transport across membranes

• What does partially permeable mean?

  The membrane lets some molecules in but not others
• What are the 3 ways in which a substance can move across the cell surface membrane?

  Diffusion, osmosis, active transport
• What are membranes composed of?

  Lipids , proteins and carbs
• What does the fluid mosaic model describe?

  Arrangement of molecules in the membrane
• Describe the fluid mosaic model

  Phospholipids form continuous bilayer which is fluid because they are constantly moving

  Proteins are scattered through like tiles in a mosaic these include channel and carrier proteins
• What do receptor proteins in the cell surface membrane do?

  Allow cell to detect chemicals released from other cells
• When do liver cells absorb glucose?


  When hormone insulin binds to proteins on the liver cells
• What is a glycoprotein?


  A protein with a carbohydrate attached
• What is a glycolipid?

  Lipid molecule with attached carbohydrate
• Where can you find cholesterol molecules?

  Cell membrane bilayer
• What molecules form a barrier to dissolved substances in the cell membrane?

  Phospholipids
• The head of a phospholipid is.......... and the tail is........

  Hydrophilic (attracts water) Hydrophobic(repels water)
• Describe the arrangement of phospholipids in the bilayer

  Heads face out towards the water on either side of membrane, the centre is hydrophobic so doesn't allow water soluble substances to diffuse through
• Explain the importance of cholesterol in cell membrane

  - gives membrane stability and helps animal cells which don't have cell walls . Important in unsupported free cells like red blood cells

  - has hydrophobic regions so is able to create a further barrier to polar substances
• How does cholesterol give the membrane stability?

  Binds to hydrophobic tails of phospholipids causing them to pack more closely together - restricts the movement making it less fluid and more rigid
• How does temperature affect cell membranes?

  Affects how much the phospholipids move which affects membrane structure and permeability
• Suggest 3 things that can happen to the cell membrane when it's under 0 degrees centigrade

  - phospholipids don't have much energy to move so they pack closely together making the membrane rigid

  - channel and carrier proteins denature increasing the permeability of the membrane

  - ice crystals may form and pierce the membrane making it highly permeable when it thaws
• What happens to the permeability of membrane when temp is between 0 - 45 degrees

  Phospholipids can move so membrane is partially permeable , as temp rises becomes more permeable
• What happens to the permeability of cell membranes when temp is above 45 degrees? 3 reasons why

  Increases - bilayer starts to melt, water inside cell expands putting pressure on membrane, channel and carrier proteins denature
• Why is beetroot good for investigating cell permeability?

  Pigmented - the higher the permeability the more pigment leaks out
• Define diffusion


  The movement of molecules from high concentration to low concentration
• Particles diffuse ....... a concentration gradient

  Down
• What is a concentration gradient?

  The path from an area of high concentration to an area of low concentration
• Explain what passive process means


  No energy is needed for it to happen
• Diffusion is a ................. process

  Passive
• What is simple diffusion?

  Molecules pass directly through the plasma membrane without the assistance of another molecule.
• Why can oxygen and co2 diffuse easily through cell membranes?

  They're small - can pass through spaces between phospholipids

  Non polar- soluble in lipids so they can dissolve in hydrophobic bilayer
• What 3 factors affect the rate of diffusion?

  - concentration gradient - higher it is the faster

  - thickness of exchange surface - thinner it is the faster

  - surface area - the larger the faster
• What is facilitated diffusion?


  the movement of specific molecules across cell membranes through protein channels
• In facilitated diffusion, particles move......... the concentration gradient

  Down
• facilitated diffusion is a ................ process

  Passive
• What are the two types of protein in facilitated diffusion?

  Carrier and channel
• What do carrier proteins do?

  Move large molecules across membrane down concentration gradient
• describe how a carrier protein moves large molecules across a membrane?

  1. molecule attaches to membrane 2. protein changes shape 3.releases molecule on opposite side
• what do protein channels do?


  form pores in membrane for charged particles to diffuse through down the concentration gradient
• which 2 factors affect facilitated diffusion?


  concentration gradient and number of proteins
• what are aquaporins?

  channel proteins for water
• what is osmosis?

  net movement of water across a partially permeable membrane from an are of high water potential to an area of low water potential
• what is water potential?

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


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