Cell Membrane and Plasma Membrane Difference
1.2 Cell wall and plasma Structure of Cell membrane
Cell wall
The cell wall is the outermost nonliving covering present in Plants, Algae, Fungi, and Prokaryotes. It is secreted by the protoplasm of the cell. Its thickness and composition vary in different groups of organisms. Here we will discuss the details of plants' cell walls.
Structure and composition of cell wall
The plant cell wall consists of three layers i.e. primary cell wall, middle lamella, and secondary cell wall. The primary cell wall is a true wall formed in developing cells. Some plant cells possess only primary cell walls such as leaves, storage cells, and young growing cells.
The primary cell wall is composed of cellulose, hemicellulose, and pectin. The outer part of the primary cell wall of the plant epidermis is usually impregnated with cutin and wax, forming a permeability barrier known as plant cuticle.
The cellulose microfibrils are arranged in a crisscross manner. The microfibrils are held together by hydrogen bonds to provide high tensile strength.
Middle lamella
It is the first layer that is deposited at the time of cell division between two adjacent cells. It is formed of sticky gel like magnesium and calcium salts of proteins which help to stick the neighbouring cells together.
Forme Secondary cell wall is thick between the primary cell wall and plasm membrane. The secondary cell wall is formed when the cell is fully grown. It is composed of cellulose hemicellulose and lignin which is used to strengthen the wall.
In the secondary cell wall, the microfibrils also have a cross arrangement Cells with secondary cell walls are rigid.
1.2.2 Plasma Membrane
It is the outermost living boundary of animal cells while in plant cells, it is always present after the cell wall. There are many other membrane-bound organelles, like mitochondria, and Golgi. bodies, Endoplasmic reticulum. All these membranes are chemically composed of 60- 80% proteins, 20 to 40% lipids, and small amounts of carbohydrates.
Fluid mosaic model:
This model of the plasma membrane was developed by Jonathan Singer and Garth Nicolson in 1972. According to this model, the plasma membrane is a fluid mosaic of protein, floating within a bilayer of phospholipid and cholesterol. The phospholipid molecule contains a hydrophilic head and two hydrophobic tails.
The hydrophobic tails face each other while hydrophilic heads are directed towards water which is present outside and inside the cell. The cholesterol molecules are embedded in the interior of the membrane which makes the membrane less permeable for water soluble It also provides stability to the plasma membrane.
Protein and intrinsic or embedded proteins (either wholly or partially embedded in bilayer). Some amount of carbohydrates are also present in the plasma membrane.
These may either attach with protein as glycoproteins or attach with lipids as glycolipids.
There are two kinds of membrane proteins, extrinsic or
The role of glycoproteins and glycolipids:
They provide receptor sites for hormones, nerve impulses, and recognition of antigens and are also responsible for endocytosis. Therefore, these are called cell surface markers. Cell-to-cell recognition, sticking, correct cells together. They are just like signboards in a shop.
Protein of plasma membranes:
Channel proteins and carrier proteins:
These are involved in the passage of molecules through the membranes. Some proteins have channels through which substances can move across the membrane while other molecules combine with carrier proteins to move across the membrane.
Enzymes:
Some proteins of the plasma membrane act like enzymes, e.g., the epithelial cells lining, and some parts of the digestive tract contain digestive enzymes on their cell surface membrane.
Receptor molecules:
Some proteins of the plasma membrane act as receptors e.g., hormones are chemical messengers, circulating in the blood but only bind to specific target cells that have the correct receptor site.
Antigens:
Antigens are glycoproteins that have shapes, so each cell can have its own specific marker. e.g., foreign antigens can be recognized to defend the cell.
0 Comments