Photosynthesis

Summary

What photosynthesis does

Photosynthesis uses light energy to convert water and carbon dioxide into sugars and oxygen. Sugars provide plants with energy and building materials, and plants can also break them down through cellular respiration. Photosynthesis occurs in plants, algae, and some bacteria.

Where it happens

In eukaryotic cells, photosynthesis takes place in chloroplasts:

• Thylakoid membranes are where the light-dependent reactions occur. Stacks of thylakoids are called grana.
• Stroma, the fluid surrounding the thylakoids, is where the Calvin cycle occurs.

Plants contain several pigments, including chlorophyll a and b, carotenoids, and xanthophylls. These pigments work together to absorb light. Chlorophyll absorbs mainly blue and red wavelengths and reflects much of the green light, which is why plants look green.

Two stages of photosynthesis

1. Light-dependent reactions

   • Take place in the thylakoid membrane.
   • Light energizes electrons in photosystems. Photosystem II was named before Photosystem I because it was discovered later.
   • Water is split, releasing oxygen and supplying electrons and hydrogen ions.
   • As electrons move through an electron transport chain, hydrogen ions are pumped into the thylakoid’s interior.
   • The movement of hydrogen ions back through ATP synthase produces ATP. These reactions also produce NADPH.
   • ATP and NADPH carry energy and reducing power to the Calvin cycle.

2. Calvin cycle

   • Takes place in the stroma.
   • Carbon dioxide enters the cycle and is attached to the five-carbon molecule RuBP by the enzyme Rubisco.
   • The resulting compounds are processed using ATP and NADPH to produce G3P, a three-carbon sugar.
   • Some G3P leaves the cycle and can be used to make sugars such as glucose. Much of it is used to regenerate RuBP so the cycle can continue.
   • The cycle depends on carbon dioxide, ATP, and NADPH.

The Calvin cycle is sometimes incorrectly called the “dark reaction,” but it is not restricted to nighttime.

Photorespiration and plant adaptations

When carbon dioxide is scarce, Rubisco can react with oxygen instead. This process, called photorespiration, diverts resources into producing a compound the plant must break down rather than making useful sugar. It is especially a problem for many C3 plants, whose initial sugar product has three carbon atoms.

Plants may close their stomata during hot conditions to reduce water loss, but doing so also limits the carbon dioxide entering the leaf. Two adaptations help some plants manage this trade-off:

• CAM plants, such as jade plants and pineapple, open their stomata at night, when it is cooler, and take in carbon dioxide. They store it as malic acid in cell vacuoles. During the day, they can keep their stomata closed and release the stored carbon dioxide for the Calvin cycle.
• C4 plants, such as corn, use enzymes to capture carbon dioxide in a four-carbon compound. This compound moves to bundle sheath cells inside the leaf, where carbon dioxide is released for the Calvin cycle.

Both strategies concentrate or store carbon dioxide so it can be used while stomata are closed. They require extra steps and energy, so they are advantageous mainly in hot environments.

Speakers and sources featured

• Paul Andersen (Mr. Anderson) — presenter and narrator.
• Melvin Calvin — the scientist after whom the Calvin cycle is named; he is not a speaker in the video.