BIOLOGY Volume 1 - A Guide to General Biology - 2004

7. AUTOTROPHIC NUTRITION

7.9. C4 Photosynthesis

7.9.5. Significance of the C4 Pathway

It is believed that the C4 pathway evolved after the C3 pathway, leading to an improved mechanism for carbon dioxide fixation. C4 plants increase their dry mass faster than C3 plants and are higher-yielding crops grown in certain PARTS OF THE world (see below).

C4 plants are predominantly found in arid subtropical and tropical regions. The adaptation of these plants to prevailing climatic conditions occurred through Two main mechanisms. First, their maximum rate of carbon dioxide fixation became higher; consequently, higher light intensity and Temperature could also be utilized more efficiently. Light saturation is achieved at higher light intensities than in C3 plants. In other words, The rate of Photosynthesis increases with light intensity to a higher level than in C3 plants. Second, C4 plants are more resistant to drought conditions. Typically, to reduce moisture loss through Transpiration, plants decrease their stomatal aperture, thereby reducing the surface area available for carbon dioxide entry. Carbon dioxide is fixed so rapidly in C4 plants that a steep carbon dioxide concentration gradient is established between the atmosphere and the internal environment, ensuring a higher growth rate than in C3 plants. Compared with C3 plants, C4 plants lose half as much Water per fixed carbon dioxide molecule. The optimum growth temperature for C4 plants is also higher than that for C3 plants.

However, in cooler and wetter temperate regions, where high light intensity lasts only a few hours a day, the additional energy (over 15%) required by C4 plants for carbon dioxide fixation is likely a limiting factor. Under such conditions, C3 plants may even have an advantage over C4 plants. In temperate climates, C3 crops such as wheat, potatoes, tobacco, sugar beet, and soybeans grow more efficiently than C4 crops such as maize, sugarcane, sorghum, and millet. The main differences between C3 and C4 plants are presented in Table 7.6.

Class="center">Table 7.6. Comparison of C3 and C4 Plants

C3 plants

C4 plants

Representative species

Most agricultural crops, e.g., cereals, tobacco, beans

Maize, sugarcane

Light intensity required to achieve maximum photosynthetic rate

10,000–30,000 FOOT-candles

Not saturated at 105 lux

Effect of temperature increase from 25 to 35 °C

Rate remains unchanged or decreases

Rate increases by 50% at 35 °C

CO2 concentration at which uptake ceases

40–60 ppm

About 0 ppm

Water loss per 1 g of dry matter produced

450–950

250–350

Carbon dioxide fixation

Occurs once

Occurs twice, initially in mesophyll Cells, then in bundle sheath cells

Carbon dioxide acceptor

RuBP — 5C compound

Mesophyll cells

PEP — 3C compound

Bundle sheath cells

RuBP

Carbon dioxide-fixing enzyme

RuBP carboxylase, which is inefficient in action

PEP carboxylase, which is highly efficient

RuBP carboxylase, functioning efficiently due to high carbon dioxide concentration

First product of photosynthesis

PGA (C3 acid)

C4 acid (oxaloacetic)

Leaf anatomy

METABOLISM/14.html">Chloroplasts of only one type present

Kranz anatomy, i.e., two distinct Cell types exist with chloroplasts characteristic of each type

Efficiency

Photosynthesis is less efficient than in C4 plants. Productivity is generally lower

Photosynthesis is more efficient than in C3 plants, but requires more energy. Productivity is generally significantly higher


Last update: 06/08/2026

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