Exchange and Transport mechanisms

• the surface area must be large in comparison to the volume for efficient exchange of substances
• where the surface area is small compared to the volume, specialised exchange and transport mechanisms are required to maximise the rate of diffusion
• diffusion: the movement of molecules from an area of high concentration to an area of low concentration
• efficient exchange requires: a large surface area for exchange, short diffusion distance and a high concentration gradient
• specialised exchange and transport mechanisms are required when there is a low surface area to volume ratio
• the phospholipid bilayer acts as a barrier to diffusion and only allows small, non-polar molecules through
• Polar substances require specialised transport systems involving proteins and glycoproteins embedded in the phospholipid bilayer
• the epithelial cells that line the small intestine are specialised cells, the plasma membrane is folded into many tiny finger-like projections called microvilli - this greatly increases the surface area for absorption of the products of digestion.
• Fluid mosaic model: The model that describes the structure of the cell membrane.
• facilitated diffusion: diffusion is assisted by proteins in the membrane, small polar molecules are transported by channel proteins and large polar molecules use carrier proteins
• channel proteins act like pores in the membrane, allowing ions and small molecules to pass through
• channel proteins that open and close are called gated channels
• carrier proteins are specific to the substance being transported, the substance binds to the carrier protein
• osmosis: a type of facilitated diffusion where water moves across a partially permeable membrane from a high concentration of water molecules to a low concentration of water molecules
• active transport: movement of molecules from a low concentration to a high concentration against a concentration gradient
• active transport involves carrier proteins which use ATP as a source of energy these types of active transport proteins are often called pumps
• co-transport mechanisms is where a molecule cannot be transported across the membrane unless it is transported with another molecule
• glycolipid: a sugar chain that is attached to a phospholipid

  
• peripheral proteins: proteins that are attached to the surface of the phospholipid bilayer

  
• transmembrane protein: a protein that spans the membrane and is exposed at both surfaces

  
• glycoprotein: a sugar chain that is attached to a protein

  
• protein on one side of the membrane

  
• channel protein with pore: a protein that has a channel that allows ions to pass through it

  
• factors that affect the rate of diffusion: diffusion distance, temperature and metabolic rate
• how does diffusion distance affect the rate of diffusion?

  the shorter the distance the substances have to move, the faster the rate of diffusion
• how does temperature affect the rate of diffusion?

  as the temperature increases, particles gain more kinetic energy and so can diffuse across a membrane more quickly. Therefore, the higher the temperature, the faster the rate of diffusion.
• certain molecules cannot diffuse through the cell membrane such as:

  • large polar molecules
  • ions
• certain molecules can only cross the cell membrane with the help of channel proteins or carrier proteins
• channel proteins and carrier proteins are highly specific, they only allow one type of molecule or ion through
• channel proteins:

  • contain water-filled pores
  • they allow charged substances (e.g. ions) to diffuse through the cell membrane
  • channel proteins are gated which means they can control the exchange of ions
• carrier proteins:

  • carrier proteins can switch between 2 shapes
  • the binding site of the protein is only open on one side at a time
• osmosis:

  • the net movement of water molecules through a semi-permeable membrane, from a region of high water potential to a region of low water potential
  • water is a polar molecule therefore it can only diffuse across a membrane via facilitated diffusion
• active transport:

  • the movement of molecules and ions through a cell membrane from a region of low concentration to a region of high concentration using energy from respiration
• active transport requires carrier proteins
• energy is required to make the cell change shape, allowing it to transfer the molecules or ions across the cell membrane in active transport
• co-transport mechanisms:

  • the coupled movement of substances across a cell membrane via a carrier protein
  • it involved a combination of facilitated diffusion and active transport