Where Does the Oxygen Released by a Plant Actually Come From?

A green leaf takes in carbon dioxide and water and, using light, makes sugar and releases oxygen.

This part covers the experiments behind the equation of photosynthesis, the chloroplast and its pigments, the light reaction and Z-scheme, and how ATP is made.

Together these experiments showed that green plants restore air used up by burning or breathing, need light to do it, make glucose stored as starch, work best in blue and red light, and release oxygen that comes from water, not carbon dioxide.

What each showed:

- Priestley — a candle or a mouse in a closed bell jar kept going only with a mint plant inside; plants restore the air
- Ingenhousz — this needs sunlight and happens only in the green parts of plants
- Julius von Sachs — glucose is made in green parts, where chlorophyll lies in chloroplasts, and is usually stored as starch
- T.W. Engelmann — light split by a prism fell on the alga Cladophora among aerobic bacteria; the bacteria gathered in blue and red light, tracing an action spectrum
- Hill — isolated chloroplasts given a suitable electron acceptor released oxygen in light even without carbon dioxide
- Cornelius van Niel — sulphur bacteria using H${}_2$S release sulphur, not oxygen; so in green plants the oxygen comes from water

The overall equation:

$$6{\text{CO}}_2+12{\text{H}}_2\text{O}\underset{\text{chlorophyll}}{\overset{\text{light}}{\to }}{\text{C}}_6{\text{H}}_{12}{\text{O}}_6+6{\text{H}}_2\text{O}+6{\text{O}}_2$$

An everyday example. A tulsi plant on a sunny windowsill does what the mint in the bell jar did — it adds oxygen to the room's air in daylight.

The substance. Writing only 6 water molecules hides the key fact; the 12-water version shows where the released oxygen comes from.

The light reaction happens on the thylakoid membranes of the grana and carbon fixation in the stroma; chlorophyll a is the chief pigment, while chlorophyll b, xanthophylls and carotenoids are accessory pigments that widen the range of light used and protect chlorophyll a.

Sites:

- Grana (thylakoid membranes) — trap light, split water, make ATP and NADPH
- Stroma — enzymes use ATP and NADPH to fix carbon dioxide into sugar

Pigments on a paper chromatogram:

- Chlorophyll a — bright or blue-green; the chief pigment
- Chlorophyll b — yellow-green
- Xanthophylls — yellow
- Carotenoids — yellow to yellow-orange

Absorption versus action spectrum. The absorption spectrum shows which wavelengths a pigment absorbs; the action spectrum shows the rate of photosynthesis at each wavelength. The action spectrum does not fall to zero in between, because accessory pigments absorb other wavelengths and pass the energy to chlorophyll a.

An everyday example. An old mango leaf turning yellow before it drops reveals carotenoids that were present all along, once chlorophyll breaks down.

The substance. Leaves look green because chlorophyll absorbs little green light and reflects it.

**Light excites photosystem II, whose electrons pass down a transport chain to photosystem I, are boosted again, and finally reduce NADP${}^{+}$ to NADPH; splitting water replaces the electrons lost by PS II, so non-cyclic flow makes ATP and NADPH, while cyclic flow around PS I makes only ATP.

Photosystems:

- PS I — reaction centre absorbs at 700 nm, called P700
- PS II — reaction centre absorbs at 680 nm, called P680

The Z-scheme:

- Light excites P680; electrons pass to an acceptor, then down a chain of cytochromes
- They reach PS I, where light excites P700
- They move to another acceptor and reduce NADP${}^{+}$ to NADPH + H${}^{+}$

Splitting of water happens at PS II, on the inner side of the thylakoid membrane**:

$$2{\text{H}}_2\text{O}\to 4{\text{H}}^{+}+{\text{O}}_2+4e^{-}$$

Cyclic versus non-cyclic:

- Non-cyclic — PS II and PS I both work; makes ATP and NADPH; releases O${}_2$
- Cyclic — only PS I; electrons return to the chain; makes ATP only; happens in the stroma lamellae, which lack PS II and NADP reductase, and when only light above 680 nm is available

An everyday example. A relay race with two runners is like PS II handing electrons to PS I, which gives them a second boost before the finish.

The substance. PS II acts before PS I — the numbering does not follow the electrons' path.

The light reaction builds a high concentration of protons inside the thylakoid lumen, and as these protons flow back to the stroma through ATP synthase, the enzyme uses that flow to join ADP and phosphate into ATP.

How the gradient builds up:

- Splitting of water releases H${}^{+}$ inside the lumen
- As electrons move along the chain, a carrier pumps protons from the stroma into the lumen
- NADP reductase on the stroma side uses up H${}^{+}$ from the stroma to form NADPH

So the lumen becomes acidic and a proton gradient forms.

ATP synthase has two parts:

- **CF${}_0$ — embedded in the membrane; a channel through which protons diffuse back
- CF${}_1$ — projects into the stroma; changes shape as protons pass, making ATP

An everyday example. Water held behind a dam spins a turbine as it rushes out — the proton gradient is the dam, and ATP synthase is the turbine.

The substance. The thylakoid membrane must stay intact** — if protons leak across it, the gradient collapses and ATP synthesis stops even in bright light.