Introduction
Every meal you have ever eaten traces back to photosynthesis. Whether you are eating rice, chicken, vegetables, or fruit — the energy in that food originally came from the sun, captured by a plant through photosynthesis. It is the most important chemical process on Earth, and without it, virtually no life as we know it could exist.
Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. It is the foundation of almost every food chain on the planet and the primary source of the oxygen in our atmosphere.
What Is Photosynthesis?
Photosynthesis is the process by which green plants use light energy to convert carbon dioxide and water into glucose and oxygen. It takes place in the chloroplasts of plant cells, which contain a green pigment called chlorophyll that absorbs light energy.
The word photosynthesis comes from the Greek words photos (light) and synthesis (putting together) — literally "putting together using light."
The Equation for Photosynthesis
Word Equation: Carbon dioxide + Water → Glucose + Oxygen
(using light energy and chlorophyll)
Symbol Equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
(using light energy and chlorophyll)
Breakdown of the equation
Where Does Photosynthesis Take Place?
Photosynthesis takes place inside chloroplasts — organelles found in the cells of green plants. Chloroplasts contain the green pigment chlorophyll, which absorbs light energy (mainly red and blue wavelengths) and converts it into chemical energy used to drive the reaction.
Not all plant cells contain chloroplasts. Only cells in the green parts of the plant that are exposed to light (particularly the palisade mesophyll cells in leaves) carry out significant amounts of photosynthesis.
What Does the Plant Do With Glucose?
Glucose produced by photosynthesis is used by the plant in several ways:
- Respiration — glucose is broken down in mitochondria to release energy for all cell activities.
- Making cellulose — used to build cell walls
- Making proteins — combined with nitrogen from the soil to make amino acids and then proteins.
- Storage as starch — converted to starch for long-term energy storage (starch is insoluble and does not affect osmosis)
- Storage as oils and fats — particularly in seeds
- Making sucrose — transported around the plant in the phloem
Leaf Structure
The leaf is the primary organ of photosynthesis. Every feature of its structure is an adaptation that makes photosynthesis as efficient as possible.
External Features of a Leaf
- Broad and flat — maximises the surface area exposed to sunlight
- Thin — short diffusion distance for gases and light to reach inner cells
- Green — due to chlorophyll, which absorbs light energy
- Network of veins — transport water and minerals in (xylem) and glucose out (phloem), and provide structural support
Internal Structure of a Leaf
1. Waxy Cuticle
A thin, transparent, waterproof layer on the upper surface of the leaf. It reduces water loss by evaporation while still allowing light to pass through to the cells below.
2. Upper Epidermis
A single layer of transparent cells with no chloroplasts. Its transparency allows light to pass through easily to reach the photosynthesising cells below. It produces the waxy cuticle.
3. Palisade Mesophyll Layer
A layer of tall, column-shaped cells packed tightly together just below the upper epidermis. These are the main photosynthesising cells of the leaf — they contain the greatest number of chloroplasts of any cell type. They are positioned at the top of the leaf to receive maximum light.
4. Spongy Mesophyll Layer
A layer of loosely packed, irregular cells below the palisade layer. The large air spaces between the cells allow gases (carbon dioxide and oxygen) to diffuse freely throughout the leaf. The cells also contain chloroplasts but fewer than palisade cells.
5. Lower Epidermis
A single layer of cells on the underside of the leaf. It contains the stomata — small pores through which gas exchange takes place.
6. Stomata (singular: stoma)
Tiny pores found mainly on the underside of the leaf. Carbon dioxide diffuses in through the stomata for photosynthesis, and oxygen and water vapour diffuse out. Each stoma is surrounded by two guard cells that control its opening and closing.
7. Guard Cells
Kidney-shaped cells that surround each stoma. When the plant has enough water, the guard cells fill with water and become turgid — they curve apart and the stoma opens. When water is scarce, the guard cells become flaccid and the stoma closes, reducing water loss.
8. Xylem Vessels
Part of the vascular bundle (veins). They carry water and dissolved mineral ions from the roots up to the leaf. Water is a raw material for photosynthesis.
9. Phloem Vessels
Also part of the vascular bundle. They carry dissolved glucose (as sucrose) away from the leaf to other parts of the plant where it is needed or stored.
Leaf Structure — Summary Table
Limiting Factors of Photosynthesis
A limiting factor is any factor that, when in short supply, slows down or stops the rate of photosynthesis — even if all other conditions are ideal. The rate of photosynthesis is controlled by whichever factor is in shortest supply at any given time.
The three main limiting factors are: Light intensity, Carbon dioxide concentration and Temperature.
Limiting Factor 1: Light Intensity
Light provides the energy needed to drive photosynthesis. As light intensity increases, the rate of photosynthesis increases — up to a point. Beyond that point, another factor becomes limiting and increasing light further has no additional effect.
What happens:
- In darkness — photosynthesis stops entirely
- Low light — photosynthesis rate is slow; light is the limiting factor
- Increasing light — rate increases proportionally
- High light — rate levels off; another factor (CO₂ or temperature) becomes limiting
Graph pattern: Rate increases linearly with light intensity, then levels off into a plateau.
Real-world application: Greenhouse growers use artificial lighting to extend the hours of light available to crops, increasing photosynthesis and yield.
Limiting Factor 2: Carbon Dioxide Concentration
Carbon dioxide is a raw material for photosynthesis. Without it, the reaction cannot proceed. As CO₂ concentration increases, the rate of photosynthesis increases — until another factor becomes limiting.
What happens:
- Very low CO₂ — photosynthesis is very slow; CO₂ is limiting
- Increasing CO₂ — rate increases
- High CO₂ — rate levels off; light or temperature becomes limiting
Normal atmospheric CO₂ concentration is approximately 0.04% (400 parts per million). Greenhouse growers sometimes enrich the air with CO₂ to increase this concentration and boost photosynthesis rates.
Graph pattern: Linear increase then plateau.
Limiting Factor 3: Temperature
Temperature affects the rate of photosynthesis because the reactions of photosynthesis are controlled by enzymes — biological catalysts that are sensitive to temperature.
What happens:
- Low temperature (0–10°C) — enzymes work slowly; reaction rate is low
- Increasing temperature — enzyme activity increases; rate increases
- Optimum temperature (around 25–30°C for most plants) — maximum rate of photosynthesis
- Above optimum (above ~40°C) — enzymes begin to denature (their shape changes permanently); rate drops sharply
- Very high temperature — enzymes are fully denatured; photosynthesis stops
Graph pattern: Rate increases with temperature up to the optimum, then drops sharply as enzymes denature. This is a distinctive curved graph — different from the light and CO₂ graphs.
How Limiting Factors Interact
In real conditions, all three factors interact simultaneously. The rate of photosynthesis at any moment is determined by whichever factor is most limiting. Even if you increase one factor, the rate will not increase further unless the limiting factor is addressed.
Example: A plant is in bright light with plenty of CO₂, but the temperature is 5°C. Increasing the light intensity further will not speed up photosynthesis — temperature is the limiting factor. To increase the rate, you would need to raise the temperature.
Investigating Photosynthesis — The Pondweed Experiment
A classic school experiment uses aquatic pondweed (usually Elodea) to investigate how light intensity affects the rate of photosynthesis. Oxygen bubbles produced by the plant are counted as a measure of the rate of photosynthesis.
Method:
- Place pondweed in a beaker of water with sodium hydrogen carbonate (to provide CO₂)
- Shine a lamp at the pondweed from a measured distance
- Count the number of oxygen bubbles produced per minute
- Move the lamp further away and repeat
- Plot results on a graph
The Two Main Types of Microscope
Light Microscope
Uses visible light and a series of glass lenses to magnify a specimen. It is the type used in school laboratories. It can magnify up to approximately ×1500 and allows you to see structures such as the nucleus, cell wall, vacuole, and chloroplasts.
Electron Microscope
Uses beams of electrons instead of light, giving a far higher magnification — up to ×2,000,000. It reveals fine detail such as ribosomes, the internal structure of mitochondria, and the cell membrane. It is large, expensive, and found only in research laboratories.
Key Terms
- Magnification — how many times larger the image appears compared to the actual object
- Resolution — the ability to distinguish between two points that are close together. Higher resolution = clearer, more detailed image
- Specimen — the sample being observed under the microscope
The Magnification Formula
Magnification = Image size ÷ Actual size
Rearranged:
- Actual size = Image size ÷ Magnification
- Image size = Actual size × Magnification
Example: A cell appears 30mm under a microscope with magnification ×200.
What is the actual size?
Actual size = 30 ÷ 200 = 0.15mm
Example 1 : A cell is viewed under a microscope with a magnification of ×400. The image of the cell measures 80mm. What is the actual size of the cell?
Actual size = Image size ÷ Magnification
Actual size = 80 ÷ 400
Actual size = 0.2mm
Example 2: A student draws a cell that measures 60mm in her notebook. The actual size of the cell is 0.03mm. What magnification was the microscope set to?
Magnification = Image size ÷ Actual size
Magnification = 60 ÷ 0.03
Magnification = ×2000
Example 3: A bacterium has an actual size of 0.005mm. It is viewed under a microscope with a magnification of ×1000. How large will the image appear?
Image size = Actual size × Magnification
Image size = 0.005 × 1000
Image size = 5mm
Common Mistakes to Avoid
① Writing the photosynthesis equation the wrong way around — reactants (CO₂ and H₂O) go on the left, products (glucose and O₂) go on the right.
② Saying plants only respire at night — plants respire all the time, day and night. It is photosynthesis that only happens in light.
③ Confusing chloroplasts and mitochondria — chloroplasts are for photosynthesis (storing energy); mitochondria are for respiration (releasing energy).
④ Saying oxygen is a raw material for photosynthesis — oxygen is a product, not a reactant. CO₂ and water are the raw materials.
⑤ Forgetting that temperature affects photosynthesis through enzymes — always mention enzymes when explaining why temperature affects the rate.
⑥ Drawing the temperature graph the same shape as the light and CO₂ graphs — the temperature graph has a sharp drop after the optimum due to enzyme denaturation. Light and CO₂ graphs level off gradually into a plateau.
⑦ Saying stomata are on the top of the leaf — stomata are found mainly on the underside (lower epidermis) of the leaf to reduce water loss from direct sunlight.
Frequently Asked Questions
Q1. Do plants photosynthesise at night?
A1: No. Photosynthesis requires light energy — it cannot occur in darkness. At night, plants only carry out respiration, taking in oxygen and releasing carbon dioxide. During the day, photosynthesis occurs at a faster rate than respiration, so there is a net uptake of CO₂ and release of O₂. At a specific light level called the compensation point, the rate of photosynthesis exactly equals the rate of respiration — there is no net gas exchange.
Q2. Why is chlorophyll green?
A2: Chlorophyll appears green because it absorbs red and blue wavelengths of light for photosynthesis but reflects green wavelengths back to our eyes. The reflected green light is what we see when we look at a plant. This also means green light is the least effective wavelength for photosynthesis — most of it is reflected rather than absorbed.
Q3. What is the difference between a limiting factor and an optimum condition?
A3: A limiting factor is any factor that is in short supply and is therefore slowing down the rate of photosynthesis. The optimum condition is the ideal value of that factor at which the rate of photosynthesis is at its maximum. For temperature, the optimum is around 25–30°C for most plants. Beyond the optimum, increasing the factor further (in the case of temperature) causes damage to enzymes and the rate drops.
Q4. Why are leaves broad and flat?
A4: Leaves are broad and flat to maximise their surface area — the greater the surface area exposed to sunlight, the more light energy can be absorbed for photosynthesis. Being thin also means that light can penetrate to the inner palisade cells, and gases have only a short distance to diffuse to reach the photosynthesising cells.
Q5. How do greenhouse growers use knowledge of limiting factors?
A5: Greenhouse growers manipulate all three limiting factors to maximise the rate of photosynthesis and therefore increase crop yield. They use artificial lighting to extend the hours of light available, add CO₂ to the air to increase its concentration above the atmospheric level of 0.04%, and use heating systems to maintain the optimum temperature for their crops. Understanding limiting factors allows growers to invest in whichever factor will give the greatest return in productivity.
Q6. What is the compensation point?
A6: The compensation point is the light intensity at which the rate of photosynthesis exactly equals the rate of respiration. At this point, all the oxygen produced by photosynthesis is used up by respiration, and all the CO₂ produced by respiration is used up by photosynthesis — there is no net gas exchange. Below the compensation point, respiration exceeds photosynthesis and the plant is a net consumer of oxygen. Above the compensation point, photosynthesis exceeds respiration and the plant is a net producer of oxygen.
