What is Plasma Membrane & Cell wall? | The Cell | Biology

Plasma membrane or cell membrane is the outer most boundary of the cell. However, in most plant cells, it is covered by a cell wall. Cell membrane is chemically composed of lipids and proteins; 60 – 80% are proteins, while 20-40% are lipids. In addition, there is a small quantity of carbohydrates.

Many biologists contributed to establish the structural organization of cell membrane. It was proposed earlier that cell membrane is composed of lipid bilayer sandwiched between inner and outer layers of protein. This basic structure is called the unit membrane and is present in all the cellular organelles. The modern technology has revealed that lipid bilayers are not sandwiched between two protein layers.

The protein layers are not continuous and are not confined to the surface of the membrane but are embedded in lipid layers in a mosaic manner. This discovery led to the proposal of Fluid Mosaic Model. This model at present in the most accepted one. Cell membrane also contains charged pores through which movement of materials take place, both by active and passive transport.

Fluid Mosaic Model

Transport of materials is one of the vital roles it plays for the cell. It offers a barrier between the cell contents and their environment, allowing only selective substances to pass through it, thus it is known as differentially permeable or selectively permeable membrane. The substances which are lipid soluble cross it more easily than others, therefore, it regulates the flow of materials and ions to maintain a definite gradient. Many small gas molecules, water, glucose etc. being neutral can easily cross while ions, being charged particles, have some difficulty in crossing. Many substances which are not needed, constantly enter the cell by passive transport, others are taken up against the concentration gradient. This uphill movements of materials requires energy and is termed as active transport. The energy used for this movement is provided by ATP.

In many animal cells, cell membrane helps to take in materials by infoldings in the form of vacuoles. This type of intake is termed as endocytosis which can be either phagocytosis or pinocytosis. In neurons the cell membrane transmits nerve impulses from one part of the body to the other to keep coordination.

CELL WALL:

The outer most boundary in most of the plant cells is cell wall. The cell wall of plant cell is different from that of prokaryotes, both in structure and chemical composition. It is secreted by the protoplasm of the cell. its thickness various in different cells of the plant. It is composed of three main layers: primary wall, secondary wall and the middle lamella. The middle lamella is first to be formed in between the primary walls not the neighboring is first to be formed in between the primary the primary walls of the neighboring cells. The primary wall is composed of cellulose and some deposition of pectin and hemicellulose. Cellulose molecules are arranged in a criss cross arrangement. The primary wall is true wall and develops in newly growing cells. The secondary wall is formed on its inner surface and is comparatively thick and rigid. Chemically it is composed of inorganic salts, silica, waxes, cutin lignin etc. Prokaryotic cell walls lack cellulose; its strengthening material is peptidoglycan or murein. Fungal cell wall contains chitin.

Secret of the strength of plant structure is revealed by electron microscope photographs of the cell walls. The cellulose fibers are arranged in layers, with the fibers of each layer at the right angle of those layers.

Cell wall is very important. It provides a definite shape to the cell and keeps it rigid. It does not act as a barrier to the materials passing through it.