6.3 Photosynthesis
Key Takeaways
Photosynthesis uses carbon dioxide, water, and light to make sugar and oxygen, and the released oxygen comes from water.
Chlorophyll a is the primary reaction-center pigment; chlorophyll b and carotenoids broaden absorption.
Noncyclic electron flow in the thylakoid membrane makes ATP, NADPH, and O2; cyclic flow uses photosystem I only and makes ATP without NADPH or O2.
The Calvin cycle in the stroma uses rubisco to attach CO2 to RuBP, then spends ATP and NADPH; three CO2 molecules yield one net G3P.
Photorespiration wastes carbon when rubisco binds O2; C4 plants separate initial fixation in space, and CAM plants separate it in time.
6.3 Photosynthesis
Photosynthesis stores light energy in sugar. In words, carbon dioxide plus water plus light yields sugar plus oxygen. A balanced summary is 6 CO2 + 6 H2O plus light energy produces C6H12O6 + 6 O2. The oxygen released comes from water, not from carbon dioxide. The carbons of the sugar come from CO2. Light reactions in the thylakoid membrane capture light and make ATP and NADPH. The Calvin cycle in the stroma uses that ATP and NADPH to fix carbon.
Pigments that collect light
Chlorophyll a is the primary reaction-center pigment. In each photosystem, a special pair of chlorophyll a molecules is what actually loses electrons. Chlorophyll b and carotenoids are accessory pigments. They broaden the range of wavelengths absorbed and pass that energy to chlorophyll a. Carotenoids also dissipate excess light energy that would otherwise damage the photosystems. A leaf looks green because chlorophyll reflects and transmits green light while absorbing strongly in the blue and the red. An item that asks which pigment occupies the reaction center is asking for chlorophyll a.
Light reactions in the thylakoid membrane
Noncyclic flow makes ATP, NADPH, and O2
Noncyclic electron flow, also called linear flow, uses both photosystems. Photosystem II absorbs light, and its reaction-center chlorophyll a passes an electron into the chain. The lost electron is replaced by splitting water. That split releases oxygen gas, protons, and electrons, so the oxygen atoms in O2 come from H2O.
Electrons then move through carriers, and the flow helps build a proton gradient. Protons accumulate in the thylakoid lumen and return to the stroma through ATP synthase, forming ATP. The idea matches mitochondrial chemiosmosis, on a different membrane: mitochondria pump protons out of the matrix, whereas chloroplasts pump protons into the thylakoid lumen. In both cases ATP forms on the side where the next pathway uses it, the mitochondrial matrix or the chloroplast stroma.
Photosystem I absorbs light too. Electrons leaving its reaction center reduce NADP+ to NADPH. Noncyclic flow therefore yields ATP, NADPH, and O2. ATP and NADPH are available in the stroma for the Calvin cycle. O2 is released, not fed into that cycle as a carbon source.
Cyclic flow makes ATP only
Cyclic electron flow uses photosystem I only. Excited electrons travel back through the chain instead of reducing NADP+. The loop still supports the proton gradient, so ATP is made. NADPH is not made. Photosystem II is not splitting water, so O2 is not released. Cells use this route when they need extra ATP relative to NADPH. Cyclic flow does not fix carbon.
The Calvin cycle in the stroma
The Calvin cycle does not split water, and it does not require darkness. It spends ATP and NADPH supplied by the light reactions, so it runs while those supplies last. The old nickname dark reactions meant only that these carbon steps do not absorb photons. It did not mean the cycle is confined to night. In prolonged darkness the cycle stops because ATP and NADPH run out.
Fixation, reduction, and regeneration
Carbon fixation attaches CO2 to an organic acceptor. The enzyme rubisco, short for ribulose bisphosphate carboxylase/oxygenase, attaches CO2 to RuBP (ribulose bisphosphate). The product splits into two molecules of 3-phosphoglycerate.
Reduction then spends ATP and NADPH from the light reactions. The carbohydrate that the accounting tracks is G3P (glyceraldehyde 3-phosphate). Regeneration rearranges most of that G3P, with more ATP, back into RuBP. The pathway is a cycle because the acceptor is rebuilt.
Three CO2, one net G3P
Three CO2 fixed onto three RuBP produce six molecules of 3-phosphoglycerate and then six G3P. Five of those six G3P regenerate three RuBP. One G3P is the net output. Six CO2, and therefore two net G3P, correspond to one glucose, because two three-carbon G3P molecules supply the six carbons of glucose. The direct net export of the cycle is G3P, not a finished glucose molecule released in one stroke. Introductory counts use 9 ATP and 6 NADPH per net G3P, which is 18 ATP and 12 NADPH for the six CO2 that correspond to one glucose. Those ATP and NADPH came from the thylakoid. The cycle did not make them by splitting water.
Photorespiration, C4 leaves, and CAM leaves
Rubisco can bind O2 as well as CO2. When CO2 is low and O2 is high, often because stomata have closed on a hot, bright day, the oxygenase activity starts photorespiration. The pathway consumes ATP and releases CO2 that had already been fixed, so carbon is wasted instead of stored in sugar.
C4 plants, such as maize and sugarcane, separate the steps in space. In mesophyll cells, PEP carboxylase attaches carbon, in the form of bicarbonate, to phosphoenolpyruvate and forms a four-carbon acid. This enzyme does not use O2 as a competing substrate the way rubisco does. The four-carbon acid moves into bundle-sheath cells and releases CO2. Rubisco and the Calvin cycle operate there, where CO2 is kept high.
CAM plants, such as many cacti and pineapple, separate the steps in time. Stomata open at night, and PEP carboxylase fixes carbon into organic acids that are stored. During the day the stomata stay closed, and the stored acids release CO2 to the Calvin cycle while the light reactions are making ATP and NADPH. Both strategies reduce photorespiration. Neither one puts the Calvin cycle on the thylakoid.
| Route | Place | Main result |
|---|---|---|
| Noncyclic electron flow | Thylakoid membrane, both photosystems | ATP, NADPH, and O2 from water |
| Cyclic electron flow | Thylakoid membrane, photosystem I only | ATP, with no NADPH and no O2 |
| Calvin cycle | Stroma | One net G3P for every three CO2 |
| C4 initial fixation | Mesophyll, then the Calvin cycle in bundle-sheath cells | Carbon capture and the cycle separated in space |
| CAM carbon capture | Acids stored at night, Calvin cycle by day | Carbon capture and the cycle separated in time |
Warning
The Calvin cycle does not run only in darkness, and it does not split water. Photosystem II in the thylakoid membrane splits water, and that is the source of released O2. The stroma cycle fixes CO2 by using ATP and NADPH.
Where do the oxygen atoms in the O2 released by photosynthesis come from?
They come from carbon dioxide split by the Calvin cycle.
They come from water split by photosystem II.
They come from RuBP when rubisco fixes carbon in the stroma.
They come from NADPH when photosystem I reduces NADP+.
Which description of cyclic electron flow is correct?
It reduces NADP+ to NADPH and does not make ATP.
It is the carbon-fixation route that runs only in darkness.
It uses photosystem I only and makes ATP without making NADPH or O2.
It uses photosystem II and photosystem I, and it releases oxygen.
How do C4 plants separate the initial capture of carbon dioxide from the Calvin cycle?
Rubisco in the thylakoid membrane releases oxygen by splitting carbon dioxide.
PEP carboxylase fixes carbon in mesophyll cells, and the Calvin cycle runs in bundle-sheath cells.
Stomata open only at night, which is how C4 plants separate the steps in time.
The Calvin cycle splits water in the dark and does not use ATP or NADPH.
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