IB DP Biology- B2.1 Membranes and membrane transport - IB Style Questions For SL Paper 2 - FA 2025
▶️ Answer/Explanation
(a)
• The circular “head” region of each phospholipid should be labelled as the phosphate head, which is hydrophilic / polar / attracted to water and faces the aqueous environments on either side of the membrane.
• The two “tails” should be labelled as fatty acid (hydrocarbon) tails, which are hydrophobic / non-polar / not attracted to water and face inwards away from water.
• Phospholipids are described as amphipathic because they have both a hydrophilic region (phosphate head) and a hydrophobic region (fatty acid tails).
(b)
• Cholesterol reduces membrane fluidity and decreases permeability of the membrane to some small molecules, helping to stabilize the bilayer at higher temperatures.
• At lower temperatures, cholesterol prevents the membrane from becoming too rigid, helping maintain an appropriate level of fluidity.
• Overall, cholesterol helps to maintain membrane flexibility and structural integrity over a range of temperatures.
(c)
Any two of the following pieces of evidence (with description) gain credit:
- Own DNA: Mitochondria and chloroplasts contain their own naked, circular DNA, similar to that of prokaryotes, suggesting they were once free-living bacteria.
- 70S ribosomes: These organelles have 70S ribosomes (prokaryotic type) rather than the 80S ribosomes found in the eukaryotic cytosol.
- Double membranes: Mitochondria and chloroplasts possess a double membrane, consistent with an engulfing event by endocytosis.
- Binary-fission-like replication: They can replicate independently of the cell by a process resembling binary fission in bacteria.
- Similar size and antibiotic sensitivity: They are a similar size to bacteria and in some cases show sensitivity to certain antibiotics, further supporting a prokaryotic origin.
These shared features between mitochondria/chloroplasts and prokaryotes support the endosymbiotic theory, which proposes that eukaryotic cells originated when ancestral prokaryotes were engulfed and became permanent symbionts.