2.1.5 B) Fluid Mosaic Model of Membrane Structure + Roles of its Components

Question 1

Components in fluid mosaic model of membrane structure:

Answer 1

• phospholipids
• cholesterol
• glycolipids
• proteins
• glycoproteins

Question 2

What is the main component of membranes?

Answer 2

phospholipids

Question 3

Diagram of phospholipid structure:

Answer 3

Question 4

What are phospholipids?

Answer 4

a type of lipid made from two fatty acid chain ‘tails’ attached to a phosphate group ‘head’

Question 5

What is the shape that phospholipids form due to?

Answer 5

the shape of the structures that phospholipids form is due to their polar nature, and the way they interact with water

Question 6

How are the phosphate group and fatty acid chains in the phospholipid different?

Answer 6

• the phosphate group head is polar and hydrophilic (‘water-loving’)
• while the fatty acid chains of the tail are non-polar and hydrophobic (‘water-hating’)

Question 7

Role of membrane bound receptors:

Answer 7

sites where hormones and drugs can bind

Question 8

What does ‘fluid’ refer to in the fluid mosaic model?

Answer 8

• phospholipids are constantly moving and free to move around - not bonded together (hydrophobic section holds them together)
• molecules are in constant motion and molecules can move freely through the lipid bilayer

Question 9

What does the ease with which phospholipids can move around depend on?

Answer 9

the ease with which they do this is dependent on the number of phospholipids with unsaturated fatty acids in the phospholipids

Question 10

What does ‘mosaic’ refer to in the fluid mosaic model?

Answer 10

mosaic the way the membrane looks if viewed from above due to protein molecules scattered throughout

Question 11

Diagram of fluid mosaic model:

Answer 11

Question 12

What is the shape that phospholipids form due to?

Answer 12

the shape of the structures that phospholipids form is due to their polar nature, and the way they interact with water

Question 13

What structure do phospholipids form when exposed to water?

Answer 13

• micelle or a bilayer - in each structure the hydrophilic heads face the water and the hydrophobic tails point inwards away from the water
• Phospholipids may also form a layer on the surface of the water with heads in the water and tails sticking up out of the water - monolayer

Question 14

Why is the head of a phospholipid attracted to water?

Answer 14

• When phospholipids are surrounded by water, H+ ions dissociate from the phosphate group
• This makes the phosphate group negatively charged
• This negative charged means it is attracted to water, whereas the fatty acid tails are repelled by water as they are non-polar

Question 15

What structure do phospholipids form in the cell membrane and why?

Answer 15

• Inside and outside of a cell membrane is aqueous solution
• the head is hydrophilic (attracts water) and the tail is hydrophobic (repels water)
• the phospholipids form a bilayer (molecules automatically arrange to form a bilayer) with two rows of phospholipids, heads pointing outwards towards the water on either side of the membrane and tails pointing inwards

Question 16

How do phospholipids move around in the cell membrane and what does this provide the cell?

Answer 16

• Phospholipids move around in bilayer, but tail never moves into water etc.
• This provides stability to the membrane

Question 17

Role of phospholipids in cell membrane:

Answer 17

• forms phospholipid bilayer which is the fundamental basis of the membrane in which all other components are embedded
• form a barrier to dissolved (water-soluble) substances
• the centre of the bilayer is hydrophobic so the membrane doesn’t allow water-soluble substances (like ions and polar molecules (molecules that carry a charge)) to diffuse through it
• fat-soluble substances (e.g. fat soluble vitamins) dissolve in the bilayer and pass directly through the membrane

Question 18

What is cholesterol?

Answer 18

a type of lipid that’s present in all cell membranes (except bacterial cell membranes)

Question 19

Where is cholesterol found in the cell membrane?

Answer 19

cholesterol molecules fit between the phospholipids

Question 20

Role of cholesterol in cell membrane:

Answer 20

• helps to maintain the fluidity of the cell membrane therefore giving it stability
• at higher temps they bind to the hydrophobic tails of the phospholipids causing them to pack more closely together - prevents the membrane from becoming too fluid and makes the membrane more rigid
• at lower temps cholesterol prevents phospholipids from packing too close together prevents the membrane from becoming too rigid/stiff and increases fluidity
• cholesterol also has hydrophobic regions so it’s able to create a further barrier to polar substances moving though the membrane

Question 21

Where are glycolipids found?

Answer 21

attached to phospholipids

Question 22

What are glycolipids?

Answer 22

lipids with carbohydrates attached

Question 23

Role of glycoplipids in the cell membrane:

Answer 23

• stabilise the membrane by forming bonds with surrounding water molecules
• act as receptors for messenger molecules in cell signalling and are sites where drugs, hormones and antibodies bind
• antigens - cell surface molecules involved in self-recognition and the immune response
• helps cells attach to each other
• acts as recognition sites for specific chemicals

Question 24

Role of proteins in the cell membrane:

Answer 24

control what enters and leaves the cell

Question 25

What are the different proteins found in the cell membrane?

Answer 25

• channel protein (intrinsic protein)
• carrier protein (intrinsic protein)
• extrinsic protein

Question 26

What type of protein are channel proteins?

Answer 26

intrinsic/transmembrane proteins - span the whole width of the membrane

Question 27

Role of channel proteins in cell membrane:

Answer 27

• form hydrophilic channels through which allows passive movement of polar hydrophilic substances (e.g. polar molecules) down concentration gradient
• these channels can be opened or closed
• so allow small charge particles through the membrane

Question 28

What type of protein are carrier proteins?

Answer 28

intrinsic/transmembrane proteins - span the whole width of the membrane

Question 29

Role of carrier proteins in cell membrane:

Answer 29

• act as transporters than can transport substances (larger molecules and charged particles) across the membrane against concentration gradients by active transport (with use of energy from ATP) and facilitated diffusion
• involves the protein changing shape

Question 30

Extrinsic proteins:

Answer 30

proteins that are confined to the inner or outer surface of the membrane - occur on the membrane or are partly embedded in it

Question 31

Role of extrinsic proteins in cell membrane:

Answer 31

• act as receptors for molecules (e.g. hormones) in cells signalling → when a molecule binds to the protein a chemical reaction is triggered inside the cell
• act as enzymes

Question 32

What are glycoproteins?

Answer 32

proteins with attached carbohydrate chains

Question 33

Role of glycoproteins in the cell membrane:

Answer 33

• stabilise the membrane by forming bonds with surrounding water molecules
• act as receptors for messenger molecules in cell signalling and are sites where drugs, hormones and antibodies bind
• antigens - cell surface molecules involved in self-recognition and the immune response