Membranes control traffic

By UK A Level Biology · independent study · myAudience

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Membranes control traffic

A phospholipid bilayer forms a selective boundary containing proteins. Small nonpolar molecules can cross directly; some other substances use channels or carriers. Diffusion and facilitated diffusion have net movement down a gradient and do not directly require ATP hydrolysis.

Active transport uses an energy supply to move substances against a gradient. Osmosis concerns water moving across a selectively permeable membrane along a water-potential gradient. Co-transport can use one substance moving down its gradient to drive another across.

The fluid-mosaic model includes mobile lipids, embedded proteins and carbohydrate-bearing components involved in recognition. Cholesterol affects movement and stability in membranes where it occurs. Water moves from higher to lower water potential across a suitable membrane: −150 kPa is higher than −400 kPa. Surface area and transporter abundance affect capacity, but selectivity, the driving gradient and substrate availability also matter. A leakier membrane is not necessarily a better-regulated membrane.

Worked example: Water potentials are -150 kPa outside a cell and -400 kPa inside. Predict initial net water movement.

-150 kPa is the higher water potential.

Water moves toward the lower water potential across a suitable membrane.

Answer: Into the cell.

Check your understanding: Is facilitated diffusion a form of active transport?

Answer: No.

Why: A transport protein is involved, but net movement is down a gradient without direct ATP use.