---
title: "Cytochrome Complex — AP Bio Definition & Exam Guide"
description: "The cytochrome complex is the electron carrier between Photosystem II and I that pumps protons to power ATP synthesis in photosynthesis. Here's how it's tested."
canonical: "https://fiveable.me/ap-bio/key-terms/cytochrome-complex"
type: "key-term"
subject: "AP Biology"
unit: "Unit 3"
---

# Cytochrome Complex — AP Bio Definition & Exam Guide

## Definition

The cytochrome complex is a protein complex in the thylakoid membrane that shuttles electrons from Photosystem II to Photosystem I and uses the energy released to pump protons (H⁺) into the thylakoid space, building the gradient that drives ATP synthesis.

## What It Is

The cytochrome complex sits between [Photosystem II](/ap-bio/key-terms/photosystem-ii "fv-autolink") and Photosystem I in the [thylakoid membrane](/ap-bio/unit-3/cellular-energy/study-guide/pOhMYoE7Yc4VJi0Rk41H "fv-autolink"), right in the middle of the light-dependent reactions' electron transport chain. Think of it as the energy-collecting toll booth between the two photosystems. Electrons leaving PSII are high-energy, and as they pass through the cytochrome complex they lose energy in steps. That released energy isn't wasted. The complex uses it to pump protons (H⁺) from the stroma into the thylakoid space, building up a concentration gradient.

This matters because that [proton gradient](/ap-bio/key-terms/proton-gradient "fv-autolink") is the whole point. Protons pile up inside the thylakoid, then rush back out through ATP synthase, and that flow powers ATP production (chemiosmosis). So the cytochrome complex doesn't make ATP directly, but no cytochrome complex pumping means no gradient, which means no ATP. After electrons pass through the complex, they continue on to Photosystem I, where they get re-energized by light and eventually reduce NADP⁺ to NADPH (per EK 3.4.B.1).

## Why It Matters

This lives in [Unit 3](/ap-bio/unit-3 "fv-autolink"): Cellular Energetics, Topic 3.4 Photosynthesis, and supports learning objective [AP Bio](/ap-bio "fv-autolink") 3.4.B, explaining how cells capture light energy and transfer it to biological molecules. The cytochrome complex is the concrete link between two big ideas in EK 3.4.B.1 and EK 3.4.B.2: electrons flow through the thylakoid membrane, and that flow does work. It connects to the course-wide theme of energy transformation. Energy never appears or disappears, it just changes form, and the cytochrome complex is where the energy of moving electrons gets converted into a proton gradient you can later cash in for ATP.

## Connections

### [Electron Transport Chain (ETC) (Unit 3)](/ap-bio/key-terms/electron-transport-chain-etc)

The cytochrome complex is one stop along the photosynthetic ETC. The same logic shows up in [mitochondria](/ap-bio/key-terms/mitochondria "fv-autolink") during cellular respiration, where electron carriers also release energy in steps to pump protons. Learn the pattern once and it works in both chloroplasts and mitochondria.

### [ATP (Unit 3)](/ap-bio/key-terms/atp)

The cytochrome complex never touches ATP directly. It builds the proton gradient, and [ATP synthase](/ap-bio/key-terms/atp-synthase "fv-autolink") later uses that gradient to make ATP. If you say 'the cytochrome complex makes ATP,' you've skipped the most important step.

### [Cyclic Electron Flow (Unit 3)](/ap-bio/key-terms/cyclic-electron-flow)

When a cell needs ATP but already has plenty of NADPH, electrons from [Photosystem I](/ap-bio/key-terms/photosystem-i "fv-autolink") loop back to the cytochrome complex instead of reducing NADP⁺. This recycling keeps the proton gradient (and ATP production) going without making more NADPH.

### [Cyanobacteria (Unit 3)](/ap-bio/key-terms/cyanobacteria)

Photosynthesis first evolved in prokaryotes (EK 3.4.A.1), so the same electron-transport machinery operates across the thylakoid-like membranes of cyanobacteria. Eukaryotic chloroplasts inherited this design, which is why the chemistry is so similar.

## On the AP Exam

Expect this in the light-dependent reactions section, often as an 'inhibitor' or 'block the chain' question. A classic stem uses an herbicide like DCMU that stops electrons from moving from Photosystem II to the cytochrome complex, then asks you to explain why ATP production stops. The answer: no electron flow means no proton pumping, which means no gradient and no ATP. The 2023 free-response set (Q4) compared noncyclic and cyclic electron flow, where the cytochrome complex appears in both paths. On FRQs you may need to predict what happens downstream when one step is blocked, so practice reasoning forward through the chain: block electrons to the complex, lose the gradient, lose the ATP.

## Cytochrome complex vs ATP synthase

Both sit in the thylakoid membrane and both deal with protons, but they do opposite jobs. The cytochrome complex pumps protons INTO the thylakoid space (using energy from electrons). ATP synthase lets protons flow back OUT and uses that flow to make ATP. One builds the gradient, the other spends it.

## Key Takeaways

- The cytochrome complex transfers electrons from Photosystem II to Photosystem I in the thylakoid membrane.
- As electrons pass through, the complex pumps protons (H⁺) into the thylakoid space, building the gradient that powers ATP synthesis.
- It does not make ATP directly. ATP synthase does that later, using the gradient the cytochrome complex helped build.
- Blocking electron flow to the cytochrome complex (like the herbicide DCMU does) stops proton pumping, kills the gradient, and shuts down ATP production.
- In cyclic electron flow, electrons from Photosystem I return to the cytochrome complex to keep making ATP when the cell already has enough NADPH.

## FAQs

### What is the cytochrome complex in photosynthesis?

It's a protein complex in the thylakoid membrane that carries electrons from Photosystem II to Photosystem I and uses the energy released to pump protons into the thylakoid space, helping build the gradient for ATP synthesis.

### Does the cytochrome complex make ATP?

No. It pumps protons to create a gradient, but ATP synthase is the enzyme that actually makes ATP when those protons flow back through it. The cytochrome complex sets up the energy source, it doesn't spend it.

### How is the cytochrome complex different from ATP synthase?

The cytochrome complex pumps protons INTO the thylakoid space using electron energy, while ATP synthase lets protons flow back OUT to make ATP. One builds the proton gradient; the other uses it.

### What happens if electrons can't reach the cytochrome complex?

If an inhibitor like DCMU blocks electron transfer from Photosystem II, the complex can't pump protons, the gradient never forms, and ATP production stops. This is a common FRQ and MCQ setup on the AP exam.

### What role does the cytochrome complex play in cyclic electron flow?

In cyclic flow, electrons from Photosystem I cycle back to the cytochrome complex instead of reducing NADP⁺. This keeps protons pumping and ATP forming when the cell has high NADPH but needs more ATP.

## Related Study Guides

- [3.4 Photosynthesis](/ap-bio/unit-3/cellular-energy/study-guide/pOhMYoE7Yc4VJi0Rk41H)

## Structured Data

```json
{"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/ap-bio/key-terms/cytochrome-complex#resource","name":"Cytochrome Complex — AP Bio Definition & Exam Guide","url":"https://fiveable.me/ap-bio/key-terms/cytochrome-complex","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/ap-bio/key-terms/cytochrome-complex#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-06-11T05:27:40.651Z","isPartOf":{"@type":"Collection","name":"AP Biology Key Terms","url":"https://fiveable.me/ap-bio/key-terms"},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/ap-bio/key-terms/cytochrome-complex#term","name":"Cytochrome complex","description":"The cytochrome complex is a protein complex in the thylakoid membrane that shuttles electrons from Photosystem II to Photosystem I and uses the energy released to pump protons (H⁺) into the thylakoid space, building the gradient that drives ATP synthesis.","url":"https://fiveable.me/ap-bio/key-terms/cytochrome-complex","inDefinedTermSet":{"@type":"DefinedTermSet","name":"AP Biology Key Terms","url":"https://fiveable.me/ap-bio/key-terms"}},{"@type":"FAQPage","mainEntity":[{"@type":"Question","name":"What is the cytochrome complex in photosynthesis?","acceptedAnswer":{"@type":"Answer","text":"It's a protein complex in the thylakoid membrane that carries electrons from Photosystem II to Photosystem I and uses the energy released to pump protons into the thylakoid space, helping build the gradient for ATP synthesis."}},{"@type":"Question","name":"Does the cytochrome complex make ATP?","acceptedAnswer":{"@type":"Answer","text":"No. It pumps protons to create a gradient, but ATP synthase is the enzyme that actually makes ATP when those protons flow back through it. The cytochrome complex sets up the energy source, it doesn't spend it."}},{"@type":"Question","name":"How is the cytochrome complex different from ATP synthase?","acceptedAnswer":{"@type":"Answer","text":"The cytochrome complex pumps protons INTO the thylakoid space using electron energy, while ATP synthase lets protons flow back OUT to make ATP. One builds the proton gradient; the other uses it."}},{"@type":"Question","name":"What happens if electrons can't reach the cytochrome complex?","acceptedAnswer":{"@type":"Answer","text":"If an inhibitor like DCMU blocks electron transfer from Photosystem II, the complex can't pump protons, the gradient never forms, and ATP production stops. This is a common FRQ and MCQ setup on the AP exam."}},{"@type":"Question","name":"What role does the cytochrome complex play in cyclic electron flow?","acceptedAnswer":{"@type":"Answer","text":"In cyclic flow, electrons from Photosystem I cycle back to the cytochrome complex instead of reducing NADP⁺. This keeps protons pumping and ATP forming when the cell has high NADPH but needs more ATP."}}]},{"@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"AP Biology","item":"https://fiveable.me/ap-bio"},{"@type":"ListItem","position":2,"name":"Key Terms","item":"https://fiveable.me/ap-bio/key-terms"},{"@type":"ListItem","position":3,"name":"Unit 3","item":"https://fiveable.me/ap-bio/unit-3"},{"@type":"ListItem","position":4,"name":"Cytochrome complex"}]}]}
```