Essentials of Biochemistry Pankaja Naik
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Cell and Membrane TransportCHAPTER 1

Chapter outline
  • ➢ Types of Living Cell
  • ➢ Structure and Functions of a Cell and its Subcellular Components
  • ➢ Cytoskeleton
  • ➢ Membrane Transport
  • ➢ Cell Fractionation
  • ➢ Marker Enzymes
 
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.
 
TYPES OF LIVING CELL
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 (Greek: Eue = true, karyon = nucleus), which have a membrane enclosed nucleus encapsulating their DNA, (deoxyribonucleic acid). Animals, plants and fungi belong to the eukaryotes. Eukaryotic cells are much larger than prokaryotes. Eukaryotes may be multicellular as well as unicellular, are far more complex than prokaryotes and are characterized by having numerous membrane enclosed organelles (subcellular elements) in their cytoplasm, including:
    • Mitochondria
    • Lysosomes
    • Endoplasmic reticulum
    • Golgi complexes
  • Prokaryotes have no typical nucleus (Greek: Pro = before) instead consists of nucleoid in which the genome, the complete set of genes, composed of DNA is replicated and stored with its associated proteins. The nucleoid, in bacteria and archaea, is not separated from the cytoplasm by a membrane (Fig. 1.1). Bacteria and blue green algae belong to the prokaryotes. Prokaryotes lack membrane enclosed organelles (subcellular elements) in their cytoplasm. Prokaryotes; which comprise the various types of bacteria, have relatively small structures and are invariably unicellular.
Table 1.1 and Figure 1.1 describe some of the major structural features of the prokaryote and eukaryote cells.
 
STRUCTURE AND FUNCTIONS OF A CELL AND ITS SUBCELLULAR COMPONENTS
A cell has three major components:
  1. Plasma membrane (Cell membrane)
  2. Cytoplasm with its organelles:
    • Endoplasmic reticulum
    • Golgi apparatus
    • Mitochondria
    • Lysosomes
    • Peroxisomes
  3. Nucleus.
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Fig. 1.1: Cell structure of eukaryotic and prokaryotic cell.
Table 1.1   Structural features of prokaryotes and eukaryotes.
Organelle
Eukaryotes
Prokaryotes
Nucleus
Present
No define nucleus. DNA present but not separated from rest cell
Plasma membrane
Present
Present
Mitochondria
Present
Absent. Enzymes for oxidation reactions located on plasma membrane
Endoplasmic reticulum
Present
Absent
Ribosomes
Present
Present
Chromosomes
Linear
Circular
Cytoplasm
Contains various membrane bound organelles, such as mitochondria, lysosomes, peroxisomes and Golgi apparatus
Undifferentiated
Reproduction
Mitosis
By binary division
Table 1.2 shows biochemical functions of subcellular organelles of the eukaryotic cell.
 
Plasma Membrane
  • The cell is enveloped by a thin membrane called cell membrane or plasma membrane.
  • Plasma membranes mainly consist of lipids, proteins and smaller proportion of carbohydrates that are linked to lipids and proteins. The basic organization of biologic membranes is illustrated in Figure 1.2.
    Table 1.2   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
    Smooth endoplasmic reticulum
    Biosynthesis of steroid hormones and phospholipids, metabolism of foreign compounds
    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 rRNA and formation of ribosomes
    Mitochondrion
    ATP synthesis, site for tricarboxylic acid cycle, fatty acid oxidation, oxidative phosphorylation, part of urea cycle and part of heme synthesis
    Cytosol
    Site for glycolysis, pentose phosphate pathway, part of gluconeogenesis, urea cycle and heme synthesis, purine and pyrimidine nucleotide synthesis
    3
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    Fig. 1.2: The basic organization of biological membrane.
  • The Plasma membrane is an organized structure consisting of a lipid bilayer primarily of phospholipids and penetrated protein molecules forming a mosaiclike pattern (Fig. 1.3).
 
Membrane Lipids
  • The major classes of membrane lipids are:
    • Phospholipids
    • Glycolipids
    • Cholesterol.
      They all are amphipathic molecules, i.e. they have both hydrophobic and hydrophilic ends.
  • Membrane lipids spontaneously form bilayer in aqueous medium, burying their hydrophobic tails and leaving their hydrophilic ends exposed to the water (Fig. 1.2).
 
Membrane Proteins
  • Proteins of the membrane are classified into two major categories:
    • Integral proteins or intrinsic proteins or transmembrane proteins and
    • Peripheral or extrinsic 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 (Fig. 1.2). Transmembrane proteins act as enzymes and transport carriers for ions as well as water soluble substances, such as glucose.
  • 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 (Fig. 1.2). Peripheral proteins function almost entirely as enzymes and receptors.
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Fig. 1.3: The fluid mosaic model of cell membrane.
 
Membrane Carbohydrates
  • 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 portion of these molecules protrudes to the outside of the cell, dangling outward from the cell surface (Fig. 1.2).
  • Many of the carbohydrates act as receptor for hormones. Some carbohydrate moieties function in antibody processing.
 
Functions of Cell Membrane
  • The plasma membrane maintains the physical integrity of the cell by preventing the contents of the cell from leaking into the outside fluid environment and at the same time facilitating the entry of nutrients, inorganic ions and most other charged or polar compounds from the outside. It permits only some substances to pass in either direction, and it forms a barrier for other substances.
  • The cell membrane protects the cytoplasm and the organelles of the cytoplasm.
  • It maintenance of shape and size of the cell.
 
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.4
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    Figs. 1.4A and B: Structure of endoplasmic reticulum.
  • According to this model, the membrane structure is a lipid bilayer made of phospholipids.
  • The bilayer is fluid because the hydrophobic tails of phospholipids consist of an appropriate mixture of saturated and unsaturated fatty acids that is fluid at normal temperature of the cell.
  • Proteins are interspersed in the lipid bilayer, of the plasma membrane, producing a mosaic effect (see Fig. 1.3).
  • The peripheral proteins literally float on the surface of ‘sea’ of the phospholipid molecules, whereas the integral proteins are like icebergs, almost completely submerged in the hydrophobic region.
  • There are no covalent bonds between lipid molecules of the bilayer or between the protein components and the lipids.
  • Thus, there is a mosaic pattern of membrane proteins in the fluid lipid bilayer.
  • Fluid mosaic model allows the membrane proteins to move around laterally in two dimensions and that they are free to diffuse from place to place within the plane of the bilayer. Whereas, they cannot outflow from one side of the lipid bilayer to the other.
  • 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.
Six important organelles that are suspended in the cytoplasm are:
  1. Endoplasmic reticulum
  2. Golgi apparatus
  3. Lysosomes
  4. Peroxisomes
  5. Mitochondria
  6. Nucleus.
 
Endoplasmic Reticulum (ER)
  • Endoplasmic reticulum is the interconnected network of tubular and flat vesicular structures in the cytoplasm (Figs. 1.4A and B).
  • Endoplasmic reticulum forms 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 Fig. 1.1).
  • 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.
  • During the process of cell fractionation, rough ER is disrupted to form small vesicles known as microsomes. It may be noted that microsomes as such do not occur in the cell.
  • Part of the ER, which has no attached ribosomes, is known as smooth endoplasmic reticulum.
 
Functions of the ER
  • Rough ER functions in the biosynthesis of protein.
  • The smooth endoplasmic reticulum functions in the synthesis of steroid hormones and cholesterol.
  • Smooth endoplasmic reticulum is the site of the metabolism of certain drugs, toxic compounds and carcinogens (cancer producing substances).5
 
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:
  • Proteins synthesized in the ER are transported to the Golgi apparatus where these are processed by addition of carbohydrate, lipid or sulfate moieties. These chemical modifications are necessary for the transport of proteins across the plasma membrane.
  • Golgi apparatus are also involved in the synthesis of intracellular organelles, e.g. lysosomes and peroxisomes.
 
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, prevent their digestive actions.
  • Many small granules are present in the lysosome. The granules contain more than 40 different hydroxylases (hydrolytic enzymes). All the enzymes are collectively called lysozymes.
 
Functions of lysosomes
Lysozymes present in lysosomes digest proteins, carbohydrates, lipids and nucleic acids. Apart from the digestive functions, the enzymes in the lysosomes are responsible for the following activities in the cell:
  • Destruction of bacteria and other foreign bodies.
  • 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 their synthesis.
  • 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. Thus, the peroxisomes are involved in the detoxification of peroxide.
  • Peroxisomes are also capable of carrying out β-oxidation of fatty acid.
 
Mitochondria (Powerhouse of Cell)
  • Mitochondria are called “Power Plant” of the cell since they convert energy to form ATP that can be used by cell.
  • A mitochondrion is a double-membrane organelle (Fig. 1.5) that is fundamentally different in composition and function:
    • – The outer membrane forms a smooth envelope. It is freely permeable for most metabolites.
    • – 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.
  • The space within the inner membrane is called the mitochondrial matrix. It contains the enzymes of the:
    • Citric acid cycle
    • β-oxidation of fatty acid
    • Some other degradative enzymes.
 
Functions of mitochondria
  • The mitochondrial matrix is the site of most of the reactions of the citric acid cycle and fatty acid oxidation. In contrast oxidative phosphorylation takes place in the inner mitochondrial membrane.
  • The outer membrane is permeable to most small molecules and ions because it contains many mitochondrial 6porin (pore forming protein) also known as voltagedependent anion channel (VDAC) that permit access to most molecules. In contrast inner membrane is impermeable to nearly all ions and polar molecules. Many transporters shuttles metabolites such as ATP, pyruvate, and citrate across the inner mitochondrial membrane.
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Fig. 1.5: 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 in shape and situated near the center of the cell. The nucleus is surrounded by the nuclear envelope.
  • 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 fibers. At mitosis, chromatin is condensed into discrete structures called chromosomes.
 
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:
  1. Ribosomal RNA (rRNA)
  2. Messenger RNA (mRNA)
  3. Transfer RNA (tRNA).
 
CYTOSKELETON
  • 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 plasma membrane is anchored to the cytoskeleton. The cytoskeleton is not a rigid permanent framework of the cell but is a dynamic, changing structure.
  • The cytoskeleton consists of three primary protein filaments:
    1. Microfilaments
    2. Microtubules
    3. Intermediate filaments.
  • Microfilaments are about 5 nm in diameter. They are made up of protein actin. Actin filaments form a meshwork just underlying the plasma membrane of cells and are referred to as cell cortex, which is labile. They disappear as cell motility increases or upon malignant transformation of cells. The function of microfilaments is:
    • To help muscle contraction
    • To maintain the shape of the cell
    • To help cellular movement.
  • Microtubules are cylindrical tubes, 20 to 25 nm in diameter. They are made up of protein tubulin.
    Microtubules are necessary for the formation and function of mitotic spindle. They provide stability to the cell. They prevent tubules of ER from collapsing. These are the major components of axons and dendrites.
  • Intermediate filaments 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 varies with different tissue type.
    • They play role in cell-to-cell attachment and help to stabilize the epithelium. They provide strength and rigidity to axons.
 
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 separation of chromosomes during cell division.
  • The internal movement of the cell organelles as well as cell locomotion and muscle fiber contraction could not take place without the cytoskeleton. It acts as “track”7on which cells can move organelles, chromosomes and other things.
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Fig. 1.6: Types of membrane transport mechanism.
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Fig. 1.7: Uniport, symport and antiport transport of substance across the cell membrane.
 
MEMBRANE TRANSPORT
  • One of the functions of the plasma membrane is to regulate the passage of a variety of small molecules across it.
  • 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.
  • The two types of transport mechanisms are (Fig. 1.6):
    1. Passive transport or passive diffusion
    2. Active transport.
 
Passive Transport or Passive Diffusion
  • Passive transport is the process by which molecules move across a membrane without energy (ATP).
  • The direction of passive transport is always from a region of higher concentration to one of lower concentration.
  • There are two types of passive transport as follows:
    1. Simple diffusion
    2. 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 is called simple 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.
  • An example of facilitated diffusion is the movement of glucose and most of the amino acids across the plasma membrane.
  • These diffusion processes are not coupled to the movement of other ions, they are known as uniport transport processes (Fig. 1.7).
 
Active Transport
  • If a molecule moves against a concentration gradient, an external energy source is required; this movement is referred to as active transport.
    8
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    Fig. 1.8: Mechanism of sodium-potassium pump (primary active transport).
  • Substances that are actively transported through cell membranes include, Na+, K+, Ca++, H+, CI-, several different sugars and most of the amino acids.
  • Active transport is classified into two types according to the source of energy used as follows:
    1. Primary active transport
    2. Secondary active transport.
  • In both instances, transport depends on the carrier proteins; like facilitated diffusion. However, in active transport, the carrier proteins function differently from the carrier in facilitated diffusion. Carrier protein for active transport is capable of transporting substance against the concentration gradient.
 
Primary Active Transport
  • In primary active transport, the energy is derived directly from hydrolysis of ATP.
  • 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 an 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 has ATPase activity (Fig. 1.8).
  • 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.
 
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.
  • It carries the active transport of sugars and amino acids.
 
Secondary Active Transport
Secondary active transport uses an energy generated by an electrochemical gradient. It is not directly coupled with hydrolysis of ATP. Secondary active transport is classified into two types:
  1. Cotransport or symport, in which both substances move simultaneously across the membrane in the same direction (see Fig. 1.7), e.g. transport of Na+ and glucose to the intestinal mucosal cells from the gut.
  2. Counter transport or antiport, in which both substances move simultaneously in opposite direction (see Fig. 1.7), e.g. transport of Na+ and H+ occurs in the renal proximal tubules and exchange of Cl and HCO3 in the erythrocytes.
 
Transport of Macromolecules Across the Plasma Membrane
  • The process by which cells take up large molecules is called endocytosis (Fig. 1.9) and the process by which cells release large molecules from the cells to the outside is called exocytosis (Fig. 1.10).
 
Endocytosis
  • There are two types of endocytosis:
    • Pinocytosis (cellular drinking)
    • 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 macromolecules, such as most proteins, polysaccharides and polynucleotides can enter cells (Fig. 1.9).
  • These molecules first attach to specific receptors on the surface of the membrane.
  • The receptors are generally concentrated in small pits on the outer surface of the cell membrane. These receptors are coated on the cytoplasmic side with a fibrillar protein called clathrin and contractile filaments of actin and myosin.
    9
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    Fig. 1.9: Three stages of the absorption of macromolecules by endocytosis.
  • Once the macromolecules (which are to be absorbed) have bound with the receptors, the entire pit invaginates inward, and the fibrillar protein by surrounding the invaginating pit causes it to close over the attached macromolecule along with a small amount of extracellular fluid.
  • Then immediately, the invaginated portion of the membrane breaks away from the surface of the cell forming endocyte vesicle inside the cytoplasm of the cell.
 
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.
    zoom view
    Fig. 1.10: Stages in exocytosis.
  • Phagocytosis occurs in much the same way as pinocytosis.
 
Exocytosis
  • Most of the endocytic vesicles formed from pinocytosis 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 and reused in the cytoplasm.
  • Undigestible substances called residual body is finally excreted through the cell membrane by a process called exocytosis, opposite to endocytosis (Fig. 1.10).
  • The undigestible substances produced within the cytoplasm may be enclosed in membranes to form vesicles called exocytic vesicles.
  • These cytoplasmic exocytic vesicles fuse with the internal surface of the plasma membrane.
  • The vesicle then ruptures releasing their contents into the extracellular space and their membranes are retrieved (left behind) and reused.
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Fig. 1.11: Subcellular fractionation of cell by differential centrifugation.
 
CELL FRACTIONATION
  • To obtain purified preparations of organelles, the tissue is first carefully broken up in a homogenizing apparatus using isotonic 0.25 M sucrose solution.
  • 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.
  • Then homogenate is centrifuged at a series of increasing centrifugal force (Fig. 1.11).
  • The subcellular organelles, which differ in size and specific gravity, sediment at different rates and can be isolated from homogenate by differential centrifugation.
  • The dense nuclei are sediment 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 this way are contaminated with lysosome and peroxisomes. These may be separated by isopycnic 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 and acid phosphatase, the catalase and acid phosphatase activities being due to contamination with peroxisomes and lysosomes respectively.
  • Some typical subcellular markers are given in Table 1.3.
Table 1.3   Marker enzymes of subcellular fractions
Fraction
Enzymes
Plasma membrane
5 Nucleotidase, Na+-K+-ATPase
Nucleus
DNA polymerase
RNA polymerase
Endoplasmic reticulum Golgi bodies
Glucose-6-phosphatase
Galactosyl transferase
Lysosomes
Acid phosphatase
β-glucuronidase
Mitochondria
Succinate dehydrogenase
Cytochrome C-oxidase
Peroxisomes
Catalase
Cytosol
Lactate dehydrogenase
Glucose-6-phosphate dehydrogenase
 
11EXAM QUESTIONS
 
Short Notes
  1. Diagrammatic representation of cell with functions of the subcellular organelles.
  2. Give structure and function of:
    1. Mitochondria
    2. Endoplasmic reticulum
    3. Golgi apparatus
    4. Plasma membrane
    5. Nucleolus
    6. Lysosomes
    7. Peroxisomes.
 
Multiple Choice Questions (MCQs)
  1. The following is the metabolic function of ER:
    1. RNA processing
    2. Fatty acid oxidation
    3. Synthesis of plasma protein
    4. ATP-synthesis
  1. In biologic membranes, integral proteins and lipids interact mainly by:
    1. Covalent bond
    2. Both hydrophobic and covalent bond
    3. hydrogen and electrostatic bond
    4. None of the above
  1. Plasma membrane is:
    1. Composed entirely of lipids
    2. Mainly made up of proteins
    3. Mainly made up of lipid and protein
    4. Composed of only carbohydrates and lipids
  1. Select the subcellular component involved in the formation of ATP:
    1. Nucleus
    2. Plasma membrane
    3. Mitochondria
    4. Golgi apparatus
  1. Mitochondrial DNA is:
    1. Maternal inherited
    2. Paternal inherited
    3. Maternal and paternal inherited
    4. None of the above
  1. 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
  1. Golgi apparatus is present in all of the following, except:
    1. RBC
    2. Parenchymal cells
    3. Skeletal muscle cells
    4. Pancreatic cell
  1. Peroxisomes arise from:
    1. Golgi membrane
    2. Lysosomes
    3. Mitochondria
    4. Pre-existing peroxisomes and budding off from the smooth ER
  1. Na+ – K+ ATPase is the marker enzyme of:
    1. Nucleus
    2. Plasma membrane
    3. Golgi bodies
    4. Cytosol
  1. Exocytosis:
    1. Is always employed by cells for secretion
    2. Is used to deliver material into the extracellular space
    3. Take up large molecules from the extracellular space
    4. Allows the salvage of elements of the plasma membrane
  1. The approximate number of cells in a normal human body is:
    1. 10
    2. 100
    3. 1014
    4. 10144
  1. The largest cell in the human body is:
    1. Nerve cell
    2. Muscle cell
    3. Liver cell
    4. Kidney cell
  1. Which one of the following eukaryotic cell structures does not contain DNA?
    1. Nucleus
    2. Mitochondrion
    3. The endoplasmic reticulum
    4. Chloroplast
  1. The cytoskeleton includes all of the following, except:
    1. Microtubules
    2. Intermediate filaments
    3. Myosin filaments
    4. Action filaments12
  1. Ribosomes are found:
    1. Only in the nucleus
    2. In the cytoplasm
    3. Attached to the smooth endoplasmic reticulum
    4. Both b and c
  1. The Golgi apparatus is involved in:
    1. Packaging proteins into vesicles
    2. Altering or modifying proteins
    3. Producing lysosomes
    4. All of the above
  1. All of the following are functions of the cell membrane, except:
    1. Participating in chemical reactions
    2. Participating in energy transfer
    3. Being freely permeable to all substances
    4. Regulating the passage of materials
  1. Who proposed the fluid mosaic model of cell membrane structure in 1972?
    1. Davidson and Singer
    2. Frye and Edidin
    3. Brown and Goldstein
    4. Singer and Nicholson
  1. Which of the following are involved with the movement or transport of materials or organelles throughout the cell?
    1. Rough endoplasmic reticulum
    2. Cytoskeleton
    3. Smooth endoplasmic reticulum
    4. All of the choices are true
  1. Lysosomes are produced by the:
    1. Nucleus
    2. Mitochondria
    3. Golgi apparatus
    4. Ribosomes
  1. Glucose-6-phosphatase is the marker enzyme of:
    1. Nucleus
    2. Plasma membrane
    3. Golgi bodies
    4. Endoplasmic reticulum
  1. Galactosyltransferase is the marker enzyme of:
    1. Nucleus
    2. Peroxisomes
    3. Golgi bodies
    4. Cytosol
  1. Acid phosphatase is the marker enzyme of:
    1. Mitochondria
    2. Lysosomes
    3. Golgi bodies
    4. Cytosol
  1. Succinate dehydrogenase is the marker enzyme of:
    1. Mitochondria
    2. Plasma membrane
    3. Golgi bodies
    4. Endoplasmic reticulum
  1. Lactate dehydrogenase and Glucose-6-phosphate dehydrogenase are the marker enzymes of:
    1. Nucleus
    2. Peroxisomes
    3. Golgi bodies
    4. Cytosol