Photorespiration and C4 Plants

• adding carbon dioxide (CO₂) to a phosphorylated 5-carbon sugar called ribulose bisphosphate.
• This reaction is catalyzed by the enzyme ribulose bisphosphate carboxylase oxygenase (RUBISCO).
• The resulting 6-carbon compound breaks down into two molecules of 3-phosphoglyceric acid (PGA).
• These 3-carbon molecules serve as the starting material for the synthesis of glucose and other food molecules.
• The process is called the Calvin cycle and the pathway is called C3 photosynthesis.

Photorespiration

• adding CO₂ to ribulose bisphosphate — the carboxylase activity
• adding O₂ to ribulose bisphosphate — the oxygenase activity.

• high CO₂, low O₂ favoring the carboxylase action,
• high O₂, low CO₂ favoring the oxygenase action.

• high light intensities and
• high temperatures (above ~ 30°C)

The details of photorespiration

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

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

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.

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.)

• Some C4 plants — called CAM plants — separate their C3 and C4 cycles by time. CAM plants are discussed below.
• Other C4 plants have structural changes in their leaf anatomy so that
  • their C4 and C3 pathways are separated in different parts of the leaf with
  • RUBISCO sequestered where the CO₂ level is high; the O₂ level low.
  These adaptations are described now.

The details of the C4 pathway

• 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).
• Instead the CO₂ is inserted into a 3-carbon compound called phosphoenolpyruvic acid (PEP) forming
• the 4-carbon compound oxaloacetic acid.
• 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 (3 carbons), which is transported back to a mesophyll cell where it is converted back into PEP.

• high daytime temperatures
• intense sunlight.

• 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

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.

• 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.

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

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

C4 Diatoms