CHAPTER 8Cell (Latin word) = a little roomCell is the structural and functional unit of life.because –all living beings are madeup of cell/cells and each living cell has the capacity to perform certain basic functions that are characteristic of all living forms.Discovery:Cells were first discovered by Robert Hooke in 1665. He observed the cells in a cork slice with the help of a primitive microscope.Leeuwenhoek (1674), with the improved microscope, discovered the free living cells in pond water for the first time. Robert Brown in 1831 discovered the nucleus in the cell. Purkinje in 1839 coined the term protoplasm for the fluid substance of the cell. Cell theory:Presented by: Schleiden (1838) and Schwann (1839) Expanded by: Virchow (1855)All the plants and animals are composed of cells The cell is the basic unit of life.All cells arise from pre-existing cells. Shape and size:The shape and size of cells are related to the specific function they perform. Cell with changing shapes: Amoeba, mycoplasma

unicellular organisms multicellular organisms(uni = single)organisms with only one cell (multi = many) many cells group together in

a single bodySingle cell perform all life processes Cells may assume different functions in it to

form various body partsChlamydomonas, Paramoecium and bacteria, amoeba, yeast, plasmodium etc.

some fungi, plants and animals

Division of labour: It means different parts perform different functions. Each cell has specific components within it known as cell organelles (plasma membrane, nucleus and cytoplasm) to perform special function, such as ER for making new material in the cell, lysosomes for clearing up the waste material from the cell etc.prokaryotes EukaryotesSize: generally small ( 1-10 μm) Size: generally large ( 5-100 μm)(Pro = primitive or primary; karyote ≈ karyon = nucleus)= Nucleoidcells lack a nuclear membrane

Organisms with cells having a nuclear membrane= well defined nucleus

Prokaryotic cells also lack most of the cytoplasmic organelles (membrane bounded)present in eukaryotic cells

eukaryotic cells have membrane bound cell organelles like ER, golgi complex, lysosomes, mitochondria, microbodies and vacuoles.

Chromosomes lack histone proteins histone proteins present Ribosomes = 70 S type Ribosomes = 80 S typeThe chlorophyll in photosynthetic prokaryotic bacteria is associated with membranous vesicles

plastids are present in eukaryotic cells

The prokaryotic cells are represented by bacteria, blue-green algae, mycoplasma and PPLO

Cells of all plants, animals and fungi are eukaryotic cell

PROKARYOTIC CELLS

Cell Envelope:

The cell envelope consists of –o outermost glycocalyx o cell wall o plasma membrane

They act together as a single protective unit. The plasma membrane is similar structurally to that of the eukaryotes and semi-permeable in

nature and interacts with the outside world. A special membranous structure is the mesosome which is formed by the extensions of plasma

membrane into the cell. These extensions are in the form of vesicles, tubules and lamellae. They help in cell wall formation, DNA replication and distribution to daughter cells. They also help in respiration, secretion processes, to increase the surface area of the plasma membrane and enzymatic content. In some prokaryotes like cyanobacteria, there are other membranous extensions into the cytoplasm called chromatophores which contain pigments.

Bacterial cells may be motile or non-motile. If motile, they have thin filamentous extensions from their cell wall called flagella. Bacteria show a range in the number and arrangement of flagella. Bacterial flagellum is composed of three parts – filament, hook and basal body. The filament is the longest portion and extends from the cell surface to the outside. Besides flagella, Pili and Fimbriae are also surface structures of the bacteria but do not play a role in motility. The pili are elongated tubular structures made of a special protein. The fimbriae are small bristle like fibres sprouting out of the cell. In some bacteria, they are known to help attach the bacteria to rocks in streams and also to the host tissues.

Ribosomes and Inclusion Bodies: In prokaryotes ribosomes are associated with the plasma membrane of the cell. They are about 15 nm by 20 nm in size and are made of two subunits - 50S and 30S units which when present together form 70S prokaryotic ribosomes. Ribosomes are the site of protein synthesis. Several ribosomes may attach to a single mRNA and form a chain called polyribosomes or polysome. The ribosomes of a polysome translate the mRNA into proteins.

Inclusion bodies: Reserve material in prokaryotic cells are stored in the cytoplasm in the form of inclusion bodies. These are not bounded by any membrane system and lie free in the cytoplasm, e.g., phosphate granules, cyanophycean granules and glycogen granules. Gas vacuoles are found in blue green and purple and green photosynthetic bacteria.

EUKARYOTIC CELLS

Cell Membrane: Each cell is covered by an asymmetrical, porous, thin, semi permeable sheet called cell membrane or plasmalemma.

CHARACTERISTICS OF CELL MEMBRANE

Living part of the cell, consist of lipid + protein.

1.5 micron in thickness. Consist of two layers of lipid. Lipid of plasma membrane are,

1. Phospho-lipids2. Glycolipids3. Sterol4. Cholesterol

STRUCTURE OF CELL MEMBRANE

FLUID MOSAIC MODEL

INTRODUCTIONThe fluid mosaic, bilayer model was proposed by “Singer and Nicolson (1972).

POSTULATES OF FLUID MOSAIC MODEL

Important postulates of this model are,(a) The cell membrane consists of lipid bilayer, in which a variety of proteins are present.(b) These proteins float in the fluid matrix of lipid (as ice bergs in the sea)

ARRANGEMENT OF PROTEINSAccording to the fluid mosaic model proteins are:

1. INTRINSIC/INTEGRAL PROTEINSThese proteins peneterate the membrane surface and enter the lipid layers (partially or wholly)

2. EXTRINSIC/PERIPHERAL PROTEINSThese are located adjacent to outer and inner surface of membrane and float like ice-berg in the sea.

FUNCTION OF MEMBRANE PROTEIN

Certain proteins themselves act as enzymes. Some protein act as carrier for active transport. Provide elasticity to membrane. Pores are lined by the proteins.

ARRANGEMENT OF LIPIDSphospho-lipid molecules form bilayer

each phospho-lipid molecules consists of ,1. Head (hydrophilic end): Present towards the surface and formed of phosphates.2. Tail (hydrophobic end) : Present towards the center and formed of fatty acids.

The non-polar end face each other while their polar ends are towards the surface.

FUNCTION OF LIPIDS PRESENT IN MEMBRANE

The lipids give rigidity to cell membrane. They lower the surface tension.

FUNCTIONS OF CELL MEMBRANE

It performs the two main functions:

Protection of Protoplasm. Regulation of material (In and Out of cell) through its permeabality.

PERMEABILITY OF MEMBRANE

The permeability of membrane is regulated by two processes.(1) Passive Transport (Osmosis and Diffusion)(2) Active Transport (Endocytosis, Exocytosis)

1. PASSIVE TRANSPORTSuch type of molecules transport which does not require energy. It is further divided into,

DIFFUSIONSpreading and free movement of molecules (or ions) from the region of higher concentration to the region of lower concentration (till equilibrium state)

The rate of diffusion is influenced by: o concentration gradient o cross-sectional area through which diffusion occurs o temperature o molecular weight of a substance o distance through which diffusion occurs

SIGNIFICANCE

Movement of oxygen and digested food (glucose, amino acids, fatty acids) into the cell. Movement of excretory waste out of cell.

OSMOSISDiffusion of water by semipermeable membrane or the movement of solvent molecules from higher to lower concentration across semi permeable membrane.

SIGNIFICANCE

Liquids, primarily water molecules enter and leave the cell by Osmosis.

It helps to maintain a balance (osmotic pressure) in and out of cell.

Facilitated diffusion = movement of a substance across a cell membrane from an area of high concentration to an area of low concentration through special proteins.

2. ACTIVE TRANSPORTSuch type of molecule transport which require energy. Or Movement of molecules against the concentration by the expenditure of energy through a carrier (i.e. movement of molecules from the region of lower concentration to higher concentration by protein using ATP as energy.

SIGNIFICANCEAbsorption of excess food (glucose), ions (K+ and Na+) takes place by Active transport.

CONDITIONS

It is unidirectional. ATP provides energy. Protein act as carrier.

Active transport is further subdivided into,(1) Phagocytosis and Pinocytosis (Endocytosis).(2) Exocytosis.

PHAGOCYTOSISProcess of picking and ingestion of large solid particle by plasma membrane (which can not enter by diffusion, osmosis or active transport).

SIGNIFICANCEIngestion of solid food particles.WBCs pick foreign particles (certain bacteria)

PINOCYTOSISProcess of fluid intake, for absorbing fluid by forming pinocytic vesicle (the fluid which cannot be absorbed by osmosis, enters through it)

SIGNIFICANCEHelps in absorption of harmones, lipids etc.

Cells are joined by a variety of intracellular junctions

In multicellular organisms, adajcent cells are held together by several types of specialized junctions.

1. Tight junctions (found in animals): specialized "belts" that bind two cells tightly to each other, prevent fluid from leaking into intracellular space.

2. Desmosomes (found in animals): intercellular "rivets" that create tight bonds between cells, but allow fluids to pass through intracellular spaces.

3. Gap junctions (found in animals): formed by two connecting protein rings embedded in cell membrane of adjacent cells. Allows passage of water, small solutes, but not macromolecules (proteins, nucleic acids).

4. Plasmodesmata (found in plants): channels connecting cells; allow free passage of water and small solutes, but not macromolecules (proteins, nucleic acids).

Cell Wall

Plant cells have an additional covering on the outside in addition to the cell membrane. This is the cell wall.

It is mainly composed of cellulose. Cellulose provides structural strength to the cell. (Cellulose is a complex carbohydrate.)

Other substances that constitute the cell wall are pectin and lignin. Algae have cell wall, made of cellulose, galactans, mannans and minerals like calcium carbonate,

while in other plants it consists of cellulose, hemicellulose, pectins and proteins. Advantage of cell wall:

It is rigid, non living and gives additional strength to the cell.

It protects the cell from mechanical damage and infection it also helps in cell-to-cell interaction and provides barrier to undesirable macromolecules. The cell wall and middle lamellae may be traversed by plasmodesmata which connect the

cytoplasm of neighbouring cells. When a living plant cell loses water through osmosis, there is shrinkage of the cell cytoplasm and

cell contents away from the cell wall. This is known as Plasmol ysis. cell wall permits the plant cells, fungi and bacteria to withstand pressures created when these cells are placed in very dilute (hypotonic) media without bursting. Plant cells, cells of fungi and bacteria are able to withstand greater changes in the surrounding

medium than the animal cells.

Structure: There are three layer of cell wall.

A. MIDDLE (LAMELLA) a) First formed cell plate. b) Cementing layer between two daughter cells. c) Composed of Ca++ and Mg++ pectate. d) Cells are separated when this layer is dissolved.

B. PRIMARY WALL a) First product of cell synthesized by protoplast.

b) In young cells it is thin and elastic while it becomes thick and rigid on maturity. c) Made up of Hemicellulose (50%), cellulose (25%) and pectate substances. d) The primary wall is capable of growth

C. SECONDARY WALL a) Composed of cellulose. b) Present inside the primary wall. c) Can be modified through the deposition of lignin and other substances.

Endomembrane System

The endomembrane system include endoplasmic reticulum (ER), golgi complex, lysosomes and vacuoles. Since the functions of the mitochondria, chloroplast and peroxisomes are not coordinated with the above components, these are not considered as part of the endomembrane system.

The Endoplasmic Reticulum (ER)

It is a network of tiny tubular structures scattered in the cytoplasm

ER divides the intracellular space into two distinct compartments, i.e., luminal (inside ER) and extra luminal (cytoplasm) compartments.

Rough ER Smooth ERThe endoplasmic reticulum bearing ribosomes on their surface is called rough endoplasmic reticulum (RER).

In the absence of ribosomes ER is called smooth endoplasmic reticulum (SER).

RER is frequently observed in the cells actively involved in protein synthesis and secretion.

The smooth endoplasmic reticulum is the major site for synthesis of lipid. In animal cells lipid-like steroidal hormones are synthesised in SER.

Golgi Apparatus

Camillo Golgi (1898) first observed densely stained reticular structures near the nucleus. These were later named Golgi bodies after him. They consist of many flat, disc-shaped sacs or cisternae of 0.5 m to μ1.0 m diameter. These are stacked parallel to each other. Varied number of cisternae are present in a μGolgi complex. The Golgi cisternae are concentrically arranged near the nucleus with distinct convex cis or the forming face and concave trans or the maturing face. The cis and the trans faces of the organelle are entirely different, but interconnected.

Functions: The golgi apparatus principally performs the function of packaging materials, to be delivered either to the intra-cellular targets or secreted outside the cell. Materials to be packaged in the form of vesicles from the ER fuse with the cis face of the golgi apparatus and move towards the maturing face. This explains, why the golgi apparatus remains in close association with the endoplasmic reticulum. A number of proteins synthesised by ribosomes on the endoplasmic reticulum are modified in the cisternae of the golgi apparatus before they are released from its trans face. Golgi apparatus is the important site of formation of glycoproteins and glycolipids.

Lysosomes

These are membrane bound vesicular structures formed by the process of packaging in the golgi apparatus. The isolated lysosomal vesicles have been found to be very rich in almost all types of

hydrolytic enzymes (hydrolases – lipases, proteases, carbohydrases) optimally active at the acidic pH. These enzymes are capable of digesting carbohydrates, proteins, lipids and nucleic acids.

Vacuoles

The vacuole is the membrane-bound space found in the cytoplasm. It contains water, sap, excretory product and other materials not useful for the cell. The vacuole is bound by a single membrane called tonoplast. In plant cells the vacuoles can occupy up to 90 per cent of the volume of the cell. In plants, the tonoplast facilitates the transport of a number of ions and other materials against concentration gradients into the vacuole, hence their concentration is significantly higher in the vacuole than in the cytoplasm.

In Amoeba the contractile vacuole is important for excretion. In many cells, as in protists, food vacuoles are formed by engulfing the food particles.

Mitochondria

Mitochondria (sing.: mitochondrion), unless specifically stained, are not easily visible under the microscope. The number of mitochondria per cell is variable depending on the physiological activity of the cells. In terms of shape and size also, considerable degree of variability is observed. Typically it is sausage-shaped or cylindrical having a diameter of 0.2-1.0 m (average 0.5 m) and length 1.0-4.1 m. μ μ μEach mitochondrion is a double membrane-bound structure with the outer membrane and the inner membrane dividing its lumen distinctly into two aqueous compartments, i.e., the outer compartment and the inner compartment. The inner compartment is called the matrix. The outer membrane forms the continuous limiting boundary of the organelle. The inner membrane forms a number of infoldings called the cristae (sing.: crista) towards the matrix. The cristae increase the surface area. The two membranes have their own specific enzymes associated with the mitochondrial function.

Function: Mitochondria are the sites of aerobic respiration. They produce cellular energy in the form of ATP, hence they are called ‘power houses’ of the cell. The matrix also possesses single circular DNA molecule, a few RNA molecules, ribosomes (70S) and the components required for the synthesis of proteins. The mitochondria divide by fission.

Plastids

Plastids are found in all plant cells and in euglenoides. These are easily observed under the microscope as they are large. They bear some specific pigments, thus imparting specific colours to the plants. Based on the type of pigments plastids can be classified into chloroplasts, chromoplasts and leucoplasts.

Chloroplasts: The chloroplasts contain chlorophyll and carotenoid pigments which are responsible for trapping light energy essential for photosynthesis.

Chromoplasts: In the chromoplasts fat soluble carotenoid pigments like carotene, xanthophylls and others are present. This gives the part of the plant a yellow, orange or red colour.

Leucoplasts: The leucoplasts are the colourless plastids of varied shapes and sizes with stored nutrients: Amyloplasts store carbohydrates (starch), e.g., potato; elaioplasts store oils and fats whereas the aleuroplasts store proteins.

Shape & Size of Chloroplasts: Majority of the chloroplasts of the green plants are found in the mesophyll cells of the leaves. These are lens-shaped, oval, spherical, discoid or even ribbon-like organelles having variable length (5-10mm) and width (2-4mm). Their number varies from 1 per cell of the Chlamydomonas, a green alga to 20-40 per cell in the mesophyll.

Structure of Chloroplasts: Like mitochondria, the chloroplasts are also double membrane bound. Of the two, the inner chloroplast membrane is relatively less permeable. The space limited by the inner membrane of the chloroplast is called the stroma. A number of organised flattened membranous sacs called the thylakoids, are present in the stroma. Thylakoids are arranged in stacks like the piles of coins called grana (singular: granum) or the intergranal thylakoids. In addition, there are flat membranous tubules called the stroma lamellae connecting the thylakoids of the different grana. The membrane of the thylakoids enclose a space called a lumen. The stroma of the chloroplast contains enzymes required for the synthesis of carbohydrates and proteins. It also contains small, double-stranded circular DNA molecules and ribosomes. Chlorophyll pigments are present in the thylakoids. The ribosomes of the chloroplasts are smaller (70S) than the cytoplasmic ribosomes (80S).

Ribosomes

Ribosomes are the granular structures first observed under the electron microscope as dense particles by George Palade (1953). They are composed of ribonucleic acid (RNA) and proteins and are not surrounded by any membrane. The eukaryotic ribosomes are 80S while the prokaryotic ribosomes are 70S. Here ‘S’ stands for the sedimentation coefficient; it indirectly is a measure of density and size. Both 70S and 80S ribosomes are composed of two subunits.

Cytoskeleton

An elaborate network of filamentous proteinaceous structures present in the cytoplasm is collectively referred to as the cytoskeleton. The cytoskeleton in a cell are involved in many functions such as mechanical support, motility, maintenance of the shape of the cell.

Cilia and Flagella

Cilia (sing.: cilium) and flagella (sing.: flagellum) are hair-like outgrowths of the cell membrane. Cilia are small structures which work like oars, causing the movement of either the cell or the surrounding fluid.

Flagella are comparatively longer and responsible for cell movement. The prokaryotic bacteria also possess flagella but these are structurally different from that of the eukaryotic flagella.

Structure: The electron microscopic study of a cilium or the flagellum show that they are covered with plasma membrane. Their core called the axoneme, possesses a number of microtubules running parallel to the long axis. The axoneme usually has nine pairs of doublets of radially arranged peripheral microtubules, and a pair of centrally located microtubules. Such an arrangement of axonemal microtubules is referred to as the 9+2 array. The central tubules are connected by bridges and are also enclosed by a central sheath, which is connected to one of the tubules of each peripheral couplet by a radial spoke. Thus, there are nine radial spokes. The peripheral doublets are also interconnected by linkers. Both the cilium and flagellum emerge from centriole-like structure called the basal bodies.

Centrosome and Centrioles

Centrosome is an organelle usually containing two cylindrical structures called centrioles. They are surrounded by amorphous pericentriolar materials. Both the centrioles in a centrosome lie perpendicular to each other in which each has an organisation like the cartwheel. They are made up of nine evenly spaced peripheral fibrils of tubulin. Each of the peripheral fibril is a triplet.The adjacent triplets are also linked. The central part of the centriole is also proteinaceous and called the hub, which is connected with tubules of the peripheral triplets by radial spokes made of protein. The centrioles form the basal body of cilia or flagella, and spindle fibres that give rise to spindle apparatus during cell division in animal cells.

Nucleus

Nucleus as a cell organelle was first described by Robert Brown as early as 1831. Later the material of the nucleus stained by the basic dyes was given the name chromatin by Flemming.

The interphase nucleus (nucleus of a cell when it is not dividing) has highly extended and elaborate nucleoprotein fibres called chromatin, nuclear matrix and one or more spherical bodies called nucleoli (sing.: nucleolus). Electron microscopy has revealed that the nuclear envelope, which consists of two parallel membranes with a space between (10 to 50 nm) called the perinuclear space, forms a barrier between the materials present inside the nucleus and that of the cytoplasm. The outer membrane usually remains continuous with the endoplasmic reticulum and also bears ribosomes on it.

At a number of places the nuclear envelope is interrupted by minute pores, which are formed by the fusion of its two membranes. These nuclear pores are the passages through which movement of RNA and protein molecules takes place in both directions between the nucleus and the cytoplasm. Normally, there is only one nucleus per cell, variations in the number of nuclei are also frequently observed. Some mature cells even lack nucleus, e.g., erythrocytes of many mammals and sieve tube cells of vascular plants.

The nuclear matrix or the nucleoplasm contains nucleolus and chromatin. The nucleoli are spherical structures present in the nucleoplasm. The content of nucleolus is continuous with the rest of the nucleoplasm as it is not a membrane bound structure. It is a site for active ribosomal RNA synthesis. Larger and more numerous nucleoli are present in cells actively carrying out protein synthesis.

During different stages of cell division, cells show structured chromosomes in place of the nucleus. Chromatin contains DNA and some basic proteins called histones, some non-histone proteins and also RNA. A single human cell has approximately two metre long thread of DNA distributed among its forty six (twenty three pairs) chromosomes.

Every chromosome essentially has a primary constriction or the centromere on the sides of which disc shaped structures called kinetochores are present. Based on the position of the centromere, the chromosomes can be classified into four types.

Metacentric Chromosome: The metacentric chromosome has middle centromere forming two equal arms of the chromosome.

Sub-metacentric Chromosome: The sub-metacentric chromosome has centromere nearer to one end of the chromosome resulting into one shorter arm and one longer arm.

Acrocentric Chromosome: In case of acrocentric chromosome the centromere is situated close to its end forming one extremely short and one very long arm,

Telocentric Chromosome: The telocentric chromosome has a terminal centromere.

Sometimes a few chromosomes have non-staining secondary constrictions at a constant location. This gives the appearance of a small fragment called the satellite.

Microbodies

Many membrane bound minute vesicles called microbodies that contain various enzymes, are present in both plant and animal cells.

 

The organisation of the prokaryotic cell is fundamentally similar eventhough prokaryotes exhibit a wide variety of shapes and functions. Allprokaryotes have a cell wall surrounding thecell membrane. The fluid matrix filling the cellis the cytoplasm. There is no well-definednucleus. The genetic material is basically naked,not enveloped by a nuclear membrane. Inaddition to the genomic DNA (the singlechromosome/circular DNA), many bacteria havesmall circular DNA outside the genomic DNA.These smaller DNA are called plasmids. Theplasmid DNA confers certain unique phenotypiccharacters to such bacteria. One such characteris resistance to antibiotics. In higher classes youwill learn that this plasmid DNA is used tomonitor bacterial transformation with foreignDNA. Nuclear membrane is found in eukaryotes.No organelles, like the ones in eukaryotes, arefound in prokaryotic cells except for ribosomes.Prokaryotes have something unique in the formof inclusions. A specialised differentiated formof cell membrane called mesosome is the characteristic of prokaryotes.They are essentially infoldings of cell membrane.8.4.1 Cell Envelope and its ModificationsMost prokaryotic cells, particularly the bacterial cells, have a chemicallycomplex cell envelope. The cell envelope consists of a tightly bound threelayered structure i.e., the outermost glycocalyx followed by the cell wall andthen the plasma membrane. Although each layer of the envelope performsdistinct function, they act together as a single protective unit. Bacteria canbe classified into two groups on the basis of the differences in the cell envelopesand the manner in which they respond to the staining procedure developedby Gram viz., those that take up the gram stain are Gram positive and theothers that do not are called Gram negative bacteria.Glycocalyx differs in composition and thickness among differentbacteria. It could be a loose sheath called the slime layer in some, whilein others it may be thick and tough, called the capsule. The cell walldetermines the shape of the cell and provides a strong structural supportto prevent the bacterium from bursting or collapsing.The plasma membrane is semi-permeable in nature and interacts withthe outside world. This membrane is similar structurally to that of theeukaryotes.A special membranous structure is the mesosome which is formedby the extensions of plasma membrane into the cell. These extensions arein the form of vesicles, tubules and lamellae. They help in cell wallTypical bacteria(1-2 _m)PPLO

(about 0.1 _m)Viruses(0.02-0.2 _m)A typical eukaryotic cell(10-20 _m)Figure 8.2 Diagram showing comparison ofeukaryotic cell with otherorganismsCELL: THE UNIT OF LIFE 129formation, DNA replication and distribution to daughter cells. They alsohelp in respiration, secretion processes, to increase the surface area ofthe plasma membrane and enzymatic content. In some prokaryotes likecyanobacteria, there are other membranous extensions into the cytoplasmcalled chromatophores which contain pigments.Bacterial cells may be motile or non-motile. If motile, they have thinfilamentous extensions from their cell wall called flagella. Bacteria show arange in the number and arrangement of flagella. Bacterial flagellum iscomposed of three parts – filament, hook and basal body. The filamentis the longest portion and extends from the cell surface to the outside.Besides flagella, Pili and Fimbriae are also surface structures of thebacteria but do not play a role in motility. The pili are elongated tubularstructures made of a special protein. The fimbriae are small bristle likefibres sprouting out of the cell. In some bacteria, they are known to helpattach the bacteria to rocks in streams and also to the host tissues.8.4.2 Ribosomes and Inclusion BodiesIn prokaryotes ribosomes are associated with the plasma membrane ofthe cell. They are about 15 nm by 20 nm in size and are made of twosubunits - 50S and 30S units which when present together form 70Sprokaryotic ribosomes. Ribosomes are the site of protein synthesis. Severalribosomes may attach to a single mRNA and form a chain calledpolyribosomes or polysome. The ribosomes of a polysome translate themRNA into proteins.Inclusion bodies: Reserve material in prokaryotic cells are stored inthe cytoplasm in the form of inclusion bodies. These are not bounded byany membrane system and lie free in the cytoplasm, e.g., phosphategranules, cyanophycean granules and glycogen granules. Gas vacuolesare found in blue green and purple and green photosynthetic bacteria.8.5 EUKARYOTIC CELLSThe eukaryotes include all the protists, plants, animals and fungi. Ineukaryotic cells there is an extensive compartmentalisation of cytoplasmthrough the presence of membrane bound organelles. Eukaryotic cellspossess an organised nucleus with a nuclear envelope. In addition,eukaryotic cells have a variety of complex locomotory and cytoskeletalstructures. Their genetic material is organised into chromosomes.All eukaryotic cells are not identical. Plant and animal cells are differentas the former possess cell walls, plastids and a large central vacuole whichare absent in animal cells. On the other hand, animal cells have centrioleswhich are absent in almost all plant cells (Figure 8.3).130 BIOLOGYRough endoplasmicreticulum

LysosomeSmoothendoplasmicreticulumPlasmodesmataMicrotubuleNucleusNucleolusGolgiapparatusNuclearenvelopeVacuoleMiddle lamellaPlasmamembraneCell wallMitochondrionChloroplast RibosomesCytoplasmPeroxisomeFigure 8.3 Diagram showing : (a) Plant cell (b) Animal cellGolgiapparatusSmoothendoplasmicreticulumNuclearenvelopeNucleolusNucleusMicrovilliPlasmamembraneCentriolePeroxiomeLysosomeRibosomesMitochondrionRoughendoplasmicreticulumCytoplasm(a)(b)CELL: THE UNIT OF LIFE 131Figure 8.4 Fluid mosaic model of plasma membraneCholesterolSugar ProteinLipid bilayerLet us now look at individual cell organelles to understand their

structure and functions.8.5.1 Cell MembraneThe detailed structure of the membrane was studied only after the adventof the electron microscope in the 1950s. Meanwhile, chemical studies onthe cell membrane, especially in human red blood cells (RBCs), enabledthe scientists to deduce the possible structure of plasma membrane.These studies showed that the cell membrane is composed of lipidsthat are arranged in a bilayer. Also, the lipids are arranged within themembrane with the polar head towards the outer sides and thehydrophobic tails towards the inner part.This ensures that the nonpolartail of saturated hydrocarbons is protected from the aqueous environment(Figure 8.4). The lipid component of the membrane mainly consists ofphosphoglycerides.Later, biochemical investigation clearly revealed that the cell membranesalso possess protein and carbohydrate. The ratio of protein and lipid variesconsiderably in different cell types. In human beings, the membrane of theerythrocyte has approximately 52 per cent protein and 40 per cent lipids.Depending on the ease of extraction, membrane proteins can beclassified as integral or peripheral. Peripheral proteins lie on the surfaceof membrane while the integral proteins are partially or totally buried inthe membrane.132 BIOLOGYAn improved model of the structure of cell membrane was proposedby Singer and Nicolson (1972) widely accepted as fluid mosaic model(Figure 8.4). According to this, the quasi-fluid nature of lipid enableslateral movement of proteins within the overall bilayer. This ability to movewithin the membrane is measured as its fluidity.The fluid nature of the membrane is also important from the point ofview of functions like cell growth, formation of intercellular junctions,secretion, endocytosis, cell division etc.One of the most important functions of the plasma membrane is thetransport of the molecules across it. The membrane is selectively permeableto some molecules present on either side of it. Many molecules can movebriefly across the membrane without any requirement of energy and thisis called the passive transport. Neutral solutes may move across themembrane by the process of simple diffusion along the concentrationgradient, i.e., from higher concentration to the lower. Water may also moveacross this membrane from higher to lower concentration. Movement ofwater by diffusion is called osmosis. As the polar molecules cannot passthrough the nonpolar lipid bilayer, they require a carrier protein of themembrane to facilitate their transport across the membrane. A few ionsor molecules are transported across the membrane against theirconcentration gradient, i.e., from lower to the higher concentration. Sucha transport is an energy dependent process, in which ATP is utilised andis called active transport, e.g., Na+/K+ Pump.8.5.2 Cell WallAs you may recall, a non-living rigid structure called the cell wall formsan outer covering for the plasma membrane of fungi and plants. Cell wallnot only gives shape to the cell and protects the cell from mechanicaldamage and infection, it also helps in cell-to-cell interaction and providesbarrier to undesirable macromolecules. Algae have cell wall, made of

cellulose, galactans, mannans and minerals like calcium carbonate, whilein other plants it consists of cellulose, hemicellulose, pectins and proteins.The cell wall of a young plant cell, the primary wall is capable of growth,which gradually diminishes as the cell matures and the secondary wall isformed on the inner (towards membrane) side of the cell.The middle lamella is a layer mainly of calcium pectate which holdsor glues the different neighbouring cells together. The cell wall and middlelamellae may be traversed by plasmodesmata which connect the cytoplasmof neighbouring cells.8.5.3 Endomembrane SystemWhile each of the membranous organelles is distinct in terms of itsstructure and function, many of these are considered together as anCELL: THE UNIT OF LIFE 133NucleusNuclear pore RoughRibosomeendoplasmicEndoplasmicreticulumSmoothreticulumendomembrane system because their functionsare coordinated. The endomembrane systeminclude endoplasmic reticulum (ER), golgicomplex, lysosomes and vacuoles. Since thefunctions of the mitochondria, chloroplast andperoxisomes are not coordinated with the abovecomponents, these are not considered as part ofthe endomembrane system.8.5.3.1 The Endoplasmic Reticulum (ER)Electron microscopic studies of eukaryotic cellsreveal the presence of a network or reticulum oftiny tubular structures scattered in the cytoplasmthat is called the endoplasmic reticulum (ER)(Figure 8.5). Hence, ER divides the intracellularspace into two distinct compartments, i.e., luminal(inside ER) and extra luminal (cytoplasm)compartments.The ER often shows ribosomes attached totheir outer surface. The endoplasmic reticulunbearing ribosomes on their surface is called roughendoplasmic reticulum (RER). In the absence ofribosomes they appear smooth and are calledsmooth endoplasmic reticulum (SER).RER is frequently observed in the cells activelyinvolved in protein synthesis and secretion. Theyare extensive and continuous with the outermembrane of the nucleus.The smooth endoplasmic reticulum is the majorsite for synthesis of lipid. In animal cells lipid-likesteroidal hormones are synthesised in SER.

8.5.3.2 Golgi apparatusCamillo Golgi (1898) first observed densely stainedreticular structures near the nucleus. These werelater named Golgi bodies after him. They consistof many flat, disc-shaped sacs or cisternae of0.5μm to 1.0μm diameter (Figure 8.6). These arestacked parallel to each other. Varied number ofcisternae are present in a Golgi complex. The Golgicisternae are concentrically arranged near thenucleus with distinct convex cis or the formingface and concave trans or the maturing face.Figure 8.5 Endoplasmic reticulumCisternaeFigure 8.6 Golgi apparatus134 BIOLOGYThe cis and the trans faces of the organelle are entirely different, butinterconnected.The golgi apparatus principally performs the function of packagingmaterials, to be delivered either to the intra-cellular targets or secretedoutside the cell. Materials to be packaged in the form of vesicles fromthe ER fuse with the cis face of the golgi apparatus and move towardsthe maturing face. This explains, why the golgi apparatus remains inclose association with the endoplasmic reticulum. A number of proteinssynthesised by ribosomes on the endoplasmic reticulum are modifiedin the cisternae of the golgi apparatus before they are released from itstrans face. Golgi apparatus is the important site of formation ofglycoproteins and glycolipids.8.5.3.3 LysosomesThese are membrane bound vesicular structures formed by the processof packaging in the golgi apparatus. The isolated lysosomal vesicleshave been found to be very rich in almost all types of hydrolyticenzymes (hydrolases – lipases, proteases, carbohydrases) optimallyactive at the acidic pH. These enzymes are capable of digestingcarbohydrates, proteins, lipids and nucleic acids.8.5.3.4 VacuolesThe vacuole is the membrane-bound space found in the cytoplasm. It containswater, sap, excretory product and other materials not useful for the cell. Thevacuole is bound by a single membrane called tonoplast. In plant cells thevacuoles can occupy up to 90 per cent of the volume of the cell.In plants, the tonoplast facilitates the transport of a number of ionsand other materials against concentration gradients into the vacuole, hencetheir concentration is significantly higher in the vacuole than in thecytoplasm.In Amoeba the contractile vacuole is important for excretion. In manycells, as in protists, food vacuoles are formed by engulfing the foodparticles.8.5.4 MitochondriaMitochondria (sing.: mitochondrion), unless specifically stained, are noteasily visible under the microscope. The number of mitochondria per cellis variable depending on the physiological activity of the cells. In terms ofshape and size also, considerable degree of variability is observed. Typically

it is sausage-shaped or cylindrical having a diameter of 0.2-1.0μm (average0.5μm) and length 1.0-4.1μm. Each mitochondrion is a doublemembrane-bound structure with the outer membrane and the innerCELL: THE UNIT OF LIFE 135membrane dividing its lumen distinctly into two aqueous compartments,i.e., the outer compartment and the inner compartment. The innercompartment is called the matrix. The outer membrane forms thecontinuous limiting boundary of the organelle. The inner membrane formsa number of infoldings called the cristae (sing.: crista) towards the matrix(Figure 8.7). The cristae increase the surface area. The two membraneshave their own specific enzymes associated with the mitochondrialfunction. Mitochondria are the sites of aerobic respiration. They producecellular energy in the form of ATP, hence they are called ‘power houses’ ofthe cell. The matrix also possesses single circular DNA molecule, a fewRNA molecules, ribosomes (70S) and the components required for thesynthesis of proteins. The mitochondria divide by fission.8.5.5 PlastidsPlastids are found in all plant cells and in euglenoides. These are easilyobserved under the microscope as they are large. They bear some specificpigments, thus imparting specific colours to the plants. Based on thetype of pigments plastids can be classified into chloroplasts,chromoplasts and leucoplasts.The chloroplasts contain chlorophyll and carotenoid pigments whichare responsible for trapping light energy essential for photosynthesis. Inthe chromoplasts fat soluble carotenoid pigments like carotene,xanthophylls and others are present. This gives the part of the plant ayellow, orange or red colour. The leucoplasts are the colourless plastidsof varied shapes and sizes with stored nutrients: Amyloplasts storecarbohydrates (starch), e.g., potato; elaioplasts store oils and fats whereasthe aleuroplasts store proteins.Outermembrane InnermembraneMatrix CristaFigure 8.7 Structure of mitochondrion (Longitudinal section)Inter-membranespace136 BIOLOGYMajority of the chloroplasts of thegreen plants are found in the mesophyllcells of the leaves. These are lens-shaped,oval, spherical, discoid or even ribbon-likeorganelles having variable length(5-10mm) and width (2-4mm). Theirnumber varies from 1 per cell of theChlamydomonas, a green alga to 20-40per cell in the mesophyll.Like mitochondria, the chloroplastsare also double membrane bound. Of thetwo, the inner chloroplast membrane isrelatively less permeable. The space

limited by the inner membrane of the chloroplast is called the stroma. Anumber of organised flattened membranous sacs called the thylakoids,are present in the stroma (Figure 8.8). Thylakoids are arranged in stackslike the piles of coins called grana (singular: granum) or the intergranalthylakoids. In addition, there are flat membranous tubules called thestroma lamellae connecting the thylakoids of the different grana. Themembrane of the thylakoids enclose a space called a lumen. The stromaof the chloroplast contains enzymes required for the synthesis ofcarbohydrates and proteins. It also contains small, double-strandedcircular DNA molecules and ribosomes. Chlorophyll pigments arepresent in the thylakoids. The ribosomes of the chloroplasts are smaller(70S) than the cytoplasmic ribosomes (80S).8.5.6 RibosomesRibosomes are the granular structures first observed under the electronmicroscope as dense particles by George Palade (1953). They arecomposed of ribonucleic acid (RNA) and proteins and are not surroundedby any membrane.The eukaryotic ribosomes are 80S while the prokaryotic ribosomesare 70S. Here ‘S’ stands for the sedimentation coefficient; it indirectly is ameasure of density and size. Both 70S and 80S ribosomes are composedof two subunits.8.5.7 CytoskeletonAn elaborate network of filamentous proteinaceous structures present inthe cytoplasm is collectively referred to as the cytoskeleton. Thecytoskeleton in a cell are involved in many functions such as mechanicalsupport, motility, maintenance of the shape of the cell.Outer membraneInner membraneGranumThylakoidStromalamellaStromaFigure 8.8 Sectional view of chloroplastCELL: THE UNIT OF LIFE 1378.5.8 Cilia and FlagellaCilia (sing.: cilium) and flagella (sing.: flagellum) are hair-like outgrowthsof the cell membrane. Cilia are small structures which work like oars,causing the movement of either the cell or the surrounding fluid. Flagellaare comparatively longer and responsible for cell movement. Theprokaryotic bacteria also possess flagella but these are structurallydifferent from that of the eukaryotic flagella.The electron microscopic study of a cilium or the flagellum show thatthey are covered with plasma membrane. Their core called the axoneme,possesses a number of microtubules running parallel to the long axis.The axoneme usually has nine pairs of doublets of radially arrangedperipheral microtubules, and a pair of centrally located microtubules.Such an arrangement of axonemal microtubules is referred to as the 9+2array (Figure 8.9). The central tubules are connected by bridges and isalso enclosed by a central sheath, which is connected to one of the tubulesof each peripheral doublets by a radial spoke. Thus, there are nine radial

spokes. The peripheral doublets are also interconnected by linkers. Boththe cilium and flagellum emerge from centriole-like structure called thebasal bodies.8.5.9 Centrosome and CentriolesCentrosome is an organelle usually containing two cylindrical structurescalled centrioles. They are surrounded by amorphous pericentriolarmaterials. Both the centrioles in a centrosome lie perpendicular to eachother in which each has an organisation like the cartwheel. They arePlasmamembranePeripheralmicrotubules(doublets)InterdoubletbridgeCentralRadial microtublespokeCentralsheathFigure 8.9 Section of cilia/flagella showing different parts : (a) Electron micrograph(b) Diagrammatic representation of internal structure(a) (b)138 BIOLOGYmade up of nine evenly spaced peripheral fibrils of tubulin. Each of theperipheral fibril is a triplet.The adjacent triplets are also linked. Thecentral part of the centriole is also proteinaceous and called the hub,which is connected with tubules of the peripheral triplets by radial spokesmade of protein. The centrioles form the basal body of cilia or flagella,and spindle fibres that give rise to spindle apparatus during cell divisionin animal cells.8.5.10 NucleusNucleus as a cell organelle was first described by Robert Brown as earlyas 1831. Later the material of the nucleus stained by the basic dyes wasgiven the name chromatin by Flemming.The interphase nucleus (nucleus of acell when it is not dividing) has highlyextended and elaborate nucleoproteinfibres called chromatin, nuclear matrix andone or more spherical bodies callednucleoli (sing.: nucleolus) (Figure 8.10).Electron microscopy has revealed that thenuclear envelope, which consists of twoparallel membranes with a space between(10 to 50 nm) called the perinuclear space,forms a barrier between the materialspresent inside the nucleus and that of thecytoplasm. The outer membrane usuallyremains continuous with the endoplasmicreticulum and also bears ribosomes on it.At a number of places the nuclear envelope is interrupted by minute

pores, which are formed by the fusion of its two membranes. These nuclearpores are the passages through which movement of RNA and proteinmolecules takes place in both directions between the nucleus and thecytoplasm. Normally, there is only one nucleus per cell, variations in thenumber of nuclei are also frequently observed. Can you recollect namesof organisms that have more than one nucleus per cell? Some maturecells even lack nucleus, e.g., erythrocytes of many mammals and sievetube cells of vascular plants. Would you consider these cells as ‘living’?The nuclear matrix or the nucleoplasm contains nucleolus andchromatin. The nucleoli are spherical structures present in thenucleoplasm. The content of nucleolus is continuous with the rest of thenucleoplasm as it is not a membrane bound structure. It is a site foractive ribosomal RNA synthesis. Larger and more numerous nucleoli arepresent in cells actively carrying out protein synthesis.Figure 8.10 Structure of nucleusNucleoplasmNucleolusNuclear poreNuclearmembraneCELL: THE UNIT OF LIFE 139KinetochoreFigure 8.11 Chromosome withkinetochoreCentromereCentromereSecondaryconstrictionSatelliteShort arm Short armLong armFigure 8.12 Types of chromosomes based on the position of centromereYou may recall that the interphase nucleus has a looseand indistinct network of nucleoprotein fibres calledchromatin. But during different stages of cell division, cellsshow structured chromosomes in place of the nucleus.Chromatin contains DNA and some basic proteins calledhistones, some non-histone proteins and also RNA. Asingle human cell has approximately two metre longthread of DNA distributed among its forty six (twenty threepairs) chromosomes. You will study the details of DNApackaging in the form of a chromosome in class XII.Every chromosome essentially has a primaryconstriction or the centromere on the sides of which discshaped structures called kinetochores are present(Figure 8.11). Based on the position of the centromere,the chromosomes can be classified into four types (Figure8.12). The metacentric chromosome has middlecentromere forming two equal arms of the chromosome.The sub-metacentric chromosome has centromere nearerto one end of the chromosome resulting into one shorter

arm and one longer arm. In case of acrocentricchromosome the centromere is situated close to its endforming one extremely short and one very long arm,whereas the telocentric chromosome has a terminalcentromere.140 BIOLOGYSometimes a few chromosomes have non-staining secondaryconstrictions at a constant location. This gives the appearance of a smallfragment called the satellite.8.5.11 MicrobodiesMany membrane bound minute vesicles called microbodies that containvarious enzymes, are present in both plant and animal cells.