IMAT Biology · Bioenergetics and Metabolism

Photosynthesis: Light Reactions and the Calvin Cycle

In the chloroplast, light energy splits water and makes ATP and NADPH on the thylakoid membranes; in the stroma, the Calvin cycle uses them to turn CO₂ into sugar.

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Why this matters

The ministry papers asked for the balanced equation (2023) and the organelle (2026). The older papers went deeper: the products of photolysis, the site of the light-dependent reactions and the carrier they reduce, where proton pumps sit in the chloroplast, an outline of carbon fixation, and whether plants release CO₂ by day and by night.

Concept 1 of 4: Photosynthesis: the overall equation and the chloroplast

Photosynthesis runs the chemistry of respiration uphill. Light energy is used to pull electrons out of water and push them onto

\mathrm{CO_2}

, building sugar and releasing oxygen. In plants and algae it all happens in the chloroplast: light is captured on the thylakoid membranes, and sugar is built in the stroma around them.

Definition

Overall:
$6\mathrm{CO_2} + 6\mathrm{H_2O} \xrightarrow{\text{light, chlorophyll}} \mathrm{C_6H_{12}O_6} + 6\mathrm{O_2}$
It is endergonic and anabolic: light supplies the energy.
The $\mathrm{O_2}$ released comes from water, not from $\mathrm{CO_2}$ (shown by labelling water with the isotope oxygen-18).
Chloroplast: a double membrane (the envelope); the stroma, a fluid holding enzymes such as RuBisCO, circular DNA, 70S ribosomes and starch grains; and thylakoids, flattened membrane sacs stacked into grana.
Chlorophyll and the photosystems sit in the thylakoid membranes, not in the envelope. Chlorophyll absorbs mainly red and blue light and reflects green.
Two stages: the light-dependent reactions (thylakoid membranes) and the light-independent reactions, or Calvin cycle (stroma).
Cyanobacteria photosynthesise without chloroplasts, on folded internal membranes.

Part	Structure	What happens there
Envelope	Two membranes, outer and inner	Controls exchange with the cytoplasm; holds no chlorophyll
Thylakoid membrane	Membrane of the flattened sacs	Light absorbed by chlorophyll, water split, electron transport, ATP synthase
Granum	A stack of thylakoids	Gives a large membrane area for the light-dependent reactions
Thylakoid lumen	The space inside each thylakoid	Protons collect here; oxygen from water is released here
Stroma	Fluid around the thylakoids	Calvin cycle: CO₂ fixed by RuBisCO and sugar made; starch stored

Practice this conceptself-check · 4 quick reps

The oxygen comes from water

In the equation, the 6

\mathrm{O_2}

all come from the split water molecules. The oxygen atoms of

\mathrm{CO_2}

end up in the sugar and in new water.

Concept 2 of 4: The light-dependent reactions: photolysis, NADPH and ATP

Light gives electrons in chlorophyll so much energy that they leave it. Photosystem II replaces its lost electrons by splitting water, which releases oxygen and protons. The electrons flow down a chain to photosystem I, get a second boost from light, and end up on

\mathrm{NADP^+}

. On the way, protons are pumped into the thylakoid, and their flow back out through ATP synthase makes ATP, just as in mitochondria.

Definition

Site: the thylakoid membranes (in the grana).
Photolysis at photosystem II: $2\mathrm{H_2O} \rightarrow 4\mathrm{H^+} + 4e^- + \mathrm{O_2}$ . Its direct products are oxygen, protons and electrons; no $\mathrm{CO_2}$ is involved.
Non-cyclic electron flow: water $\rightarrow$ photosystem II $\rightarrow$ plastoquinone $\rightarrow$ cytochrome $b_6f$ complex (a proton pump) $\rightarrow$ plastocyanin $\rightarrow$ photosystem I $\rightarrow$ ferredoxin $\rightarrow$ $\mathrm{NADP^+}$ , which is reduced to NADPH.
Protons build up in the thylakoid lumen (from photolysis and from pumping). ATP synthase in the thylakoid membrane lets them flow out to the stroma and makes ATP there: photophosphorylation, by chemiosmosis.
Products sent to the Calvin cycle: ATP and NADPH. Oxygen leaves the leaf or is used in the plant's own respiration.
Cyclic electron flow uses photosystem I only: electrons return from ferredoxin to the cytochrome complex. It makes ATP but no NADPH and no $\mathrm{O_2}$ .

Feature	Mitochondrion	Chloroplast
Membrane holding the chain and ATP synthase	Inner membrane (cristae)	Thylakoid membrane
Source of the electrons	NADH and FADH₂ from food	Water, split using light energy
Protons pumped into	Intermembrane space	Thylakoid lumen
ATP made in	Matrix	Stroma
Final electron acceptor	Oxygen, forming water	NADP⁺, forming NADPH
Name of the ATP synthesis	Oxidative phosphorylation	Photophosphorylation

Both organelles make ATP by chemiosmosis, so ATP is made in both photosynthesis and respiration.

Practice this conceptself-check · 4 quick reps

Proton pumps are in membranes, not in spaces

A pump has to sit in a membrane to move protons across it. In the chloroplast the pumping complex and ATP synthase are in the thylakoid (granum) membrane. The stroma and the lumen are spaces: they hold protons but contain no pumps.

Photosynthesis reduces NADP⁺, not NAD⁺

The carrier reduced in the light-dependent reactions is

\mathrm{NADP^+}

, and it ends up reduced (NADPH). NAD⁺ and FAD are the carriers of respiration.

Concept 3 of 4: The Calvin cycle: fixing carbon dioxide in the stroma

The Calvin cycle spends the ATP and NADPH from the light-dependent reactions to turn

\mathrm{CO_2}

into sugar. The enzyme RuBisCO joins

\mathrm{CO_2}

to a 5-carbon acceptor; the product splits in two, and each half is reduced to a 3-carbon sugar phosphate. Most of that sugar is used to rebuild the acceptor, so the cycle keeps turning as long as light keeps supplying ATP and NADPH.

Definition

Site: the stroma. It is called light-independent because light does not drive it directly, but it stops within a minute of darkness, when ATP and NADPH run out.
Fixation: $\mathrm{CO_2}$ + RuBP (ribulose bisphosphate, 5C) $\rightarrow$ an unstable 6C compound $\rightarrow$ 2 GP (glycerate 3-phosphate, 3C). The enzyme is RuBisCO, probably the most abundant protein on Earth.
Reduction: GP is reduced to triose phosphate (TP) using ATP and NADPH. The hydrogen that came from water enters the sugar here.
Regeneration: 5 of every 6 TP are rearranged into 3 RuBP, using more ATP.
Per 3 $\mathrm{CO_2}$ fixed: 9 ATP and 6 NADPH, giving 1 TP to export. Per glucose (6 $\mathrm{CO_2}$ ): 18 ATP and 12 NADPH.
TP is used to make glucose, sucrose, starch and cellulose, and also amino acids and lipids.
The cycle releases no $\mathrm{O_2}$ and no $\mathrm{CO_2}$ . RuBisCO can also react with $\mathrm{O_2}$ instead of $\mathrm{CO_2}$ (photorespiration), which wastes energy, more so in hot, dry conditions.

Stage	What happens	Uses	Carbon atoms
Fixation	CO₂ joins RuBP, catalysed by RuBisCO	No ATP or NADPH	1 + 5 → 6, which splits into 2 × 3
Reduction	GP is reduced to triose phosphate	ATP and NADPH	3 → 3
Regeneration	Five triose phosphates are rearranged into three RuBP	ATP	5 × 3 → 3 × 5
Export	One triose phosphate in six leaves the cycle	Nothing more	2 × 3 → 6 (two make one glucose)

Practice this conceptself-check · 4 quick reps

Light-independent does not mean it runs at night

The Calvin cycle needs a constant supply of ATP and NADPH, which only the light-dependent reactions make. In the dark it stops within a minute or so.

Light splits water, not carbon dioxide

\mathrm{CO_2}

is never split by light. It is joined whole to RuBP by RuBisCO, and the product is then reduced with hydrogen that came from water.

Concept 4 of 4: Limiting factors, the compensation point and respiration in plants

The rate of photosynthesis is set by whichever input is in shortest supply, like a production line held back by its slowest worker. Raising that factor speeds the process up; raising anything else does nothing. Meanwhile the plant respires all the time, day and night, so what we measure in the light is photosynthesis minus respiration.

Definition

Law of limiting factors: the rate is limited by the factor in shortest supply relative to need.
Light intensity limits the light-dependent reactions. At high light the rate levels off, because another factor takes over.
$\mathrm{CO_2}$ concentration (about 0.04% of air) limits fixation by RuBisCO; on a bright day it is often the limiting factor, which is why growers add $\mathrm{CO_2}$ to greenhouses.
Temperature affects the Calvin cycle enzymes: the rate rises to an optimum, then falls as enzymes denature.
Water shortage acts indirectly: stomata close and $\mathrm{CO_2}$ intake falls.
Plant cells have mitochondria and respire day and night, releasing $\mathrm{CO_2}$ . In the light, that $\mathrm{CO_2}$ can be fixed by the plant's own chloroplasts.
The compensation point is the light intensity at which the rate of photosynthesis equals the rate of respiration: there is no net exchange of $\mathrm{CO_2}$ or $\mathrm{O_2}$ , though both processes are still running.

Condition	Photosynthesis	Respiration	Net gas exchange with the air
Darkness	Stopped	Running	CO₂ released, O₂ taken in
Dim light, below the compensation point	Slower than respiration	Running	Small net release of CO₂
At the compensation point	Equal to respiration	Running	None
Bright light	Faster than respiration	Running	O₂ released, CO₂ taken in

Practice this conceptself-check · 4 quick reps

Plants respire all the time

Photosynthesis does not replace respiration. A plant releases

\mathrm{CO_2}

from respiration in the light as well as in the dark; in bright light the release is simply hidden, because photosynthesis takes in more than respiration gives out.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (4)

Photosynthesis: the overall equation and the chloroplast5 rows

Part	Structure	What happens there
Envelope	Two membranes, outer and inner	Controls exchange with the cytoplasm; holds no chlorophyll
Thylakoid membrane	Membrane of the flattened sacs	Light absorbed by chlorophyll, water split, electron transport, ATP synthase
Granum	A stack of thylakoids	Gives a large membrane area for the light-dependent reactions
Thylakoid lumen	The space inside each thylakoid	Protons collect here; oxygen from water is released here
Stroma	Fluid around the thylakoids	Calvin cycle: CO₂ fixed by RuBisCO and sugar made; starch stored

The light-dependent reactions: photolysis, NADPH and ATP6 rows

Feature	Mitochondrion	Chloroplast
Membrane holding the chain and ATP synthase	Inner membrane (cristae)	Thylakoid membrane
Source of the electrons	NADH and FADH₂ from food	Water, split using light energy
Protons pumped into	Intermembrane space	Thylakoid lumen
ATP made in	Matrix	Stroma
Final electron acceptor	Oxygen, forming water	NADP⁺, forming NADPH
Name of the ATP synthesis	Oxidative phosphorylation	Photophosphorylation

Both organelles make ATP by chemiosmosis, so ATP is made in both photosynthesis and respiration.

The Calvin cycle: fixing carbon dioxide in the stroma4 rows

Stage	What happens	Uses	Carbon atoms
Fixation	CO₂ joins RuBP, catalysed by RuBisCO	No ATP or NADPH	1 + 5 → 6, which splits into 2 × 3
Reduction	GP is reduced to triose phosphate	ATP and NADPH	3 → 3
Regeneration	Five triose phosphates are rearranged into three RuBP	ATP	5 × 3 → 3 × 5
Export	One triose phosphate in six leaves the cycle	Nothing more	2 × 3 → 6 (two make one glucose)

Limiting factors, the compensation point and respiration in plants4 rows

Condition	Photosynthesis	Respiration	Net gas exchange with the air
Darkness	Stopped	Running	CO₂ released, O₂ taken in
Dim light, below the compensation point	Slower than respiration	Running	Small net release of CO₂
At the compensation point	Equal to respiration	Running	None
Bright light	Faster than respiration	Running	O₂ released, CO₂ taken in

Watch out for (6)

The oxygen comes from water
→ Photosynthesis: the overall equation and the chloroplast
Proton pumps are in membranes, not in spaces
→ The light-dependent reactions: photolysis, NADPH and ATP
Photosynthesis reduces NADP⁺, not NAD⁺
→ The light-dependent reactions: photolysis, NADPH and ATP
Light-independent does not mean it runs at night
→ The Calvin cycle: fixing carbon dioxide in the stroma
Light splits water, not carbon dioxide
→ The Calvin cycle: fixing carbon dioxide in the stroma
Plants respire all the time
→ Limiting factors, the compensation point and respiration in plants

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