Photosynthesis

Core Concepts

Autotrophs and phototrophs

• Autotrophs ("self-feeders") are organisms that produce their own organic molecules from inorganic sources. They are the producers in ecosystems.

• Phototrophs (also called photoautotrophs) use light energy to drive the synthesis of organic molecules. They include plants, algae, and cyanobacteria.

• Chemoautotrophs use energy from the oxidation of inorganic chemicals (e.g., H2S, NH3) to drive synthesis. They are found in deep-sea hydrothermal vents and other extreme environments.

Chloroplast structure

Chloroplasts are double-membrane organelles found in plants and algae. Internal structure:

• Thylakoids: Flattened, membrane-bound sacs. The thylakoid membrane contains chlorophyll, accessory pigments, and the proteins of the electron transport chain and ATP synthase. Stacks of thylakoids are called grana (singular: granum).

• Thylakoid lumen: The interior space of the thylakoid.

• Stroma: The fluid-filled space surrounding the thylakoids. Contains enzymes of the Calvin cycle, chloroplast DNA, and ribosomes.

Chlorophyll and pigments

Chlorophyll is the primary photosynthetic pigment. It absorbs light most strongly in the blue-violet and red regions of the spectrum and reflects green light — which is why leaves appear green. There are two main types:

• Chlorophyll a: The primary pigment; directly participates in the light reactions.

• Chlorophyll b: An accessory pigment; broadens the range of light wavelengths that can be absorbed.

Accessory pigments (carotenoids, xanthophylls) absorb light at wavelengths chlorophyll misses and pass the energy to chlorophyll. They also protect the photosynthetic apparatus from damage by excess light (photoprotection).

Absorption spectrum vs. action spectrum

• Absorption spectrum: Graph of the wavelengths of light absorbed by a pigment.

• Action spectrum: Graph of the rate of photosynthesis at different wavelengths. The action spectrum closely matches the combined absorption spectrum of all photosynthetic pigments — confirming that absorbed light drives photosynthesis.

Photosystems

A photosystem is a protein complex in the thylakoid membrane that contains chlorophyll molecules, accessory pigments, and a reaction center. There are two types, working in series:

• Photosystem II (PSII): Absorbs light best at 680 nm (P680). Splits water (photolysis), releasing O2, H+, and electrons.

• Photosystem I (PSI): Absorbs light best at 700 nm (P700). Produces NADPH.

The two photosystems are connected by an electron transport chain. This arrangement is called noncyclic electron flow and is the predominant pathway.

The light reactions

Location: Thylakoid membranes

Process

1. Light energy is absorbed by chlorophyll in PSII, exciting electrons to a higher energy level.

2. The excited electrons are passed to the primary electron acceptor and then through an electron transport chain (similar in concept to the respiratory ETC). As electrons pass through, H+ is pumped from the stroma into the thylakoid lumen, creating a proton gradient.

3. PSII replaces its lost electrons by splitting water: 2 H2O → 4 H+ + O2 + 4 e−. Oxygen is released as a byproduct.

4. At PSI, light re-excites the electrons. They are passed through a short electron transport chain and ultimately transferred to NADP+ reductase, which reduces NADP+ to NADPH.

5. The proton gradient across the thylakoid membrane drives ATP synthase — protons flow back into the stroma, and ATP is produced. This process is called photophosphorylation because it is driven by light energy.

Products of the light reactions: ATP, NADPH, O2 (released as waste).

Note the parallel with oxidative phosphorylation in mitochondria: electron transport chains pump protons; proton gradients drive ATP synthase. In mitochondria, the energy comes from food molecules; in chloroplasts, it comes from light.

The Calvin cycle (light-independent reactions)

Location: Stroma

The Calvin cycle uses ATP and NADPH from the light reactions to fix CO2 and produce sugar. It does NOT require light directly (hence "light-independent"), but it depends on the products of the light reactions and ceases in prolonged darkness.

The cycle has three phases:

1. Carbon fixation: CO2 is attached to a five-carbon molecule, ribulose bisphosphate (RuBP), by the enzyme RuBisCO (ribulose bisphosphate carboxylase/oxygenase). The resulting six-carbon intermediate immediately splits into two molecules of 3-phosphoglycerate (3-PGA, 3 carbons each).

2. Reduction: ATP and NADPH from the light reactions are used to convert 3-PGA to glyceraldehyde 3-phosphate (G3P), a three-carbon sugar. For every 6 CO2 fixed, 12 G3P are produced.

3. Regeneration of RuBP: Of the 12 G3P, 2 are used to make glucose and other organic molecules. The remaining 10 G3P are used in a series of reactions that consume ATP and regenerate 6 RuBP, allowing the cycle to continue.

Net reaction (summarized): 6 CO2 + 18 ATP + 12 NADPH + 12 H2O → C6H12O6 + 18 ADP + 18 Pi + 12 NADP+ + 6 H+

Note that water appears on both sides of photosynthesis equations depending on how you tally; the key point is that water is split in the light reactions and produced in the Calvin cycle.

Photorespiration: RuBisCO can bind O2 instead of CO2, consuming O2 and releasing CO2 without producing ATP or sugar. Favored when CO2 is low relative to O2. C4 and CAM plants have adaptations to minimize this, beyond our introductory scope.

Photosynthesis and respiration are complementary. Photosynthesis: 6 CO2 + 6 H2O → C6H12O6 + 6 O2 (stores energy, chloroplasts, NADPH). Respiration: reverse (releases ATP, mitochondria, NADH). Products of one are reactants of the other. Plants perform BOTH. During the day, photosynthesis > respiration (net O2 release). At night, respiration continues (net CO2 release).

ELI Example

Photosynthesis is a solar-powered bakery. Solar panels (chlorophyll) capture sunlight → electricity (ATP, NADPH). The chef (Calvin cycle) uses this power to turn CO2 and water into sugar pastries. Oxygen is the pleasant smell drifting out. At night, the solar panels stop; production halts until sunrise.

Do Not Confuse

Lab Link

Photosynthesis laboratories commonly investigate: (1) pigment separation by paper chromatography (separating chlorophyll a, chlorophyll b, carotenoids, and xanthophylls); (2) the effect of light wavelength or intensity on photosynthetic rate (measuring O2 production by aquatic plants or CO2 consumption using pH indicators); (3) the requirement for CO2 or light (using control experiments with sodium bicarbonate as a CO2 source); (4) starch production in leaves as evidence of photosynthesis (iodine staining of leaves that have been partially covered to block light).

High-Yield Memory Anchors

• Light reactions: thylakoid membrane. Split H2O → O2 + ATP + NADPH.

• Calvin cycle: stroma. Fix CO2 → G3P (sugar). Uses ATP + NADPH.

• Chlorophyll absorbs blue + red; reflects green.

• O2 comes from H2O, not CO2.

• Plants do BOTH photosynthesis AND respiration.

Quick Check

Q1 (Foundational): In which specific parts of the chloroplast do the light reactions and the Calvin cycle occur? What are the three main products of the light reactions?

Q2 (Application): A plant is placed in a sealed chamber with a CO2 sensor. During the day, CO2 levels decrease. At night, CO2 levels increase. Explain these observations in terms of photosynthesis and cellular respiration.

Q3 (Comparison/Reasoning): Compare the roles of the electron transport chain and chemiosmosis in mitochondria (cellular respiration) and chloroplasts (photosynthesis). Identify at least one similarity and one difference.

Quick Check Answers

A1: Light reactions: thylakoid membranes. Calvin cycle: stroma. Three main products of the light reactions: ATP, NADPH, and O2.

A2: During the day, both photosynthesis and cellular respiration occur, but the rate of photosynthesis exceeds the rate of respiration. The plant takes up more CO2 for photosynthesis than it releases from respiration, so net CO2 in the chamber decreases. At night, photosynthesis stops (no light), but cellular respiration continues. The plant releases CO2 from respiration without taking any up, so CO2 levels in the chamber increase.

A3: Similarity: Both organelles use an electron transport chain to pump protons (H+) across a membrane, creating a proton gradient, and both use ATP synthase to harness the energy of protons flowing back down their gradient to produce ATP. Differences: (1) The energy source for the ETC differs — in mitochondria, electrons come from NADH/FADH2 (derived from food); in chloroplasts, electrons come from water and are energized by light absorbed by chlorophyll. (2) The location differs — in mitochondria, protons are pumped from the matrix into the intermembrane space; in chloroplasts, protons are pumped from the stroma into the thylakoid lumen. (3) The final electron acceptor differs — in mitochondria, it is O2 (forming H2O); in chloroplasts, it is NADP+ (forming NADPH).

Chapter Summary

Light reactions (thylakoid): H2O → O2 + ATP + NADPH. Calvin cycle (stroma): CO2 → G3P using ATP + NADPH. Photosynthesis and respiration are complementary — together they form the energy cycle sustaining most life. Plants perform both processes.

Common Mistakes

Mistake: "Plants do not perform cellular respiration — they only photosynthesize."

Reality: Plants perform BOTH photosynthesis and cellular respiration. Photosynthesis produces glucose and O2; respiration breaks down glucose (consuming O2) to produce ATP. At night, plants are net O2 consumers and CO2 producers.

Mistake: "The Calvin cycle requires darkness (because it is called the dark reactions)."

Reality: The Calvin cycle is "light-independent" — it does not directly require light. But it requires ATP and NADPH, which are produced by the light reactions. In prolonged darkness, ATP and NADPH run out, and the Calvin cycle stops. The Calvin cycle is more accurately called the light-independent reactions.

Mistake: "Oxygen released by photosynthesis comes from CO2."

Reality: The oxygen released comes from the splitting of WATER (H2O) in the light reactions, NOT from CO2. This was demonstrated using radioactive oxygen-18 tracer experiments. The oxygen atoms in CO2 end up in the sugar produced and in the water released during the Calvin cycle.

Mistake: "Photosynthesis and respiration are exact reverse reactions."

Reality: While the overall equations are reverses, the pathways are completely different — different locations, different enzymes, different electron carriers, different mechanisms. They are complementary but not identical pathways running in reverse.