According to this model, the cell membrane consists of a highly viscous fluid matrix of two layers of phospholipid molecules. These serve as relatively impermeable barrier to the passage of most water soluble molecules. Protein molecules occur in the

membrane, but not in continuous layer; Instead, these occur as separate particles asymmetrical arranged in a mosaic pattern.

Some of these are loosely bound at the polar surfaces of lipid layers, called peripheral or extrinsic proteins. Others penetrate deeply into the lipid layer called integral or intrinsic proteins. Some of the

Lipid bilayer

Boundary lipid

Polar end Non-polar end

integral proteins penetrate through the phospholipid layers and project on both the

Intrinsic protein

surface. These are called trans membrane or tunnel proteins (glycophorins). Singly or in

Lipid

Hydrophobic tail

Hydrophilic head                              Intrinsic protein

Extrinsic proteins

groups, they function as channels for passage of water ions and other solutes. The channels may have gate mechanism

Fig : Fluid-mosaic model of the plasma membrane. Proteins floating in a sea of lipid. Some proteins span the  lipid  bilayer, others are  exposed only to one surface or the other (Modified after De Robertis et al.; 1975).

for opening in response to specific condition. The carbohydrates occur only at the outer surface of the membrane. Their molecules are covalently linked to the polar heads of some lipid molecules (forming glycolipids) and most of the proteins exposed at outer surface (forming glycoproteins).

The sugar protions of glycolipids and glycoproteins are involved in recognition mechanisms :–

1. Sugar recognition sites of two neighbouring cells may bind each other causing cell to cell adhesion. This enables cells to orientate themselves and to form tissues.
2. Through glycoproteins, bacteria recognise each other. e.g., female bacteria are recognised by male bacteria.
3. These provide the basis of immune response and various control system, where glycoproteins act as antigens. Lipid and integral proteins are amphipathic in nature i.e., they have hydrophilic and hydrophobic groups with in the same molecules. The NMR (Nuclear magnetic resonance) and ESR (Electron spin resonance) studies showed that the membrane is dynamic. The lipid tails show flexibility. The molecule can rotate or show flip flop motion.

Difference between protein types

5. Membrane protein can be of following types with different functions
   1. Carrier molecules : These bind with the specific molecules into or out of the cell. This provides selective exchange of materials. The carrier protein molecules are called “permeases” e.g., Na⁺ – K⁺ pump, Na⁺– sugar transport.
   2. Receptor molecules : The glycoproteins on the cell surface act as receptors that recognize and bind with specific molecules.
   3. Enzyme molecules : The inner mitochondrial membrane carrier enzyme comprising the electron transport chain for cellular respiration.

(6)Cell membranes are fluid and dynamic due to

1. The constituent molecules can move freely in the membrane.
2. The cell membranes are constantly renewed during the cells life.
3. They can repair minor injuries.
4. They expand and contract during cell movement and during change in shape.
5. They allow interactions of cells such as recognition of self and fusion of cells.

7. Membrane permeability : According to permeability, membranes are classified as –
   1. Permeable membrane : They allow both solvent and solute molecules or ions through them. e.g.,

cellulose wall, lignified cell walls.

2. Impermeable membrane : They do not allow solute and solvent molecules. e.g., heavily cutinised or suberinised cell walls in plants.
3. Semi-permeable membrane : They allow solvent molecules only. e.g., membranes of colloidion, parchment paper and copper ferrocyanide membranes.
4. Differentially permeable membrane : All membranes found in plants allow some solutes to pass through them along with the solvent molecules. e.g., plasma membrane, tonoplast (vacuolar membrane) etc.

(8)Intercellular   communications/modification   of   plasma   membrane/following   structures   are derived from plasma membrane

1. Microvilli : They are fingers like evaginations of 0.1

m m diameter, engaged in absorption. e.g., intestinal

cells, hepatic cell, mesothelial cells. The surface having microvilli is called striated border or brush border.

2. Lomasomes : They are plasmalemma foldings found in fungal cells.
3. Mesosomes : It serves as site for cellular respiration in prokaryotes.
4. Tight junctions : Plasma membrane of two adjacent cells are fused at a series of points with a network of ridges or sealing strands. e.g., capillaries, brain cells collecting tubules etc.
5. Plasmodesmata : They are protoplasmic bridges amongst plant cells, which occur in area of cell wall pits. It was discovered and reported by Tangle and Strasburger respectively.
6. Desmosomes : concerned with cell adherence.

(9)Functions

1. They control the flow of material through them and provides passage for different substances.
2. It is differentially permeable, solute particles (1-15 Å) can pass through it.
3. It is not only provides mechenical strength but also acts as a protective layer.
4. Plasma membrane is responsible for the transportation of materials, molecules, ions etc.
5. It helps in osmoregulation.
6. Diffusion of gases take place through plasma membrane by simple and facilitated diffusion.
7. Transport of ions, small polar molecules through active (energy used) and passive transport (energy not used).
8. Gases like O₂ and CO₂ diffuse rapidly in solutions through membranes.
9. Ions and small polar molecules diffuse slowly through the membranes.
10. Some solute molecules or ions first bind with certain specific carrier or transport proteins called permeases.
11. Water as well as some solute molecules and ion pass through membranes pores; pores are always bordered by channel proteins.
12. When diffusion takes place through channel, called simple diffusion and through carrier proteins, called facilitated diffusion.

10. Membrane transport : It is passage of metabolites, by-products and biochemicals across biomembrane. Membrane transport occurs through four methods–passive, facilitated, active and bulk. Size of the particles passing through plasmalemma is generally 1 – 15 Å.

1. Passive transport : No energy spent. Passive transport occurs through diffusion and osmosis.

1. Diffusion : It is movement of particles from the region of their higher concentration or electrochemical potential to the region of their lower concentration or electrochemical potential. Electrochemical potential operates in case of charged particles like ions. Diffusion can be observed by opening a bottle of scent or ammonia in one

corner, placing a crystal of copper sulphate in a beaker of water or a crystal of diffusion does not require carrier molecules.

KMnO₄ on a piece of gelatin. Simple

Independent Diffusion : In a system having two or more diffusion substances, each individual substance will diffuse independent of others as per gradient of its own concentration, diffusion pressure or partial pressure form region of higher one to region of lower one.

Rate of diffusion is proportional to difference in concentration and inversely to distance between the two ends of the system, inversely to square root of relative density of substance and density of medium, directly to temperature and pressure.

2. Osmosis is diffusion of water across a semipermeable membrane that occurs under the influence of an osmotically active solution.
3. Mechanism of passive transport : Passive transport can continue to occur if the absorbed solute is immobilised. Cations have a tendency to passively pass from electropositive to electronegative side. While anions can pass from electronegative to electropositive side. There are two modes of passive transports.

Lipid matrix permeability : Lipid soluble substances pass through the cell membrane according to their solubility and concentration gradient, e.g., triethyl citrate, ethyl alcohol, methane.

Hydrophillic membrane channels : They are narrow channels formed in the membrane by tunnel proteins. The channels make the membrane semipermeable. Water passes inwardly or outwardly from a cell

through these channels according to osmotic gradients. CO₂ and O₂ also diffuse through these channels as per their

concentration gradients. Certain small ions and other small water soluble solutes may also do so.

4. Ultrafiltration is fine filtration that occurs under pressure as from blood capillaries, epithelia and endothelia. It is of two types : –

• Paracellular through leaky junctions or gaps in between cells.
• Transcellular through fenestrations in the cells. ‘Dialysis’ is removal of waste products and toxins from blood by means of diffusion between blood and an isotonic dialysing solution.

5. Facilitated transport or Facilitated diffusion : It is passage of substances along the concentration gradient without expenditure of energy that occurs with the help of special permeating substances called permeases. Permeases form pathways for movement of certain substances without involving any expenditure of energy. At times certain substances are transported alongwith the ones requiring active transport. The latter phenomenon called cotransport. Facilitated transport occurs in case of some sugars, amino acids and nucleotides.

2. Active transport : It occurs with the help of energy, usually against concentration gradient. For this, cell membranes possess carriers and gated channels.

1. Carrier particles or Proteins : They are integral protein particles which have affinity for specific solutes. A solute particles combines with a carrier to form carrier solute complex. The latter undergoes conformational change in such a way as to transport the solute to the inner side where it is released into cytoplasm.
2. Gated channels : The channels are opened by either change in electrical potential or specific substances,

e.g., Calcium channels.

Active transport systems are also called pumps, e.g., H ⁺ pump,

K ⁺ pump,

Cl ^- pump,

Na⁺ - K ⁺ pump. The

pumps operate with the help of ATP. K ⁺ - H ⁺ exchange pump occurs in guard cells. Na⁺ - K ⁺ exchange pump

operates across many animal membranes. For every ATP hydrolysed, three

Na⁺ ions are passed out while two

K ⁺ ions are pumped in. Sea Gulls and Penguins operate glands.

Na ⁺ - K ⁺

pump for excreting NaCl through their nasal

Active transport of one substance is often accompanied by permeation of other substances. The phenomenon is called secondary active transport. It is of two main types, cotransport (e.g., glucose and some amino acids

alongwith inward pushing of excess

Na⁺ passes inwardly).

Na⁺ ) and counter-transport ( Ca²⁺ and

H ⁺ movement outwardly as excess

3. Bulk transport : It is transport of large quantities of micromolecules, macromolecules and food particles through the membrane. It is accompanied by formation of transport or carrier vesicles. The latter are endocytotic and perform bulk transport inwardly. The phenomenon is called endocytosis. Endocytosis is of two types, pinocytosis and phagocytosis. Exocytic vesicle perform bulk transport outwardly. It is called exocytosis. Exocytosis performs secretion, excretion and ephagy.

1. Pinocytosis : (Lewis, 1931). It is bulk intake of fluid, ions and molecules through development of small

endocytotic vesicles of 100 – 200 nm in diameter. ATP,

Ca²⁺ , fibrillar protein clathrin and contractile protein actin

are required. Fluid-phase pinocytosis is also called cell drinking. It is generally nonselective. For ions and molecules the membrane has special receptor or adsorptive sites located in small pits. They perform adsorptive pinocytosis. After coming in contact with specific substance, the area of plasma membrane having adsorptive sites, invaginates and forms vesicle. The vesicle separates. It is called pinosome. Pinosome may burst in cytosol, come in contact with tonoplast and pass its contents into vacuole, form digestive vacuole with lysosome or deliver its contents to Golgi apparatus when it is called receptosome.

2. Phagocytosis : (Metchnikoff, 1883). It is cell eating or ingestion of large particles by living cells, e.g., white blood corpuscles (neutrophils, monocytes), Kupffer’s cells of liver, reticular cells of spleen, histiocytes of connective tissues, macrophages, Amoeba and some other protists, feeding cells of sponges and coelentrates. Plasma membrane has receptors. As soon as the food particle comes in contact with the receptor site, the edges of the latter evaginate, form a vesicle which pinches off as phagosome.

One or more lysosomes fuse with a phagosome, form digestive vacuole or food vacuole. Digestion occurs inside the vacuole. The digested substances diffuse out, while the residual vacuole passes out, comes in contact with plasma membrane for throwing out its contents through exocytosis or ephagy.

Important tips

• E. Grater and H. Grendel (1926) : Proposed leaflet model which states that plasma membrane is formed of bilayer sheet of phospholipids.
• Wolpers (1941) : Proposed lattice model which states lipids are distributed in a framework of proteins.
• Hilleir and Hoffman (1953) : Proposed micellar model. Plasma membrane is formed of micelles of lipid molecules.
• Sandwich model of Danielli and Davson (1935) is based on physical and chemical properties.
• Proteins of plasma membrane provide functional specificity, elasticity and mechanical support.
• The arrangement of phospholipid molecules in bilayer forms a water resistant barrier.
• Glycoproteins of plasma membrane determine antigen specificity of cell. These glycoproteins from major histocompatible complex (MHC) which are of specific type in every individual so act as finger print of the cell.
• Negative charge of the membrane is due to N – acetyl neuraminic acid (NANA)/sialic acid.
• Lehninger described the percentage of extrinsic and intrinsic protein.
• Harmone receptor proteins of plasma membrane of target cells act as signal transduction.
• Phospholipids show asymmetric distribution in plasma membrane lacithin and sphingomycelin mainly found in outer phospholipids layer while cephalin and phosphatidyl serine are mainly present in inner phosphalipid layer.
• Lomasomes : Infolds of plasma membrane found in fungi. These were reported by Moore and Mclean.
• Transosomes found in follicular cells of ovary of birds and have triple unit membrane. First reported by Press(1964).
• Lipid soluble substances pass through the plasma membrane move readily than the water soluble substances.
• Term biomembrane was coined by Singer and Nicolson.
• Nehar and Sakmann discovered ion-channels in plasma membrane and they were awarded Noble prize for it in 1971.
• Pinocytosis and phagocytosis do not take place in prokaryotic cells.
• Singer and Nicolson’s model differs from Robertson’s model in the arrangement of proteins.
• Plasma membrane contains ATPase enzymes.
• Plasma gel or ectoplasm are the synonyms of plasma membrane.
• The secondary structure of the integral protein buried in the lipid bilayer of a cell membrane is nature.

 Protoplasm.                                                                                                                                                           

1. Definition : Protoplasm is a complex, granular, elastic, viscous and colourless substance. It is selectively or differentially permeable. It is considered as “Polyphasic colloidal system”.

(2)Discoveries

1. J. Huxley defined it as “physical basis of life”.
2. Dujardin (1835) discovered it and called them “sarcode”.
3. Purkinje (1837) renamed it as “Protoplasm”.
4. Hugo Von Mohl (1844) gave the significance of it.
5. Max Schultz (1861) gave the protoplasmic theory for plants.
6. Fischer (1894) and Hardy (1899) showed its colloidal nature.
7. Altman (1893) suggested protoplasm as granular.

3. Composition : Chemically it is composed of

Trace elements – 5% ( Ca, P, Cl, S, K, Na, Mg, I, Fe, etc.)

Maximum water content in protoplasm is found in hydrophytes, i.e. 95% where as minimum in seeds, spores (dormant organs) i.e. 10 – 15%. In animals water is less (about 65%) and proteins are more (about 15%).

4. Physical properties of protoplasm : Cyclosis movement are shown by protoplasm. These are of two types.
   1. Rotation : In one direction, either clockwise or anticlockwise e.g., Hydrilla, Vallisneria. Found only in eukaryotes.
   2. Circulation : Multidirectional movements around vacuole e.g. Tradescantia.

1. It shows stimulation or irritability.
2. Protoplasm is polyphasic. Colloidal substance or true solution because true solution act as dispersion medium and different colloidal particles constitute dispersed phase.
3. It shows increased surface area and adsorption.
4. It shows sol – gel transformation.
5. It is highly viscous.
6. It coagulates at 60^o C or above or if treated with concentrated acids or bases.
7. It shows Brownian movements.
8. It’s specific gravity is slightly more than 1.
9. It’s pH is on acidic side, but different vital activities occur at neutral pH which is considered as 7, injury decreases the pH of the cell (i.e. 5.2 – 5.5) and if it remains for a long time, the cell dies.
10. Scattering and dispersion of light is shown by protoplasm i.e. Tyndall effect.

  Cytoplasm.                                                                                                                                                           

The substance occur around the nucleus and inside the plasma membrane containing various organelles and inclusions is called cytoplasm.

1. The cytoplasm is a semisolid, jelly – like material. It consists of an aqueous, structureless ground substance called cytoplasmic matrix or hyaloplasm or cytosol.
2. It forms about half of the cell’s volume and about 90% of it is water.
3. It contains ions, biomolecules, such as sugar, amino acid, nucleotide, tRNA, enzyme, vitamins, etc.
4. The cytosol also contains storage products such as glycogen/starch, fats and proteins in colloidal state.
5. It also forms crystallo – colloidal system.
6. Cytomatrix is differentiated into ectoplasm or plasmagel and endoplasm or plasmasol.
7. Cytomatrix is three dimensional structure appear like a network of fine threads and these threads are called microfilaments (now called actin filaments or microtrabecular lattice) and it is believed to be a part of cytoskeleton. It also contains microtubules and inter mediate cytoplasmic filaments.
8. Hyaloplasm contains metabolically inactive products or cell inclusions called deutoplast or metaplasts.
9. Cytoplasmic organelles are plastid, lysosome, sphaerosome, peroxisome, glyoxysomes, mitochondria, ribosome, centrosome, flagellum or cilia etc.
10. The movement of cytoplasm is termed as cyclosis (absent in plant cells).

 Mitochondria.                                                                                                                                                       

1. Definition : (Gk – mito = thread ; chondrion = granule) Mitochondria are semi autonomous having hollow sac like structures present in all eukaryotes except mature RBCs of mammals and sieve tubes of phloem. These are absent in all prokaryotes like bacteria and cyanobacteria. Mitochondria are also called chondriosome, chondrioplast, plasmosomes, plastosomes and plastochondriane.

(2)Discoveries

1. These were first observed in striated muscles (Voluntary) of insects as granules by Kolliker (1850), he called them “sarcosomes”.
2. Flemming (1882) called them “fila” for thread like structure.
3. Altman (1890) called them “bioplast”.
4. C. Benda (1897) gave the term mitochondria.
5. F. Meves (1904) observed mitochondria in plant (Nymphaea).
6. Michaelis (1898) demonstrated that mitochondria play a significant role in respiration.
7. Bensley and Hoerr (1934) isolated mitochondria from liver cells.
8. Seekevitz called them “Power house of the cell”.
9. Nass and Afzelius (1965) observed first DNA in mitochondria.

3. Number of mitochondria : Presence of mitochondria depends upon the metabolic activity of the cell. Higher is the metabolic activity, higher is the number

e.g., in germinating seeds.

1. Minimum number of mitochondria is one in Microasterias,       Trypanosoma,  Chlorella, Chlamydomonas (green alga) and Micromonas. Maximum numbers are found (up to 50,000) in giant Amoeba called Chaos – Chaos. These are 25 in human sperm, 300 - 400 in kidney cells and 1000 –

1600 in liver cells.

Outer membrane

Inner membrane Cristae

Matrix

Intermembranous space

Crista

DNA

2. Mitochondria of a cell are collectively called

Inclusions

Intercristaeal space Ribosomes

A

Inner membrane F1

Particles or

chondriome.

4. Size of mitochondria : Average size is 0.5–1.00 mm and length up to 1 – 10 m m.
   1. Smallest sized mitochondria in yeast cells

(1 m m³ ).

F₁ Particles or Oxysomes

Tubuli

Oxysomes

B

Outer membrane

2. Largest sized are found in oocytes of Rana pipiens and are 20 – 40 m m.

Intermembranous space

Matrix            Inclusions

Ribosomes

3. A dye for staining mitochondria is Janus B – green.

(5)Ultrastructure     of      mitochondria      :

C

F₁ Particles or Oxysomes

Matrix Outer

chamber DNA

Mitochondrion is bounded by two unit membranes

separated by perimitochondrial space (60 – 80 Å).             D

The outer membrane is specially permeable because of presence of integral proteins called porins. The

Intratubuli space

Intermembranous

space         Inner Inner membrane chember Outer membrane

inner membrane is selective permeable. The inner membrane is folded or convoluted to form mitochondrial crests. In animals these are called cristae and in plants these folding are called tubuli or microvili.

The matrix facing face is called ‘M’ face and face towards perimitochondrial space is called ‘C’ face. The ‘M’ face have some small stalked particles called oxysomes or F₁ particle or elementory particle or Fernandez – Moran Particles. Each particle is made up of base, stalk and head and is about 10nm in length. Number of oxysomes varies to 10⁴ to 10⁵ per mitochondrion and chemically they are made of

Fig : Three dimentional structure of mitochondrion.

A. From an animal cell. B. From plant cell, C. T.S. mitechondrion, D. One tubule

Perimitochondrial space

Fig : Molecular organization of inner membrane of mitochondria

phospholipid core and protein cortex. Oxysomes have ATPase enzyme molecule (Packer, 1967) and therefore, responsible for ATP synthesis. These elementary particles are also called F₀ – F₁ particles.

In the matrix 2–6 copies of naked, double stranded DNA (circular) and ribosome of 70 S type are present. It is rich in G-C ratio. Basic histone proteins are absent in mitochondrial DNA. The synthesis of ATP in mitochondria is called oxidative phosphorylation, which is O₂ dependent and light independent. Cristae control dark respiration. F₀ particles synthesize all the enzymes required to operate Kreb’s cycle. Inner membrane contains cytochrome.

6. Semi-autonomous nature of mitochondrion : Mitochondria contain all requirements of protein synthesis :
   1. 70 S ribosomes.
   2. DNA molecules to form mRNA and also replicate.
   3. ATP molecules to provide energy.

The mitochondria can form some of the required proteins but for most of proteins, these are dependent upon nuclear DNA and cytoplasmic ribosomes, so the mitochondria are called semi-autonomous organelles.

7. Two states of mitochondria : When ATP synthesis is low or the respiratory chain of mitochondrion is inhibited, it is called inactive or orthodox state, and has large amount of matrix and only a few cristae. But when mitochondria are active or condensed state, and have small amount of matrix and highly developed cristae. This shows that the development of mitochondria depends upon the physiological activity of the cell.
8. Chemical composition : Cohn gave the chemical composition of mitochondrion: Proteins = 65 – 70%

Lipids = 25 – 30% (90% phospholipids and 10% cholesterol, Vit. E., etc) 2 – 5% RNA Some amount of DNA

The mitochondrial matrix has many catabolic enzymes like cytochrome oxidase and reductases, fatty acid oxidase, transaminase, etc.

(9)Enzymes of Mitochondria

1. Outer membrane : Monoamine oxidase, glycerophosphatase, acyltransferase, phospholipase A.
2. Inner membrane : Cytochrome b,c₁,c,a, (cyt.b, cyt.c₁, cyt.c, cyt.a, cyt.a₃) NADH, dehydrogenase, succinate dehydrogenase, ubiquinone, flavoprotein, ATPase.
3. Perimitochondrial space : Adenylate kinase, nucleoside diphosphokinase.
4. Inner matrix : Pyruvate dehydrogenase, citrate synthase, aconitase, isocitrate dehydrogenase, fumarase,

a - Ketoglutarate dehydrogenase, malate dehydrogenase.

10. Origin : Mitochondria are self-duplicating organelles due to presence of DNA molecules so new mitochondria are always formed by growth and division of pre-existing mitochondria by binary fission.

Difference between outer and inner membrane of mitrochondria

11. Functions of mitochondria

1. Mitochondria are called power house or storage batteries or ATP mills as these are sites of ATP formation.
2. Intermediate products of cell respiration are used in the formation of steroids, cytochromes, chlorophyll, etc.
3. These are also seat of some amino acid biosynthesis.
4. Mitochondria also regulate the calcium ion concentration inside the cell.
5. Site of Krebs cycle and electron transport system.
6. Site of thermiogenesis.
7. Yolk nucleus (a mitochondrial cloud and golgi bodies) controls vitellogenesis.
8. Mitochondria of spermatid form nebenkern (middle piece) of sperm during spermiogenesis.
9. It is capable of producing its own DNA.
10. Mitochondria release energy during respiration.
11. Mitochondria contain electron transport system.

Important Tips

• Petite character in yeast and cytoplasmic male sterility in maize are examples of mitochondrial inheritance.
• Mitochondria are believed to be bacterial endosymbionts.
• Mitochondria show a large degree of autonomy or independence in their functioning.
• Mitochondria as a place of cellular respiration were first observed by Hogeboom. Enzymes of Kreb’s cycle or TCA cycle or citric acid cycle are present in matrix except succinic dehydrogenase which is found attached to inner mitochondrial membrane.
• With the help of phase contrast microscope mitochondria has been studied well.
• Mitochondria can be separated by centrifugation.
• Mitochondria are called as “cell inside cell” by Schiff (1982).
• Life of mitochondria is not more than 5 days.
• Mitochondria are yellowish due to riboflavin.
• 70% of total enzymes of a cell are found in mitochondria.
• Mitochondrial genome has 200 kilobase pairs.
• Mitochondria has the similarity , with bacteria as both have 70 S ribosome, circular DNA and RNA.
• Mitochondria are rich in manganese.
• It has its own electron transport system.
• Mitochondria and chloroplasts have many resemblances.
• According to endosymbiotic origin of mitochondria by Kirns Altman, mitochondria were intially a free living, aerobic bacteria which during to the process of evolution entered an anaerobic cell and become established as mitochondria. This theory is supported by many similarities which exist between bacteria and mitochondria.
• Lehninger discovered oxysomes.
• Percentage of mitochondrial DNA in cells is 1% of the total cellular DNA.
• Parson discovered stalkless and hollow spherical particles present on outer surface of outer mitochondrial membrane.
• When mitochondria treated with detergents like digitonin or lubral, their outer unit membrane is removed and remaining part is called Mitoplast
• The F₁ particle is made up of five types of subunits namely a, b , g , d and e. of these a is heaviest and e is lightest.
• In prokaryotic cell, plasma membrane infolding makes a structure mesosome. Which is analogous structure of mitochondria of eukaryotic cell (both part in respiration).

 Plastids.                                                                                                                                                                 

1. Definition : Plastids are semiautonomous organelles having DNA, RNA, Ribosomes and double membrane envelope which store or synthesize various types of organic compounds as ATP and NADPH + H⁺ etc. These are largest cell organelles in plant cell.

(2)History

1. Haeckel (1865) discovered plastid, but the term was first time used by Schimper (1883).
2. A well organised system of grana and stroma in plastid of normal barley plant was reported by de Von Wettstein.
3. Park and Biggins (1964) gave the concept of quantasomes.
4. The term chlorophyll was given by Pelletier and Caventou, and structural details were given by Willstatter and Stall.
5. The term thylakoid was given by Menke (1962).
6. Fine structure was given by Mayer.
3. Types of plastids : According to Schimper, Plastids are of 3 types: Leucoplasts, Chromoplasts and Chloroplasts.

Leucoplasts : They are colourless plastids which generally occur near the nucleus in nongreen cells and possess internal lamellae. Grana and photosynthetic pigments are absent. They mainly store food materials and occur in the cells not exposed to sunlight e.g., seeds underground stems, roots, tubers, rhizomes etc. These are of three types.

1. Amyloplast : Synthesize and store starch grains. e.g., potato tubers, wheat and rice grains.
2. Elaioplast (Lipidoplast, Oleoplast) : They store lipids and oils e.g. castor endosperm, tube rose, etc.
3. Aleuroplast (Proteinoplast) : Store proteins e.g., aleurone cells of maize grains.

Chromoplasts : Coloured plastids other than green are kown as chromoplasts. These are present in petals and fruits, imparting different colours (red, yellow, orange etc) for attracting insects and animals. These also carry on photosynthesis.

These may arise from the chloroplasts due to replacement of chlorophyll by other pigments e.g. tomato and chillies or from leucoplasts by the development of pigments.

All colours (except green) are produced by flavins, flavenoids and cyanin. Cyanin pigment is of two types one is anthocyanin (blue) and another is erythrocyanin (red). Anthocyanin express different colours on different pH value. These are variously coloured e.g. in flowers. They give colour to petals and help in pollination. They are water soluble. They are found in cell sap.

Green tomatoes and chillies turn red on ripening because of replacement of chlorophyll molecule in chloroplasts by the red pigment lycopene in tomato and capsanthin in chillies. Thus, chloroplasts are changed into chromatophores.

Chloroplast : Discovered by Sachs and named by Schimper. They are greenish plastids which possess photosynthetic pigments.

1. Number : It is variable. Number of chloroplast is 1 in Spirogyra indica, 2 in Zygnema, 16 in S.rectospora, up to 100 in mesophyll cells. The minimum number of one chloroplast per cell is found in Ulothrix and species of Chlamydomonas.
2. Shape : They have various shapes

3. Size : It ranges from 3 – 10 m m (average 5 m m) in diameter. The discoid chloroplast of higher plants are

4 – 10 m m in length and 2– 4 m m in breadth. Chloroplast of spirogyra may reach a length of 1 mm. Sciophytes

(Shade plant) have larger chloroplast.

(iv)Chemical composition  :

1. Proteins 50 – 60%,
2. Lipids 25 – 30% ,
3. Chlorophyll – 5- 10 %,
4. Carotenoids (carotenes and xanthophylls) 1 –2%, (e) DNA – 0.5%, RNA 2 – 3%,

6. Vitamins K and E,
7. Quinines, Mg, Fe, Co, Mn, P, etc. in traces.

5. Ultrastructure : It is double membrane structure. Both membranes are smooth. The inner membrane is less permeable than outer but rich in proteins especially carrier proteins. Each membrane is 90 – 100 Å thick. The inter-membrane space is called the periplastidial space. Inner to membranes, matrix is present, which is divided into two parts.

1. Grana : Inner plastidial membrane of the chloroplast is invaginated to form a series of parallel membranous sheets, called lamellae, which form a

number of oval – shaped closed sacs, called thylakoids. Thylakoids are structural and functional elements of chloroplasts. These thylakoids contain all the requirements of light reactions e.g., pigments like chlorophyll, carotenoids, plastoquinone, plastocyanin, etc. that are involved in photosynthesis. Each thylakoid

has an intrathylakoid space, called loculus (size 10-30Å)

Outer membrane

Inner membrane

Granum in L.S.

Frets or Lamellae

Granum

Stroma

Thylakoid

bounded by a unit membrane. Along the inner side of

Fig : A chloroplast in section (diagrammatic)

thylakoid membrane, there are number of small rounded para-crystalline bodies, called quantasomes (a

quantasome is the photosynthetic unit) which can trap a mole of quantum of light and can bring about photosynthetic act. Each quantasome contains about 230 chlorophyll molecules and 50 carotenoid molecules.

In eukaryotic plant cells, a number of thylakoids are superimposed like a pile of coins to form a granum. The number of thylakoids in a granum ranges from 10-100 (average number is 20-50). The number of grana per chloroplast also v