IGCSE Biology 0610
6.1 Photosynthesis
- Photosynthesis is the process by which plants make carbohydrates from raw materials using energy from light
Word equation for photosynthesis:
carbon dioxide + water → glucose + oxygen
in the presence of light and chlorophyll
Balanced chemical equation for photosynthesis:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
- Chlorophyll is a green pigment that is found in chloroplasts
- Chlorophyll transfers energy from light into energy in chemicals, for the synthesis of carbohydrates
Uses and Storage of Carbohydrates made in Photosynthesis
- Starch → Energy source
- Cellulose → Build cell walls
- Glucose → Respiration for energy
- Sucrose → Transport in phloem
- Nectar → Attract insects for pollination
Bonus: Use the mnemonic “Some Cool Guys Sing Nicely”
Mineral Requirements
- Nitrate ions → A source of nitrogen to make amino acids. Nitrate deficiency causes stunted growth and yellow leaves
- Magnesium ions → To make chlorophyll. Magnesium deficiency causes yellowing between leaf veins (chlorosis)
Investigating the need for Chlorophyll, Light, and Carbon Dioxide
- Chlorophyll
-Destarch a variegated plant by leaving the plant in darkness for 48 hours
-Leave the plant in light for a few hours
-Pluck a leaf from the plant
-Test leaf for starch
Result: Only the green parts which contain chlorophyll turn blue-black
Explanation: The green parts contain chlorophyll which is needed for photosynthesis to make starch. The white parts do not contain chlorophyll, so no photosynthesis occurs.
Control: Use a non-variegated plant - Light
-Destarch a plant by leaving the plant in darkness for 48 hours
-Cover part of a leaf from the plant with aluminium foil to prevent light from passing through
-Leave plant in light for a few hours
-Test leaf for starch
Result: Only uncovered parts of the leaf will turn blue-black
Explanation: The parts of the leaf that were covered did not receive light and could not carry out photosynthesis, and so could not make starch
Control: Do not cover leaf with foil - Carbon Dioxide
-Destarch a plant by leaving the plant in darkness for 48 hours
-Enclose the de-starched plant in a plastic bag with soda lime (a chemical that absorbs carbon dioxide)
-Leave the plant in light for a few hours
-Test leaf for starch
Result: Negative result (iodine remains yellow-brown)
Explanation: Without carbon dioxide, the leaf is not able to photosynthesise and make starch
Control: Do not use soda lime
Investigating Factors for Rate of Photosynthesis
- Determine rate of photosynthesis by measuring oxygen. Either count bubbles or measure gas
- Light intensity
-Cut a 5cm long piece of pondweed (aquatic plant)
-Attach a paperclip to the pondweed so that it doesn’t float
-Place pondweed into a beaker of water
-Place the lamp close to the plant, measure distance between lamp and beaker, count bubbles for 5 minutes. Repeat and calculate average
-Repeat previous step for different distances
-Maintain constant temperature by checking with a thermometer and adding hot/cold water when necessary
Increasing light intensity increases the energy available to the plant for photosynthesis - Carbon dioxide concentration
– Place pondweed in test-tubes (at least 5)
-Attach a paperclip to the pondweed so that it doesn’t float
-Add different masses of sodium hydrogen carbonate in each test-tube
-Constant light intensity and temperature (keep lamp at same distance throughout, add hot/cold water when necessary)
-Count the number of bubbles produced in 5 minutes
- Temperature
-Place pondweed into test-tubes with water
-Attach a paperclip to the pondweed so that it doesn’t float
-Test-tubes are put into beakers of water. The minimum is five different temperatures across a range of 10°C to 40°C
-Lamp is at a constant distance to ensure constant light intensity
-Add same amount of sodium hydrogen carbonate to water so there is constant supply of carbon dioxide
-Count the number of bubbles produced in 5 minutes
Photosynthesis decreases at high temperatures as enzymes in chloroplasts are denatured
Investigating Light & Dark Conditions for Gas Exchange
- Plants carry out respiration all the time, where they take in oxygen and release carbon dioxide
- During the day, plants also carry out photosynthesis, where they take in carbon dioxide and give out oxygen. Plants carry out photosynthesis at a faster rate than they carry out respiration, so there is a net intake of carbon dioxide and net output of oxygen
- At night, plants do not carry out photosynthesis so they take in oxygen and give out carbon dioxide
- We can investigate the effect of light on plant gas exchange using hydrogencarbonate indicator (a pH indicator)
- Hydrogencarbonate indicator shows the concentration of carbon dioxide in a solution as carbon dioxide is an acidic gas
- Hydrogencarbonate indicator colours:
High CO₂ -> atmospheric level -> Low CO₂
yellow orange red magenta purple - When photosynthesis>respiration, there is a net intake of carbon dioxide. The solution becomes more alkaline and the indicator turns orange/yellow
- When respiration>photosynthesis, there is a net output of carbon dioxide. The solution becomes more acidic and the indicator turns magenta/purple
Procedure:
-Add hydrogencarbonate indicator to 3 test-tubes
-Cut 2 pieces of pondweed equal in length and place them in 2 of the test-tubes (1 test-tube will be empty)
-Fully cover one of the test-tubes with pondweed with foil
-Place the test-tubes near a light for a few hours
Result:
In the covered test-tube, the indicator turned yellow because carbon dioxide was released by the pondweed during respiration.
In the other plant test-tube, the indicator turned purple because carbon dioxide was taken in for photosynthesis.
In the empty test-tube which was a control, the indicator remained red.
Limiting Factors in Photosynthesis
A limiting factor is something present in the environment in such short supply that it restricts life processes
- Light intensity
Determines energy available for photosynthesis
If there is low light intensity but lots of carbon dioxide and water and warm temperature, light intensity is the limiting factor - Temperature
Influences enzyme activity in chloroplasts.
When temperature increases, rate of photosynthesis increases until a certain temperature then decreases. This is because increasing temperature increases the rate of reaction for the enzymes in chloroplasts. At too high temperatures, enzymes become denatured. - Carbon dioxide
Raw material for photosynthesis
Identifying the limiting factor:
Let’s go back to our earlier graph
We can see that as carbon dioxide increases, rate of photosynthesis increases. The rate increases as carbon dioxide is the limiting factor.
However, at a certain point, the rate becomes constant no matter how much the carbon dioxide increases. This means that carbon dioxide is no longer the limiting factor and it must be something else like light intensity or temperature.