Chapter 20: Photosynthesis Flashcards
What is photosynthesis?
Photosynthesis is an anabolic process by which complex organic substances (C₆H₁₂O₆) are made from simple inorganic substances (CO₂, H₂O) using light energy. Oxygen is released as a by-product.
What are the examples of photosynthetic organisms?
Plants, some protists (e.g. algae), some prokaryotes (e.g. Cyanobacteria)
How do plants obtain energy?
Plants are autotrophs (Make their own organic substances from simple inorganic substances).
- Plants have photosynthetic pigments, e.g. chlorophyll (green pigments) & carotenoids, to absorb light energy.
- During photosynthesis, light energy absorbed by photosynthetic pigments is converted to chemical energy stored in organic substances formed.
What is the word equation for the overall process of photosynthesis?
State the by-product and raw materials of photosynthesis.
Equation: Carbon dioxide + Water (light energy; chlorophyll) → Glucose + Oxygen
By-product: Oxygen; Raw materials: Carbon dioxide, Water
What is the significance of photosynthesis?
Providing the basic food source in ecosystem
- Plants are producers of the food chain which transform light energy into biomass (chemical energy).
- Most organisms depend directly or indirectly on plants for food.
Maintaining energy flow in ecosystem
- During photosynthesis, light energy is converted into chemical energy which is passed along the food chain to support other organisms.
- Energy is continuously lost during energy transfer along the food chain, thus photosynthesis is important in providing constant input of energy to the ecosystem.
Maintaining oxygen and carbon dioxide balance
- Plants maintain steady air composition by releasing O₂ and absorbing CO₂ so that the ecosystem can be self-sustaining.
Why should we detect the occurrence of photosynthesis by the presence of starch?
We can test the presence of starch with iodine solution to determine a plant has carried out photosynthesis.
∵ One of the products of photosynthesis, glucose, is converted to starch, as the storage form of glucose in leaves.
Why we cannot test the presence of starch by adding iodine solution onto an untreated leaf directly?
- Cell membrane is impermeable to iodine solution. (1 mark)
- Chlorophyll will mask up the colour change of iodine solution. (1 mark)
What are the procedures to detect the presence of starch produced in photosynthesis?
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Boil the leaf in a beaker of water for 1 minute.
Purpose: (1) To destroy the differentially permeable nature of cell membrane allowing the iodine solution to pass into the cells. (2) To kill the leaf cells and stop chemical reactions e.g. photosynthesis. -
Turn off the Bunsen burner.
Why? Alcohol is inflammable → catch fire easily. -
Immerse the boiled leaf into a boiling tube half filled with alcohol.
Purpose: To dissolve and remove the chlorophyll (delourize) allowing the colour change in iodine test clearly be observed → renew the alcohol until the leaf turns white in colour. -
Put the tube into a beaker of hot water to heat up the alcohol.
Purpose: To speed up the diffusion of chlorophyll out the cells. -
Wash the decolourized leaf in a beaker of hot water.
Purpose: To remove the alcohol and soften the leaf.
(Alcohol is dehydrating! The leaf will become hard and brittle.) -
Spread the leaf on a white tile. Add a few drops of iodine solution on the decolourized leaf surface.
→ If the result is positive, iodine solution changes from reddish brown to blue-black all over the leaf.
What are the procedures to detect the presence of oxygen produced in photosynthesis? Is it a qualitative or quantitative test?
Set-up: Refer to book p.5 / Class notes
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Set up the apparatus as shown.
Use of the stoppers: To raise the funnel allowing free circulation of solution.
Purpose of using sodium hydrogencarbonate solution: ↑ conc. of CO₂, to provide sufficient CO₂ for photosynthesis to take place, such that oxygen gas can be collected in a shorter time. - Expose the set-up to bright light for at least 24 hours to collect enough gas released from the plant.
- Stopper the mouth of the test tube under water.
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Put a glowing splint over the mouth of the test tube to test for oxygen.
If it relights → the gas released is O₂.
Qualitative test ∵ It only tests the production of O₂ and the consumption of CO₂ by the plant.
How to destarch a plant + What is the purpose, principle & significance of destarching?
Method:
- Put the plant in the dark for at least 24 hours.
Purpose:
- To remove starch from the plant before the experiment.
Principle:
- In darkness, photosynthesis stops, starch stored in leaves is hydrolysed into glucose, which is oxidized in respiration to release energy. It may also combine with fructose and is converted to sucrose, which is transported by phloem away from the leaves.
Significance:
- To make sure any starch detected after the experiment is formed during the experiment by photosynthesis in leaves, but not previously stored.
How do we test whether a plant is destarched?
Refer to the procedures for testing the presence of starch produced in photosynthesis.
How do the two set-ups: experimental set-up and control set-up, determine the requirements for photosynthesis to take place?
If photosynthesis has occured in the experimental set-up, but not in the control set-up, indicates that the factor under investigation is needed for photosynthesis.
What are the procedures to investigate the need for light in photosynthesis?
^Experimental setup, control setup, procedures, results + conclusion
Experimental setup:
- The part of the leaf exposed to light, not covered with black paper
Control setup:
- The part of the leaf covered by black paper
Procedures (Refer to notes p.9):
- Destarch a plant by putting it in the dark for at least 24 hours.
- Attach two pieces of black papers with similar size as the plant, and a hole punched at the centre, to both sides of a leaf on the plant. Fix them with paper clips.
- Put the plant under bright light for about 5 hours.
- Test if starch is present with iodine test.
Results + Conclusion:
- The exposed part of the leaf turns blue-black in the iodine test, but the masked part does not.
- This indicates that starch is not present in the masked part. Light is required for photosynthesis.
What are the procedures to investigate the need for carbon dioxide in photosynthesis?
^Experimental setup, control setup, procedures, results + conclusion
Experimental setup:
- Leaf with CO₂
Control setup:
- Leaf without CO₂ (Soda lime, Potassium hydroxide pellet, Sodium hydroxide pellet added)
Procedures:
- Destarch a plant by putting it in the dark for at least 24 hours.
- Set up the apparatus as shown.
- Put the plant under bright light for about 5 hours.
- Test if starch is present with iodine test.
Results + Conclusion:
- Leaf with CO₂ turns blue-black in the iodine test but leaf without CO₂ does not.
- This shows that starch present in leaf with CO₂, but absent in Leaf without CO₂. CO₂ is required for photosynthesis.
What are the procedures to investigate the need for chlorophyll in photosynthesis?
Experimental setup:
- Green part of the variegated leaf (have chlorophyll)
Control setup:
- Non-green part of the variegated leaf (chlorophyll is absent)
Procedures:
- Destarch a variegated plant by putting it in the dark for at least 24 hours.
- Put the plant under bright light for about 5 hours.
- Test if starch is present with iodine test.
Results + Conclusion:
- The green parts of the leaf turn blue-black in the iodine test, but the non-green parts do not.
- This shows that starch is present only in the green parts which contain chlorophyll. Chlorophyll is required for photosynthesis.
