Chapter 2 Cell- The Unit of Life Part 2 by Teaching Care online coaching classes

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 :–

• Sugar recognition sites of two neighbouring cells may bind each other causing cell to cell This enables cells to orientate themselves and to form tissues.
• Through glycoproteins, bacteria recognise each other. g., female bacteria are recognised by male bacteria.
• These provide the basis of immune response and various control system, where glycoproteins act as 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

 

Extrinsic Protein	Intrinsic Protein
These are associated with surface only.	These lie throughout phospholipid matrix and project on both surfaces, also called transmembrane or tunnel protein.
They form about 30% of the total membrane protein.	They form about 70% of total membrane proteins.
Example – Spectrin in red blood cells & ATPase in mitochondria.	Example – Rhodopsin in retinal rod cells.

 

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

(6)  Cell membranes are fluid and dynamic due to

• The constituent molecules can move freely in the
• The cell membranes are constantly renewed during the cells
• They can repair minor
• They expand and contract during cell movement and during change in
• They allow interactions of cells such as recognition of self and fusion of cells.

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

cellulose wall, lignified cell walls.

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

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

 

• Microvilli : They are fingers like evaginations of 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.

• Lomasomes : They are plasmalemma foldings found in fungal
• Mesosomes : It serves as site for cellular respiration in
• Tight junctions : Plasma membrane of two adjacent cells are fused at a series of points with a network of ridges or sealing e.g., capillaries, brain cells collecting tubules etc.
• 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
• Desmosomes : concerned with cell

(9)  Functions

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

• 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.
• Passive transport : No energy Passive transport occurs through diffusion and osmosis.
• 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 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.

• Osmosis is diffusion of water across a semipermeable membrane that occurs under the influence of an osmotically active
• Mechanism of passive transport : Passive transport can continue to occur if the absorbed solute is 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.

• Ultrafiltration is fine filtration that occurs under pressure as from blood capillaries, epithelia and It is of two types : –
• Paracellular through leaky junctions or gaps in between
• 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
• 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 Facilitated transport occurs in case of some sugars, amino acids and nucleotides.
• Active transport : It occurs with the help of energy, usually against concentration For this, cell membranes possess carriers and gated channels.
• Carrier particles or Proteins : They are integral protein particles which have affinity for specific 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.
• 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

 

• Bulk transport : It is transport of large quantities of micromolecules, macromolecules and food particles through the 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.

 

• 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.

• 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 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

• 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
• Hilleir and Hoffman (1953) : Proposed micellar Plasma membrane is formed of micelles of lipid molecules.
• Sandwich model of Danielli and Davson (1935) is based on physical and chemical
• Proteins of plasma membrane provide functional specificity, elasticity and mechanical
• The arrangement of phospholipid molecules in bilayer forms a water resistant
• Glycoproteins of plasma membrane determine antigen specificity of 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
• Lehninger described the percentage of extrinsic and intrinsic
• Harmone receptor proteins of plasma membrane of target cells act as signal
• 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
• Lomasomes : Infolds of plasma membrane found in These were reported by Moore and Mclean.
• Transosomes found in follicular cells of ovary of birds and have triple unit First reported by Press(1964).
• Lipid soluble substances pass through the plasma membrane move readily than the water soluble
• Term biomembrane was coined by Singer and
• Nehar and Sakmann discovered ion-channels in plasma membrane and they were awarded Noble prize for it in
• Pinocytosis and phagocytosis do not take place in prokaryotic
• Singer and Nicolson’s model differs from Robertson’s model in the arrangement of
• Plasma membrane contains ATPase
• Plasma gel or ectoplasm are the synonyms of plasma
• The secondary structure of the integral protein buried in the lipid bilayer of a cell membrane is

 

 

 Protoplasm.

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

(2)  Discoveries

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

• Composition : Chemically it is composed of

 

Water	75 – 85%	Carbon	20%
Proteins	10 – 25%	Oxygen	62%
Lipids	2 – 3%	Hydrogen	10%
Inorganic Materials	1%	Nitrogen	3%

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%).

• Physical properties of protoplasm : Cyclosis movement are shown by These are of two types.
  • Rotation : In one direction, either clockwise or anticlockwise g., Hydrilla, Vallisneria. Found only in eukaryotes.
  • Circulation : Multidirectional movements around vacuole g. Tradescantia.
• It shows stimulation or
• Protoplasm is Colloidal substance or true solution because true solution act as dispersion medium and different colloidal particles constitute dispersed phase.
• It shows increased surface area and
• It shows sol – gel
• It is highly
• It coagulates at 60^o C or above or if treated with concentrated acids or
• It shows Brownian movements.
• It’s specific gravity is slightly more than
• 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 (e. 5.2 – 5.5) and if it remains for a long time, the cell dies.
• Scattering and dispersion of light is shown by protoplasm e. Tyndall effect.

 

  Cytoplasm.

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

• The cytoplasm is a semisolid, jelly – like It consists of an aqueous, structureless ground substance called cytoplasmic matrix or hyaloplasm or cytosol.
• It forms about half of the cell’s volume and about 90% of it is
• It contains ions, biomolecules, such as sugar, amino acid, nucleotide, tRNA, enzyme, vitamins,
• The cytosol also contains storage products such as glycogen/starch, fats and proteins in colloidal
• It also forms crystallo – colloidal
• Cytomatrix is differentiated into ectoplasm or plasmagel and endoplasm or
• 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
• Hyaloplasm contains metabolically inactive products or cell inclusions called deutoplast or
• Cytoplasmic organelles are plastid, lysosome, sphaerosome, peroxisome, glyoxysomes, mitochondria, ribosome, centrosome, flagellum or cilia
• The movement of cytoplasm is termed as cyclosis (absent in plant cells).

 Mitocondria.

• 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 These are absent in all prokaryotes like bacteria and cyanobacteria. Mitochondria are also called chondriosome, chondrioplast, plasmosomes, plastosomes and plastochondriane.

(2)  Discoveries

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

• 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

• Mitochondria of a cell are collectively called

Inclusions

Intercristaeal space Ribosomes

A

Inner membrane F1

Particles or

 

chondriome.

• Size of mitochondria : Average size is 5–1.00 mm and length up to 1 – 10 m m.
  • Smallest sized mitochondria in yeast cells

(1 m m³ ).

F₁ Particles or Oxysomes

Tubuli

Oxysomes

 

B

Outer membrane

 

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

Intermembranous space

Matrix            Inclusions

 

Ribosomes

 

• A dye for staining mitochondria is Janus B –

(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.

1. 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.

• Semi-autonomous nature of mitochondrion : Mitochondria contain all requirements of protein synthesis :
  • 70 S
  • DNA molecules to form mRNA and also
  • ATP molecules to provide

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.

• 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 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.
• 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

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

a – Ketoglutarate dehydrogenase, malate dehydrogenase.

• 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

Difference between outer and inner membrane of mitrochondria

 

Outer membrane	Inner membrane
It is smooth having less area.	It is infolded to form cristae hence large surface area.
It is freely permeable.	Semipermeable, impermeable to coenzyme A and NAD.
It consist 50% lipid and 50% protein.	It consist 80% protein and 20% lipid.
Sialic acid is more (4 – 5 time).	Sialic acid is less.
Near about 14% enzymes are present.	Near about 60 enzymes are present.

 

 

• Functions of mitochondria
• Mitochondria are called power house or storage batteries or ATP mills as these are sites of ATP
• Intermediate products of cell respiration are used in the formation of steroids, cytochromes, chlorophyll,
• These are also seat of some amino acid
• Mitochondria also regulate the calcium ion concentration inside the
• Site of Krebs cycle and electron transport
• Site of
• Yolk nucleus (a mitochondrial cloud and golgi bodies) controls vitellogenesis.
• Mitochondria of spermatid form nebenkern (middle piece) of sperm during
• It is capable of producing its own
• Mitochondria release energy during
• Mitochondria contain electron transport

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
• 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
• With the help of phase contrast microscope mitochondria has been studied
• Mitochondria can be separated by
• Mitochondria are called as “cell inside cell” by Schiff (1982).
• Life of mitochondria is not more than 5
• Mitochondria are yellowish due to
• 70% of total enzymes of a cell are found in
• Mitochondrial genome has 200 kilobase
• Mitochondria has the similarity , with bacteria as both have 70 S ribosome, circular DNA and
• Mitochondria are rich in
• It has its own electron transport
• Mitochondria and chloroplasts have many
• 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 This theory is supported by many similarities which exist between bacteria and mitochondria.
• Lehninger discovered
• Percentage of mitochondrial DNA in cells is 1% of the total cellular
• Parson discovered stalkless and hollow spherical particles present on outer surface of outer mitochondrial
• 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
• In prokaryotic cell, plasma membrane infolding makes a structure mesosome. Which is analogous structure of mitochondria of eukaryotic cell (both part in respiration).

 

 Plastids.

• 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⁺ These are largest cell organelles in plant cell.

(2)  History

• Haeckel (1865) discovered plastid, but the term was first time used by Schimper (1883).
• A well organised system of grana and stroma in plastid of normal barley plant was reported by de Von
• Park and Biggins (1964) gave the concept of quantasomes.
• The term chlorophyll was given by Pelletier and Caventou, and structural details were given by Willstatter and Stal
• The term thylakoid was given by Menke (1962).
• Fine structure was given by

• Types of plastids : According to Schimper, Plastids are of 3 types: Leucoplasts, Chromoplasts and

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.

• Amyloplast : Synthesize and store starch grains. g., potato tubers, wheat and rice grains.
• Elaioplast (Lipidoplast, Oleoplast) : They store lipids and oils g. castor endosperm, tube rose, etc.
• Aleuroplast (Proteinoplast) : Store proteins 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.

 

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

 

Shape	Example
Cup shaped	Chlamydomonas sp.
Stellate shaped	Zygnema.
Collar or girdle shaped	Ulothrix
Spiral or ribbon shaped	Spirogyra
Reticulate	Oedogonium
Discoid	Voucheria

• 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  :

• Proteins 50 – 60%,
• Lipids 25 – 30% ,
• Chlorophyll – 5- 10 %,
• Carotenoids (carotenes and xanthophylls) 1 –2%, (e) DNA – 5%, RNA 2 – 3%,
• Vitamins K and E,
• Quinines, Mg, Fe, Co, Mn, P, in traces.
• Ultrastructure : It is double membrane 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.
• 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 (

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 varies widely e.g., one granum per chloroplast in Euglena while there are 40-60 grana per chloroplast in spinach. The size of each granum varies from 0.2 – 0.6 m m in diameter. But in blue-green algae, the

thylakoids are not organised to form granum.

Adjacent grana are interconnected by branched tubules, called stromal lamellae or Fret-channel or Fret membrane’s.

• Stroma : It is transparent, proteinaceous and watery substance. Dark reaction of photosynthesis occurs in

 

this portion. Stroma is almost filled with “Rubisco” (about 15% of total enzyme, protein) enzyme

CO₂ is accepted

 

by this enzyme. CO₂ assimilation results in carbohydrate formation. It has 20 – 60 copies of naked circular double stranded DNA. Each DNA copy is 40 m in length, which can code for 125 amino acids. All plastids of a cell called as

“Plastidome” (Dangeared 1920) in stroma. Amount of DNA per chloroplast is 10^–15 g. Chloroplast genome has 145 kilobase pairs. It shows semiautonomous nature and ribosomes are of 70 S type.

• Pigments of chloroplast : Willsttater and Stall observed the following pigments:

 

• Chlorophyll a : C55  H72 O5 N 4 Mg

(with methyl group)

 

• Chlorophyll b : C55 H70 O6 N 4 Mg (with aldehyde group)
• Chlorophyll c : C35 H 32 O5 N 4 Mg

 

(d)  Chlorophyll d :

C54  H70 O6 N 4 Mg

 

• Carotenes, Xanthophylls :

Difference between Chl. a and Chl. b

 

Chl. a	Chl. b
Absorption peak at 430, 662.	It is 453, 642.
Bluish green in colour.	Yellowish green.
Soluble in petroleum, ether.	Soluble in methyl alcohol.
Functional group at C3 position is CH3	Functional group attached to pyrrol ring is CHO.
Present in all green plants excepts autotrophic bacteria.	Present in all green plants except blue green, brown and red algae.
In chloroplast it is 75%.	It is 25%
In reflected light Chl. a shows blood red colour while in transmitted light, it shows blue green colour.	In reflected light it show dull brown colour while in transmitted light, it shows yellowish green colour.

 

• Chlorophylls and their presence : Term by Cavantou (1818). It’s molecule has tetrapyrollic or porphyrin head ( 15 Å ´15 Å ) and phytol tail (20 Å long). Mg⁺⁺ is present in the centre of porphyrin head. If chlorophyll is burnt only Mg is
• Chlorophyll b : It is found in members of
• Chlorophyll c : It is found in members of phaeophyceae,
• Chlorophyll d : It is found in members of
• Chlorophyll e : It is found in members of
• Phycoerythrin and phycocyanin (phycobilins) are the red and blue green pigments in rhodophyceae and cyanophyceae
• Fucoxanthin (brown pigment) in

 

(g)   Bacteriochlorophyll

(C55 H74 O6 N 4 Mg)

or chlorobium chlorophyll present in photosynthetic bacteria.

 

These pigment are red in acidic and blue in alkaline medium.

• Carotenoids : These are hydrocarbons, soluble in organic These are of 2 types:

 

• Carotenes : C40 H56 derivatives of vitamin Carrot coloured a, b ,g

carotene, lycopene, etc.

 

(b)    Xanthophyll :

C40 H56 O2 , yellowish in colour, fucoxanthin, violaxanthin. Molar ratio of carotene and

 

xanthophyll in young leaves is 2 : 1.

(ix)  Plastids are interchangeable

Leucoplast  Chloroplast

Chromoplast (degenerate chloroplast)

The leucoplast and chloroplast are interconvertible but once they have converted into chromoplast, the reverse can not take place. Because, chromoplasts are aged or degenerated form of chloroplast e.g. in tomato.

Young ovary (colourless)                      –        Leucoplast

Young fruit (green)                             –        Chloroplast Mature fruits (red) (due to Lycopene)        –        Chromoplast.

In carrot leucoplast                             –        Chromoplast (carotene) etc.

• Origin of chloroplast : Plastids, like the mitochondria, are self duplicating These develop from colourless precursors, called proplastids. They are believed to be evolved from endosymbiont origination.

(4)  Function of plastids

• It is the site of photosynthesis, (light and dark reaction).
• Photolysis of water, reduction of NADP to NADPH₂ take place in
• Photophosphorylation through cytochrome b₆ f, plastocyanine and plastoguinone
• They store starch or factory of synthesis of sugars.
• Chloroplast store fat in the form of
• They can be changed into chromoplasts to provide colour to many flowers and fruits for attracting
• They maintain the percentage of CO₂ and O₂ in

Important Tips

 

• Murphy and Leech (1978) have reported the synthesis of fatty acids in the spinach
• Proplastids are precursor of all type of
• Capasanthin is the pigment in carotenoids found in bacteria, fungi and
• Solar energy is trapped in lamella by chlorophylls but in bacteria trapping centre is B₈₉₀.
• The chloroplast with nitrogen fixing genes (nif genes) constitute
• Pyrenoids : A proteinaceous core around which starch is deposited mostly found in the chloroplast of algae and in some
• Algal classification is based on pigmentation
• Eye spot or stigma is photosensitive carotenoid
• Intact chloroplast can be separated by sugar solution (2.5 M).
• Mitochondria and plastids both have own DNA molecules which is called as Extranuclear/ Extrachromosomal DNA.
• Plastids are absent from monerans, fungi and animals these are also absent from gametes and zoospores of
• Ris and Plaut (1962) reported DNA in chloroplast and was called plastidome. It forms about 5% of total cellular DNA and is rich in G-C pairs.
• Plastidoribosomes : Ribosomes of plastids and are of 70S These were reported by Jacobson et. al. (1963)
• Thylakoid term was given by Menke (1961).
• Transducers : Structure which are involved in energy transformations g. mitochondria and plastids.
• Plastids are the largest cell organelles. The plastids in the order of their increasing size are Chloroplast ® Chromoplast ® Elaioplast ® Aleuroplast ® Amyloplast
• Quantasome is formed of 160 chlorophyll a + 70 chlorophyll b molecules and 50 carotenoid
• Scattered thylakoids in the cytoplasm of cyanobacteria and photosynthesis bacteria are known as
•       Chromatophore term was given by Schmitz.                                                                                                                                                 

 

 Endoplasmic reticulum (ER).

• Definition : It is well developed electron microscopic network of interconnected cisternae, tubules and vesicles present throughout the cytoplasm, especially in the
• Discovery : Garnier (1897) was first to observe the ergastoplasm in a The ER was first noted by

Porter, Claude, and Fullman in 1945 as a network. It was named by Porter in 1953.

• Occurrence : The ER is present in almost all eukaryotic cells. A few cells such as ova, embryonic cells, and mature RBCs, however, lack It is also absent in prokaryotic cell.

In muscle cells, it is called sarcoplasmic reticulum, myeloid bodies and nissel granules are believed to be formed from ER. ER is little develop in meristematic cells.

• Chemical composition : All the components of ER are lipoperoteins and trilaminar like the plasma membrane but differ in following
• Thinner (50 – 60 Å) than plasma
• With less
• With more
• The lumen is filled with fluid containing 70% phospholipids lecithin and cephalin
  • Ultrastructure : The ER is made up of three components :
    • Cisternae : These are flattened, unbranched, sac like They lie in stacks (piles) parallel to one another. They bear ribosomes. They contain glycoproteins named ribophorin-I and ribophorin-II that bind the ribosomes. Found in protein forming cells.
    • Vesicles : These are oval or rounded, vacuole like elements, scattered in cytoplasm. These are also studded with
    • Tubules : Wider, tubular, branched elements mainly present near the cell membrane. They are free from These are more in lipid forming cells.

Fig : Elements of Endoplasmic Reticulum

 

All the three structures are bound by a single unit membrane.

• Types of ER : Depending upon the presence of ribosomes, the ER has been categorised into two types:
  • A smooth or Agranular endoplasmic reticulum (SER) : It consists mainly of tubules and vesicles. It has no ribosomes associated to it. It is well developed in the muscle cells, adipose tissue cells, interstitial cells, glycogen storing liver cells, etc. and the cells that synthesize and secrete steroids. SER also takes part in synthesis of vitamins, carbohydrates and Detoxification of pollutants carcinogens and drugs is carried out SER of

liver cells and mitochondria, SER is associated with storage and release of Ca²⁺ ions. It gives rise to spherosomes.

• Rough or Granular endoplasmic reticulum (RER) : It mainly consists of cisternae. It has ribosomes attached on its cytoplasmic It is abundant in cells engaged in production and excertion of proteins, e.g., plasma cells, goblets cells, pancreatic acinus cells and certain liver cells. The RER is more stable than SER. The RER is basophilic due to the presence of ribosomes. Ribosomes are attached to ER through hydrophobic interaction.

The proteins synthesised by the ER membrane bound ribosomes pass into the ER lumen, where most of the

 

proteins are glycosylated. For this, an oligosaccharide is always linked to the

• NH₂ group on side chain of an

 

asparagine residue. The ER lumen serves as a compartment to contain substances which must be kept separate from cytosol. In the ER lumen, the enzymes modify the proteins.

Differences between SER and RER

 

SER	RER
SER or smooth endoplasmic reticulum does not possesses ribosomes over the surface of its membrane.	RER possesses ribosomes attached to its membrane.
It is mainly formed of vesicles and tubules.	It is mainly formed of cisternae and few tubules.
It is engaged in the synthesis of glycogen lipids and steroids.	The reticulum takes part in the synthesis of proteins.
Pores are absent so that materials synthesised by SER do not pass into its channels.	RER possesses narrow pores below its ribosomes for the passage of synthesised polypeptides into ER channels.
SER is often peripheral. It may be connected with plasmalemma.	It is often internal and connected with nuclear envelope.
Ribophorins are absent.	RER contains Ribophorins I and II for providing attachment to ribosomes.
SER gives rise to sphaerosomes.	It helps in the formation of lysosome through the agency of golgi apparatus.

• Origin : RER is formed from nuclear membrane while SER is formed from RER by loss of ribosomes. Rough vesicles originate only from RER after homogenisation of RER breaks in small fragments (Vesicles) and it is called microsome (This is not a cell organelle). ER constitute cytoskeleton and also help as intracellular transport system. And it is sensitive to irritation.

(8)  Functions

• Synthesis and secretion of specific proteins via – golgi
• Formation of protein Which helps in attachment of ribosome.
• Give rise to
• Provides surface for synthesis of cholesterol, steroid, ascorbic acid and visual
• It helps in synthesis of harmones g., testosterone and estrogen.
• It helps in glycogenolysis in the liver cells and brings about detoxification (SER).
• Gastric cells secreting zymogen have well developed
• ER is a component of cytoskeleton (Spread as a net) of cell and provides mechanical support and shape to the
• ER acts as segregation apparatus and divides the cytoplasm into Compartmentalisation is most necessary for cellular life.
• It participates in the formation of cell-plate during cytokinesis in the plant cells by the formation of
• ER has many types of enzymes g. ATPase, reductases, dehydrogenases and phosphatases.

• Sacroplasmic reticulum : It is a modified SER striated muscle fibres which forms a network of interconnected tubules in the It helps in conduction of motor nerve impulses throughout the muscle fibre and in the removal of lactic acid so prevents muscle fatigue. It is called “ergastoplasm” in muscle and “nisslegranules” in nerve cells.

 

Important Tips

• Annullated lamellae : It was first reported by Mc Culloch (1952) in the egg of sea Formed by blebbing of outer nuclear membrane.
• Transitional ER : It is RER without
• Microsome : This term was used by Claude (1941). It probably refers to these fragments of ER which are associated to
• Sjostrand gave the term a – cytomembrane for
• Veratti (1902) reported sacroplasmic recticulum in the muscle
• Nissl’s granules are the masses of RER in the cyton of
• Myeloid bodies are the masses of tubules (S0 SER) found in retinal cells and are related with
• Total ER in the cell – 2/3 RER + 1/3 SER.
• In rapidly dividing cells endoplasmic reticulum is poorly developed.

 

 Golgi complex.

• Definition : Golgi complex is made up of various membranous system e.g. cisternae, vesicles and These are also called golgi bodies, golgisomes, lipochondrion, dictyosomes, Dalton complex, idiosomes or Baker’s body. These are also called “traffic police” of the cell.
• Discovery : First observed by George (1867) but it’s morphological details were given by Camillo Golgi (1898), in nerve cells of barn fowl and
• Occurence : It is present in all eukaryotic cells. They form 2% of total cell volume. In a cell these are found above centriole or near In plants,

these are scattered irregularly in the cytoplasm and called as “dictyosomes”. These are absent in bacteria and blue green algae, RBCs, spermatozoa of bryophytes and pteridophytes, and sieve tube cells of phloem of angiosperm.

• Size and number : The size of the golgi body varies with the metabolic state of cell and hence it is called pleomorphic. Large in mature functional and secretary cell g., germinal cells, goblet cells, but small size in non-secretary cells. There may be

 

25,000 dictysomes present in rhizoidal cells of Chara. Average number 10 – 20 per cell. Number increases during cell division.

Fig : Arrangement of membrane, tubles and vesicles in golgi complex

 

• Structure : Under transmission electron microscope the of golgibodies was study by Dalton and Felix (1954), golgi body is made of 4 parts.
  • Cisternae : Golgi apparatus is made up of stack of Sac like structure called cisternae. The margins of each cisterna are gently curved so that the entire golgi body takes on a cup like appearance. The golgi body has a definite polarity. The cisternae at the convex end of the dictyosome comprises forming face (F. face) or cis face. While the cisternae at the concave end comprises the maturing face (M. face) or trans face. The forming face is located next to either the nucleus or endoplasmic reticulum. The maturing face is usually directed towards the plasma membranes. It is the functional unit of golgi body.
  • Tubules : These arise due to fenestration of cisternae and it forms a complex of

 

• Secretory vesicles : These are small sized components each about 40 Å in diameter presents along convex surface of edges of These are smooth and coated type of vesicles. Smooth or secretory vesicles, which have a smooth surface and contain secretions of the cell and coated vesicles, that have rough surface. They carry materials to or from the cisternae.
• Golgian vacuoles : These are spherical components each about 600 Å in These are produced by vesiculation of saccules of cisternae. Scattered cisternae are called dictyosomes and condition is called diffused.

(6)  Function

• The main function of golgi body is secretion, so it is large sized among the secretory Secretion are released either by exocytosis or reverse pinocytosis.
• Glycosidation of lipids e. addition of oligosaccharides to produce glycolipids.
• Glycosylation of proteins e. addition of carbohydrate to produce glycoproteins.
• Formation of
• Golgi body forms the cell During cell division by secreting hemicellulose formation of enzyme and hormones (Thyroxine) etc.
• Matrix of connective tissue is formed by golgi
• In oocytes of animal, golgi apparatus functions as the centre around which yolk is deposited e. vitellogenesis.
• Membrane of the vesicles produced by golgi apparatus join in the region of cytokinesis to produce new
• It is also called export house of
• Golgi body contains phospholipids, proteins, enzymes and vitamin-c.
• The golgi complex gives rise to the acrosome in an animal

• Origin : Most accepted view is that golgi body originates from RER-that has lost its ribosomes from this RER arise transport vesicles that contain Golgi membrane and fuse with the saccule on the forming face of Golgi This is why this face is called the forming face.

Important Tips

• According to Camillo Golgi “Apparato reticulare interno” (internal reticular apparatus) is Golgi
• Cellulose, hemicellulose and pectin are synthesized by Golgi
• Metal silver impregnation technique was used by Camillo
• Sperm acrosome is made of golgi
• The main enzyme of golgi complex are glycosyl transferase, nucleoside diphosphatase and thiamine
• Zymogen is processed in
• Term “trophospongium” given by Holmgen.
• The number of golgi bodies increase during cell Phragmoplast is the precursor of cell plate.
• The basophilic ergastoplasm in gland cells indicate the richness of golgi
• Root cap cells are rich in golgi complex secreting mucilage, which lubricates the root
• Proteins and fats are stored in vacuoles and vesicles of golgi
• In fungi, unicisternal dictyosomes are
• Zone of exclusion : A zone of clear cytoplasm with no ribosomes, mitochondria around the golgi body.
• Perner gave the term
• Mollenhaver and Whaley (1963): Reported polarity in golgi
• GERL : Golgi-endoplasmic reticulum-lysosome
• GER : Golgi associated endoplasmic