plasma membrane structure

What Is the Plasma Membrane?

• The plasma membrane, also called the cell membrane, surrounds all cells.

• The same membrane structure is found around membrane-bound organelles such as:

  • Smooth and rough endoplasmic reticulum

  • Golgi apparatus

  • Lysosomes and vesicles

  • Double-membrane organelles like the nucleus, mitochondria, and chloroplasts

Why Are Membranes Important?

Membranes perform several essential functions:

📡 Communication & Signalling

• Enable communication between cells and with the external environment

• Allow cells to respond to hormones and signals

🚧 Compartmentalisation

• Create separate internal environments (e.g. nucleus vs cytoplasm)

• Allow specialised conditions and concentration gradients

🚪 Selective Permeability

• Partially permeable: controls the entry and exit of substances

• Maintains homeostasis by regulating what enters and exits the ce

The Fluid Mosaic Model

The fluid mosaic model describes the structure of the cell membrane:

• Fluid: Phospholipids in the bilayer are constantly moving

• Mosaic: Proteins are randomly embedded like tiles in a mosaic

Phospholipid Bilayer

The main component of the membrane is the phospholipid bilayer.

Structure of the phospholipid bilayer

• Each phospholipid has a:

  • Hydrophilic (water-attracting) head

  • Hydrophobic (water-repelling) tail

• Phospholipids arrange themselves into a bilayer:

  • Heads face outwards towards water

  • Tails face inwards, away from water

What Can & Can’t Pass Through the phospholipid bilayer

• Cannot pass through:

  • Water-soluble substances

  • Charged ions (e.g. Na⁺, Cl⁻)

  • Polar molecules

• Can pass through:

  • Small, non-polar molecules (e.g. O₂, CO₂)

  • Lipid-soluble substances (e.g. steroid hormones)

Proteins in the Membrane

Membrane proteins play key roles in transport, signalling, and recognition.

1⃣Channel & Carrier Proteins

• Help charged or large polar molecules cross the membrane

• Involved in:

  • Facilitated diffusion

  • Active transport

2⃣Peripheral Proteins

• Sit on the inner or outer surface of the membrane

• Receptor proteins can bind to:

  • Hormones

  • Signalling molecules

• Trigger responses inside the cell

3⃣Glycoproteins

• Proteins with carbohydrate chains attached

• Functions:

  • Antigens for cell recognition (e.g. immune response)

  • Cell signalling

Lipids in the Membrane

Glycolipids

• Phospholipids with carbohydrate groups

• Similar in function to glycoproteins:

  • Cell recognition

  • Signalling

  • Adhesion

2⃣ Cholesterol

• A type of lipid that fits between phospholipids

• Functions:

  • Restricts movement of phospholipids, making the membrane less fluid

  • Increases membrane stability, especially in animal cells (which lack cell walls)

  • Vital for free-floating cells like:

    • Red blood cells

    • White blood cells

  • Helps them maintain shape while moving through the bloodstream

Key Features of the Plasma Membrane

Component	Role
Phospholipids	Form the bilayer, control basic permeability
Proteins	Transport, signalling, communication
Glycoproteins	Recognition, immune function, cell adhesion
Glycolipids	Recognition, adhesion, signalling
Cholesterol	Membrane stability and fluidity control, especially in animal cells