3.2.3 Transport across cell membranes

• The fluid mosaic model is molecules which are free to move laterally in phospholipid bilayer. In this model there are many components such as phospholipid, proteins, glycoproteins and glycolipids
• Cell membrane

  • Contains phospholipids which form a bilayer - fatty acid tail faces inwards, phosphate heads face outwards
  • Contains proteins which are intrinsic/integral (span the bilayer) or extrinsic/peripheral (on the surface of the membrane)
  • Contains glycolipids (lipids with polysaccharide chains attached on the exterior surface)
  • Contains glycoproteins (proteins with polysaccharide chains attached on the exterior surface)
  • Sometimes contains cholesterol which bonds to phospholipids hydrophobic fatty acid tails
• Phospholipids in a cell membrane have a bilayer, which has water present on either side

  • Hydrophobic fatty acid tail which repels water so it points away from water
  • Hydrophillic phosphate heads attract water, so it is pointed to the direction of water
• Cholesterol in the cell membrane:

  • Restricts movement of other molecules making up the membrane
  • Decreases fluidity and increases rigidity
• Cell membranes are adapted for other functions by:

  • Phospholipid bilayer being fluid -> membrane can bend for vesicle formation or phagocytosis
  • Glycoproteins and glycolipids act as receptors or antigens -> involved in cell signalling or recognition
• Movement across membranes occurs by simple diffusion by:

  • Lipid soluble (non-polar)/ very small substances such as oxygen or steroid hormones
  • Move from an area of high conc. to an area of low conc. down a conc. gradient
  • Across a phospholipid bilayer
  • It is passive - does not require energy from ATP or respiration, only uses kinetic energy from substances
• The limitations of the nature of phospholipid bilayers are that it restricts the movement of water-soluble (polar) and larger substances such as sodium or glucose. This is due to hydrophobic fatty acid tails in the interior of the bilayer.
• Movement across membranes through facilitated diffusion by:

  • Water-soluble (polar) and slightly larger substances
  • Move down a conc. gradient
  • Through specific channel or carrier proteins
  • Passive - does not require energy from ATP or respiration, only through kinetic energy through substances
• Carrier proteins facilitate the diffusion of slightly larger substances. These are complementary substances which attaches to the binding site. The protein then changes shape to transport substances.
• Channel proteins facilitate the diffusion of water-soluble substances, using a hydrophilic pore filled with water and also may be gated, so they either be open or closed
• Movement across membranes through osmosis occurs by:

  • Water diffusing or moving
  • From an area of high to low water potential down a water potential gradient
  • Through a partially permeable membrane
  • It is passive so it doesn't require ATP or respiration
• Movement across membrane through active transport occurs by:

  • Substances moving from an area of low to high conc. against a conc. gradient
  • This requires the hydrolysis of ATP and specific carrier proteins
• Carrier proteins are important in the hydrolysis of ATP in active transport as:

  • Complementary substances binds to specific carrier proteins
  • ATP binds, and hydrolysed into ADP + Pi, releasing energy
  • Carrier protein therefore changes shape, releasing substances on the side of higher concentration
  • Pi is then released and the protein returns to it's original shape
• Movement across membranes occurs through co-transport as:

  • 2 different substances bind to and move simultaneously through a co-transporter protein which is a type of carrier protein
  • Movement of one substance against its concentration gradient is coupled with the movement of another down its concentration gradient
• An example that illustrates co transport is absorption of sodium ions and glucose by cells lining in the mammalian ileum
• Absorption:

  1. Na+ is actively transported from the epithelial cells to the blood through a sodium-potassium pump. This forms a conc. gradient of sodium
  2. Sodium enters the epithelial cell down its concentration gradient with glucose against its conc. gradient through a co-transporter protein
  3. Glucose moves down a conc. gradient into blood through facilitated diffusion
• Increasing surface area of membrane increases the rate of movement
• Increasing number of channel or carrier proteins increases rate of facilitated diffusion and active transport
• Increasing concentration gradient increases rate of simple and facilitated diffusion and osmosis
• Increasing the concentration gradient increases the rate of facilitated diffusion -> this is until several channel or carrier proteins become a limiting factor as all are in use
• Increasing water potential gradient increases rate of osmosis
• The adaptations of some specialised cells in relation to the rate of transport across their internal and external membranes:

  • Membranes folded e.g, microvilli in the ileum which increases surface area
  • More protein channels and carriers for facilitated diffusion
  • Large number of mitochondria which makes more ATP by aerobic respiration for active transport