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Flashcards · 9700 · AS Topic 4

Cell membranes and transport (AS Level) flashcards

14 cards on AS Topic 4, subtopics 4.1 to 4.2. Pick a subtopic or study them all.

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4.1 Fluid mosaic membranes

Fluid mosaic model
A phospholipid bilayer in which molecules move sideways (fluid), with proteins scattered through it (mosaic).
Role of cholesterol in a membrane
Sits between phospholipids in both layers. It regulates fluidity, adds stability and reduces permeability to ions and polar molecules.
Glycoprotein vs glycolipid
Carbohydrate chain attached to a protein vs to a lipid. Both face outwards and act in cell recognition (as antigens) and stability.
Ligand
A signalling chemical that binds to a specific receptor.
Outline cell signalling across a membrane
The signalling cell secretes a ligand, which is transported (for example in blood) to target cells. It binds to a complementary cell surface receptor, and a specific response follows. Cells without the receptor do not respond.
Channel protein vs carrier protein
A channel protein is a fixed, water-filled pore. A carrier protein binds the substance and changes shape to move it across.

4.2 Movement into and out of cells

Diffusion
The net movement of molecules or ions from a region of higher concentration to a region of lower concentration, by random movement.
Osmosis
The net movement of water from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.
Active transport
Movement of molecules or ions against a concentration gradient through carrier proteins, using energy from ATP.
Facilitated diffusion vs active transport
Both use membrane proteins. Only active transport uses ATP and moves substances against a concentration gradient.
Water potential rules
Pure water = 0 kPa, the highest value. Solutes lower ψ, so solutions are negative. Water moves from less negative to more negative ψ (−200 kPa is higher than −800 kPa).
Plant and animal cells in a solution of lower water potential
Animal cell: loses water, shrinks (crenated). Plant cell: loses water, plasmolysed; the protoplast pulls away from the wall. (In higher ψ: animal cell may burst; plant cell becomes turgid.)
Why does a larger object rely less on diffusion alone?
Larger objects have a smaller surface area to volume ratio, so diffusion reaches a smaller fraction of their volume. Write SA:V as n : 1.
Finding the water potential of a plant tissue
Weigh equal tissue pieces, immerse in a range of sucrose concentrations for a fixed time, reweigh. Plot percentage change in mass against concentration; at 0 % change the tissue ψ equals the solution ψ.

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