Photosynthesis

Photosynthesis — Light Reactions

Part 1 of 7

Photosynthesis is the process by which autotrophs — primarily plants, algae, and cyanobacteria — convert light energy into chemical energy stored in glucose. It is the ultimate source of virtually all organic matter and molecular oxygen on Earth.

The overall equation is:

$6\,\text{CO}_2 + 6\,\text{H}_2\text{O} + \text{light energy} \longrightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\,\text{O}_2$

Photosynthesis occurs in two main stages:

1. Light-dependent reactions (the "photo" part) — occur in the thylakoid membrane
2. Light-independent reactions / Calvin cycle (the "synthesis" part) — occur in the stroma

This lesson focuses on the light-dependent reactions.

Chloroplast Anatomy

The chloroplast is a double-membrane organelle with its own DNA (evidence of endosymbiotic origin):

Connection to Respiration: The chloroplast uses chemiosmosis in much the same way as the mitochondrion — protons are pumped across a membrane to create a gradient, and ATP synthase uses the gradient to make ATP. However, in chloroplasts, protons accumulate in the thylakoid lumen (instead of the intermembrane space), and ATP is produced in the stroma (instead of the matrix).

Checkpoint — Chloroplast Structure

Light and Pigments

Visible light is a small portion of the electromagnetic spectrum with wavelengths from about 380-750 nm. When light strikes a pigment molecule, certain wavelengths are absorbed (their energy excites electrons) while others are reflected or transmitted (which gives the pigment its visible color).

Key photosynthetic pigments:

The absorption spectrum shows which wavelengths a pigment absorbs. The action spectrum shows the rate of photosynthesis at each wavelength. These two spectra closely match, confirming that light absorption drives photosynthesis.

Photoexcitation: When a chlorophyll molecule absorbs a photon, an electron is boosted from a ground state to an excited state. This excited electron can follow three fates:

1. Return to ground state, releasing energy as heat or fluorescence
2. Transfer energy to a neighboring pigment (resonance energy transfer) — this is how the antenna complex funnels energy
3. Be transferred to an electron acceptor, initiating the light reactions (this occurs at the reaction center)

Checkpoint — Light and Pigments

The Light-Dependent Reactions: Overview

The light reactions use light energy to produce ATP and NADPH, which then power the Calvin cycle. They also split water and release O$_2$ as a byproduct.

The process involves linear (noncyclic) electron flow through two photosystems:

$\text{H}_2\text{O} \xrightarrow{\text{PSII}} \text{PQ} \xrightarrow{\text{Cyt b6f}} \text{PC} \xrightarrow{\text{PSI}} \text{Fd} \rightarrow \text{NADP}^+ \rightarrow \text{NADPH}$

1. Photosystem II (PSII): Light excites electrons in the P680 reaction center. The electrons are passed to the primary electron acceptor. The "hole" left behind is filled by electrons from water splitting (photolysis): $2\text{H}_2\text{O} \rightarrow 4\text{H}^+ + 4e^- + \text{O}_2$
2. Electron Transport Chain: Electrons flow from PSII through plastoquinone (PQ), the cytochrome b6f complex (which pumps H$^+$ into the thylakoid lumen), and plastocyanin (PC)
3. Photosystem I (PSI): Light re-energizes electrons at the P700 reaction center. Electrons pass through ferredoxin (Fd) to NADP$^+$ reductase
4. NADPH production: NADP$^+$ reductase reduces NADP$^+$ to NADPH

Chemiosmosis in Chloroplasts:

As H$^+$ accumulates in the thylakoid lumen (from water splitting and proton pumping by cytochrome b6f), it flows back into the stroma through ATP synthase, driving ATP production.

Key Terms — Light Reactions

Match the Light Reaction Component

Exit Ticket — Light Reactions

Photosystems in Detail

Part 2 of 7

The two photosystems — PSII and PSI — are sophisticated multi-protein complexes embedded in the thylakoid membrane. They work in series during linear electron flow to produce both ATP and NADPH, or PSI can operate alone in cyclic electron flow to produce only ATP.

Understanding the structure and function of each photosystem is essential for explaining how light energy is converted to chemical energy.

Photosystem II (PSII) — Water Splitting

Despite its name, PSII acts first in linear electron flow (it was named "II" because it was discovered second).

Structure:

• Antenna complex (LHC II): ~250 chlorophyll and carotenoid molecules that absorb photons and funnel energy to the reaction center
• P680 reaction center: A pair of chlorophyll a molecules with peak absorption at 680 nm
• Oxygen-evolving complex (OEC): A manganese-containing cluster (Mn$_4$CaO$_5$) that catalyzes water splitting
• Primary electron acceptor (pheophytin): Accepts the excited electron from P680

Function:

1. A photon is absorbed by the antenna complex and energy is funneled to P680
2. An electron in P680 is excited and transferred to pheophytin (the reaction center is now oxidized: P680$^+$)
3. P680$^+$ is the strongest biological oxidizing agent — it pulls electrons from water via the OEC
4. Water splitting: $2\text{H}_2\text{O} \rightarrow 4\text{H}^+ + 4e^- + \text{O}_2$ (occurs in the lumen)
5. The excited electron passes to plastoquinone (PQ) and onward through the ETC

Key Fact: The O$_2$ we breathe is a byproduct of PSII splitting water. Every O$_2$ molecule requires two water molecules and four photons of light.

Photosystem I (PSI) — NADPH Production

Structure:

• Antenna complex (LHC I): ~200 pigment molecules
• P700 reaction center: A pair of chlorophyll a molecules with peak absorption at 700 nm
• Primary electron acceptor (A0): A modified chlorophyll molecule

Function:

1. A photon energizes P700, and an electron is transferred to the primary acceptor
2. The "hole" in P700$^+$ is filled by an electron arriving from PSII via plastocyanin (PC)
3. The excited electron passes through a series of iron-sulfur (Fe-S) proteins to ferredoxin (Fd)
4. Ferredoxin-NADP$^+$ reductase (FNR) transfers electrons from ferredoxin to NADP$^+$:

$\text{NADP}^+ + 2e^- + \text{H}^+ \longrightarrow \text{NADPH}$

NADPH is produced on the stroma side of the membrane, where it will be used by the Calvin cycle.

Checkpoint — Photosystems

Cyclic Electron Flow

In addition to linear electron flow, PSI can operate independently in cyclic electron flow:

$\text{PSI} \rightarrow \text{Fd} \rightarrow \text{Cyt b6f} \rightarrow \text{PC} \rightarrow \text{PSI}$

In cyclic flow:

• Electrons from PSI are passed to ferredoxin
• Instead of going to NADP$^+$, ferredoxin passes them back to the cytochrome b6f complex
• Cytochrome b6f pumps H$^+$ into the thylakoid lumen (contributing to the proton gradient)
• Electrons return to PSI via plastocyanin

Products of cyclic electron flow:

• ATP (via chemiosmosis) $\checkmark$
• NADPH ✗ (electrons return to PSI, not to NADP$^+$)
• O$_2$ ✗ (no water splitting — PSII is not involved)

Why is cyclic electron flow important?

The Calvin cycle consumes ATP and NADPH in a ratio of 3:2. Linear electron flow produces approximately equal amounts of each. Cyclic electron flow supplements the ATP supply to maintain the correct 3:2 ratio.

It also plays a role in photoprotection — under high light conditions, cyclic flow can dissipate excess energy safely.

Checkpoint — Cyclic vs. Linear Electron Flow

Key Terms — Photosystems

Exit Ticket — Photosystems

The Calvin Cycle — Carbon Fixation

Part 3 of 7

The Calvin cycle (named after Melvin Calvin, who traced its steps using radioactive $^{14}\text{C}$) uses the ATP and NADPH produced by the light reactions to fix atmospheric CO$_2$ into organic molecules. It occurs in the stroma of the chloroplast and does not directly require light — though it depends on the light reactions for its energy inputs.

The Three Phases of the Calvin Cycle

The Calvin cycle can be divided into three phases. For each molecule of CO$_2$ fixed, the cycle uses 3 ATP and 2 NADPH. Three complete turns of the cycle fix 3 CO$_2$ and produce one net molecule of glyceraldehyde-3-phosphate (G3P).

Phase 1: Carbon Fixation

The enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzes the attachment of CO$_2$ to the 5-carbon sugar RuBP (ribulose-1,5-bisphosphate), forming an unstable 6-carbon intermediate that immediately splits into two molecules of 3-PGA (3-phosphoglycerate):

$\text{CO}_2 + \text{RuBP (5C)} \longrightarrow 2 \times \text{3-PGA (3C)}$

RuBisCO is the most abundant protein on Earth, comprising up to 50% of leaf protein.

Phase 2: Reduction

3-PGA is phosphorylated by ATP and then reduced by NADPH to produce G3P (glyceraldehyde-3-phosphate):

$\text{3-PGA} \xrightarrow{\text{ATP}} \text{1,3-BPG} \xrightarrow{\text{NADPH}} \text{G3P}$

This is the step that converts the energy of ATP and NADPH into the chemical bonds of an organic molecule.

Phase 3: Regeneration of RuBP

Five of every six G3P molecules are rearranged and phosphorylated (using ATP) to regenerate 3 RuBP molecules, allowing the cycle to continue. Only one out of every six G3P molecules represents net carbon gain and exits the cycle.

Summary for 3 turns of the cycle (3 CO$_2$ fixed):

Two net G3P molecules (from 6 turns / 6 CO$_2$) can be combined to make one glucose.

Checkpoint — Calvin Cycle Steps

RuBisCO: The Most Important (and Imperfect) Enzyme

RuBisCO has a critical flaw: it can react with O$_2$ as well as CO$_2$. When O$_2$ binds to RuBP instead of CO$_2$, the process is called photorespiration:

$\text{RuBP} + \text{O}_2 \longrightarrow \text{3-PGA (3C)} + \text{Phosphoglycolate (2C)}$

Phosphoglycolate is toxic and must be recycled in a complex pathway involving the chloroplast, peroxisome, and mitochondrion — consuming ATP and releasing CO$_2$ without producing useful sugar.

Photorespiration:

• Wastes energy (ATP and NADPH are consumed without net carbon fixation)
• Increases when O$_2$ concentration is high relative to CO$_2$
• Is more severe at high temperatures (RuBisCO has lower affinity for CO$_2$ at higher temperatures, and O$_2$ solubility decreases less than CO$_2$ solubility)
• Reduces photosynthetic efficiency by as much as 25-50% in C3 plants on hot days

Why does RuBisCO bind O$_2$? RuBisCO evolved ~3.5 billion years ago when Earth had very little atmospheric O$_2$. It never needed to distinguish between CO$_2$ and O$_2$. Today, with ~21% O$_2$ in the atmosphere, this ancient inability to discriminate is a significant liability.

Checkpoint — Photorespiration

Key Terms — Calvin Cycle

Exit Ticket — Calvin Cycle

C3, C4, and CAM Photosynthesis

Part 4 of 7

Plants have evolved different strategies to deal with the problem of photorespiration. Three major carbon fixation pathways are recognized, named after the first stable product of carbon fixation:

• C3 plants — initial product is a 3-carbon molecule (3-PGA)
• C4 plants — initial product is a 4-carbon molecule (oxaloacetate)
• CAM plants — use crassulacean acid metabolism (temporal separation)

All three types ultimately use the Calvin cycle for sugar synthesis, but C4 and CAM plants have evolved mechanisms to concentrate CO$_2$ around RuBisCO, minimizing photorespiration.

C3 Photosynthesis

C3 plants (e.g., rice, wheat, soybeans, most trees) use only the Calvin cycle for carbon fixation. RuBisCO directly fixes CO$_2$ from the air in the mesophyll cells.

Characteristics:

• Most common pathway (~85% of plant species)
• Initial fixation product: 3-PGA (3-phosphoglycerate), a 3-carbon molecule
• Susceptible to photorespiration, especially in hot, dry, or bright conditions
• Optimal environment: Cool, moist climates with moderate light

When stomata close to conserve water, CO$_2$ levels drop and O$_2$ rises inside the leaf, dramatically increasing photorespiration.

C4 Photosynthesis — Spatial Separation

C4 plants (e.g., corn/maize, sugarcane, sorghum, crabgrass) have evolved a two-step carbon fixation process that spatially separates initial carbon fixation from the Calvin cycle:

Step 1 — Mesophyll cells:

• The enzyme PEP carboxylase (not RuBisCO) fixes CO$_2$ by attaching it to PEP (phosphoenolpyruvate, 3C) to form oxaloacetate (OAA, 4C)
• OAA is quickly converted to malate (4C)
• Malate is transported to the bundle-sheath cells via plasmodesmata

Step 2 — Bundle-sheath cells:

• Malate is decarboxylated, releasing CO$_2$ inside the bundle-sheath cells
• This CO$_2$ is then fixed by RuBisCO in the normal Calvin cycle
• The remaining 3C molecule (pyruvate) returns to the mesophyll to regenerate PEP (costs 2 ATP)

Why this works:

• PEP carboxylase has a much higher affinity for CO$_2$ than RuBisCO and does not bind O$_2$
• CO$_2$ is concentrated to high levels around RuBisCO in the bundle-sheath cells
• Photorespiration is virtually eliminated

Cost: 2 extra ATP per CO$_2$ fixed (for PEP regeneration), so C4 photosynthesis is only advantageous when photorespiration would otherwise be significant.

Checkpoint — C3 vs C4

CAM Photosynthesis — Temporal Separation

CAM (Crassulacean Acid Metabolism) plants — e.g., cacti, pineapple, jade plant, many succulents — face extreme water stress and have evolved a different strategy: temporal separation of carbon fixation and the Calvin cycle.

Night (stomata OPEN):

• CO$_2$ enters through open stomata
• PEP carboxylase fixes CO$_2$ into oxaloacetate, which is converted to malate
• Malate is stored in large vacuoles (as malic acid)

Day (stomata CLOSED):

• Stomata close to prevent water loss
• Malate is released from the vacuole and decarboxylated, releasing CO$_2$
• CO$_2$ is fixed by RuBisCO in the Calvin cycle using ATP and NADPH from the light reactions

Key difference from C4:

• C4 plants separate fixation and Calvin cycle spatially (different cell types)
• CAM plants separate them temporally (different times of day)
• Both use PEP carboxylase for initial fixation and concentrate CO$_2$ around RuBisCO

Checkpoint — CAM Plants

Match the Plant Type

Exit Ticket — Carbon Fixation Strategies

Factors Affecting Photosynthesis

Part 5 of 7

The rate of photosynthesis is influenced by several environmental variables. Understanding how each factor affects the light reactions and Calvin cycle is essential for predicting plant productivity and interpreting experimental data on the AP exam.

The three main limiting factors are:

1. Light intensity
2. CO$_2$ concentration
3. Temperature

Light Intensity

As light intensity increases from zero:

• The rate of photosynthesis increases linearly at first (light is the limiting factor)
• The curve gradually levels off and reaches a plateau (the light-saturation point)
• Beyond the saturation point, increasing light does not increase the rate — another factor (CO$_2$, temperature, or enzyme capacity) becomes limiting

Compensation point: The light intensity at which the rate of photosynthesis equals the rate of cellular respiration (net gas exchange = 0). Below this point, the plant consumes more O$_2$ than it produces.

Very high light intensity can actually cause photoinhibition — damage to PSII reaction centers from excess absorbed energy, reducing photosynthetic efficiency.

Experiment Tip: The leaf disc flotation assay is a common AP lab technique that measures photosynthetic rate by counting how quickly leaf discs float to the surface (O$_2$ production makes them buoyant). Light intensity is varied by changing the distance between the light source and the beaker.

CO2 Concentration

CO$_2$ is a substrate for RuBisCO in the Calvin cycle:

• At low CO$_2$: The rate of carbon fixation is limited because RuBisCO is not saturated
• As CO$_2$ increases: The rate increases linearly
• At high CO$_2$: The rate plateaus when RuBisCO is fully saturated or when the light reactions cannot supply enough ATP/NADPH

Current atmospheric CO$_2$ (~420 ppm) is below the saturation point for most C3 plants, meaning CO$_2$ enrichment can increase photosynthetic rates in greenhouses.

C4 and CAM plants are less responsive to CO$_2$ enrichment because their carbon-concentrating mechanisms already saturate RuBisCO under normal conditions.

Temperature

Temperature affects the rate of enzyme-catalyzed reactions:

• From 0 to the optimum (~25-30 °C for most C3 plants, ~30-40 °C for C4 plants): The rate increases as molecular kinetic energy increases
• At the optimum temperature: The rate is maximized
• Above the optimum: Enzymes begin to denature, active sites lose shape, and the rate drops sharply
• At extremely high temperatures: Enzymes are completely denatured and photosynthesis stops

Temperature also affects the ratio of RuBisCO carboxylation to oxygenation:

• Higher temperatures decrease the relative solubility of CO$_2$ vs O$_2$
• RuBisCO also has lower affinity for CO$_2$ at higher temperatures
• Both effects increase photorespiration in C3 plants

This is why C4 plants (which circumvent photorespiration) dominate in tropical grasslands, while C3 plants dominate in temperate forests.

Checkpoint — Limiting Factors

Interactions Between Factors

In real ecosystems, multiple factors interact:

Liebig's Law of the Minimum applies: the rate of photosynthesis is determined by whichever factor is most limiting, regardless of the abundance of other factors.

Key Terms — Photosynthetic Factors

Exit Ticket — Factors Affecting Photosynthesis

Problem-Solving Workshop — Photosynthesis

Part 6 of 7

This workshop applies photosynthesis concepts to experimental scenarios and data interpretation problems commonly tested on the AP Biology exam.

Scenario 1: Hill Reaction Experiment

In 1937, Robert Hill demonstrated that isolated chloroplasts could produce O$_2$ in the presence of an artificial electron acceptor (like DCPIP, a dye that changes from blue to colorless when reduced), even without CO$_2$.

Experimental setup:

• Tube A: Chloroplasts + DCPIP + light → DCPIP decolorizes; O$_2$ produced
• Tube B: Chloroplasts + DCPIP + dark → No color change; no O$_2$
• Tube C: Boiled chloroplasts + DCPIP + light → No color change; no O$_2$
• Tube D: Chloroplasts + no DCPIP + light → Minimal O$_2$ (DCPIP is needed as electron acceptor)

This experiment demonstrated that:

1. The light reactions can occur independently of the Calvin cycle
2. O$_2$ comes from water splitting, not from CO$_2$
3. Light and functional proteins are both required

Scenario 1 Questions

Scenario 2: The Lollipop Experiment (Calvin and Benson)

Melvin Calvin and Andrew Benson used the "lollipop" apparatus and $^{14}\text{C}$-labeled CO$_2$ to trace the path of carbon through photosynthesis:

1. Algae (Chlorella) were grown in a thin, flat flask illuminated continuously
2. $^{14}\text{CO}_2$ was injected into the culture
3. At various time intervals (5 seconds, 30 seconds, 5 minutes), samples were killed in hot methanol
4. Radioactive compounds were separated by two-dimensional paper chromatography and identified by autoradiography

Results:

• After 5 seconds: Nearly all $^{14}\text{C}$ was in 3-PGA (confirming it as the first stable product)
• After 30 seconds: $^{14}\text{C}$ appeared in G3P, RuBP, and several sugar phosphates
• After 5 minutes: $^{14}\text{C}$ was found in glucose, amino acids, and lipids

This experiment mapped out the complete Calvin cycle and earned Calvin the 1961 Nobel Prize.

Scenario 2 Questions

Scenario 3: Comparing C3 and C4 Productivity

Researchers measure net photosynthesis rates in a C3 grass and a C4 grass under varying temperatures:

Scenario 3 Questions

Apply Your Knowledge

Exit Ticket — Workshop

AP Review — Photosynthesis

Part 7 of 7

This final section presents comprehensive AP-exam-style questions integrating concepts from all parts of the photosynthesis unit.

Master Summary

$6\text{CO}_2 + 6\text{H}_2\text{O} + \text{light} \longrightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

Light Reactions (thylakoid membrane):

• Inputs: H$_2$O, light, NADP$^+$, ADP + P$_i$
• Outputs: O$_2$, ATP, NADPH
• Key complexes: PSII, cytochrome b6f, PSI, ATP synthase

Calvin Cycle (stroma):

• Inputs: CO$_2$, ATP, NADPH
• Outputs: G3P, ADP + P$_i$, NADP$^+$
• Key enzyme: RuBisCO
• 3 phases: fixation, reduction, regeneration

Connections to respiration:

• Products of photosynthesis (glucose, O$_2$) are the reactants of respiration
• Products of respiration (CO$_2$, H$_2$O) are the reactants of photosynthesis
• Both use chemiosmosis (proton gradients + ATP synthase)
• Both involve electron transport chains

AP-Style Questions — Set 1

AP-Style Questions — Set 2

Comprehensive Matching

Final Key Facts

Final Exit Ticket