Chapter summary
Membranes and transport, at a glance
Scan the key ideas, or hide the answers and try to recall them.
01
The fluid mosaic membrane
How can a membrane be a barrier and a communication surface?
Key idea and reminders
A mobile phospholipid bilayer forms the barrier; embedded molecules provide selective transport, recognition and signalling.
- Hydrophobic interior: barrier to ions.
- Proteins: transport, receptors and enzymes.
- Cholesterol buffers fluidity.
Keep in mind: Fluid describes lateral movement within the membrane; permeability across it remains selective.
02
Diffusion, channels and osmosis
When does membrane transport need a protein but no ATP input?
Key idea and reminders
Passive transport follows the relevant gradient; needing a membrane protein does not itself make transport active.
- Passive: down the relevant gradient.
- Facilitated diffusion needs a protein, not direct ATP input.
- Osmosis follows water potential.
Keep in mind: Carriers can mediate facilitated diffusion down a gradient or active transport against it.
03
Move a solute against its gradient
Where does the energy for uphill transport come from?
Key idea and reminders
Active transport couples an energetically unfavourable movement to an energy source, using selective membrane proteins.
- Pumps can couple transport to ATP hydrolysis.
- Cotransport can use energy stored in an ion gradient.
- Stopping a pump does not stop diffusion.
Keep in mind: A passive channel offers a downhill route; active pumping needs coupling and regulated protein changes.
04
Endocytosis and exocytosis
How does a cell move material too large for a carrier?
Key idea and reminders
Endocytosis encloses external material in a vesicle; exocytosis fuses a vesicle with the surface membrane to release material.
- Endocytosis: membrane encloses material.
- Exocytosis: vesicle fuses and releases.
- Vesicle lumen corresponds to the extracellular side.
Keep in mind: The membranes fuse; the vesicle does not pass bodily through an unchanged bilayer.