Cell Wall

The cell wall is secreted by the protoplasm. It was first observed by Robert Hooke in 1665 in a thin slice of cork.
              The cell wall is chiefly composed of insoluble polysaccaride, cellulose, certain other compound such as hemicellulose, pectin, lignin, suberin, xylan and silicates of calcium and magnesium also occur in the cell wall.

Structure:- the cell wall is composed of four layers. a) middle lamella b) primary wall c) secondary wall and d) tertiary wall. 
a) Middle lamella:- It is the first formed wall layer, formed at the time of cytokinesis ( division of cytoplasm is called cytokinesis ). It is made up of Ca and Mg pectate. It is common to all the cells, popularly known as the cementing layer.The softening of ripe fruits is caused by partial solubilisation of pectic compounds to produce jelly like consistency.
b) Primary wall:- It is laid down along the middle lamella. It is made up of cellulose and pectin and is about 1-3 micro meter in thickness. Meristematic ( those cells who can divide are called meristematic cells in plants ) and parenchyma cells have primary cell wall only.
c) Secondary wall:- It is thick and rigid and laid down next to the primary wall. It again consists of three sub-layers called S1, S2 and S3. It is made up of cellulose, lignin or suberin. It develops in those cells which cease to grow e.g. in tracheary elements and sclerenchyma.
d) Tertiary wall:- internal to secondary wall is another wall material. It is chiefly composed of xylans. this is found in the tracheids of gymnosperms.

Functions of cell wall:- i) It maintains the shape of the cell.
ii) It protects the cell from mechanical injury.
iii) It allows the material to pass in and out of the cell.
iv) Many enzymatic activities are known to occur in the cell wall.
v) Cutin and suberin check loss of water by transpiration.
vi) It provides mechanical support aganist gravity. It is due to the rigid cell wall that the aerial parts of the plants are able to keep erect and expose their leaves to sunlight.
vii) It counteracts osmotic pressure.

Diffusion

The movement of molecules, atoms, ions of gases, liquid or solids from region of higher concentration to region of lower concentration until the molecules get uniformly distributed irrespective of the law of gravitation.

Diffusion Pressure:- It can be defined as the potential ability of a molecule or ions ( solid, liquid or gas ) to diffuse from the area of greater concentration to an area of lesser concentration. The diffusion pressure is directly proportional to the concentration or number of diffusing particles.

Factors affecting diffusion:- i) Rate of diffusion is directly proportional to the temperature.
ii) The rate of diffusion is inversely proportional to sequare root of the density of the diffusing substance.  
iii) The rate of diffusion is directly proportional to the surface area from which diffusion occurs.
iv) Rate of diffusion is directly proportional to the difference of diffusion pressure at the two ends.

Importance:- i) The exchange of gases e.g CO2 intake and O2 output in photosynthesis and CO2 output or O2 intake in respiration takes place by the principle of independent diffusion.

ii) The process of diffusion involved in the transpiration of water vapours.

iii) The ions are absorbed by the simple diffusion during passive salt intake.

iv) Diffusion is an effective means of transport of substances over a very short distances.

Imbibition

Imbibition is the phenomena of adsorption of water or any other liquid by the solid particles of a substance without forming a solution. The solid particles which adsorb water are called imbibants. The imbibants are rich in hydrophilic colloids and have strong affinity with water.
                                                                                     Imbibition results in as increase in volume, liberation of heat and development of pressure called imbibition pressure.

Importance:- i) Absorption of water by young cells is mostly through imbibition.
ii) Imbibition plays important role in germination of seeds.
iii) Imbibition causes the swelling of seeds results in the breaking of seed coat.
iv) Seedling is able to come out of soil due to development of imbibition pressure.
v) Jamming of wooden frames during rains is caused by swelling of wood due to imbibition.
vi) In older times, it was used in breaking the rocks.

Factors affecting imbibition:-
i)  The rate of imbibition increases with rise in temperature.
ii) The rate of imbibition decreases with increasing the concentration of solutes in medium.
iii) Imbibition also influenced by texture like more looseness more imbibition and compactness less imbibition.

Imbibition pressure:- The imbibant, after imbibition of water or liquid, exerts the pressure which is called imbibition pressure. The germinating seeds have imbibition pressure of about 1000 aiomspheres.

Plant-Water Relations

Water is one of the most important constituent of the protoplasm and body fluids. It forms 50 - 90% of the total body weight of living beings and is the universal solvent.
                                                                            Gases, minerals and other solutes enter the plants and move from cell to cell dissolved in water. It provides turgidity to growing cells which is essential for plants to maintain their form and structure. It helps in regulating the body temperature.

Osmoregulation in plants:- Osmoregulation is the homeostatic process in which organisms maintain their internal conditions constant or in which organisms maintain the concentration of their body fluids and temperature at a steady state. In plants this is maintained by absorption, transport and retention of water in their bodies. Transpiration ( loss of water in the form of water vapour from the exposed surface of the plant ) results in the loss of water by plants but this loss is made up by absorption of water.
                The forces like imbibition, diffusion and osmosis are responsible for exchange of material between cells and environment and between cells.

Membrane Transport

Passage of substances across biomembranes occur by three methods -
1) Passive transport
2) Active transport
3) Bulk transport

1) Passive transport:- It is the mode of membrane transport in which cell does not spend any energy. it is of two types:-
a) Passive diffusion by tunnel protein molecules:- The tunnel protein molecules have the channels. these channels permit water and water soluble substances to pass through them. The channels only permit the molecules to pas from higher concentration to lower concentration gradient.
b) passive transport by carrier proteins:- The proteins are known as permeases. They transport the material down the concentration gradient. A carrier protein combines with a specific substances to be transported and moves it from one side of the membrane to another through a channel in it.
                                                             Facilitated diffusion moves glucose molecules in liver and it occurs without expenditure of energy.

2) Active transport:- Active transport moves substances against the concentration by utilizing energy which is privided by ATP ( Adenosine triphosphate ). The carries protein has a binding site for ATP in addition to the binding site for the substrate. ATP molecules binds to the carrier protein. The energy set free brings the substrate binding site of the protein to the surface of the membrane. The substrate present in the medium joins the binding site of protein, forming carrier-substrate complex. It undergoes conformational changes and carries the substrate through a channel in it to the cytoplasmic site of the membrane. Here the substrate is released. This type of transport by using energy is supported by various evidences:- i) Absorption is reduced with the decrease in oxygen content of the surrounding environment ( cells are using oxygen in the respiration involved in the liberation of energy-ATP).
ii) Metabolic inhibitors like cyanides inhibit absorption.
iii) Absorption of different substances is selective.
iv) Decrease in temperature decreases absorption. ( again respiration process is regulated by temperature)
v) Active transport is more rapid then diffusion.

3) Bulk transport:- It occurs by two methods:- pinocytosis and phagocytosis. They involve the enclosure of the material under transport in the vesicles of the membrane. The inward transport by means of carrier vesicles is called endocytosis. The outward transport of substance by means of carrier vesicles is known as exocytosis.
Pinocytosis:- It is the bulk trasport of fluid matter and substances dissolved in it. pinocytosis is also called cell drinking.
Phagocytosis:- It is also called cell eating. Phagocytosis is the transport of solid matter like food, foreign particles, pathogens etc. across the membrane by forming vesicles. These vesicles are called phagosomes. phagosomes fuses with lysosomes to produce a digestive vacuole. The solid food is digested. The digested food diffuses into the cytoplasm. The vacuole with indigestible substance is called residual vacuole. The undigested part are usually thrown out of the cell in the process of exocytosis called ephagy or cell vomiting. Phagocytosis by some white blood cells is an important defence mechanism of the animal body.

Function of Cell Membrane

These are the important functions of cell membrane.
1) The cell membrane cause compartmentalisation. As plasma membranes they separates the cell from external environment.
2) Plasma membrane protect the cell from injury.
3) The plasma membrane allow the flow of materials and information between one cell and another, the cell membrane makes metabolism possible.
4) Plasma membranes of the adjacent cells form various types of junctions for keeping the cells together.
5) plasma membranes as well as other membranes of the organelles have selective permeability, they allow only selected substances to pass inwardly. The membranes are impermeable to others.
6) Membranes have carrier proteins for active transport.
It helps certain cells in movement by forming psudopodia as in Amoeba and leucocytes.
7) Substances attached to cell membrane determine antigen specificity.
8) In nerve cells the cell membrane takes part in transmission of impulses.
9) Membrane infolds are used for bulk intake of materials by endocytosis.
10) Cell membranes contain enzymes for performing certain reactions on their surfaces. e.g. ATPase
11) As microvilli the membrane becomes specialized for the absorption of substances.

Different types of membranes:-
a) Permeable membrane :- These membranes allow all types of solute, solvent and ions through them e.g. cellulose wall of cells.
b) Impermeable membrane :- These types of membranes does not allow the diffusion of both solute and solvent molecules. e.g. heavily cutinised cell walls in plants.
c) Semipermeable membranes :- These types of membranes allow diffusion of solvent molecules but do not allow the passage of solute particles. e.g. parchment paper.
d) Differentially permeable membranes :- These are also called selectively permeable membranes. These type of mmbranes allow only selected solute molecules and slovent molecules to pass through them. e.g. plasma membrane.



 

Cell Membrane

The term was originally used by Nageli and Cramer (1855) for the membranous covering of the protoplast. Plasmalemma or plasma membrane was discovered by Schwann (1838). Membranes also occur inside the cytoplasm of eucaryotic cells as covering of several organelles like nucleus, mitochondria, plastids, lysosomes, Golgi bodies etc. Vacuoles are separated from cytoplasm by a membrane called tonoplast.
All membranes whether external or internal are now called biomembranes. Average thickness is 75 Angstrom. Biomembranes are selectively permeable membranes.

Composition:- Chemically biomembrane consists of lipids (20-79%), proteins (20-70%), carbohydrates (1-5%) and water.The important lipids of the membrane are phospholipids, sterols, glycolipids, sphingolipids.
                                     The lipid molecules are amphiatic or amphipathic, they posses both polar hydroplilic (water loving) and nonpolar hydrophobic (water repelling) ends. The hydrophilic region is in the form of head and hydrophobic tails usually occurs towards the center of the membrane. It results in the formation of lipid bilayer.
                                                   Sveral types of models have been put forward to explain the structure of biomembrane.
Lamellar Models :- They are the early molecular models of biomembrane. according to these models biomembranes are stable layered structures.
Danielli and Davson Model :- This model can be described in the following points:-

1) According to this model the plasma membrane consists of a biomolecular layer of phospholipids which is sandwiched between the two layers of proteins.
2) The molecules of phospholipids are arranged in the two parallel chains.
3) The polar hydrophilic heads of the phospholipid molecules are facing towards outer surface while hydrophilic tails lie toward the centre.
4) The protein layers are asymmetrical. On one side folded beta chain proteins are present while on the other side globular proteins are present.
Drawbacks :- a) this model fail to explain how the water and water soluble materials pass through cell membrane.
b) How the biomembranes are flexible.
c) How active transportation occurs through biomembranes.

Robertson Model :- J. David Robertson (1959) modified the model of Danielli and Davson. This model can be described in the following points:- 

a) The cell membrane consists of three layers - outer dark, middle light and inner dark. The outer and inner proteins layers and middle one is layer of phospholipids. 

b) According to this theory the cell membrane is a unit membrane, consisting of a biomolecular lipid layer between the two protein layers.

                                       

                                                    Robertson Model of cell membrane

c) Each protin layer is 20 angtsrom in thickness and the bimolecular lipid layer is 35 angstrom. 

                      It is simply the modification of sandwich model therefor it faces the same drawbacks as of sandwich hypothesis. 

Mosaic Model :- It is most recent model of biomembrane proposed by Singer and Nicolson in 1972.

According to this model the membrane dors not have a uniform disposition of lipids and proteins. Further the membrane is not solid but is quasifluid. 

                                      Fluid-mosaic model postulates that the lipid molecules are present in a viscous bilayer as in lipid layer. Protein molecules occur at place both inside and on the outer side of lipid bilayer --  Proteins icebergs in a sea of lipids. The internal proteins are called intrinsic or integral proteins while the external ones are known as extrinsic or peripheral proteins.The intrinsic proteins account for 70% of the total membrane proteins. Some of the intrinsic proteins run throughout the lipid bilayer. They are called tunnel proteins. The extrinsic proteins are present superficially on the two surfaces of the membrane. Many membranous proteins function as enzymes. Some of them behaves as permeases. Some lipids on the outer side possess small carbohydrate molecules to form glycolipids. 

Evidences in support of Fluid Mosaic Model:- 

1) The model provides the occurrence of protein particles both on the surface and interior of cell membranes. 

2) Fluid mosaic model can explain the presence of different types of permeability of cell membrane.

3) It accounts for dynamic nature of biomembranes.

4) The model explain the passage of both electrolytes and non-electrolytes.

                                       Fluid-mosaic model of a membrane