PsynDark-2

Photosynthesis-Carbon Fixation-2

The C4 Cycle

Evapo/Transpiration is the principal way in which leaves cool themselves.

The latent heat of evaporation produces a significant cooling effect.

This occurs best when the stomata are open.

However, leaf water loss leads to stomatal closure.

Since water loss is greatest when leaf temperatures are the highest, the stomata are typically closed under these circumstances. This greatly limits evaporative cooling!

Closed stomata also prevent CO2 uptake from the atmosphere.

All of these factors favor Photorespiration &

Diminish Photosynthesis!

Some plants have Carboxylating enzymes that have a higher affinity for CO2, compared to RUBISCO, especially at low CO2 concentrations & high temperatures.

These plants use 3-Carbon acceptors like Phosphoenolpyruvate (PEP) rather than the 5-Carbon acceptor RUBP.

Carboxylation produces a 4-Carbon Acid like Malic Acid (Malate).

Consequently, this is called C4 photosynthesis.

C4 Plants have a distinctive Leaf Anatomy compared to C3 Plants.

C3 Leaf Anatomy
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Section through a Typical C3 Dicot Leaf: Note the Spongy - Palisade Organization. A non-photosynthetic Bundle Sheath surrounds the Vascular Bundle	Wheat (Triticum) is a C3 Monocot. It has a wide-sread, Uniform Photosynthetic Mesophyll. Its Bundle Sheath cells lack Chloroplasts.

C4 Leaf Anatomy

The Vascular Bundles of C4 leaves have large Photosynthetic Bundle Sheath Cells.

One to three layers of Photosynthetic Mesophyll cells that surround the Bundle Sheath and radiate away from the Bundle Sheath.

These resemble a Wreath and this has been called Kranz (wreath) Anatomy.

This is very distinctive and occurs in Monocots like Corn & Sugarcane plus some Dicots.

Typical C4 Leaves
Cross-section of a Corn (Zea mays) Leaf: This is a classic case of Kranz Anatomy!
The picture on the right is from Sugarcane (Saccharum) which has C4 Photosynthesis & Kranz Anatomy. Note the large Photosynthetic Bundle Sheath Cells and the densely stained, Photosynthetic Mesophyll that surround the Bundle Sheath.
The picture on the right is from a grass in the genus Poa. It also has Kranz Anatomy & C4 Photosynthesis Note the Chloroplasts in the Bundle Sheath Cells. The Stomata are precisely arranged to provide optimal diffusion paths for the Photosynthetic Mesophyll Cells. This allows for extremely efficient gas exchange with the atmosphere and the Mesophyll when the stomata are open.