AP Biology

## Learning Outcomes

- Describe where the light-dependent reactions occur in plant cells and photosynthetic prokaryotes.
- Explain how photosystems II and I absorb light and transfer excited electrons.
- Trace the source and pathway of electrons from water to NADPH.
- Explain how photolysis of water produces oxygen gas and contributes protons and electrons.
- Describe how an electron transport chain builds a proton gradient for ATP production.
- Distinguish the products of the light-dependent reactions from the products of the Calvin cycle.
- Predict how changes in light intensity, pigments, or thylakoid membrane integrity affect photosynthesis.

## AP Biology Syllabus

In AP Biology Unit 3, cellular energetics questions often test photosynthesis as an energy transformation process: light energy is converted into chemical energy stored temporarily in ATP and NADPH, which then support carbon fixation in the Calvin cycle. For the light-dependent reactions, focus on locations, inputs, outputs, electron flow, proton gradients, and links to the Calvin cycle rather than memorizing every carrier protein name.

- Light-dependent reactions occur in the thylakoid membrane of chloroplasts.
- Photosystems contain chlorophyll and accessory pigments that absorb photons.
- Photosystem II acts before photosystem I in linear electron flow.
- Water is split during photolysis, producing electrons, protons, and oxygen gas.
- Electron transport helps generate a proton gradient across the thylakoid membrane.
- ATP synthase uses the proton gradient to produce ATP by chemiosmosis.
- $$\mathrm{NADP^+}$$ is the final electron acceptor and is reduced to NADPH.
- ATP and NADPH from the light-dependent reactions are used by the Calvin cycle.

## 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. What are the three main products of the light-dependent reactions?
2. Why would puncturing the thylakoid membrane reduce ATP production?
3. Which photosystem acts first in linear electron flow: photosystem I or photosystem II?
4. Where does the oxygen gas released during photosynthesis come from?
5. Why does the Calvin cycle slow or stop if a plant is kept in the dark for a long time?

## Introduction

Photosynthesis allows photoautotrophs, such as plants, algae, and cyanobacteria, to use light energy to build organic molecules. The overall simplified equation is:

$$
6\mathrm{CO_2} + 6\mathrm{H_2O} \xrightarrow{\text{light energy}} \mathrm{C_6H_{12}O_6} + 6\mathrm{O_2}
$$

This equation hides two linked stages. The light-dependent reactions capture light energy and convert it into ATP and NADPH, while releasing oxygen gas. The Calvin cycle then uses ATP, NADPH, and carbon dioxide to produce carbohydrate molecules such as G3P, which can be used to build sugars.

> **Key Term: Light-dependent reactions**
> The stage of photosynthesis that uses light energy, water, ADP, and $$\mathrm{NADP^+}$$ to produce oxygen gas, ATP, and NADPH.

The light-dependent reactions are not simply “making energy.” Energy is transformed. Photons excite electrons in pigments, electron transfers release energy, and that energy is used to form ATP and NADPH. AP Biology questions often test whether you can connect these steps logically: light absorption leads to excited electrons, electron movement leads to a proton gradient, and the proton gradient drives ATP synthesis.

### Chloroplast location and pigment absorption

In eukaryotic photosynthetic cells, photosynthesis occurs in chloroplasts. A chloroplast has an outer and inner membrane surrounding a fluid-filled space called the stroma. Inside the stroma are stacks of membrane sacs called grana; each individual sac is a thylakoid. The light-dependent reactions occur in the thylakoid membrane, while the Calvin cycle occurs in the stroma.

The inside of the thylakoid is called the thylakoid lumen. This distinction matters because the light-dependent reactions depend on separating protons across a membrane. Protons accumulate in the thylakoid lumen, then flow back toward the stroma through ATP synthase.

Photosynthetic prokaryotes, such as cyanobacteria, do not have chloroplasts. They still perform photosynthesis, but the light-dependent reactions occur on infoldings of the plasma membrane, and the Calvin cycle occurs in the cytosol. This is a useful AP Biology example because it connects photosynthesis with cell structure and evolution: prokaryotic photosynthesis contributed to oxygen in Earth’s atmosphere and came before eukaryotic chloroplast-based photosynthesis.

Light is absorbed by pigments. Chlorophylls are the major photosynthetic pigments, with chlorophyll $$a$$ and chlorophyll $$b$$ absorbing strongly in the violet-blue and red regions of visible light. Carotenoids absorb mainly in the blue-green range and often appear yellow, orange, or red because they reflect other wavelengths.

> **Key Term: Absorption spectrum**
> A graph showing how strongly a pigment absorbs different wavelengths of light.

Absorption spectra are tested because they connect light wavelength to photosynthetic rate. If a pigment absorbs a wavelength strongly, that wavelength can support a higher rate of photosynthesis. If a wavelength is poorly absorbed, less light energy is captured. For example, plants rich in chlorophyll usually appear green because green wavelengths are reflected more than they are absorbed. In an experiment measuring photosynthesis by oxygen production, a plant exposed to poorly absorbed light would usually produce less oxygen than one exposed to strongly absorbed light.

> **Test Tip:**
> If a question gives an absorption spectrum, do not rely only on memorized color names. Read the graph: the wavelength with the highest absorbance for a pigment is the wavelength most likely to support the highest photosynthetic activity for that pigment.

Pigments are organized into photosystems in the thylakoid membrane. A photosystem contains proteins, chlorophyll, and accessory pigments. Accessory pigments absorb light and transfer the energy toward a reaction center, where a special chlorophyll molecule passes excited electrons to an electron acceptor.

> **Key Term: Photosystem**
> A protein-pigment complex in the thylakoid membrane that absorbs light and transfers excited electrons into an electron transport pathway.

There are two main photosystems in the light-dependent reactions: photosystem II and photosystem I. Their names can be misleading. Photosystem II acts first in the normal linear pathway, while photosystem I acts second. Photosystem II contains a reaction center often called P680 because it absorbs best at about 680 nm. Photosystem I contains P700, which absorbs best at about 700 nm.

### Linear electron flow from water to NADPH

The main AP Biology version of the light-dependent reactions is linear electron flow, also called noncyclic electron flow. “Linear” means electrons move in one direction: from water, through photosystem II, through an electron transport chain, through photosystem I, and finally to $$\mathrm{NADP^+}$$.

The process begins when photons strike photosystem II. Light energy excites electrons in the P680 reaction center to a higher energy level. These excited electrons are transferred to an electron acceptor and then passed along an electron transport chain. As electrons move through carriers, energy is released.

Photosystem II has now lost electrons, so they must be replaced. The replacement electrons come from water. Light energy drives the splitting of water molecules in a process called photolysis.

> **Key Term: Photolysis**
> The light-driven splitting of water during photosynthesis, producing electrons, protons, and oxygen gas.

A simplified way to show water splitting is:

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

The products of photolysis have three different roles. The electrons replace those lost from photosystem II. The protons contribute to the proton gradient across the thylakoid membrane. The oxygen gas is released as a waste product.

This point is a common source of exam mistakes. The oxygen gas released during photosynthesis comes from water, not from carbon dioxide. Carbon dioxide is used in the Calvin cycle to build carbohydrates, while water is split during the light-dependent reactions.

> **Exam Warning:**
> Do not say that $$\mathrm{O_2}$$ released by photosynthesis comes from $$\mathrm{CO_2}$$. The released oxygen gas comes from the splitting of water in the light-dependent reactions.

After electrons pass through the first electron transport chain, they reach photosystem I. Light then excites these electrons again in the P700 reaction center. The re-energized electrons move through another, shorter set of carriers. At the end of this pathway, the electrons are transferred to $$\mathrm{NADP^+}$$, along with a proton, forming NADPH. The enzyme NADP+ reductase carries out this reduction.

> **Key Term: NADP+**
> The final electron acceptor in the light-dependent reactions; when it accepts electrons and a proton, it becomes NADPH.

NADPH is a reduced molecule, meaning it carries high-energy electrons and hydrogen. It supplies reducing power to the Calvin cycle. The Calvin cycle uses NADPH to reduce carbon-containing molecules during sugar formation.

In AP multiple-choice questions, $$\mathrm{NADP^+}$$ is the final electron acceptor of the light-dependent reactions. Do not confuse this with cellular respiration, where oxygen is the final electron acceptor in the mitochondrial electron transport chain. Also do not choose $$\mathrm{NAD^+}$$ for photosynthesis; $$\mathrm{NAD^+}$$ is associated with cellular respiration, while $$\mathrm{NADP^+}$$ is used in photosynthesis.

### Proton gradients, chemiosmosis, and ATP production

As electrons move from photosystem II through the electron transport chain, their energy helps move protons across the thylakoid membrane into the thylakoid lumen. Photolysis also adds protons to the lumen. The result is a high concentration of $$\mathrm{H^+}$$ inside the thylakoid lumen compared with the stroma.

This difference in proton concentration is a proton gradient. It stores potential energy because protons tend to move down their concentration gradient. However, the thylakoid membrane is not freely permeable to protons, so they pass mainly through ATP synthase.

> **Key Term: Chemiosmosis**
> The production of ATP using energy from the movement of protons down their gradient through ATP synthase.

ATP synthase allows protons to move from the thylakoid lumen back into the stroma. As protons flow through ATP synthase, the enzyme uses the energy of that movement to join ADP and inorganic phosphate, producing ATP.

> **Key Term: Photophosphorylation**
> The use of light-driven electron flow and a proton gradient to produce ATP from ADP and inorganic phosphate during photosynthesis.

This ATP is produced on the stroma side of the thylakoid membrane, which is useful because the Calvin cycle occurs in the stroma and needs ATP. In the Calvin cycle, ATP provides energy for reduction reactions and for regeneration of RuBP.

A strong membrane is required for chemiosmosis. If the thylakoid membrane is punctured or made leaky, protons can diffuse freely between the lumen and the stroma. The cell can no longer maintain a steep proton gradient, so ATP synthase has less proton flow to use. Light may still be absorbed by pigments, but ATP production by chemiosmosis will be directly reduced.

> **Test Tip:**
> For questions about membrane damage in chloroplasts, look for answer choices involving the proton gradient, chemiosmosis, or ATP synthesis. Light absorption by pigments may still occur, but the gradient cannot be maintained if the membrane is leaky.

Chemiosmosis in chloroplasts is similar in principle to chemiosmosis in mitochondria during cellular respiration. In both cases, electron transport builds a proton gradient, and ATP synthase uses proton flow to make ATP. The locations and electron sources differ: chloroplasts use light-excited electrons and thylakoid membranes, while mitochondria use electrons from food molecules and the inner mitochondrial membrane.

### Products, Calvin cycle links, and cyclic electron flow

The light-dependent reactions produce three main products: ATP, NADPH, and oxygen gas. ATP and NADPH are sent to the Calvin cycle. Oxygen gas is released.

The Calvin cycle returns ADP, inorganic phosphate, and $$\mathrm{NADP^+}$$ to the light-dependent reactions. This exchange makes the two stages interdependent. The light-dependent reactions need ADP and $$\mathrm{NADP^+}$$ to continue making ATP and NADPH. The Calvin cycle needs ATP and NADPH to continue reducing carbon-containing molecules and regenerating RuBP.

A common AP Biology question asks what happens if a plant is kept in the dark for a long time. Without light, the light-dependent reactions stop. ATP and NADPH are no longer produced by those reactions. The Calvin cycle may continue briefly using any remaining ATP and NADPH, but it will stop once those supplies are depleted. This is why “light-independent” does not mean “can run indefinitely without light.” It means the Calvin cycle does not directly require photons at each step, but it depends on products made by the light-dependent reactions.

The inputs and outputs can be summarized in words:

- Light-dependent reactions use light, water, ADP, inorganic phosphate, and $$\mathrm{NADP^+}$$.
- Light-dependent reactions produce oxygen gas, ATP, and NADPH.
- The Calvin cycle uses carbon dioxide, ATP, and NADPH.
- The Calvin cycle returns ADP, inorganic phosphate, and $$\mathrm{NADP^+}$$.

> **Exam Warning:**
> Do not list G3P as a product of the light-dependent reactions. G3P is produced by the Calvin cycle. Do not list oxygen as a product of the Calvin cycle; oxygen is produced when water is split in the light-dependent reactions.

Some plants can also use cyclic electron flow. In cyclic electron flow, electrons from photosystem I pass through electron carriers and return to photosystem I rather than reducing $$\mathrm{NADP^+}$$. This pathway produces ATP but not NADPH and does not release oxygen, because water is not split in the same way as in linear flow.

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

Cyclic electron flow is useful when a cell needs extra ATP relative to NADPH. For most AP Biology questions, linear electron flow is the standard pathway to know in detail: water is split, oxygen is released, ATP is made by chemiosmosis, and $$\mathrm{NADP^+}$$ is reduced to NADPH.

Free-response questions may ask you to predict effects of environmental changes. For example, lower light intensity reduces the excitation of electrons in photosystems, which lowers electron transport, proton gradient formation, ATP production, and NADPH production. In organisms that rely partly on photosynthetic cells or chloroplasts, reduced light can reduce available food production. Similarly, mutations that remove chlorophyll $$a$$ and chlorophyll $$b$$ would reduce light absorption and lower photosynthetic rate, often measured by reduced oxygen production.

When writing AP Biology explanations, link cause and effect clearly. A strong answer does not simply say, “less light means less photosynthesis.” It explains that less light means fewer electrons are excited, causing less electron transport, a weaker proton gradient, less ATP synthesis, less NADPH production, and therefore reduced support for the Calvin cycle.

## Key Point Checklist

This article has covered the following key knowledge points:

- The light-dependent reactions occur in the thylakoid membrane of chloroplasts.
- In photosynthetic prokaryotes, light-dependent reactions occur on infolded plasma membranes.
- Photosystems contain chlorophyll and accessory pigments that absorb photons.
- Photosystem II acts before photosystem I in normal linear electron flow.
- Photolysis splits water to replace electrons in photosystem II.
- Oxygen gas released during photosynthesis comes from water, not carbon dioxide.
- Electron transport helps create a proton gradient across the thylakoid membrane.
- ATP synthase uses proton flow from the thylakoid lumen to the stroma to make ATP.
- $$\mathrm{NADP^+}$$ is the final electron acceptor and is reduced to NADPH.
- The products of the light-dependent reactions are ATP, NADPH, and oxygen gas.
- The Calvin cycle uses ATP and NADPH and returns ADP, inorganic phosphate, and $$\mathrm{NADP^+}$$.
- A damaged thylakoid membrane directly disrupts the proton gradient and ATP production.

## Key Terms and Concepts

- Light-dependent reactions
- Absorption spectrum
- Photosystem
- Chlorophyll
- Accessory pigments
- Photosystem II
- Photosystem I
- Photolysis
- Electron transport chain
- Proton gradient
- Chemiosmosis
- Photophosphorylation
- ATP synthase
- NADP+
- NADPH
- Thylakoid membrane
- Thylakoid lumen
- Stroma
- Calvin cycle
- Cyclic electron flow


Photosynthesis - Light-dependent reactions