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Biomembrane Structure and Function. Paul D. Brown, PhD BC21D: Bioenergetics & Metabolism. Learning Objectives. Describe the structural relationship among the components of the membrane and general functional roles served by each of them

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biomembrane structure and function

Biomembrane Structure and Function

Paul D. Brown, PhD

BC21D: Bioenergetics & Metabolism

learning objectives
Learning Objectives
  • Describe the structural relationship among the components of the membrane and general functional roles served by each of them
  • Describe the processes by which small solutes, ions and macromolecules cross biomembranes
  • Describe various membrane transport pumps including their energy source, stoichiometry and functional significance
biomembrane structure
Biomembrane structure
  • Cell (plasma) membrane: defines cell boundaries
  • Internal membranes define a variety of cell organelles
    • Nucleus
    • Mitochondria
    • Endoplasmic reticulum (rough and smooth)
    • Golgi apparatus
    • Lysosomes
    • Peroxisomes
    • Chloroplasts
    • Other
membrane functions
Membrane functions
  • Form selectively permeable barriers
  • Transport phenomena
    • Passive diffusion
    • Mediated transport
      • Facilitated diffusion
        • Carrier proteins
        • Channel proteins
          • Gated or non-gated channels
    • Active transport
  • Cell communication and signalling
  • Cell-cell adhesion and cellular attachment
  • Cell identity and antigenicity
  • Conductivity
fluid mosaic model
Fluid mosaic model
  • Mosaic
  • Membrane lipids: supporting structure
    • Phospholipids
    • Glycolipids
    • Cholesterol
  • Membrane proteins: bits and pieces
    • Integral (integral) proteins
    • Peripheral (extrinsic) proteins
biomembrane composition
Biomembrane composition
  • Phospholipid bilayer (basic structure)
  • Various membrane proteins, depending on membrane function
  • Glycolipids and glycoproteins (lipids and proteins with attached carbohydrates)
  • Cholesterol (in animal cells)
membrane lipids
Membrane lipids
  • Phospholipids
    • Major lipid component of most biomembranes
    • Amphipathic: hydrophobic and hydrophilic
    • Examples
      • Phosphatidylcholine
      • Sphingomyelin
      • P-serine
      • P-ethanolamine
      • P-inositol
membrane lipids9
Membrane lipids
  • Glycolipids
    • Least common of the membrane lipids (ca. 2%)
    • Always found on outer leaflet of membrane
    • Carbohydrates covalently attached
  • Involved in cell identity (blood group antigens)
membrane lipids10
Membrane lipids
  • Cholesterol
    • Steroid; lipid-soluble
    • Found in both leaflets of bilayer
    • Amphipathic
    • Found in animal cells
    • Membrane fluidity “buffer”
    • Synthesized in membranes of ER
membrane proteins
Membrane proteins
  • Integral (intrinsic) proteins
    • Penetrate bilayer or span membrane
    • Can only be removed by disrupting bilayer
    • Types
      • Transmembrane proteins
        • Single-pass or Multiple-pass
      • Covalently tethered integral proteins
    • Many are glycoproteins
      • Covalently-linked via asparagine, serine, or threonine to sugars
    • Synthesized in rough ER
    • Function: enzymatic, receptors, transport, communication, adhesion
membrane proteins12
Membrane proteins
  • Five types of associations
membrane proteins13
Membrane proteins
  • Peripheral (extrinsic) proteins
    • Do not penetrate bilayer
    • Not covalently linked to other membrane components
    • Form ionic links to membrane structures
      • Can be dissociated from membranes
      • Dissociation does not disrupt membrane integrity
    • Located on both extracellular and intracellular sides of membrane
    • Synthesis
      • Cytoplasmic (inner) side – cytoplasm
      • Extracellular (outer) side – made in ER and exocytosed
membrane dynamics
Membrane dynamics
  • Asymmetry
  • Lateral mobility
  • Fluidity
membrane asymmetry
Membrane asymmetry
  • The inner and outer leaflets of the membrane have different compositions of lipids and proteins
fluid mosaic model16
Fluid mosaic model
  • Biomembranes are a two-dimensional “mosaic” of lipids and proteins
  • Most membrane lipids and protein can freely move through the membrane plane
membrane fluidity
Membrane fluidity
  • Movement of hydrophobic tails
  • Depends on temperature and lipid composition

How does lipid composition affect fluidity?

lipids and membrane fluidity
Lipids and membrane fluidity
  • Interactions between hydrophobic tails decrease fluidity (movement):
    • Shorter tails have fewer interactions
    • Unsaturated fatty acids are kinked and decrease interactions
  • Cholesterol “buffers” fluidity
    • Prevents interactions
    • Restricts tail movement
  • Surrounds cell
  • Separates cell from environment
    • Allows cellular specialization
  • Separate some of the cellular organelles
    • Allows specialization within the cell
  • Continuity of membranes between adjoining cells (tight junctions) can separate two extracellular compartments
    • Important in organ function
accessory structures
“Accessory” structures
  • Extracellular matrix (ECM)
    • Outside animal cells
    • Composed of proteins and carbohydrates
    • Attached to plasma membrane
  • Cell wall
    • Surrounds plant cells
    • Composed of cellulose (carbohydrate)
    • Adds rigidity
transport across membranes
Transport across membranes
  • Nutrients in and waste out
  • Specific ion gradients
  • Signals relayed
  • Mediated by membrane proteins