Plasma membranes

Question 1

Structure of phospholipid

Answer 1

• Phosphate group (polar, hydrophillic head) bonded to glycerol molecule
• 2 fatty acid tails (hydrophobic, non-polar)

Question 2

function of plasma membranes

Answer 2

• keep cellular components in cell
• allow selected molecules to move in and out
• isolating organelles from the rest of cytoplasm allowing cellular reactions to occur separately
• allows cell to change shape
• site of chemical reactions eg. respiration

Question 3

fluid mosaic model

Answer 3

• mosaic - proteins are scattered within 2 phospholipid layers (bilayer)
• fluid because phospholipids can move around - flexible
• hydrophobic fatty acid tail on inside
• hydrophilic phosphate heads on outside

Question 4

When exposed to water, what are the 2 structures phospholipds can form?

Answer 4

micelle - circular shape
bilayer - straight layer
- both have hydrophobic tails on inside and hydrophilic heads on outside

Question 5

is it easier for hydrophobic or hydrophilic molecules to pass through the membrane?

Answer 5

• hydrophobic because the centre (fatty acids) is hydrophobic
  eg. steroid hormones
• prevents hydrophilic molecules passing through easily because they’re polar and can’t pass through non-polar membrane (apart from water because they’re small)

Question 6

what substances can directly pass through the phospholipid bilayer?

Answer 6

• small non-polar molecules eg. oxygen and carbon dioxide
• small uncharged polar molecules eg. water

Question 7

what substances cannot directly pass through phospholipid bilayer?

Answer 7

• larger uncharged polar molecules eg. glucose, amino acids
• ions

Question 8

glycolipid structure and function in membrane

Answer 8

• lipid with carbohydrate chain attached
• antigens - are recognised by immune system as self

Question 9

glycoprotein structure and function in membrane

Answer 9

• embedded in membrane with carbohydrate chain attached
• cell adhesion - allow cells to attach to each other to form tissues such as nervous tissue
• receptors for chemicals - important in cell signalling eg. insulin binds to it

Question 10

cholesterol structure and function in membrane

Answer 10

• hydrophobic tail and hydrophilic head interact with head and tail of phospholipids to hold it in place
• stability, flexibility and fluidity - disrupts close packing of phospholipids increasing rigidity - less flexible
• acts as a barrier, preventing water-soluble molecules passing through
• the more cholesterol the less fluidity and less permeability

Question 11

cholesterol structure

Answer 11

• hydrophobic region one end - attracts non-polar fatty acids in phospholipids
• hydrophilic group other end - attracts the polar head of phospholipid

Question 12

carrier protein function in membrane

Answer 12

• passive and active transport into cells - can change shape to allow molecules to pass through

Question 13

channel protein function and how they’re held in place in plasma membrane

Answer 13

• lined with hydrophilic amino acids allowing passive movement of polar molecules and ions down conc grad
• held in place by interactions between hydrophobic core of membrane and hydrophobic R groups on outside

Question 14

What effect does heat have on the plasma membrane?

Answer 14

• as temperature increases, kinetic energy of phospholipid increases - membrane more fluid and loses structure
• if temp too high, cell proteins denature
• molecules can pass through gaps increasing permeability

Question 15

effect of low pH on membrane

Answer 15

• increases permeability
• acidity changes tertiary structure of proteins in membrane
• can cause them to denature

Question 16

what effect do solvents (water / alcohol and benzene) have on the plasma membrane?

Answer 16

• water is polar so interacts with hydrophilic heads and creates stability with phospholipids
• strong alcohols and benzene are less polar so can dissolve the membrane bilayer structure - making it more permeable

Question 17

method of investigating effect of temp. on membrane permeability

Answer 17

• prepare beetroot cylinders by blotting and washing them until they’re free of pigment
• put 5cm^3 distilled water into 6 test tubes of varying temps
• put 3 beetroot discs in the first test tube and leave for 1 min
• put water from test tube in a colourimeter

Question 18

what do temperatures below freezing do to the phospholipid bilayer?

Answer 18

• phospholipids don’t have much kinetic energy so don’t move
• they pack close together forming a rigid layer
• channel proteins and carrier proteins denature
• ice crystals may pierce the bilayer increasing permeability significantly

Question 19

diffusion

Answer 19

net movement of particles down a concentration gradient from a high concentration to a low concentration

Question 20

non-polar molecules across plasma membrane

Answer 20

• hydrophobic eg. oxygen, CO2 freely diffuse down a conc gradient

Question 21

polar molecules across plasma membrane

Answer 21

• hydrophobic interior repels ions - cannot easily pass through
• water (partially +ve and -ve) can diffuse through at a very slow rate
• small polar pass through easier than large polar - membrane is partially permeable

Question 22

factors affecting rate that molecules or ions diffuse across membrane

Answer 22

• surface area - larger, higher rate
• thickness of membrane - thinner, faster

Question 23

facilitated diffusion definition

Answer 23

• the movement of polar molecules or ions across a membrane via channel or carrier proteins
• passive process - doesn’t require energy

Question 24

facilitated diffusion in channel proteins

Answer 24

• lined with hydrophilic amino acids
• polar substances and ions can pass through the channel passively
• they are selective - only certain chemicals can pass through each type of protein channel

Question 25

where does active transport occur in membranes?

Answer 25

carrier proteins

Question 26

active transport in carrier protein

Answer 26

- molecule/ion being transported binds to receptor on carrier protein - ATP from inside the cell binds to carrier protein and undergoes hydrolysis to phosphate and ADP - phosphate binds to carrier protein and causes a change in shape transporting the molecule to the other side of the membrane - phosphate detaches and recombines to ADP and carrier protein returns to original shape

Question 27

osmosis definition

Answer 27

movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane

Question 28

units for water potential

Answer 28

kilopascals

Question 29

what happens when both sides of the membrane have the same water potential?

Answer 29

- reached equilibrium and osmosis stops - same no. water molecules moving in each direction so net movement is 0

Question 30

what happens to animal cell when water pot. outside cell is higher than cytoplasm?

Answer 30

- water moves in cell via osmosis and hydrostatic pressure increases - cell may burst (cytolysis)

Question 31

what happens when water pot. inside cell is higher than outside? (animal cells)

Answer 31

- water moves out by osmosis - cell shrivels - crenation

Question 32

what happens when water pot. outside cell is higher than inside? (plant cell)

Answer 32

- water moves into cell by osmosis - protoplast pushes against cell wall as hydrostatic pressure increases - cell wall is strong so doesn't burst, cell just becomes turgid

Question 33

what happens when water pot. inside cell is higher than outside? (plant cell)

Answer 33

- water moves out cell by osmosis - protoplast pulls away from cell wall - shrivels - plasmolysis - there is a gap between protoplast and cell wall which is filled with solution surrounding cell

Question 34

2 processes involved in bulk transport

Answer 34

endocytosis exocystosis (both require energy to move vesicles)

Question 35

endocytosis

Answer 35

- cell membrane folds inwards forming cavity around particles (invagination) - membrane forms vesicle around particles and vesicle pinches off into cytoplasm

Question 36

2 types of endocytosis

Answer 36

- phagocytosis - solid materials taken into cell - pinocytosis - liquid brought into the cell

Question 37

exocytosis

Answer 37

- proteins found in Golgi apparatus - vesicles bud off Golgi containing proteins and move towards cell membrane - vesicle fuses with membrane and proteins secreted

Question 38

what is ATP used for in bulk transport?

Answer 38

- movement of vesicles along cytoskeleton - changing shapr of cell to engulf materials - fusing of vesicles with cell membrane