Photosynthesis
Every gram of sugar in your food, every breath of oxygen you take — all of it traces back to a single biochemical process in the thylakoid membranes of plant cells. Understanding photosynthesis means understanding where almost all energy on Earth originally comes from.
Think First — Case Entry
A biology student is examining two leaves under a microscope. Leaf A is from a plant grown in full sunlight. Leaf B is from the same plant species, but grown in a dimly lit room. Leaf B is noticeably paler and thinner than Leaf A.
Before reading on: What differences would you expect to find inside the cells of these two leaves? Focus on organelles. Why might a plant grown in low light look different at the cellular level?
Hold onto this — you will revisit it at the end of the lesson.
Know
- Light as a suitable energy form for cells
- Inputs and outputs of photosynthesis
- Where light reactions and Calvin cycle occur
- Role of chlorophyll and chloroplast structure
Understand
- How light energy is converted to chemical energy
- Why oxygen is a by-product, not the main product
- How chloroplast structure suits its function
- Why low light limits photosynthesis at the cellular level
Can Do
- Write and explain the overall equation for photosynthesis
- Draw and label a chloroplast with functional annotations
- Explain the two-stage process with inputs/outputs at each stage
Core Content
The Challenge Every Living Cell Faces
All living things need energy to survive — for movement, growth, repair, reproduction, and maintaining internal conditions. But energy doesn't come from nowhere. It must be captured from the environment and converted into a form cells can actually use: ATP (adenosine triphosphate).
There are two forms of usable energy that cells can work with:
- Light energy — electromagnetic radiation from the sun, captured directly by photosynthetic pigments
- Chemical energy in complex molecules — stored in bonds of glucose, fats, and other organic molecules, released through cell respiration
Photosynthesis is the process that bridges these two: it takes light energy — which cannot be used directly by most cellular machinery — and converts it into chemical energy stored in glucose. This glucose then becomes the fuel for virtually all life on Earth, either directly (for the plant itself) or indirectly (for every organism that eats plants, or eats something that ate plants).
The Overall Equation
Before diving into the mechanism, it helps to know what photosynthesis achieves at the simplest level. The overall summary equation is:
Three things worth noting about this equation:
- Carbon dioxide enters through stomata (pores in the leaf surface). It provides the carbon atoms that become the carbon skeleton of glucose.
- Water is absorbed through roots and transported to leaves. Crucially, the oxygen released as a by-product comes from the splitting of water — not from CO₂.
- Glucose is the main product — a stable molecule for storing chemical energy. It can be used immediately for respiration, converted to starch for storage, or used to build other organic molecules.
Chloroplast cross-section. Thylakoid membranes (stacked into grana) = site of light-dependent reactions. Stroma = site of Calvin cycle. ATP and NADPH produced in Stage 1 drive glucose synthesis in Stage 2.
Where It Happens — Chloroplast Structure
Photosynthesis occurs in chloroplasts — organelles found in plant cells (and algae). Their structure is precisely suited to carrying out two distinct stages of photosynthesis in two separate compartments.
A plant grown in low light (like the Leaf B in your Think First) will have chloroplasts with fewer and smaller grana — less stacked thylakoid membrane means less chlorophyll, less light capture capacity. The cell adapts by spreading chloroplasts to maximise light interception, which is why the leaf appears thinner and paler.
The Two Stages of Photosynthesis
Photosynthesis is not a single reaction — it is two connected stages, each occurring in a different part of the chloroplast:
Stage 1 — Light-Dependent Reactions
Location: Thylakoid membranes (grana)
Inputs: Light energy, water (H₂O)
Outputs: ATP, NADPH (electron carrier), O₂ (released as by-product)
What happens:
- Chlorophyll absorbs light energy
- Light energy is used to split water molecules (photolysis)
- Electrons from water are used to produce ATP and NADPH
- Oxygen is released as a by-product of splitting water
Key point: This stage captures light energy and converts it to chemical energy (ATP + NADPH) — but no glucose yet.
Stage 2 — Light-Independent Reactions (Calvin Cycle)
Location: Stroma
Inputs: CO₂, ATP, NADPH (from Stage 1)
Outputs: Glucose (C₆H₁₂O₆)
What happens:
- CO₂ from the air is fixed (attached to organic molecules)
- ATP and NADPH from Stage 1 drive the reactions
- Carbon atoms are built up into glucose over multiple cycles
- The cycle regenerates the starting molecules
Key point: This stage uses the energy from Stage 1 to build glucose. It doesn't directly require light — but it stops if Stage 1 stops.
Photosynthesis two-stage process. Stage 1 (thylakoid): light energy + H₂O → ATP + NADPH + O₂. Stage 2 / Calvin cycle (stroma): CO₂ + ATP + NADPH → glucose. The stages are tightly coupled — Stage 2 cannot run without Stage 1 products.
Factors That Limit Photosynthesis
The rate of photosynthesis is not constant — it depends on the availability of its raw materials and the conditions in which it operates. Three key limiting factors are:
| Factor | How it limits photosynthesis | What happens at the cellular level |
|---|---|---|
| Light intensity | More light = more energy for Stage 1. Below a threshold, the rate is limited by insufficient light energy to drive photolysis of water. | Fewer ATP and NADPH molecules produced per unit time. Stage 2 slows because it lacks these inputs. |
| CO₂ concentration | More CO₂ = more carbon available for Stage 2. At low CO₂ levels, the Calvin cycle cannot run at full speed. | Carbon fixation slows; glucose output falls even if Stage 1 is running normally. |
| Temperature | Enzymes catalyse Stage 2 reactions (including RuBisCO, the enzyme that fixes CO₂). Too cold: enzyme activity is low. Too hot: enzymes denature. | Optimal temperature ~25–35°C for most plants. Above ~40°C enzyme denaturation causes rapid decline. |
In practice, the factor in shortest supply is called the limiting factor — increasing it will increase the rate of photosynthesis; increasing any other factor will have no effect until the limiting factor is addressed.
Overall Equation
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Requires: light energy + chlorophyll
O₂ comes from splitting H₂O (not CO₂)
Two Stages
- Stage 1 (thylakoids): light + H₂O → ATP + NADPH + O₂
- Stage 2 / Calvin cycle (stroma): CO₂ + ATP + NADPH → glucose
- Stage 2 depends on Stage 1 products
Chloroplast Structure
- Thylakoids — light reactions, chlorophyll
- Grana — stacked thylakoids, increases SA
- Stroma — Calvin cycle, enzymes
- Double membrane — controls entry/exit
Limiting Factors
- Light intensity → limits Stage 1
- CO₂ concentration → limits Stage 2
- Temperature → limits enzyme activity (Stage 2)
- Limiting factor = factor in shortest supply
Activities
Labelled Chloroplast Diagram
In your book, draw and label a chloroplast. Your diagram must show:
- Outer and inner membranes
- Thylakoids and grana (at least 3 grana with thylakoids visible)
- Stroma
- For each structure: name → key feature → function in photosynthesis
- Annotate clearly where Stage 1 occurs and where Stage 2 occurs
Then, alongside your diagram, answer this: Why does increasing the number of grana increase the rate of photosynthesis? Your answer must connect structure to function — don't just say "more chlorophyll."
Write your explanation here.
Trace the Carbon
A carbon atom enters a leaf as part of a CO₂ molecule. Trace this carbon atom's complete journey through photosynthesis, step by step. Your answer should:
- State where the CO₂ enters the leaf (which structure?)
- Identify which stage of photosynthesis uses CO₂ and where in the chloroplast this occurs
- Describe what happens to the carbon atom in Stage 2 — what molecule does it end up in?
- Explain what happens to glucose after it is produced — name two possible fates
- Now trace an oxygen atom from the same water molecule that was split in Stage 1 — where does it end up?
Write your trace here or in your book.
Assessment
Multiple Choice — 5 marks
1. Which of the following correctly identifies where the light-dependent reactions of photosynthesis occur?
2. In the overall equation for photosynthesis, what is the source of the oxygen released as a by-product?
3. A plant is placed in conditions where CO₂ concentration is very low but light intensity is very high. Which of the following best predicts the effect on photosynthesis?
4. Which of the following is the correct role of ATP and NADPH produced in the light-dependent reactions?
5. A plant is moved from a warm environment (28°C) to a cold environment (5°C). Light and CO₂ levels remain constant. Which stage of photosynthesis is most directly affected, and why?
Short Answer — 9 marks
1. Describe the role of the thylakoid membranes in photosynthesis. In your answer, explain how their structure suits their function. (3 marks)
1 mark: thylakoids as site of light-dependent reactions + contain chlorophyll; 1 mark: stacking into grana increases surface area; 1 mark: structure–function link (more SA = more chlorophyll = more light absorption = more ATP/NADPH)
2. Explain what would happen to the rate of glucose production in a plant if all light was suddenly removed. In your answer, refer to both stages of photosynthesis. (3 marks)
1 mark: Stage 1 stops (no light to drive photolysis/electron transport); 1 mark: ATP and NADPH production ceases; 1 mark: Stage 2 (Calvin cycle) stops because it depends on ATP and NADPH from Stage 1
3. A student conducts an iodine test on sections of a variegated leaf (green and white patches). The green sections turn blue-black; the white sections remain brown. Explain these results with reference to chloroplast structure and the two stages of photosynthesis. (3 marks)
1 mark: iodine tests for starch — blue-black = starch present; 1 mark: white sections lack chlorophyll so cannot absorb light, Stage 1 cannot proceed, no ATP/NADPH produced; 1 mark: without Stage 1 products, Stage 2 cannot fix CO₂ into glucose, no glucose means no starch stored
Answers
- Q1 — C: Light-dependent reactions occur on the thylakoid membranes (within the grana). The stroma is where Stage 2 (Calvin cycle) occurs. The outer membrane is structural only.
- Q2 — B: Isotope-labelling experiments with ¹⁸O confirmed that the oxygen released comes from water, not CO₂. Water is split during photolysis in Stage 1.
- Q3 — C: CO₂ is only used in Stage 2 (Calvin cycle). Stage 1 uses light and water — it can still run. But without CO₂, Stage 2 is blocked, so glucose cannot be produced. High light intensity cannot compensate for low CO₂.
- Q4 — D: ATP provides energy and NADPH provides energised electrons (reducing power) for the Calvin cycle in Stage 2. They are the link between the two stages — produced in Stage 1, consumed in Stage 2.
- Q5 — B: Stage 2 is enzyme-driven (RuBisCO catalyses CO₂ fixation). Low temperature significantly reduces enzyme activity. Stage 1 involves light absorption by chlorophyll — a physical/photochemical process — which is much less temperature-sensitive than enzyme reactions.
SA1: Thylakoid membranes are the site of the light-dependent reactions of photosynthesis. They contain chlorophyll and other photosynthetic pigments embedded within the membrane, which absorb light energy (primarily red and blue wavelengths). The thylakoids are arranged in stacks called grana, which dramatically increases the surface area of thylakoid membrane within a small chloroplast volume. This greater surface area means more chlorophyll molecules can be packed in, allowing more light energy to be absorbed per unit time — producing more ATP and NADPH to drive Stage 2.
SA2: If all light is suddenly removed, Stage 1 (light-dependent reactions) would stop immediately because it requires light energy to drive the splitting of water (photolysis) and the electron transport chain in the thylakoid membranes. This means ATP and NADPH — the products of Stage 1 — would no longer be produced. Stage 2 (the Calvin cycle) depends entirely on ATP and NADPH from Stage 1 to fix CO₂ and build glucose. Without these inputs, Stage 2 would also stop very quickly, and glucose production would cease. The two stages are tightly coupled — Stage 2 cannot continue independently.
SA3: Iodine solution turns blue-black in the presence of starch, indicating glucose has been produced and stored. The green sections of the variegated leaf contain chlorophyll in their chloroplasts and can absorb light energy — Stage 1 runs normally, producing ATP and NADPH, Stage 2 fixes CO₂ to form glucose, which is stored as starch. The white sections lack chlorophyll due to a genetic mutation preventing its synthesis. Without chlorophyll, these cells cannot absorb light energy, so Stage 1 cannot proceed — no photolysis of water, no ATP, no NADPH are produced. Without the products of Stage 1, Stage 2 cannot fix CO₂ into glucose. With no glucose produced, no starch accumulates, and the iodine test remains brown.
Revisit Your Thinking
You were asked to predict what differences you'd expect in the cells of Leaf A (full sun) versus Leaf B (low light). What's the verdict?
Leaf A cells would have chloroplasts with more and larger grana — more stacked thylakoids, more chlorophyll, greater light-capture capacity. Leaf B cells would have fewer, smaller grana with less chlorophyll. The cells themselves might be positioned differently to maximise light interception, and the leaf might be thinner (less mesophyll tissue) and paler because chlorophyll production is reduced when light is consistently low.
This is a real adaptation — plants in shaded environments produce "shade leaves" with larger surface areas, thinner blades, and chloroplasts repositioned parallel to the leaf surface to catch more diffuse light. Everything traces back to thylakoid membrane structure and chlorophyll concentration.
Boss Battle — Photosynthesis Showdown!
Challenge the boss using your knowledge of photosynthesis reactions and pathways. Pool: lessons 1–10.
🎮 Fight the Boss →