What is the role of cholesterol in membranes?

It regulates fluidity and stability, fitting between phospholipids.

Which types of molecule cross the bilayer by simple diffusion?

Small, non-polar, lipid-soluble molecules such as oxygen, carbon dioxide and steroid hormones.

Why can't ions cross the bilayer by simple diffusion?

They are charged and cannot pass through the hydrophobic fatty acid tails.

What is facilitated diffusion?

The passive movement of polar molecules or ions through channel or carrier proteins, down a concentration gradient.

Why does the rate of facilitated diffusion level off?

All carrier proteins are in use (saturated), so their number limits the rate.

The net movement of water from a higher to a lower water potential through a partially permeable membrane.

What is the water potential of pure water?

0 kPa, the highest possible value; solutions are negative.

What happens to a plant cell in pure water?

Water enters by osmosis, the cell becomes turgid and P rises until ψ = 0 and net movement stops.

Why does beetroot pigment leak at high temperature?

Membrane proteins denature and the bilayer is disrupted, so the membranes become more permeable.

How is the water potential of plant tissue found in Core Practical 6?

Find the sucrose concentration where the percentage change in mass is zero, and read off its water potential.

Cell membranes and passive transportEdexcel A-Level Biology B: Flashcards

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Describe the fluid mosaic model.

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Describe the fluid mosaic model.: A phospholipid bilayer in which phospholipids and proteins can move (fluid) and proteins are scattered through it (mosaic).
What is the role of cholesterol in membranes?: It regulates fluidity and stability, fitting between phospholipids.
Which types of molecule cross the bilayer by simple diffusion?: Small, non-polar, lipid-soluble molecules such as oxygen, carbon dioxide and steroid hormones.
Why can't ions cross the bilayer by simple diffusion?: They are charged and cannot pass through the hydrophobic fatty acid tails.
What is facilitated diffusion?: The passive movement of polar molecules or ions through channel or carrier proteins, down a concentration gradient.
Why does the rate of facilitated diffusion level off?: All carrier proteins are in use (saturated), so their number limits the rate.
Does facilitated diffusion need ATP?: No, it is passive.
Define osmosis.: The net movement of water from a higher to a lower water potential through a partially permeable membrane.
What is the water potential of pure water?: 0 kPa, the highest possible value; solutions are negative.
State the equation linking water potential, pressure and osmotic potential.: ψ = P + π
What happens to a plant cell in pure water?: Water enters by osmosis, the cell becomes turgid and P rises until ψ = 0 and net movement stops.
Why does beetroot pigment leak at high temperature?: Membrane proteins denature and the bilayer is disrupted, so the membranes become more permeable.
How is the water potential of plant tissue found in Core Practical 6?: Find the sucrose concentration where the percentage change in mass is zero, and read off its water potential.

Exam questions on Cell membranes and passive transport

A liver cell takes up and releases many substances across its cell surface membrane, including testosterone (a lipid-soluble steroid hormone), glucose, sodium ions and amino acids. The membrane is described by the fluid mosaic model.
Explain why sodium ions cannot cross the membrane by simple diffusion, and state how they can cross it by a passive process.2 marks
A student investigated the effect of temperature on the permeability of beetroot cell membranes. She cut discs of beetroot of identical size, rinsed them, and placed one in each tube of distilled water at 20, 30, 40, 50 and 60 °C for 20 minutes. She then measured the absorbance of the water in each tube with a colorimeter, as a measure of the red pigment, betalain, that had leaked out. The absorbance readings were 0.05, 0.08, 0.15, 0.62 and 1.10.
Explain the large increase in absorbance between 40 °C and 50 °C.2 marks
Glucose enters red blood cells through carrier proteins in the cell surface membrane. A scientist measured the rate of glucose uptake at different external glucose concentrations. The rate rose in proportion to the concentration up to 5 mmol dm⁻³, but at higher concentrations it levelled off at a maximum. The uptake was not changed when a respiratory inhibitor was added to the cells.
Explain why the rate of glucose uptake levels off at higher glucose concentrations.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).