Cell Membrane M5

• Plasma membrane


  Biological membrane present in both prokaryotic and eukaryotic cells, acts as a barrier between outer and inner surface of a cell
• Cell/bio membrane


  Has extracellular face (facing outer environment) and intracellular face (facing cytoplasm)

  
• Plasma membrane


  • Appears trilaminar (Protein-phospholipid bilayer protein) when viewed under electron microscope

  
• Protoplasm


  Surrounded by plasma membrane, which in eukaryotic cells extend into the interior of the cell to divide the protoplasm into a number of compartments to form cell organelles
• Chemical composition of plasma membrane

  • Lipids (phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, sphingomyelin, cardiolipin, etc.)
  • Proteins
  • Carbohydrates (as glycolipids or glycoproteins)
  • Cholesterol

  
• Ratio of protein to lipids

  80:20 (Bacteria), 20:80 (nerve cells), 50:50 in most cells
• Proteins


  Major component, provide 60% mechanical strength and help in transportation
• Components of plasma membrane

  • Phospholipids
  • Peripheral proteins
  • Carbohydrates (forming glycoproteins and glycolipids)
  • Cholesterol

  
• Glycolipids


  Help the cell to recognize other cells of the body
• Common sugars present in glycoproteins and glycolipids

  • D-Glucose
  • D-mannose
  • D-Galactose
  • N-acetyle glucosamine
  • N-acetyleneuramic acid
  • Sialic acid
• Cholesterol is not found in the membranes of plant cells
• Fluid Mosaic Model


  Main component is the phospholipid bilayer with hydrophilic polar heads oriented outwards and hydrophobic tails towards inside, proteins are arranged as extrinsic/peripheral or intrinsic/integral

  
• Fluidity of the membrane

  Lipid molecules can move laterally within the bilayer keeping the orientation intact, depends on degree of saturation of fatty acids, chain length and temperature

  
• Increase in membrane fluidity is related with the increase of unsaturated fatty acids and decrease in fatty acid chain length and cholesterol content
• Membrane proteins are capable of rapid lateral migration in fluid lipid layer, their random distribution depends on the membrane fluidity
• Mitochondria and chloroplast membranes contain 50% unsaturated fatty acids which help in oxidation reduction through electron transport chain (ETC)
• Eukaryotic plasma membrane contains large amount of cholesterol (animal cell) which enhances the permeability barrier properties of the lipid bilayer

  
• Phospholipid asymmetry in lipid bilayer

  Can be functionally important, enzyme protein kinase binds to the cytoplasmic face where phosphatidylserine is concentrated

  
• Membrane proteins based on orientation

  • Integral (intrinsic) - embedded, can be transmembranous or unilateral
  • Peripheral (extrinsic) - attached to the membrane surface only by weak ionic bonds

  
• Polar amino acids are exposed at the surface of membranes, allowing interaction with ions, hormones, antigens
• Peripheral proteins are bound to the hydrophilic head groups of the lipid and they do not form a continuous layer, but are randomly distributed

  
• Other membrane proteins are located in the cytosol, associated with bilayer only by means of one or more covalently attached fatty acid chains
• Spectrin is a cytoskeletal protein
• Membrane proteins


  • Carried out by proteins
  • Number of proteins in membrane highly variable (25% in nerve cell, 75% in mitochondria/chloroplast, 50% in usual plasma membrane)

  
• Integral (intrinsic) proteins


  Embedded in lipid bilayer (can be transmembranous or unilateral)

  
• Peripheral (extrinsic) proteins


  Attached to membrane surface only by weak ionic bonds
• Peripheral proteins are bound to the hydrophilic head groups of the lipid and are randomly distributed
• Membrane proteins


  • Spectrin
  • Glycophorin

  
• Glycocalyx


  Sugar groups of glycolipids and glycoproteins exposed at the cell surface, play a role in interaction of cell with its surroundings
• Cholesterol


  Selectively dispersed in both leaflets of animal cell membranes, prevents phospholipids from being too closely packed
• Lipid bilayer (core) of plasma membrane serves as a barrier to the passage of most polar molecules due to its hydrophobic nature
• Membrane transport proteins


  Responsible for transferring solutes across cell membranes, occur in many forms and are present in all types of biological membranes
• Simple diffusion


  Requires no energy, molecules move from high to low concentration
• Osmosis


  Diffusion of water across a membrane, from high water potential (low solute) to low water potential (high solute)
• Aquaporins


  Water channels, protein pores used during osmosis
• Cell in isotonic solution


  No net movement of water
• Cell in hypotonic solution


  Water moves into the cell (cytolysis)
• Cell in hypertonic solution


  Water moves out of the cell (plasmolysis)
• Passive transport


  • Simple diffusion (no energy, high to low concentration)
  • Facilitated diffusion (no energy, uses transport proteins, high to low concentration)
• Transport proteins


  • Channel proteins (have pores)
  • Carrier proteins (change shape to move materials)