Understand Of Prof. Sabrie Ideas

The details of photorespiration

• The uptake of O₂ by RUBISCO forms:
• the 3-carbon molecule 3-phosphoglyceric acid — just as in the Calvin cycle
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• the 2-carbon molecule glycolate.
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• The glycolate    enters peroxisomes where it uses O₂ to form intermediates that
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• enter mitochondria where they are broken down to CO₂.

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.


Knowing this is important because it has someting to do with plant productivity

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.

The details of the C₄ cycle

• After entering through stomata, CO₂ diffuses into a mesophyll cell.
  • Being close to the leaf surface, these cells are exposed to high levels of O₂, but
  • have no RUBISCO so cannot start photorespiration (nor the dark reactions of the Calvin cycle).
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• Instead the CO₂ is inserted into a 3-carbon compound (C₃) called phosphoenolpyruvic acid (PEP) forming
• the 4-carbon compound oxaloacetic acid (C₄).
• Oxaloacetic acid is converted into malic acid or aspartic acid (both have 4 carbons), which is
• transported (by plasmodesmata) into a bundle sheath cell. Bundle sheath cells
  • are deep in the leaf so atmospheric oxygen cannot diffuse easily to them;
  • often have thylakoids with reduced photosystem II complexes (the one that produces O₂).
  • Both of these features keep oxygen levels low.
• Here the 4-carbon compound is broken down into
  • carbon dioxide, which enters the Calvin cycle to form sugars and starch.
  • pyruvic acid (C₃), which is transported back to a mesophyll cell where it is converted back into PEP.

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

• high daytime temperatures
• intense sunlight.

Some examples:

• crabgrass
• corn (maize)
• sugarcane
• sorghum

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,

• CAM plants take in CO₂ through their open stomata (they tend to have reduced numbers of them).
• The CO₂ joins with PEP to form the 4-carbon oxaloacetic acid.
• This is converted to 4-carbon malic acid that accumulates during the night in the central vacuole of the cells.

In the morning,

• the stomata close (thus conserving moisture as well as reducing the inward diffusion of oxygen).
• The accumulated malic acid leaves the vacuole and is broken down to release CO₂.
• The CO₂ is taken up into the Calvin (C₃) cycle.

These adaptations also enable their owners to thrive in conditions of

• high daytime temperatures
• intense sunlight
• low soil moisture.

Some examples of CAM plants:

• cacti
• Bryophyllum
• the pineapple and all epiphytic bromeliads
• sedums
• the "ice plant" that grows in sandy parts of the scrub forest biome

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.