BIO2030 Chap.3 Photosynthesis, Source and Sink, and Rising Carbon Dioxide
Photosynthesis, Source and Sink, and Rising Carbon Dioxide
Photosynthesis, Source and Sink, and Rising Carbon Dioxide follows one atom of carbon from the air to the part of the plant you eat. It maps a physical route rather than a vocabulary: carbon enters at a single enzyme, is loaded into a single transport tissue, and is unloaded wherever a sink can take it.
The chapter then uses that route to answer the week's real question, which is why a rising atmospheric carbon dioxide concentration raises the growth of some crops far more than others, and why the harvested organ often gains less than the leaf does. Source and sink are treated as roles a tissue can hold at different times rather than as fixed places, which is the distinction most answers get wrong.
What this chapter covers
- 01
The two reaction sets of photosynthesis and why neither runs without the other
- 02
The carbon fixing enzyme: the most plentiful protein on earth, and one of the slowest
- 03
The competing reaction that costs the plant carbon, and the two ways plants avoid it
- 04
Source and sink as roles rather than places, and the tissue that reverses role during development
- 05
The transport pathway: what is loaded, what actually travels, and how the pressure gradient works
- 06
Evidence for the pathway, including the isotope argument that identifies the source
- 07
The carbon dioxide fertilisation effect, and why the response is limited by sink capacity
- 08
Why a storage organ crop gains more from elevated carbon dioxide than a grain crop does
Predict which of two crops gains more from elevated carbon dioxide, and justify it
- +1State what elevated carbon dioxide changes: it raises the rate at which the leaf can fix carbon, so it acts on the source side only.
- +1State the constraint that decides the outcome: extra assimilate has to be deposited somewhere, so the response is bounded by the capacity of the sinks to accept it.
- +1Classify the two sinks. A tuber is a large storage organ that can keep accepting assimilate for as long as it is supplied. A grain head has a number of seeds largely determined earlier in development, so its capacity is fixed before the extra carbon arrives.
- +1Predict: the tuber crop gains proportionally more in the harvested organ, because its sink capacity is not the binding constraint.
- +1Add the qualification that separates a good answer from a correct one: the leaf may respond similarly in both, so a gain measured on total biomass can look larger than the gain in the part that is eaten.
- +1Give the falsifying result: if the grain crop matched the tuber crop in harvested organ gain while its seed number was fixed, the sink limitation account would be wrong for that crop.
Key terms
- Carbon fixing enzyme
- The enzyme that attaches atmospheric carbon dioxide to an acceptor molecule, beginning the reactions that build sugar. It is abundant because it is slow, so a plant compensates with quantity.
- Source tissue
- A tissue that exports more assimilate than it uses, typically a mature leaf. Source is a role held at a point in time, not a permanent property of an organ.
- Sink tissue
- A tissue that imports assimilate, such as a root, a developing seed or a storage organ. A young leaf is a sink before it becomes a source.
- Sink capacity
- The total amount of assimilate the importing tissues can accept and store. Where it is the binding constraint, extra photosynthesis does not become extra harvested yield.
- Carbon dioxide fertilisation
- The rise in photosynthetic rate and growth caused by a higher atmospheric carbon dioxide concentration. It acts on the supply side, so its effect on yield depends on what the sinks can take.
Photosynthesis, Source and Sink, and Rising Carbon Dioxide FAQ
Why is the main carbon fixing enzyme so abundant if it is inefficient?
Because abundance is the compensation for slowness. Each molecule of the enzyme processes very few reactions per second compared with most enzymes, so a leaf that needs a given rate of carbon fixation has to carry a very large number of copies.
That has a cost the unit returns to in the nutrition chapter: building all that protein consumes a large share of the plant's nitrogen, which links photosynthetic capacity directly to nitrogen supply and therefore to fertiliser.
What does the isotope evidence actually show?
It identifies which molecule the oxygen released by photosynthesis comes from. Because the two candidate inputs, water and carbon dioxide, both contain oxygen, the balanced equation alone cannot settle it. Labelling the oxygen in one input and then measuring where the label appears in the products does settle it, and the answer is that the released oxygen comes from water rather than from carbon dioxide.
The argument is worth learning as a method as much as a result, since it is a clean example of a tracer answering a question a mass balance cannot.
If source and sink are roles, can a tissue change role?
Yes, and that is the point of teaching them as roles. A young expanding leaf is a net importer, because it costs more to build than it produces, so it is a sink. Once expanded, it exports more than it consumes and becomes a source. A storage organ can also reverse: a tuber is a sink while it fills and can become a source when its reserves are mobilised for new growth.
Answers that treat leaf as a synonym for source lose the ability to explain either of these cases.
Assessment move
Draw the route once, from the air to the harvested organ, and keep redrawing it from memory until every station has a name and a reason. Most questions in this chapter are answered by pointing at a station on that route and saying what limits it there.
Practise the source and sink discrimination on cases the unit did not use, such as a young leaf and a mature leaf on the same plant, because the marked skill is applying the roles rather than reciting them.
For the carbon dioxide material, rehearse one sentence you can deploy immediately: elevated carbon dioxide raises supply, and yield responds only if the sinks can take the extra, so the answer depends on the harvested organ. Keep the isotope argument as a four step story rather than as a conclusion, since it is the one piece of experimental reasoning in the chapter and it is examined as reasoning.
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