Biological Membranes

• Describe the fluid mosaic model of membranes:

  • Fluid: phospholipid bilayer where individual phospholipids can move, giving the membrane a flexible shape
  • Mosaic: extrinsic and intrinsic proteins of different sizes and shapes are embedded
• Explain the role of cholesterol and glycolipids in membranes:

  • Cholesterol: a steroid molecule in some plasma membranes that connects phospholipids and reduces fluidity to stabilize the bilayer
  • Glycolipids: involved in cell signaling and cell recognition
• Explain the functions of extrinsic proteins in membranes:

  • Binding sites/receptors for hormones and drugs
  • Antigens (glycoproteins)
  • Binding cells together
  • Involved in cell signaling
• Explain the functions of intrinsic transmembrane proteins in membranes:

  • Electron carriers (respiration/photosynthesis)
  • Channel proteins (facilitated diffusion)
  • Carrier proteins (facilitated diffusion/active transport)
• Explain the functions of membranes within cells:

  • Provide an internal transport system
  • Selectively permeable to regulate the passage of molecules into/out of organelles or within organelles
  • Provide a reaction surface
  • Isolate organelles from cytoplasm for specific metabolic reactions
• Name and explain 3 factors that affect membrane permeability:

  • Temperature: high temperature denatures membrane proteins/phospholipid molecules have more kinetic energy and move further apart
  • pH: changes the tertiary structure of membrane proteins
  • Use of a solvent: may dissolve the membrane
• Outline how colorimetry could be used to investigate membrane permeability:

  1. Use plant tissue with soluble pigment in the vacuole. Tonoplast & cell-surface membrane disrupted = increased permeability = pigment diffuses into the solution.

  2. Select a colorimeter filter with a complementary color.

  3. Use distilled water to set the colorimeter to 0. Measure absorbance/% transmission value of the solution.

  4. High absorbance/low transmission = more pigment in the solution
• Define osmosis:

  • Water diffuses across semi-permeable membranes from an area of higher water potential to an area of lower water potential until a dynamic equilibrium is established
• What is water potential (ψ)?

  • Pressure created by water molecules measured in kPa
  • Ψ of pure water at 25℃ & 100 kPa: 0
  • More solute = ψ more negative
• How does osmosis affect plant and animal cells?

  • Osmosis INTO cell:
  • Plant: protoplast swells = cell turgid
  • Animal: lysis
  • Osmosis OUT of cell:
  • Plant: protoplast shrinks = cell flaccid
  • Animal: crenation
• Define simple diffusion:

  • Passive process that requires no energy from ATP hydrolysis
  • Net movement of small, lipid-soluble molecules directly through the bilayer from an area of high concentration to an area of lower concentration
• Define facilitated diffusion:

  • Passive process where specific channel or carrier proteins transport large and/or polar molecules/ions down the concentration gradient
• Explain how channel and carrier proteins work:

  • Channel: hydrophilic channels bind to specific ions, one side of the protein closes & the other opens
  • Carrier: binds to complementary molecule, conformational change releases molecule on the other side of the membrane; in facilitated diffusion, passive process; in active transport, requires energy from ATP hydrolysis
• Define active transport:

  • Active process where ATP hydrolysis releases a phosphate group that binds to a carrier protein, causing it to change shape
  • Specific carrier protein transports molecules/ions from an area of low concentration to an area of higher concentration (against the concentration gradient)
• Define exocytosis and endocytosis:

  • Active processes involved in bulk transport & transporting large particles
  • Vesicles fuse with the cell surface phospholipid membrane
• Name 5 factors that affect the rate of diffusion:

  • Temperature
  • Diffusion distance
  • Surface area
  • Size of molecule
  • Difference in concentration (how steep the concentration gradient is)
• Extrinsic proteins purpose is to provide a surface for the attachment of other proteins.
• Intrinsic proteins are embedded within the lipid bilayer and have functions such as ion pumps, receptors, enzymes, and structural support.
• NaK pump is a proton pump that uses ATP to pump protons across the membrane
• How does glucose travel across membranes?

  Facilitated diffusion
• BULK transport is the movement of large molecules across the cell membrane
• glycoproteins


  Involved in cell adhesion and cell recognition
• Define cell recognition: The process by which a cell recognizes another cell and responds to it.
• Define cell signalling: A process by which cells communicate with each other to coordinate their activities.
• Define cell adhesion molecules: Molecules that bind to the cell surface of other cells