1.4 membrane transport

Question 1

what is Simple diffusion?

Answer 1

• passive movement of particles from a region of high concentration to a region of low concentration
• passing between the phospholipid molecules
• not all particles are
  able to pass through the membrane in this way.
• Examples include oxygen, urea, steroids and ethanol

Question 2

what is Facilitated diffusion?

Answer 2

• channels are made from proteins and have holes through their centre for the particles to pass through
• Carrier proteins transfer particles across the membrane by a conformational change
• Each channel or carrier has a structure that allows only one type of particle to pass through
• Particles diffuse through the protein channels along the concentration gradient.
• Examples include glucose, amino acids, magnesium ions and potassium ions

Question 3

what is Osmosis?

Answer 3

• passive movement of water molecules across a partially permeable membrane
• lower solute concentration to a region of higher solute concentration
• osmosis is due to differences in the concentration of substances dissolved in water (the solute)
• Some cells that require a greater movement of water, such as the nephron in the kidney, also have water channels called aquaporins that greatly increase the permeability of the membrane to water

Question 4

what is active transport?

Answer 4

• The movement of particles against the concentration gradient
• ATP is needed for this transport
• done by globular proteins in the membrane called pump proteins
• allows cells to take in substances even though the cell may already have these in high supply, or to pump out substances into a region outside the cell with a higher concentration

Question 5

how do pump proteins work?

Answer 5

• change shape, using ATP, when a specific molecule or ion enters the
  pump.
• The molecule or ion reaches into the centre of the pump protein and then passes to the opposite side of the membrane.
• Then the pump protein returns to its original form.
• eg. sodium, potassium and calcium ions.

Question 6

what is the Active transport of sodium and potassium in axons?

Answer 6

• Nerve impulses are caused by movements of sodium (Na+) and potassium (K+) ions across the membrane of the axon
• movement of these ions is by active transport and facilitated diffusion
• The sodium-potassium pump follows a repetitive cycle of steps resulting in three sodium ions pumped out of the axon and two potassium ions being pumped in

Question 7

what is the cycle of the transport of sodium and potassium in axons?

Answer 7

1. The pump opens to the inside of the axon and 3 sodium ions enter the pump.
2. ATP transfer one of its phosphates to the pump, so changes the shape of the pump, so closing open.
3. The pump then opens to the outside of the axon, and the sodium ions are released.
4. As the pump is opened to the outside, 2 potassium ions can enter and bind to its binding spots.
5. This causes the phosphate group from the original ATP to be released, which changes the shape of the pump.
6. The pump is then open to the inside of the axon, allowing the potassium ions to be released into the axon.
7. The cycle is then repeated.

Question 8

what are Potassium voltage-gated channels?

Answer 8

• potassium channels travel through potassium channels that are voltage-gated due to the imbalance of positive and negative charges across the membrane.
• When the outside of the axon more positively charged than the inside of the axon, potassium channels close
• When a nerve impulse is passing through the axon, the inside becomes more positive, causing potassium channels to open.
• The potassium ions can diffuse through the channels to the outside of the axon.

Question 9

different osmolarity?

Answer 9

• An isotonic solution has the
  same osmolarity as a tissue.
• A hypertonic solution has a
  higher osmolarity.
• A hypotonic solution has a
  lower osmolarity

Question 10

how are removed organs protected?

Answer 10

• soak in isotonic solution
• Saline solution (NaCl) is usually used during medical procedures as it has approximately the same osmolarity as human tissues (300 mOsm).
• Saline is also used to rinse wounds, keep damaged skin moist, used as eye drops or bathing organs for transplants.

Question 11

what does membrane fluidity allow?

Answer 11

• allows the change of shape and thus the formation of vesicles used in endocytosis and exocytosis processes
• partly due to the weak bonding between the phospholipid tails and bends in the tails themselves that prevent close packing.
• Cholesterol also affects membrane fluidity.

Question 12

what is exocytosis?

Answer 12

• remove things from cells
• requires ATP
• Vesicles containing waste products or unwanted materials move to and fuse with the plasma membrane of the cell the expel material