How are phospholipids arranged in a membrane?

As a bilayer, with hydrophilic heads outwards and hydrophobic tails inwards.

Why can ions not easily cross the bilayer?

They are charged and are repelled by the hydrophobic core.

Which molecules pass through the bilayer easily?

Small non-polar molecules such as oxygen.

What is a glycoprotein?

A protein with a carbohydrate chain attached.

What is a glycolipid?

A lipid with a carbohydrate chain attached.

Why is the fluid-mosaic model called fluid?

Phospholipids and many proteins can move within their layer.

Why is it called a mosaic?

Proteins of different sizes and shapes are scattered through the bilayer.

What does cholesterol do in a membrane?

It restricts the movement of the other molecules.

In the beetroot practical, what does a higher absorbance show?

More pigment has leaked, so the membrane is more permeable.

Why does high temperature increase membrane permeability?

Proteins are denatured and the bilayer is disrupted.

Why does ethanol increase membrane permeability?

It dissolves or disrupts the phospholipids.

Why are beetroot discs rinsed before the experiment?

To remove pigment released from cells damaged by cutting.

Cell membrane structureAQA A-Level Biology: Flashcards

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What is the same about all membranes in a eukaryotic cell?

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What is the same about all membranes in a eukaryotic cell?: They have the same basic structure.
How are phospholipids arranged in a membrane?: As a bilayer, with hydrophilic heads outwards and hydrophobic tails inwards.
What does hydrophilic mean?: Attracted to water.
Why can ions not easily cross the bilayer?: They are charged and are repelled by the hydrophobic core.
Which molecules pass through the bilayer easily?: Small non-polar molecules such as oxygen.
What is a glycoprotein?: A protein with a carbohydrate chain attached.
What is a glycolipid?: A lipid with a carbohydrate chain attached.
Why is the fluid-mosaic model called fluid?: Phospholipids and many proteins can move within their layer.
Why is it called a mosaic?: Proteins of different sizes and shapes are scattered through the bilayer.
What does cholesterol do in a membrane?: It restricts the movement of the other molecules.
In the beetroot practical, what does a higher absorbance show?: More pigment has leaked, so the membrane is more permeable.
Why does high temperature increase membrane permeability?: Proteins are denatured and the bilayer is disrupted.
Why does ethanol increase membrane permeability?: It dissolves or disrupts the phospholipids.
Why are beetroot discs rinsed before the experiment?: To remove pigment released from cells damaged by cutting.

Exam questions on Cell membrane structure

A technician cut equal-sized discs from a beetroot with a cork borer and then rinsed them in distilled water before starting an experiment on membrane permeability.
The technician placed other discs in ethanol. Explain why the solution around these discs became redder than the solution around discs in water.2 marks
Phospholipid molecules have a hydrophilic head containing a phosphate group and two hydrophobic fatty acid tails. A cell-surface membrane has an aqueous solution on both sides: tissue fluid outside the cell and cytoplasm inside it.
Explain why sodium ions cannot easily diffuse through the middle of the membrane.2 marks
The cell-surface membrane of a liver cell contains phospholipids, cholesterol, proteins and short carbohydrate chains. In 1972 Singer and Nicolson proposed the fluid-mosaic model of membrane structure.
Describe the arrangement of the phospholipids, proteins and carbohydrates in this membrane.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).