Plasma Membranes

• Name 3 examples of membranes with the plasma membrane structure.

  Nuclear membrane, mitochondrial membrane, cell membrane
• What is the function of membranes?

  To control the movement of substances across it.
• What does semi-permeable/partially permeable mean?

  Only some substances can be transported across.
• What is the phospholipid bilayer?

  A continuous double layer of phospholipids with hydrophilic heads on the outside and hydrophobic tails on the inside, creating a hydrophobic region that provides a barrier to polar (water-soluble) substances. The bilayer forms the main structure of a plasma membrane.
• What does cholesterol do in the bilayer?

  It binds to the tails of phospholipids and therefore reduces fluidity of the membrane.
• What are intrinsic proteins and the two types?

  Proteins that pass all the way through the membrane.

  (carrier proteins and channel proteins)
• What are extrinsic proteins?

  Proteins found on the surface of the membrane or which passes part the way through it (e.g. receptors).
• What are glycoproteins?

  Extrinsic proteins in the membrane that have carbohydrates attached to them.
• What is a receptor?

  An extrinsic protein with a specific shape (tertiary structure) that other substances such as a hormone or neurotransmitter bind to. They can open/close carrier proteins when attached.
• Which substances can dissolve through the phospholipid bilayer and by what transport?

  Lipid-soluble molecules (e.g. some hormones) by simple diffusion.
• Which substances can pass through the gaps between phospholipids and by what transport?

  Small and non-polar molecules (e.g. oxygen and water) by simple diffusion.
• Which substances must pass through using a complementary shaped channel or carrier protein and by what transport?

  Larger and polar (water-soluble) molecules (e.g. amino acids, glucose, ions) by facilitated diffusion or active transport.
• How does a channel protein work?

  Has a space (channel) in which a substance with a complementary shape can pass through - facilitated diffusion.
• How does a carrier protein work?

  Has a receptor site that substances with a complementary shape can bind to. The protein then changes shape allowing the molecule to pass through to the other side. This transport can either by facilitated diffusion or active transport (however the latter requires energy).
• Where in the cell can ATP synthase be found?

  In the cristae membrane of mitochondria (intrinsic protein).
• What is meant by the 'fluid-mosaic model'?

  Used to describe the structure of a plasma membrane - the continuous phospholipid bilayer that is constantly moving with various proteins scattered throughout it.
• How does temperature effect permeability of a membrane?


  As temperature increases, permeability increases as protein tertiary structures within the membrane begin to denature.

  Below zero, permeability also increases as ice crystals form, widening the gaps in the phospholipid bilayer.
• How does pH affect permeability of a membrane?

  If pH strays too far from optimum, protein tertiary structures in the membrane begin to denature.
• How does concentration of ethanol affect permeability of a membrane?

  The higher the concentration of ethanol, the more permeable a membrane becomes as the ethanol dissolves phospholipids in the bilayer.
• What is diffusion as a whole?

  The net movement of particles from an area of higher concentration to an area of lower concentration (along a concentration gradient).

  This is a passive process.
• What is the difference between simple and facilitated diffusion?

  Simple diffusion doesn't require a membrane, whereas facilitated diffusion does, as well as an intrinsic protein (channel or carrier) with a complementary shape/receptor site to the molecule being transported.
• What is active transport?

  The movement of particles across a plasma membrane from an area of lower concentration to an are of higher concentration (against a concentration gradient). This process requires a carrier protein with a complementary-shaped receptor site to the molecule and also energy in the form of ATP from respiration.
• What limits the rate of transport across a membrane?

  The number of carrier/channel proteins - they reach a point of saturation where all are 'being used' at the same time.
• What factors increase the rate of diffusion across a membrane?

  . steeper concentration gradient

  . more complementary-shaped carrier/channel proteins (facilitated)

  . increased surface area
• What factors increase the rate of active transport across a membrane?

  . high rate of respiration (to release ATP to provide energy)

  . more complementary-shaped carrier proteins

  . increased surface area
• What is osmosis?

  The net movement of water molecules from an area of higher (less negative) water potential to an area of lower (more negative) water potential (down the water potential gradient), across a partially permeable membrane.
• In which two ways can water diffuse across a membrane?

  . through the phospholipid bilayer

  . through channel proteins called aquaporins
• What is water potential?

  A pressure exerted by water molecules on a membrane, measured in kPa.
• What is the water potential of pure water?

  0 kPa
• What happens when a solute is added to pure water?

  Water molecules are bound to the solute molecules as a hydration shells and therefore cannot pass across a membrane. Increasing solute concentration decreases the number of freely moving water molecules and so water potential decreases.
• What does hypotonic mean?

  The solution has a higher water potential than the cytoplasm in the cell.

  As a result water diffuses into the cell causing it to swell.

  In animal cells, too much swelling leads to cell lysis (bursting).

  In plant cells, cell walls are able to resist pressure, preventing cell lysis and so the cell just becomes turgid.
• What does isotonic mean?

  Two solutions have the same water potential so there is no net movement of water (osmotic effect).

  Animal cells remain normal size, plant cells are described as flaccid.
• What does hypertonic mean?

  The solution has a lower water potential than the cytoplasm in the cell.

  As a result, water diffuses out of the cell causing it to shrink.

  In plant cells the cell membrane as a result shrinks away from the cell wall - this is called plasmolysis.
• How does cystic fibrosis relate to osmosis?


  Cystic fibrosis means that an individual is unable to water down thick mucus, causing issues.

  This occurs through a mutation in the gene for making chloride ion carrier proteins which leads to an altered tertiary structure and a receptor site that is no longer complementary to the ions. As a result, chloride ions cannot be actively transported across the membrane into the mucus, reducing its water potential and therefore triggering water to move into it by osmosis. As a result the mucus remains thick and sticky.