Membrane Structure and Function

Introduction to Membrane Structure and Function

  • Focus on the plasma membrane surrounding the cell.

  • Every organelle consists of one or more phospholipid bilayers.

Key Concepts

1. Diffusion and Concentration Gradients

  • Definition of Concentration Gradient: Established when there is an area of higher concentration and an area of lower concentration.

  • Spontaneous Movement: Particles move from high to low concentration without energy, also termed passive.

  • Non-Spontaneous Movement: Movement from low to high concentration requires energy.

  • Real-world Example: Drop of dye in water, showing natural diffusion into a red tinge.

2. Selectively Permeable Membranes

  • Definition: Membranes allow certain materials to cross while restricting others.

  • Example of Solute vs. Water in a Beaker: Solute molecules may be too large to cross a selectively permeable membrane, while smaller water molecules can.

3. Types of Movement Across Membranes

3.1. Diffusion

  • Movement of matter from high to low concentration.

3.2. Osmosis

  • Definition: Diffusion of water across a selectively permeable membrane.

  • Does not require energy due to its passive nature.

4. Tonicity

  • Definitions:

    • Hypertonic: Solution with higher solute concentration.

    • Hypotonic: Solution with lower solute concentration.

    • Isotonic: Solutions with equal solute concentrations.

  • Tonicity and Water Movement:

    • Water moves from hypotonic to hypertonic areas.

    • Hypertonic environment causes cell shrinkage; hypotonic causes cell swelling.

5. Biological Implications of Tonicity

5.1. Animal Cells

  • Hypertonic Environment: Water moves out, cell shrinks, leading to potential death.

  • Isotonic Environment: No net change in mass; water moves in and out equally.

  • Hypotonic Environment: Water enters leading to potential bursting.

5.2. Plant Cells

  • Prefer hypotonic environment; water enters, filling the central vacuole and maintaining turgidity due to the cell wall.

  • Isotonic Environments: Plant cells become flaccid.

  • Hypertonic Environments: Causes plasmolysis (shrinking of the cell).

Dialysis

  • Definition: Diffusion of solute molecules across a selectively permeable membrane.

  • Example with proteins: Removal of salt from a protein solution using a dialysis bag.

Structure of Biological Membranes

1. Composition

  • Membranes consist of phospholipids arranged in a bilayer, often referred to as a fluid mosaic.

  • Contains not just phospholipids but also proteins, carbohydrates, and sometimes cholesterol.

2. Membrane Proteins

2.1. Types and Functions

  • Glycoproteins: Proteins with oligosaccharides attached, important for cell-cell recognition.

  • Glycolipids: Lipids with oligosaccharides attached.

  • Cholesterol: Stabilizes membrane fluidity in animal cells.

2.2. Functions of Membrane Proteins

  • Enzymes: Catalyze chemical reactions.

  • Receptors: Receive environmental signals.

  • Transport Proteins: Facilitate movement of materials across the membrane.

  • Intercellular Joining: Connects cells together.

  • Cell Recognition: Helps identify 'self' from 'non-self' (important for immune response).

Viral Interactions with Membranes

  • HIV: An enveloped virus with glycoproteins on the surface that interact with human cell receptors, allowing entry.

Membrane Fluidity and Structure

  • The composition of fatty acids in phospholipids (saturated vs. unsaturated) affects membrane fluidity.

  • Cholesterol helps to maintain the balance between rigidity and fluidity.

Transport Processes

1. Passive Transport

1.1. Simple Diffusion

  • Movement from high to low concentration without energy.

1.2. Facilitated Diffusion

  • Requires membrane proteins to help larger, polar molecules or ions cross the membrane passively.

    • Example: Channel proteins like aquaporins for water, carrier proteins for glucose.

2. Active Transport

  • Movement against the concentration gradient which requires ATP (energy).

  • Example: Proton pumps moving H⁺ ions out of the cell to create a gradient.

3. Secondary Active Transport (Cotransport)

  • Coupling the transport of one molecule with the movement of another molecule down its gradient to facilitate intake (e.g., sucrose-H⁺ cotransport in plant cells).

Bulk Transport

1. Exocytosis

  • Bulk transport of materials out of the cell.

2. Endocytosis

  • Bulk transport of materials into the cell.

2.1. Types of Endocytosis

  • Pinocytosis: Cell drinking, engulfing fluids and small solutes.

  • Phagocytosis: Cellular eating, engulfing large particles such as pathogens (e.g. macrophages).

  • Receptor-Mediated Endocytosis: Specific uptake of molecules, enabled by receptors (e.g. LDL uptake by cells).