HSC Exam Practice

Sample response. Cell differentiation is the process by which a cell permanently becomes structurally and functionally specialised. It occurs through selective gene expression: specific genes are switched on while others are permanently silenced, causing the cell to produce only the proteins needed for its function and to develop a structure suited to that role.

Marking notes. 1 mark for defining differentiation as permanent structural/functional specialisation of a cell; 1 mark for explicitly linking differentiation to selective gene expression (not gene loss).

Sample response. Both cell types developed from the same fertilised egg, which contained the organism’s complete genome. As the embryo developed, chemical signals activated specific transcription factors in each developing cell lineage. In a cell destined to become a neuron, genes for axon formation, ion channels, and myelination proteins were switched on; in the red blood cell precursor, genes for haemoglobin production were activated and most other genes (including those for the nucleus and organelle maintenance) were permanently silenced. Both cells still carry all 20,000 genes, but only different subsets are expressed in each. The structural differences arise from which proteins are produced, not from any difference in the DNA sequence.

Marking notes. 1 mark for stating that both cells contain the same/identical DNA (from the same fertilised egg / same complete genome); 1 mark for explaining that differentiation is caused by selective gene expression (different genes switched on/off in each lineage), not loss of genes; 1 mark for explaining the consequence: different proteins are produced, giving different structures/functions.

Sample response. Animal cell, neuron: the long axon (up to 1 m) enables transmission of electrical signals over large distances because it physically bridges distant parts of the nervous system, allowing communication that could not occur by diffusion alone. Plant cell, root hair cell: the long thin root hair extension increases surface area for absorption by up to 10 times, enabling more water and minerals to be taken up by osmosis and active transport because a greater membrane area is in contact with the soil solution.

Marking notes. 1 mark per cell for naming a correct structural feature; 1 mark per cell for correctly explaining how that feature enables the cell’s function (including the mechanism). Any valid animal cell (RBC, neuron, muscle cell, goblet cell, sperm) and plant cell (palisade mesophyll, root hair, guard cell) are acceptable. “Because” reasoning required for second mark.

Sample response. This cell is a sperm cell (spermatozoon). The whip-like projection is a flagellum that propels the cell toward the egg; mitochondria concentrated near the flagellum supply ATP directly at the site where it is consumed by the flagellum’s motor proteins, enabling continuous movement; the membrane-capped structure at the head is the acrosome, which contains hydrolytic enzymes released to penetrate the egg’s outer membrane during fertilisation. Minimal cytoplasm reduces mass, increasing motility efficiency.

Marking notes. 1 mark for correctly identifying the cell as a sperm cell; 1 mark for correctly explaining any two of the three structural clues (flagellum = propulsion; mitochondria near flagellum = ATP supply; acrosome = enzymes for egg penetration). Award both marks only if the explanation connects the structure to a specific function.

Sample response. Mitochondria counts vary greatly between cell types. The liver cell and skeletal muscle cell both have the highest counts (approximately 1,500 each), while the root hair cell has very few (~50) and the mature red blood cell has none. Heart muscle cells are also shown at ~5,000 (the highest bar on the graph). In general, cells that carry out high-energy processes continuously have more mitochondria than cells with low or no aerobic metabolic activity.

Marking notes. 1 mark for correctly describing the overall pattern (metabolically active cells have more mitochondria; red blood cell = 0; root hair = few) and identifying the cells at the extremes. 1 mark for quoting at least two specific figures from the graph to support the description.

Sample response. Root hair cells are specialised for water and mineral absorption: they extend a long thin projection into the soil to increase surface area, and they use osmosis to take in water and active transport for mineral ions. Their energy demand is relatively low, mainly for the active transport of minerals across the cell membrane, so few mitochondria are needed [1]. Liver cells (hepatocytes), by contrast, are among the most metabolically active cells in the body: they synthesise enormous quantities of protein for export (requiring ~13 million ribosomes), detoxify substances, and carry out hundreds of biochemical reactions continuously. These high-energy processes require a sustained, very high rate of ATP production, which is why liver cells maintain ~1500 mitochondria compared to the root hair cell’s ~50 [1]. In general, cells with a high ATP demand have many mitochondria; cells with a low ATP demand have few, structure suits function [1].

Marking notes. 1 mark for explaining that root hair cells have a low ATP demand (absorption/osmosis requires little energy compared to the liver’s metabolic workload). 1 mark for explaining that liver cells have a very high ATP demand because of continuous, high-volume protein synthesis and metabolic activity. 1 mark for stating the organelle inference principle: more mitochondria = higher ATP demand / more metabolically active cell.

Sample response. Mature red blood cells have zero mitochondria because they were expelled during maturation along with the nucleus and other membrane-bound organelles. This structural loss maximises the internal volume available for haemoglobin, the iron-containing protein that binds and transports oxygen. Since the cell’s sole function is oxygen transport, not energy-intensive active transport or cell division, the need for large-scale ATP production (the function of mitochondria) is absent, making the trade-off beneficial: more haemoglobin space means more oxygen carried per cell.

Marking notes. 1 mark for explaining that organelles including mitochondria are expelled/lost during maturation as part of the differentiation program; 1 mark for linking this absence to improved function, more internal volume for haemoglobin = greater oxygen-carrying capacity (and/or explaining that the cell does not require mitochondrial ATP production for its primary function).

Sample response. The student’s claim is largely correct but the use of the word “trade-off” requires careful evaluation.

The process of cell differentiationby which cells permanently specialise through selective gene expression, does enable extreme efficiency in a single function. For example, a red blood cell has permanently silenced genes for nucleus maintenance, cell division, and mitochondrial production, devoting all available internal volume to haemoglobin. This makes it extraordinarily efficient at oxygen transport, more so than any generalised cell could be. Similarly, a palisade mesophyll cell has expressed genes that produce 40–50 chloroplasts per cell and adopted a columnar shape to maximise light capture, while silencing genes for functions unrelated to photosynthesis. The student is right that specialisation enables exceptional performance.

The “trade-off” language is partially useful: a differentiated cell does permanently commit to one function at the expense of flexibility. A red blood cell cannot suddenly start photosynthesising, and a neuron cannot revert to a stem cell under normal conditions. The permanent silencing of genes (through chromatin condensation and methylation) means the cell’s gene expression pattern is locked in for its lifetime.

However, the word “cost” is potentially misleading. The silenced genes are not lost, damaged, or wasted, they remain intact in every cell’s nucleus and are expressed in the other cell types that need them. The “cost” to any individual cell is not a cost to the organism as a whole, because the full genome is deployed across the organism’s different cell types. Division of labour means the organism as a whole gains the combined benefit of all specialised cells performing their functions at maximum efficiency.

The student’s claim that this makes multicellular organisms “more complex but more capable” is accurate: the coordination of many specialised cells allows a multicellular organism to perform functions, directed movement over large distances, active immune responses, photosynthesis at scale, that no unicellular organism could achieve. The “trade-off” framing works best when applied at the cell level, not the organism level.

Marking criteria.

• 1 markStates an overall evaluative judgement on the student’s claim (e.g. “largely correct” with a qualification about the “trade-off” language).
• 1 markCorrectly defines cell differentiation as permanent specialisation through selective gene expression (not gene loss).
• 1 markNames a first specialised cell (animal) and correctly explains how selective gene expression produces its specific structure and function.
• 1 markNames a second specialised cell (plant or animal) and correctly explains how selective gene expression produces its specific structure and function.
• 1 markCorrectly identifies the sense in which “trade-off” is useful at the cell level: permanent silencing means the cell commits fully to one role and cannot take on others.
• 1 markCorrectly identifies the limitation of “trade-off” language: silenced genes are not lost to the organism, they remain in the genome, expressed in other cell types; the organism as a whole deploys the full genome across specialised cells.
• 1 markExplains how division of labour across specialised cells makes multicellular organisms more capable than unicellular organisms (with reference to a specific function or comparison).
• 1 markResponse is well-organised, uses correct terminology throughout, and reaches an explicit, justified conclusion on whether “trade-off” is a useful or misleading framing.