Cell Membranes

Question 1

What are the two types of membranes?

Answer 1

• plasma membranes; at the surface of cells
• membranes within cells

Question 2

What is compartmentalisation and why is it important?

Answer 2

• the formation of separate membrane bound areas inside a cell
• vital as metabolism includes many different and often incompatible reactions so this separation allows specific conditions required for cellular reactions such as chemical gradients being maintained

Question 3

Outline 3 functions of plasma membranes

Answer 3

• they allow recognition by other cells
• they allow cell communication/cell signalling
• they act as barriers between the cell and environment controlling what leaves and enters

Question 4

Describe the permeability of plasma membranes

Answer 4

• they are partially permeable
• substances can move across via diffusion, osmosis or active transport

Question 5

Outline the Cell Membrane Theory

Answer 5

• the fluid-mosaic model
• describes the arrangement of molecules within the membrane
• phospholipids form a continuous bilateral and are constantly moving (fluid)
• protein molecules vary in shapes and sizes and are scattered through the bilayer (mosaic)
• cholesterol, glycoproteins and glycolipids are present

Question 6

What are intrinsic proteins

Answer 6

• transmembrane proteins embedded through both layers of a membrane
• have amino acids with hydrophobic R-groups which interact with the hydrophobic core of the membrane, keeping them in place

Question 7

What are examples of intrinsic proteins

Answer 7

• channel proteins
• carrier proteins
• glycoproteins
• glycolilpids

Question 8

What are channel proteins

Answer 8

• intrinsic proteins
• they provide a hydrophilic channel that allows the diffusion of polar molecules and ions

Question 9

What are carrier proteins

Answer 9

• intrinsic proteins
• play an important role in the passive/active transport into cells; often involves the protein changing shape

Question 10

What are glycoproteins? Descrive their structure and function

Answer 10

• intrinsic proteins which are embedded in the plasma membrane with attached carbohydrate chains of varying lengths and shapes
• play a role in cell adhesion and as receptors for chemical signals (cell communication/signalling)

Question 11

Give 3 examples where glycoproteins act as receptors for cell signalling

Answer 11

• receptors for neurotransmitters (eg acetylcholine) at synapses where the binding triggers/prevents an impulse
• receptors for peptide hormones (eg insulin and glucagon) in the storage and uptake of glucose
• drugs such as b-blockers to reduce the response of the heart to stress

Question 12

What are glycolipids? Describe their structure and function

Answer 12

• intrinsic proteins; lipids with attached carbohydrate chains
• known as cell markers or antigens and can be recognised by cells of the immune system as self or non-self

Question 13

What are extrinsic proteins?

Answer 13

• are present in one side of the bilayer
• have hydrophilic R groups on their surface which interact with the polar heads of the phospholipids or intrinsic proteins
• can be in either layer and can move between layers

Question 14

What is an example of an extrinsic protein

Answer 14

• cholesterol
• regulates membrane fluidity
• has a hydrophobic end which interact/binds with the hydrophilic tail of the phospholipid
• has a hydrophilic end which interact/binds with the hydrophobic head of the phospholipid
• make the membrane less fluid by binding and packing closely together
• makes the membrane more fluid by preventing the phospholipids from packing too close together

Question 15

How does temperature affect membrane permeability

Answer 15

• increased temp means phospholipids will have more kinetic energy and therefore move more, making the membrane more fluid as it will begin to lose structure, if it gets too high the membrane will break down
• the loss in structure increases the permeability so particles can easily cross it
• high temperatures can denature the proteins, affecting permeability as transport of substances may be impacted

Question 16

How do solvents affect membrane permeability

Answer 16

• organic solvents such as alcohol can dissolve membranes
• pure/strong alcohol is toxic and will destroy cells but less concentrated alcohol will just cause damage

Question 17

How can less concentrated alcohol damage membranes

Answer 17

• the non polar alcohol molecules can enter the membrane and the presence of them between the phospholipids disrupts the membrane making it more fluid and permeable
• this prevents the membrane from being able to carry out functions (e.g the transmission of nerve impulses)

Question 18

Define diffusion

Answer 18

• the net movement of particles from a region of high concentration to a region of low concentration until it reaches equilibrium
  (down a concentration gradient)

Question 19

What factors affect rate of diffusion

Answer 19

• temperature; higher temp means higher rate of diffusion as particles have more kinetic energy and move at higher speeds
• concentration difference; the greater conc difference means faster rate of diffusion as overall movement will be larger

Question 20

Why are membranes described as partially permeable

Answer 20

• non polar molecules (oxygen) can diffuse freely
• ions cannot easily pass through
• polar molecules (water) can diffuse through membranes but at a slow rate

Question 21

How does SA and thickness affect rate of diffusion

Answer 21

• the larger the SA, the higher rate of diffusion
• the thinner the exchange surface, the higher rate is diffusion

Question 22

What is facilitated diffusion

Answer 22

• the diffusion across a membrane through protein channels
• can also involve carrier proteins which change shape when a specific molecule binds

Question 23

Why are membranes with protein channels selectively permeable

Answer 23

• most protein channels are specific to one molecule or ion

Question 24

What other factors affect the rate of facilitated diffusion

Answer 24

• the number of protein channels present
• the more present the higher rate of diffusion overall

Question 25

Define active transport

Answer 25

- movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration - particles are moving up a concentration gradient so therefore the process requires energy which is supplied by ATP

Question 26

Describe the general process of active transport

Answer 26

- the carrier protien spans the membrane - the molecule will bind to receptors in the channel of the carrier protien - on the inside of the cell ATP will bind to the carrier protein and is hydrolysed into APD and phosphate - the phosphate binds to the carrier protein causing it to change shape and opens up the inside of the cell - the molecule is then released into the cell - the phosphate molecule is released from the carrier protein and recombines with ADP to form ATP - the carrier protein returns to its original shape

Question 27

Why is active transport selective

Answer 27

specific substances are transported by specific carrier proteins

Question 28

What is bulk transport

Answer 28

- form of active transport - large molecules such as enzymes are too large to fit through channel or carrier proteins so are transported by bulk transport

Question 29

What is endocytosis? What are the two types?

Answer 29

- the bulk transport of material into cells - phagocytosis (solids) and pinocytosis (liquids)

Question 30

Describe the process of endocytosis

Answer 30

- the material will come into contact with the membrane which then invaginates (bends inwards) - the membrane enfolds the material until it eventually fuses, forming a vesicle - the vesicle pinched off and moves into the cytoplasm to transfer the material for further processing

Question 31

Define exocytosis

Answer 31

- vesicles (normally formed by the golgi) move towards and fuse with the membrane - contents of the vesicle are then released outside of the cell