B5 - Plasma membranes

Question 1

What is the role of membranes at the surface of cells?

Answer 1

• they are partially permeable barriers between the cell and its environment (limits what enters/leaves the cell)
• allow cell recognition (cells of immune system)
• site of cell communication (cell signalling)

Question 2

What is the role of membranes within cells?

Answer 2

• acts as a barrier between organelles and the cytoplasm/within organelles
• can form vesicles to transport substances around the cell
• compartmentalise the cell (separate processes can occur in specialised areas of the cell)
• can form conc. gradients
• site of chemical reactions (e.g. mitochondria’s inner membrane contains enzymes for respiration)

Question 3

What are the components of the fluid mosaic model membrane?

Answer 3

• phospholipid bilayer
• cholesterol
• intrinsic proteins (channel/carrier/glycoproteins/glycolipids)
• extrinsic proteins

Question 4

What is the role of the phospholipid bilayer?

Answer 4

• forms a barrier that limits the movement of some substances into/out of the cell and organelles
• only small, fat-soluble substances can dissolve into the bilayer/diffuse across the membrane
• water-soluble substances/ions cannot pass through as the centre of the membrane is hydrophobic/non-polar

Question 5

What is the role of cholesterol?

Answer 5

• fits between the tails of the phospholipid bilayer and holds them together (mechanical stability)
• it inhibits the movement of the phospholipids, regulating the membrane’s fluidity
• makes it less permeable to water/ions

Question 6

What is the role of intrinsic proteins (channel/carrier)?

Answer 6

• channel :
  
  • provides a hydrophilic channel
  • allows passive movement of polar molecules down conc. gradient
  • held in position by interactions between the hydrophobic core and hydrophobic R-groups outside of proteins
• carrier :
  
  • helps with both passive/active transport (against conc. gradient) into cells
  • shape of protein changes during this

Question 7

What is the role of glycoproteins?

Answer 7

• intrinsic proteins
• embedded in cell-surface membrane with attached carbohydrate chain
• they play a role in cell adhesion/as receptors for chemical signals
• the binding of the receptor and the chemical signal elicits a cell response

Question 8

What is the process of cell signalling?

Answer 8

• one cell releases a messenger molecule (e.g. hormone)
• this travels (e.g. in the blood) to another cell
• the molecule is then detected by the cell and binds to the receptor on the membrane
• receptor proteins have specific shapes (complementary to the messenger molecules)
• a target cell responds to the particular messenger molecule
• drugs can also bind to receptors to trigger a response/to block the receptor to prevent it from working

Question 9

What is the role of glycolipids?

Answer 9

• similar to glycoproteins
• they are lipids with attached carbohydrate chains
• called ‘antigens’/cell markers and can be recognised by cells of immune system as self or non-self

Question 10

What is the role of extrinsic proteins?

Answer 10

• present on one side of bilayer
• consist of hydrophilic R-groups on outer surface (interacts with polar phosphate heads/intrinsic proteins)

Question 11

What are the factors affecting membrane structure?

Answer 11

• temperature
• solvents

Question 12

How does temperature below 0°C affect membrane structure?

Answer 12

• phospholipids are packed closely together (rigid membrane)
• channel/carrier proteins lose their structure/function, which increases membrane permeability
• ice crystals also may pierce the membrane

Question 13

How do temperatures between 0°C and 45°C affect membrane structure?

Answer 13

• phospholipids gain kinetic energy and are able to move around (not as tightly packed)
• membrane is partially permeable
• as they move about, the phospholipids leave temporary gaps (allows small molecules to enter the membrane)

Question 14

How do temperatures above 45°C affect membrane structure?

Answer 14

• phospholipid bilayer may melt and lose its mechanical stability
• this further increases membrane permeability
• carrier/channel proteins denature so they cannot control what goes into/out of the cell

Question 15

How do solvents affect membrane structure?

Answer 15

• organic solvents (e.g. alcohol/ethanol) can increase membrane permeability
• they are able to dissolve the lipids in the bilayer causing the membrane to lose its structure
• as conc. of alcohol increases, permeability of membrane increases

Question 16

What is passive transport?

Answer 16

• the movement of particles that does not require energy from respiration
• makes use of the natural motion of the particles

Question 17

What are the passive methods of transport?

Answer 17

• diffusion
• facilitated diffusion

Question 18

What is diffusion?

Answer 18

• the net movement of particles from a region of higher conc. to a region of lower conc.
• occurs when a membrane is fat-soluble/when the molecules are able to fit between the phospholipids

Question 19

Factors affecting the rate of diffusion

Answer 19

• temperature
  
  • as temp. increases, kinetic energy of particles also increases so there is a higher rate of diffusion
• conc. difference
  
  • greater the difference = faster rate of diffusion
  • overall movement from high conc. to low conc. is larger

Question 20

What factors affect diffusion across (partially permeable) membranes?

Answer 20

• surface area = larger the area, the higher the rate of diffusion
• thickness of membrane (thinner)

Question 21

What is facilitated difffusion?

Answer 21

• phospholipid bilayers are barriers to polar molecules/ions
• diffusion across protein channels allows for them to be transported
  
  • these membranes are selectively permeable
    ‎
• facilitated diffusion can also include carrier proteins
• they change shape when a specific molecule binds
  ‎
• does not require external energy as it is the movement down a conc. gradient

Question 22

What factors affect facilitated diffusion?

Answer 22

• temp.
• conc. gradient
• membrane surface area/thickness
• no. of proteins channels present (more = higher rates of diffusion)

Question 23

What is active transport?

Answer 23

• movement of molecules/ions into/out of a cell from a region of lower conc. to a higher conc.
• this requires energy and carrier proteins
• metabolic energy is supplied by ATP
  ‎
• the molecule/ion binds to receptors in the channel of carrier proteins
• ATP binds to the carrier proteins and is hydrolysed (ADP and Pi)
• the Pi molecule binds to the protein causing a change in shape
• molecule/ion is released into the cell
• Pi is released and carrier protein returns to its original shape

Question 24

What is bulk transport?

Answer 24

• another form of active transport
• larger molecules are unable to move through channel/carrier proteins

Question 25

What is endocytosis?

Answer 25

• transport of materials into cells
• phagocytosis (solids)
• pinocytosis (liquids)
  ‎
• cell-surface membrane bends inwards and enfolds the material until it forms a vesicle
• it then pinches off and moves into the cytoplasm to transfer the material

Question 26

What is exocytosis?

Answer 26

• the vesicle is formed by the Golgi apparatus
• fuses with the cell surface membrane
• its contents are released outside of the cell
  ‎
• energy (ATP) is required to move the vesicles along the cytoskeleton

Question 27

What is water potential?

Answer 27

• pressure exerted by water molecules as the collide with a membrane/container
• measured in Pa/kPa
  ‎
• pure water has a water potential of 0 kPa
  
  • highest possible value
  • presence of solute in water lowers the w.p. below 0
    ‎
• solutions of different w.ps. are separated by a partially permeable membrane
• net movement = higher w.p. to a lower w.p.
• this continues until equilibrium is reached

Question 28

What is hydrostatic pressure?

Answer 28

• pressure created by water in an enclosed system
• cellular level = relatively large and potentially damaging

Question 29

How does osmosis affect animal cells?

Answer 29

• higher water potential outside:
  
  • water moves in by osmosis
  • thin cell membranes cannot withstand the high pressure
  • cytolysis will occur as cell bursts
    ‎
• lower water potential inside:
  
  • loses water by osmosis
  • causes crenation as the cell-surface membrane ‘puckers’
    ‎
• multicellular animals have control mechanisms to ensure surrounding solutions are constantly at equal w.p.

Question 30

How does osmosis affect plant cells?

Answer 30

• unlike animals, plants are unable to control the w.p. of the fluid around them
• higher water potential outside:
  
  • water enters cell by osmosis
  • due to strong cellulose cell walls, the membrane pushes against it (turgor = pressure against it)
    ‎
• lower water potential outside:
  
  • water is lost by osmosis
  • cell-surface membrane is pulled away from the cell wall
  • cell is plasmolysed