Biology • Year 11 • Module 2 • Lesson 2
Cell Specialisation and Differentiation
Apply structure–function reasoning to real data, organelle counts, and unfamiliar cell descriptions, the skills tested in HSC Section I and short-response questions.
1. Interpret organelle count data
The table below shows organelle counts in three cell types taken from a Year 11 Biology study. Use the data to answer the questions that follow. 8 marks
| Organelle | Liver cell (hepatocyte) | Skeletal muscle cell | Mature red blood cell |
|---|---|---|---|
| Mitochondria | ~1,000–2,000 | ~1,000–2,000 | 0 |
| Ribosomes | ~13,000,000 | Moderate | 0 |
| Lysosomes | ~300 | Few | 0 |
| Nucleus | 1 | Multiple (50–100) | 0 |
| Rough ER | Extensive | Moderate | None |
1.1 Explain why liver cells have significantly more ribosomes than skeletal muscle cells. Refer to the liver cell’s function in your answer. 2 marks
1.2 Explain why skeletal muscle cells have multiple nuclei while liver cells typically have only one. 2 marks
1.3 Justify why mature red blood cells lack all of the organelles listed. Link your answer to the cell’s function. 2 marks
1.4 Predict which organelles would be present in high numbers in a goblet cell. Justify your prediction by linking organelle function to the goblet cell’s role. 2 marks
2. Interpret graph, surface area and cell specialisation
The figure below shows the estimated surface area available for absorption in a section of plant root, comparing cells with and without root hair extensions. A standard epidermal cell has a surface area of 1 unit; a fully developed root hair cell reaches approximately 10 units. 6 marks
Figure 2. Modelled relative surface area available for absorption per cell. Based on mean values reported in plant anatomy literature.
2.1 Describe the difference in surface area between a standard epidermal cell and a root hair cell shown by the graph. Include figures in your answer. 2 marks
2.2 Using lesson content, explain why a larger surface area is advantageous for a root hair cell’s function. Refer to the mechanism involved in water uptake. 2 marks
2.3 Explain why the same surface area increase seen in root hair cells would not benefit a red blood cell. Link your reasoning to the different functions of the two cells. 2 marks
3. Mystery cell identification
Four mystery cells are described below. For each one: (a) identify the most likely cell type, and (b) explain how two named structural features support your identification. 8 marks, 2 per cell
Cell A: Enormous numbers of ribosomes and rough ER; prominent Golgi apparatus; no chloroplasts; cup-shaped cytoplasm filled with granules.
(a) Cell type: ___________________________________________
(b) Two supporting features:
Cell B: No nucleus; no mitochondria; biconcave disc shape; densely packed with an iron-containing protein.
(a) Cell type: ___________________________________________
(b) Two supporting features:
Cell C: Many branching dendrites; a single very long axon covered in a myelin sheath; synaptic vesicles at the terminal end; many mitochondria.
(a) Cell type: ___________________________________________
(b) Two supporting features:
Cell D: Has a cell wall; 40+ chloroplasts; elongated columnar shape; positioned near the surface of a flat structure exposed to sunlight.
(a) Cell type: ___________________________________________
(b) Two supporting features:
4. Apply to a new scenario, sperm cell structure
An electron micrograph shows an unidentified cell with: an elongated shape, mitochondria concentrated near a long whip-like projection, a membrane-capped structure at the head end, and very little cytoplasm. 4 marks
4.1 Identify the cell and justify your identification by explaining how two structural features are linked to the cell’s function. 3 marks
4.2 This cell type develops from a stem cell that originally contained all genes in the organism’s genome. Explain how it is possible for this highly specialised cell to develop from a cell that had the potential to become any cell type. 1 mark
Q1.1, Liver cells vs skeletal muscle: ribosomes (2 marks)
Liver cells (hepatocytes) produce enormous quantities of protein, including plasma proteins, clotting factors, and enzymes involved in metabolism, for export to the bloodstream [1]. This requires a very high rate of protein synthesis, which depends on a large ribosome population. Skeletal muscle cells also produce protein (actin and myosin) but not at the same extraordinary rate as the liver [1].
Q1.2, Multiple nuclei in skeletal muscle (2 marks)
Skeletal muscle cells are extremely long (up to several centimetres) and contain many hundreds of sarcomeres requiring coordinated protein synthesis across their entire length [1]. Multiple nuclei allow protein synthesis instructions to be issued closer to where proteins are needed, as a single nucleus could not efficiently control gene expression across such a large cell volume [1].
Q1.3, Mature red blood cells: no organelles (2 marks)
Mature red blood cells have discarded their nucleus and all membrane-bound organelles during maturation [1]. This maximises the internal volume available for haemoglobin, the oxygen-carrying protein, so each cell can carry more O&sub2; per trip through the circulation. Red blood cells do not need to divide, synthesise proteins or generate ATP via aerobic respiration in the same way, so the organelles supporting those functions are no longer required [1].
Q1.4, Goblet cell organelle prediction (2 marks)
A goblet cell would have many ribosomes, extensive rough ER, and a prominent Golgi apparatus [1]. Goblet cells specialise in secreting mucus (made of mucin proteins); ribosomes and rough ER are needed to synthesise these proteins, and the Golgi apparatus packages them into mucin granules for secretion [1].
Q2.1, Describe the graph difference (2 marks)
The root hair cell has approximately 10 times the surface area of a standard epidermal cell (10 units vs 1 unit) [1]. The root hair extension accounts for this approximately 10-fold increase [1].
Q2.2, Advantage of larger surface area for root hair cell (2 marks)
Water moves into the root hair cell by osmosis across the plasma membrane, from an area of higher water potential (soil solution) to lower water potential (cell contents) [1]. A larger surface area increases the area of membrane across which osmosis can occur simultaneously, so more water and dissolved minerals can be absorbed per unit time, improving the rate of uptake [1].
Q2.3, Why surface area increase would not benefit a red blood cell (2 marks)
A red blood cell’s primary function is transporting oxygen, not absorbing substances from its environment [1]. Its biconcave disc shape already provides an increased SA:V ratio suited to rapid O&sub2; and CO&sub2; diffusion. Adding extensions like root hairs would increase cell mass and reduce the flexibility of the membrane needed to squeeze through narrow capillaries, impairing rather than improving the cell’s function [1].
Q3, Mystery cell identifications
Cell A: Goblet cell. Features: (1) Enormous ribosomes + rough ER, required for synthesis of large quantities of mucin protein; (2) prominent Golgi apparatus, packages mucin into secretory granules for release by exocytosis. The cup shape and granules confirm this is a secretory cell, consistent with goblet cell morphology.
Cell B: Mature red blood cell (erythrocyte). Features: (1) No nucleus and no mitochondria, these were lost during maturation, freeing space for haemoglobin and eliminating anaerobic competition for O&sub2;; (2) Biconcave disc shape, increases SA:V ratio for faster gas diffusion across the membrane. The densely packed iron-containing protein is haemoglobin.
Cell C: Neuron. Features: (1) Long myelinated axon, transmits electrical signals rapidly over large distances via saltatory conduction; (2) Synaptic vesicles at the terminal, store neurotransmitters for release at synapses to communicate with the next cell in the pathway.
Cell D: Palisade mesophyll cell. Features: (1) 40+ chloroplasts per cell, maximises light capture for photosynthesis; (2) Columnar shape positioned near the leaf surface, ensures maximum light exposure before it is absorbed by lower layers, further boosting photosynthesis rate.
Q4.1, Sperm cell identification (3 marks)
This cell is a sperm cell (spermatozoon) [1]. The whip-like projection is a flagellum; its function is to propel the cell toward the egg via wave-like movements [1]. Mitochondria concentrated near the flagellum supply ATP directly where it is consumed by the flagellum’s motor proteins, enabling continuous movement [1]. (Accept also: acrosome at the head contains enzymes to penetrate the egg’s outer membrane; minimal cytoplasm reduces mass and increases motility.)
Q4.2, Differentiation from a totipotent stem cell (1 mark)
Cell differentiation occurs through selective gene expression, not loss of genes. The sperm cell’s stem cell ancestor contained the full genome, but as chemical signals activated specific transcription factors during development, certain genes were switched on (producing sperm-specific proteins like flagellar proteins) while others were permanently silenced. The cell’s final structure reflects only the subset of genes expressed, not the loss of other genes [1].