Short Diffusion Distances and Step-wise Oxidation
Plants have no lungs, no gills and no circulatory pump, yet even a massive tree must supply oxygen to every living cell and remove carbon dioxide. This quiz explores the elegant geometric solution that makes diffusion sufficient, and the chemical strategy that lets a cell capture energy instead of losing it as heat.
What you will practise
- Why the diffusion path stays short even in bulky plants
- How living cells are arranged in thin layers beneath the bark, with dead supporting cells inside
- How loosely packed parenchyma creates an interconnected network of air spaces
- Why complete combustion of glucose releases most energy as heat
- How step-wise oxidation couples energy release to ATP synthesis
Work carefully through each question — the reasoning behind the answer matters as much as the answer itself.
Q1. In even the largest and bulkiest plants, the distance that gases must diffuse is best described as:
The lesson states directly that the distance gases must diffuse even in large, bulky plants is not great, because each living cell is located quite close to the surface of the plant.
Q2. In thick woody stems, where are the living cells organised?
In stems the living cells are organised in thin layers inside and beneath the bark, and they are served by openings called lenticels.
Q3. What is the role of the cells in the interior of a thick stem?
The lesson explains that the cells in the interior are dead and provide only mechanical support, so life is confined to thin, well-ventilated layers.
Q4. The openings in the bark that serve the living cells of a stem are called:
The lesson names these openings lenticels, which serve the thin layers of living cells inside and beneath the bark.
Q5. The loose packing of parenchyma cells in leaves, stems and roots is important because it:
Loose packing of parenchyma cells provides an interconnected network of air spaces, which facilitates contact of cell surfaces with air.
Q6. Which statement best summarises the contact between plant cells and air?
The lesson concludes that most cells of a plant have at least a part of their surface in contact with air, helped by lenticels and the interconnected air spaces.
Q7. The complete combustion of glucose produces carbon dioxide and water, and yields energy that is:
The lesson states that complete combustion of glucose yields energy, most of which is given out as heat — useful in a furnace but not directly useful to the cell.
Q8. Which equation represents the complete combustion of glucose as given in the lesson?
The lesson gives ##\ce{C6H12O6 + 6O2 -> 6CO2 + 6H2O + Energy}## as the complete combustion reaction of glucose.
Q9. Why is the heat released by complete combustion of glucose not directly useful to a cell?
Most of the energy of combustion escapes as heat; for the energy to be useful the cell must utilise it to synthesise molecules it requires, which is achieved through ATP synthesis.
Q10. The plant cell's strategy for making glucose energy useful is to:
The lesson states the strategy is to catabolise the glucose molecule in such a way that not all the liberated energy goes out as heat.
Q11. The key feature of the cell's oxidation of glucose is that it occurs:
The key is to oxidise glucose not in one step but in several small steps, enabling some steps to be just large enough for the released energy to be coupled to ATP synthesis.
Q12. During respiration, which substances are released as products?
The lesson states that during respiration oxygen is utilised, and carbon dioxide, water and energy are released as products.
Q13. The breakdown of glucose to pyruvic acid without the help of oxygen is called:
The lesson states that all living organisms retain the enzymatic machinery to partially oxidise glucose without oxygen, and this breakdown of glucose to pyruvic acid is called glycolysis.
Q14. Which statement about glycolysis is supported by the lesson?
Glycolysis occurs in the cytoplasm of the cell and is present in all living organisms; in anaerobic organisms it is the only process in respiration.
Q15. In plants, the glucose used in glycolysis is derived from:
The lesson states that in plants this glucose is derived from sucrose, which is the end product of photosynthesis, or from storage.