BIOL1030 Chap.7 Carbon Gain, Phloem and C4 and CAM Plants
Carbon Gain, Phloem and C4 and CAM Plants
Carbon Gain, Phloem and C4 and CAM Plants follows Lecture 5 of Unit 6, on how plants turn carbon dioxide into sugar, move that sugar and cope with the limits of photosynthesis. Mature leaves act as sources and growing tissues, fruits and seeds as sinks; sugars are loaded into the phloem with ATP and carried by pressure-driven bulk flow, as the pressure-flow hypothesis explains.
The chapter then shows why net photosynthesis is fixation minus photorespiratory and respiratory losses, why Rubisco's reaction with oxygen becomes costly in hot, dry conditions, and how C4 plants with Kranz anatomy and CAM plants concentrate carbon dioxide around Rubisco in space or in time.
What this chapter covers
- 01
Sources, sinks and changing source-sink relations
- 02
Xylem and phloem compared
- 03
Symplastic and apoplastic phloem loading
- 04
The pressure-flow hypothesis and grain filling in wheat
- 05
Net photosynthesis, Rubisco and photorespiration
- 06
Evolution of C4 photosynthesis and Kranz anatomy
- 07
C3, C4 and CAM compared
Worked example · free
Trace sugar from a leaf to a filling grain
- 1Sugar is loaded into sieve tubes at the source leaf, using ATP for transfer from the apoplast into companion cells and sieve tubes.
- 1Loading lowers the osmotic potential in the sieve tube, so water enters from the neighbouring xylem.
- 1The inflow raises pressure, and bulk flow carries sap along the phloem towards the lower-pressure sink, at up to 1 m per hour.
- 1At the grain the sugar is unloaded for storage and water leaves, keeping pressure low at the sink end.
Key terms
- Sink tissue
- A plant part that imports more sugar than it makes, such as a root, fruit, seed or growing shoot.
- Pressure-flow hypothesis
- The explanation that sugar loading at sources creates pressure that drives bulk flow of phloem sap to sinks.
- Photorespiration
- Uptake of oxygen by Rubisco that leads to the release of CO2 and lowers net carbon gain.
- Kranz anatomy
- The C4 leaf arrangement in which bundle-sheath cells around the veins hold concentrated CO2 for Rubisco.
Carbon Gain, Phloem and C4 and CAM Plants FAQ
What is the difference between a source and a sink?
A source exports more sugar than it uses, usually a mature leaf. A sink imports sugar, such as roots, stems, fruits, seeds and meristems. The roles change with development, so a young leaf begins as a sink and later becomes a source once it is fully expanded.
Why is photorespiration a problem for C3 plants?
Rubisco can bind oxygen instead of carbon dioxide, and when it does the plant loses carbon that it had already fixed. The loss grows with temperature and falls when carbon dioxide is high, so hot, dry conditions with stomata partly closed are the worst case.
How do C4 and CAM plants differ?
Both first fix carbon dioxide into a four-carbon acid and both raise carbon dioxide around Rubisco to cut photorespiration. C4 plants separate the two steps between cell types in space, while CAM plants separate them in time, capturing at night and fixing by day, which gives the highest water-use efficiency.
Why did C4 photosynthesis evolve?
Uptake by C3 plants lowered atmospheric carbon dioxide over millions of years, and low carbon dioxide combined with heat increased photorespiration. Selection then favoured a carbon-concentrating pathway, which arose about 35 million years ago and has evolved independently more than 45 times.
Where are C4 grasses common in Australia?
Their share of the regional grass flora rises with growing-season temperature, so C4 grasses dominate northern Australia. They do best in hot, bright and periodically dry places because concentrating CO2 around Rubisco suppresses photorespiration and lets stomata stay partly closed, giving them high water-use efficiency.
Exam move
Start by drawing the source-to-sink system once from memory, with every arrow labelled, then explain it aloud in six steps. Next, practise net photosynthesis arithmetic with your own numbers, halving the oxygen taken up to find the photorespiratory loss before subtracting.
Finish with a comparison table you build yourself for C3, C4 and CAM, then test it by naming the pathway of maize, wheat and pineapple and explaining which environment each suits and why. Add the xylem and phloem contrast as a final check, naming what each carries, which way it flows and what drives it.
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