(a) A student constructed a table to compare the structural features of a plant cell, a prokaryotic cell and a virus.
Complete Table 6.1.
| feature | plant cell | prokaryotic cell | virus |
|---|---|---|---|
| external structure | cell wall composed of cellulose | cell wall composed of ………… | capsid composed of ………… |
| size of ribosomes | 80S and 70S | ………… | no ribosomes |
| nucleic acids | DNA and RNA | DNA and RNA | ………… |
(b) The cholera bacterium releases a protein toxin called choleragen. The toxin causes the loss of chloride ions and water from epithelial cells into the lumen of the intestine.
Fig. 6.1 shows the events that occur in cells lining the intestine when choleragen binds to the membrane of one of these cells.
(i) State one likely source of an outbreak of cholera.
(ii) With reference to Fig. 6.1, state why choleragen molecules are described as having quaternary structure.
(iii) State the type of cell membrane component that forms the receptor for choleragen.
(iv) The process by which chloride ions leave the epithelial cell requires energy.
Name the phosphorylated nucleotide that is needed for this process.
(v) Explain why water also moves from epithelial cells into the lumen of the intestine when choleragen is present.
▶️ Answer/Explanation
(a)
| feature | plant cell | prokaryotic cell | virus |
|---|---|---|---|
| external structure | cell wall composed of cellulose | cell wall composed of peptidoglycan/murein | capsid composed of protein/polypeptides |
| size of ribosomes | 80S and 70S | 70S | no ribosomes |
| nucleic acids | DNA and RNA | DNA and RNA | DNA or RNA |
Explanation: The table compares key structural features of different cell types. Prokaryotic cells have peptidoglycan/murein cell walls, unlike plant cells with cellulose. Viruses have protein capsids instead of cell walls. Prokaryotic ribosomes are smaller (70S) compared to eukaryotic ones (80S). Viruses uniquely contain either DNA or RNA, never both, unlike cells which contain both nucleic acids.
(b)(i) Untreated/raw sewage or contaminated drinking water.
Explanation: Cholera outbreaks typically originate from water sources contaminated with Vibrio cholerae bacteria, often due to poor sanitation systems where human waste mixes with drinking water supplies. This was notably seen in the 2010 Haiti outbreak linked to UN peacekeepers.
(b)(ii) Because choleragen is composed of more than one polypeptide (typically 6-7 subunits).
Explanation: Quaternary structure refers to protein complexes made of multiple polypeptide chains. Choleragen’s toxic activity depends on this multi-subunit arrangement, with different subunits performing binding and enzymatic functions.
(b)(iii) Glycolipid or glycoprotein.
Explanation: These membrane components with carbohydrate groups serve as common receptors for bacterial toxins. The specific GM1 ganglioside receptor for choleragen is a glycolipid.
(b)(iv) ATP (adenosine triphosphate).
Explanation: The active transport of chloride ions against their concentration gradient requires energy from ATP hydrolysis, which provides both the phosphate group and energy for membrane pumps.
(b)(v) Chloride ions decrease the water potential in the intestinal lumen, creating an osmotic gradient that draws water out of cells by osmosis.
Detailed Explanation: When choleragen activates chloride ion channels, massive efflux of Cl⁻ ions into the intestinal lumen occurs. These ions lower the water potential (make it more negative) in the lumen compared to epithelial cells. Water then follows the ions by osmosis, moving from the higher water potential in cells to the lower water potential in the lumen. This results in the characteristic watery diarrhea of cholera as the intestine cannot reabsorb this fluid.