Category: Biology Second Paper

In 1972 scientists S. J. Singer and G. L. Nicholson described the most widely accepted model of cell membrane structure called the fluid-mosaic model. In this model, protein molecules in the lipid layer are compared to icebergs floating in the ocean. This is called the iceberg model. Cell membrane structure is discussed according to fluid-mosaic model

1. Phospholipid: There are five types of phospholipids in the cell membrane. Lecithin, cephalin, glycolipid, glycophosphotide and phosphotidic acid. Phospholipids are arranged in two layers. Each layer is composed of numerous lipid molecules. Each lipid molecule has two parts. head and tail The large outer part is called the head. It is spherical or oval and composed of phosphates. It is polar and hydrophilic. The two fine filament-like parts attached to the head are called tails. It is composed of fatty acids. The tail is non-polar and hydrophobic. Between the head and the tail is the gyserol. Lipid molecules are always moving, shaking and bouncing around each other. This type of movement is called flip flop movement. As the flip flop motion changes the space between the two layers, the membrane feels like a fluid.
2. Protein: There are three types of proteins in the plasma membrane. These are-

(i) Peripheral protein: The protein located at the edge of the lipid layer is called peripheral protein. It looks round. It is enzyme in nature. It remains attached to the cytoskeleton. Adenylate cyclase produces cyclin AMP from ATP by the action of the membrane bound enzyme Adenylate cyclase.

(ii) Integral protein: The protein that is embedded inside the lipid layer is called integral protein. It looks oval. Acts as a carrier and transports essential substances. It maintains cell-to-cell adhesion.

(iii) Inter membrane protein: The protein that extends from one end of the lipid layer to the other end is called intermembrane protein. It is the largest protein. It takes energy from ATP to change its conformation. As a result, a hole is created through it. Essential substances move through these pores. It acts as a receptor for hormones, neurotransmitters, mediated endocytosis, insulin etc. Various molecules, ions and electrons move through channels, pumps and electron transport chains.

3. Glycocalyx: Carbohydrate chains on the outside of the cell membrane. Carbohydrate chains are attached to lipids to form glycolipids and proteins to form glycoproteins. Glycoproteins and glycolipids together are called glycocalyx or cell coat. It acts as a cell recognizer. It helps in the movement of essential substances.
4. Cholesterol: Cholesterol is a type of steroid. It has an OH group at the head and is hydrophilic. The other part is water repellent. Cholesterol is sandwiched between phospholipid molecules. It is irregularly arranged. Cholesterol is higher in animal cells and lower in plant cells.

Protein molecules are scattered between phospholipids in the cell membrane. Because of this, the protein molecules look like a mosaic when viewed from the surface of the cell membrane. To explain this situation in one word, the cell membrane model has been named fluid mosaic model.

In 1959, scientist Robertson proposed the unit membrane model of cell membrane structure. According to him, the molecular structure of all biological membranes is similar. It consists of three levels. The three levels together are called units or units. It is composed of phospholipid bilayers. Each lipid molecule has two parts. head and tail Molecules are polar and water absorbing (hydrophilic). The tail is non-polar and hydrophobic. Protein is grown in its place. A phospholipid bilayer with proteins is called a unit membrane. Its structure is protein-lipid-protein (P-L-P) with a total thickness of 75 Å. The outer layer is 20 Å thick and the inner layer is 35 Å thick.

In 1935 scientists Danielli & Davson first proposed a precise model of cell membrane structure. It is called sandwich model. According to this model the cell membrane is bi-layered. Each layer consists of protein and lipid sub-layers. The upper and lower layers contain proteins and the middle layer contains lipids.

1. Lipid Membrane Model – Gorter & Grendel (1925)
2. Sandwich Model or Bilayer Model – Danielli and Davson (1935)
3. Micellar Model – Hiller and Hoffman (1953)
4. Single Screen Hypothesis – Robertson (1959)
5. Benson’s model (1966)
6. Lenard & Singer’s model (1966)
7. Protein crystal model – Venderkoff and Green (1970)
8. Fluid-mosaic model – Singer and Nicholson (1972)

The fine, thin, elastic and permeable membrane that surrounds the protoplasm is called the plasma membrane. It is composed of lipids and proteins. It is called cytoplasmic membrane, cytomembrane, cell membrane, plasmalemma, biomembrane etc. It is called biomembrane as it exists in plant and animal body i.e. in all living cells. Chemically it consists of three layers: protein-lipid-protein. Carl Nageli and Cramer (1855) first discovered the cell membrane and named it plasma membrane. J. Q. Plower (1931) called it plasmalemma.

1. Protoplasm is the physical basis of life. It acts as the physical basis of life.
2. It is the main component of cells. Contains all the organelles of the cell.
3. It holds water for cells.
4. It causes reproduction.
5. It causes physical growth.
6. Maintains ionic balance in cells.

Protoplasm always exhibits dynamic movement. The dynamic movement of protoplasm is called cyclosis or cyclosis. It exhibits zig zag oscillations, called Brownian oscillations. It exhibits two types of movement.
1. Unidirectional Cyclosis: The movement in which the protoplasm rotates in one direction in a specific direction around a single cavity is called unidirectional cyclosis. The movement of the protoplasm of leafhoppers is unidirectional.
2. Multidirectional Cyclosis: The movement in which the protoplasm rotates irregularly in different directions around several cavities is called multidirectional cyclosis. The movement of the protoplasm of Tradescantia is unidirectional.

1. Reticular theory: According to the scientist Frommann (1865), protoplasm is formed by reticular fibers on the base material.
2. Granular theory: According to the scientist Altman (1886), protoplasm is a granular fluid with numerous small and large biomass.
3. Alveolar theory: According to the scientist Butschli (1892), protoplasm is like soap foam with numerous alveoli or tiny bubbles.
4. Colloidal theory: According to scientists Wilson and Fisher (1894), protoplasm is partly liquid and partly colloidal.

1. The main chemical constituent of protoplasm is water. It contains 70-90% water.
2. Dry protoplasm contains 45% proteins, 25% lipids, 25% carbohydrates and 5% other substances.
3. The main organic constituents of protoplasm are proteins and nitrogenous compounds.
4. It contains small amounts of liquid fats and oils.
5. Its minerals are calcium, magnesium, potassium, sodium, iron etc.

1. Protoplasm is translucent, colorless, gelatinous and sticky.
2. It is granular and colloidal.
3. Its colloidal particles carry homogeneous electrical charges.
4. Its relative importance is greater than that of water.
5. It performs all the basic functions of the body. The basic functions of organisms are photosynthesis, growth, movement, digestion, excretion, growth, reproduction, absorption, diffusion, respiration etc.
6. It coagulates under the influence of heat, acid and alcohol
7. It is always in motion and its dynamic movement is called cyclosis (eg Tradescantia).
8. It exhibits a zig zag pattern. It is called Brownian movement.
9. Plasmagel is formed from plasmasol of protoplasm and plasmasol from plasmagel.
10. Particles of protoplasm can be separated by Millipore filter paper.
11. It responds to any stimulus. Responds to light, heat, pressure, touch etc.
12. Its density is always variable.
13. After a certain period of time the protoplasm dies.