1.4 (Membrane Transport) Flashcards
Explain the phospholipid bilayer.
- Selectively permeable
- Some molecules pass through easily (diffusion), go through a ‘tunnel’ (facilitated diffusion), need energy to get them through (active transport) or use their own membrane to get them through (endo-/exo- cytosis)
Define diffusion.
The passive net movement of particles from areas of high concentration to low concentration, often through a partially permeable membrane.
Compare high and low concentration gradient.
High concentration gradient = faster diffusion, low concentration gradient = slower diffusion
What factors affect diffusion rate?
- Concentration gradient
- Surface area
- Length of diffusion path
How can the length of the diffusion path be reduced?
- Membranes are incredibly thin
- Folded membranes increase the SA:Vol ratio
- More membrane in a smaller volume means shorter distances across which molecules must diffuse
How can the length of the diffusion path be maximised?
- Alveoli in lungs
- Membrane folds in mitochondria and in cristae in the chloroplasts
- Root hairs for water and mineral ion uptake
- Villi for absorption of digested food molecules
What is osmosis?
- May occur when there is a partially permeable membrane, such as acell membrane.
- When a cell is submerged inwater, the water molecules pass through the cell membrane from an area of low solute concentration (outside the cell) to one of high solute concentration (inside the cell)
What is aquaporin?
An integral protein that acts as a pore in the membrane that speeds the movement of water molecules
Importance of osmotic control.
Common medical procedures in which an isotonic saline solution is useful:
- fluids introduction to a patient’s blood system via an intravenous drip, e.g for rehydration
- used to rinse wounds, skin abrasions etc.
- keep areas of damaged skin moist before applying skin grafts
- eye drops/wash
- frozen and used pack donor organs for transportation
What is facilitated diffusion?
- Large and polar molecules can’t get across the membrane via simple diffusion
- Transmembrane (polytopic) proteins recognise a particular molecule and help it to move across the membrane. The direction it moves is dependent on the concentration gradient
Outline how potassium channels enable facilitated diffusion.
- At one stage during a nerve impulse there are relatively more positive charges inside
- This voltage change causes potassium channels to open, allowing potassium ions to diffuse out of the axon
- Once the voltage conditions change the channel rapidly closes again
What is the role of integral protein pumps in primary active transport?
- Primary active transport requires ATP.
- Integral protein pumps use the energy from the hydrolysis of ATP to move ions or large molecules across the cell membrane.
- Molecules are moved against their concentration gradient
Outline the sodium-potassium pump cycle.
- The interior of the pump is open to the inside of the axon; three sodium ions enter the pump and attach to their binding sites
- ATP transfers a phosphate group from itself to the pump; this causes the pump to change shape and the interior is then closed
- The interior of the pump opens to the outside of the axon and the three sodium ions are released
- Two potassium ions from outside can then enter and attach to their binding sites
- Binding of potassium causes release of the phosphate group; this causes the pump to change shape again so that it is again only open to the inside of the axon
- The interior of the pump opens to the inside of the axon and the two potassium ions are released. Sodium ions can now enter and bind to the pump again
Where is the required energy for secondary active transport from?
It is derived from energy stored in the form of concentration differences in a second solute
Define endocytosis.
The taking in of external substances by an inward pouching of the plasma membrane, forming a vesicle
