Question
Although simple in structure, bacteria as a group show a wide range of diversity.
a. Outline the roles bacteria play in the carbon cycle.
b. Describe the evolution of antibiotic resistance in bacteria.
c. Explain the process of genetically modifying bacteria.
Answer/Explanation
Ans:
a)
a. decomposition of dead organic material «by saprotrophic bacteria» $\checkmark$
b. «decomposition» leads to $\mathrm{CO}_2$ formation/regeneration due to respiration $\checkmark$
c. «saprotrophic bacteria only» partially decompose dead organic matter in acidic/anaerobic conditions in waterlogged soil $\checkmark$
d. results in peat formation in bogs/swamps $\checkmark$
e. photosynthetic bacteria/cyanobacteria fix $\mathrm{CO}_2$ in photosynthesis $\checkmark$
b)
a. problem results from excessive use of antibiotics by doctors/veterinarians/in livestock OR
low antibiotic doses taken by patients (not finishing treatment) $\checkmark$
b. natural variation exists in any population of bacteria making some resistant to a specific antibiotic $\checkmark$
c. variation arises from mutation OR
antibiotic resistance can be transferred between bacteria by plasmids $\checkmark$
d. antibiotic kills all bacteria except those that are resistant $\checkmark$
e. resistant bacteria survive, reproduce and pass on resistance to offspring $\checkmark$
f. soon population is made of mainly antibiotic resistant bacteria $\checkmark$
g. this is an example of natural selection «increasing frequency of characteristics that make individuals better adapted to environment» $\checkmark$
c)
a. genetic modification carried out by gene transfer between species $\checkmark$
b. genes transferred from one organism to another produce the same protein/amino acid sequence $\checkmark$
c. due to universality of genetic code
OR
organisms use same codons of mRNA to code for specific amino acids $\checkmark$
d. mRNA for required gene extracted/identified $\checkmark$
e. DNA copies of mRNA made using reverse transcriptase $\checkmark$
f. PCR used (to amplify DNA to be transferred) $\checkmark$
g. genes/DNA transferred from one species to another using a vector $\checkmark$
h. plasmid acts as vector to transfer genes to bacteria/E. coli $\checkmark$
i. plasmid cut open at specific base sequences using restriction endonuclease
OR
plasmid cut to produce blunt ends then extra cytosine/C nucleotides added
OR
sticky ends made by adding extra guanine/G nucleotides
OR
mention of sticky ends if not gained already $\checkmark$
j. cut plasmids mixed with DNA copies stick together (due to complementary base pairing) $\checkmark$
k. DNA ligase makes sugar-phosphate bonds to link nucleotides of gene with those of plasmid $\checkmark$
I. bacteria that take up plasmid are identified $\checkmark$
m. (genetically modified) bacteria will reproduce carrying the transferred gene $\checkmark$
Question
Every cell is surrounded by a cell surface membrane which regulates the movement of materials into and out of the cell.
a. Draw an annotated diagram of the fluid mosaic model of membrane structure.
b. Describe the processes involved in absorbing different nutrients across the cell membrane of villus epithelium cells lining the small intestine. [4]
c. Explain the events that occur during a nerve impulse and how the impulse is propagated along an axon.
Answer/Explanation
Ans:
a)
a. phospholipid bilayer – with head and tails $\checkmark$
b. hydrophilic/phosphate/polar heads AND hydrophobic/hydrocarbon/fatty acid/non-polar tails labelled $\checkmark$
c. integral/intrinsic protein – embedded in the phospholipid bilayer $\checkmark$
d. protein channel – integral protein showing clear channel/pore $\checkmark$
e. peripheral/extrinsic protein – on the surface $\checkmark$
f. glycoprotein with carbohydrate attached $\checkmark$
b)
a. «simple» diffusion of nutrients along/down a concentration gradient $\checkmark$
b. example of simple diffusion, eg: fatty acids $\checkmark$
c. facilitated diffusion of nutrients involves movement through channel proteins $\checkmark$
d. example of nutrient for facilitated diffusion eg: fructose $\checkmark$
e. active transport of nutrients against a concentration gradient / involving protein pumps $\checkmark$
f. example of active transport, eg: (iron) ions/glucose/amino acids $\checkmark$
g. endocytosis / by means of vesicles $\checkmark$
h. example of nutrient for endocytosis, eg: cholesterol in lipoprotein particles $\checkmark$
c)
a. nerve impulses are action potentials propagated along axons of neurons $\checkmark$
b. resting potential is $-70 \mathrm{mV}$ OR
relatively negative inside in comparison to the outside $\checkmark$
c. $\mathrm{Na}^{+} / \mathrm{K}^{+}$pumps maintain/re-establish «the resting potential» $\checkmark$
d. more sodium ions outside than inside «when at the resting potential» OR
more potassium ions inside than outside «when at the resting potential» $\checkmark$
e. action potential stimulates «wave of» depolarization along the membrane/axon $\checkmark$
f. “when neuron is stimulated” if threshold potential is reached $\mathrm{Na}^{+}$channels open $\checkmark$
g. sodium ions diffuse/move in $\checkmark$
h. «$Na^{+}$ move in» causing depolarization / inside of the neuron becomes more positively charged than the outside of the neuron $\checkmark$
i. potassium ion channels open
OR
potassium ions diffuse/move out $\checkmark$
j. «K ${ }^{+}$move out» causing repolarization $\checkmark$
k. local currents
OR
description of $\mathrm{Na}^{+}$ion diffusion between depolarized region and next region of axon to depolarize $\checkmark$