Plasma Membrane Structure and Functions | Free Biology Notes

In this article we will discuss about:- Plasma Membrane Structure, Function of Plasma Membrane and Transport through Plasma Membrane

What is Plasma Membrane?

• Plasma membrane is thin, elastic semi permeable living membrane that serves as boundary for the cytoplasm
• Plasma membrane was coined by Nageli in 1855
• Also known as Cell membrane or plasmalemma
• Made up of protein and phospholipids
• Selectively permeable in nature
• Dynamic membrane

Plasma Membrane Structure

• Three important models explaining the structure of plasma membrane are
  1. Sandwiched model
  2. Unit membrane model
  3. Fluid mosaic model

1. Sandwiched Model

• In 1935, proposed by Davson & Danielli
• Oldest model on the structure of plasma membrane
• Plasma membrane is made up of three layers
• Outer protein layer, middle lipid layer & inner protein layer
• Proteins are alpha globular
• Lipids are amphipathic
• Lipids arranged in bilayer in such a manner that tails face each others
• Van der vaal force helping to 2 layers stay together
• Proteins on either side of phospholipids bilayers

2. Unit Membrane Model

• In 1959, Proposed by Robertson
• Plasma membrane is made up of three layers
• Outer protein layer, Middle lipid layer & inner protein layer
• Proteins are beta fibers
• Thickness of plasma membrane is 7.5nm (75Å)
• Lipids are amphipathic
• Lipids arranged in bilayer in such a manner that tails face each others
• Van der vaal force helping to 2 layers stay together
• Proteins on either side of phospholipids bilayers

3. Fluid Mosaic Model

• In 1972, Proposed by Singer & Nicolson
• Made up of protein and phospholipids
• Proteins are alpha globular & two types

I) Extrinsic Proteins

• Arranged on surface of lipid heads
• Loosely attached

II) Intrinsic Proteins

• Deeply embedded
• Tightly bound to lipids

Phospholipids have two types of movements

I) Transition

• Movement of phospholipids molecules in the same layer

Ii) Flip Flop

• Movement of phospholipids molecules between two layers
• Protein iceberg in a sea of phospholipids

Function of Plasma Membrane

a) Mechanical Support

• It gives shape to the cell
• Protects all cell contents

b) Exchange of Materials

• Regulate the exchange of materials
• It allows need materials to enter the cell
• It Send out unwanted materials from the cell

c) Endocytosis

• Engulfing of food or foreign particles through the
• Plasma membrane
• Endocytosis differentiated into two types

I) Phagocytosis

• Engulfing of solid particles through the plasma membrane
• g. Amoeba & WBC

II) Pinocytosis

• Engulfing of fluid particles through the plasma membrane
• E.g. Epithelial cells of intestine

d) Exocytosis

• Process of exudating secretary products from the cells to outside of the cytoplasm
• E.g. Pancreatic cells

[ Also Read: Cell Wall Structure and Function ]

Transport through Plasma Membrane

1. I) Passive Transport

• No need ATP
• Movement of substance according to concentration gradient
• Higher concentration to lower concentration

Osmosis: Movement of solvent through selectively permeable membrane

Diffusion: Movement of molecules through selectively permeable membrane

Facilitated diffusion: Movement of molecules through transport protein in plasma membrane

Ii) Active Transport

• Need ATP
• Movement of substance against to concentration gradient
• Lower concentration to Higher concentration
• E.g. Na+/K+ Pump and Ca Pump etc.