Transport across membranes

    Cards (21)

    • The fluid mosaic model of membranes consists of:
      • Fluid: phospholipid bilayer allowing individual phospholipids to move for a flexible shape
      • Mosaic: extrinsic & intrinsic proteins of different sizes and shapes embedded in the membrane
    • Cholesterol in membranes:
      • Acts as a steroid molecule in some plasma membranes
      • Connects phospholipids & reduces fluidity to stabilize the bilayer
      Glycolipids in membranes:
      • Involved in cell signalling & cell recognition
    • Extrinsic proteins in membranes:
      • Serve as binding sites/receptors (e.g. for hormones)
      • Act as antigens (glycoproteins)
      • Bind cells together
      • Participate in cell signalling
      Intrinsic proteins in membranes:
      • Function as electron carriers (in respiration/photosynthesis)
      • Serve as channel proteins (for facilitated diffusion)
      • Act as carrier proteins (for facilitated diffusion/active transport)
    • Functions of membranes within cells:
      • Provide internal transport system
      • Regulate passage of molecules into/out of organelles
      • Provide a reaction surface
      • Isolate organelles from cytoplasm for specific metabolic reactions
    • Functions of the cell-surface membrane:
      • Isolate cytoplasm from extracellular environment
      • Regulate transport of substances
      • Involved in cell signalling/cell recognition
    • Factors affecting membrane permeability:
      • Temperature: high temperature denatures membrane proteins
      • pH: changes tertiary structure of membrane proteins
      • Use of a solvent may dissolve the membrane
    • Osmosis is the movement of water across semi-permeable membranes from an area of higher water potential to an area of lower water potential until dynamic equilibrium is reached
    • Water potential (ψ) is the pressure created by water molecules measured in kPa, with pure water at 25°C & 100 kPa having a ψ of 0. More solute leads to a more negative ψ
    • To produce a desired concentration of solution from a stock solution:
      • Calculate volume of stock solution needed
      • Calculate volume of distilled water needed
    • Simple diffusion is a passive process where small, lipid-soluble molecules move directly through the bilayer from high to low concentration without requiring ATP energy
    • Osmosis affects plant and animal cells differently:
      • Osmosis INTO cell:
      • Plant: protoplast swells, cell becomes turgid
      • Animal: cell lysis
      • Osmosis OUT of cell:
      • Plant: protoplast shrinks, cell becomes flaccid
      • Animal: crenation
    • Facilitated diffusion is a passive process where specific channel or carrier proteins transport large and/or polar molecules/ions down their concentration gradient
    • Channel proteins work by binding specific ions, causing one side of the protein to close and the other to open.
      Carrier proteins work by binding complementary molecules, leading to a conformational change that releases the molecule on the other side of the membrane
    • Factors affecting the rate of diffusion:
      • Temperature
      • Diffusion distance
      • Surface area
      • Size of molecule
      • Difference in concentration gradient
    • Fick’s law states that the rate of diffusion is determined by the surface area multiplied by the difference in concentration, divided by the diffusion distance
    • Cells are adapted to maximize transport across membranes by having many carrier/channel proteins and increasing surface area through folding
    • Graph shape for concentration vs rate:
      • Simple diffusion: straight diagonal line
      • Facilitated diffusion: line levels off when proteins are saturated
    • Active transport is an active process requiring ATP energy to transport molecules/ions against their concentration gradient using specific carrier proteins
    • Comparison of active transport and facilitated diffusion:
      • Both may involve carrier proteins
      • Active transport requires ATP energy, facilitated diffusion is passive
      • Facilitated diffusion may involve channel proteins
    • Co-transport in the absorption of glucose/amino acids in the small intestine:
      • Na+ actively transported out of epithelial cells
      • Transport of glucose/amino acids coupled to Na+ facilitated diffusion
    • Co-transport involves moving a substance against its gradient while coupling it with the movement of another substance down its gradient using symport or antiport proteins
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