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 (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:

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:

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

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

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):

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

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 (OAA, 4C)

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

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)

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

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