What is Photosynthesis?
Overview☀️ The Solar Factory Analogy
Think of a leaf as a solar-powered factory. Raw materials (CO₂ and H₂O) come in, solar energy powers the production line, and the finished products (glucose and O₂) come out. The chloroplast is the factory floor where all the work happens.
📥 Reactants (Raw Materials)
- Carbon dioxide (CO₂) — enters through stomata in leaves
- Water (H₂O) — absorbed by roots, transported via xylem
- Light energy — absorbed by chlorophyll pigments
📤 Products (Output)
- Glucose (C₆H₁₂O₆) — stored chemical energy; used in respiration, growth, storage as starch
- Oxygen (O₂) — released through stomata as a by-product
- Water — small amount released
| Word Equation | Symbol Equation |
|---|---|
| Carbon dioxide + Water → Glucose + Oxygen | 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ |
| In the presence of light energy, absorbed by chlorophyll | |
📍 Where
In chloroplasts — found mainly in the mesophyll cells of leaves. More chloroplasts = more photosynthesis.
⚡ Energy
Light energy (from sun) is converted to chemical energy and stored in glucose molecules. This is energy transduction.
🌍 Why it matters
Photosynthesis is the foundation of almost all food chains on Earth. It also produces the oxygen in our atmosphere.
⚠️ Common Exam Mistake
Photosynthesis does NOT happen in all plant cells — only in cells that contain chloroplasts. Root cells have no chloroplasts and cannot photosynthesise. Also: oxygen is a BY-PRODUCT, not the main product. Glucose is the main product.
📌 Autotroph vs Heterotroph
Plants are autotrophs (producers) — they make their own food using light energy. Animals are heterotrophs (consumers) — they must eat other organisms to get energy. Photosynthesis is what makes autotrophy possible.
The Chloroplast
Structure🔬 The Factory Floor
The chloroplast is the organelle where photosynthesis takes place. Its structure is perfectly designed for the job — a double membrane for protection, thylakoids stacked into grana for the light reactions, and a fluid-filled stroma for the dark reactions.
Chloroplast Structure — Label the Parts
| Structure | Description | Role in Photosynthesis |
|---|---|---|
| Outer membrane | Smooth double membrane surrounding the chloroplast | Controls what enters and exits the chloroplast |
| Inner membrane | Second membrane layer inside the outer | Forms the boundary of the stroma |
| Stroma | Fluid-filled matrix inside inner membrane | Site of the DARK REACTIONS (Calvin cycle / carbon fixation) |
| Thylakoid | Flattened, disc-shaped membrane sac | Contains chlorophyll; site of the LIGHT REACTIONS |
| Granum (pl. grana) | Stack of thylakoids (like a pile of coins) | Increases surface area for light absorption |
| Lamella (pl. lamellae) | Membrane connecting different grana | Links grana together; allows transport between stacks |
| Chlorophyll | Green pigment embedded in thylakoid membranes | Absorbs light energy (mainly red and blue wavelengths) |
📌 Why leaves are green
Chlorophyll absorbs red and blue light wavelengths for photosynthesis but reflects green light — which is why we see leaves as green. It does not absorb green light efficiently.
Light & Dark Reactions
Two Stages⚡ Two Production Lines
Photosynthesis happens in two main stages. The light reactions happen in the thylakoids and need direct sunlight. The dark reactions (Calvin cycle) happen in the stroma — they don't directly need light but do use the products of the light reactions.
Light Reactions
In the thylakoid membranes
- Light absorbed by chlorophyll
- Water molecules split (photolysis)
- H₂O → 2H⁺ + ½O₂ + 2e⁻
- ATP produced
- NADPH produced
- O₂ released as by-product
→
Dark Reactions (Calvin Cycle)
In the stroma
- CO₂ is fixed (carbon fixation)
- Uses ATP + NADPH from light reactions
- CO₂ + RuBP → GP → GALP
- Glucose (C₆H₁₂O₆) produced
- RuBP regenerated for next cycle
- Can happen without direct light
⚠️ "Dark" Reactions Misconception
Dark reactions are NOT called "dark" because they happen at night! They are called dark reactions because they do not directly require light. However, they still depend on the ATP and NADPH produced by the light reactions — so they effectively stop when there is no light.
☀️ Photolysis of Water
- Photo = light; lysis = splitting
- Light energy splits water molecules
- 2H₂O → 4H⁺ + O₂ + 4e⁻
- Oxygen released is the source of O₂ in our atmosphere
- Hydrogen ions (H⁺) used to make NADPH
- Electrons replace those lost from chlorophyll
🌿 Carbon Fixation
- CO₂ from the air is "fixed" into organic molecules
- CO₂ combines with RuBP (5-carbon compound)
- Produces GP (3-carbon)
- GP reduced to GALP using ATP + NADPH
- GALP used to make glucose
- RuBP is regenerated to accept more CO₂
📌 What to remember for exams
Light reactions: location = thylakoid membranes; needs = light + water; produces = ATP, NADPH, O₂ (by-product)Dark reactions: location = stroma; needs = CO₂ + ATP + NADPH; produces = glucose
Limiting Factors
Environmental📊 What Controls the Rate?
The rate of photosynthesis is controlled by limiting factors — the factor in shortest supply controls the overall rate. Even if light is abundant, if CO₂ is low, the rate cannot increase. This is Blackman's Law of Limiting Factors.
Light Intensity
As light increases, rate of photosynthesis increases — up to a point. Beyond the light saturation point, other factors become limiting.
Can be limiting at low light
CO₂ Concentration
Increasing CO₂ increases the rate of dark reactions. Normal atmospheric CO₂ (~0.04%) is often a limiting factor for plants.
Often limiting in atmosphere
Temperature
Affects enzyme activity in dark reactions. Optimal range ~25–35°C. Too hot denatures enzymes. Too cold slows enzyme activity.
Affects enzymes (dark reactions)
Water Availability
Water is a raw material for photolysis. Severe drought causes stomata to close (reducing CO₂ entry) and limits the light reactions.
Rarely limiting alone
| Factor | Effect on rate if increased | Plateau reason |
|---|---|---|
| Light intensity | Rate increases (more energy for light reactions) | CO₂ or temperature becomes limiting; chlorophyll saturated |
| CO₂ concentration | Rate increases (more substrate for dark reactions) | Light or enzyme capacity becomes limiting |
| Temperature (up to optimum) | Rate increases (faster enzyme activity) | Above optimum → enzymes denature → rate drops sharply |
| Chlorophyll concentration | Rate increases (more light-absorbing pigment) | Limited by CO₂, light, or temperature |
📌 Blackman's Law of Limiting Factors
The rate of a physiological process is limited by the factor that is present at its minimum. Even if you increase all other factors, the rate cannot increase beyond the most limiting factor. On a graph, when the rate plateaus, a different factor has become limiting.
⚠️ Temperature & Enzymes
Temperature affects the DARK reactions (enzyme-controlled) more than the light reactions. Light reactions are physical (photochemical) processes — less sensitive to temperature. So: low temp mainly slows dark reactions; high temp denatures enzymes in both stages.
Experiments
Practical Work🧪 Testing Photosynthesis
Several classic experiments demonstrate photosynthesis. Know the method, variables, and conclusions for each — particularly the starch test and the bubble counting experiment.
🍃 Iodine Starch Test (Testing for glucose/starch)
- Purpose: Show that a leaf has been photosynthesising
- 1. Boil leaf in water (kill cells, stop reactions)
- 2. Boil in ethanol (remove chlorophyll — decolourise)
- 3. Rinse in water (soften leaf)
- 4. Add iodine solution
- Positive result: Blue-black = starch present → photosynthesis occurred
- Negative result: Remains orange-brown = no starch
🫧 Bubble Counting Experiment (Rate of O₂ production)
- Purpose: Measure rate of photosynthesis by counting O₂ bubbles from aquatic plant (Elodea/Cabomba)
- IV: Light intensity (vary distance of lamp)
- DV: Number of bubbles per minute
- Controlled: Temperature, CO₂ concentration, same plant
- Result: Closer lamp = more bubbles = faster photosynthesis
- Conclusion: Light intensity is a limiting factor
🌿 Variegated Leaf Experiment
- Uses a leaf with green (chlorophyll) and white (no chlorophyll) sections
- After iodine test: only green parts turn blue-black
- Conclusion: Chlorophyll is necessary for photosynthesis
🖤 Covered Leaf Experiment
- Part of leaf covered with black paper (no light), part exposed
- After iodine test: covered part = no blue-black
- Exposed part = blue-black (starch present)
- Conclusion: Light is necessary for photosynthesis
📌 Why boil in ethanol?
Ethanol removes the green chlorophyll from the leaf. This is called decolourising. If you don't do this step, the dark green colour of the leaf makes it impossible to see the blue-black colour change from iodine. Boiling in water first kills the cells to stop any further reactions and makes the leaf permeable to iodine.
⚠️ Exam Watch — Variables
Always identify ALL three variable types: Independent Variable (what you change), Dependent Variable (what you measure), Controlled/Fixed Variables (what you keep the same). For validity: use the word SAME and be specific — e.g. "same species of plant", "same volume of water", "same time period."
Test Yourself
Quiz🎯 Photosynthesis Quiz
IEB and CAPS style questions. Select your answer and get instant feedback with full explanations.
Q1
Where exactly in the chloroplast do the LIGHT REACTIONS take place?
Q2
What is photolysis and what does it produce?
Q3
A student increases the light intensity on a plant but the rate of photosynthesis does not increase further. What is the most likely explanation?
Q4
A leaf is decolourised in ethanol, then tested with iodine. It turns ORANGE-BROWN. What does this indicate?
Q5
Why are the dark reactions still dependent on light, even though they don't directly use light?
Q6 — Application
A farmer wants to increase crop yield in a greenhouse. She has already maximised light intensity with artificial lights. What TWO other factors should she adjust?