Question
Explain how hydrophobic and hydrophilic properties contribute to the arrangement of molecules in a membrane. [7]
▶️Answer/Explanation
Ans
a. hydrophilic is attracted to/soluble in water and hydrophobic not attracted/insoluble ✔
b. hydrophilic phosphate/head and hydrophobic hydrocarbon/tail in phospholipids ✔
c. phospholipid bilayer in water/in membranes ✔
d. hydrophilic heads «of phospholipids» face outwards/are on surface ✔
e. hydrophobic tails «of phospholipids» face inwards/are inside/are in core ✔
f. cholesterol is «mainly» hydrophobic/amphipathic so is located among phospholipids/in hydrophobic region of membrane ✔
g. some amino acids are hydrophilic and some are hydrophobic ✔
h. hydrophobic «amino acids/regions of» proteins in phospholipid bilayer «core» ✔
i. hydrophilic «amino acids/regions of» proteins are on the membrane surface ✔
j. integral proteins are embedded in membranes due to hydrophobic properties/region OR
transmembrane proteins have a hydrophobic middle region and hydrophilic ends ✔
k. peripheral proteins on are on the membrane surface/among phosphate heads due to being «entirely» hydrophilic
OR
«carbohydrate» part of glycoproteins is hydrophilic so is outside the membrane ✔
l. pore of channel proteins is hydrophilic ✔
Question
(a) (i) Outline how the amphipathic properties of phospholipids play a role in membrane structure. [2]
(ii) State the role of cholesterol in animal cell membranes. [1]
(b) Describe what happens to the membranes of an animal cell during mitosis. [2]
▶️Answer/Explanation
Ans
a i
a part hydrophobic/not attracted to water/non-polar AND part hydrophilic/attracted to water/polar;
b bilayer formed (formed naturally by phospholipids in water);
c hydrophilic heads/parts face outwards and hydrophobic tails/parts face inwards; Do not allow water loving/hating in mpa or mpc
a ii
a controls/regulates/reduces fluidity of membrane / prevents crystallization of phospholipids;
b reduces permeability to some substances; Do not accept ‘stabilizes membrane’
b
a nuclear membrane breaks down/disappears (in prophase/at start of mitosis);
b nuclear membrane reforms around two new nuclei (in telophase/at end of mitosis);
c plasma membrane pulled inwards at equator / cleavage furrow formed; d membrane pinches apart to form two cells / cytoplasm divided / cytokinesis;
Question
Draw a labelled diagram to show the structure of the plasma membrane.
The light-dependent reactions in photosynthesis take place on the thylakoid membranes. Explain the light-dependent reactions.
Outline two factors that affect the rate of photosynthesis.
▶️Answer/Explanation
Markscheme
Remember, up to TWO “quality of construction” marks per essay.
Award [1] for each structure clearly drawn and correctly labelled.
a. phospholipid bilayer – with head and tails;
b. hydrophilic/phosphate/polar heads and hydrophobic/hydrocarbon/fatty acid/non-polar tails labelled;
c. integral/intrinsic protein – embedded in the phospholipid bilayer;
d. protein channel – integral protein showing clear channel/pore;
e. peripheral/extrinsic protein – not protruding into the hydrophobic region;
f. glycoprotein with carbohydrate attached – carbohydrate should be outside the bilayer;
g. cholesterol – positioned across one half of bilayer and not protruding;
h. thickness indicated (10 nm); (allow answers in the range of 7 nm to 13 nm)
Remember, up to TWO “quality of construction” marks per essay.
a. (chlorophyll/pigments/antenna complex) in photosystem II absorb light;
b. light/photoactivation produces an excited/high energy/free electron;
c. electrons pass from carrier to carrier/along electron transport chain/e.t.c.;
d. protons pumped across thylakoid membrane/into thylakoid space;
e. ATP produced (by the light dependent reactions);
f. ATP production by chemiosmosis/by ATP synthase/ATP synthetase;
g. electrons from photosystem II passed to photosystem I;
h. light/photoactivation excites electrons in photosystem I (to higher energy level);
i. production of NADPH/reduction of NADP(+) (using electrons from photosystem I); (reject NAD in place of NADP. Accept reduced NADP instead of NADPH)
j. electrons from photolysis (needed) for photosystem II;
k. oxygen from photolysis is a waste product/by-product/passes out/excreted;
l. in cyclic photophosphorylation electrons from photosystem I return to it;
Remember, up to TWO “quality of construction” marks per essay.
a. (increase in) light (intensity) increases rate (of photosynthesis);
b. until a plateau is reached at higher light intensities/when another factor is limiting;
c. light needed for light dependent reactions/example of light dependent reaction;
d. (increase in) temperature/heat increases the rate (of photosynthesis);
e to an optimum temperature above which the rate drops;
f. temperature/heat affects rate of Calvin cycle/enzyme activity/rubisco activity;
g. (increase in) carbon dioxide (concentration) increases rate (of photosynthesis);
h. until a plateau is reached at higher CO2 levels/when another factor is limiting;
i. CO2 needed for light independent reactions/Calvin cycle/carboxylation of RuBP/production of glycerate phosphate;
If the candidate outlines more than two factors, only mark the first two.
Accept the first two points relating to each factor if clearly shown on a graph with both axes appropriately labelled.
Accept level instead of concentration, intensity or rate.
Do not accept enzyme denaturation as a reason for reductions in photosynthesis at higher temperatures.
Question
Draw a labelled diagram to show the molecular structure of a membrane.
Some proteins in membranes act as enzymes. Describe a model that accounts for the ability of enzymes to catalyse reactions.
Membranes of pre-synaptic and post-synaptic neurons play an important role in transmission of nerve impulses. Explain the principles of synaptic transmission.
▶️Answer/Explanation
Markscheme
Award [1] for each of the following clearly drawn and correctly labelled.
phospholipid bilayer; (double row of opposing phospholipids, tails to inside)
hydrophilic/phosphate/polar (heads) and hydrophobic/hydrocarbon/fatty acid/nonpolar (tails) labeled;
integral protein; (embedded in the phospholipid bilayer)
protein channel/channel protein; (integral protein showing clear channel/pore)
peripheral protein; (shown on surface or slightly embedded on either side)
glycoprotein; (with carbohydrate attached on outer side)
cholesterol; (shown embedded in bilayer and smaller than the hydrophobic tail)
induced fit model; (do not accept lock and key hypothesis)
accounts for ability of some enzymes to bind to several substrates;
enzyme with active site to which substrate(s) binds;
enzyme active site and substrate do not match up exactly;
enzyme-substrate complex forms;
enzyme changes shape once bound / enzyme moulds to substrate/ hand in glove;
change in shape strains bonds/facilitates bonds breaking/product formation;
reduces activation energy;
once reaction is complete, products leave and enzyme can work again;
Award any of the above points for a clearly drawn correctly annotated diagram.
synapse is gap between adjacent neurons;
(arriving) action potential depolarizes pre-synaptic membrane;
opens (voltage-gated) calcium channels in membrane;
causes influx of calcium ions;
causes synaptic vesicles to fuse with pre-synaptic membrane;
vesicles release/exocytose neurotransmitter into the synaptic cleft;
neurotransmitter diffuses/moves across synaptic cleft;
neurotransmitter binds to receptors on post-synaptic membrane;
opens channels allowing sodium ions/potassium ions to diffuse;
initiation of action potential/depolarization in post-synaptic membrane;
removal/breakdown of neurotransmitter stops effect on post-synaptic membrane;
Award any of the above points for a clearly drawn correctly annotated diagram.
(Plus up to [2] for quality)