5.1 Structure and function of membranes

Question 1

What is the function of the membrane?

Answer 1

1. Selectively permeable barrier between cell + external environment, organelles + cytoplasm
2. Site of chemical reactions
3. Sites of cell signalling

Question 2

What are membranes formed from?

Answer 2

Phospholipid bilayer

Question 3

Describe the structure of the phospholipid bilayer.

Answer 3

Hydrophilic phosphate heads form inner + outer surface of membrane.
Fatty acid tails form a hydrophobic core inside the membrane

Question 4

What are the 2 types of proteins in plasma membranes?

Answer 4

Intrinsic and extrinsic

Question 5

Where are intrinsic proteins?

Answer 5

Embedded through both layers of the membrane

Question 6

What is the structure of intrinsic proteins?

Answer 6

They have amino acids with hydrophobic R-groups on their external surfaces.

Question 7

What do the hydrophobic amino acids with hydrophobic R groups on the external surface of intrinsic proteins do?

Answer 7

Interact with the hydrophobic core of the membrane, which keeps them in place.

Question 8

What are the 2 types of intrinsic proteins?

Answer 8

Channel proteins

Carrier proteins

Question 9

What is the function of channel proteins?

Answer 9

Provide hydrophilic channel - allows passive movement of polar molecules + ions down a conc. gradient through membranes.

Question 10

What are channel proteins held in position by?

Answer 10

Interactions between hydrophobic core of membrane + hydrophobic R-groups on the outside of proteins.

Question 11

What is the function of carrier proteins?

Answer 11

Enable passive transport down a concentration gradient + active transport against a concentration gradient. This often involves the protein’s shape changing.

Question 12

What type of protein are glycoproteins?

Answer 12

Intrinsic

Question 13

Where can glycoproteins be found?

Answer 13

Embedded in the plasma membrane

Question 14

What is the structure of glycoproteins?

Answer 14

Carb. chains attached to their surface

Question 15

What is the function of glycoproteins?

Answer 15

Cell adhesion (when cells join to form tight junctions in certain tissues).
Receptors for chemical signals.

Question 16

What are glycolipids?

Answer 16

Lipids w/ attached carb. chains

Question 17

Why are glycolipids referred to as ‘Cell markers’ or ‘Antigens’?

Answer 17

They can be recognised by the immune system as self (of orgasm) or non-self (belonging to other organisms)

Question 18

Where are extrinsic proteins?

Answer 18

Either side of bilayer.

Can move between layers.

Question 19

Describe the structure of intrinsic proteins.

Answer 19

Hydrophilic R-groups on outer surfaces + interact w/ polar heads of phospholipids / intrinsic proteins.

Question 20

What is cholesterol?

Answer 20

A lipid with a hydrophilic end and hydrophobic end, like a phospholipid

Question 21

What is the function of cholesterol?

Answer 21

Regulate membrane fluidity

Question 22

Where can cholesterol molecules be found? How does this relate to its function?

Answer 22

Between phospholipids in the membrane bilayer.

The hydrophilic end interacts w/ hydrophilic heads.
The hydrophobic end interacts w/ hydrophobic tails, pulling them together.
Due to this, cholesterol adds stability to membranes without making them too rigid.

Question 23

How do cholesterol molecules prevent membranes from becoming too solid?

Answer 23

By stopping phospholipids grouping too closely and cystallising