AP Biology

## Learning Outcomes

- Explain the roles of Photosystem II and Photosystem I in the light-dependent reactions of photosynthesis.
- Describe how light energy excites electrons in chlorophyll and drives electron transport.
- Connect water splitting, oxygen release, proton gradients, ATP production, and NADPH formation.
- Compare linear electron flow with cyclic electron flow.
- Predict the effects of blocking a photosystem or electron carrier on ATP, NADPH, and oxygen production.
- Interpret diagrams and data involving photosynthetic pigments, thylakoid membranes, and light-dependent reactions.

## AP Biology Syllabus

Photosystems I and II are tested as part of Unit 3: Cellular Energetics, especially photosynthesis and energy capture. AP Biology questions often ask you to explain how organisms capture energy from light and store it temporarily in ATP and NADPH. You should be able to read pathway diagrams, predict outcomes when parts of the pathway are disrupted, and connect the light-dependent reactions to the Calvin cycle.

- Photosystems are protein-pigment complexes in the thylakoid membrane of chloroplasts.
- Photosystem II acts before Photosystem I in linear electron flow.
- Photosystem II splits water, releasing electrons, hydrogen ions, and oxygen gas.
- Electron transport between the photosystems helps build a proton gradient.
- Photosystem I re-energizes electrons and helps reduce NADP+ to NADPH.
- ATP synthase uses the proton gradient to make ATP by chemiosmosis.
- Cyclic electron flow uses Photosystem I to make extra ATP without producing NADPH or oxygen.
- AP questions may test pathway disruptions, graph interpretation, experimental controls, and predictions.

## Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision.

1. Why does Photosystem II act before Photosystem I even though it is called “II”?
2. What molecule supplies replacement electrons to Photosystem II, and what waste product is released?
3. How does electron transport help produce ATP in the light-dependent reactions?
4. What is the main product associated with Photosystem I?
5. How would blocking electron movement from Photosystem II affect oxygen production, ATP production, and NADPH production?

## Introduction

Photosynthesis converts light energy into chemical energy that can be used to build carbohydrates. In plants and algae, this process occurs in chloroplasts. The light-dependent reactions take place in the thylakoid membrane and use light energy to make ATP and NADPH. These two products then supply energy and reducing power for the Calvin cycle, where carbon dioxide is fixed into organic molecules.

Photosystems I and II are the main light-capturing units of the light-dependent reactions. They do not “make glucose” directly. Instead, they capture photons, excite electrons, pass electrons through carriers, and help generate ATP and NADPH. AP Biology often tests this subtopic through diagrams: if you can trace where electrons, protons, oxygen, ATP, and NADPH come from and where they go, you can answer most photosystem questions.

> **Key Term: Photosystem**
> A protein-pigment complex in the thylakoid membrane that absorbs light energy and uses it to excite electrons during the light-dependent reactions.

### Chloroplast Location and Photosystem Structure

Photosystems are found in the thylakoid membrane of chloroplasts. A chloroplast has an outer membrane, an inner membrane, and internal flattened sacs called thylakoids. The fluid inside the chloroplast but outside the thylakoids is the stroma. The space inside the thylakoid is the thylakoid lumen.

This location matters because the light-dependent reactions depend on separation of spaces. Hydrogen ions are moved into the thylakoid lumen, creating a higher proton concentration inside the thylakoid than in the stroma. ATP synthase then allows protons to flow back toward the stroma, using that flow to make ATP.

Each photosystem contains many pigment molecules, including chlorophyll a, chlorophyll b, and accessory pigments such as carotenoids. These pigments absorb different wavelengths of visible light. Energy absorbed by antenna pigments is passed inward to a special chlorophyll a pair in the reaction center.

> **Key Term: Reaction center**
> The part of a photosystem containing a special pair of chlorophyll a molecules that transfers excited electrons to a primary electron acceptor.

Photosystem II has a reaction center called P680 because it best absorbs light with a wavelength of about 680 nm. Photosystem I has a reaction center called P700 because it best absorbs light around 700 nm. The names Photosystem I and Photosystem II can be confusing: Photosystem I was discovered first, but Photosystem II acts first in the main light-dependent pathway.

> **Exam Warning:**
> Do not assume that Photosystem I happens first because of its name. In linear electron flow, the order is Photosystem II, electron transport chain, Photosystem I, then NADPH formation.

When a reaction-center chlorophyll absorbs light energy, one of its electrons becomes excited to a higher energy level. That energized electron is passed to a primary electron acceptor. The chlorophyll molecule is now missing an electron and must be supplied with a replacement. The source of that replacement electron differs between Photosystem II and Photosystem I.

### Photosystem II: Water Splitting and the Start of Electron Flow

Photosystem II begins linear electron flow. Light energy excites electrons in P680, and these high-energy electrons are transferred to a primary electron acceptor. Because P680 has lost electrons, it becomes a strong oxidizing agent and pulls replacement electrons from water.

Water splitting is one of the most tested parts of photosynthesis. In Photosystem II, enzymes split water molecules into electrons, hydrogen ions, and oxygen gas. The electrons replace those lost from P680. The hydrogen ions contribute to the proton gradient in the thylakoid lumen. The oxygen atoms combine to form oxygen gas, which leaves the plant as a waste product.

A simplified equation for water splitting is:

$$
2H_2O \rightarrow 4H^+ + 4e^- + O_2
$$

> **Key Term: Photolysis**
> The splitting of water using light energy during photosynthesis, producing electrons, hydrogen ions, and oxygen gas.

This reaction explains a classic AP Biology point: the oxygen released during photosynthesis comes from water, not from carbon dioxide. Carbon dioxide is used later in the Calvin cycle to build sugars, but the oxygen gas released by the light-dependent reactions originates from water.

After electrons leave Photosystem II, they enter an electron transport chain in the thylakoid membrane. As electrons move through carriers, they lose energy. That released energy is used to move protons from the stroma into the thylakoid lumen. This builds a proton gradient.

> **Key Term: Electron transport chain**
> A series of membrane proteins and electron carriers that pass electrons from one molecule to another, releasing energy used to move protons across a membrane.

A helpful sequence to memorize is:

1. Light excites electrons in Photosystem II.
2. Water is split to replace the lost electrons.
3. Oxygen is released.
4. Electrons move through an electron transport chain.
5. Energy from electron movement helps pump protons into the thylakoid lumen.
6. The proton gradient powers ATP synthase.

> **Test Tip:**
> In a pathway-disruption question, track both electrons and protons. If electron transport slows or stops, the proton gradient usually decreases, so ATP production also decreases.

### Electron Transport, Chemiosmosis, and ATP Formation

The movement of electrons from Photosystem II to Photosystem I is linked to ATP production. Electrons do not directly become ATP. Instead, their energy helps create a proton gradient, and the gradient drives ATP synthase.

The thylakoid lumen becomes more acidic than the stroma because hydrogen ions accumulate there. Protons enter the lumen from two main sources: water splitting and active movement of protons from the stroma into the lumen by electron transport proteins. This difference in proton concentration stores potential energy.

> **Key Term: Chemiosmosis**
> The production of ATP using energy stored in a proton gradient across a membrane.

ATP synthase is a membrane protein that allows protons to move down their concentration gradient. As protons flow through ATP synthase from the thylakoid lumen into the stroma, ATP synthase uses that energy to add inorganic phosphate to ADP, forming ATP.

$$
ADP + P_i \rightarrow ATP
$$

> **Key Term: ATP synthase**
> A membrane protein that uses energy from proton movement down a gradient to make ATP from ADP and inorganic phosphate.

This process is similar in concept to oxidative phosphorylation in cellular respiration. In both photosynthesis and respiration, an electron transport chain helps create a proton gradient, and ATP synthase uses chemiosmosis to make ATP. The locations differ: in photosynthesis, the gradient is across the thylakoid membrane; in cellular respiration, it is across the inner mitochondrial membrane.

For AP Biology, focus on the relationship, not every carrier name. You may see names such as plastoquinone, cytochrome complex, plastocyanin, ferredoxin, or NADP+ reductase in a diagram. If those names are provided, use the diagram to follow electron movement. You usually do not need to memorize every carrier in order.

> **Exam Warning:**
> Avoid saying that ATP is made because electrons “turn into ATP.” Electrons provide energy for proton pumping. The proton gradient powers ATP synthase, which makes ATP.

A common experimental setup uses chloroplasts or isolated thylakoids exposed to light. If a chemical blocks electron transport after Photosystem II, electrons may not reach Photosystem I efficiently. As a result, the proton gradient decreases, ATP production decreases, and NADPH production also decreases. Oxygen production may continue briefly if Photosystem II can still split water, but it will soon be limited because downstream electron acceptors become unavailable.

### Photosystem I: NADPH Production and Electron Re-Energizing

After electrons pass through the electron transport chain, they reach Photosystem I. By this point, they have lost much of the energy they originally gained in Photosystem II. Light energy absorbed by Photosystem I re-energizes these electrons.

In Photosystem I, P700 absorbs light, and excited electrons are passed to a primary electron acceptor. These electrons then move through carriers to NADP+ reductase, an enzyme that transfers electrons to NADP+. NADP+ also receives a hydrogen ion, forming NADPH.

A simplified equation is:

$$
NADP^+ + 2e^- + H^+ \rightarrow NADPH
$$

> **Key Term: NADPH**
> An electron carrier produced in the light-dependent reactions that supplies high-energy electrons for the Calvin cycle.

NADPH is used in the Calvin cycle to reduce carbon-containing molecules. ATP supplies energy, while NADPH supplies electrons. The Calvin cycle needs both. If Photosystem I is disrupted, NADPH production falls, and the Calvin cycle will slow because it lacks reducing power.

The full linear pathway from water to NADPH is sometimes called noncyclic electron flow because electrons do not return to their starting point. They begin in water, move through Photosystem II, pass along electron carriers, enter Photosystem I, and finally reduce NADP+ to NADPH.

> **Key Term: Noncyclic electron flow**
> The light-dependent pathway in which electrons move from water through Photosystem II and Photosystem I to NADP+, producing oxygen, ATP, and NADPH.

Linear electron flow produces three major outputs:

- Oxygen gas from water splitting
- ATP from chemiosmosis
- NADPH from reduction of NADP+

These products connect directly to the Calvin cycle. ATP and NADPH are consumed in the stroma during carbon fixation and sugar-building reactions. Oxygen is released as a by-product.

> **Test Tip:**
> If an AP question asks which products of the light-dependent reactions are used in the Calvin cycle, choose ATP and NADPH. Oxygen is produced by the light-dependent reactions but is not used by the Calvin cycle.

### Cyclic Electron Flow and Balancing ATP Needs

Plants sometimes need more ATP relative to NADPH. The Calvin cycle uses more ATP than NADPH, so chloroplasts can use cyclic electron flow to make extra ATP without making extra NADPH.

In cyclic electron flow, only Photosystem I is used. Electrons excited from P700 move through electron carriers and then cycle back to Photosystem I instead of reducing NADP+ to NADPH. As electrons pass through part of the electron transport chain, protons are pumped into the thylakoid lumen. This supports ATP production through chemiosmosis.

> **Key Term: Cyclic electron flow**
> A photosynthetic pathway involving Photosystem I in which electrons cycle back to the electron transport chain, producing ATP but not NADPH or oxygen.

Cyclic electron flow does not split water. Therefore, it does not release oxygen. It also does not reduce NADP+ to NADPH. Its main purpose is ATP production.

Compare the outcomes in words:

- Linear electron flow uses Photosystem II and Photosystem I.
- Linear electron flow moves electrons from water to NADP+.
- Linear electron flow produces oxygen, ATP, and NADPH.
- Cyclic electron flow uses Photosystem I only.
- Cyclic electron flow sends electrons back through carriers.
- Cyclic electron flow produces ATP only.

This comparison is useful in multiple-choice questions. If a question describes ATP production without oxygen release or NADPH production, cyclic electron flow is likely involved. If a question describes water splitting and oxygen production, Photosystem II and linear electron flow are involved.

### Common AP Biology Applications and Question Styles

Photosystems are often tested through models, graphs, or experiments rather than recall alone. You may be given a diagram of the thylakoid membrane and asked to identify what happens if a carrier is inhibited. You may also be given data showing oxygen production at different wavelengths of light and asked to connect pigment absorption to photosynthetic rate.

For example, chlorophyll absorbs red and blue light better than green light. A plant exposed to red or blue light often has a higher photosynthetic rate than a plant exposed only to green light. However, accessory pigments expand the range of usable wavelengths, so do not assume green light has no effect at all unless the data show that.

In experimental questions, remember the required design skills. Identify the independent variable, dependent variable, and control group. For a photosynthesis experiment, the independent variable might be light intensity, wavelength, carbon dioxide concentration, temperature, or presence of an inhibitor. The dependent variable might be oxygen production, carbon dioxide uptake, ATP concentration, NADPH concentration, pH change in the thylakoid lumen, or rate of floating leaf disks.

A common lab-style measure is oxygen production. Since oxygen comes from water splitting at Photosystem II, oxygen production can be used as evidence for Photosystem II activity. If oxygen levels rise in the light, Photosystem II is active. If oxygen production drops after adding an inhibitor, the inhibitor may affect Photosystem II, water splitting, or electron acceptors downstream of Photosystem II.

Another common measure is pH. In the light, protons accumulate in the thylakoid lumen. If a diagram or experiment shows that the lumen becomes less acidic, that suggests fewer protons are being moved into the lumen or the membrane has become leaky to protons. Either change would reduce ATP production.

> **Test Tip:**
> On free-response questions, make cause-and-effect links explicit. A strong answer says: “Blocking electron transport reduces proton pumping into the thylakoid lumen, which decreases the proton gradient and lowers ATP production by ATP synthase.”

Be careful with direction. ATP and NADPH are produced on the stromal side of the thylakoid membrane, where the Calvin cycle enzymes are located. Protons build up inside the thylakoid lumen and then flow back to the stroma through ATP synthase.

Also avoid mixing up photosynthesis and cellular respiration. In photosynthesis, water is split and oxygen is produced. In aerobic respiration, oxygen is consumed as the final electron acceptor, and water is produced. Both processes use electron transport chains and chemiosmosis, but the sources and destinations of electrons differ.

## Key Point Checklist

This article has covered the following key knowledge points:

- Photosystems are protein-pigment complexes in the thylakoid membrane.
- Photosystem II acts before Photosystem I in linear electron flow.
- Photosystem II contains P680; Photosystem I contains P700.
- Light excites electrons in reaction-center chlorophyll molecules.
- Water splitting at Photosystem II replaces lost electrons and releases oxygen gas.
- The oxygen released during photosynthesis comes from water, not carbon dioxide.
- Electron transport between Photosystem II and Photosystem I helps pump protons into the thylakoid lumen.
- A proton gradient across the thylakoid membrane powers ATP synthase.
- Photosystem I re-energizes electrons and helps reduce NADP+ to NADPH.
- Noncyclic electron flow produces oxygen, ATP, and NADPH.
- Cyclic electron flow uses Photosystem I to produce extra ATP without producing NADPH or oxygen.
- AP questions often test predictions about inhibitors, light wavelength, proton gradients, and pathway diagrams.

## Key Terms and Concepts

- Photosystem
- Reaction center
- Photolysis
- Electron transport chain
- Chemiosmosis
- ATP synthase
- NADPH
- Noncyclic electron flow
- Cyclic electron flow


Photosynthesis - Photosystems I and II