Photosynthesis

The big picture

Photosynthesis uses light energy to build sugar from CO₂ and water. The summary equation is often written 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. A more honest version is 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O. It has water on both sides, which makes it clear that all 12 oxygen atoms in the 6 O₂ come from water, while the oxygen atoms of CO₂ end up in the sugar and in new water.

In plants and algae it all happens in the chloroplast. Inside is a system of flattened membrane sacs called thylakoids, often stacked into piles called grana. The space inside each thylakoid is the thylakoid lumen. The fluid around the thylakoids is the stroma.

• The light-dependent reactions take place in the thylakoid membranes. They capture light energy and store it in ATP and NADPH, splitting water and releasing O₂ along the way.
• The Calvin cycle takes place in the stroma. It uses that ATP and NADPH to turn CO₂ into sugar.

Catching light

Light comes in packets of energy called photons, and molecules called pigments absorb certain wavelengths of them. The main pigment, chlorophyll a, absorbs mostly blue-violet and red light. Chlorophyll b and orange carotenoids absorb some of the wavelengths it misses. Green light is mostly reflected or passed through, which is why leaves look green.

The pigments are organised into photosystems in the thylakoid membrane. Each photosystem has a large antenna of pigment molecules that pass the absorbed energy inward, like a satellite dish, to a special pair of chlorophyll a molecules at its reaction centre. There the energy is enough to knock an electron loose and hand it to a nearby acceptor. That's the moment light energy becomes chemical energy.

Following the electrons from water to NADPH

There are two photosystems, working one after the other. Confusingly, photosystem II (PSII) comes first; it was just discovered second. Its reaction centre is called P680 and that of photosystem I (PSI) is P700, after the wavelengths (in nanometres) they absorb best.

1. 1Light excites P680 in PSII, and it loses an electron to an acceptor. P680 is left short of an electron and becomes an extremely strong oxidiser.
2. 2PSII refills P680 by pulling electrons out of water. Splitting two water molecules gives 4 electrons, 4 H⁺ and one O₂: 2H₂O → 4H⁺ + 4e⁻ + O₂. The O₂ is released as a by-product, and the H⁺ are left in the thylakoid lumen.
3. 3The excited electrons pass to a mobile carrier, plastoquinone, and then to the cytochrome complex. As electrons pass through it, the cytochrome complex pumps H⁺ from the stroma into the lumen.
4. 4A small copper protein, plastocyanin, carries the electrons on to PSI.
5. 5Light excites P700 in PSI, lifting the electron to a much higher energy again. It passes to ferredoxin.
6. 6The enzyme NADP⁺ reductase uses two electrons and an H⁺ from the stroma to turn NADP⁺ into NADPH, which is released into the stroma.

If you draw the energy of the electron at each step, it rises at PSII, falls along the chain, rises again at PSI, then falls a little to NADP⁺. The shape looks like a sideways letter Z, so it's called the Z-scheme. Two light-driven boosts are needed because one photosystem on its own can't span the whole gap: it can't both pull electrons off water and push them high enough to reduce NADP⁺.

Making ATP: the same trick as mitochondria

Three things build up the H⁺ gradient across the thylakoid membrane: the H⁺ released by splitting water, the H⁺ pumped by the cytochrome complex, and the removal of H⁺ from the stroma when NADPH is made. The thylakoid membrane won't let H⁺ leak back, so a steep gradient builds.

The only easy way back into the stroma is through ATP synthase. As H⁺ flows through, the enzyme makes ATP from ADP and Pᵢ, and the ATP is released into the stroma, right where the Calvin cycle needs it. Making ATP with light energy is called photophosphorylation, and the mechanism is chemiosmosis, exactly as in mitochondria.

Chloroplasts can also run cyclic electron flow. Electrons from PSI go back to the cytochrome complex instead of to NADP⁺, so more H⁺ is pumped and more ATP is made, but no NADPH and no O₂. This lets the chloroplast top up its ATP, because the Calvin cycle needs more ATP than NADPH.

The Calvin cycle: building sugar

The Calvin cycle (or Calvin–Benson cycle) is named after Melvin Calvin, who worked it out with Andrew Benson and James Bassham by tracking radioactive carbon-14 through photosynthesising cells, and won the 1961 Nobel Prize in Chemistry. It turns CO₂ into sugar in the stroma, in three stages. It's easiest to follow three turns, which fix three CO₂:

1. 1Fixation. The enzyme rubisco attaches each CO₂ to a five-carbon sugar, RuBP (ribulose bisphosphate). The six-carbon product splits at once into two three-carbon molecules of 3-PGA. Three CO₂ + three RuBP give six 3-PGA. Turning CO₂ into organic molecules like this is called carbon fixation.
2. 2Reduction. Six ATP and six NADPH turn the six 3-PGA into six G3P (glyceraldehyde 3-phosphate), a three-carbon sugar. This is where the energy of the light reactions is put into the sugar.
3. 3Export. One of the six G3P leaves the cycle. The plant uses it to make glucose, sucrose, starch, cellulose, amino acids and more.
4. 4Regeneration. The other five G3P (15 carbons) are rearranged, using three more ATP, into three RuBP (15 carbons), ready to fix more CO₂.

So fixing three CO₂ costs 9 ATP and 6 NADPH, or 3 ATP and 2 NADPH per CO₂. The ADP, Pᵢ and NADP⁺ go back to the thylakoids to be recharged.

Two stages, one system

The two stages depend on each other, which makes them good for prediction questions.

• Turn off the light. ATP and NADPH run out, so 3-PGA can no longer be reduced to G3P and RuBP can't be regenerated. 3-PGA piles up and RuBP falls. Sugar production stops.
• Remove the CO₂. Rubisco has nothing to fix, so RuBP piles up and 3-PGA falls. The Calvin cycle stops using ATP and NADPH, so NADP⁺ and ADP run out. With nowhere to put electrons, the light reactions slow down too, even though the light is still on.

Plants respire too, and rubisco's weakness

Plants don't photosynthesise instead of respiring. Their cells have mitochondria and respire all the time, day and night, to make ATP for everything outside the chloroplast. In bright light, photosynthesis usually runs much faster than respiration, so the leaf releases O₂ overall. In the dark, it only takes O₂ in.

Rubisco has a flaw: it sometimes grabs O₂ instead of CO₂. This wasteful side reaction, photorespiration, uses energy and releases fixed carbon as CO₂. In ordinary (C₃) plants roughly a quarter of rubisco's reactions can be with O₂, and the problem gets worse in hot, dry weather, when leaves close their pores (stomata) to save water and CO₂ inside runs low.

• C₄ plants such as maize fix CO₂ first into a four-carbon molecule in outer leaf cells, then release it in inner bundle-sheath cells around rubisco. They separate the steps in space and keep CO₂ high around rubisco.
• CAM plants such as cacti open their stomata at night, store CO₂ as a four-carbon acid, and release it to the Calvin cycle during the day with stomata closed. They separate the steps in time.

Key terms

Thylakoid / lumen / stroma

Membrane sacs where the light reactions happen / the space inside them / the fluid around them, where the Calvin cycle happens.

Photosystem

A complex of pigments and proteins that captures light and uses it to energise an electron at its reaction centre.

Photosystem II / I

PSII (P680) splits water and starts the chain; PSI (P700) re-energises electrons and passes them toward NADP⁺.

Photolysis of water

Splitting water at PSII: 2H₂O → 4H⁺ + 4e⁻ + O₂, the source of the O₂ released.

Photophosphorylation

Making ATP using the H⁺ gradient built by light-driven electron transport.

Calvin cycle

The cycle in the stroma that fixes CO₂ and uses ATP and NADPH to make G3P.

Rubisco

The enzyme that attaches CO₂ to RuBP; it can also react with O₂ (photorespiration).

Carbon fixation

Incorporating CO₂ from the air into an organic molecule.

Check yourself

Try answering in your head before you open each answer.

• 1.A plant is given CO₂ labelled with ¹⁸O, while its water is ordinary. Where will the ¹⁸O end up?Show answerHide

  In the sugar (and in the water made by the Calvin cycle), not in the O₂. All the O₂ released comes from water split at PSII, so only a label in the water would appear in the O₂.

• 2.Some herbicides block electrons from leaving photosystem II. Predict the effects on O₂ release, NADPH and the Calvin cycle.Show answerHide

  O₂ release stops, because PSII can't keep taking electrons from water if its own electrons can't move on. No electrons reach PSI and NADP⁺, so NADPH production stops, and without linear electron flow far less H⁺ is pumped, so ATP falls too. The Calvin cycle then stalls for lack of ATP and NADPH, and the plant can't make sugar.

• 3.A leaf releases O₂ in bright light but takes in O₂ in the dark. Does that mean it respires only in the dark?Show answerHide

  No. It respires all the time. In bright light, photosynthesis makes O₂ faster than respiration uses it, so there's a net release. In the dark photosynthesis stops, and respiration's O₂ uptake is all that's left.