Cell Membrane – Structure & Function (NEET Biology Notes)

Chapter: Cell – The Unit of Life
Topic: Cell Membrane (Plasma Membrane)
Exam Focus: NEET (Very High Yield)

Introduction

The cell membrane, also called the plasma membrane, is a thin, living, flexible boundary that surrounds the cell.
It separates the internal environment of the cell from the external environment and regulates the movement of substances in and out of the cell.

The structure of the cell membrane is explained by the Fluid Mosaic Model, which is extremely important for NEET MCQs.

Fluid Mosaic Model (Singer & Nicolson, 1972)

The Fluid Mosaic Model states that the cell membrane is made of a fluid phospholipid bilayer with proteins embedded in it, giving a mosaic appearance.

Key Points

Proposed by Singer and Nicolson (1972)
Membrane is dynamic and flexible
Proteins can move laterally within the bilayer

📌 Most accepted model of plasma membrane (NCERT line)

Structure of Cell Membrane

1️⃣ Phospholipid Bilayer

Made of phospholipid molecules
Each phospholipid has:
- Hydrophilic (polar) head → faces outward
- Hydrophobic (non-polar) tail → faces inward
Forms a bilayer

2️⃣ Membrane Proteins

Proteins are of two types:

🔹 Integral (Intrinsic) Proteins

Embedded within the lipid bilayer
Act as channels, carriers, receptors

🔹 Peripheral (Extrinsic) Proteins

Present on the surface of membrane
Provide structural support

3️⃣ Carbohydrates

Present as glycoproteins and glycolipids
Found only on the outer surface
Function in cell recognition and cell adhesion

4️⃣ Cholesterol (in Animal Cells)

Present only in animal cell membranes
Maintains membrane fluidity
Absent in plant cell membranes

Properties of Cell Membrane

🔹 Selectively Permeable

Allows some substances to pass while restricting others
Small non-polar molecules pass easily

🔹 Fluid Nature

Lipids and proteins move laterally
Gives flexibility to the membrane

Transport Across Cell Membrane

1️⃣ Passive Transport

Occurs without energy (ATP)

a) Diffusion

Movement from higher to lower concentration
Example: Oxygen, Carbon dioxide

b) Facilitated Diffusion

Requires carrier proteins
No energy required

2️⃣ Active Transport

Movement against concentration gradient
Requires ATP
Uses carrier proteins

📌 Example: Sodium-potassium pump

3️⃣ Bulk Transport

a) Endocytosis

Entry of material into cell
Types:
- Phagocytosis (solid)
- Pinocytosis (liquid)

b) Exocytosis

Removal of material from cell

NEET Important Points ⭐

Fluid Mosaic Model → Singer & Nicolson
Glycoproteins & glycolipids → outer surface only
Cholesterol → animal cells only
Plasma membrane is living
Transport proteins are specific

Common NEET Traps ⚠️

❌ Cell membrane is not rigid
❌ Cholesterol is absent in plant cells
✔ Endocytosis occurs only in eukaryotic cells
✔ Facilitated diffusion does not require ATP

Quick Revision Box ⏱️

Plasma membrane = selectively permeable
Fluid Mosaic Model = Singer & Nicolson
Hydrophilic head → outside
Hydrophobic tail → inside
Cholesterol → animal cells
Endocytosis → eukaryotes

NEET Practice MCQs

Q1. Fluid Mosaic Model was proposed by:
A) Watson & Crick
B) Singer & Nicolson
C) Robert Hooke
D) Schleiden

Answer: B

Q2. Cholesterol is present in membrane of:
A) Plant cell
B) Bacterial cell
C) Animal cell
D) Fungal cell

Answer: C

Q3. Which transport does not require ATP?
A) Active transport
B) Endocytosis
C) Diffusion
D) Sodium pump