How are phospholipids arranged in a membrane?

A bilayer, with heads facing the watery surroundings and tails facing inwards.

Why can oxygen cross the bilayer but ions cannot?

Oxygen is small and non-polar and dissolves in the hydrophobic core; ions are charged and are repelled by it.

What is the function of cholesterol in a membrane?

It regulates fluidity and stability, and reduces leakiness.

What is the role of channel and carrier proteins?

They provide routes for specific ions and polar molecules to cross the membrane.

What is the role of glycoproteins?

They act as receptors, for example for hormones, and in cell recognition.

Why is it called the fluid mosaic model?

Phospholipids and proteins can move (fluid), and proteins are scattered through the bilayer (mosaic).

What was the Davson and Danielli model?

A phospholipid layer between two continuous layers of protein.

What evidence led to the fluid mosaic model replacing it?

Freeze-fracture electron microscopy showed proteins embedded in the bilayer.

What did Frye and Edidin show?

Labelled proteins from fused mouse and human cells mixed at 37 °C, so membrane proteins can move and the membrane is fluid.

Why did mixing slow at 1 °C?

Phospholipids have less kinetic energy and pack more closely, making the membrane less fluid.

Why is the model described as an interpretation of data?

It is the best explanation of the evidence available, and may be revised if new evidence arises.

In the beetroot practical, why does absorbance rise with more ethanol?

Ethanol disrupts the membrane, so more pigment leaks out.

Why are beetroot discs rinsed before use?

To remove pigment released from cells damaged by cutting.

Cell membrane structureEdexcel A-Level Biology A: Flashcards

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Describe the structure of a phospholipid.

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Describe the structure of a phospholipid.: A hydrophilic phosphate head and two hydrophobic fatty acid tails.
How are phospholipids arranged in a membrane?: A bilayer, with heads facing the watery surroundings and tails facing inwards.
Why can oxygen cross the bilayer but ions cannot?: Oxygen is small and non-polar and dissolves in the hydrophobic core; ions are charged and are repelled by it.
What is the function of cholesterol in a membrane?: It regulates fluidity and stability, and reduces leakiness.
What is the role of channel and carrier proteins?: They provide routes for specific ions and polar molecules to cross the membrane.
What is the role of glycoproteins?: They act as receptors, for example for hormones, and in cell recognition.
Why is it called the fluid mosaic model?: Phospholipids and proteins can move (fluid), and proteins are scattered through the bilayer (mosaic).
What was the Davson and Danielli model?: A phospholipid layer between two continuous layers of protein.
What evidence led to the fluid mosaic model replacing it?: Freeze-fracture electron microscopy showed proteins embedded in the bilayer.
What did Frye and Edidin show?: Labelled proteins from fused mouse and human cells mixed at 37 °C, so membrane proteins can move and the membrane is fluid.
Why did mixing slow at 1 °C?: Phospholipids have less kinetic energy and pack more closely, making the membrane less fluid.
Why is the model described as an interpretation of data?: It is the best explanation of the evidence available, and may be revised if new evidence arises.
In the beetroot practical, why does absorbance rise with more ethanol?: Ethanol disrupts the membrane, so more pigment leaks out.
Why are beetroot discs rinsed before use?: To remove pigment released from cells damaged by cutting.

Exam questions on Cell membrane structure

In 1935 Davson and Danielli proposed that a cell membrane is a layer of phospholipid with a continuous layer of protein covering both of its surfaces. In 1972 Singer and Nicolson proposed the fluid mosaic model after freeze-fracture electron microscopy showed protein molecules lying within the phospholipid bilayer.
Explain how scientific understanding of the structure of cell membranes changed between 1935 and 1972.2 marks
The cell surface membrane of an animal cell contains phospholipids, cholesterol, channel proteins, carrier proteins and glycoproteins. Cholesterol molecules fit between the fatty acid tails of the phospholipids.
Explain why oxygen can diffuse directly through the phospholipid bilayer, but sodium ions cannot.2 marks
A student investigates the effect of ethanol on cell membranes using beetroot, which contains a red-purple pigment (betalain) inside its cells. She cuts equal discs of beetroot, rinses them and places each in 5 cm³ of ethanol solution for 30 minutes. She then measures the absorbance of each solution in a colorimeter with a blue-green filter. Her results for ethanol concentrations of 0, 10, 20, 30 and 40% were absorbances of 0.05, 0.08, 0.20, 0.52 and 0.95.
Describe the trend in the results and explain it.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).