Describe photophosphorylation (cyclic and non-cyclic)

WHAT is it? (Definition)

WHY do plants need BOTH ATP and NADPH?

HOW non-cyclic works — step by step (the Z-scheme)

Figure — Describe photophosphorylation (cyclic and non-cyclic)

Worked example Walking the electron through non-cyclic flow

Step 1 — Light hits PSII (P680). An electron is excited to a high energy level. Why this step? Light energy is captured as the energy of an excited electron — everything else is that energy being released usefully.

Step 2 — Water is split (photolysis). $2H_2O \rightarrow 4H^+ + 4e^- + O_2$ . The released electrons replace the ones PSII lost. Why this step? PSII just gave away an electron; it must be refilled or it can't fire again. Water is the electron source — and O₂ is the by-product we breathe.

Step 3 — Electron travels down the carriers (plastoquinone → cytochrome b₆f → plastocyanin). As it drops in energy, H⁺ are pumped into the thylakoid lumen. Why this step? The "downhill" energy isn't wasted — it builds a proton gradient (chemiosmosis).

Step 4 — Light hits PSI (P700). The now-lower-energy electron is re-excited. Why this step? It lost energy on the way down; a second photon "re-lifts" it high enough to reduce NADP⁺.

Step 5 — NADP⁺ is reduced. $NADP^+ + 2e^- + H^+ \rightarrow NADPH$ . Why this step? This is the final electron destination — NADPH carries reducing power to the Calvin cycle.

Step 6 — ATP synthase. H⁺ flow back out of the lumen down their gradient through ATP synthase → $ADP + Pi \rightarrow ATP$ . Why this step? The stored gradient energy is cashed in as ATP. This coupling is chemiosmosis.

HOW cyclic works

Derivation: where does the ATP energy actually come from?

Formula Chemiosmotic free energy (first principles)

The energy stored per mole of H⁺ moved across the membrane (proton-motive force) has two parts — a concentration part and an electrical part: $\Delta G = RT \ln\frac{[H^+]_{in}}{[H^+]_{out}} + zF\Delta\psi$ Build-up:

Moving a solute against a concentration ratio costs $RT\ln(\text{ratio})$ — standard chemical potential.
H⁺ carries charge $z=+1$ , so moving it across a voltage $\Delta\psi$ costs $zF\Delta\psi$ (work = charge × voltage). Using $pH = -\log_{10}[H^+]$ and converting $\ln \to 2.303\log$ : $\Delta G = -2.303\,RT\,\Delta pH + F\Delta\psi$ Why it matters: a big $\Delta pH$ (lumen acidic, stroma alkaline) stores energy. ATP synthase releases this $\Delta G$ to drive $ADP + Pi \rightarrow ATP$ ( $+30.5$ kJ/mol). About 3–4 H⁺ pass per ATP because one ATP needs more energy than one H⁺ provides.

Compare them at a glance

Feature	Non-cyclic	Cyclic
Photosystems used	PSII + PSI	PSI only
Electron path	Linear (open)	Loop (closed)
Water split?	Yes	No
O₂ released?	Yes	No
NADPH made?	Yes	No
ATP made?	Yes	Yes
Electron source	Water	The same electron (recycled)

Common mistake Steel-manning the common errors

Mistake 1: "Cyclic flow makes NADPH too." Why it feels right: both pathways involve PSI and produce ATP, so it seems symmetric. The fix: In cyclic flow the electron is sent back to the chain instead of to NADP⁺. No final electron acceptor → no NADPH. Only ATP.

Mistake 2: "Splitting water happens at PSI." Why it feels right: PSI is "Photosystem One", so it feels like it should go first. The fix: The numbering is by discovery order, not flow order. In the Z-scheme the electron goes PSII → PSI, and water is split at PSII.

Mistake 3: "ATP is made directly by the photosystems." Why it feels right: light → ATP seems immediate. The fix: ATP is made by ATP synthase, driven by the proton gradient — light only builds the gradient. This indirect step is chemiosmosis.

Mistake 4: "O₂ comes from CO₂." The fix: The O₂ released in photosynthesis comes from water (photolysis), not CO₂ — proven by isotope (¹⁸O) labelling.

Recall Feynman: explain it to a 12-year-old

Imagine a water slide. You climb up the ladder (that's light giving an electron energy). You whoosh down the slide and the splashing water you push turns a little water-wheel (that wheel = ATP synthase making ATP). Then you have two choices: (A) Non-cyclic — you don't climb back; instead a friend from a bucket of water (the split water molecule) takes your place at the bottom, and you go off to deliver a parcel (NADPH) to the kitchen (Calvin cycle). The bucket also pops out a bubble of air — that's the oxygen you breathe. (B) Cyclic — you grab a rope and swing right back to the top of the same slide and go again, just to keep turning the wheel for more ATP. No bucket needed, no air bubble, no parcel. So: cyclic = ATP only; non-cyclic = ATP + parcel + oxygen.

Flashcards

What two molecules combine to form ATP in photophosphorylation?

ADP + inorganic phosphate (Pi)

Where in the chloroplast does photophosphorylation occur?

The thylakoid membrane

Which photosystems are used in non-cyclic photophosphorylation?

PSII and PSI

Which photosystem is used in cyclic photophosphorylation?

PSI only

What are the products of non-cyclic photophosphorylation?

ATP, NADPH, and O₂

What is the only product of cyclic photophosphorylation?

ATP

In which photosystem is water split (photolysis)?

PSII (P680)

What is the source of the O₂ released in photosynthesis?

Water (via photolysis), proven by ¹⁸O labelling

Why does the cyclic pathway produce no NADPH?

The electron loops back to the chain instead of reducing NADP⁺

What directly drives ATP synthase?

The proton (H⁺) gradient across the thylakoid membrane (chemiosmosis)

Why do plants run cyclic flow alongside non-cyclic?

To make extra ATP for the Calvin cycle without making extra NADPH

The equation for photolysis of water?

2H₂O → 4H⁺ + 4e⁻ + O₂

Final electron acceptor in non-cyclic flow?

NADP⁺ (becomes NADPH)

Wavelengths absorbed by P680 and P700?

680 nm (PSII) and 700 nm (PSI)

Name the carrier between cytochrome b₆f and PSI.

Plastocyanin

Connections

Light Dependent Reactions
Z-Scheme of Electron Transport
Chemiosmosis and ATP Synthase
Photolysis of Water
Calvin Cycle — the customer that uses the ATP & NADPH
Oxidative Phosphorylation — same chemiosmotic principle in mitochondria
Chloroplast Structure

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, photophosphorylation ka matlab simple hai: light energy ka use karke ATP banana, ADP + Pi se. Yeh sab chloroplast ke thylakoid membrane par hota hai. Light electron ko "excited" kar deti hai, aur jab woh electron carriers se hote hue neeche girta hai, toh H⁺ (protons) thylakoid ke andar (lumen) pump ho jaate hain. Ab yeh proton gradient ek battery ki tarah kaam karta hai — H⁺ wapas bahar nikalte hain ATP synthase ke through, aur ATP ban jaata hai. Isko chemiosmosis kehte hain.

Do types hain. Non-cyclic mein electron seedha jaata hai: paani (H₂O) split hota hai PSII par, electron PSII → PSI → NADP⁺ tak jaata hai. Yahan teen cheezein banti hain — ATP, NADPH, aur O₂ (oxygen paani se aati hai, CO₂ se nahi, yeh yaad rakhna). Cyclic mein sirf PSI use hota hai, electron loop maar ke wapas aa jaata hai, isliye sirf ATP banta hai — na NADPH, na oxygen, na paani split.

Yeh important kyun hai? Kyunki Calvin cycle (dark reaction) ko 3 ATP aur 2 NADPH chahiye har CO₂ ke liye. Non-cyclic flow se kabhi-kabhi ATP kam pad jaata hai, isliye plant cyclic flow chalata hai sirf extra ATP banane ke liye, bina extra NADPH waste kiye. Yahi balance puri photosynthesis ko chalta rakhta hai.

Ek common galti: "PSI" naam sun ke lagta hai pehle aata hoga — par nahi, flow mein PSII pehle (paani wahin split hota hai), PSI baad mein. Numbering sirf discovery ke order se hai. Aur ATP photosystem khud nahi banata — woh sirf gradient banata hai, ATP toh ATP synthase banati hai.

Test yourself — Photosynthesis

Connections