Membrane+Structure

Membrane Structure and Synthesis

  • Professor G. Whitley

  • Email: g.whitley@sgul.ac.uk

  • Key Features of Membranes:

    • Flexible

    • Self-sealing

    • Selectively permeable

  • Functionality:

    • Transport

    • Cell recognition

    • Cellular communication

    • Metabolic regulation

  • Define boundaries between cells and within cells

Biological Membrane Bilayers

  • Composed largely of lipids and proteins

  • Lipids are amphipathic (hydrophilic and hydrophobic parts)

  • Form non-covalent assemblies:

    • Hydrophilic head

    • Hydrophobic tail

  • Phospholipids assemble spontaneously

Lipid Composition

  • Key Lipids:

    • Choline

    • Serine

    • Ethanolamine

    • Inositol

  • Structure:

    • Polar head group

    • Hydrophilic head

    • Hydrophobic tail

    • Phospholipid consists of phosphate, glycerol, and fatty acids

  • Other Important Lipids:

    • Cholesterol

    • Glycolipid

    • Sphingomyelin

Membrane Asymmetry

  • RBC Plasma Membrane Composition:

    • PC = Phosphatidylcholine

    • PS = Phosphatidylserine

    • PE = Phosphatidylethanolamine

    • PI = Phosphatidylinositol

    • Cl = Cholesterol

    • SM = Sphingomyelin

  • Sphingomyelin: A phospholipid with sphingosine instead of glycerol backbone

Membrane Synthesis

  • Locations:

    • Lipids synthesized in ER, Golgi, Cytosol

    • Newly synthesized phospholipids

  • Transfer Mechanisms:

    • Floppase: Moves phospholipids from inner to outer leaflet

    • Flippase: Moves phospholipids from outer to inner leaflet

    • Scramblase: Bidirectional movement

  • Requires ATP for active transport (e.g., ABC transporters)

Membrane Key Facts

  • Lipids are amphipathic; major classes: phospholipids, glycolipids, and cholesterol

  • Lipids spontaneously form bilayers in aqueous solutions

  • Phospholipid synthesis occurs on the cytosolic surface of the endoplasmic reticulum

  • Lipid distribution involves flippase and floppase enzymes

Membrane Fluidity

  • Lateral Movement: Rapid

  • Transverse Movement: Slow, requires enzymatic action

  • Factors Affecting Fluidity:

    • Temperature

    • Fatty acid composition

    • Chain length

    • Degree of saturation

    • Cholesterol content

Cholesterol's Effect on Fluidity

  • Low Temperature: Increases fluidity

  • High Temperature: Decreases fluidity

  • Increases cholesterol content in spur cell anemia leads to reduced membrane fluidity (by 25-65%)

Fluid Mosaic Model

  • Membrane Proteins:

    • Flexible, self-sealing, selectively permeable

    • Define internal boundaries in cells

Integral Membrane Proteins

  • Types:

    • Single or multi-pass through the membrane

    • Established through strong non-covalent bonds

    • Often form a-helical transmembrane domains

    • Predicted from sequence analysis

Peripheral Membrane Proteins

  • Located on both extracellular and cytosolic sides

  • Interact through non-covalent bonds

Lipid-Attached Membrane Proteins

  • Covalently linked to lipids, such as glycosyl-phosphatidylinositol (GPI)

Membrane Carbohydrates

  • Associated with both lipids and proteins

  • Comprise 2-10% of the membrane weight

  • Primarily externally facing, involved in:

    • Cell-cell interactions

    • Cellular recognition

  • Blood group antigens are glycolipids

  • In red blood cells, carbohydrates constitute 8% of weight

  • Selectins: Important membrane glycoproteins

Summary

  • Lipid composition affects membrane fluidity

  • Membrane lipid synthesis involves specific enzymes

  • Differentiation in roles between integral and peripheral proteins

  • Role of carbohydrates in membranes

  • Next Discussion: Membrane Function, Biological and Clinical Significance of Membranes

Exam Style Questions

Short Answer Questions

  • Definition: Open-ended questions requiring a brief, concise response.

  • Purpose: Assess understanding, reasoning, and ability to articulate concepts clearly.

  • Example: "Describe the role of integral membrane proteins in cellular function."

Single Best Answer Questions

  • Definition: Multiple-choice questions where only one option is the best answer among several choices.

  • Purpose: Evaluate critical thinking and ability to discriminate between closely related concepts.

  • Example: "Which of the following is a characteristic of phospholipids?A) Hydrophobic tailB) Hydrophilic tailsC) Insoluble in waterD) Forms bilayers spontaneouslyCorrect answer: D - Forms bilayers spontaneously.

Creating Effective Questions

  1. Clarity: Ensure questions are clear and unambiguous.

  2. Relevance: Align questions with key learning objectives.

  3. Balance: Include a mix of question types to assess different levels of knowledge.

  4. Difficulty Level: Vary difficulty to challenge students appropriately.

  5. Feedback Opportunity: Design questions that help identify areas needing further review.