The cell membrane, also called as the plasma membrane or cell surface membrane, is the outermost borderline of animal cells. The cell membrane acts as a hurdle between the intracellular and extracellular fluids.
The cell membrane is selective, which allows only certain molecules to pass through it, and that’s why the cell membrane is also called a selectively permeable membrane.
The model of the structure of the cell membrane was presented by Singer and Nicholson in 1972, which is called the “Fluid Mosaic Model”. This model describes that the nature of the cell membrane is fluid type, which means that its molecules can easily move within it, and the proteins are irregularly arranged in it, hence, the term mosaic is used for the arrangement of proteins.
Cell membrane composition
The cell membrane is phospholipid bilayer that is composed of:
- Membrane lipids
- Membrane proteins
- Glycocalyx (glycoproteins and glycolipids)
Membrane lipids
Membrane phosphates: The outer membrane is made up of phosphatidylcholine and sphingomyelin while the inner membrane is composed of phosphatidylserine and phosphatidylethanolamine. Both of these sides are polar and hydrophilic.
Fatty acid chains: These are the thread-like structures coming out from phosphate heads which may be saturated or straight chain compounds and unsaturated or double bonded compounds. These are nonpolar and hydrophobic.
Cholesterol: The deposited structures in fatty acid chains are called cholesterols which play an important role in maintaining membrane fluidity.
Membrane proteins
These are the structures that are completely or partially embedded in the lipid bilayer.
Integral proteins: These are the specific types of transmembrane proteins that are largely embedded by covalent bonds in the whole cell membranes. These are called integral proteins or intrinsic proteins. They have the ability to interact with intracellular and extracellular fluids.
Peripheral proteins: Peripheral proteins are partially embedded in the phospholipid bilayer by hydrogen bonding. These are slightly positively charged due to which they can slightly interact with phosphate groups. These are also called extrinsic proteins.
Glycocalyx
Glycocalyx is a mesh-like structure present on the outer surface of the cell membrane. It includes glycoproteins and glycolipids.
Glycoproteins: These are some proteins having some sugar molecules especially oligosaccharides attached to them on the surface of the cell membrane.
Glycolipids: Glycolipids are lipid molecules having some sugar molecules attached to them.
Cell Membrane Functions
Membrane lipids
Membrane lipids help in maintaining the fluidity of the cell and help in transporting lipid-soluble molecules across the cell membrane.
1. Fluidity: The fluidity of the cell is its ability to adapt its shape and movement. Fluidity is greatly influenced by three main factors. These factors are temperature, cholesterol and type of fatty acids. Below is the detail of how these factors affect the fluidity of the membrane.
Temperature
- Fluidity increases with the increase in temperature. A rise in temperature causes the fatty acids to move away from each other which causes the membrane to move easily.
- Fluidity decreases with low temperature. Low temperature causes the molecules to get closer to each other and the membrane becomes rigid.
Cholesterol Cholesterol is responsible for maintaining the fluidity of the cell membrane.
- High cholesterol causes the fatty acids to come close to each other.
- Low cholesterol causes an increase in membrane fluidity.
Due to high temperature, when fluidity increases, the cholesterol molecules do not allow the fatty acids to move away from each other which is beneficial for maintaining membrane fluidity.
Similarly, when low temperature causes the molecules to get closer, cholesterol helps molecules to move away from each other due to which fluidity of the membrane is maintained.
Type of Fatty Acids
- In saturated fatty acids, due to straight chains, molecules can have spaces between them and fluidity may increase.
- In unsaturated fatty acids, due to double or triple bonds between the molecules, there is less space in molecules due to which fluidity may decrease.
2. Transport
Another function of membrane lipids is the transport of molecules across the membrane.
- Membrane lipids allow simple diffusion of small molecules. Oxygen, carbon dioxide, tiny molecules and steroid hormones are easily transported in and out of the cell.
- Phospholipids can easily travel along the cell membrane in a lateral pattern or they can flip-flop back and forth between the outer and inner membrane.
Membrane Proteins
1. Transport
Small molecules can easily transport through the phospholipid bilayer but larger molecules are unable to diffuse. Water molecules can also not enter due to the hydrophobic tail of fatty acids. Similarly, some polar molecules are not allowed to enter due to the nonpolar nature of fatty acids. Therefore, these molecules are transported inside and outside the cell through the membrane.
- Integral proteins or channel proteins allow large molecules or water-soluble molecules to move across the cell membrane. This process is referred to as facilitated diffusion.
- Peripheral proteins can move and attach to the cell membrane to release molecules to particles by exocytosis or they can bring some molecules inside the cell by endocytosis.
2. Enzymatic Function
Membrane proteins are also responsible for performing enzymatic functions. For example, reactants are converted into products by enzymes that speed up chemical reactions e.g. digestive enzymes present in small intestinal cells help in hydrolysis of disaccharides.
3. Cytoskeleton composition
Membrane proteins also take part in the composition of the cytoskeleton. For example, microtubules present in the cell get attached to peripheral proteins and a network is formed within the cell which is important for the proper shape and maintenance of the cell.
4. Cell Adhesion
Membrane proteins such as integral proteins on the outer surface of the cells adhere to the integral proteins on the outer surface of other cells and play an important function in cell adhesion.
5. Cell-to-Cell Communication
Membrane proteins also allow cell communication by transportation from one cell to the other by gap junctions, especially in muscle cells.
6. Hormonal Receptors
Membrane proteins play an important function in triggering the nucleus to produce specific proteins by acting as receptionists for hormones. These hormones attach to the surface of membrane proteins and activate the nucleus by a specific pathway. The nucleus then produces desirable proteins that perform specific functions.
Glycocalyx
1. Prevention From Dehydration
Glycocalyx regulates the movement of water inside and outside of the cell and helps prevent cell dehydration.
2. Cell Signaling
Glycolipids and glycoproteins can also act as receptionists for specific molecules. For example, hormones and neurotransmitters attach to the surface of glycolipids and glycoproteins due to which the cell gets activated and performs its functions with the help of enzymatic activities.
3. Antigenic Function
Glycocalyx allows the cells especially our immune cells to recognize harmful substances inside the body or foreign substances. The cells detect these substances and kill them. Hence, glycocalyx helps maintain cells’ health. For example, immune cells recognize the patterns of glycolipids and glycoproteins in the body cells. In the case of blood transfusion, if the blood from the donor does not match the antigens of the receiver, the immune cells kill the donor blood cells and a reaction starts in the body.
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