Lecture on Cell Structure: Focus on Cell Membrane

Introduction

• Beginning a series on the physical structure of a cell, focusing on the cell membrane.
• The cell membrane, also known as the plasma membrane, forms the boundary of the cell.

Structure of the Cell Membrane

• Composed mainly of phospholipids and proteins.
• Phospholipid Bilayer: A double layer of phospholipid molecules, also known as a lipid bilayer.
  • Phospholipids have a polar head and hydrophobic fatty acid tails.
  • Aqueous Environment: Both extracellular and intracellular environments are aqueous, influencing the orientation of phospholipids.

Orientation and Function

• Hydrophilic heads face outward towards the aqueous environment, while hydrophobic tails face inward, avoiding water.
• The arrangement creates a permeability barrier:
  • Lipid-soluble substances and small molecules can pass through easily.
  • Large or charged molecules require special mechanisms.

Role of Cholesterol

• Intercalated between the phospholipids, it affects membrane fluidity.
• Has a polar end that is oriented outward.

Membrane Fluidity

• Fluid Mosaic Model: Phospholipids and proteins can move laterally, contributing to membrane fluidity.
• Factors Affecting Fluidity:
  • Temperature increase enhances fluidity.
  • Cholesterol decreases fluidity.
  • Unsaturated fatty acids increase fluidity due to kinks in their structure.

Asymmetry in Cell Membranes

• Phospholipid Composition: Different types are found in the outer and inner leaflets.
• Proteins and Carbohydrates: Outer leaflet often has carbohydrates, while inner has different proteins.

Protein Association in Cell Membranes

• Integral Proteins: Span the membrane completely (transmembrane proteins).
  • Function as channels, carriers, and receptors.
• Peripheral Proteins: Loosely attached to the membrane surface.
  • Often involved in signaling or structural support.

Integral Proteins and Their Functions

• Channels: Allow passage of water-soluble substances.
• Carriers: Transport substances across the membrane by changing shape.
• Enzymes: Facilitate chemical reactions.
• Linker Proteins: Connect intracellular cytoskeleton to extracellular matrix.
• Receptors: Bind to specific molecules (e.g., hormones) to trigger intracellular responses.

Peripheral Proteins

• Associated with the inner or outer surface.
• Can be part of the cytoskeleton or involved in signaling pathways.

Lipid Rafts

• Specialized membrane areas with tightly packed phospholipids and cholesterol.
• Often host to receptors and associated proteins, playing roles in signal transduction.

Conclusion

• Cell membranes are dynamic structures crucial for protecting cell integrity and facilitating communication and transport.
• Understanding the cell membrane's structure and function is foundational for further studies in cell biology.