bio mem

• Types of movement through cell membranes:

  • Diffusion
  • Facilitated diffusion
  • Osmosis
  • Active transport
• Diffusion:

  passive movement of small, non-polar lipid-soluble molecules from high to low concentration through the phospholipid bilayer
• Facilitated diffusion:

  requires channel proteins to transport polar, charged, and water-soluble molecules
• Osmosis:

  diffusion of water molecules from high water potential to low water potential through a partially permeable membrane
• Active transport:

  transports molecules through carrier proteins from low to high concentration, requiring ATP energy
• Fluid mosaic model of membranes


  • Phospholipid bilayer in which individual phospholipids can move = membrane has flexible shape
  • Extrinsic & intrinsic proteins of different sizes and shapes are embedded
• Glycolipids


  Cell signalling & cell recognition
• Functions of extrinsic and transmembrane proteins in membranes


  • Binding sites/ receptors e.g. for hormones & drugs
  • Antigens (glycoproteins)
  • Bind cells together
  • Involved in cell signalling
• Functions of intrinsic transmembrane proteins in membranes

  • Electron carriers (respiration/photosynthesis)
  • Channel proteins (facilitated diffusion)
  • Carrier proteins (facilitated diffusion / active transport)
• Functions of membranes within cells

  • Provide internal transport system
  • Selectively permeable to regulate passage of molecules into / out of organelles or within organelles
  • Provide reaction surface
  • Isolate organelles from cytoplasm for specific metabolic reactions
• Functions of the cell-surface membrane

  • Isolates cytoplasm from extracellular environment
  • Selectively permeable to regulate transport of substances
  • Involved in cell signalling / cell recognition
• Factors that affect membrane permeability

  • Temperature: high temperature denatures membrane proteins / phospholipid molecules have more kinetic energy & move further apart
  • pH: changes tertiary structure of membrane proteins
  • Use of a solvent: may dissolve membrane
• Investigating membrane permeability using colorimetry

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

  2. Select colorimeter filter with complementary colour

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

  4. High absorbance/ low transmission = more pigment in solution
• 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
• 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 osmosis affects 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
• Simple diffusion

  • Passive process 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 (i.e. down a concentration gradient)
• Facilitated diffusion

  • Passive process
  • Specific channel or carrier proteins with complementary binding sites transport large and/ or polar molecules/ ions (not soluble in hydrophobic phospholipid tail) down concentration gradient
• How channel and carrier proteins work

  1. Channel: hydrophilic channels bind to specific ions = one side of the protein closes & the other opens

  2. Carrier: binds to complementary molecule = conformational change releases molecule on other side of membrane; in facilitated diffusion, passive process; in active transport, requires energy from ATP hydrolysis
• Active transport

  • Active process: ATP hydrolysis releases phosphate group that binds to carrier protein, causing it to change shape
  • Specific carrier protein transports molecules/ ions from area of low concentration to area of higher concentration (i.e. against concentration gradient)
• Exocytosis and endocytosis


  • Active process
  • Involved in bulk transport & transporting large particles
  • Vesicles fuse with cell surface phospholipid membrane
• Factors that affect the rate of diffusion

  • Temperature
  • Diffusion distance
  • Surface area
  • Size of molecule
  • Difference in concentration (how steep the concentration gradient is)
• Protein components of a cell membrane include: 

  • Glycoproteins
  • Extrinsic proteins
  • Intrinsic proteins
  • Transport/Channel/Carrier proteins
• What proteins are used in these forms of transport?

  • Facilitated diffusion - carrier and channel
  • Active transport - carrier
• The words ‘fluid’ and ‘mosaic’ refer to:


  Fluid:

  • phospholipids and proteins can move around freely and laterally within their layers

  Mosaic:

  • There are multiple different kinds of molecule in the membrane
  • eg. glycolipids /glycoproteins
• Cell membranes are affected by organic solvents in the following way:


  • They dissolve the lipids in the cell membrane
  • Disrupting/changing membrane structure
• What does net movement mean?

  The overal. movement of particles from an ares of high conc to low conc
• Two requirements for active transport would include:

  • Carrier protein
  • Energy/ATP
• processes in living organisms where active transport is important:


  • Transport of inorganic  ions into - root hair cells
  • Absorption of digestion products in the - small intestine
  • Loading of sugars into plant - phloem
  • Reabsorption of useful molecules and ions in the - kidney nephron
• Bulk transport into cells would include:

  • Endocytosis
  • Phagocytosis
  • Pinocytosis
  • Micropinocytosis
• Pinocytosis: 


  The bulk uptake of liquids into the cell using energy in the form of ATP.
• Hypertonic solution

  • Surrounding solution has a higher solute concentration compared to inside the cell
  • Surrounding solution has low solvent concentration compared to the inside of the cell
• What happens in a hypertonic solution


  Solvent moves from the cell to the outside
• Hypotonic solution

  • Solution in the surrounding has a lower solute concentration as compared to the solute concentration inside the cell
  • Surrounding solution has high solvent concentration compared to the inside of the cell
• What happens in a hypotonic solution

  Solvent moves from the surrounding towards the cells
• Isotonic solution


  • Solution that has the same concentration of solute in both surrounding solution and solution inside the cell
  • Solution that has the same concentration of solvent in both surrounding solution and solution inside the cell
• What happens in an isotonic solution


  No movement of solvent as there is an equilibrium between the surrounding solution and inside of the cell
• What is cell signalling?


  Communication between cells
• Explain how cell surface membranes contribute to the process of cell signalling.

  • release of signal molecule by, exocytosis
  • glycoproteins act as receptors to specific signal
  • shape of receptor and signal are complementary
  • attachment of signal molecule causes change (inside cell / on cell surface)
• Describe the routes that water molecules take through the cell surface membrane.

  fit between phospholipids via, protein channels / aquaporins ;