Photosynthesis
Section: Biology | Syllabus: Cambridge IGCSE Combined Science (0653)
What is Photosynthesis?
Photosynthesis is the process by which plants synthesise carbohydrates from raw materials using energy from light. It is the basis of almost all food chains on Earth.
Word Equation
carbon dioxide + water → glucose + oxygen
This reaction happens in the presence of light and chlorophyll.
Symbol Equation (Supplement)
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Chlorophyll and Light Absorption
- Chlorophyll is a green pigment found in the chloroplasts
- Chlorophyll transfers energy from light into energy in chemicals, which is used for the synthesis of carbohydrates
- Chlorophyll reflects green light - which is why leaves appear green
- Without chlorophyll, a plant cannot photosynthesise
Chlorophyll, held within the grana of the chloroplast, absorbs light energy to drive photosynthesis
Investigating the Requirements for Photosynthesis
Photosynthesis requires light, chlorophyll and carbon dioxide. Each requirement can be tested using an appropriate control, using the iodine test for starch as evidence that photosynthesis has taken place.
Testing the Need for Light and Chlorophyll
- Destarch the plant by keeping it in the dark for 24-48 hours, so any starch already present is used up
- Cover part of a leaf with foil to exclude light, or use a variegated leaf (with green and white areas) to compare chlorophyll presence directly
- Leave the plant in bright light for several hours
- Remove the leaf, boil it in water to kill the cells, then boil it in ethanol to remove the chlorophyll (decolourise it)
- Rinse the leaf with water, spread it flat, and add iodine solution
- Result: areas that received light and contained chlorophyll turn blue-black (starch present). Covered areas, and white (non-green) areas of a variegated leaf, remain yellow-brown (no starch) - showing that both light and chlorophyll are needed for photosynthesis
Only the parts of the leaf with chlorophyll and access to light turn blue-black with iodine
Testing the Need for Carbon Dioxide
- Destarch two similar plants. Enclose one leaf in a flask containing soda lime, which absorbs carbon dioxide from the air; enclose a similar leaf on the second plant in a flask with no soda lime, as a control
- Leave both leaves in bright light for several hours, then test each with iodine solution
- Result: the control leaf turns blue-black (starch present, since CO₂ was available), but the leaf enclosed with soda lime remains yellow-brown (no starch) - showing that carbon dioxide is needed for photosynthesis
Common Mistakes
- ❌ Skipping the destarching step
- ✓ if the plant already contains starch before the experiment, any starch found afterwards cannot be reliably linked to new photosynthesis. Destarching first ensures that any starch detected was made during the experiment
Effect of Light Intensity, Carbon Dioxide and Temperature on Rate
The rate of photosynthesis in an aquatic plant, such as Elodea, can be investigated by counting the number of oxygen bubbles it produces per minute.
- Light intensity: increasing light intensity increases the rate of photosynthesis, up to a point beyond which the rate levels off
- Investigated by varying the distance between the plant and a lamp - light intensity is inversely proportional to the square of the distance from the source (I ∝ 1/d²), so doubling the distance reduces light intensity to one quarter
- Carbon dioxide concentration: increasing CO₂ concentration increases the rate of photosynthesis, up to a point beyond which the rate levels off
- Investigated by varying the concentration of sodium hydrogencarbonate dissolved in the water around the plant, which releases CO₂ into solution
- Temperature: increasing temperature increases the rate of photosynthesis up to an optimum, after which the rate decreases as the enzymes controlling the reactions become denatured
- Investigated by placing the apparatus in water baths set to different temperatures
In each investigation, all variables other than the one being tested must be kept constant as control variables - including the species and length of the plant used.
Investigating Gas Exchange Using Hydrogencarbonate Indicator
Hydrogencarbonate indicator solution changes colour depending on the carbon dioxide concentration around it, and can be used to show whether an aquatic plant is photosynthesising:
- Red/orange - normal atmospheric CO₂ concentration
- Yellow - high CO₂ concentration (more acidic)
- Purple - low CO₂ concentration (more alkaline)
An aquatic plant is placed in a test tube of hydrogencarbonate indicator solution:
- In the light: the plant photosynthesises faster than it respires, so overall it removes more CO₂ than it produces - the indicator turns purple
- In the dark: the plant cannot photosynthesise, so it only respires, releasing CO₂ - the indicator turns yellow
- If photosynthesis and respiration are roughly balanced, the CO₂ produced and used are approximately equal, so the indicator shows little colour change
Limiting Factors of Photosynthesis (Supplement)
A limiting factor is the factor present at the lowest level that restricts the rate of photosynthesis, even if other factors are in good supply.
Once a graph of rate against one factor plateaus, a different factor has become limiting
When a curve flattens (plateaus), increasing that factor no longer increases the rate - a different factor is now limiting. For example, if a graph of rate against light intensity plateaus early, increasing the carbon dioxide concentration or temperature may raise the plateau further.
Worked Example: Identifying the Limiting Factor
- Question: a plant is in bright light with plenty of water, but its rate of photosynthesis is low. What is the most likely limiting factor?
- Answer: since light is not limiting, the most likely limiting factor is low carbon dioxide concentration (or possibly low temperature). In typical indoor or enclosed environments, CO₂ is often the limiting factor
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