Transport across membranes

• 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