AP Biology photosynthesis: Light reactions, Calvin cycle and C3/C4/CAM

Photosynthesis sits in Unit 3 of AP (Advanced Placement) Biology, Cellular Energetics, worth 12-16% of the exam. It shows up on FRQs (free-response questions) almost as often as respiration, and it tests the same set of skills: Pathway recall, location of stages, energy transfer, and the ability to predict what happens when a step is blocked.

This guide covers the light-dependent reactions, the Calvin cycle, and the three plant adaptations (C3, C4 and CAM), plus the FRQ angles that come up most often. If you want structured content to match, Cognito's AP Biology notes walk through photosynthesis in short video lessons.

The overall reaction for photosynthesis:

6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2

Carbon dioxide and water are the inputs, with light energy driving the process. Glucose and oxygen are the outputs. Notice this is the reverse of aerobic respiration on paper, but the mechanism is completely different, using entirely different enzymes and locations.

Photosynthesis happens in chloroplasts, which are organelles found in plant leaf cells and algae. A chloroplast has an outer membrane, an inner membrane, and inside that a network of flattened membrane sacs called thylakoids. Stacks of thylakoids are called grana (singular: granum). The fluid outside the thylakoids but inside the inner membrane is the stroma.

Location matters for the AP: The light reactions happen in the thylakoid membrane, and the Calvin cycle happens in the stroma. That split is examinable.

The light reactions happen in the thylakoid membrane and require light. Their purpose is to convert light energy into chemical energy in the form of ATP (adenosine triphosphate) and NADPH, which the Calvin cycle then uses to build glucose.

The process starts at Photosystem II (PSII), where chlorophyll absorbs light and becomes excited. Excited electrons are passed to an electron transport chain in the thylakoid membrane. To replace the lost electrons, PSII splits water (photolysis): 2 H2O → 4 H+ + 4 electrons + O2. This is where the oxygen you breathe comes from.

As electrons flow down the transport chain from PSII to Photosystem I (PSI), the energy released is used to pump H+ from the stroma into the thylakoid lumen, building a proton gradient across the thylakoid membrane. At PSI, electrons get another light boost and are eventually used to reduce NADP+ to NADPH.

The H+ gradient drives ATP synthesis via ATP synthase, protons flow back into the stroma through ATP synthase, and the enzyme phosphorylates ADP to ATP. This is chemiosmosis, the same mechanism as in respiration but in a different membrane and with the gradient built the opposite way.

Outputs of the light reactions: ATP, NADPH, O2 (byproduct).

The Calvin cycle happens in the stroma and uses the ATP and NADPH from the light reactions to fix CO2 into glucose. It doesn't need light directly (that's why it's called light-independent), but it does need the products of the light reactions, so it stops in the dark once ATP and NADPH run out.

The cycle has three phases:

Carbon fixation: CO2 combines with a 5-carbon molecule called RuBP (ribulose bisphosphate) in a reaction catalyzed by the enzyme rubisco. The resulting 6-carbon molecule immediately splits into two 3-carbon molecules of 3-PGA (3-phosphoglycerate). This is the actual moment carbon enters the organic world from the atmosphere.

Reduction: Each 3-PGA molecule is phosphorylated by ATP and reduced by NADPH to form G3P (glyceraldehyde-3-phosphate), a 3-carbon sugar. This is where the ATP and NADPH from the light reactions get spent.

Regeneration of RuBP: Most of the G3P is used to regenerate RuBP so the cycle can continue, again using ATP. Only a small fraction of G3P leaves the cycle, but that's the useful sugar that gets built into glucose, sucrose and starch.

Stoichiometry to know: 3 turns of the cycle fix 3 CO2 and produce 1 G3P (net). 6 turns produce 2 G3P, which combine to form 1 glucose.

About 85% of plant species are C3 plants, so-called because the first stable product of carbon fixation is a 3-carbon molecule (3-PGA). C3 photosynthesis works well in cool, moist conditions but suffers in hot, dry environments, when it's hot, plants close their stomata to conserve water, CO2 levels inside the leaf drop, O2 levels rise, and rubisco starts binding O2 instead of CO2. This process (photorespiration) wastes fixed carbon and drops photosynthetic efficiency.

C4 plants (corn, sugarcane, sorghum) evolved a workaround. They first fix CO2 into a 4-carbon molecule (oxaloacetate) in the mesophyll cells using a different enzyme (PEP carboxylase), which doesn't bind O2. The 4-carbon molecule is then transported into bundle-sheath cells, where it releases CO2 for rubisco to use in a concentrated environment. This spatial separation minimizes photorespiration and works well in hot, sunny climates.

CAM plants (cacti, succulents, pineapples) do the same trick but separate the two steps in TIME rather than space. They open their stomata at night to take in CO2 and fix it into a 4-carbon molecule, store it, then run the Calvin cycle during the day with stomata closed. This lets them conserve water in extreme desert conditions.

A few question patterns come up repeatedly on released AP FRQs, and knowing them ahead of time helps you spot what the graders want.

Light vs dark scenarios: You're told a plant is placed in the dark, or the light is turned off partway through an experiment. Predict what happens to ATP, NADPH, RuBP, G3P and 3-PGA concentrations. Reasoning: Without light, ATP and NADPH stop being made, so the reduction step of Calvin can't run, so G3P production halts. 3-PGA accumulates (because carbon fixation keeps going briefly), then also stops. RuBP is depleted because it isn't being regenerated.

Inhibitor questions: A specific step of photosynthesis is blocked (rubisco inhibitor, ATP synthase inhibitor, photosystem inhibitor). Predict the effect on outputs. Same logic as respiration inhibitor questions, trace the pathway downstream.

Leaf disk floating assay: A common lab-based FRQ. Leaf disks are submerged in bicarbonate solution and exposed to light. As they photosynthesize, they produce O2 bubbles that make them float. You're given a data table of time-to-float under different light intensities or temperatures and asked to graph, describe and explain.

Comparative environments: You're given two plant species in different climates and asked which is likely C3, C4 or CAM, with justification. Anchor your reasoning in stomatal opening times, photorespiration risk and water availability. Our AP Biology FRQ guide walks through the complete rubric structure and scoring strategy across all question types.

Plants do both photosynthesis and cellular respiration, all the time. In the light, photosynthesis rate exceeds respiration rate, so the plant nets O2 output and glucose accumulation. In the dark, only respiration runs, so the plant nets CO2 output.

The compensation point is the light intensity at which the two rates are equal, and net gas exchange is zero. Below the compensation point, the plant is a net CO2 producer; above it, a net O2 producer. FRQs sometimes give you a graph of gas exchange vs light intensity and ask you to identify the compensation point and justify what's happening on either side.

Structurally, the two processes mirror each other in useful ways. Both involve electron transport chains, proton gradients across a membrane, and ATP synthesis by chemiosmosis. Both use redox reactions with NAD+/NADH (respiration) or NADP+/NADPH (photosynthesis). Both happen in double-membrane organelles thought to descend from ancient bacteria (endosymbiotic theory). Knowing these parallels makes both pathways easier to remember, and it's a common Argumentation-type FRQ.

Photosynthesis rewards diagram work more than reading. Draw a chloroplast with labels for thylakoid membrane, stroma and lumen, then map the light reactions onto the membrane and the Calvin cycle onto the stroma. Do it three times over a week without looking between attempts.

Pair the diagram work with 3-4 released FRQs from AP Central on photosynthesis. Score against the official rubric with a highlighter, marking every point you actually hit. This calibrates how specific your vocabulary needs to be, "light provides energy" is not a rubric point; "photons excite electrons in PSII, which are passed to an ETC that pumps H+ into the lumen" is. Our three-month AP Biology study plan integrates this kind of targeted practice with content review, timed for the fall and spring semester breakdown.

Cognito's AP Biology course covers photosynthesis alongside respiration and enzyme kinetics with animated pathway diagrams, which is useful if you learn better by watching the process than by reading it. If you're taking multiple AP sciences, Cognito's AP hub covers Chemistry and Environmental Science too.