Photosynthesis
Section: Biology | Syllabus: Cambridge Lower Secondary Checkpoint Science (0893)
What is Photosynthesis?
- Photosynthesis is the process by which plants make carbohydrates (glucose) using light energy from the sun, using carbon dioxide and water
- It is essential for life on Earth
- it is the starting point of almost every food chain, and it produces the oxygen we breathe
- Takes place inside chloroplasts
- a chloroplast is the organelle in plant cells where photosynthesis happens, it contains the green pigment chlorophyll
- chloroplasts are found in the green parts of plants, mainly leaves
- Converts light energy into chemical energy
- the energy doesn't disappear — it's stored in the bonds of glucose molecules
- Provides food for plants and oxygen for other organisms
- this is why photosynthesis supports almost all life on Earth, not just plants
- Only occurs in cells containing chloroplasts
- this is why roots and other non-green parts of a plant cannot photosynthesise
The Photosynthesis Equation
- Word equation:
- carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll)
- Reading left to right: carbon dioxide and water are the raw materials the plant absorbs, and they are chemically rebuilt into glucose, with oxygen released as a by-product
- light and chlorophyll are written underneath the arrow rather than as reactants, because they make the reaction happen but are not themselves used up or turned into anything new
- Light energy and chlorophyll are required for photosynthesis but are not reactants
- they enable the reaction to occur without being used up
Word equation for photosynthesis
| Reactants (What's Needed) | Products (What's Made) |
|---|---|
| Carbon dioxide (CO₂) | Glucose (C₆H₁₂O₆) |
| Water (H₂O) | Oxygen (O₂) |
| Light energy | - |
| Chlorophyll (catalyst) | - |
Exam Tip
- If a question asks you to "write the word equation for photosynthesis," you only need carbon dioxide + water → glucose + oxygen
- light and chlorophyll are conditions, not part of the equation itself, so don't write them as reactants
Requirements for Photosynthesis
Light Energy
- Absorbed by chlorophyll, the green pigment in chloroplasts
- chlorophyll is what makes leaves appear green, because it reflects green light while absorbing red and blue light
- Provides energy to drive the chemical reactions
- without this energy input, carbon dioxide and water cannot be joined together into glucose
- More light intensity increases photosynthesis rate, up to a limit
- beyond that limit, some other factor (like CO₂ or temperature) becomes the one holding the rate back
Carbon Dioxide
- Enters leaves through tiny pores called stomata
- stomata are mostly found on the underside of the leaf, which reduces water loss
- Diffuses into leaf cells and chloroplasts
- this movement happens because CO₂ is at a lower concentration inside the leaf than in the air, so it moves down its concentration gradient
- Provides carbon atoms needed to make glucose
- glucose (C₆H₁₂O₆) needs a carbon "skeleton," and CO₂ is the only source of that carbon
Water
- Absorbed by roots from the soil
- root hair cells give the root a large surface area, which speeds up water uptake
- Transported through xylem vessels to leaves
- xylem forms a continuous tube from root to leaf, allowing a steady water supply
- Provides hydrogen atoms needed to make glucose
- the oxygen released during photosynthesis actually comes from this water, not from the CO₂
Chlorophyll
- Green pigment found in chloroplasts
- Absorbs light energy for photosynthesis
- Acts as a catalyst
- it speeds up the reaction but isn't used up or changed by it, which is why the same chlorophyll molecules keep working day after day
- Magnesium is needed to make chlorophyll
- magnesium is a mineral absorbed by the roots from the soil and is a required component for building chlorophyll molecules
- a magnesium deficiency means the plant cannot make enough chlorophyll, so the leaves turn yellow — this is called chlorosis
Cross-section of a leaf showing where light, carbon dioxide and water each enter and reach the chloroplasts
Uses of Glucose
- Glucose produced during photosynthesis is not stored directly
- plants convert it into various substances depending on what they need at the time
- Glucose is converted to starch for storage
- starch is insoluble and doesn't affect water concentration in cells, unlike glucose
| Use | Purpose |
|---|---|
| Respiration | Broken down to release energy for life processes |
| Stored as starch | Converted to starch for long-term storage (in roots, stems, seeds) |
| Making cellulose | Used to build cell walls for structure and strength |
| Making proteins | Combined with minerals (nitrates) to form proteins for growth |
| Making fats/oils | Converted and stored in seeds |
Factors Affecting Photosynthesis Rate
Light Intensity
- Low light: slow photosynthesis
- reason: there isn't enough energy available to drive the reaction
- High light: fast photosynthesis
- reason: more energy is available to power the reaction
- Very high light: rate plateaus
- reason: another factor, such as CO₂ or temperature, becomes limiting instead
- Light intensity decreases as distance from a light source (such as a lamp) increases — this is an inverse relationship
- the further a plant is from the lamp, the lower the light intensity reaching it, so the slower the rate of photosynthesis — seen experimentally as fewer bubbles of oxygen (or a smaller volume of gas) produced in a fixed time
Carbon Dioxide Concentration
- Low CO₂: slow photosynthesis
- reason: there isn't enough raw material to build glucose
- High CO₂: fast photosynthesis
- reason: more raw material is available for the reaction
- Normal air contains about 0.04% CO₂
- this is naturally quite low, which is why CO₂ is very often the limiting factor outdoors
Temperature
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