Understand Photosynthesis

Historical Context & Motivation

For thousands of years, people assumed that plants gained their mass from soil. It seemed logical — roots draw material from the ground, so surely that's where a tree's bulk comes from. Yet careful experiments showed that this intuition was wrong, and the real answer turned out to be far more elegant. The story of photosynthesis — the process by which organisms convert light energy into chemical energy — unfolded over centuries, driven by scientists who asked simple questions and designed clever experiments to answer them.

These discoveries raised a central question that the IB Biology syllabus asks you to grapple with: How exactly do plants capture photons and use that energy to build glucose from carbon dioxide and water? The answer involves two tightly linked stages — the light-dependent reactions and the light-independent reactions — and understanding them reveals why almost every food web on Earth begins with photosynthesis.

Core Principles of Photosynthesis

Photosynthesis is fundamentally an endergonic reaction — it absorbs energy (from light) to build complex molecules from simpler ones. The process occurs primarily in the chloroplasts of plant cells, which contain the pigment chlorophyll. Before diving into the details, it helps to grasp the overarching principles that govern the process.

Inside the Chloroplast — A Visual Tour

To truly understand photosynthesis, you need to see how the chloroplast's structure supports its function. The diagram below shows a cross-section of a chloroplast with the key structures labelled. Notice how the thylakoid membranes are stacked into grana (singular: granum), which are connected by intergranal lamellae. This arrangement dramatically increases the surface area available for the light-dependent reactions, which rely on pigment molecules embedded in these membranes.

In the diagram, you can see how the thylakoid discs stack to form grana. Each thylakoid membrane contains photosystem I and photosystem II, plus the electron transport chain and ATP synthase. The fluid-filled interior of each thylakoid disc is called the thylakoid lumen, where protons accumulate to drive ATP synthesis by chemiosmosis. The stroma surrounding the grana contains all the enzymes needed for the Calvin cycle, including the critical enzyme RuBisCO.

The Two Stages of Photosynthesis

Overall Equation

Stage 1: Light-Dependent Reactions

The light-dependent reactions occur across the thylakoid membrane. Here is a summary of the key events. First, photosystem II (PSII) absorbs photons and uses that energy to split water molecules in a process called photolysis: 2H₂O → 4H⁺ + 4e⁻ + O₂. The oxygen is released, and the electrons pass along an electron transport chain (ETC), losing energy at each carrier. That energy pumps H⁺ ions into the thylakoid lumen, creating a proton gradient. H⁺ ions then flow back through ATP synthase by chemiosmosis, driving the production of ATP.

Meanwhile, the electrons arriving at photosystem I (PSI) are re-energised by another photon and then used to reduce NADP⁺ to NADPH. Both ATP and NADPH are sometimes called assimilatory power because they carry the energy and reducing power needed for the next stage.

Stage 2: Light-Independent Reactions (Calvin Cycle)

The Calvin cycle takes place in the stroma and does not directly require light, although it depends on ATP and NADPH produced by the light-dependent reactions. The cycle has three main phases: carbon fixation, reduction, and regeneration of RuBP. In the fixation step, the enzyme RuBisCO catalyses the attachment of CO₂ to a 5-carbon molecule called ribulose bisphosphate (RuBP), producing two molecules of glycerate 3-phosphate (GP). GP is then reduced to glyceraldehyde 3-phosphate (G3P) using ATP and NADPH. Most G3P is recycled to regenerate RuBP, but some exits the cycle to form glucose and other organic molecules.

Light Absorption & Photosynthetic Pigments

Not all wavelengths of light are equally useful for photosynthesis. Chlorophyll a absorbs most strongly in the blue-violet (around 430 nm) and red (around 662 nm) regions, while reflecting green light — which is why leaves appear green. Chlorophyll b and carotenoids serve as accessory pigments, broadening the range of wavelengths a plant can use and funnelling energy to chlorophyll a in the reaction centre.

The action spectrum of photosynthesis — a graph showing the rate of photosynthesis at each wavelength — closely mirrors the absorption spectrum. This tight correlation is strong evidence that chlorophyll and accessory pigments are directly responsible for driving the reactions. Wavelengths near 430 nm and 662 nm yield the highest rates, while green light (around 550 nm) produces relatively little photosynthesis.

Worked Example — Tracking Molecules Through Photosynthesis

IB Biology often tests whether you can follow specific atoms or molecules through the two stages of photosynthesis. Let's work through a classic example step by step.

Limiting Factors & Environmental Influences

The rate of photosynthesis is not constant — it depends on environmental conditions. According to Blackman's law of limiting factors, the rate of a physiological process is limited by the factor that is closest to its minimum value. In photosynthesis, three factors are most important: light intensity, carbon dioxide concentration, and temperature.

Connecting to Advanced Concepts

At the IB level, you're expected to know the overall process and limiting factors well. However, photosynthesis connects to many advanced topics that you may encounter in HL Biology or university-level courses. The table below previews these connections, giving you a sense of where this topic leads.

One of the most fascinating extensions involves C₄ photosynthesis and CAM photosynthesis. These are adaptations found in plants such as maize and cacti, respectively, that minimise water loss and photorespiration in hot environments. Both involve spatially or temporally separating the initial CO₂ fixation from the Calvin cycle. While you won't be tested on the details at SL, being aware of these adaptations helps you understand the ecological diversity of photosynthetic strategies.

Practice Problems

Photosynthesis — Summary