Question
Draw a labelled diagram to show the structure of a membrane.
Outline how vesicles are used to transport materials secreted by a cell.
Explain how the structure of a villus in the small intestine is related to its function.
▶️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;
phospholipid – with head and two tails;
hydrophilic/phosphate/polar (heads) and hydrophobic/hydrocarbon/fatty acid/non-polar (tails) labelled;
integral protein – embedded in the phospholipid bilayer;
protein channel – integral protein showing clear channel/pore;
peripheral protein – on the surface;
glycoprotein – with carbohydrate attached on one side;
cholesterol – shown embedded in bilayer;
vesicles formed from rER transport proteins to Golgi apparatus;
these vesicles fuse with membranes of Golgi apparatus;
proteins are processed as they move through Golgi apparatus;
(transport) vesicles bud off/leave Golgi apparatus;
vesicles move through cytoplasm;
(vesicles) fuse with plasma membrane;
contents released to outside of cell / exocytosis;
cells use vesicles to secrete substances such as hormones/digestive enzymes/other appropriate example;
vesicles may contain cell products other than proteins;
Credit drawings which fully explain the points above.
To achieve [1] answer must name the structure and relate it to its function.
the villus has a large surface area to volume ratio;
microvilli increase surface area for absorption;
thin surface (epithelial) layer so products of digestion can pass easily through;
channel proteins located in plasma membrane used for facilitated diffusion;
network of capillaries inside each villus (so only short distance) for movement of absorbed products;
capillaries transport absorbed nutrients/sugars and amino acids away from small intestine;
blood flow in capillaries maintains concentration gradient;
central lymph vessel/lacteal to transport absorbed fats/fatty acids away from small intestine;
large number of mitochondria provide ATP needed for active transport;
protein pumps in membrane (of microvilli) carry out active transport;
pinocytosis occurs at surface (epithelial) layer;
Accept any of the points above shown in a drawing.
Question
Draw a labelled diagram to show the structure of membranes.
Explain passive transport and active transport across membranes.
▶️Answer/Explanation
Markscheme
Award [1] for each structure clearly drawn and correctly labelled.
phospholipid bilayer – with head and tails;
hydrophilic / phosphate / polar heads and hydrophobic / hydrocarbon / fatty acid / non-polar tails labelled;
integral/intrinsic protein – embedded in the phospholipid bilayer;
protein channel – integral protein showing clear channel/pore;
peripheral/extrinsic protein – on the surface;
glycoprotein with carbohydrate attached; (carbohydrate should project outwardly from membrane protein)
cholesterol – shown embedded in bilayer; (must appear in hydrophobic region)
thickness indicated (10 nm); (allow answers in the range of 7 nm to 13 nm)
diffusion/facilitated diffusion and osmosis are passive;
do not require energy/ATP;
diffusion is movement from high to low concentration/down a (concentration) gradient;
facilitated diffusion uses (protein) channels/carrier proteins;
osmosis is water movement from lower to higher solute concentration / from higher to lower water potential/concentration;
across a partially permeable membrane;
active transport/formation of vesicles require energy;
in the form of ATP;
active transport moves materials up/against the (concentration) gradient/from low to high concentration;
protein pumps required;
endocytosis into cells / exocytosis out of cells;
example of active or passive transport; (e.g. sodium potassium pump for active transport / oxygen exchange in alveoli for passive transport)
Question
List two functions of membrane proteins.
Explain why digestion of large food molecules is essential.
Outline why antibiotics are effective against bacteria but not against viruses.
Outline the use of polymerase chain reaction (PCR) to copy and amplify minute quantities of DNA.
▶️Answer/Explanation
Markscheme
a. hormone binding sites / receptors;
b. (immobilized) enzymes;
c. cell adhesion;
d. cell (to cell) communication;
e. passive transport/channels;
f. active transport/pumps;
g. facilitate diffusion;
h. carry electrons;
a. many molecules are too large to be absorbed (by the villi) / small molecules are soluble and can be absorbed;
b. large food molecules are broken down so they can be reorganized/rearranged;
a. antibiotics block/inhibit specific metabolic pathways/cell functions found in bacteria;
Accept specific examples of inhibition such as cell protein synthesis, cell wall formation
b. viruses must use host/eukaryotic cell metabolism / viruses do not have their own metabolic pathways;
c. host/eukaryotic cell metabolism/pathways not blocked/inhibited by antibiotics;
a. strands of DNA (fragments) split/denatured with heat;
b. complementary nucleotides added to split stands (when cooling);
c. with the use of (DNA) polymerase (and primers);
d. process/heating and cooling cycle is repeated (until enough DNA is obtained);
Accept example of PCR application e.g. paternity cases or forensic investigations.
Question
The diagram shows how vesicles are used to transport materials in a cell.
State the name of organelle A.
State the process occurring at B.
Describe how the structure of the membrane allows the formation of vesicles.
Explain active transport across membranes.
▶️Answer/Explanation
Markscheme
Golgi apparatus/complex/body
Reject Golgi vesicle and Golgi unqualified.
endocytosis/phagocytosis/pinocytosis
Reject exocytosis.
a. fluidity of membrane allows change of shape/invagination/formation of vesicles;
b. phospholipids can move / phospholipid bilayer makes membrane fluid/flexible;
c. weak bonding between phospholipid tails;
d. bends/kinks in the phospholipid tails prevent close packing;
e. cholesterol affects membrane fluidity;
a. moves substances up/against a concentration gradient / from lower to higher concentration;
b. protein/pump (in membrane) that moves material; (reject channels)
c. ATP is used; (reject energy alone)
d. example/labeled diagram showing mechanism;
Question
Draw a labelled diagram to show the molecular structure of a membrane.
Some proteins in membranes act as enzymes. Outline enzyme-substrate specificity.
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)
enzyme shape is specific to (particular) substrate;
lock and key analogy/model;
example of specific enzyme and substrate;
has specific 3-D/tertiary configuration/3-D/tertiary shape essential to functioning;
active site on enzyme binds to substrate;
substrate and active site complementary/fit together;
(substrate and active site) are complementary due to structure/chemical attraction;
enzyme-substrate complex forms;
denaturation changes enzyme’s binding ability (to specific substrate);
Award [6] for the above points clearly shown in an 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)
Question
The image shows a phospholipid bilayer that is a component of the cell membrane.
(a) Annotate the diagram to illustrate the amphipathic nature of phospholipids. [2]
(b) Outline a function of cholesterol in cell membranes. [1]
(c) Describe two pieces of evidence that show that eukaryotic cells originated by endosymbiosis. [2]
▶️Answer/Explanation
a a. line to circle labelled phosphate (head) and (tail) labelled fatty acid/hydrocarbon/lipid (tail);
b. label hydrophilic/polar/attracted to water/ and hydrophobic/non polar/not attracted to water;
b reduces fluidity of membrane / reduces permeability of membrane (to some molecules);
c a. mitochondria/chloroplasts have their own DNA;
b. mitochondria can self-replicate/undergo a process like binary fission;
c. mitochondria/chloroplasts have double membranes;
d. mitochondria/chloroplasts have(70s) ribosomes;
e. mitochondria/chloroplasts are sensitive to antibiotics;
f. similar in size to bacteria
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