Membrane Structure & Function

Membrane Structure & Function

Fluid Mosaic Model

  • Glycoprotein: A protein with a carbohydrate attached to it.
  • Glycolipid: A lipid with a carbohydrate attached to it.
  • Phospholipid bilayer: The structural foundation of the cell membrane.
  • Peripheral membrane protein: A protein attached to the membrane but does not span it.
  • Integral membrane protein: A protein that spans the membrane, embedded in the lipid bilayer.
  • Cholesterol: A sterol that maintains membrane fluidity.
  • Protein channel: A type of integral protein that facilitates the passage of ions or molecules.
  • Cytoskeletal filaments: Structures that help maintain cell shape and organize internal components.

The Structure

A. Lipids

  • Consist of hydrophobic (water-repelling) tails and hydrophilic (water-attracting) heads.
  • Phospholipids: Key components of cell membranes. They form a bilayer with hydrophilic heads facing outward and hydrophobic tails facing inward.
    • Represented structurally as:
      ext{RO - OP - O - C=O - CH2}
    • Contain a glycerol backbone, fatty acid tails, and a phosphate group.

B. Proteins

  • Peripheral proteins: Reside on the exterior or interior surfaces of membranes.
  • Integral proteins: Span the membrane and can be either:
    • Transmembrane proteins: Extends across the membrane (e.g., receptor proteins).
    • Monotopic proteins: Inserted into the membrane and do not span it.
    • Bitopic proteins: Span the membrane in two regions.
    • Polytopic proteins: Cross the bilayer multiple times.

C. Carbohydrates

  • Present on the extracellular surface as glycoproteins or glycolipids, facilitating cell recognition and signaling.
    • Can contain various sugars such as:
    • N-acetylgalactosamine
    • N-acetylglucosamine
    • Galactose
    • Fucose
    • Form the glycocalyx, which protects and facilitates communication.

Membrane Properties

Membrane "Sidedness"

  • Distinctions exist between the extracellular and cytoplasmic sides of the membrane.
    • Apical plasma membrane: Regulates nutrient intake.
    • Lateral plasma membrane: Involved in cell adhesion and communication.
    • Basal membrane: Contacts the extracellular matrix and contributes to ion balance.

Membrane Fluidity

  • Movement of phospholipids can occur laterally (~$10^7$ times per second) and flip-flop across membranes approximately once per month.
  • Fluidity is influenced by
    • The degree of saturation of fatty acid tails (saturated tails are less fluid than unsaturated).
    • Presence of cholesterol, which stabilizes membrane structure and fluidity.

Membrane Function

Transport Mechanisms

  • Selectively Permeable: Membranes allow specific substances to cross while restricting others.
  • Solutes/Molecules that can move: Depends largely on size, polarity, and concentration gradient.

Types of Transport Proteins

  1. Channel proteins: Allow passive movement of molecules and ions down their concentration gradient.
  2. Carrier proteins: Bind molecules and undergo conformational changes to ferry them across the membrane.

Direction of Transport Across Membranes

  • Passive Movement: Movement occurs from regions of high to low concentration, including:
    1. Simple Diffusion: Direct spread of particles down a concentration gradient directly across the membrane.
    2. Facilitated Diffusion: Requires proteins to assist it across the membrane.
    3. Osmosis: Movement of water through a selectively permeable membrane from a region of lower solute concentration to higher.

Tonicity

  • Describes how an extracellular solution can affect cell volume by osmosis:
    1. Hypertonic: Higher concentration of solutes outside the cell; cells lose water and shrink.
    2. Isotonic: Equal concentration of solutes; no net movement of water.
    3. Hypotonic: Lower concentration of solutes outside the cell; cells gain water and may burst.

Active Transport

Characteristics

  • Requires ATP to move substances against their concentration gradient (from low to high concentration).
    • Carrier proteins: Utilize ATP to transport ions, such as Na+ and K+ across the plasma membrane.
  • Examples include:
    • Proton pumps: Move H+ ions out of the cell.
    • Cotransport mechanisms: Utilize one gradient (like H+) to help transport another molecule (like sucrose) across the membrane.

Transport of Large Molecules

  1. Exocytosis: Process where vesicles fuse with the plasma membrane to release their contents outside the cell.
  2. Endocytosis: Cellular uptake of material through vesicle formation, which includes:
    • Phagocytosis: Cell engulfs large particles or cells.
    • Pinocytosis: Cell engulfs liquid substances.
    • Receptor-Mediated Endocytosis: Specific uptake triggered by ligand-receptor interactions.

Conclusion

  • Ability to discuss membrane composition, fluidity influencers, and distinctions between transport types and mechanisms.
    • Contrast passive and active transport and elucidate how large molecules traverse membranes effectively.
  • Understand how tonicity affects cellular dynamics and the implications for cell behavior in different environments.