How do phospholipids arrange themselves in a membrane?

As a bilayer, with hydrophilic heads facing the water and hydrophobic tails facing inwards.

Why can sodium ions not diffuse through the bilayer?

They are charged, so they are repelled by the hydrophobic tails; they need channel or carrier proteins.

Which substances diffuse directly through the bilayer?

Small, non-polar molecules such as oxygen and carbon dioxide.

What is the role of cholesterol in membranes?

It fits between phospholipid tails and regulates the fluidity of the membrane.

What are glycoproteins used for?

Cell recognition and as receptors, with carbohydrate chains on the outer surface.

Describe the fluid mosaic model.

A fluid phospholipid bilayer with proteins scattered through it like a mosaic.

Why is the membrane described as fluid?

Phospholipids and proteins can move sideways within the layer.

What did the cell fusion experiment show?

Labelled proteins mixed over the hybrid cell, so membrane proteins can move within the membrane.

Why did the labels not mix at 0 °C?

The phospholipids have less kinetic energy, so the membrane is less fluid.

In Core Practical 3, why does the liquid become red?

The membranes are damaged, so red betalain pigment leaks out of the beetroot vacuoles.

How does ethanol increase membrane permeability?

It dissolves lipids and disrupts the bilayer, and it can denature membrane proteins.

How does high temperature increase permeability?

It denatures membrane proteins and makes the bilayer more fluid and disrupted.

Cell membrane structureEdexcel International A Level Biology: Flashcards

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Describe a phospholipid.

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Describe a phospholipid.: A hydrophilic phosphate head and two hydrophobic fatty acid tails.
How do phospholipids arrange themselves in a membrane?: As a bilayer, with hydrophilic heads facing the water and hydrophobic tails facing inwards.
Why can sodium ions not diffuse through the bilayer?: They are charged, so they are repelled by the hydrophobic tails; they need channel or carrier proteins.
Which substances diffuse directly through the bilayer?: Small, non-polar molecules such as oxygen and carbon dioxide.
What is the role of cholesterol in membranes?: It fits between phospholipid tails and regulates the fluidity of the membrane.
What are glycoproteins used for?: Cell recognition and as receptors, with carbohydrate chains on the outer surface.
Describe the fluid mosaic model.: A fluid phospholipid bilayer with proteins scattered through it like a mosaic.
Why is the membrane described as fluid?: Phospholipids and proteins can move sideways within the layer.
What did the cell fusion experiment show?: Labelled proteins mixed over the hybrid cell, so membrane proteins can move within the membrane.
Why did the labels not mix at 0 °C?: The phospholipids have less kinetic energy, so the membrane is less fluid.
In Core Practical 3, why does the liquid become red?: The membranes are damaged, so red betalain pigment leaks out of the beetroot vacuoles.
How does ethanol increase membrane permeability?: It dissolves lipids and disrupts the bilayer, and it can denature membrane proteins.
How does high temperature increase permeability?: It denatures membrane proteins and makes the bilayer more fluid and disrupted.

Exam questions on Cell membrane structure

Cell surface membranes are made mainly of phospholipids and proteins. Each phospholipid molecule has a phosphate-containing head that attracts water and two fatty acid tails that repel water.
Explain why sodium ions cannot diffuse directly through the phospholipid bilayer.2 marks
In an experiment on cell fusion, a mouse cell and a human cell were joined to make one hybrid cell. The membrane proteins of the mouse cell were labelled with a red fluorescent dye and those of the human cell with a green dye. Immediately after fusion the red and green labels were in separate halves of the hybrid cell. After 40 minutes at 37 °C the two labels were completely mixed over the whole surface. When the experiment was repeated at 0 °C the labels stayed in separate halves.
Explain how the results at 37 °C support the fluid mosaic model of membrane structure.2 marks
A student investigates the effect of temperature on the permeability of beetroot cell membranes. She cuts identical discs of beetroot, rinses them, and places one disc in 10 cm³ of distilled water in each of five test tubes held in water baths at 20, 30, 40, 50 and 60 °C for 10 minutes. She then removes the discs and measures the absorbance of the liquid in each tube with a colorimeter. Absorbance rose slightly between 20 °C and 40 °C and then rose sharply above 50 °C.
Explain why the absorbance of the liquid increases when the membranes of the beetroot cells become more permeable.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).