Why Leaves Look Green When Chlorophyll Absorbs Red and Blue Light
Understand photosynthetic pigments and the difference between absorption and action spectra, the light reactions with photosystems I and II and cyclic and non-cyclic photophosphorylation, and how chemiosmosis makes ATP in chloroplasts.
How does a leaf turn sunlight into chemical energy?
A mango leaf in sunshine is running a tiny solar power plant. Its pigments trap light, split water, release oxygen and build two energy carriers, ATP and NADPH, that later power the making of sugar.
This lesson covers photosynthetic pigments with absorption and action spectra, the light reactions and photophosphorylation, and how chemiosmosis makes ATP.
This lesson covers photosynthetic pigments with absorption and action spectra, the light reactions and photophosphorylation, and how chemiosmosis makes ATP.
What are photosynthetic pigments, and how is an absorption spectrum different from an action spectrum?
Photosynthetic pigments — chlorophyll a, chlorophyll b, xanthophylls and carotenoids — absorb light of particular wavelengths; an absorption spectrum shows how much light a pigment absorbs at each wavelength, while an action spectrum shows how much photosynthesis happens at each wavelength.
Pigments:
- Chlorophyll a — bright or blue-green; the chief pigment, found at the reaction centres
- Chlorophyll b — yellow-green; an accessory pigment
- Xanthophylls — yellow; accessory pigments
- Carotenoids — yellow to yellow-orange; accessory pigments
Accessory pigments absorb wavelengths that chlorophyll a misses, pass the energy on to chlorophyll a, and protect it from photo-oxidation.
Absorption versus action spectrum:
- Absorption spectrum — measured for a pigment; chlorophyll a absorbs most strongly in the blue and red regions
- Action spectrum — measured as the rate of photosynthesis; it also peaks in blue and red light
- The two graphs largely overlap, showing that chlorophyll a drives most photosynthesis, while the small differences reveal the work of accessory pigments
An everyday example. Neem and peepal leaves look green because chlorophyll absorbs blue and red light strongly and reflects much of the green.
The substance. Yellow leaves in the dry season do not make new yellow pigment — as chlorophyll breaks down, the carotenoids that were present all along become visible.
Pigments:
- Chlorophyll a — bright or blue-green; the chief pigment, found at the reaction centres
- Chlorophyll b — yellow-green; an accessory pigment
- Xanthophylls — yellow; accessory pigments
- Carotenoids — yellow to yellow-orange; accessory pigments
Accessory pigments absorb wavelengths that chlorophyll a misses, pass the energy on to chlorophyll a, and protect it from photo-oxidation.
Absorption versus action spectrum:
- Absorption spectrum — measured for a pigment; chlorophyll a absorbs most strongly in the blue and red regions
- Action spectrum — measured as the rate of photosynthesis; it also peaks in blue and red light
- The two graphs largely overlap, showing that chlorophyll a drives most photosynthesis, while the small differences reveal the work of accessory pigments
An everyday example. Neem and peepal leaves look green because chlorophyll absorbs blue and red light strongly and reflects much of the green.
The substance. Yellow leaves in the dry season do not make new yellow pigment — as chlorophyll breaks down, the carotenoids that were present all along become visible.
What happens in the light reactions, and how do cyclic and non-cyclic photophosphorylation differ?
In the photochemical phase, light absorbed by photosystems II and I drives electrons from water to NADP, splitting water, releasing oxygen and making ATP and NADPH; non-cyclic photophosphorylation uses both photosystems and makes both products, while cyclic photophosphorylation uses only photosystem I and makes only ATP.
Pigment systems:
- Photosystem I (PS I) — its reaction centre absorbs best at 700 nm and is called P700
- Photosystem II (PS II) — its reaction centre absorbs best at 680 nm and is called P680
- Each has a light-harvesting complex of many pigment molecules that funnel energy to the reaction centre
Non-cyclic photophosphorylation (the Z scheme):
- Light excites P680 in PS II, which passes electrons to an electron transport chain
- Water is split near PS II, replacing those electrons:
- Electrons move down the chain to PS I, and ATP is made along the way
- Light excites P700 in PS I, and its electrons reduce NADP to NADPH
Cyclic photophosphorylation:
- Only PS I works, in the stroma lamellae
- Excited electrons return to PS I through the chain instead of reaching NADP
- Only ATP is made — no NADPH and no oxygen
An everyday example. Bubbles rising from Hydrilla kept in a beaker of water in sunlight are oxygen released as water is split in the light reactions.
The substance. The oxygen given out in photosynthesis comes from water, not from carbon dioxide.
Pigment systems:
- Photosystem I (PS I) — its reaction centre absorbs best at 700 nm and is called P700
- Photosystem II (PS II) — its reaction centre absorbs best at 680 nm and is called P680
- Each has a light-harvesting complex of many pigment molecules that funnel energy to the reaction centre
Non-cyclic photophosphorylation (the Z scheme):
- Light excites P680 in PS II, which passes electrons to an electron transport chain
- Water is split near PS II, replacing those electrons:
- Electrons move down the chain to PS I, and ATP is made along the way
- Light excites P700 in PS I, and its electrons reduce NADP to NADPH
Cyclic photophosphorylation:
- Only PS I works, in the stroma lamellae
- Excited electrons return to PS I through the chain instead of reaching NADP
- Only ATP is made — no NADPH and no oxygen
An everyday example. Bubbles rising from Hydrilla kept in a beaker of water in sunlight are oxygen released as water is split in the light reactions.
The substance. The oxygen given out in photosynthesis comes from water, not from carbon dioxide.
How does the chemiosmotic hypothesis explain ATP synthesis in chloroplasts?
According to the chemiosmotic hypothesis, the light reactions build up hydrogen ions inside the thylakoid lumen, and as these ions flow back to the stroma through the enzyme ATP synthase, their movement powers the formation of ATP.
How the proton gradient forms:
- Splitting of water releases hydrogen ions inside the lumen
- Electron transport — as electrons pass along the chain, a carrier moves hydrogen ions from the stroma into the lumen
- NADP reductase on the stroma side uses hydrogen ions from the stroma to make NADPH, lowering their concentration there
How ATP is made:
- The thylakoid membrane lets hydrogen ions out only through ATP synthase
- ATP synthase has two parts: , a channel in the membrane, and , a knob facing the stroma
- As hydrogen ions rush through , changes shape and joins ADP and phosphate into ATP
Where the products go. ATP and NADPH are released into the stroma, where the Calvin cycle uses them to turn carbon dioxide into sugar.
An everyday example. Water held behind a dam spins turbines as it flows down — in the same way, hydrogen ions held inside the thylakoid turn ATP synthase as they flow out.
The substance. Chemiosmosis needs a sealed compartment — if the thylakoid membrane leaks hydrogen ions, electrons can still flow, but no ATP is made.
How the proton gradient forms:
- Splitting of water releases hydrogen ions inside the lumen
- Electron transport — as electrons pass along the chain, a carrier moves hydrogen ions from the stroma into the lumen
- NADP reductase on the stroma side uses hydrogen ions from the stroma to make NADPH, lowering their concentration there
How ATP is made:
- The thylakoid membrane lets hydrogen ions out only through ATP synthase
- ATP synthase has two parts: , a channel in the membrane, and , a knob facing the stroma
- As hydrogen ions rush through , changes shape and joins ADP and phosphate into ATP
Where the products go. ATP and NADPH are released into the stroma, where the Calvin cycle uses them to turn carbon dioxide into sugar.
An everyday example. Water held behind a dam spins turbines as it flows down — in the same way, hydrogen ions held inside the thylakoid turn ATP synthase as they flow out.
The substance. Chemiosmosis needs a sealed compartment — if the thylakoid membrane leaks hydrogen ions, electrons can still flow, but no ATP is made.
Exam tip
What earns full marks on the light reactions?
Draw the Z scheme with PS II on the left and PS I on the right, mark water splitting beside PS II, and end the electron path at NADPH.
- Pigments: chlorophyll a at the reaction centre; chlorophyll b, xanthophylls and carotenoids as accessory pigments
- PS I: P700; PS II: P680
- Non-cyclic: both photosystems, ATP and NADPH, oxygen released
- Cyclic: PS I only, ATP only, no oxygen
The trap. Writing that PS I works before PS II. The numbers are only names — electrons pass from PS II to PS I.
- Pigments: chlorophyll a at the reaction centre; chlorophyll b, xanthophylls and carotenoids as accessory pigments
- PS I: P700; PS II: P680
- Non-cyclic: both photosystems, ATP and NADPH, oxygen released
- Cyclic: PS I only, ATP only, no oxygen
The trap. Writing that PS I works before PS II. The numbers are only names — electrons pass from PS II to PS I.
Did you know
Why do seedlings grown in the dark turn pale yellow?
A potato or a gram seed sprouting in a dark cupboard produces long, thin, yellowish-white shoots. This is etiolation.
Without light, a flowering plant cannot finish making chlorophyll, so the yellow carotenoids show. It also spends its stored food on stretching the stem as fast as possible, as if searching for light.
Within a day or so of reaching sunlight, the shoots turn green as chlorophyll forms — a quick demonstration that making chlorophyll itself depends on light.
Without light, a flowering plant cannot finish making chlorophyll, so the yellow carotenoids show. It also spends its stored food on stretching the stem as fast as possible, as if searching for light.
Within a day or so of reaching sunlight, the shoots turn green as chlorophyll forms — a quick demonstration that making chlorophyll itself depends on light.
Exam relevance
How does NEET test photosynthetic pigments and the light reactions?
Photosynthesis in Higher Plants is a recurring NEET chapter, and the light reactions supply many of its conceptual questions.
What gets asked. Absorption and action spectra, reaction centres P680 and P700, the source of the oxygen released, differences between cyclic and non-cyclic photophosphorylation, and where the proton gradient forms in chemiosmosis.
Question types. Mostly statement-based and assertion-reason questions, with diagram-based questions on the Z scheme.
Why it matters later. Chemiosmosis returns in Respiration in Plants, where the same ATP synthase mechanism works across the inner mitochondrial membrane.
The trap that costs marks. Placing the proton build-up in the stroma — hydrogen ions gather inside the thylakoid lumen, and ATP forms on the stroma side.
What gets asked. Absorption and action spectra, reaction centres P680 and P700, the source of the oxygen released, differences between cyclic and non-cyclic photophosphorylation, and where the proton gradient forms in chemiosmosis.
Question types. Mostly statement-based and assertion-reason questions, with diagram-based questions on the Z scheme.
Why it matters later. Chemiosmosis returns in Respiration in Plants, where the same ATP synthase mechanism works across the inner mitochondrial membrane.
The trap that costs marks. Placing the proton build-up in the stroma — hydrogen ions gather inside the thylakoid lumen, and ATP forms on the stroma side.
Key takeaways
What must you be able to do from this lesson?
- Pigments and spectra: chlorophyll a at the reaction centre with accessory pigments; absorption and action spectra both peaking in blue and red
- Light reactions: PS II and PS I, water splitting, and non-cyclic versus cyclic photophosphorylation
- Chemiosmosis: a proton gradient inside the thylakoid lumen driving ATP synthase
If a chloroplast ran only cyclic photophosphorylation, which product would it make — and which two would be missing?
- Light reactions: PS II and PS I, water splitting, and non-cyclic versus cyclic photophosphorylation
- Chemiosmosis: a proton gradient inside the thylakoid lumen driving ATP synthase
If a chloroplast ran only cyclic photophosphorylation, which product would it make — and which two would be missing?