Question
Many biological processes are cyclical. Examples of cycles can be found at the level of the cell, the organism and the ecosystem.
(a) Explain how changes to the cell cycle can result in tumour formation.
(b) Outline the role of the right atrium in the cardiac cycle.
(c) Describe processes in the carbon cycle that produce or use carbon dioxide.
Answer/Explanation
Answer:
(a)
a. cell cycle is (repeated) sequence of cytokinesis/cell division, (then) interphase
(then) mitosis / cell cycle includes the sequence of interphase, mitosis and
cytokinesis/cell division (to form new cells which repeat the cycle);
b. cyclins control/regulate this cycle / ensure the cell moves on to the next stage
of the cycle when it is appropriate;
c. extra cells produced when they are needed;
d. tumour formation is the result of uncontrolled cell division/ cells growing and
dividing endlessly;
e. repeated mitoses/high mitotic index;
f. due to mutations in oncogenes / oncogenes may become active and contribute
to the development of a cancer cell;
g. carcinogens/radiation/mutagenic chemicals/mutagens/smoking cause
mutations/tumours;
(b)
a. right atrium collects (deoxygenated) blood from the body;
b. blood drains into atrium through/from the vena cava;
c. deoxygenated blood present in vena cava/right atrium/right ventricle;
d. right atrium pumps blood into the right ventricle;
e. during atrial systole/ventricular diastole/at the start of the cardiac cycle;
f. sinoatrial node is in the right atrium/sinoatrial node acts as the (natural)
pacemaker / sinoatrial node initiates the heartbeat;
g. SA node sends out electrical signal to stimulate contraction in the (walls of the)
atria/then propagated to the AV node / (walls of the) ventricles;
(c)
a. photosynthesis uses carbon dioxide / reduces carbon dioxide concentration of the atmosphere;
b. autotrophs/plants/cyanobacteria convert/fix carbon dioxide into carbon/organic
compounds;
c. cell respiration produces/releases carbon dioxide;
d. glucose/carbon/organic compounds oxidised/broken down to produce/release
carbon dioxide;
e. carbon dioxide released from aerobic (cell) respiration AND anaerobic
respiration in yeast/plants (but not animals);
f. carbon dioxide released from saprotrophs/detritivores/decomposers from dead
organic matter / during decay/decomposition/respiration;
g. (partially) decomposed organic matter can lead to the formation of peat /
fossilized organic matter (coal/oil/natural gas)
h. carbon dioxide released when carbon/organic compounds burn / during
combustion (of biomass/fossil fuels) / forest fires;
i. carbon dioxide dissolves in aquatic ecosystems / can form carbonic
acid/hydrogen carbonate ions;
j. reef-building corals/molluscs use calcium carbonate to make/build
shells/exoskeletons or other body parts;
k. hard parts/shells/exoskeletons / precipitation of calcium carbonate to form
limestone/tufa;
Question
The diagram shows a section through the melatonin receptor, with melatonin attached to its binding site. Darker grey areas show the surface of the protein and paler areas are internal. The membrane in which this receptor is located is also shown.
(a) Draw one phospholipid molecule on the diagram to show a possible position in the membrane.
(b) The receptor contains seven alpha helices and one other secondary structure.
Deduce what this other secondary structure is.
(c) Discuss briefly whether amino acids on the surface of the protein are likely to be polar or non-polar.
(d) Outline the role of melatonin in humans.
Answer/Explanation
Answer:
(a) phospholipid shown with circular head and 2 tails;
e.g.
phospholipid shown in either the upper or lower half of the membrane only with its
head at the margin of the membrane and tails towards centre;
(b) beta pleated sheet/beta-loop/beta strands;
(c) polar/hydrophilic where exposed to the cytoplasm/to fluid outside cell/to polar phospholipid heads;
non-polar/hydrophobic where exposed to the (core of the) membrane/hydrophobic tails (of phospholipids);
(d)
a. control/maintain/regulate circadian rhythms;
b. secreted/released late evening/end of day/in dark/night time/dim light/absence of blue light;
c. helps to induce sleep/sleepiness/influences timing of sleeping/waking/control sleep cycle;
Question
Hemoglobin is a conjugated protein consisting of four polypeptide chains with attached heme groups.
(a) State the level of protein structure at which the polypeptide chains of hemoglobin
are combined.
(b) Explain the shape of the polypeptide chain at X.
(c) Outline the role of tRNA in hemoglobin synthesis.
Answer/Explanation
Ans:
(a) quaternary / fourth level;
(b)
a. alpha helix / α helix
b. type of secondary structure / second level of protein structure;
c. maintained by hydrogen bonds;
d. between C=O group and an N–H group;
(c)
a. decoding/translation (of the genetic code/RNA base sequence);
b. carries/brings one amino acid/a specific amino acid/the amino acid (corresponding to
codon/anticodon);
c. tRNA has an anticodon which pairs with mRNA/is complementary to a codon (on mRNA);