$\displaystyle \small \bullet$ The eukaryotes include all the protists, plants, animals and fungi.
$\displaystyle \small \bullet$ In eukaryotic cells there is an extensive compartmentalization of cytoplasm through the presence of membrane bound organelles.
$\displaystyle \small \bullet$ They possess an organized nucleus with a nuclear envelope.
$\displaystyle \small \bullet$ Genetic material is organized into chromosomes.
$\displaystyle \small \bullet$ Cell to cell communication takes place by gap junction.
$\displaystyle \small \bullet$ Plant cell possess cell walls, plastids and a large central vacuole, which are absent in animal cell.
$\displaystyle \small \bullet$ Animal cells have centrioles which are absent in plant cells
DIFFERENCE BETWEEN PLANT CELL AND ANIMAL CELL
| Plant Cell | Animal Cell |
|---|---|
| The cell is enclosed by a cell wall and a plasma membrane | the cell is enclosed by a plasma membrane. The cell wall is absent |
| Centrosome is absent. Just two clear areas called polar caps are present | Centrosome is present. It is in the form of two granule like centrioles |
| Plastids are present | Plastids are absent |
| Instead of Golgi apparatus, plant cells have several, scattered and unconnected unit called dictyosomes. | Have highly complex and prominent Golgi apparatus near the nucleus |
| A mature plant cell has a large central vacuole and cytoplasm forms a thin layer around it | Cytoplasm contains a number of small vacuoles |
COMPONENTS OF A EUKARYOTIC CELL
Cell Membrane
$\displaystyle \small \bullet$ Every living cell is covered by a thin, elastic, transparent, semi-permeable and regenerative membrane called cell membrane (plasma membrane or plasmalemma).
$\displaystyle \small \bullet$ The plasma membrane separates the internal environment of the cell from external environment.
$\displaystyle \small \bullet$ The cell membrane is composed of lipids that are arranged in a bilayer.
$\displaystyle \small \bullet$ The lipids are also arranged within the membrane with the polar head towards the outer sides and the hydrophobic tails towards the inner part.
$\displaystyle \small \bullet$ The lipid component of the membrane mainly consists of phosphoglycerides.
$\displaystyle \small \bullet$ Cell membranes also possess protein and carbohydrate.
$\displaystyle \small \bullet$ The proportion of lipid molecules varies in plasma membrane of different cell types. These are formed of cholesterol (25-32%) and mainly of phospho-glycerides or phospholipids
$\displaystyle \small \bullet$ The membrane proteins can be classified as
(i) Integral Proteins (intrinsic protein): They have stronger association and bound firmly to the membrane. These proteins are buried partially or totally in the phospholipid bilayer.
(ii) Peripheral Proteins (extrinsic protein): They have weaker association and are bound to lipids of membrane by electrostatic interactions.
Fluid Mosaic Model
$\displaystyle \small \bullet$ This model was given by Singer and Nicholson (1972).
$\displaystyle \small \bullet$ According to this model, the lipid bilayer and integral proteins appear like a mosaic arrangement and the quasi-fluid nature of lipid enables the lateral movement of the proteins within the overall bilayer.
$\displaystyle \small \bullet$ This ability of proteins to move within the membrane indicate the fluidity of the lipid part.
$\displaystyle \small \bullet$ The word ‘fluid’ refers to the fact that the lipids and proteins are not rigid structures but have mobility. The word ‘mosaic’ refers to distribution of molecules to produce a pattern.
$\displaystyle \small \bullet$ Plasma membrane is trilaminar (three layers), there are two thick and one thin layer(Robertson 1965)
$\displaystyle \small \bullet$ It is made up of lipids, proteins, carbohydrates and water. The lipids are about 30—70%, proteins 40-60%, carbohydrates 2% and water 20%.
$\displaystyle \small \bullet$ The lipids are in the form of phospholipids, they have polar and non-polar ends. These are also called amphipathic. The polar hydrophilic ends are towards margin (intact water) and non-polar hydrophobic fatty acid tails face each other (do not intact water).
$\displaystyle \small \bullet$ The peripheral or extrinsic proteins are present on the surface membrane. An integral or intrinsic proteins are buried in the membrane.
$\displaystyle \small \bullet$ Some large protein project both the sides are called channel proteins
$\displaystyle \small \bullet$ Major component of proteins that float on or in the lipid layer are called transmembrane protein or carrier proteins.
$\displaystyle \small \bullet$ Carbohydrates are short branched or unbranched chain of sugars attached either to proteins or to lipids.
1. The fluid nature of the membrane is helpful in cell growth, formation of intercellular junctions, secretion, endocytosis, cell division etc.
2. Most important functions of the plasma membrane is the transport of the molecules across it. The membrane is selectively permeable to some molecules present on either side of it.
3. Many molecules can move briefly across the membrane without any requirement of energy and this is called the passive transport. Passive transportation takes place by Osmosis, Diffusion and facilitated diffusion
4. Some transport is an energy dependent process, in which ATP is utilized and is called active transport. e.g., $\displaystyle Na^{+}$ /$\displaystyle K^{+}$ Pump.
5. Intake of solids through cell membrane is called phagocytosis and intake of liquid is called pinocytosis.
Cell Wall
$\displaystyle \small \bullet$ The cell wall is a non-living rigid structure which forms an outer covering for the plasma membrane of fungi and plants.
$\displaystyle \small \bullet$ Cell wall gives shape to the cell, protects the cell from mechanical damage and infection.
$\displaystyle \small \bullet$ The chief structural component of cell wall is cellulose in plants and chitin in fungi.
$\displaystyle \small \bullet$ Cell wall can be differentiated into three regions
Middle lamellae: It holds the neighbouring cell walls together and is composed of gel like cementing material composed of calcium and magnesium pectate. The cells get separated if this layer dissolves.
Primary wall: It is formed over the middle lamellae on either side. It is thin wall of 1-3µ thickness, elastic and extensible in growing cells. The cell wall is chemically made up of hemicellulose, pectin and glycoproteins.
Secondary wall: It is a thick wall of 5-10µ thickness and deposited over the primary wall. It is composed of several layers by deposition of pectin, lignin, subarin, hemicellulose, fats and waxes.
$\displaystyle \small \bullet$ The cell wall and middle lamellae may be traversed by plasmodesmata which connect the cytoplasm of neighbouring cells.
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