normal functioning to occur, e.g. exocytosis and endocytosis and lysosomal activity
  • The cell membrane is anchored to the cytoskeleton which is a network of microfilaments and microtubules that interact with each other and with the components of the plasma membrane.
    (Discussed latter in detail).
    Table 1.2   Chemical composition of cell membranes
    Chemical composition by weight
    Membrane
    Protein%
    Lipid%
    Carbohydrate%
    Erythrocyte
    49
    43
    8
    Outer mitochondrial membrane
    50
    46
    4
    Inner mitochondrial membrane
    75
    23
    2
    Myelin
    20
    75
    5
 
MEMBRANE LIPIDS
  • The major classes of membrane lipids are,
    • phospholipids,
    • glycolipids and
    • cholesterol
      They are all amphipathic molecules, that is, they have both hydrophobic and hydrophilic ends
      5
      zoom view
      Figure 1.3: The basic organisation of biological membrane
  • Membrane lipids spontaneously form bilayer in aqueous medium, burrying their hydrophobic tails and leaving their hydrophilic ends exposed to the water (Figure 1.3).
 
Phospholipids
  • Each phospholipid molecule consists of head in which phosphoric acid group is located and two thin tails which consists of fatty acid portion (Figure 1.4)
  • The head end is polar and hydrophilic while the tail end is the non-polar and hydrophobic
    zoom view
    Figure 1.4: Structure of phospholipidA = A common glycerophospholipidB = Diagrammatic representation of phospholipid
  • Two major classes of phospholipid present in membranes are
    • Phosphoglycerides and
    • Sphingomyelins
  • Phosphoglycerides consists of glycerol backbone to which two fatty acids and phosphorylated alcohol is attached (Figure 1.4). The common alcohol moieties of phosphoglycerides are serine, ethanolamine, choline, glycerol and inositol
  • Sphingomyelins which contain a sphingosine back bone rather than glycerol to which fatty acids and phosphorylcholine is attached. Sphingomyelins are prominent in myelin sheaths
  • The phospholipid molecules are not attached to their neighbouring phospholipid molecule by any chemical bonds. Thus to some extent, they can move laterally and such movement are called lateral movements. Also a phospholipid molecule can be twisted or folded to some extent. This is how a cell can alter its shape
  • The lipid bilayer in the middle of the membrane is impermeable to the usual water soluble substances, such as ions, glucose and urea
  • Conversely, fat soluble substances such as oxygen, carbon dioxide and alcohol can penetrate this portion of the membrane with ease.
 
Functions of cell membrane phospholipids
Functions of phospholipid molecule of cell membrane are as follows:
  • Provide the membrane its form
  • Provide a selective barrier that prevents movement of water and water soluble substances from one cell component to other.
 
Glycolipids
  • Glycolipids are sugar containing lipids derived from sphingosine, e.g. cerebroside and gangliosides
  • Glycolipids are present only over the outer surface of the cell membrane
  • Glycolipids are inserted into the membrane lipid bilayer in such a manner that the carbohydrate group projects out from the surface of the bilayer, interacting with the phospholipid head groups.
 
Cholesterol
  • Cholesterol exists almost exclusively in the plasma membranes of mammalian cells
  • It provides stability to the membrane
  • Cholesterol is inserted into the lipid bilayer between phospholipid molecules in both leaflets of the lipid bilayer. Its hydroxyl group is oriented towards the aqueous interface and interacts with the polar head groups of the phospholipids and remainder of the molecule interact with the hydrocarbon chains of the phospholipid fatty acyl groups (Figure 1.5).
zoom view
Figure 1.5: Inseration of cholesterol into the membrane lipid bilayer
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PROTEINS OF THE CELL MEMBRANE
  • Proteins of the membrane are classified into two major categories
    • Integral proteins or intrinsic proteins or transmembrane proteins and
    • Peripheral or extrinsic proteins.
  • Peripheral proteins are attached to the surface of the lipid bilayer by electrostatic and hydrogen bonds. They bound loosely to the polar head groups of the membrane phospholipid bilayer (see Figure 1.3). Peripheral proteins comprise about 30% of the membrane proteins.
  • Integral proteins are either partially or totally immersed in the lipid bilayer. Many integral membrane proteins span the lipid bilayer from one side to the other and are called transmembrane protein whereas others are partly embedded in either the outer or inner leaflet of the lipid bilayer (see Figure 1.3).
  • Transmembrane proteins are exposed on both the extracellular fluid and the cytoplasmic surfaces of the bilayer and in this way can form a channel or pore through the lipid phase. They act as enzymes and transport carriers for ions as well as water soluble substances such as glucose.
  • Most of these membrane proteins are glycoproteins. They bear short chains of carbohydrates on the exterior side of the membrane (see Figure 1.3).
 
Functions of Integral Proteins
Integral protein serve as:
Channels (pores): Through which water, and water soluble substances especially ions diffuse between extracellular and intracellular fluid. These protein channels also have selective properties, that allow preferential diffusion of some substances more than others.
Carrier proteins: Transport substances that otherwise could not penetrate the lipid bilayer. Sometimes these even transport substances in the direction opposite to their natural direction of diffusion, which is called “active transport”.
Enzymes: That activate or inactivate various metabolic processes.
 
Functions of Peripheral Proteins
Peripheral proteins function almost entirely as:
  • Enzymes and
  • Receptors. Receptors bind neurotransmitters and hormones initiating physiological and biochemical changes inside the cell.
 
MEMBRANE CARBOHYDRATES (The Cell Glycocalaxy)
Membrane carbohydrate is not free. It occurs in combination with proteins or lipids in the form of glycoproteins or glycolipids. Most of the integral proteins are glycoproteins and about one tenth of the membrane lipid molecules are glycolipids. The carbohydrate (Glyco) portion of these molecules protrude to the outside of the cell, dangling outward from the cell surface (Figure 1.3).
Many other carbohydrate compounds called proteoglycans which are mainly carbohydrate substances bound to small protein cores. These proteoglycans are loosely attached to the outer surface of the cell as well. Thus, the entire outer surface of the cell which has a loose carbohydrate layer called “the glycocalaxy”. The carbohydrate moieties attached to the outer surface of the cell have several important functions.
 
Functions of Membrane Carbohydrate
  • Many of them are electrically negatively charged, which gives most cells an overall negatively charged surface that repels other negative compounds and do not permit the negatively charged particles of the cell to move outside
  • Glycocalaxy of some cells is attached to the glycocalayx of other cells thus attaching cells to one another
  • Many of the carbohydrates act as receptor substances for binding hormones such as insulin
  • Some carbohydrate moieties enter into immune reactions and functions in antibody processing.
 
FUNCTIONS OF CELL MEMBRANE
  • Protective function: The cell membrane protects the cytoplasm and the organelles of the cytoplasm.
  • Maintenance of shape and size of the cell.
  • As a semipermeable membrane: The cell membrane permits only some substances to pass in either direction, and it forms a barrier for other substances.
 
DISORDER OF MEMBRANE TRANSPORT PROTEIN
 
Cystic Fibrosis (CF)
  • Cystic fibrosis is the autosomal recessive inherited disorder of electrolyte transfer across cell membrane by7 a transport protein, CFTR (cystic fibrosis transmembrane conductance regulator).
    zoom view
    Figure 1.6: The fluid mosaic model of cell membrane
  • CFTR acts as an ATP-regulated chloride channel in the plasma membrane of epithelial cells.
  • Cystic fibrosis is characterized by
    • Chronic bacterial infections of the lungs
    • Fat maldigestion due to pancreatic exocrine insufficiency.
    • Infertility in males due to abnormal development of the vas deferens and
    • Elevated levels of chloride in sweat (>60 mmol/L)
 
Cause of cystic fibrosis
  • The gene encodes a protein CFTR is mutated in cystic fibrosis. This mutation in CFTR leads to,
    • reduced amount of CFTR proteins
    • reduced ability of the Cl¯ channel to open
    • reduced rate of ion flow through a channel generating loss of Cl¯ ions.
  • An abnormality of membrane Cl¯ permeability is due to increased viscosity of many body secretions, through the precise mechanisms are still under investigation.
  • The most serious and life threatening complication is recurrent pulmonary infections due to overgrowth of various pathogens in the viscous secretions of the respiratory tract.
 
Diagnosis
  • Mutation analysis can be performed for prenatal diagnosis and for carrier testing in families in which one child already has the condition.
  • Efforts are in progress to use gene therapy to restore the activity of CFTR.
  • An aerosolized preparation of human DNase tract has proved helpful in therapy.
 
FLUID MOSAIC MODEL
 
The Fluid Mosaic Model of Cell Membrane
  • In 1972 Singer and Nicolson postulated a theory of membrane structure called the fluid mosaic model, which is now widely accepted
  • A mosaic is a structure made up of many different parts. Likewise, the plasma membrane is composed of different kinds of macromolecules like phospholipid, integral proteins, peripheral proteins, glycoproteins, glycolipids and cholesterol
  • According to this model the matrix or continuous part of membrane structure, is a polar lipid bilayer
  • The bilayer is fluid because the hydrophobic tails of its polar lipids consist of an appropriate mixture of saturated and unsaturated fatty acids that is fluid at normal temperature of the cell
  • This lipid bilayer has a dual role; it is both a solvent for integral membrane proteins and a permeability barrier
  • Proteins are interspersed in the lipid bilayer, of the plasma membrane, producing a mosaic effect (Figure 1.6)
  • The fluid mosaic model proposes that the integral proteins of membranes have hydrophobic non-polar amino acid side chain e.g. valine and leucine, which would cause such proteins to dissolve in the central hydrophobic portion of the bilayer and thus they are embedded within the lipid bilayer
  • On the other hand, peripheral membrane proteins have essentially hydrophilic polar amino acid side chains, such as glutamate and serine, which are bound by electrostatic attraction to the hydrophilic electrically charged polar heads of the bilayer lipids
  • 8The peripheral proteins literally float on the surface of ‘sea□ of the predominantly phospholipid molecules, whereas the integral proteins are like icebergs, almost completely submerged in the hydrocarbon core
  • There are no covalent bonds between lipid molecules of the bilayer or between the protein components and the lipids
  • Fluid mosaic model allows the membrane proteins to move around laterally in two dimensions unless restricted by special interactions and that they are free to diffuse from place to place within the plane of the bilayer. Whereas they cannot tumble from one side of the lipid bilayer to the other
  • Thus there is a mosaic pattern of membrane proteins in the fluid lipid bilayer
  • The Singer-Nicolson model can explain many of the physical, chemical and biological properties of membranes and has been widely accepted as the most probable molecular arrangement of lipids and proteins of membranes.
 
CYTOPLASM AND ITS ORGANELLES
Cytoplasm is the internal volume bounded by the plasma membrane. The clear fluid portion of the cytoplasm in which the particles are suspended is called cytosol. This contains mainly dissolved proteins, electrolytes and glucose.
Five important organelles that are suspended in the cytoplasm are:
  • Endoplasmic reticulum
  • Golgi apparatus
  • Mitochondria
  • Lysosomes and
  • Peroxisomes
 
ENDOPLASMIC RETICULUM (ER)
Endoplasmic reticulum is the inter-connected network of tubular and flat vesicular structures in the cytoplasm (Figure 1.7). Their walls are constructed of lipid bilayer membranes, which contain a large amount of proteins, similar to the cell membrane.
The lumen of the endoplasmic reticulum contains of fluid medium called endoplasmic matrix, which is different from the fluid in the cytosol, outside the endoplasmic reticulum.
zoom view
Figure 1.7: Structure of endoplasmic reticulum
Endoplasmic reticulum form the link between nucleus and cell membrane by connecting the cell membrane at one end and the outer membrane of the nucleus at the other end (see Figure 1.2).
Substances formed in some parts of the cell enter the space (lumen) of endoplasmic reticulum and are then carried to other parts of the cell. Also, the vast surface area of the reticulum and multiple enzyme systems attached to its membranes provide the machinery for metabolic functions of the cell. The endoplasmic reticulum is of two types namely:
  • Rough or granular endoplasmic reticulum and
  • Smooth or agranular endoplasmic reticulum.
 
Rough or Granular Endoplasmic Reticulum
A large number of minute granular particles called ribosomes are attached to the outer surface of many parts of the endoplasmic reticulum, this part of the ER is known as rough or granular ER.
 
Function of rough or granular endoplasmic reticulum
The ribosomes are composed of a mixture of RNA and proteins and they function in the synthesis of new protein molecules in the cell.
 
Smooth or Agranular Endoplasmic Reticulum
Part of the ER, which has no attached ribosomes, is known as smooth endoplasmic reticulum.
 
Functions of Smooth Endoplasmic Reticulum
The smooth endoplasmic reticulum functions in the synthesis of:
  • Lipid substances like steroid hormones, sebum, cholesterol, etc.
  • Many enzymes are present on the outer surface of the smooth endoplasmic reticulum. These enzymes are concerned with various metabolic processes of the cell.
  • Smooth endoplasmic reticulum is the site of the metabolism of certain drugs, toxic compounds and carcinogens (cancer producing substances).
9
zoom view
Figure 1.8: A Golgi apparatus and its relationship to the endoplasmic reticulum and nucleus
 
GOLGI APPARATUS
Golgi apparatus is present in all cells except in red blood cells. It is situated near the nucleus and is closely related to the endoplasmic reticulum. It consists of four or more membranous sacs. This apparatus is prominent in secretory cells.
 
Functions of Golgi Apparatus
The Golgi apparatus functions in association with the endoplasmic reticulum as shown in Figure 1.8.
  • Small “transport vesicles” (ER-vesicles), are continually formed from the endoplasmic reticulum, and thereafter they fuse with Golgi apparatus. In this way, substances entrapped in the ER-vesicles are transported from the endoplasmic reticulum to the Golgi apparatus
  • The transported substances are then processed in the Golgi apparatus to form lysosomes, a cytoplasmic component.
 
LYSOSOMES
Lysosomes are vesicular organelles formed from Golgi apparatus and dispersed throughout the cytoplasm. Among the organelles of the cytoplasm, the lysosomes have the thickest covering membrane to prevent the enclosed hydrolytic enzymes from coming in contact with other substances in the cell and therefore, prevents their digestive actions.
 
Functions of Lysosomes
Many small granules are present in the lysosome. The granules contain more than 40 different hydroxylases (hydrolytic enzymes). The enzymes can digest proteins, carbohydrates, lipids and nucleic acids. All the enzymes are collectively called lysozymes. These enzymes are synthesized in rough endoplasmic reticulum and processed and packed into lysosomal vesicles in Golgi apparatus. Apart from the digestive functions, the enzymes in the lysosomes are responsible for the following activities in the cell:
  • Destruction of bacteria and other forgein bodies. Normally, the lysosomes are intact. When bacteria or other foreign particles enter the cell, the lysosomes are ruptured and the hydrolytic enzymes are released into the cytoplasm. The enzymes immediately digest the invaded bacteria or foreign particles.
  • Removal of excessive secretory products in the cells of the glands.
  • Removal of unwanted cells in embryo.
 
PEROXISOMES
These organelles resemble the lysosomes in their appearance, but they differ both in function and in biogenesis. They do not arise from Golgi membranes, but rather from the division of pre-existing peroxisomes. Or perhaps through budding off from the smooth endoplasmic reticulum.
 
Functions of Peroxisomes
  • Peroxisomes contain enzymes peroxidases and catalase which are concerned with the metabolism of peroxide. Hydrogen peroxide is formed from poisons or alcohol, which enters the cell. Whenever hydrogen peroxide is produced in the cell, the peroxisomes are ruptured and the oxidative enzymes are released. These oxidases destroy hydrogen peroxides. Thus, the peroxisomes are involved in the detoxification of peroxide.
  • Peroxisomes are also capable of carrying out β-oxidation of fatty acid (See Chapter Lipid Metabolism).
 
MITOCHONDRIA (POWER HOUSE OF CELL)
These membrane bound organelles vary in size, but typically have a diameter of about 1 mm. Mitochondria also vary widely in shape, number and location depending on the cell type or tissue function.
Mitochondria are called “Power Plant” of the cell since they convert energy to form ATP that can be used by cell. Erythrocytes which derive their ATP from glycolysis (due to lack of mitochondria) are an exception.
 
Structure of Mitochondria
A mitochondrion is a double membrane organelle that are fundamentally different in composition and function.
  • The outer membrane forms a smooth envelope. It is freely permeable for most metabolites, since it contains protein-lipid pores which permit the passage of small molecules. It has relatively few enzymic activities.
  • The inner membrane is folded to form cristae, which give it a large surface area and are the site of oxidative phosphorylation.
The components of the electron transport chain are located on the inner membrane. This membrane also contains numerous permeases which permit interchange of some metabolites□ with the cytoplasm.
10The space within the inner membrane is called the mitochondrial matrix. It contains the soluble enzymes of the:
  • Citric acid cycle
  • β-Oxidation pathway of fatty acid catabolism and
  • Some other degradative enzymes.
The space between the two membranes is called the intermembrane or cristae space and enzyme such as adenylate kinase, is located here. The organization of mitochondrion is shown in Figure 1.9.
Mitochondria contain DNA, which encodes a few polypeptides involved in oxidative phosphorylation. However, the majority of mitochondrial protein are encoded by nuclear DNA, which is translated into the cytosol and transported to the mitochondria.
It is worth noting that sperm contribute no mitochondria to the fertilized egg, so that mitochondrial DNA is inherited exclusively through the female line. Thus mitochondria are maternally inherited.
zoom view
Figure 1.9: Structure of mitochondria
 
NUCLEUS
The cells with nucleus are called eukaryotes and those without nucleus are known as prokaryotes. Most of the cells have only one nucleus but cells of skeletal muscles have many nuclei. The matured red blood cell contains no nucleus.
 
STRUCTURE OF NUCLEUS
The nucleus is spherical and situated near the centre of the cell. The nucleus is surrounded by the nuclear envelope, which consists of:
  • An outer membrane, continuous with the endoplasmic reticulum and containing attached ribosomes (Figure 1.10)
  • An inner membrane, separated by a gap of 15 to 30 nm; perinuclear space.
Nuclear pores, occur at points where the outer and inner membranes are connected. Nuclear pores form openings between the cytoplasm and nucleoplasm (see Figure 1.10). Each pore is surrounded by multiprotein molecules. The exchange of material between nucleoplasm and cytoplasm occurs through the pores of nuclear membrane.
The space enclosed by the nuclear envelope is called nucleoplasm, within this the nucleolus is present. Nucleolus is an organized structure of DNA, RNA and protein that is involved in the synthesis of ribosomal RNA. The remaining nuclear DNA is dispersed throughout the nucleoplasm in the form of chromatin fibres.
Chromatin are complexes of DNA with specific proteins such as histones. In the nucleus, these chromatin fibres are associated with nuclear lamina, a fibrous network made of three protein lamins, A, B and C; lying beneath the inner nuclear membranes (see Figure 1.10).
At mitosis chromatin is condensed into discrete structures called chromosomes. During mitosis, phosphorylation of the lamins leads to the breakdown of the nuclear envelope. The organization of the nuclear envelope, nucleolus, and chromatin is shown in Figure 1.10.
zoom view
Figure 1.10: General structure of nucleus
 
FUNCTIONS OF NUCLEUS
The major functional role of the nucleus is that of:
  • Replication: Synthesis of new DNA.
  • Transcription: The synthesis of the three major types of RNA:
    • ribosomal RNA (r-RNA)
    • messenger RNA (m-RNA) and
    • transfer RNA (t-RNA).
      All of the RNA molecules operate functionally outside the nucleus and seem to leave via the nuclear pores.
  • The most prominent area of transcription in the nucleus is the nucleolus where the synthesis of r-RNA takes place and the r-RNA formed in the nucleolus interacts with proteins to form ribosomes and it is these, that make the nucleolus so prominent.
 
CYTOSKELETON
  • High resolution electron microscopy has shown that the cytoplasm of most eukaryotic cells contains network of protein filaments, that interact extensively with each other and with the component of the plasma membrane. Such an extensive intracellular network of protein has been called cytoskeleton.
  • The outer cellular membrane called the plasma membrane is anchored to the cytoskeleton.
  • The cytoskeleton is not a rigid permanent frame work of the cell but is a dynamic, changing structure.
11
 
FUNCTIONS OF CYTOSKELETON
  • The cytoskeleton gives cells their characteristic shape and form, provides attachment points for organelles, fixing their location in cells and also makes communication between parts of the cell possible
  • It is also responsible for the seperation of chromosomes during cell division
  • The internal movement of the cell organelles as well as cell locomotion and muscle fibre contraction could not take place without the cytoskeleton. It act as “track” on which cells can move organelles, chromosomes and other things.
 
STRUCTURE OF CYTOSKELETON
The cytoskeleton is an organized network of three primary protein filaments.
  1. Microfilaments.
  2. Microtubules and
  3. Intermediate filaments.
 
Microfilaments
  • These are about 5 nm in diameter. They are made up of protein actin of the non-muscle cells G-actin protein present in most cells of the body polymerizes to form double helical F-actin filaments like those seen in muscle.
  • In non-muscle cells there are two types of actin,
    • β-actin and
    • γ-actin
    Both types of actin can co-exist in the same cell and even copolymerise in the same filament.
  • Actin filament form a meshwork just underlying the plasma membrane of many cells and are referred to stress fibres or cell cortex which is labile. They disappear as cell motility increases or upon malignant transformation of cells by chemical or oncogenic viruses.
 
Function of microfilaments
  • The function of microfilaments is
    • to help muscle contraction
    • to maintain the shape of the cell
    • to help cellular movement
  • The microviili contain bundles of actin filaments and that is how they are maintained.
 
Microtubules
  • Microtubules are cylindrical tubes, 20 to 25 nm in diameter. They are made up of protein tubulin. Tubulin composed of subunits α and β.
  • α β dimers (one α-tubulin molecule and one β-tubulin molecule) polymerised into protofilaments.
  • A microtubule is made up of 13 protofilaments that are arranged longitudinally.
  • Numerous other proteins, called microtubule-associated proteins (MAPs), associated with microtubules to stabilise the protofilaments.
 
Functions of microtubules:
  • Microtubules are necessary for the formation and function of the mitotic spindle.
  • As part of the cytoskeleton, they provide stability to the cell. They prevent tubules of ER from collapsing.
  • Microtubules form tracks on which intracellular vesicles and organelles move. They also involved in the intracellular movement of endocytic and exocytic vesicles and form the major structural components of cilia and flagella.
  • These are major components of axons and dendrites in which they maintain structure and participate in the axoplasmic flow of material along the neuronal processes.
 
Intermediate Filaments
  • These are so called as their diameter 10 nm is intermediate between that of microfilaments (5 nm) and of microtubules (25 nm).
  • Intermediate filaments are formed from fibrous protein, which cannot be easily disassembled as either the microtubules or the microfilaments can (except lamin).
  • The protein structure of intermediate filaments varies with different tissue type as indicated in Table 1.3. There are major seven classes of intermediate filaments.
 
Functions of intermediary filaments
  • They are important structural component of cells which helps to cell strength and to maintain its shape.
  • They play a role in cell to cell attachment and help to stabilise the epithelium.
  • They provide strength and rigidity to axons.
  • They maintain the correct register to contractile units in muscle cells.
  • They play a major structural role in skin and hair cells.
 
MEMBRANE TRANSPORT
  • One of the function of the plasma membrane is to regulate the passage of a variety of small molecules that need to be taken up by or extruded from the cell or cell compartment.
  • Biological membranes are semipermeable membranes through which certain molecules freely diffuse across membranes but the movement of the others is restricted because of size, charge or solubility.
  • They allow the passage of small uncharged or hydrophobic molecules, but not of large polar molecules (whether charged or not). Several mechanisms exist for the substances in the latter category, to cross the biological membranes (Figure 1.11) in which permeability is conferred by membrane proteins, these are called channel proteins and carrier proteins (see Figure 1.11).
  • Channel proteins have watery spaces all the way through the molecules and allow free movement of water as well as selected ions or molecules.
    12
    Table 1.3   Types of cytoskeleton proteins and their characteristics
    Cytoskeleton protein
    Types of protein present
    Characteristic
    Microfilaments
    Actin filament
    Formed by polymerisation of G-actin subunits
    Microfilaments
    Tubulin (α and β)
    Formed by 13 protofilaments which are formed by polymersiation of αβ-dimers
    Differ in different types of cells, e.g.,
    Intermediate filament
    Keratin
    Present in epithelial cells, hair and nails.
    Vimentin
    Present in various mesenchymal cells.
    Desmin
    Present in muscle
    Glial fibrillary acidic protein (GFAP)
    Present in glial cells which surrounds neurons
    Peripherin
    Present in neurons
    Neurofilament
    Present in neuron
    Lamins
    Present in nuclear lamina of the cell.
    zoom view
    Figure 1.11: Transport pathways across the cell membrane by channel proteins and carrier proteins
  • Carrier proteins bind with molecules or ions that are to be transported, and leads to conformational charges in the protein molecules which then move the substance through the interstices of the protein to the other side of the membrane.
  • Both the channel and carrier proteins are highly selective in the types of molecules or ions that are allowed to cross the membrane.
  • The two types of transport mechanisms are (Figure 1.12).
    1. Passive transport or passive diffusion and
    2. Active transport.
 
PASSIVE TRANSPORT OR PASSIVE DIFFUSION
  • Passive transport is the process by which molecules move, across a membrane without energy supplied by ATP.
  • The direction of passive transport is always from a region of higher concentration to one of lower concentration.
  • It is called passive transport, because the energy provided for the transport of ions or molecules originates from the ion gradient itself, without any contribution by the transport system.
  • There are two types of passive transport,
    • Simple diffusion and
    • Facilitated diffusion.
 
Simple Diffusion
  • Lipid soluble, i.e. lipophilic molecules can pass through cell membrane, without any interaction with carrier proteins in the membrane. Such molecules will pass through membrane along the concentration gradient, i.e. from a region of higher concentration to one of lower concentration. This process in called simple diffusion.
  • Simple diffusion can occur through the cell membrane by two pathways.
    1. Through the interstices of the lipid bilayer if the diffusing substance is lipid soluble and
    2. Through watery channels that penetrate all the way through some of the large transport proteins as shown in (Figure 1.11). These protein channels are often selectively permeable to certain substances and many of the channels can be opened or closed by gates.
  • Small, nonpolar lipophilic molecules such as oxygen, carbondioxide, nitrogen and benzene and uncharged polar molecules such as urea, ethanol and small organic acids move through membrane by simple diffusion without the aid of membrane proteins.
  • In simple diffusion, the rate of transport of a molecule across a membrane is directly proportional to its concentration and the process is not saturable at high concentration of diffusible molecules (see Figure 1.13).
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zoom view
Figure 1.12: Types of membrane transport mechanism
zoom view
Figure 1.13: Effect of concentration of a substance on rate of diffusion through a membrane by simple diffusion and facilitated diffusion
 
Facilitated Diffusion
  • The movement of water soluble molecules and ions across the membrane requires specific transport system. They pass through specific carrier proteins. A carrier protein binds to a specific molecule on one side of the membrane and releases it on the other side. This type of crossing the membrane is called facilitated diffusion or carrier mediated diffusion because a substance transported in this manner diffuses through the membrane using a specific carrier protein to help. That is, the carrier facilitates diffusion of the substance to the other side of the membrane (see Figure 1.11)
  • In facilitated diffusion as the concentration of diffusible molecules increases, a point will be reached when all of the carrier proteins are occupied with diffusible molecules and no further increase in rate of diffusion (transport) is possible, the process is said to be saturable as shown in Figure 1.13. Thus the rate of transport by facilitated diffusion is limited by the number of carrier proteins in the membrane
  • An example of facilitated diffusion is the movement of glucose and most of the amino acids across the plasma membrane
  • Glucose carrier protein can also transport several other monosaccharides that have structures similar to that of glucose e.g., galactose and mannose
  • These diffusion processes are not coupled to the movement of other ions, they are known as uniport transport processes (Figure 1.14).
zoom view
Figure 1.14: Uniport, symport and antiport transport of substance across the cell membrane
 
ACTIVE TRANSPORT
  • If a molecule moves against a concentration gradient, an external energy sources is required; This movement is referred to as active transport. Because an input of energy from another sources is required, this type of crossing the membrane is called active transport
  • Substances that are actively transported through cell membranes include, Na+, K+, Ca++, Fe++, H+, CI, I, several different sugars and most of the amino acids.
  • 14Active transport is classified into two types according to the source of energy used as follows :
    1. Primary active transport and
    2. Secondary active transport.
  • In primary active transport, the energy is derived directly from hydrolysis of ATP
  • In secondary active transport ATP provides the energy from transport indirectly. Secondary active transport uses an energy of an electrochemical gradient or membrane potential produced originally by primary active transport process by using ATP
  • In both instances transport depends on the carrier proteins like facilitated diffusion. However in active transport, the carrier protein function differently from the carrier in facilitated diffusion. Carrier protein for active transport is capable of transporting substance against the concentration (electrochemical) gradient.
 
Examples of Primary Active Transport
  • Sodium, potassium, calcium hydrogen and chloride ions are transported by primary active transport.
 
Primary active transport of Na+ and K+ / Sodium-potassium pump
  • Na+-K+ Pump, a primary active transport process that pumps Na+ ions out of the cell and at the same time pumps K+ ions from outside to the inside generating a electrochemical gradient
  • Carrier protein of Na+-K+ pump has three receptor sites for binding sodium ions on the inside of the cell and two receptor sites for potassium ions on the outside. The inside portion of this protein near the sodium binding sites has ATPase activity (Figure 1.15)
  • The pump is called Na+-K+ ATPase because the hydrolysis of ATP occurs only when three Na+ ions bind on the inside and two K+ ions bind on the outside of the carrier proteins. The energy liberated by the hydrolysis of ATP leads to conformational change in the carrier protein molecule, extruding the three Na+ ions to the outside and the two K+ ions to the inside.
zoom view
Figure 1.15: Mechanism of sodium-potassium pump (primary active transport)
 
Physiological Importance of Na+-K+ Pump
  • The active transport of Na+ and K+ is of great physiological significance. The Na+-K+ gradient created by this pump in the cells, controls cell volume.
  • Renders neurons and muscles electrically excitable and
  • Drives the active transport of sugars and amino acids.
 
Primary Active Transport of Calcium/Calcium Pump
  • Ca++-ATPase, the enzyme that transports Ca++ out of the cytoplasm and into the sarcoplasmic reticulum of muscle cells. Sarcoplasmic reticulum is a specialized compartment for calcium storage
  • Ca++-ATPase of sarcoplasmic reticulum plays an important role in muscle contraction, which is triggered by an increase in cytosolic calcium level
  • The relaxation of skeletal muscle requires the transport of calcium from cytoplasm of muscle cells into the sarcoplasmic reticulum. Sarcoplasmic reticulum Ca++-ATPase decreases the concentration of Ca++ in the sarcoplasm and promotes skeletal muscle relaxation
  • Ca-pump maintains a Ca2+ concentration of approximately 0.1 μm in the cytosol compared with 1.5 mm in the sarcoplasmic reticulum.
 
Primary Active Transport of H+ions / Proton Pump
  • Primary active transport of H+ ions is important in stomach. The pump transports two cytoplasmic protons and two extracellular K+ ions, coupled with hydrolysis of a molecule of ATP and thus is called H+-K+ ATPase.
  • The gastric H+-K+ ATPase, the enzyme responsible for pumping proton (H+) into the stomach and lowers the pH below 1.0; which is essential for gastric digestion.
 
P-Type ATPase
A family of membrane proteins that uses ATP hydrolysis to pump ions across membranes are called P-type ATPase. P-type ATPase pump ions against a concentration gradients, e.g.:
  • Na+-K+ ATPase
  • Ca++ ATPase
  • H+-K+-ATPase are referred to as P-type ATPase because they form a phosphorylated intermediate that drive the active transport of ion. ‘P□ refers to the phosphorylation.
  • In the formation of this intermediate, a phosphate group obtained from the hydrolysis of ATP is linked to the side chain of a specific aspartate residue in the ATPase
  • The free energy of ATP hydrolysis drives membrane transport by effecting conformational changes associated with the addition and removed of phosphate group to an specific aspartate residue in ATPase protein
  • All members of this protein family have the same fundamental mechanism.
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SECONDARY ACTIVE TRANSPORT
Many active transport processes are not directly coupled with hydrolysis of ATP. Instead ATP provides energy indirectly for active transport of ions or molecules against concentration gradient, is termed as secondary active transport.
Secondary active transport is classified into two types:
  1. Co-transport or symport, in which both substances move across the membrane in the same direction (see Figure 1.14)
  2. Counter transport or antiport, in which each substance moves in opposite direction (see Figure 1.14).
 
Co-transport or Symport
  • By primary active transport, a large concentration gradient of Na+ ions across the cell membrane is generated, with high concentration outside the cell and very low concentration inside. An unequal distribution of molecules is an energy rich condition. This ion gradients are storage forms of energy and the excess sodium outside of the cell membrane is always try to diffuse to the interior
  • Under appropriate condition this energy of sodium can pull other substances along with the sodium through the cell membrane. This phenomenon is called co-transport or symport (Figure 1.16)
  • For sodium to pull another substance along with it requires a carrier protein in the cell membrane. This carrier protein has two binding site, on its exterior side for both the Na+ ion and the substance to be co-transported.
    zoom view
    Figure 1.16: Mechanism of sodium co-transport of glucose (secondary active transport)
  • When they both become attached, the conformational change in carrier protein take place and sodium and the other substance are transported to the interior of the cell at the same time
  • Glucose and many amino acids are transported against concentration gradient by this co-transport or symport mechanism
  • Other examples of co-transport mechanisms are, cotransport of chloride ions, iodine ions, iron and urate ions.
 
Counter Transport or Antiport Mechanism
  • As described above sodium ions again attempt to diffuse to the interior of the cell because of their large concentration gradient. However, in this case, the substance to be transported is on the inside of the cell and must be transported to the outside
  • Therefore sodium ions binds exterior portion of the carrier protein while the substance to be counter transported binds to the interior portion
  • After binding of both, conformational change take place and energy released by the Na+ ion while moving to the interior causes the other substance to move to the exterior.
  • Examples of counter transport or antiport mechanisms are
    • Sodium-calcium counter transport and
    • Sodium-hydrogen counter transport.
  • Sodium-calcium counter transport uses the electrochemical gradient of Na+ ions to pump Ca2+ out of the cell. Three sodium ions enters the cell for each Ca2+ ions that is extruded
  • Sodium-hydrogen counter transport occurs in several tissues but is especially important in the renal proximal tubules where Na+ ions move from the lumen of the tubule, to the interior of the tubular cell, while H+ ions are counter transported into the tubular lumen.
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TRANSPORT OF MACROMOLECULES ACROSS THE PLASMA MEMBRANE
  • The process by which cells take up large molecules is called endocytosis, a specialized function of cell membrane.
  • There are two types of endocytosis;
    1. Pinocytosis (cellular drinking) and
    2. Phagocytosis (cellular eating).
 
Pinocytosis
  • Pinocytosis is the cellular uptake of fluid and fluid contents and is a cellular drinking process.
  • Pinocytosis is the only process by which most macro-molecules, such as most proteins, polysaccharides and polynucleotides can enter cells (Figure 1.17).
  • These molecules first attach to specific receptors on the surface of the membrane.
  • The receptors generally are concentrated in a small pits on the outer surface of the cell membrane. These receptors are coated on the cytoplasmic side with a fibrillar protein called calthrin and contractile filaments of actin and myosin.
    zoom view
    Figure 1.17: Three stages of the absorption of macromolecules by endocytosis
  • Once the macromolecules (which is to be absorbed) have bound with the receptors, the surface properties of the local membrane change in such a way that the entire pit invaginates inward, and the fibrillar protein by surrounding the invaginating pit cause to close over the attached macromolecule along with a small amount of extraceullar fluid
  • Then immediately the invaginated portion of the membrane breaks away from the surface of the cell forming pinocytic vesicle inside the cytoplasm of the cell
  • For the process of endocytosis it requires,
    1. Energy in the form of hydrolysis of ATP.
    2. Presence of calcium ions in the extracellular fluid, which react with contractile protein filaments beneath the coated pits to provide force for pinching the vesicles away from the cell membrane.
 
Phagocytosis
  • Phagocytosis involves the ingestion of large particles such as viruses, bacteria, cells, tissue debris or a dead cell
  • It occurs only in specialized cells such as macrophages and some of the white blood cells
  • Phagocytosis occurs in much the same way as pinocytosis
  • In the case of bacteria, each bacterium usually is attached to a specific antibody, and it is the antibody that attaches to the phagocyte receptors, dragging the bacterium along with it. This intermediation of antibodies is called opsonization.
 
Digestion of Endocytic Vesicles
  • Most of the endocytotic vesicles fuse with lysosomes. Lysosomes empty their acid hydrolases to the inside of the vesicle and begin hydrolyzing the proteins, carbohydrate, lipids and other substances in the vesicle
  • The macromolecular contents are digested to yield amino acids, simple sugars or nucleotides and they diffuse out of the vesicle to be reused in the cytoplasm
  • Undigestible substances called residual body is finally excreted through the cell membrane by a process called exocytosis, opposite to endocytosis (Figure 1.18).
 
Exocytosis
  • Molecules produced within the cytoplasm may be enclosed in membranes to form vesicles called exocytic vesicle
  • These cytoplasmic secretary excocytic vesicles fuse with the internal surface of the plasma membrane in a Ca2+ dependent process
  • The vesicle then ruptures releasing their contents (protein or small molecules such as neurotransmitters) into the extracellular space and their membranes are retrieved (left behind) and reused.
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zoom view
Figure 1.18: Stages in exocytosis
 
CELL FRACTIONATION
  • Investigation of the biochemical properties of organelles requires subcellular fractionation in which the cell is disrupted by mechanical means and the organelles are purified
  • To obtain purified preparations of organelles the tissue is first carefully broken up in a homogenizing apparatus using isotonic 0.25 M sucrose solution
  • Before homogenization the tissue is coarsely minced using buffer to maintain the pH at its optimum value for organelle stability
  • Sucrose solution is used because it is not metabolized in most tissues and it does not pass through membranes readily and thus does not cause inter organelles to swell
  • By gently homogenization in an isotonic sucrose solution the cell membrane is ruptured by the shearing forces developed by the rotating homogenizer pestle; keeping most of the internal organelles intact. However, large fragile structures such as the endoplasmic reticulum, is broken into pieces that spontaneously form vesicles called microsomes
  • Homogenate is strained to remove connective tissue and fragments of blood vessels by a stainless steel sieve.
  • Then homogenate is centrifuged at a series of increasing centrifugal force. In differential centrifugation, the homogenate is subjected to a series of centrifugation steps of increasing time and gravitational force (Figure 1.19)
    zoom view
    Figure 1.19: Subcellular fractionation of cell by differential centrifugation
  • The subcellular organelles, e.g., nuclei and mitochondria, which differ in size and specific gravity and thus sediment at different rates in a centrifugal field and can then be isolated from homogenate by differential centrifugation
  • The dense nuclei are sedimented first, followed by the mitochondria, and finally the microsomal fraction at the highest forces. After, all the particulate matter has been removed, the soluble remnant is the cytosol
  • Organelles of similar sedimentation coefficient obviously cannot be separated by differential centrifugation. For example, mitochondria isolated in his way are contaminated with lysosome and peroxisomes. These may be separated by isopyknic centrifugation technique.
 
ISOPYKNIC CENTRIFUGATION TECHNIQUE
  • In this technique, a density gradient is set up in a centrifuge tube; that is, the density of the solution in the tube increases from the top to the bottom. Sucrose is often used as a medium. Colloidal materials such as Percoll, which form density gradients with a low osmotic pressure, are often preferred
  • Particles are sedimented to an equilibrium position at which their density equals that of the medium at that point in the tube (Figure 1.20)
  • Different organelles are thus separated according to their density, their size and shape being immaterial.
  • After centrifugation to equilibrium, the gradient is fractionated and the separated organelles recovered. Macromolecules, such as large proteins, nucleic acids and nucleoprotein complexes can also be separated by density gradient centrifugation technique.
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zoom view
Figure 1.20: Separation of organelles by isopyknic centrifugation technique
 
MARKER ENZYMES
  • The purity of isolated subcellular fraction is assessed by the analysis of marker enzymes
  • Marker enzymes are the enzymes that are located exclusively in a particular fraction and thus become characteristic of that fraction
  • Analysis of marker enzymes confirms the identity of the isolated fraction and indicates the degree of contamination with other organelles. For example, isolated mitochondria have a high specific activity of cytochrome oxidase but low catalase acid phosphatase, the catalase and acid phosphatase activities being due to contamination with peroxisomes and lysosomes respectively.
  • Some typical sub-cellular markers are given in Table 1.4
Table 1.4   Marker enzymes of subcellular fractions
Fraction
Enzymes
Plasma membrane
5 – Nucleotidase Na+-K+-ATPase
Nucleus
DNA polymerase
RNA polymerase
Endoplasmic reticulum
Glucose 6 – phosphatase
Cytochrome b5 reductase
Golgi bodies
Galactosyl transferase
Manosidase
Lysosomes
Acid phosphatase
β-Glucuronidase
Mitochondria
Succinate dehydrogenase
Cytochrome c-oxidase
Oligomycin – sensitive ATPase
Peroxisomes
Catalase
Cytosol
Lactate dehydrogenase
Glucose 6 – phosphate dehydrogenase
 
Review of the Contents
  • Cells are the structural and functional units of living organism
  • In eukaryotes the genetic material is surrounded by a nuclear envelop; prokaryotes have no such membrane.
  • Cell membranes mainly consist of lipids, proteins and smaller proportion of carbohydrates that are linked to lipids and proteins
  • Electron microscopy has revealed the cell membrane as a organized structure consisting of a lipid bilayer primarily of phospholipids and penetrated protein molecules forming a mosaic like pattern
  • Membrane proteins are classified as integral and peripheral proteins
  • Peripheral membrane proteins bind to the membrane surface and integral membrane proteins are embedded in the bilayer
  • The plasma membrane is a tough, flexible, permeability barrier, which contains numerous transporters as well as receptors for a variety of extracellular signals
  • The cytoplasm consists of the cytosol and organelles such as rough endoplasmic reticulum, smooth endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, mitochondria and nucleus
  • Proteins synthesized on ribosomes bound to rough endoplasmic reticulum enter the lumen of ER and travel through the Golgi apparatus on their way to organelles or to the cell surface, where they are secreted by exocytosis
  • The genetic material in eukaryotic cell is organized into chromosomes, highly ordered complexes of DNA and histone proteins
  • Lysosomes control the intracellular digestion of macromolecules
  • Peroxisome contains enzymes peroxidase and catalase which converts hydrogen peroxide to water and oxygen
  • Mitochondria are called power plant of the cell. They are maternally inherited
  • The cytoplasm is permeated by a number of fibriller elements that collectively form a supporting network. This network is called the cytoskeleton
  • Cytoskeleton include microfilaments (actin filaments), microtubules (composed primarily of α-tubulin and β-tubulin), and intermediate filaments
  • The molecules freely diffuse across membranes but the movement of others is restricted because of size, charge or solubility
  • Various passive and active mechanisms are employed to transport such molecules across membrane
  • Facilitated diffusion permit the movement of ions and molecules along a downhill gradient from high to low concentration. Specific carrier proteins are involved in such processes. Whereas uphill gradient from low to high concentration by active transport requires energy.
  • Primary active transport is catalyzed by ATPase that use energy produced by ATP hydrolysis.
  • 19Secondary active transport uses electrochemical gradient of Na+ and H+ or membrane potential produced by primary active transport processes
  • Co-transport or symport and counter transport or antiport are examples of secondary active transport
  • Macromolecules can enter or leave cells through mechanisms such as endocytosis or exocytosis
  • The properties of subcellular organelles can be investigated by cell fractionation, components being separated by differential centrifugation, on the basis of differences in sedimentation rates or density
  • Purity of subcellular fractions may be assessed by analyzing marker enzymes.
 
Review Questions
 
SHORT NOTES
  1. Diagrammatic representation of cell with functions of the subcellular organelles.
  2. Give structure and function of:
    1. Mitochondria
    2. ER
    3. Golgi apparatus
    4. Plasma membrane
    5. Nucleolus
    6. Lysosomes
    7. Peroxisomes
 
MULTIPLE CHOICE QUESTIONS (MCQ)
1. The following is the metabolic function of ER:
  1. RNA processing
  2. Fatty acid oxidation
  3. Synthesis of plasma protein
  4. ATP-synthesis
2. In biologic membranes, integral proteins and lipids interact mainly by:
  1. Covalent bond
  2. Both hydrophobic and covalent bond
  3. Hydrophobic interactions
  4. Hydrogen bond
3. Plasma membrane is made up of:
  1. Lipid bilayer
  2. Protein bilayer
  3. Carbohydrate bilayer
  4. Lipid single layer
4. Select the subcellular component involved in the formation of ATP:
  1. Nucleus
  2. Plasma membrane
  3. Mitochondria
  4. Golgi apparatus
5. Mitochondrial DNA is:
  1. Maternal inherited
  2. Paternal inherited
  3. Maternal and paternal inherited
  4. None of the above
6. Which of the following events leads to a breakdown of the nuclear envelope?
  1. A loss of cholesterol from the nuclear membrane
  2. Degradation of integral proteins in the nuclear membrane
  3. Modification of lipid bilayer of the nuclear membrane
  4. Phosphorylation of the lamins in the nuclear membrane
7. All of the following statements about the nucleus are true, except:
  1. Outer nuclear membrane is connected to ER
  2. It is the site of storage of genetic material
  3. Nucleolus is surrounded by a bilayer membrane
  4. Outer and inner membranes of nucleus are connected at nuclear pores
 
CORRECT ANSWERS FOR MCQ
1-c
2-c
3-c
4-c
5-a
6-d
7-c