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Biomembrane Structure and Function

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

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  1. Biomembrane Structure and Function Paul D. Brown, PhD BC21D: Bioenergetics & Metabolism

  2. 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

  3. 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

  4. 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

  5. Fluid mosaic model • Mosaic • Membrane lipids: supporting structure • Phospholipids • Glycolipids • Cholesterol • Membrane proteins: bits and pieces • Integral (integral) proteins • Peripheral (extrinsic) proteins

  6. 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)

  7. Membrane lipids • Phospholipids • Major lipid component of most biomembranes • Amphipathic: hydrophobic and hydrophilic • Examples • Phosphatidylcholine • Sphingomyelin • P-serine • P-ethanolamine • P-inositol

  8. Phospholipid bilayer

  9. 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)

  10. 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

  11. 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

  12. Membrane proteins • Five types of associations

  13. 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

  14. Membrane dynamics • Asymmetry • Lateral mobility • Fluidity

  15. Membrane asymmetry • The inner and outer leaflets of the membrane have different compositions of lipids and proteins

  16. 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

  17. Membrane fluidity • Movement of hydrophobic tails • Depends on temperature and lipid composition How does lipid composition affect fluidity?

  18. 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

  19. Biomembranes • 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

  20. “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

  21. Transport across membranes • Nutrients in and waste out • Specific ion gradients • Signals relayed • Mediated by membrane proteins

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