Cell Membrane 

The cell membrane, also known as the plasma membrane, is a vital structure in all living cells. It acts as a selective barrier, controlling the movement of substances in and out of the cell. Understanding the structure and function of the cell membrane is essential in NEET Biology, as it plays a key role in maintaining cellular integrity and communication.

1.0Introduction

• The cell membrane is a thin, flexible layer that surrounds the cytoplasm of every cell. 
• It separates the cell's internal components from its external environment and maintains homeostasis.
• It is found in both plant and animal cells — in plant cells, it lies just beneath the cell wall, while in animal cells, it forms the outermost boundary.

2.0Discovery of the Cell Membrane

• The existence of a cell membrane was first suggested by Charles Overton in 1895, who proposed that it is made of lipids. 
• Later, Gorter and Grendel (1925), Davson and Danielli (1935), and Singer and Nicolson (1972) contributed to the understanding of its structure.
• The Fluid Mosaic Model, proposed by Singer and Nicolson, is the most widely accepted model today.

3.0Cell Membrane Structure

• According to the Fluid Mosaic Model, the cell membrane is composed of a phospholipid bilayer with proteins embedded in it. 
• Lipids and proteins move freely within the membrane, making it flexible and dynamic.

4.0Main Components of the Cell Membrane

Phospholipids:

• Form the basic structure of the membrane.
• Each phospholipid molecule has a hydrophilic (water-loving) head and a hydrophobic (water-repelling) tail.
• These molecules arrange themselves in a bilayer, with heads facing outward and tails inward.

Proteins:

• Embedded within or attached to the lipid bilayer.
• Integral proteins span the membrane, while peripheral proteins are found on the surface.
• They assist in transport, signal transmission, and cell recognition.

Carbohydrates:

• Present as glycolipids or glycoproteins on the outer surface.
• Help in cell recognition and communication.

Cholesterol:

• Found in animal cell membranes.
• Maintains fluidity and stability of the membrane by preventing it from becoming too rigid or too fluid.

Also Read: Cell Structure and Function

5.0Functions of Cell Membrane

The cell membrane performs several crucial functions essential for the survival of the cell.

Selective Permeability

• The cell membrane allows only specific molecules to pass through, while blocking others.
• This selective transport maintains the proper balance of nutrients, ions, and water inside the cell.

Protection and Support

• Acts as a protective barrier, preventing harmful substances from entering the cell.
• Provides structural support and helps maintain the cell’s shape.

Transport of Materials

Substances move across the cell membrane through:

• Passive Transport (no energy required): Diffusion, osmosis, facilitated diffusion
• Active Transport (energy required): Uses ATP to move substances against the concentration gradient

Cell Communication

• The membrane contains receptor proteins that detect and respond to external signals such as hormones or nutrients.

Cell Recognition

• Glycoproteins and glycolipids act as identification tags, helping the immune system recognize the body’s own cells.

Endocytosis and Exocytosis

• The cell membrane helps the cell take in materials (endocytosis) and release substances (exocytosis).
• Example: Absorption of nutrients and release of waste products.

6.0Types of Transport Across the Cell Membrane

7.0Characteristics of the Cell Membrane

• Flexible and dynamic – adapts to changes in the cell’s environment
• Semi-permeable – allows selective movement of substances
• Self-healing – can repair minor damages automatically
• Fluid nature – due to the movement of lipids and proteins within the layer
• Maintains equilibrium – regulates homeostasis by balancing internal and external conditions

8.0Difference Between Cell Membrane and Cell Wall