Photosynthesis and Carbon Fixation

Photosynthesis converts light energy into chemical energy and uses it to fix atmospheric carbon dioxide into sugars, the process on which almost all life and the breathable atmosphere depend.

Definition

Photosynthesis is the light-driven synthesis of organic compounds from carbon dioxide and water, and carbon fixation is the incorporation of inorganic carbon dioxide into organic molecules, principally through the Calvin–Benson cycle.

Scope

This topic covers the light reactions of the thylakoid membrane (photosystems, electron transport, and ATP synthesis), the Calvin–Benson cycle of carbon fixation by Rubisco, photorespiration, and the C4 and CAM adaptations that concentrate carbon dioxide.

Core questions

• How do the light reactions convert light into ATP and NADPH while releasing oxygen?
• How does the Calvin–Benson cycle fix carbon dioxide into carbohydrate?
• Why have C4 and CAM mechanisms evolved to overcome the limitations of Rubisco?

Key theories

Z-scheme of photosynthetic electron transport

Light energizes electrons through photosystems II and I in series, splitting water to release oxygen and generating the NADPH and proton gradient that power ATP synthesis.

Carbon-concentrating mechanisms

Because Rubisco also reacts with oxygen, causing wasteful photorespiration, C4 and CAM plants spatially or temporally concentrate carbon dioxide around Rubisco to improve efficiency in hot or dry conditions.

Mechanisms

In the thylakoid membrane, photosystem II oxidizes water to oxygen and feeds electrons through the cytochrome b6f complex to photosystem I, which reduces NADP+ to NADPH; the linked proton gradient drives ATP synthase. In the stroma, Rubisco fixes carbon dioxide onto ribulose-1,5-bisphosphate, and the Calvin–Benson cycle reduces the product to triose phosphate using ATP and NADPH while regenerating the acceptor. C4 plants prefix carbon dioxide into four-carbon acids in mesophyll cells and release it around Rubisco in bundle-sheath cells, while CAM plants fix carbon dioxide at night, both suppressing photorespiration. Chlorophyll fluorescence provides a non-invasive probe of these reactions.

Clinical relevance

Photosynthetic efficiency sets the ceiling on crop productivity and biomass, making it a central target for improving food security; the process also governs how much carbon dioxide vegetation removes from the atmosphere, linking it to climate.

History

Hill showed isolated chloroplasts could evolve oxygen, Calvin and Benson mapped the carbon-fixation cycle with carbon-14, and Hatch and Slack described the C4 pathway in the 1960s, completing the modern picture of photosynthesis.

Key figures

• Melvin Calvin
• Andrew Benson
• Robert Hill
• Marshall Hatch

Seminal works

• buchanan2015
• taiz2015

Frequently asked questions

Where does the oxygen released by plants come from?

The oxygen comes from water, which photosystem II splits during the light reactions; the released oxygen is a byproduct, while the hydrogen and electrons are used to build NADPH.

Why are C4 plants more efficient in hot climates?

C4 plants concentrate carbon dioxide around Rubisco, suppressing the oxygen-fixing reaction (photorespiration) that becomes costly at high temperatures, so they photosynthesize more efficiently in hot, bright conditions.