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Photosynthesis
Light Reactions & Calvin Cycle

Discover how plants convert light energy into chemical energy, producing the glucose that fuels nearly all life on Earth.

AP Biology

What Is Photosynthesis?

Photosynthesis is the process by which autotrophs (primarily plants, algae, and cyanobacteria) convert light energy into chemical energy stored in glucose. It occurs mainly in the chloroplasts of plant cells.

This single equation conceals two distinct stages: the light reactions (which capture energy) and the Calvin cycle (which builds sugars).

Inside the Chloroplast

Stroma Granum (thylakoid stack) Thylakoid Thylakoid Membrane Light Reactions: Thylakoid Membrane Calvin Cycle: Stroma Outer & Inner Membranes

Key Molecules to Know

ATP
Energy currency of the cell. Stores energy in its phosphate bonds.
NADPH
Electron carrier. Delivers high-energy electrons to the Calvin cycle.
Chlorophyll
Green pigment that absorbs light (mainly red and blue wavelengths).

Two Stages of Photosynthesis

Light Reactions (Thylakoid Membrane) Inputs:
Light, \( \text{H}_2\text{O} \), ADP, \( \text{NADP}^+ \)
 Outputs:
ATP, NADPH, \( \text{O}_2 \)
 ATP NADPH Calvin Cycle (Stroma) Inputs:
\( \text{CO}_2 \), ATP, NADPH
 Outputs:
G3P (→ Glucose), ADP, \( \text{NADP}^+ \)
ADP + \( \text{NADP}^+ \) recycled
 Light

The Light Reactions in Detail

The light reactions occur across two photosystems embedded in the thylakoid membrane. Together, they use light energy to split water, generate a proton gradient, and produce ATP and NADPH.

1
Photosystem II (PSII): Absorbs light at 680 nm. Water is split (\( 2\text{H}_2\text{O} \to 4\text{H}^+ + 4e^- + \text{O}_2 \)), releasing \( \text{O}_2 \) as a byproduct.
2
Electron Transport Chain (ETC): Excited electrons pass through carriers (plastoquinone, cytochrome b6f, plastocyanin), pumping \( \text{H}^+ \) into the thylakoid lumen.
3
Photosystem I (PSI): Absorbs light at 700 nm. Electrons are re-energized and passed to ferredoxin, then to \( \text{NADP}^+ \) reductase, producing NADPH.
4
Chemiosmosis: The \( \text{H}^+ \) gradient drives ATP synthase, producing ATP from ADP + \( \text{P}_i \).

Photosystems & Electron Flow

Thylakoid Membrane
Thylakoid Lumen (\( \text{H}^+ \) accumulates)
 Stroma PS II P680
\( \text{H}_2\text{O} \to \text{O}_2 + \text{H}^+ + e^- \)
 Light
\( e^- \) →
 Cyt b6f
\( \text{H}^+ \) pump
\( e^- \) →
 PS I P700 Light NADPH
\( \text{NADP}^+ + \text{H}^+ + e^- \)
 ATP Synthase ATP
\( \text{H}^+ \) flows through

The Calvin Cycle in Detail

The Calvin cycle uses the ATP and NADPH from the light reactions to fix carbon dioxide into organic molecules. It has three main phases:

1
Carbon Fixation: The enzyme RuBisCO attaches \( \text{CO}_2 \) to a 5-carbon molecule (RuBP), forming two 3-carbon molecules (3-PGA).
2
Reduction: ATP and NADPH convert 3-PGA into G3P (glyceraldehyde-3-phosphate). This is the energy-storing step.
3
Regeneration of RuBP: ATP is used to rearrange the remaining G3P molecules back into RuBP, so the cycle can continue.

Calvin Cycle Diagram

3 \( \text{CO}_2 \)
3 RuBP (5C)
 RuBisCO
6 3-PGA (3C)
 6 ATP 6 NADPH
6 G3P (3C)
 1 net G3P → Glucose 5 G3P recycled 3 ATP 1. Fixation 2. Reduction 3. Regeneration

Light Reactions vs. Calvin Cycle

Feature Light Reactions Calvin Cycle
Location Thylakoid membrane Stroma
Light required? Yes Not directly
Inputs \( \text{H}_2\text{O} \), light, \( \text{NADP}^+ \), ADP \( \text{CO}_2 \), ATP, NADPH
Outputs \( \text{O}_2 \), ATP, NADPH G3P (→ glucose), ADP, \( \text{NADP}^+ \)
Energy conversion Light → chemical (ATP/NADPH) Chemical → organic (glucose)
Key enzyme Photosystems I & II RuBisCO

Why Photosynthesis Matters

\( \text{O}_2 \) Production
Photosynthesis generates virtually all atmospheric oxygen that aerobic organisms breathe.
Food Webs
Glucose from photosynthesis is the base of nearly every food chain on Earth.
Carbon Cycling
Plants remove \( \text{CO}_2 \) from the atmosphere, playing a critical role in regulating climate.

How to Trace Photosynthesis Step-by-Step

1
Identify the stage: Is the question asking about the light reactions (thylakoid) or the Calvin cycle (stroma)?
2
List inputs and outputs: Write down what goes in and what comes out of each stage.
3
Follow the electrons: Track electron flow from \( \text{H}_2\text{O} \) → PS II → ETC → PS I → NADPH.
4
Follow the carbon: Track \( \text{CO}_2 \) → 3-PGA → G3P → glucose through the Calvin cycle.
5
Connect the stages: ATP and NADPH link the light reactions to the Calvin cycle. ADP and \( \text{NADP}^+ \) are recycled back.

Example: Counting the Carbons

Problem: If 3 molecules of \( \text{CO}_2 \) enter the Calvin cycle, how many carbon atoms are involved in each phase?

1
Carbon fixation: 3 \( \text{CO}_2 \) (each 1C) combine with 3 RuBP (each 5C) = 3 + 15 = 18 carbons total, forming 6 molecules of 3-PGA (each 3C = 18C).
2
Reduction: 6 × 3-PGA (18C) are reduced to 6 × G3P (still 18C). No carbon is gained or lost here.
3
Output: 1 G3P (3C) exits the cycle. The remaining 5 G3P (15C) are rearranged back into 3 RuBP (15C).

Example: ATP and NADPH Budget

Problem: How many ATP and NADPH are needed to produce one molecule of glucose?

Per turn ATP NADPH
Reduction (3-PGA → G3P) 2 2
Regeneration (G3P → RuBP) 1 0
Total per turn 3 2
Total for 6 turns 18 12

Example: What Happens Without Light?

Scenario: A plant is suddenly moved into complete darkness. Predict the immediate effects on the concentrations of key molecules.

ATP/NADPH
Decrease. Light reactions stop, so no new ATP or NADPH is made. The Calvin cycle continues briefly, consuming remaining stores.
3-PGA
Increases initially. RuBisCO still fixes \( \text{CO}_2 \), producing 3-PGA, but reduction to G3P stalls (no NADPH).
G3P
Decreases. Without ATP/NADPH, 3-PGA cannot be reduced to G3P. Existing G3P is depleted.

Common Mistakes

✕ "Calvin cycle is a dark reaction"
The Calvin cycle doesn't require light directly, but it depends on ATP and NADPH from the light reactions. It stops soon after light stops.
✕ "\( \text{O}_2 \) comes from \( \text{CO}_2 \)"
The \( \text{O}_2 \) released in photosynthesis comes from the splitting of water (photolysis) in PS II, not from carbon dioxide.
✕ "Glucose is the direct product"
The Calvin cycle directly produces G3P, not glucose. Two G3P molecules are later combined outside the cycle to make one glucose.

Question 1 of 5

Where do the light reactions of photosynthesis take place?

Question 2 of 5

What is the source of the oxygen released during photosynthesis?

Question 3 of 5

Which enzyme catalyzes the carbon fixation step of the Calvin cycle?

Question 4 of 5

How many turns of the Calvin cycle are required to produce one molecule of glucose?

Question 5 of 5

If a plant is suddenly placed in complete darkness, what happens to the concentration of 3-PGA in the short term?