5.1 Structure & Function of Plasma Membranes

Question 1

What is the main function of membranes?

Answer 1

separate their contents

from environment

Question 2

What is compartmentalisation?

Answer 2

formation of separate, membrane-bound areas

within a cell

Question 3

What is the importance of compartmentalisation within a cell?

Answer 3

Allows cytoplasm to be divided into separate membrane-bound compartments

can provide separate environment & conditions for different reactions

which require specific pH, temperature, enzymes etc

Question 4

Describe the structure of a phospholipid:

Answer 4

• AMPHIPATHIC - contain both hydrophilic & hydrophobic parts
  
  • hydrophilic phosphate group “head”
    
    • hydrophilic due to negatively charged phosphate
  • hydrophobic fatty acid “tails”
    
    • 1 saturated + 1 unsaturated
    • hydrophobic due to no charge

Question 5

Describe the structure of phospholipids within the phospholipid bilayer with reference to their environment:

Answer 5

• 2 LAYERS OF PHOSPHOLIPIDS
• HYDROPHILIC HEADS FACE OUTWARDS
  
  • can interact with water in aqueous environment
• HYDROPHOBIC TAILS FACE INWARDS

Question 6

Do hydrophilic or hydrophobic molecules interact with water molecules? Explain why.

Answer 6

Hydrophilic molecules interact with water:

Water is a polar molecule, hydrophilic molecules also have (at least partial) separation of charge

∴ opposite charges on neighbouring molecules can attract

Question 7

What is facilitated diffusion?

Answer 7

net movement of particles

across a membrane

down a concentration gradient

via a membrane protein

Question 8

What are intrinsic membrane proteins?

Answer 8

transmembrane proteins

embedded through both layers within the phospholipid bilayer

Question 9

What are examples of intrinsic membrane proteins & what are their functions?

Answer 9

• CHANNEL PROTEINS - provides hydrophilic channel through protein
  
  • hydrophilic channel
  • selective, specific to certain type of molecule
• CARRIER PROTEINS - transport substances through membrane
  
  • Active transport:
    
    • Target molecules bind to active site,
    • ATP binds to carrier protein >> conformational change
    • Allows molecules into cell
  • Facilitated diffusion:
    
    • Target molecules bind to active site
    • Conformational change occurs without ATP, due to molecules naturally diffusing down concentration gradient

Question 10

What are extrinsic membrane proteins?

Answer 10

proteins present on only 1 side of phospholipid bilayer

can move between layers

Question 11

What is the function of glycolipids?

Answer 11

act as antigens

allows cells to be recognised by immune cells as self or non-self

Question 12

What is the function of glycoproteins?

Answer 12

• CELL ADHESION
  
  • cells join using glycoproteins
• RECEPTORS during CELL SIGNALLING
  
  • cell_A releases chemical by exocytosis
  • diffuses & binds to complementary glycoprotein receptor on cell_B
  • elicits response

Question 13

What are examples of extrinsic membrane proteins & what are their functions?

Answer 13

• GLYCOLIPIDS - lipid with attached carbohydrate chain
  
  • ANTIGENS: molecules which can be recognised by immune cells as self or non-self
• GLYCOPROTEINS - protein with attached carbohydrate chain
  
  • CELL ADHESION: cells join using glycoproteins to form tight junctions
  • RECEPTORS during CELL SIGNALLING: cell_A releases chemical by exocytosis, binds to specific complementary glycoprotein receptor on cell_B, eliciting response

Question 14

Carrier proteins can transport molecules through the membrane either by _____ or

_____ depending on whether ATP is required.

Answer 14

active transport, facilitated diffusion

Question 15

What is the difference between active transport & facilitated diffusion within carrier proteins?

Answer 15

Conformational change occurs due to different reasons:

In facilitated diffusion, caused by molecules diffusing naturally down concentration gradient

In active transport, caused by ATP binding to carrier protein.

∴ transport via carrier proteins can be active or passive

Question 16

What is cholesterol?

Answer 16

amphipathic lipid

regulates fluidity of membrane

Question 17

What makes cholesterol molecules amphipathic?

Answer 17

Hydrophilic due to polar OH

Hydrophobic due to non-polar carbon rings

Question 18

How does cholesterol regulate membrane fluidity?

Answer 18

cholesterol is embedded in phospholipid bilayer

hydrophilic & hydrophobic parts on cholesterol interact with heads & tails of phospholipids respectively

decreases fluidity by pulling together phospholipids

increases fluidity by stopping phospholipids from grouping too closely together

Question 19

Compare how these molecules can pass through the plasma membrane:

SMALL, NON-POLAR

LARGE

POLAR

Answer 19

small, non-polar: simple diffusion through gaps in phospholipids

large: endocytosis / carrier proteins (FD/AT)
polar: channel proteins (FD) / carrier protein (FD/AT)