Membrane Transport (SL IB Biology)

• Simple diffusion is a type of membrane transport that involves particles passing directly between the phospholipids in the plasma membrane
• It can be defined as:

The net movement, as a result of the random motion of molecules or ions, of a substance from a region of higher concentration to a region of lower concentration

• The random movement is caused by the kinetic energy of the molecules or ions
• The molecules or ions are said to move down a concentration gradient
• If diffusion takes place for a long enough time period, molecules eventually reach equilibrium, where they are evenly distributed on either side of a membrane

• Examples of molecules that move by simple diffusion include
  • Oxygen
    • Oxygen diffuses into cells from the surrounding capillaries
    • Respiration uses up oxygen, resulting in a low concentration inside cells and so generating a concentration gradient
  • Carbon dioxide
    • Carbon dioxide diffuses out of cells and into the surrounding capillaries
    • Respiration produces carbon dioxide as a product, resulting in a high concentration inside cells and so generating a concentration gradient

Simple diffusion diagram

Simple diffusion involves the movement of molecules directly between the phospholipids of a cell membrane

• The rate at which a substance diffuses across a membrane depends on several factors:
  • 'Steepness' of the concentration gradient
    • The greater the difference in concentration across a membrane, the higher the rate of diffusion
  • Temperature
    • The higher the temperature the higher the rate of diffusion
    • The molecules have more kinetic energy at high temperatures, so random movement of molecules is faster
  • Surface area
    • The greater the surface area the higher the rate of diffusion
  • Properties of the molecules or ions 
    • Large molecules diffuse more slowly as they require more energy to move
    • Uncharged molecules, e.g. oxygen, diffuse faster as they move directly across the phospholipid bilayer
    • Non-polar molecules diffuse more quickly as they are soluble in the non-polar phospholipid bilayer
    • Although polar molecules cannot easily pass through the hydrophobic part of the membrane, smaller polar molecules (e.g. urea) can diffuse at low rates

• Osmosis can be defined as:

The diffusion of water molecules, from a dilute solution to a solution with a higher solute concentration, across a partially permeable membrane

• In doing this, water is moving down its concentration gradient, and so osmosis can be said to be a type of diffusion
  • A dilute solution has a high concentration of water molecules and a concentrated solution has a low concentration of water molecules
• As with facilitated diffusion, osmosis occurs as the result of the random movement of molecules, so is technically the net movement of water
• While water can move directly in between the phospholipids, channel proteins called aquaporins allow water to pass through membranes more freely
  • Water is unusual for a polar molecule in its ability to pass directly across cell membranes

Movement of water molecules diagram

Osmosis diagram

• Some substances cannot diffuse through the phospholipid bilayer of cell membranes, e.g.:
  • Large molecules
  • Polar molecules
  • Ions
• These substances can only cross the phospholipid bilayer with the help of transport proteins
• This form of diffusion is known as facilitated diffusion
• There are two types of proteins that enable facilitated diffusion:
  • Channel proteins
  • Carrier proteins
• Transport proteins are highly specific, meaning that they only allow one type of molecule or ion to pass through
• During facilitated diffusion the net diffusion of molecules or ions into or out of a cell will occur down a concentration gradient
  • Facilitated diffusion is a passive form of transport; it does not require energy
  • The direction of movement of molecules through a transport protein depends on their relative concentration on each side of the membrane

Channel proteins

• Channel proteins are pores that allow the passage of specific substances across a membrane
• They allow charged substances (eg. ions) to diffuse through the cell membrane
• Some channel proteins are gated, meaning that part of the channel protein on the inside surface of the membrane can move in order to close or open the pore
  • This allows the channel protein to control the exchange of ions

Channel protein diagram

Carrier proteins

• Unlike channel proteins, which have a fixed shape, carrier proteins can switch between two shapes
  • The substance to be transported attaches to a binding site, causing a shape change in the carrier protein
  • Initially the binding site of the carrier protein is open to one side of the membrane
  • When the carrier protein switches shape it opens to the other side of the membrane

Carrier protein diagram

• Active transport can be defined as:

The movement of molecules and ions across a cell membrane, from a region of lower concentration to a region of higher concentration, using energy from respiration

• Active transport occurs against, or up, a concentration gradient
• Active transport requires carrier proteins
  • Carrier proteins in active transport are sometimes known as pumps
  • Although facilitated diffusion also uses carrier proteins, active transport is different as it requires energy
• Energy is required to allow the carrier protein to change shape, allowing it to transfer the molecules or ions across the cell membrane
  • The energy required is provided by ATP (adenosine triphosphate), produced during respiration.
  • The ATP is hydrolysed to release energy

Active transport diagram

Active transport is the transport of substances across cell membranes from low to high concentration

• Facilitated diffusion and active transport are mechanisms that allow cell membranes to be selectively permeable
  • Selective permeability is the ability of the membrane to differentiate between different types of molecules, only allowing some molecules through while blocking others
• Simple diffusion provides less control for cell membranes, as it is dependent only on the size and hydrophobic or hydrophilic nature of the molecules diffusing
  • Simple diffusion provides no ability for membranes to be selective with regard to small, polar molecules 
    • Small, non-polar molecules can diffuse across the membrane with ease so this is not selective
  • Simple diffusion does allow for selective permeability with regard to large or polar molecules
    • Large or polar molecules cannot cross the phospholipid bilayer without transport proteins