final_membranes_presentation_grade11_AP_LP

Membranes Grade 11

5.1 The Structure of Membranes Learning Outcomes:

• Describe the components of biological membranes.

• Explain the fluid mosaic model of membrane structure.

• Describe the factors involved in membrane fluidity.

Key Concepts Components of Biological Membranes:

• Composed of 2 phospholipid sheets 5 to 10 nm thick.

• Not only lipids; also encase proteins and other molecular components.

Cellular Membranes:

• Consist of four component groups:

  • Cellular structures: Nucleus, Cytoplasm, Cell membrane.

  • Molecules: Carbohydrates, Glycoproteins, Proteins (Channel proteins, Cholesterol, Glycolipids, Surface proteins), Cytoskeleton (Integral and peripheral proteins)

Table 5.1: Components of the Cell MembranePhospholipid Bilayer:

• Composition: Phospholipid.

• Function: Provides permeability barrier.

• Mechanism: Excludes water-soluble molecules from proteins.

Transmembrane Proteins:

• Actively or passively transport molecules.

• Channel Proteins: Allow passive transport, creating selective tunnels.

• Receptor Proteins: Transmit information into the cell by binding signal molecules.

Fluid Mosaic Model of Membrane Structure Description: A dynamic arrangement of various proteins within the fluid lipid bilayer.

• Proteins float in or on the lipid bilayer similar to boats on water.

Phospholipid Structure

• Made of glycerol linked to two fatty acids and a phosphate group.

• The structure is amphipathic: polar head (hydrophilic) and nonpolar tails (hydrophobic).

Factors Affecting Membrane Fluidity

• Lipid Composition: Sphingolipids and phospholipids can affect membrane rigidity.

• Temperature: Increased temperature enhances fluidity; decreased temperature reduces it.

• Presence of Cholesterol: Cholesterol stabilizes membranes at moderate temperatures and prevents solidification at low temperatures.

Experiment: Membrane Fluidity Test Objective: Observe mixing of membrane proteins between mouse and human cells over time. Result: Over time, intermixed proteins in hybrid cells.

Membrane Transport Mechanisms Passive Transport:

• Simple Diffusion:

  • Movement of nonpolar molecules (e.g., oxygen, carbon dioxide) across the membrane down their concentration gradient without energy input.

• Facilitated Diffusion:

  • Involves specific protein channels and carriers allowing polar molecules (e.g., glucose, ions) to cross the membrane down their concentration gradient without energy input.

• Osmosis:

  • The diffusion of water through specific channel proteins called aquaporins, allowing cells to maintain osmotic balance by responding to solute concentrations.

Active Transport:

• Requires energy (usually from ATP) to move molecules against their concentration gradient (from low to high concentration).

• Sodium-Potassium Pump:

  • An example of active transport that maintains concentration gradients of Na+ and K+ across membranes by moving three sodium ions out of the cell and two potassium ions into the cell for each ATP molecule used.

• Other Active Transport Mechanisms:

  • Includes primary and secondary active transport where primary directly uses ATP, and secondary utilizes the energy from the electrochemical gradient created by primary active transport, such as the glucose-sodium symporter.

Endocytosis and Exocytosis Definitions:

• Endocytosis: Intake of materials by engulfing them in vesicles; can be further classified into phagocytosis (cell eating) and pinocytosis (cell drinking).

• Exocytosis: Removal of materials by vesicles fusing with the plasma membrane to release substances outside the cell, essential for processes like neurotransmitter release.

Summary of Key Components and Functions:

• Glycolipids and Glycoproteins: Role in cell recognition and signaling.

• Transport Proteins: Specificity in moving substances across membranes.

• Membrane Fluidity: Essential for membrane function, allowing proteins to move and function properly within the lipid bilayer.

• Aquaporins: Specialized channels for efficient water transport.

Questions for Review

• What are the main components of the cell membrane?

• How does the fluid mosaic model explain membrane structure?

• What factors influence membrane fluidity?

• Explain the differences between active and passive transport, including specific examples of each.