PhotosynthesisIB MYP Biology: Revision notes
Section 1
What is photosynthesis?
Photosynthesis is the process by which plants make glucose from carbon dioxide and water, using light energy. It takes place in chloroplasts, which contain the green pigment chlorophyll that absorbs the light.
Word equation: carbon dioxide + water → glucose + oxygen
Balanced symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
(light energy and chlorophyll are needed, written above the arrow). Light energy is converted into chemical energy stored in the glucose, so photosynthesis is an endothermic process. Oxygen is released as a by-product.
Check the equation balances: 6 carbons, 12 hydrogens and 18 oxygens on each side.
Section 2
What happens to the glucose?
Plants use glucose in several ways:
- Respiration: to release energy for life processes.
- Starch: stored in leaves, roots and tubers. Starch is insoluble, so it does not affect the water balance of the cells.
- Cellulose: to build strong cell walls.
- Proteins: glucose combined with nitrates from the soil makes amino acids and then proteins.
- Lipids (fats and oils): stored in seeds as an energy store.
Section 3
How is the leaf adapted?
A leaf is designed to capture light and gases:
- Broad and flat: a large surface area to absorb light.
- Thin: a short distance for gases to diffuse.
- Palisade layer: tightly packed cells near the top with many chloroplasts.
- Stomata and guard cells: tiny pores, mostly on the underside, that open and close to let carbon dioxide in and oxygen out.
- Air spaces in the spongy layer for gas diffusion.
- Veins: xylem brings water to the leaf, and phloem carries sugars away.
- Transparent upper epidermis lets light reach the palisade cells.
Section 4
Testing a leaf for starch
Starch is a sign that photosynthesis has taken place. To test a leaf:
- Dip the leaf in boiling water for about a minute to kill it and soften it.
- Heat the leaf in hot ethanol in a water bath (never over a naked flame, as ethanol is flammable) to remove the chlorophyll.
- Rinse the leaf in water to soften it again.
- Add iodine solution.
A blue-black colour shows starch is present. If iodine stays orange-brown, there is no starch. A variegated leaf shows starch only in the green parts, because only they have chlorophyll.
Remove the chlorophyll first, or the green colour hides the blue-black result.
Must know
- Equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, needing light and chlorophyll.
- Photosynthesis occurs in chloroplasts.
- Glucose is used for respiration, starch, cellulose, proteins and lipids.
- Leaves are thin, broad and have palisade cells, stomata and veins.
- Starch test: boil, ethanol, rinse, iodine; blue-black means starch.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Photosynthesis
- A market gardener near Marrakech in Morocco grows spinach in sunny fields. The dark green spinach leaves make glucose in their cells using light energy, and the gardener wants to understand the process so that she can grow healthier plants.Write the balanced symbol equation for photosynthesis.2 marks
- Potato plants grow in cool, humid highland areas of Peru. During the growing season the leaves make large amounts of glucose. By harvest time, the underground tubers of the potato plants are full of stored food, but the leaves contain very little glucose.Explain why the potato plants convert glucose into starch for storage in the tubers.2 marks
- A student wants to show that chlorophyll is needed for photosynthesis. She uses a plant with variegated leaves, which have green patches and white patches. She keeps the plant in the dark for 48 hours, then in bright light for six hours. She then tests one leaf for starch with iodine solution. The green patches of the leaf turn blue-black, and the white patches stay orange-brown.Describe how the student should test the leaf for starch after the six hours in light.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).