C4 plants can bypass photorespiration

In plants such as roses, wheat, and rice, the mesophyll cells, which lie just below the surface of the leaf, are full of chloro-plasts that contain abundant rubisco (Figure 8.16a). On a hot day, these leaves close their stomata to conserve water. The level of CO2 in the air spaces of the leaves falls, and that of O2 continues to rise, as photosynthesis goes on. As we have

(a) Arrangement of cells in a C3 leaf m

Upper epidermis

(a) Arrangement of cells in a C3 leaf

Upper epidermis

And Leaf Anatomy Picture

Stoma

Spongy mesophyll cell Lower epidermis

Stoma

(b) Arrangement of cells in a C4 leaf

Spongy mesophyll cell Lower epidermis

(b) Arrangement of cells in a C4 leaf

Leaf Anatomy
8.16 Leaf Anatomy of C3and C4 Plants Carbon dioxide fixation occurs in different organelles and cells of the leaves in (a) C3 and (b) C4 plants.

'1 PEP carboxylase in C4

C4 mesophyll cells catalyzes the formation of the 4-carbon compound oxaloacetate.

'1 PEP carboxylase in C4

C4 mesophyll cells catalyzes the formation of the 4-carbon compound oxaloacetate.

Calvin Benson Cycle

Starch grains in the bundle sheath cell indicate that the Calvin-Benson cycle is active and that glucose (and then starch) is being produced.

Oxaloacetate diffuses through plasmo-desmata to a bundle sheath cell, where it is decarboxylated, releasing CO2.

Mesophyll cell

Starch grains in the bundle sheath cell indicate that the Calvin-Benson cycle is active and that glucose (and then starch) is being produced.

8.17 The Anatomy and Biochemistry of C4 Carbon Fixation

Carbon dioxide is fixed initially in the mesophyll cells, but enters the Calvin-Benson cycle in the bundle sheath cells. (b) There is an interconnected biochemical pathway for CO2 assimilation between the two cell types.

Cam Plants Co2 Fixation

seen, rubisco acts as an oxygenase, and photorespiration occurs, under these conditions. Because the first product of CO2 fixation in these plants is the three-carbon molecule 3PG, they are called C3 plants.

Corn, sugarcane, and other tropical grasses (Figure 8.16b) also close their stomata on a hot day, but their rate of photosynthesis does not fall, nor does photorespiration occur. They keep the ratio of CO2 to O2 around rubisco high so that rubisco continues to act as a carboxylase. They do this in part by making a four-carbon compound, oxaloacetate, as the first product of CO2 fixation, and so are called C4 plants.

C4 plants perform the normal Calvin-Benson cycle, but they have an additional early reaction that fixes CO2 without losing carbon to photorespiration, greatly increasing the overall photosynthetic yield. Because this initial CO2 fixation step can function even at low levels of CO2 and high temperatures, C4 plants very effectively optimize photosynthesis under conditions that inhibit it in C3 plants.

C4 plants have two separate enzymes for CO2 fixation, located in two different parts of the leaf (Figure 8.17). One enzyme, present in the cytosol of mesophyll cells near the surface of the leaf, fixes CO2 to a three-carbon acceptor compound, phosphoenolpyruvate (PEP), to produce the four-carbon fixation product, oxaloacetate. This enzyme, PEP car-boxylase, has two advantages over rubisco:

► It does not have oxygenase activity.

► It fixes CO2 even at very low CO2 levels.

8.1 Comparison of Photosynthesis

in C3 and C4 Plants

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Responses

  • kristian
    How does carbon dioxide enter the calvinbenson cucle?
    8 years ago
  • livia
    Does photorespiration occur in c4 plants?
    8 years ago
  • uriele
    How C4 plants bypass photorespiration?
    8 years ago
  • Bercilac Brandagamba
    How do plants overcome photorespiration?
    8 years ago
  • alvisa
    What do c4 plants do to prevent photorespiration?
    8 years ago
  • lewis
    How do plants circumvent photorespiration?
    8 years ago
  • Sesuna
    When do cam plants perform the calvin benson cycle?
    8 years ago
  • christine
    How does c4 photosynthesis circumvent photorespiration?
    8 years ago
  • melilot
    When photoresperation plants close stoma what builds up?
    8 years ago
  • FORTINBRAS
    Where are starch grains in c4 plants located?
    8 years ago
  • Amber
    Do photorespiration occur in mesophyll cells?
    8 years ago
  • alighiero
    When does CO2 enter the leaf in C4 plants?
    7 years ago
  • Genoveffa
    How plants C4 circunvent photorespiration?
    7 years ago
  • CHRISTINA
    Is there epidermis on the outside of a leaf for c4 plants?
    7 years ago
  • Emanuele
    How do cam plants and c4 plants bypass rubisco?
    7 years ago
  • Kent
    How do C4 plant bypass photorespiration?
    3 years ago

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