What is in the cell wall of plant cells?

Answer 1

Plant cells have cell walls composed mainly of cellulose.

CELL MEMBRANE:- A thin, transparent, electron microscopic, elastic, regenerative and selectively permeable membrane, which forms the outermost covering of the cell organelles, called cell membrane or plasma membrane or plasma lamella. It lies just below the cell wall in the plant cell and forms outermost covering in the animal cell.

Chemical composition of cell membrane:- Chemically, cell membrane are lipoproteinous i.e. they are chiefly composed of lipids and proteins. Lipid constitutes (40-79) %, protein (20-70) %, carbohydrates (1-5) % and water 20%. Phospholipids are the major lipids present in the cell membrane. The other lipids are Sphignolipids, Glycerol, Sterol, etc. The proteins are globular, structural, carrier and enzymatic. Carbohydrates are branched or unbranched Oligosaccharides.

Some important structural model of cell membrane:-

A. Lamellar Model:-

i.Sandwich model:-This model was proposed by Danielli and Davson in 1935. This model was based on the physical and the chemical properties of the cell membrane. It states that: →cell membrane is a trilaminar structure having a phospholipid bilayer sandwiched by the two surfaces layer of the protein molecules.

→ protein molecules are globular structures and provide stability to the cell membrane. →each phospholipid molecules have hydrophilic heads and hydrophobic tails. → the hydrophilic head of phospholipid molecules faces towards the opposite direction associated with protein molecules, and the hydrophobic tails held together by Vander Waal Force.

ii.Unit membrane concept:-This model was proposed by Robertson in 1959. It is the improvement over Danielli and Davson model. It states that:- → all cell membrane have trilaminar structure. → the mucoid protein lie towards the external surface and the non-mucoid protein lie towards the internal surface. → the thickness of cell membrane is 75 Å having 35 Å phospholipid bilayer and 20 Å each protein layer.

The lamellar model has not explained: →permeability to the water →protein and lipid ratio →active transport →unit membrane concept.

B.Fluid mosaic Model: - It is the recent model of cell membrane proposed by Singer and Nicolson in 1972. It states that "plasma membrane is formed of protein icebergs in the sea of lipids". It's statements are:

→cell membrane is trilaminar and proteinous structure.

→phospholipid bilayer is fluidy in nature in which phospholipid molecules show: - transitional and flip-flop movement.

→each phospholipid molecules have two specific ends; ●hydrophilic head of glycerol ●hydrophobic tail of fatty acid.

→hydrophilic head of phospholipid molecules are directed oppositely, while the hydrophobic tails faces each other.

→proteins are globular and are of two types: - ●Extrinsic or peripheral proteins ●Intrinsic or integral proteins.

→carbohydrates (Oligosaccharides) are associated with outer surface of plasma membrane in the form of glycolipids or glycoproteins. They play important role in blood grouping, immune response, cancer and rejection of transplanted organs.

Functions of Cell membrane:- Cell membrane is selectively permeable membrane which has following functions:-

1. Cell adhesion:-

ü Compartmentalization: Cell wall help in the partition of the different cells, which is known as compartmentalization.

ü Bounding semi-fluid substances of the cell

2. Ion conductivity:-

ü Passive transport: It is a mode of membrane transport which occurs according to concentration gradient without any energy. It is of following types:-

*Osmosis: It is a process of movement of mater molecules from higher water concentration to lower water concentration through semi permeable membrane.

*Diffusion: It is the movement of ions or molecules from higher concentration to the lower concentration due to the tangency of molecules to spread uniformly. Both simple and facilitated diffusion occur through the cell membrane. E.g. Exchange of gases in cellular respiration (simple diffusion), entry of glucose into RBC through the agency of membrane protein (facilitated diffusion).

ü Active transport: It is a mode of membrane transport which is energy gradient and requires energy. E.g. Salt uptake by plant cell.

üEndocytosis: It is the process of active intake of material by membrane. It is of two types:

*Pinocytosis: It is the intaking process of large sized liquid particles through membrane. It is also known as cell-drinking.

*Phagocytosis: It is the intaking process of large sized solid particles through membrane. It is also known as cell-eating.

ü Exocytosis: It is the expelling of materials from cell through cell membrane. It is also known as cell-vomiting or Ephagy.

3. Cell signaling: - Cell signaling is a part of a complex system communication that governs basic cellular activities and coordinates cell actions. The ability of cells to perceive and correctly respond to their microenvironment is the basis of development, tissue repair, and immunity as well as normal tissue homeostasis. Errors in the cellular information processing are responsible for diseases such as cancer, autoimmunity and Diabetes. By understanding cell signaling, diseases may be treated effectively and, theoretically, artificial tissues may be created.

4. Locomotion:- The extension and folding of cell membrane help in the cellular pseudopodial locomotion in Amoeba, W.B.C. of blood, macrophages (phagocytes),etc. and undulatory locomotion shown by some mammalian cells such as fibroblasts.