C3 and C4 Plants.

C3 plants

C3 plants are plants in which, after fixing CO₂, the first product has three carbons.

Most C3 plants are found in temperate and cold regions, so they don't need any modifications since the environment supports them.

The enzyme for fixing CO₂ is Rubisco. Under high light intensity and high concentration of O₂, C3 plants can fix O₂ instead of CO₂. This condition is called photorespiration. This reveals that C3 plants are not efficient for photosynthesis.

Photorespiration is a wasteful oxidation process since, in the normal Calvin cycle, oxygen is used instead of CO₂, forming nothing (no food formed).

When O₂ is fixed instead of CO₂, the enzyme is ribulose biphosphate oxygenase. This shows that Rubisco has a higher affinity for O₂ than for CO₂.

Examples: potato, tobacco, beans, wheat, etc. (have only one fixing enzyme)

C4 plants

Scientists Krantz and Hatch-Slack discovered C4 plants, and so the name Krantz and Hatch-Slack pathway.

C4 plants are plants in which the first compound formed after fixing CO₂ has four carbon atoms (oxaloacetate). E.g. maize, sorghum, etc.

There is a distinct arrangement of chloroplasts in mesophyll cells and bundle sheath cells; each one has its own chloroplast. The mesophyll cells have grains but few starch grains compared to the bundle sheath.

This arrangement of chloroplasts is called kranz anatomy.

The role of this is to fix CO₂ twice as much as C3 plants.

This is an adaptation since C4 plants live in drought areas (with no water).

There is rapid opening and closing of stomata to conserve water.

(C4 pathway has two fixing enzymes: i.e. PEP (phosphoenol pyruvate) in mesophyll cells and Rubisco in bundle sheath cells (normal Calvin cycle).

PEP has a high affinity for carbon dioxide since it can fix it at low concentrations and high temperatures. This is an adaptation.

NB: Carbon dioxide fixation and Calvin cycle are separated in space.

• The role of this is to conserve water. C4 plants have PEP which can fix carbon dioxide 120 times faster than C3 plants.
• Another adaptation is that it can fix carbon dioxide even when stomata are closed. This occurs in all mesophyll cells.
• The compound oxaloacetate acts as a compound for fixing CO₂.
• Uses a lot of water.
• PEP can work above 25°C, while Rubisco is affected by high temperature.

Significance of C4 plants

1. They have a maximum rate of CO₂ fixation at high light intensity and high temperatures. C4 plants increase in dry areas more rapidly than C3 plants.
2. They are more tolerant to dry conditions in order to reduce water loss. C4 plants can adapt to drought conditions.

	C3 plants	C4 plants
Examples	Most crop plants, e.g. cereals, tobacco, beans	Maize and sugar cane
Light intensity	10,000 – 30,000 foot candles	Not saturated at 10⁵
CO₂ fixation	Occurs once	Occurs twice: 1st in mesophyll, 2nd in bundle sheath cells
Effects of temperature rise from 25°C – 35°C	No change in rate or a low rate	50% greater at 35°C
CO₂ acceptor	Mesophyll cell – RuBP, a 5-carbon compound	Mesophyll cell – PEP, a 3-carbon compound; bundle sheath cell – RuBP
CO₂ fixation enzyme	Only one type of chloroplast, no bundle sheath	Two types: kranz anatomy (mesophyll cells and bundle sheath cells, each with its own chloroplast)
Leaf anatomy	RuBP carboxylase, which is very efficient	Mesophyll cell – PEP carboxylase, which is efficient
Amount of energy used	Low energy	A lot of energy
Efficiency	Less efficient in photosynthesis	More efficient in photosynthesis