AP BIOLOGY · UNIT 3

Photosynthesis

// Energy Conversion in Plants

The process by which autotrophs convert solar energy into chemical energy. Occurs in two coupled stages: the Light-Dependent Reactions (thylakoid membrane) generate ATP and NADPH, which the Calvin Cycle (stroma) uses to fix CO₂ into sugar.

Location: Chloroplast Inputs: CO₂, H₂O, Light Outputs: Glucose, O₂

01 The Net Equation

The balanced overall equation for photosynthesis. Tap or hover each term to see exactly where and how it is used.

Overall Photosynthesis Reaction (per 1 glucose)

6 CO₂ + 6 H₂O + Light Energy → C₆H₁₂O₆ + 6 O₂

Tap a term above to see its role in the process.

ΔG = −686 kcal/mol (endergonic — energy stored)

⚡ Key insight

The oxygen we breathe comes entirely from water, not CO₂. This was proven with isotope-labeling experiments using ¹⁸O-labeled water.

02 Chloroplast Structure

Understanding where each stage occurs requires knowledge of chloroplast anatomy. Click any labeled region below.

Click any labeled region to learn about that structure.

Location A

Thylakoid Membrane

Site of the light-dependent reactions. Protein complexes (PSII, Cyt b6f, PSI, ATP synthase) are embedded here. Stacking into grana maximizes surface area for light absorption.

Location B

Stroma

Fluid-filled space surrounding the thylakoids. Site of the Calvin Cycle. Contains Rubisco, enzymes, DNA (chloroplasts are semi-autonomous), ribosomes, and starch granules.

📍 AP Exam Tip

Thylakoid lumen = inside the thylakoid disk. This is where H⁺ accumulates to drive ATP synthase. If asked which direction protons flow, they move from lumen → stroma through ATP synthase.

03 Photosynthetic Pigments

Plants use multiple pigments to capture a wider range of the visible spectrum. Each absorbs certain wavelengths and reflects others (which we see as color).

Visible Spectrum (400–700 nm)

400 nm (Violet)450500550600650700 nm (Red)

Chlorophyll a — primary pigment (P680 / P700 reaction centers)

Absorbs: Blue-violet (~430 nm) + Red (~680 nm) · Reflects: Green (appears green)

Chlorophyll b — accessory pigment, expands light capture

Absorbs: Blue (~453 nm) + Red-orange (~642 nm) · Reflects: Yellow-green

Carotenoids (β-carotene, xanthophylls) — accessory + photoprotection

Absorbs: Blue-green (400–500 nm) · Reflects: Yellow/orange. Also quenches excess light energy (photoprotection).

📍 Paper Chromatography

Pigments are separated by paper chromatography. Less polar pigments (carotenoids) travel farthest. Chlorophylls travel less far. Rf = distance of pigment / distance of solvent front.

Antenna Complex

Light-Harvesting

Hundreds of accessory pigment molecules absorb photons and funnel energy via resonance energy transfer toward the reaction center chlorophyll (P680 or P700). This dramatically increases the effective cross-section for light capture.

Action Spectrum

Most Effective Wavelengths

Engelmann's experiment with Spirogyra and aerobic bacteria showed highest O₂ production (highest photosynthesis rate) in blue-violet and red wavelengths — matching chlorophyll's absorption spectrum.

04 The Light-Dependent Reactions

Occur in the thylakoid membrane. Two photosystems work in series (the "Z-scheme") to transfer electrons from water to NADP⁺, generating ATP and NADPH along the way. Step through the reaction below.

Light Reactions Net Equation (per 2 H₂O, 2 NADP⁺)

2 H₂O + 2 NADP⁺ + ~3 ADP + 3 Pᵢ + Light → 2 NADPH + ~3 ATP + O₂

Z-Scheme: Linear (Non-Cyclic) Electron Flow

H⁺ Pumped to Lumen

ATP Yield

NADPH Yield

O₂ Released

THYLAKOID LUMEN (high [H⁺])

STROMA (low [H⁺])

PSII
P680

Cyt
b6f

PSI
P700

ATP
Synth

Photosystem II (P680)
Ready to receive light. When a photon strikes, P680 oxidizes H₂O via the OEC → ½ O₂ + 2 H⁺ + 2 e⁻. Electrons are raised to high energy and passed to pheophytin.

Cyclic Electron Flow

PSI → Cyt b6f → PSI

When the cell needs more ATP but not NADPH, electrons cycle from PSI back through Cyt b6f, pumping more H⁺ and producing extra ATP. No NADPH or O₂ is produced. Common in C4 mesophyll cells.

Chemiosmosis

The Proton Gradient

H⁺ accumulates in the thylakoid lumen (from water splitting + Cyt b6f pumping). The proton-motive force drives H⁺ through ATP synthase (CF₀CF₁) back into the stroma, synthesizing ATP. ~4 H⁺ per ATP.

Stoichiometry per 6 CO₂ fixed (1 Glucose equivalent)

Water split: 12 H₂O → 6 O₂ + 24 H⁺ + 24 e⁻

Electron carriers: 24 e⁻ → 12 NADPH

Proton gradient: ~48 H⁺ through ATP synthase → ~18 ATP

📍 AP Exam Tip

The thylakoid lumen becomes acidic (low pH) during the light reactions because H⁺ is pumped in. This pH gradient is the driving force for ATP synthesis. Disrupting the membrane (detergent) destroys the gradient and stops ATP production.

05 The Calvin Cycle (Dark Reactions)

Occurs in the stroma. This cycle is not directly driven by light, but uses the ATP and NADPH produced by the light reactions. To produce 1 glucose, the cycle must run 6 times (fixing 6 CO₂).

Calvin Cycle Net Equation (3 CO₂ → 1 G3P, per single turn)

3 CO₂ + 3 RuBP + 9 ATP + 6 NADPH → 1 G3P + 3 RuBP + 9 ADP + 9 Pᵢ + 6 NADP⁺

× 6 turns for 1 glucose: consumes 18 ATP + 12 NADPH total

Inputs per turn: 9 ATP Inputs per turn: 6 NADPH 3 CO₂ fixed per turn Output: 1 G3P (net)

PHASE · 01 OF 03

Carbon Fixation

The enzyme Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzes the binding of CO₂ to RuBP (a 5-carbon sugar). This forms a highly unstable 6-carbon compound that immediately splits into two 3-PGA (3-phosphoglycerate) molecules. This is why C3 plants are named — their first product is a 3-carbon molecule.

3 CO₂ enter 3 RuBP consumed → 6 × 3-PGA

🔬 Rubisco — Most Abundant Protein on Earth

Rubisco is the rate-limiting enzyme of photosynthesis. It is remarkably slow (~3 reactions/sec) and can accidentally fix O₂ instead of CO₂ (photorespiration). C4 and CAM plants evolved mechanisms to concentrate CO₂ around Rubisco, largely eliminating this inefficiency.

Full Cycle Stoichiometry (per 6 CO₂ → 1 Glucose)

Input: 6 CO₂ + 12 H₂O → 6 G3P (of which 5 regenerate RuBP, 1 exits)

Cost: 18 ATP + 12 NADPH consumed total

2 G3P → 1 Glucose (C₆H₁₂O₆)

Stage	Molecules In	Energy Used	Molecules Out
Fixation (×3)	3 CO₂ + 3 RuBP	—	6 × 3-PGA
Reduction (×6)	6 × 3-PGA	6 ATP + 6 NADPH	6 × G3P
Regeneration (×5)	5 × G3P	3 ATP	3 × RuBP
Net (1 turn)	3 CO₂	9 ATP + 6 NADPH	1 G3P
Net (glucose)	6 CO₂	18 ATP + 12 NADPH	2 G3P → Glucose

06 C4 & CAM Plants

Adaptations that concentrate CO₂ around Rubisco to minimize photorespiration, especially in hot, dry, or high-light environments.

⚠️ Photorespiration

When O₂ levels are high and CO₂ levels are low (common on hot days when stomata close), Rubisco binds O₂ instead of CO₂. This wastes energy, releases CO₂ already fixed, and produces no sugar. Can reduce C3 plant productivity by up to 25–50% in hot climates.

STANDARD PHOTOSYNTHESIS

C3 Plants

First product: 3-PGA (3C) Prone to photorespiration Examples: Wheat, rice, soybean, most trees

CO₂ is fixed directly by Rubisco in mesophyll cells into 3-carbon 3-PGA. Stomata open during the day, enabling CO₂ uptake but also water loss. In cool, moist environments, C3 plants thrive — photorespiration is minimal when CO₂ concentration is relatively high.

07 Factors Affecting the Rate

Three primary environmental factors limit photosynthesis. At any moment, the most limiting factor controls the rate (Blackman's Law of Limiting Factors). Drag the sliders to see the simulated effect.

Interactive Rate Simulator

Light Intensity

CO₂ Concentration

Temperature (10–40°C)

25°C

Relative Photosynthesis Rate

% of maximum

Limiting factor: CO₂ Concentration

Light

Intensity & Quality

More photons = more electron excitation. Rate plateaus at the light saturation point. Red + blue wavelengths most effective. Too much light → photooxidation damage.

CO₂

Concentration

More CO₂ → more carbon fixation by Rubisco → more G3P. Currently atmospheric CO₂ (~420 ppm) is often limiting for C3 plants. Rising CO₂ can boost C3 growth.

Temperature

Enzyme Kinetics

Affects enzyme activity (Calvin cycle enzymes). Optimal ~25–30°C. Too hot → Rubisco + others denature. Too cold → enzyme activity slows. High temp also increases photorespiration in C3 plants.

08 Photosynthesis vs Cellular Respiration

These two processes are essentially the reverse of each other at the net level, but use completely different pathways, enzymes, and compartments.

Feature	Photosynthesis	Cellular Respiration
Net equation	6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂	C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Location	Chloroplast (thylakoid + stroma)	Cytoplasm + Mitochondria
Energy direction	Stores energy (endergonic)	Releases energy (exergonic)
O₂ role	Produced (photolysis of water)	Consumed (final electron acceptor)
CO₂ role	Fixed (Calvin cycle input)	Released (Krebs cycle output)
Electron carriers	NADPH → builds sugars	NADH, FADH₂ → drive ETC
ATP	Consumed in Calvin Cycle	Net produced (~30–32 per glucose)
Proton gradient	Thylakoid lumen → stroma	Intermembrane space → matrix
Occurs in	Plants, algae, some prokaryotes	All living organisms

09 AP Exam Key Concepts

High-frequency AP Bio exam topics related to photosynthesis. Know each of these cold.

Must Know #1

18 ATP + 12 NADPH

Required to synthesize one glucose (2 G3P) in the Calvin Cycle. Per single turn: 9 ATP + 6 NADPH per 3 CO₂ fixed.

Must Know #2

O₂ comes from H₂O

Confirmed by ¹⁸O isotope tracing. Oxygen from CO₂ ends up in G3P. This is a classic experimental design question.

Must Know #3

Rubisco fixes CO₂

Rubisco is the world's most abundant enzyme but is slow and inefficient. It can fix O₂ (photorespiration) when CO₂ is low. C4/CAM plants evolved to minimize this.

Must Know #4

PSII before PSI

Despite numerical order, PSII acts first in the Z-scheme. PSII splits water; PSI reduces NADP⁺. If PSII is blocked, linear electron flow stops.

Must Know #5

Chemiosmosis

The proton gradient across the thylakoid membrane drives ATP synthase. This is the same mechanism in mitochondria — just in reverse direction and a different compartment.

Must Know #6

C4 spatial separation

C4 plants fix CO₂ into OAA (4C) in mesophyll cells, then release CO₂ in bundle sheath cells (spatial). CAM does the same thing temporally (night vs day).

Must Know #7

G3P is the key sugar

G3P (glyceraldehyde-3-phosphate) is the direct product of the Calvin Cycle. It's used to make glucose, fructose, starch, cellulose, amino acids, and fatty acids.

Must Know #8

Leaf disk experiments

Floating leaf disk assays measure photosynthesis rate by tracking how quickly air-evacuated discs rise in a bicarbonate solution as O₂ is produced.

10 AP Exam Practice

Score: 0 / 12

Select the best answer for each question. Explanations appear after answering.

QUESTION 01 / 12

Which protein complex oxidizes water and directly releases O₂ during the light reactions?

Photosystem II contains the Oxygen-Evolving Complex (OEC), which catalyzes: 2H₂O → O₂ + 4H⁺ + 4e⁻. The electrons replace the ones lost when P680 is excited by light. ATP synthase makes ATP, PSI reduces NADP⁺, and Cyt b6f pumps H⁺ but does not split water.

QUESTION 02 / 12

How many total ATP molecules are consumed by the Calvin Cycle to produce one glucose molecule (C₆H₁₂O₆)?

Per Calvin Cycle turn (3 CO₂): 6 ATP for reduction of 3-PGA → G3P, + 3 ATP for regeneration of RuBP = 9 ATP per turn. For 1 glucose (6 CO₂, which requires 2 turns): 9 × 2 = 18 ATP total. Also 12 NADPH are consumed.

QUESTION 03 / 12

In the Calvin Cycle, what is the 3-carbon molecule produced when CO₂ is fixed by Rubisco and RuBP is cleaved?

3-PGA (3-phosphoglycerate) is the first stable product of carbon fixation (why the plants are called "C3"). It is then reduced using ATP and NADPH to produce G3P. G3P is produced later in Phase 2 (Reduction). OAA is the initial product in C4 plants, not C3.

QUESTION 04 / 12

Where exactly does the Calvin Cycle take place within the chloroplast?

The stroma is the fluid-filled space surrounding the thylakoids. It contains Rubisco, the enzymes for all three phases of the Calvin Cycle, plus chloroplast DNA and ribosomes. The thylakoid membrane is where light reactions occur.

QUESTION 05 / 12

Which of the following correctly describes the direction of H⁺ flow through ATP synthase during the light reactions?

H⁺ (protons) accumulate in the thylakoid lumen (high [H⁺]) from two sources: water splitting at PSII and active pumping by Cyt b6f. Protons then flow DOWN their concentration gradient through ATP synthase into the stroma (low [H⁺]), driving ATP synthesis. This is chemiosmosis.

QUESTION 06 / 12

A researcher inhibits Photosystem I only. Which of the following outcomes is most likely?

PSI reduces NADP⁺ to NADPH using ferredoxin (Fd) and FNR. Blocking PSI halts NADPH production. O₂ is still released (PSII still splits water). Some ATP can still be made from the H⁺ gradient established by PSII → Cyt b6f. RuBP regeneration requires ATP + NADPH, so Calvin Cycle will eventually fail, but that's indirect.

QUESTION 07 / 12

What is the primary advantage of C4 photosynthesis over C3 photosynthesis in hot, sunny climates?

C4 plants use a CO₂ concentrating mechanism: PEP carboxylase (which has a higher affinity for CO₂ than O₂) fixes CO₂ into OAA in mesophyll cells, then CO₂ is released near Rubisco in bundle sheath cells. This maintains a high CO₂:O₂ ratio around Rubisco, suppressing photorespiration. CAM plants fix CO₂ at night (temporal separation).

QUESTION 08 / 12

An experiment uses ¹⁸O-labeled CO₂ and normal H₂O. Where would the ¹⁸O label appear in the products?

The O₂ released during photosynthesis comes from H₂O (photolysis at PSII), not CO₂. The ¹⁸O from CO₂ is incorporated by Rubisco into 3-PGA and ultimately into glucose (G3P). This classic isotope tracing experiment, using ¹⁸O-labeled water, confirmed that O₂ comes from water.

QUESTION 09 / 12

During cyclic electron flow, which product(s) are made?

In cyclic electron flow, electrons from PSI are recycled back to Cyt b6f (instead of going to NADP⁺). This pumps more H⁺, generating more ATP — but no NADPH and no O₂ are produced. This mode increases the ATP:NADPH ratio when extra ATP is needed (e.g., C4 plants, CalvinCycle when ATP runs low).

QUESTION 10 / 12

A plant is exposed to only green light. What do you predict about its photosynthesis rate?

Plants appear green because chlorophyll reflects green light (~500–560 nm) rather than absorbing it. The action spectrum shows that photosynthesis is most efficient at blue (~430 nm) and red (~680 nm) wavelengths. Green light produces a very low (but not zero — carotenoids absorb some) rate of photosynthesis.

QUESTION 11 / 12

Which molecule serves as the CO₂ acceptor in the Calvin Cycle, and how many carbons does it have?

RuBP (ribulose-1,5-bisphosphate) is a 5-carbon molecule. Rubisco adds CO₂ to RuBP to form an unstable 6C intermediate, which splits into two 3-PGA molecules. RuBP is regenerated in Phase 3 of the Calvin Cycle. OAA is the 4C product in C4 plants; G3P and 3-PGA are Calvin Cycle products.

QUESTION 12 / 12

A scientist adds a chemical that makes thylakoid membranes freely permeable to H⁺. What is the most likely result?

ATP synthesis by ATP synthase depends on the proton gradient (H⁺ concentration difference) across the thylakoid membrane. Making the membrane permeable to H⁺ eliminates this gradient (H⁺ flows freely without going through ATP synthase), so ATP synthesis stops. PSII still splits water → O₂ is still released. NADPH production via linear electron flow continues. This is how protonophore uncouplers work.