The membrane and transport
How molecules get into and out of the cell: the lipid bilayer, simple and facilitated diffusion, osmosis, primary and secondary active transport, endocytosis and exocytosis, and their medical uses (infusions, rehydration, drugs).
- 34 cards
- 3 topics
- Free
The outline, topic by topic
Each topic is weighted the way the exam weighs it.
- Concepts
- Scenarios
- Understand
A few cards from the deck
Phospholipid bilayer: what is it?
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A double layer of phospholipids with water-loving heads facing outwards and water-fearing tails in the middle.
Its fatty core stops ions and large polar molecules.
What happens to red cells placed in pure water?
- They swell and burst (haemolysis)
- They shrink
- Nothing changes
- They divide
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They swell and burst (haemolysis)
Water flows in by osmosis towards the more concentrated inside: pure water is never infused.
Simple diffusion: what is it?
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A molecule crossing the bilayer on its own, from higher to lower concentration, with no energy.
That is the case for oxygen, CO₂ and small fat-soluble molecules.
Why does oxygen cross the membrane without a carrier?
- It is a small non-polar molecule, soluble in the bilayer's fatty core
- It has its own channel
- It is pumped by ATP
- It does not cross it
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It is a small non-polar molecule, soluble in the bilayer's fatty core
Ions, being charged, need channels.
Facilitated diffusion: what is it?
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Passage down the gradient through a channel or carrier, with no energy.
The carrier saturates: above a certain concentration, the rate levels off.
Why do oral rehydration solutions contain salt and sugar?
- For the taste
- The gut's sodium-glucose cotransport pulls water in with them
- To kill microbes
- To block diarrhoea with acid
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The gut's sodium-glucose cotransport pulls water in with them
Secondary transport keeps working even during cholera.