Plasma Membrane (Cell Membrane) or Plasmalemma is the outermost layer of the animal cell while in a plant cell; it is present inside of the cell wall. It consists of proteins and phospholipids.
in other words, the nucleus and cytoplasm in all types of cells are enclosed in a membrane called a cell membrane.
7 Functions of Plasma OR Cell Membrane
Plasma OR Cell Membrane performs the following functions:
- Transport of Material
The transport of material is an important function of the plasma membrane. It forms a barrier between the cell contents and the outer environment.
- Differentially permeable membrane
The plasma membrane allows only selective substances to pass through it. So, it is called a differentially permeable or selectively permeable membrane.
- Maintenance of concentration gradient
The plasma membrane regulates the flow of materials and ions. So it maintains a definite concentration gradient. Lipid soluble substances easily cross the membrane. Similarly, many small gas molecules, water, glucose, etc. are neutral compounds.
So they easily cross the membrane. While ions like Na+ and K+ are charged particles. So, they cannot cross the membrane easily. Thus, there is an unequal distribution of substances inside and outside the cell.
- Passive transport OR Diffusion
The unequal distribution of substances causes passive transport or diffusion. “The movement of molecules from the area of higher concentration to the area of low concentration is called diffusion or passive transport.”
The movement of molecules from lower concentration to higher concentration by the expenditure of energy is cell active transport. This transport takes place against the concentration gradient. The energy used for this movement is provided by ATP.
The intake of food material by in-folding of the membrane in the form of the vacuole is called Endocytosis. It takes place in many animal cells. There are two types of Endocytosis:
- Phagocytosis: or cell eating. In this case, solid particles are engulfed by the in-folding of the membrane.
- Pinocytosis: or cell drinking. In this case, the liquid material is taken inside by in-folding.
- Transmission of Nerve Impulses
The cell membranes of the neurons (nerve cells) transmit the nerve impulses from one part of the body to the other part. It keeps coordination in the body.
Structure of Plasma Membrane or Cell Membrane
The structure of cell membrane was proposed by two scientists, Singer and Nicholson. Their structure is called the Fluid Mosaic Bilayered mode of the cell membrane. Different biologists proposed different structures of the plasma membranes. These are as follows:
According to this model, the cell membrane is composed of lipid bilayers. These lipid bilayers are sandwiched between the inner and outer layers of protein. This basic structure is called a unit membrane. This unit membrane is present in all the cellular organelles like mitochondria, and Golgi bodies. Modern technology could not prove this model.
According to this model, the proteins do not form a continuous layer. It is not present only on the outer surface. Proteins are embedded in the lipid bilayers in a mosaic manner. The fluid mosaic model was proposed on the basis of this discovery. This is the most acceptable model. The cell membrane contains charged pores. The movement of material takes place through these charge pores by diffusion or by active transport.
The cell membrane has a Bilayered structure i.e. it consists of two layers of Phospholipids, an upper layer, and a lower layer. The phospholipids form the major parts. Each layer consists of two parts.
- Hydrophilic End
It is the outer end of two layers also called Polar End and is considered water-soluble. These are situated at the upper and lower parts of the cell membrane.
- Hydrophobic End
These are non-polar and water-insoluble. These are present on the inner side and are present opposite to each other.
The cell membrane is chemically composed of lipids and proteins:
Proteins are 60% to 80%
Lipids are 20% to 40%
A small amount of carbohydrates is also present.
The lipid molecules are found in a fluid condition, rotating and moving within the membrane. These lipids also contain proteins which are of two types;
- Extrinsic Proteins (peripheral proteins)
These proteins are present along the surface of the lipids. They have a loose attachment to the membrane surface.
- Intrinsic Proteins (integral proteins)
These proteins are found deep in the lipid layer. They help in the movement of water-soluble ions outside or inside the cell.
The proteins are associated with lipids, called lipoproteins, or associated with carbohydrates are called glycoproteins. The proteins are found like a mosaic within the cell membrane, so the model is a celled fluid mosaic model.