Photosynthesis

Photosynthesis is two cycles

Photosynthesis happens in two places inside the chloroplast. In the thylakoid membrane, light energy splits water and makes ATP and NADPH. In the surrounding stroma, ATP and NADPH are used to make sugar from CO₂.

The reactions in the stroma form a cycle that comes back to where it started: the Calvin-Benson cycle. ATP and NADPH go round as well. Once used, they return to the thylakoid as ADP and NADP⁺ and are made again.

Light, water and CO₂ go in, and O₂ and sugar come out. Turn off the light or the CO₂ to see where each cycle stops.

The thylakoid uses light to make ATP and NADPH

A thylakoid is a flattened sac inside the chloroplast. Its membrane holds photosystem II, the cytochrome b6f complex, photosystem I and ATP synthase.

Press Next to follow the steps from light to ATP and NADPH.

Photosystem II absorbs light

When the chlorophyll of photosystem II absorbs light, an electron gains energy and leaves.

Water is split

Photosystem II takes an electron from water to replace the one it lost. Water is split into O₂ and H⁺. The O₂ released by photosynthesis comes from here.

Electrons are passed along

The electron moves through the membrane from photosystem II to the cytochrome b6f complex. As it passes, H⁺ is carried from the stroma into the thylakoid.

Photosystem I absorbs light again

The electron reaches photosystem I and gains energy from light once more. It is then handed to NADP⁺, which becomes NADPH.

ATP synthase makes ATP

H⁺ has built up inside the thylakoid. As it flows back out to the stroma through ATP synthase, the flow makes ATP from ADP.

ATP and NADPH are used in the stroma

ATP and NADPH are used in the Calvin-Benson cycle and come back to the thylakoid as ADP and NADP⁺.

The Calvin-Benson cycle makes sugar from CO₂

The cycle starts when RuBP (ribulose bisphosphate) takes up CO₂. It turns into PGA (phosphoglycerate), then GAP (glyceraldehyde phosphate), and finally back into RuBP.

Press Next to follow fixation, reduction and regeneration in turn.

Fixation: CO₂ is taken up

RuBP combines with CO₂ and becomes PGA. The enzyme that drives this reaction is RuBisCO.

Reduction: ATP and NADPH are used

PGA uses ATP and NADPH to become GAP. This is where the ATP and NADPH from the thylakoid are spent.

Sugar leaves the cycle

Part of the GAP leaves the cycle. Outside the chloroplast it becomes sugar (sucrose), and inside it is stored as starch.

Regeneration: back to RuBP

The rest of the GAP uses ATP to become RuBP again, so RuBP can take up CO₂ again and again.

More light speeds it up, until it levels off

In weak light the thylakoid makes only a little ATP and NADPH, so the Calvin-Benson cycle keeps stopping to wait for them.

Apart from photosynthesis, a plant respires in its mitochondria and gives off CO₂. In weak light, respiration gives off more CO₂ than photosynthesis takes up. The light intensity at which the two balance is the light compensation point.

As the light gets stronger, the enzymes of the cycle eventually cannot keep up, and more light no longer speeds up photosynthesis. That light intensity is the light saturation point.

Shade plants respire less, and their cycle runs more slowly, so they reach both points in weaker light than sun plants.

Whatever is in shortest supply sets the pace

The rate of photosynthesis depends on light intensity, CO₂ concentration and temperature. Whichever of these is in shortest supply sets the rate. It is called the limiting factor.

Without enough light, ATP and NADPH run short and the cycle stops at reduction and regeneration. Without enough CO₂, RuBP waits for CO₂ and the cycle stops at fixation. When it is too cold or too hot, the enzymes of the cycle slow down.

Raising the scarce factor speeds things up, until something else runs short.

Chlorophyll absorbs red and blue-violet light

Photosynthesis runs on the light absorbed by pigments such as chlorophyll. Chlorophyll absorbs red and blue-violet light well and green light poorly.

Green light that is not absorbed is reflected by the leaf or passes through it. That is why leaves look green.

Engelmann shone light split into a spectrum onto a filamentous alga and watched where oxygen-loving bacteria gathered. They crowded where the red and blue-violet light fell, because that is where photosynthesis was releasing oxygen.

C4 and CAM plants put a CO₂-gathering cycle in front

On hot, dry days a plant closes its stomata to save water. CO₂ inside the leaf then runs low, and RuBisCO starts binding O₂ instead of CO₂. This is photorespiration, and it loses carbon that had already been fixed.

C4 plants such as maize and sugarcane take up CO₂ as a C4 compound in mesophyll cells, carry it into bundle sheath cells and release the CO₂ there. CO₂ builds up around the Calvin-Benson cycle, so photorespiration hardly happens.

CAM plants such as cacti and stonecrops open their stomata in the cool of the night and store CO₂ in the vacuole as malate. By day they keep their stomata shut and release CO₂ from the stored malate into the Calvin-Benson cycle.

Experiments revealed how it works

The two cycles of photosynthesis were worked out by tracing labelled atoms and by changing one condition at a time.

Ruben: O₂ comes from water

Ruben and colleagues used a heavier oxygen atom (¹⁸O) as a label. When the oxygen of water is labelled, the label appears in the O₂ that is given off. When the oxygen of CO₂ is labelled, it does not appear in O₂ but ends up in the sugar.

So the O₂ released by photosynthesis is made from water, not from CO₂.

Hill: O₂ is released without CO₂

Hill took chloroplasts out of leaves, added a substance that accepts electrons (iron(III) oxalate) and shone light on them. O₂ was released even without CO₂.

So splitting water to release O₂ and taking up CO₂ are separate reactions.

Calvin: the order of the cycle

Calvin and colleagues gave green algae CO₂ containing radioactive carbon (¹⁴C) for a short time and traced where the label went. It appeared first in PGA, and later spread to GAP, sugar and RuBP.

When the light was turned off, PGA rose and RuBP fell. When the CO₂ was stopped, RuBP rose and PGA fell. This revealed the order of the cycle, in which RuBP takes up CO₂, becomes PGA and is eventually made again.