▶️ Answer/Explanation
(a)(i)
According to Fig 3.1, the eye is composed of $95\%$ water.
Mass of water = $28\,\text{g} \times 0.95 = 26.6\,\text{g}$.
Rounding to two significant figures, the answer is $27\,\text{g}$.
(a)(ii)
Water acts as a critical solvent in the body, allowing substances to dissolve for transport (e.g., in blood plasma) and for metabolic reactions (e.g., digestion/hydrolysis). It is also vital for excretion (urine) and temperature regulation (sweating).
(b)(i)
L: Cell membrane (the inner line beneath the cell wall).
M: Ribosome (the small dots in the cytoplasm).
(b)(ii)
Typical prokaryotic features visible in the diagram include:
1. Circular DNA (the looped genetic material, not enclosed in a nucleus).
2. Plasmids (small circular rings of DNA).
3. Cell wall (the outer layer; note: prokaryotes do not have a nucleus).
(b)(iii)
The type of pathogen is a bacterium.
(b)(iv)
The scientific name is Vibrio cholerae. The genus name is the first part: Vibrio.
(b)(v)
The cholera bacterium produces a toxin that causes the secretion of chloride ions ($\text{Cl}^-$) into the small intestine. This accumulation of ions lowers the water potential in the gut lumen. Consequently, water moves out of the cells and blood into the gut by osmosis, leading to watery diarrhoea and dehydration.
(b)(vi)
During genetic modification:
1. Restriction enzymes create sticky ends.
2. These are joined together using an enzyme called DNA ligase (or simply ligase).
3. The modified plasmid is called a recombinant plasmid.