TRANSPORT ACROSS MEMBRANES (CELLS)

• What is the structure of the plasma membrane?

  Composed of a phospholipid bilayer with embedded proteins and other molecules.
• What is the function of the plasma membrane?

  • To regulate the movement of substances in and out of the cell
  • To maintain the cell's internal environment.
  • To keep the components of the cell isolated from the external environment
• Which 2 organelles are surrounded by a double membrane?

  Nucleus and mitochondria
• What is the fluid mosaic model?

  A concept that describes the structure of the cell membrane as 'fluid' because the phospholipids are constantly moving and 'mosaic' because proteins are scattered through the bilayer
• What structures are present within the plasma membrane?

  Phospholipids

  Proteins (intrinsic and extrinsic)

  Cholesterol

  Glycolipids and glycoproteins
• Give an example of an intrinsic and extrinsic protein

  Intrinsic - channel proteins and carrier proteins

  Extrinsic - Glycoproteins
• What is the function of the phospholipid bilayer ? Give an example


  Forms a barrier to dissolved substances

  Centre of the bilayer is hydrophobic so it does not allow water soluble substances to pass through.

  But it does allow small non polar molecules like CO2 to diffuse through

  Example:

  Water is polar but it can diffuse through the membrane by osmosis because it is so small.
• What is the function of intrinsic proteins ? Give an example

  Channel proteins and carrier proteins allow large molecules, polar molecules and ions to pass through the membrane

  Example:

  Molecules which need to travel via active transport or facilitated diffusion are moved through these proteins.
• What is the function of glycolipids and glycoproteins? Give an example

  Can act as receptors and allow the cell to detect chemicals released from other cells so that they can respond

  Example:

  Act as antigens to facilitate cellular recognition which is crucial to the immune response
• What is the function of cholesterol? Give an example

  Restricts the movement of other molecules

  Gives the membrane stability by binding to the tails of phospholipids and causing them to pack together more closely. Has a hydrophobic region so acts as a barrier to polar substances moving through the membrane.

  Example:

  Helps to maintain the shape of animal cells (which don't have cell walls)
• Describe the arrangement of phospholipids in the bilayer

  Hydrophilic heads pointing outwards

  Hydrophobic heads pointing inwards

  
• Suggest how cell membranes are adapted for other functions

  Phospholipid bilayer is fluid - membrane can bend for vesicle formation - phagocytosis
• What 3 factors affect membrane permeability?

  Temperature

  pH

  Solvents
• Explain how temperature affects the permeability of cell surface membranes

  As temperature increases, membrane permeability increases because the phospholipids gain kinetic energy and fluidity increases. Transport proteins denature at high temperatures because the hydrogen bonds are broken, which changes the tertiary structure

  As temperature decreases, membrane permeability increases because ice crystals form which pierce and damage the cell membrane which increases permeability
• Explain how pH affects the permeability of cell surface membranes

  High or low pH increases

  Transport proteins denature as hydrogen bonds break ,changing the tertiary structure
• Explain how lipid soluble solvents affect the permeability of the cell surface membrane

  As concentration increases, the permeability of the membrane also increases

  Ethanol (a lipid soluble solvent) may dissolve the phospholipid bilayer and form gaps
• What is meant by permeability?

  The ease with which a molecule can pass through the cell membrane
• What is a colorimeter

  A machine that passes light of a specific wavelength through a liquid and measures how much of that light is absorbed
• Define diffusion 


  The net movement of molecules or ions from an area of high concentration to an area of low concentration
• When does diffusion stop?

  When an equilibrium is reached
• Is diffusion passive or active? Why?

  Passive (does not require any energy) as the particles are moving down the concentration gradient.
• What 4 factors affect the rate of diffusion?

  Concentration gradient

  Membrane thickness

  Surface area

  Temperature
• How does the concentration gradient affect the rate of diffusion?

  The greater the concentration gradient, the faster the rate of diffusion

  This is because:

  A good blood or air supply helps to maintain the concentration gradient by transporting substances to and from the membrane. Otherwise diffusion slows down over time as equilibrium is reached
• How does membrane thickness affect the rate of diffusion?

  The thinner the membrane the faster the diffusion

  This is because:

  Thin membranes reduce the distance that the particles have to travel.
• How does surface area affect the rate of diffusion?

  The larger the surface area, the faster the rate of diffusion

  This is because:

  Increasing the surface area means that more particles can be exchanged in the same amount of time, increasing the rate of diffusion
• How does temperature affect the rate of diffusion?

  The higher the temperature the faster the rate of diffusion

  This is because:

  At higher temperatures, particles will have more kinetic energy so they will be able to move faster
• What is facilitated diffusion?

  When larger molecules and charged particles would move very slowly through the membrane via simple diffusion so they would need to diffuse via specialized carrier and channel proteins
• Is facilitated diffusion faster or slower?


  Faster
• What is the difference between channel proteins and carrier proteins?

  Carrier proteins are proteins that bind to molecules or ions on one side of the membrane and release them on the other. Channel proteins create holes/pores that penetrate the membrane, enabling molecules or ions to flow through without interfering with one another.

  Carrier proteins - large molecules

  Channel proteins - charged particles
• What 2 factors affect the rate of facilitated diffusion?

  Concentration gradient

  Number of channel / carrier proteins
• How does the concentration gradient affect the rate of facilitated diffusion?

  The greater the concentration difference, the faster the rate of facilitated diffusion

  This is because:

  All particles are still moving down the equilibrium. This is true until all the proteins are in use or one equilibrium is reached , then the rate will level off.
• How does the number of channel / carrier proteins affect the rate of facilitated diffusion?

  The greater the number of channel or carrier proteins in the plasma membrane the faster the rate of facilitated diffusion.

  This is because:

  Once all the proteins in the membrane are in use, facilitated diffusion cant happen any faster even if you increase the concentration gradient. So it becomes a limiting factor
• Define osmosis


  The net movement of water molecules through a partially permeable membrane from an area of high water potential to an area of low water potential
• What is water potential?

  The potential or likelihood of water molecules to diffuse out of or into a solution
• What is the water potential of pure water?

  0
• What does a negative water potential show?

  The more negative the water potential, the more concentrated the solution is
• Define hypotonic?


  The solution has a higher water potential than the inside of the cell so water will move into the cell by osmosis
• Define isotonic


  There will be no net movement of water molecules because there is no difference in water potential between the cell and the surrounding solution
• Define hypertonic


  The solution has a lower water potential than the inside of the cell so water will move out of the cell by osmosis.
• What effect will a hypotonic solution have on plant and animal cells?

  Animal cells - they will swell up and eventually burst as their plasma membranes are not strong enough to withstand the pressure of the water

  Plant cells - they will swell up but not burst because their cell walls protect them. Their vacuoles expand and they become turgid as the plasma membrane pushes on the cell wall.