Edexcel A Level (IAL) Biology -2.2 Cell Membranes- Study Notes- New Syllabus

Structure & Properties of Cell Membranes

🌱 Introduction

Every cell is surrounded by a cell membrane (plasma membrane) that controls what enters and leaves the cell. It’s often described as “partially permeable” allowing some substances through while blocking others.

🧩 (i) Structure and Properties of Cell Membranes

Basic Structure

The cell membrane is mainly made of:

• Phospholipids
• Proteins
• Cholesterol
• Carbohydrates (glycolipids & glycoproteins)

Phospholipid Bilayer

• Each phospholipid molecule has:
  • Hydrophilic head → water-loving phosphate group
  • Hydrophobic tails → two fatty acid chains (water-fearing)
• In water, phospholipids form a bilayer:
  • Heads face outwards (towards water)
  • Tails face inwards (away from water)
• Creates a selectively permeable barrier → allows small, non-polar molecules (O₂, CO₂) to diffuse but blocks ions & large polar molecules.

Proteins in the Membrane

• Intrinsic (Integral) proteins → span the bilayer → act as channels or carriers for transport.
• Extrinsic (Peripheral) proteins → attached to surface → act as receptors or enzymes.

Functions of membrane proteins:

• Transport of substances (channels/carriers)
• Cell recognition (glycoproteins act as markers)
• Receptor sites for hormones or neurotransmitters
• Enzyme activity on membrane surfaces

Cholesterol

• Fits between phospholipids in animal membranes.
• Functions:
  • Adds stability (prevents excessive fluidity/rigidity)
  • Reduces permeability to small water-soluble molecules

Carbohydrate Chains

• Attached to lipids → glycolipids or to proteins → glycoproteins.
• Functions: cell recognition, cell signalling, protection.

⚙️ Key Properties of Cell Membranes

🔬 (ii) The Fluid Mosaic Model of Membrane Structure

Proposed by: Singer & Nicolson (1972)

Description

The membrane is a fluid phospholipid bilayer with proteins floating within it – like tiles in a mosaic.

• “Fluid” → lipids & proteins move laterally within the layer.
• “Mosaic” → proteins scattered irregularly, not in a fixed pattern.

Evidence & Interpretation

• Earlier models (e.g., Davson-Danielli) suggested proteins coated both sides of the bilayer.
• Freeze-fracture electron microscopy → showed proteins embedded within the bilayer.
• Fluorescent labeling experiments → proteins move freely → confirmed fluidity.
• X-ray diffraction & biochemical studies → supported bilayer structure with embedded proteins.

Hence, scientists updated the model → Fluid Mosaic Model a more accurate, flexible representation.

📘 Summary Table

🧠 Quick Recap 
Cell membrane = phospholipid bilayer + proteins + cholesterol + carbohydrates
Selectively permeable → allows small non-polar molecules
Fluid Mosaic Model → proteins float in a sea of lipids
Evidence: freeze-fracture & fluorescent labeling
Functions → transport, recognition, communication, stability