The structure of cell membranes 2.5.1

Question 1

What are membranes?

Answer 1

They cover the surface of every cell, and also surround most organelles within cells.

Question 2

Name 5 roles of the plasma membrane

Answer 2

• Separates cell contents from the external environment
• Regulates transport of materials into and out of the cell
• Contains receptors for cell to cell signalling
• Has antigens so an organisms immune system doesn’t attack the membrane
• Site of biochemical reactions

Question 3

Why are membranes partially permeable?

Answer 3

As they do not let all types of molecules through.

Question 3

What makes up a cell membrane?

Answer 3

Phospholipids

Question 4

Describe the composition of a membrane

Answer 4

Phospholipids (phosphate head, glycerol, and two fatty acid tails), arranged in a bilayer, due to the fatty acid tails being hydrophobic and the phosphate head being hydrophilic.

Question 5

Explain the term hydrophobic

Answer 5

A molecule that is insoluble in water

Question 6

Explain the term hydrophilic

Answer 6

A molecule that is soluble in water

Question 7

Which part of the membrane faces into the external environment and the cytoplasm (aqueous)?

Answer 7

The hydrophilic phosphate heads

Question 8

Which part of the membrane faces inwards?

Answer 8

The hydrophobic phosphate tails

Question 9

Why is the plasma membrane described as being the fluid mosaic model?

Answer 9

Singer and Nicholson discovered that the membrane was fluid and had sideways movement within it, and also there were proteins within the membrane which are distributed in a mosaic pattern

Question 10

Why can’t polar molecules pass through the fluid part of the plasma membrane?

Answer 10

Due to the hydrophobic tails repelling these charged molecules.

Question 11

How do polar molecules pass through the membrane?

Answer 11

Through proteins within the bilayer

Question 12

State the many types of proteins found within a membrane

Answer 12

• Glycolipid (recognition)
• Glycoprotein (recognition)
• Receptor
• Enzymes
• Channel protein
• Carrier protein
• Peripheral protein

Question 13

State the four types of carrier protein within the membrane

Answer 13

• Carrier protein (passive)
• Gated-channel protein
• Channel protein
• Carrier protein (active - ATP)

Question 14

Other than as carrier proteins state two functions of membrane bound proteins

Answer 14

• Enzymes
• Receptors

Question 15

Which type of molecules can pass through the plasma membrane?

Answer 15

Non-polar, lipid soluble molecules

Question 16

What is the role of cholesterol within the membrane

Answer 16

It adds firmness and integrity to the membrane and prevents it from becoming overly fluid, it also helps to maintain its fluidity.

Question 17

What two structures do phospholipids form when exposed to water?

Answer 17

A micelle or bilayer

Question 18

What is the function of peripheral proteins on the extracellular side of the membrane?

Answer 18

Act as receptors or hormones, and many are glycoproteins

Question 19

What is the function of peripheral proteins on the cytosolic side of the membrane?

Answer 19

Involved in cell signalling or chemical reactions

Question 20

What is the membrane composition of a neurone?

Answer 20

• Have protein channels and carriers in the plasma membrane covering the long axon - to allow entry and exit of ions
• Have a myelin sheath formed by many layers of flattened cells

Question 21

What is the membrane composition of white blood cells?

Answer 21

• Contains protein receptors that enable them to recognise the antigens on foreign cells (pathogens)