2.5 - Biological Membranes

Question 1

Give 7 roles of membranes at the SURFACE of the cells.

Answer 1

1. SEPARATES the cell’s components from its EXTERNAL ENVIRONMENT.
2. REGULATES transport of materials into and out of the cell.
3. May contain ENZYMES involved in specific metabolic pathways.
4. Has ANTIGENS so that the organism’s immune system can recognise the cell as being ‘self’ and not attack it.
5. May release chemicals that SIGNAL to other cells.
6. Contains RECEPTORS for such chemical signals (cell communication/hormones)
7. May be the site of CHEMICAL REACTIONS

Question 2

Give 3 roles of membranes WITHIN cells.

Answer 2

1. Organelles like mitochondria have cristae that have a large surface area for some of the reactions of aerobic respiration and localise the enzymes needed for respiration to occur.
2. The inner membrane of chloroplasts, thylakoid membranes, contain chlorophyll. On these membranes, some of the reactions of photosynthesis occur.
3. There are digestive enzymes on plasma membranes of epithelial cells that line the small intestine. They catalyse the breakdown of sugars.

Question 3

Why is the cell membrane structure expressed as a ‘fluid mosaic model’?

Answer 3

“fluid” - the lipid molecules can change places with each other, and some of the proteins may move, giving fluidity.

“mosaic” - the model consists of a phospholipid bilayer with proteins floating in it, making up a mosaic pattern.

Question 4

State each component in the fluid mosaic model (8).

Answer 4

1. A PHOSPHOLIPID BILAYER (7mm in width).
2. INTEGRAL PROTEINS: CARRIER AND CHANNEL.
3. PERIPHERAL PROTEIN
4. CHOLESTEROL
5. GLYCOLIPID
6. CARBOHYDRATE MOLECULES (glycocalyx)
7. CYTOSKELETON FILAMENTS
8. PROTEINS that do not span the lipid bilayer act as enzymes.

Question 5

Why are channel and carrier proteins known as INTEGRAL proteins?

Answer 5

They are PERMANENTLY embedded within the structure of the membrane

Question 6

What is the role of channel proteins (due to their structure)?

Answer 6

They have pores and act as CHANNELS. They allow ions, which have an electrical charge and are surrounded by water molecules, to pass through.

Question 7

What is the role of CARRIER proteins (due to their structure)?

Answer 7

By changing their shape, carrier proteins CARRY specific molecules across the membrane.

Question 8

Why do eukaryotic cell membranes contain cholesterol?

Answer 8

1. helps to REGULATE the FLUIDITY
2. maintains mechanical STABILITY
3. resists the effects of TEMPERATURE CHANGES on the structure of the membrane.

Question 9

What are GLYCOCALYX?

Answer 9

• Carbohydrate molecules that are either attached to lipids (GLYCOLIPIDS) or PROTEINS (glycoproteins).
• They are hydrophilic and allow the cell to interact with the watery environment and obtain dissolved substances.

Question 10

How is the membrane different in NEURONES?

Answer 10

• There is a large distribution of protein channels and carriers covering the long axon.
• This allows entry and exit of IONS to perform conduction of electrical impulses.
• They have a myelin sheath which is formed by flattened cells wrapped over many times. The membrane forming the myelin sheath is 20% protein and 76% lipid.

Question 11

How is the plasma membrane different in WHITE BLOOD CELLS?

Answer 11

-contain special protein receptors that enable them to recognise antigens on foreign cells.

Question 12

How is the plasma membrane different in ROOT HAIR CELLS?

Answer 12

-have many carrier proteins to actively transport NITRATE IONS from the soil into the cells.