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CYTOLOGY
The cell and its structures are studied under a branch of biology called cytology. Definition :- The structural & functional unit of living beings is called cell. DISCOVERY OF CELL
1. Robert Hooke (1665) :– An English man and first curator of Royal society of London. Observed a thin transverse section of bark of a tree under self designed microscope. He noticed honey - comb like compartments. He coined the term cell. He wrote a book - Micrographia. He actually observed dead cells. 2. Antony Van Leeuwenhoek (1674) was first to observe living cells like bacteria [from tartar of teeth] erythrocytes [fish], sperms and protozoans [eg. Vorticella] 3. N. Grew (1682) :– Proposed cell concept which states that cell is unit of structure of organisms. 4. Cell is called structural & functional unit of life because – (i) All the living organisms are composed of one or more cells. (ii) All the cells have similar basic structure. (iii) Similar cell organelles of different cells perform similar functions. 5. Knoll and Ruska (1932) of Germany designed the electron microscope which was employed to study the ultrastructrue (fine structure) of cell and various cell organelles in 1940s. MICROSCOPE It is instrument which is used to study those objects that cannot be seen with the naked eye or with the help of a hand lens. A microscope has more than one lens. The 1st compound microscope was built by F. Janssen and Zacharias Janssen (1590). ² Structure of Microscope : The microscope used in schools is called compound microscope, a compound microscope has following parts:
1 Base: It is the basal, metallic, horse-shoe shaped structure. It bears the whole weight of microscope. 2 Handle: It is the curved part to hold the microscope. It is also called as arm. 3 Stage: It is a strong metallic, rectangular, horizontal plate fixed to the handle. 4.Stage Clips: Two clips are attached to stage used for holding the slide in position. 5.Condenser: Below the stage is present a condenser for concentrating the light rays.
6. Body tube: It is wide, hollow tube attached to the upper part of the arm. To this tube lenses are attached. 7.Adjustment Screw: (a) Coarse adjustment: It is bigger sized screw used to move the body tube up and down. (b) Fine adjustment: It is a smaller sized screw for fine focussing. 8 Reflecting Mirror: It is meant for reflecting the light rays, so that light passes through the object which is to be seen. CELL THEORY Two biologists, "Schleiden and Schwann' gave the "Cell theory" which was later on expanded by "Rudolf Virchow". Cell theory states that- (i) All plants and animals are composed of cells. (ii) Cells is the basic unit of life. (iii) All cells arise from pre-existing cells. Ciruses are the exceptions of cell theory. CELL SIZE & SHAPE (A) Size of cell – Normal size in human 20 μm to 30 μm in diametre. (i) Largest cell – In animals – Ostrich egg [15 cm is diametre] In plants – Acetabularia [6-10 cm] (ii) Longest cell – In animals – Nerve cell [upto 1mt] In plants – Hemp fibre.
DIFFERENCES BETWEEN PLANT CELL & ANIMAL CELL
COMPONENTS OF A CELL
There is an occurrence of division of labour within a cell as they all got certain specific components called "Cell organelles" each of them performs a specific function. The three basic components of all the cells are (i) PM (Plasma Membrane) (ii) Nucleus (iii) Cytoplasm
· Mitochondria · Vacuole · Endoplasmic Reticulum · Starch granules · Golgibody · Store food materials · Plastid · Wastes · Lysosome · Ribosome Cell Membrane or Plasma Membrane Each cell (prokaryotic as well as eukaryotic) is surrounded by a covering called plasma membrane or plasmalemma or cell membrane. Most cell organelles in eukaryotic cells (e.g., Mitochondria, Plastids, Golgi apparatus, Lysosomes, Endoplasmic reticulum, Peroxisomes, Vacuoles etc). are enclosed by subcellular unit membranes. These membranes, thus, compartmentalise the cell. Molecular Structure of Plasma membrane. Plasma membrane is a living, ultra-thin, elastic, selectively permeable membrane. Chemically, it is composed of phospholipids, proteins, oligosaccharides and cholesterol. Trilamilar or 3-layered structure :- J.D. Robertoson noted trilamilar or 3-layered structure for all membranes he studied. Based on his findings, he proposed the 'unit membrane hypothesis' in 1959. Fluid Mosaic Model :- In 1972, S.J. Singer and G. Nicolson proposed fluid mosaic model to explain the structure and functions of plasma membrane. According to this model, the plasma membrane is made up of a phospholipid bilayer and two types of protein molecules 'floating about' in the fluid phospholipid bilayer. The two types of proteins are (i) Intrinsic proteins which are embeded in the phospholipid matrix incompletely or completely, and (ii) Extrinsic proteins which occur superficially either on the outer surface or on the inner surface of the phospholipid layer. In other words, the membrane is a viscous fluid with phospholipids and protein molecules arranged as a mosaic.
bind specific molecules such as hormones. Fluid mosaic model is also described as "a number of protein icebergs floating in the sea of lipids'. Types of membranes :- (i) Impermeable membrane :- If the membrane does not allow passage of substances (solvent and solute) through it. (ii) Permeable membrane :- If the membrane allows free passage of solute and solvent through it. (iii) Semipermeable membrane :- If the membrane allows passage to solvents but prevents the passing of solutes. (iv) Selectively permeable membrane :- If the membrane allows the passage of solvent and few selected solutes. Advantage of Semi permeability membrane :- Semi permeability ensures that
- The useful molecules enter the cell,
- The metabolic intermediates remain within the cell and
- The secretions and wastes leave the cell. Thus, semi permeability of cell membranes enables the cell to maintain homeostasis, i.e., a constant internal environment inspite of the changes outside it. The substances generally drawn in the cell include : (i) Raw materials for metabolism, viz. food stuffs, water, salts and oxygen; and (ii) Regulatory substances, e.g., vitamins and hormones. The substances generally turned out of the cells include : (i) The products of metabolism, namely, nitrogenous wastes and carbon dioxide; and (ii) Secretions. Following mechanisms are involved in the entry or exit of various materials across p.m. (A).Physical processes. (B) Biological processes. A.Physical Processes :- These processes are slow and do not expend energy. These occur down the concentration gradient and do not use carrier proteins. Physical processes include. (i) Diffusion, (ii) Osmosis. B.Biological processes :- These processes are rapid and often use energy in the form of ATP. These can occur down as well as against the concentration gradient and often use carrier proteins. Biological processes include:-
- Mediated transport (i) Facilitated transport / diffusion (ii) Active transport
- Endocytosis (Pinocytosis and Phagocytosis)
- Exocytosis.
1 Diffusion :- The process by which a substance uniformly spreads into another substance by random movement of its particles from a region of higher concentration to a region of its lower concentration due to their kinetic energy is called diffusion. It is faster in gaseous phase than in liquid phase or solid phase. Significance of diffusion :- (i) Diffusion helps in the distribution of various substances throughout the cytoplasm of the cell without much delay. (ii) It helps in the exchange of respiratory gases (oxygen and carbon dioxide) between the body cells and their environment. (iii) Various materials such as gases, liquids and solids dissolve in the medium, i.e., air or liquid by diffusion. (iv) Loss of water in vapours form from the aerial parts of the plants (transpiration) occurs through diffusion. (v) Flowers of plants spread aroma through diffusion. It attracts insects and other animals for pollination. CELL MEMBRANE OR PLASMA MEMBRANE Each cell (prokaryotic as well as eukaryotic) is surrounded by a covering called plasma membrane or plasmalemma or cell membrane. Most cell organelles in eukaryotic cells (e.g., Mitochondria, Plastids, Golgi apparatus, Lysosomes, Endoplasmic reticulum, Peroxisomes, Vacuoles etc). are enclosed by subcellular unit membranes. These membranes, thus, compartmentalise the cell. Molecular Structure of Plasma membrane. Plasma membrane is a living, ultra-thin, elastic, selectively permeable membrane. Chemically, it is composed of phospholipids, proteins, oligosaccharides and cholesterol. Trilamilar or 3-layered structure :- J.D. Robertoson noted trilamilar or 3- layered structure for all membranes he studied. Based on his findings, he proposed the 'unit membrane hypothesis' in 1959. Fluid Mosaic Model :- In 1972, S.J. Singer and G. Nicolson proposed fluid mosaic model to explain the structure and functions of plasma membrane. According to this model, the plasma membrane is made up of a phospholipid bilayer and two types of protein molecules 'floating about' in the fluid phospholipid bilayer. The two types of proteins are (i) Intrinsic proteins which are embeded in the phospholipid matrix incompletely or completely, and (ii) Extrinsic proteins which occur superficially either on the outer surface or on the inner surface of the phospholipid layer. In other words, the membrane is a viscous fluid with phospholipids and protein molecules arranged as a mosaic. Oligosaccharide molecules are present on the exposed surface of the plasma membrane. They are associated with proteins as well as lipid molecules forming glycoproteins and glycolipids respectively. Cholesterol molecules are inserted between the phospholipid molecules of plasma membrane of animal cells to stabilize the membrane. Presence of lipids and proteins provides flexibility to the plasma membrane. Proteins present in the membrane serve as :-
semipermeable plasma membrane when surrounded by hypertonic solution. Experiment : Demonstration of osmosis in the laboratory. Requirements : Funnel fitted with a semipermeable membrane, beaker, sugar solution, water. Procedure : Take sugar solution in a funnel fitted with a semipermeable membrane (fish bladder or egg membrane) upto mark 'A' and place it in an inverted position in a beaker filled with clean water as shown in figure. After some time, observe the level of sugar solution in the funnel. Result :- You would find that the sugar solution has risen from level 'A' to a new level 'B'. Explanation and conclusion : Sugar solution in the funnel and water in the beaker are separated by a semipermeable membrane. The fitted membrane is permeable to small water molecules but is relatively impermeable to large sugar molecules dissolved in water. Due to difference in the concentration of solute on the two sides of semipermeable membrane, water molecules have moved from the solution having lower concentration of solutes (e.g., water in this experiment) to the solution having higher concentration of solutes [e.g. sugar solution] due to osmosis has risen to new level 'B'. Types of solutions :
**1. Isotonic solution
- Hypotonic solution, and
- Hypertonic solution. 1.Isotonic solution :-** Isotonic solution is one in which the concentration of water and solutes is the same as in the cytoplasm of the red blood cells. 0.9% salt solution and 5% glucose solution are isotonic for red blood cells. 2. Hypotonic solution :- Hypotonic solution is one in which the concentration of solutes is less and concentration of water is more as compared to inside the red blood cells. 0.66% salt solution and 0.2% glucose solution are hypotonic for red blood cells. 3. Hypertonic solution :- Hypertonic solution is one in which the concentration of solutes is more and the concentration of water is less as compared to in the cytoplasm of the red blood cell. 1.25% salt solution and 10% glucose solution are hypertonic for red blood cells. Other examples of osmosis :-
- Fresh water unicellular organisms (e.g., Amoeba, Paramecium ) continuously gain water in their bodies due to osmosis. These organisms have mechanisms (e.g., contractile vacuoles) to throw out excess of water from their bodies.
- Most plant cells have the tendency to gain water due to osmosis.
- Absorption of water by the plant roots from the soil through root hairs is also an example of osmosis.
- Certain plant movements (e.g., seismonastic movements in 'touch-me-not' plant) occur due to loss or gain of water.
- Stomata are present in the leaves. They open and close at different times of the day due to osmotic movements of water.
- In plants, cells, tissues and soft organs (leaves, young shoots, flowers) maintain turgidity or stretched form due to osmotic absorption of water. Mediated transport : Type of transport of materials across the plasma membrane with the help of carrier proteins is called mediated transport. Types of mediated transport Mediated transport is of following two types : (i) Facilitated transport (ii) Active transport (i) Facilitated transport :- In this case, transport proteins (e.g. permeases) assist molecules to diffuse through the membrane down the concentration gradient, i.e., from the region of higher concentration to the region of lower concentration across the membrane. It is, therefore, also termed as facilitated diffusion. No cellular energy is used in such transport. A carrier protein combines with a specific substance (e.g., glucose) to be transported and moves it down the concentration gradient from one side of membrane to another through a channel formed by it.
Animal cells can also actively take in and turn out materials in masses much larger than in the hither to described processes by utilizing energy. Such materials include macromolecules, lipid droplets and solid particles. Items of this size cannot cross the phospholipid bilayer by diffusion or with the help of transport proteins. Special processes are involved in the transport of such large quantities of materials. These include endocytosis (phagocytosis) and exocytosis. Endocytosis :- The term endocytosis refers to invagination of a small region of plasma membrane, and ultimately forming an intracellular membrane-bound vesicle. Endocytosis is not shown by plant cells because of their rigid cell wall and internal turgor pressure. Depending upon the intake of fluid droplet or solid particles, endocytosis is of two types : (i) Pinocytosis (ii) Phagocytosis (i) Pinocytosis :- The non-specific intake of a tiny droplet of extracellular fluid by a cell through the cell membrane which cannot otherwise pass through it. It is also, therefore, termed as cell drinking. It was first observed in Amoeba. In this process, a small region of plasma membrane invaginates and the fluid droplet passes into the pocket so formed. This pocket is called caveola. The pocket deepens and finally nips off as a fluid-filled vacuole called pinosome or pinocytotic vesicle. (ii) Phagocytosis : - Phagocytosis is the intake of solid particles by a cell through cell membrane. It is also called cell eating. Phagocytosis is the major feeding method in many unicellular organisms (e.g., Amoeba) and simple metazoa (e.g., sponges). An area of the plasma membrane, coated initially with actin-myosin, comes in contact with the food particle(s). The contact induces the cell membrane to put out tiny protoplasmic processes, the pseudopodia, around the food particle(s). The pseudopodia meet on the other side of the food particle(s) and fuse. In this way, an internal vacuole, called phagosome, containing food particle(s) in a droplet of water is acquired. INTERNAL STRUCTURE OF A MITROCHONDRION
Exocytosis :- Exocytosis is the process that involves fusion of membrane of the exocytotic vesicle with the plasma membrane to extrude its contents to the surrounding medium. This process is also called cellular vomiting or ephagy and the vesicles that turn out the materials are termed exocytotic vesicles. Exocytosis process is responsile for : (i) removal of undigested food left in the food vacuoles in the cells. (ii) secretion of substances such as hormones, enzymes, and (iii) replacement of internalized membrane by the fusion of exocytotic vesicles with the cell membrane. Functions of plasma membrane
- It gives a definite shape to the cell.
- It provides protection to the internal contents of the cell.
- It regulates entry and exit of substances in and out of the cell.
- It can internalize solid and liquid materials by infolding or extending around them. This is a process of active intake of materials.
- In animal cells, it is involved in adhesion, recognition and in the formation of vesicles, cilia, flagella, microvilli, etc. Plasma membrane acts as a mechanical barrier to protoplasm so after rupturing or breakdown of plasma membrane, the protoplasmic contents will be dispersed in the surrounding medium. CELL WALL
(b) the non-living structures are called Deutoplasmic or ergastic bodies. Role of Cytoplasm: (i) Participates in intracellular distribution of nutrients, metabolites and enzymes. (ii) Helps in exchange of materials between cell organelle. (iii) acts as a site of chemical reactions like glycolysis (step of respiration), synthesis of fatty acids. CELL ORGANELLES These are living sub-cellular structures of the cytoplasm and are also called protoplasmic bodies or organoids. These include- Single membranous : Endoplasmic reticulum, Golgi apparatus, Lysosomes, peroxisomes, Glyoxysomes etc. Double membranous : Plastid and Mitochondria. Non-membranou s: Ribosomes etc. NUCLEUS
Introduction :
(i) The nucleus is the most important component of the cell and controls all functional activities of the cell. Historical Account : (i) Robert Brown (1831) discovered a dense, spherical body in the cells of an ‘orchid’ and named it as ‘Nucleus’. Ultrastructure : Nuclear membrane/Nuclear envelope/Karyotheca Nuclear sap/ Nucleoplasm/karyolymph. Nucleolus. Chromatin threads. (a) Nuclear envelope : Nucleus is surrounded by two membranes, that separates nucleoplasm from cytoplasm. The nuclear membrane has minute pores. These are called nucleo-pores. (b) Nucleoplasm : The part of protoplasm which is enclosed by nuclear membrane is called nucleoplasm. It contains chromatin threads and nucleolus. (c) Nucleolus : Discovered by Fontaina. Usually one nucleolus is present in each nucleus but sometimes more than one nucleoli are present. It is a store house of RNA. (d) Chromation threads : A darkly stained network of long and fine threads called chromatin threads. Chromatin threads are intermingled with one another forming a network called chromation reticulum. Whenever the cell is about to divide the chromatin material gets organized into chromosomes.
Functions of Nucleus : (i) The nucleus control all metabolic activities of the cell. (ii) It regulates the cell cycle. (iii) It brings about growth of the cell by directing the synthesis of structural proteins. (iv) It takes part in the formation of ribosomes. (v) It contains genetic information and is concerned with the transmission of hereditary traits from one generation to another. Do you know? Chromatin threads are made up of – (i) DNA (ii) Protein [Histone protein] Gene :– The segment of DNA and act as unit of heredity ATP :– Adenosine triphosphate. It is also known as energy currency. It provides energy to perform bio-synthesis & mechanical work. Homologous chromosomes :– All chromosomes are found in pair and the chromosomes of a pair are called homologous chromosomes. Non-homologous chromosomes :– Chromosomes of different pair. The nucleus of prokaryotes is also known nucleoid. Nucleus is also called director of cell as it controls most of the cellular activities. Nucleus is absent in sieve tubes of vascular plants & mature RBC's of mammals. Mammalian RBC also lacks Golgibodies, mitochondria, ER, lysosomes. ENDOPLASMIC RETICULUM Introduction : (i) In the cytoplasm some closed or open, branched cavities are present which are bounded by membranes to form a network of membranous system called Endoplasmic Reticulum. Historical Account :
q Functions of Endoplasmic Reticulum : (i) It forms supporting skeleton framework of the cell. (ii) Certains enzymes present in smooth E.R. synthesis fats (lipids), steroids and cholesterol. (iii) Rough E.R. is concerned with protein synthesis. (iv) Smooth E.R. is involved in the process of detoxification. PLASTID Plants and some protists have several types of double membrane bound organelles called plastids, which harvest solar energy, manufacture nutrient molecules and store materials. Plastid term was coined by E. Haeckel. Plastids generally contian pigments and may synthesize & accumulate various substances. Depending upon the type of pigment present in them they are of following three types. ² Chloroplast: It is a double membranous discoidal structure, found only in plant cells. Chloroplast was discovered by A.V. Leeuwenhoek and named by Schimper. Besides being discoidal or rhombic in plant cells they occur in variable shapes like in algae they can be 'U' shaped, spiral, coiled, ribbon shaped etc. In each thylakoid Quantasomes are present which are called as Photosynthetic units. Each quantasome possesses 230 chlorophyll molecules. Each chloroplast consists of two parts. (i) Grana : It constitutes the lamellar system. These are found layered on top of each other, these stacks are called as Grana. Each granum of the chloroplast is formed by superimposed closed compartments called Thylakoids. Functions: Grana are the sites of light reaction of photosynthesis as they contain phtosynthetic pigment chlorophyll. (ii) Stroma : It is a granular transparent substance also called as matrix.
Grana are embedded in it. Besides grana they also contain lipid droplets, starch grains, ribosomes etc. Function : This is the site of drak reaction of photosynthesis. MITOCHONDRIA 1 It was first seen by Kolliker in insect cells and named by Benda. 2 It is a rod shaped structure found in cytoplasm of all eukaryotic cell except mammalian RBC's. 3 These are also absent in prokaryotes. 4 Maximum mitochondria are found in metabolically active cells. 5 It is also called as "Power House of the Cell" or the "Storage Battery". 6 It is double membranous structure where outer membrane has specific proteins while inner me:nbrane is folded inside to form chambers called Cnstae."Cristae" are the infoldings of inner mitochondrial membrane that possess enzymes for respiratory cycles like Kreb Cycle. ATP synthesizing units are called Oxysomes or F0 – F1 Particles. 7 Space between inner and outer mitochondrial membranes is called as perimitochondrial space. The fluid present in mitochondria is called as matrix. (a) Functions:
