Plasma Membrane

 Cellular level of organization:  Introduction and Plasma Membrane

---

Introduction:

• Cell is defined as a basic structural and functional unit of the body.

• Cytology is a branch of science that deals with the study of cells.

• The animal cell is divided into two major parts as,

1. Plasma Membrane.

2. Subcellular organelles.

1. Nucleus.

2. Nucleolus

3. Mitochondria.

4. Golgi complex.

5. Ribosomes.

6. Endoplasmic reticulum.

7. Lysosomes.

8. Centrioles.

Plasma Membrane:

• Also called as a Cell membrane.

• It is a thin barrier made of lipids and proteins that separate internal environment of the cell and external environment.

• The basic function of cell membrane is to contain the contents of the cell and its protection.

• The cell membrane is “Semipermeable” in nature means it allows passage of certain materials while restricting others.

Figure 1.1 Structure of Cell Membrane

Composition of Plasma membrane.

• It has a sandwich-like structure: The plasma membrane consists of two layers of phospholipids (fatty substances) with some protein molecules embedded in them.

• The composition of the plasma membrane is as following,

• Protein (60-80%): Structural protein, carrier protein, enzymes, etc.

• Lipid (20-40%): Phospholipid, sterols, etc.

• Carbohydrates (4-5%): Oligosaccharides

• Water and minerals: Trace amount.

Membrane Lipids:

• They constitute almost 40% of the plasma membrane.

• The lipids part of the plasma membrane contains phospholipids, cholesterol and glycolipids.

• Phospholipids form a major part of the cell membrane; they constitute around 75% of the total lipids in the cell membrane.

• The phospholipids are amphoteric in nature i.e. they are divided into two parts 

• Polar heads: Hydrophilic (Water loving).

• Nonpolar tails: Hydrophobic (Water hating).

• Polar heads are exposed to the external environment while nonpolar tails are arranged at the center.

• Glycolipids represent 5 % part of the lipids and have one or more sugar molecules attached to them.

• Glycolipids are also amphoteric in nature.

• The remaining 20% of the lipid part is made by “Cholesterol”.

• Cholesterol is distributed throughout the lipid bilayer.

• Cholesterol offers strength to the plasma membrane but loses its flexibility.

• Plant cells don't contain Cholesterol.

Membrane Proteins:

• They constitute almost 60% of the plasma membrane.

• Proteins are responsible for the most of the functions of the cell membrane.

• There are two types of proteins in the plasma membrane,

• Integral Proteins.

• Peripheral proteins.

• Integral Proteins:

• They are completely embedded in the lipid bi layer.

• Also called as transmembrane proteins (Reaching both ends of the membrane).

• They are further classified as,

1. Channel Proteins: Transport water.

2. Carrier proteins: Transport solutes.

3. Receptor proteins: Binding site for chemicals and neurotransmitters.

4. Pumps: Transport of ions against concentration gradient.

• Peripheral Proteins:

• Also called “Extrinsic Proteins”.

• They do not penetrate cell membranes completely, like integral proteins.

• They remain attached to the phospholipids or integral proteins.

• They serve as cell adhesion sites, electron carriers etc.

Functions of Plasma Membrane:

1. The plasma membrane envelops the cytoplasm of living cells.

2. It separates the internal components of the cell from the external environment. 

3. It protects the internal structure of the cell and different organelles of the cytoplasm.

4. It maintains the shape of the cell.

5. It helps to attach the cell with the extracellular matrix;

6. It helps to regulate the transport of materials within the cells due to their selectively permeable characteristics.

7. The cell membrane also allows cell identification.

8. It also plays a role in endocytosis and exocytosis.

Commonly Asked Questions

1. Write a note on Cell membrane.

2. Explain in detail the structure of the plasma membrane.

3. Draw a well labelled diagram of the human cell.