Unicellular, Colonial and Multicellular Organisms

Q1.1, Graph

Correct graph should show: x-axis labelled "Cell width (µm)" with values 1, 2, 5, 10, 20, 50; y-axis labelled "SA:V ratio" ranging from 0 to at least 6; points plotted at (1, 6.00), (2, 3.00), (5, 1.20), (10, 0.60), (20, 0.30), (50, 0.12) connected with a smooth curve. Mark allocation: 1 for correct axes and labels; 1 for correctly plotted points; 1 for correct curve shape (decreasing, non-linear).

Q1.2, Trend description

As cell width increases, the SA:V ratio decreases. The relationship is non-linear, the ratio falls steeply at first (from 6.00 to 0.60 over the first tenfold increase in width) and then more gradually.

Q1.3, SA:V and cell size limit (2 marks)

All exchange of nutrients, gases and waste occurs across the cell membrane (the surface). As a cell grows, its volume increases much faster than its surface area (volume scales with width cubed; surface area with width squared), so the SA:V ratio falls [1]. A very large cell cannot exchange materials fast enough across its membrane to supply or remove substances from its interior by diffusion, the centre becomes deprived of oxygen and nutrients and waste accumulates, limiting the maximum viable size [1].

Q1.4, How multicellularity overcomes SA:V (2 marks)

Multicellular organisms keep individual cells small, so each cell maintains a high SA:V ratio for efficient exchange [1]. Specialised internal transport systems (the circulatory system in animals, vascular tissue in plants) then distribute materials to every cell in the body, allowing the whole organism to be large without any individual cell becoming diffusion-limited [1].

Q2.1, Bar graph pattern (1 mark)

Cell survival decreases across the three categories: unicellular Amoeba cells show the highest survival (~98%), separated colonial Volvox cells show intermediate survival (~72%), and specialised human muscle cells show very low survival (~4%).

Q2.2, Why Amoeba survived but muscle cells did not (3 marks)

Amoeba is a unicellular organism, a single cell that performs all life functions independently (obtaining nutrients, gas exchange, osmoregulation, reproduction) [1]. It has no structural dependence on any other cell, so isolation in nutrient broth causes no deficit. In contrast, a human muscle cell is a permanently specialised cell within a multicellular organism [1]. Its structure is optimised for contraction and it relies on other cells and systems (the cardiovascular system for oxygen and glucose, the nervous system for signals) to supply essential materials. When isolated, these supply chains are severed and the specialised cell cannot survive alone, this is interdependence [1].

Q2.3, Volvox intermediate result (2 marks)

Volvox is a colonial organism: its cells show limited division of labour but each cell retains the ability to survive independently if separated [1]. Unlike multicellular cells, colonial cells are not permanently specialised, the separation does not irreversibly sever essential supply chains, so many cells survive. However, the ~72% (not ~100%) survival suggests some cells were partially dependent on colonial connections (e.g. coordinated flagellar beating, shared resources via cytoplasmic bridges) and struggled when isolated [1].

Q3, Diagram critique (6 marks)

3.1 Common error, "Colonial cells cannot survive alone": This is the defining feature of multicellular cells, not colonial cells. Correction: the diagram should show that colonial cells (e.g. Volvox somatic cells) CAN survive if separated from the colony, this is what distinguishes colonial from multicellular organisation. [1 + 1]

3.2 Common error, "All cells in a multicellular organism have different DNA": This is biologically wrong. Correction: nearly all cells in a multicellular organism share identical DNA; cell differences arise from selective gene expression, not from different DNA sequences. [1 + 1]

3.3 Common error, Showing Volvox as a type of multicellular organism: Colonial and multicellular organisms are fundamentally different categories. Correction: Volvox is a colonial organism; it should be shown as a separate, intermediate category between unicellular and multicellular, not as a subcategory of multicellular. [1 + 1]

Q4.1, Why coral polyps are not colonial (2 marks)

Coral polyps are multicellular organisms, not colonial ones [1]. Each polyp contains permanently specialised, interdependent cells (cnidocytes, gastrodermal cells, epidermal cells) that cannot survive independently, this is the defining feature of multicellularity. A colonial organism is made of genetically identical cells where each individual cell can survive alone; the cells within a single coral polyp cannot do this [1].

Q4.2, Symbiodinium category (2 marks)

Symbiodinium are unicellular organisms [1]. A unicellular organism is a single cell that performs all life functions independently. The fact that individual Symbiodinium cells can survive in the water column for several days confirms they are self-sufficient and fully independent, which matches the definition of unicellular [1].

Q4.3, Cnidocyte death and interdependence (2 marks)

Cnidocytes are permanently specialised cells, their structure is optimised for stinging and capturing prey, not for independent survival [1]. They depend on other cells and systems within the polyp (e.g. the gastrodermal cells to supply digested nutrients, the circulatory system for oxygen and waste removal). This mutual reliance is interdependence. When expelled, the supply chains are broken and the cnidocyte dies rapidly because it cannot independently perform all the life functions needed to sustain itself [1].