Cell Membranes

Question 1

Where are plasma membranes found?

Answer 1

Around cells and some organelles

Question 2

Are cell membranes fully or partially permeable?

Answer 2

Partially permeable ( selectively permeable)

Question 3

What do cell membranes look like under an electron microscope?

Answer 3

You can see the bilayer as two dark bands. The distance across the membrane is about 7nm

Question 4

Why are cell membranes described as fluid mosaic?

Answer 4

Fluid- phospholipids (and proteins) are free to move laterally (sideways)
Mosaic - proteins have a scattered arrangement within the phospholipid bilayer

Question 5

Describe the fluid mosaic model of membranes

Answer 5

The component molecules are not bonded together so there is some movement but it is relatively stable because of the nature of the phospholipid

Question 6

Who proposed a model of how the components in the membrane are arranged and what year?

Answer 6

Singer and Nicholson (1972)

Question 7

What are the components of the plasma membrane?

Answer 7

Phospholipids
Cholesterol
Glycolipids
Glycoproteins
Intrinsic + Extrinsic Proteins

Question 8

What is the role of phospholipids in the membrane?

Answer 8

• make up most of the plasma membrane
• act as a barrier to water soluble, polar molecules and ions
• allows lipid soluble, small, non-polar substances to diffuse through
• makes the membrane flexible and self sealing making endo and exocytosis possible

Question 9

What is cholesterol and what is its role in plasma membranes?

Answer 9

• found in eukaryotic cells for stability and fluidity of the cell membrane
• steroid molecule that fits between fatty acid tails completing the membrane barrier to water and ions
• binds to the hydrophobic tails of the phospholipids - prevent the membrane being too fluid at high temperatures but also stops the membrane solidifying at low temperatures

Question 10

What is a glycolipid and what is its role in a plasma membrane?

Answer 10

• Lipid with a carbohydrate attached
• inter cell signalling and recognition
• receptor
• helps stick cells together and to basement membranes to form tissues - cell adhesion

Question 11

What is the role of intrinsic proteins in plasma membranes?

Answer 11

Channel or carrier proteins that allow transport of hydrophilic, polar molecules (like glucose) and ions through the phospholipid bilayer

Question 12

What is the role of extrinsic proteins in plasma membranes?

Answer 12

• inter cell signalling
• cell recognition
• enzymes
• receptors for signalling between cells
• cell adhesion

Question 13

What are glycoproteins and what is their role in plasma membranes?

Answer 13

• proteins with carbohydrate attached
• receptors for signalling molecules
• cell signalling and recognition
• binding cells together to basement membrane, making tissues, cell adhesion

Question 14

Give a summary of the roles of membranes within cells

Answer 14

1. Separate cell components from cytoplasm - compartmentalisation
2. Holding the components of metabolic pathways in place eg enzymes for aerobic respiration in mitochondria
3. Control what enters or leaves the organelle
4. Sites of attachment

Question 15

Give a summary of the roles of membranes at the surface of cells

Answer 15

1. Separate cell contents from the outside environment
2. Cell recognition and signalling
3. Regulating transports of materials into or out of cells
4. Create/maintain concentration gradients

Question 16

What is cell signalling?

Answer 16

Processes that lead to communication and coordination between cells, so that they can work together to trigger a response. It allows cells to be recognised as self and so prevent cells from being destroyed by immune system. A receptor in the cells plasma membrane detects chemical signals and brings about responses within the cell.

Question 17

What is the process of cell signalling?

Answer 17

• Protein receptors found on target cells have a specific shape complementary only to a specific hormone.
• they will bind with the hormone to allow the cell to respond in a particular way
  E.g insulin receptor, insulin binds to the receptors and the cells respond by taking up more glucose to reduce blood glucose levels

Question 18

Where are insulin receptors found and what do they do?

Answer 18

Found on liver and muscle cells, insulin binds to the receptors and the new cells respond by taking up more glucose to reduce blood glucose levels

Question 19

How are plasma membranes affected by temperature?

Answer 19

Affects how much the phospholipids in the bilayer can move and this affects membrane structure and permeability

Question 20

How do temperatures of 35-50 degrees C affect plasma membranes?

Answer 20

• Increases kinetic energy or phospholipids and protein molecules in the membrane.
• Molecules being transported also have more kinetic energy and can diffuse across membrane more quickly

Question 21

How do temperatures of 50-60 degrees C affect plasma membranes?

Answer 21

• Increase in temp denatures proteins making membranes more permeable
• phospholipid membrane becomes more fluid and permeable
• causes gaps to open temporarily in the bilayer
• results in ions/molecules diffusing more quickly

Question 22

How do temperatures of 60-70 degrees C affect plasma membranes?

Answer 22

• Further increase in temperature causes the membrane to be destroyed
• ions/molecules diffuse in first few moments of experiment and reach equilibrium quickly