IB Bio: Unit 2-Cellular Membrane

Cell Membrane Structure and Function

Introduction to Cell Membrane

  • Yeast and algae have cell walls, but animal cells, including humans, do not.

  • Humans have a cell membrane which plays critical roles in cellular function.

  • The membrane separates the interior and exterior environments, allowing for higher concentrations of chemicals inside the cell.

Importance of the Cell Membrane

  • Thin Structure: Appears as a black line under standard microscopes; detailed structure visible via electron microscopy.

  • Selective Permeability: Controls the movement of substances in and out.

    • The hydrophobic center of the membrane is a key feature that restricts movement and provides selective permeability.

Composition of the Cell Membrane

  • Phospholipids: The fundamental building blocks of the membrane with hydrophilic heads and hydrophobic tails.

    • The fluid mosaic model describes the membrane as a mosaic of varied components moving fluidly.

  • Types of Phospholipids:

    • Saturated Phospholipids: Straight tails, can lead to rigidity and cell damage.

    • Unsaturated Phospholipids: Include kinked tails which enhance fluidity.

Fluid Mosaic Model

  • Mosaic Nature: Composed of different types of molecules (like proteins) embedded within a phospholipid bilayer, resembling a mosaic pattern.

  • Fluidity: The components, including phospholipids and proteins, can move laterally, which allows for flexibility in membrane structure.

Role of Cholesterol in the Membrane

  • Embedded cholesterol maintains membrane integrity and fluidity, especially in animals, preventing solidification at room temperature.

  • Balance of saturated and unsaturated phospholipids with cholesterol is crucial for maintaining appropriate cell permeability.

Proteins in the Cell Membrane

  • Types of Membrane Proteins:

    • Peripheral Proteins: Located on the surface, involved in signaling and recognition.

    • Integral Proteins: Span the membrane, with hydrophilic and hydrophobic regions; act as transport proteins.

Functions of Membrane Proteins

  1. Transport Proteins: Facilitate the passage of substances across the cell membrane.

    • Channel Proteins: Provide openings for molecules to pass through.

    • Carrier Proteins: Change shape to transport specific molecules across the membrane.

  2. Enzymatic Proteins: Catalyze biochemical reactions necessary for cell function.

  3. Receptor Proteins: Bind signaling molecules (e.g., hormones) to trigger responses in the cell.

  4. Recognition Proteins: Help cells distinguish self from non-self, crucial for immune response.

  5. Adhesion Proteins: Form tight junctions between cells to prevent leakage and maintain tissue integrity.

Conclusion of Membrane Dynamics

  • The cell membrane's capacity to be semi-permeable is largely determined by its composition and structural arrangement, allowing it to respond and adapt to environmental changes.

  • Maintaining a balance of fluidity and selectivity is vital for cell viability.