Membrane+Function

Overview of Membrane Function

• Discussed aspects:

  • Relationship between structure and function

  • Transport mechanisms: Active and Passive

  • Importance of cellular compartmentation

  • Cell recognition

Key Concepts from Previous Session

• Membrane lipid composition

• Membrane lipid synthesis

• Regulation of membrane fluidity

• Membrane proteins

Major Functions of Membranes

Plasma Membrane

• Acts as a barrier and facilitates transport and signal transduction.

Mitochondria

• Inner and outer membranes serve for energy transduction.

Endoplasmic Reticulum (ER)

• Rough ER: Translation and protein processing.

• Smooth ER: Synthesis of complex lipids.

Golgi Apparatus

• Post-translational modification and processing for secretion.

Nuclear Membranes

• Support attachment of chromatin.

Lysosomes

• Contains hydrolytic enzymes for digestion.

Peroxisomes

• Involved in fatty acid oxidation.

Chemical Composition of Cell Membranes

• Variations in different cell types:

  • Myelin: 18% protein, 79% lipid, 3% carbohydrate

  • Erythrocyte Plasma Membrane: 49% protein, 43% lipid, 8% carbohydrate

  • Hepatocyte Plasma Membrane: 54% protein, 39% lipid, 7% carbohydrate

  • Outer Mitochondrial Membrane: 50% protein, 46% lipid, 4% carbohydrate

Membrane Permeability

Characteristics

• Lipid soluble molecules can move through membranes based on concentration (simple diffusion).

• Molecule types and permeability:

  • Hydrophobic molecules

  • Small uncharged or polar molecules (e.g., H2O, Urea, Glycerol)

  • Ions (e.g., Na+, K+)

Membrane Transport Mechanisms

Passive Transport

• No energy required; follows a concentration gradient.

• Types:

  • Nonmediated

  • Carrier mediated (facilitates diffusion)

Active Transport

• Requires energy to move substances against concentration gradient.

• Involves specific transport proteins.

Carrier-Mediated Transport

Comparison: Facilitative vs. Simple Diffusion

• Simple diffusion:

  • Driven by concentration gradient

  • Energy requirement: No

  • Specificity: No

  • Speed: Slow

  • Capacity: No limit

• Carrier-mediated:

  • Driven by concentration gradient

  • Energy requirement: No

  • Specificity: Yes

  • Speed: Fast

  • Capacity: Can be saturated

Glucose Transporters

• Types of glucose transporters:

  • GLUT1: Basal glucose uptake, Kt = 1 mM (all tissues)

  • GLUT2: In liver and pancreatic B cells, Kt = 15-20 mM

  • GLUT3: Basal glucose uptake, Kt = 1 mM (all tissues)

  • GLUT4: Insulin dependent, Kt = 5 mM (muscle and fat cells)

  • GLUT5: Primarily transports fructose (small intestine)

Mechanism of Carrier-Mediated Transport

• Selective transport of D-glucose only, not L-glucose.

• GLUT4 transporter numbers increase due to insulin stimulation.

Summary of Carrier-Mediated Facilitative Diffusion

• Driven by concentration gradient; gradients maintained by phosphorylation.

• Bidirectional transport with selective transporters; they can become saturated.

• Some transporters undergo hormonal regulation.

Active Transport Mechanisms

• Example: Sodium ion transport

  • Na+ concentration outside cell: 143 mM

  • Na+ concentration inside cell: 14 mM

• Establishment of Na+ gradient requires energy (ATP).

Mechanisms of Active Transport

• Electrochemical gradients for Na+ and K+ ions.

• Illustrates energy needs and transport specificity.

Pharmacological Example: Digitalis

• Digitalis inhibits the Na+/K+ pump affecting cardiac contractility.

Sodium-Dependent Glucose Transporters

• Mechanism includes Na+-glucose symporters (SGLUT-1 and 2) with Na+ gradients crucial for function.

Membrane Key Facts

• Membranes are semi-permeable, supporting passive and active transport mechanisms.

• Passive transport follows concentration gradients, while active transport is energy-consuming.

Cellular Asymmetry

• Maintained by tight junctions and specific transporters in epithelial cells.

• Ensures proper directionality of transport in polarized cells.

Re-Hydration Therapy Strategy

• Simple formula: 8 teaspoons of sugar, 1 teaspoon salt, 1 litre of water targets sodium-glucose co-transport.

Gated Channels

• Transmembrane proteins facilitating ion movement, activated by voltage, ligands, or phosphorylation.

Disease Association with Transport Proteins

• Cystic fibrosis linked to chloride channel issues.

Additional Membrane Functions

• Tight junctions maintain asymmetry and separation of metabolic processes within cells.

• Importance in targeting enzymes to specific cell organelles, as seen with I-cell disease.

Compartmentalization in Cells

• Intracellular membranes define local environments, support reactions, and allow for electrochemical gradients.

• Ensure that reactants and enzymes are either brought together or separated based on need.

Transport Proteins and Diseases

• Understanding transport function is critical for disease research and treatment strategies.