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Photorespiration and C4 Plants

All plants carry on photosynthesis by
Link to page describing the Calvin cycle.

Photorespiration

As its name suggests, RUBISCO catalyzes two different reactions: Which one predominates depends on the relative concentrations of O2 and CO2 with The light reactions of photosynthesis liberate oxygen and more oxygen dissolves in the cytosol of the cell at higher temperatures. Therefore, favor the second reaction.

The details of photorespiration

So this process uses O2 and liberates CO2 as cellular respiration does which is why it is called photorespiration.

It undoes the good anabolic work of photosynthesis, reducing the net productivity of the plant.

For this reason, much effort — so far largely unsuccessful — has gone into attempts to alter crop plants so that they carry on less photorespiration.

The problem may solve itself. If atmospheric CO2 concentrations continue to rise, perhaps this will enhance the net productivity of the world's crops by reducing losses to photorespiration.

Link to discussion of the earth's carbon cycle.

C4 Plants

Over 8000 species of angiosperms have developed adaptations which minimize the losses to photorespiration.

They all use a supplementary method of CO2 uptake which forms a 4-carbon molecule instead of the two 3-carbon molecules of the Calvin cycle. Hence these plants are called C4 plants. (Plants that have only the Calvin cycle are thus C3 plants.)

The details of the C4 cycle

These C4 plants are well adapted to (and likely to be found in) habitats with Some examples:

Although only ~3% of the angiosperms, C4 plants are responsible for ~25% of all the photosynthesis on land.

C4 cells in C3 plants

The ability to use the C4 pathway has evolved repeatedly in different families of angiosperms — a remarkable example of convergent evolution. Perhaps the potential is in all angiosperms.

A report in the 24 January 2002 issue of Nature (by Julian M. Hibbard and W. Paul Quick) describes the discovery that tobacco, a C3 plant, has cells capable of fixing carbon dioxide by the C4 path. These cells are clustered around the veins (containing xylem and phloem) of the stems and also in the petioles of the leaves. In this location, they are far removed from the stomata that could provide atmospheric CO2. Instead, they get their CO2 and/or the 4-carbon malic acid in the sap that has been brought up in the xylem from the roots.

If this turns out to be true of many C3 plants, it would explain why it has been so easy for C4 plants to evolve from C3 ancestors.

CAM Plants

These are also C4 plants but instead of segregating the C4 and C3 pathways in different parts of the leaf, they separate them in time instead. (CAM stands for crassulacean acid metabolism because it was first studied in members of the plant family Crassulaceae.)

At night,

In the morning, These adaptations also enable their owners to thrive in conditions of Some examples of CAM plants:

C4 Diatoms

On 26 October 2000, Nature reported the discovery of both the C3 and C4 pathways in a marine diatom. In this unicellular organism, the two paths are kept separate by having the C4 path in the cytosol, and the C3 path confined to the chloroplast. The presence of a C4 pathway probably reflects the frequent low concentrations of CO2 in ocean waters.
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29 December 2010