Photosynthesis
Senior 2 · Biology
Photosynthesis
Lesson 1 of 4
22 min
Let's begin
Every meal you eat, whether it is ugali, isombe, beans or a glass of milk, can be traced back to a green plant capturing sunlight. The tea and coffee that Rwanda exports, the firewood and charcoal many families cook with, and the oxygen in every breath you take all come from one process: photosynthesis.
In this lesson you will learn what photosynthesis needs, what it makes, where in the plant it happens, and how to prove in the laboratory that a leaf has been photosynthesising.
In the 1600s the Flemish scientist Jan Baptist van Helmont planted a 2.3 kg willow shoot in a pot holding a weighed amount of dry soil. For five years he added only rainwater. At the end the tree weighed about 77 kg, yet the soil had lost only about 57 g.
Worked example
Where did the tree's extra mass come from?
Plants, algae and some bacteria make their own food using light, so they are called autotrophs ("self-feeders"). Animals, fungi and most bacteria must eat food that other organisms made, so they are heterotrophs ("other-feeders").
Photosynthesis is the process in which green plants use light energy, absorbed by chlorophyll, to make glucose from carbon dioxide and water, releasing oxygen as a by-product.
Word equation: carbon dioxide + water → glucose + oxygen (light energy absorbed by chlorophyll)
Balanced symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Check the balance: carbon 6 = 6; hydrogen 12 = 12; oxygen (12 + 6) = 18 on the left and (6 + 12) = 18 on the right.
Photosynthesis takes place mainly in the leaves, in the middle layer called the mesophyll. Inside each mesophyll cell are chloroplasts. A chloroplast contains stacks of membranes (each stack is a granum) holding chlorophyll, surrounded by a fluid called the stroma.
The process has two linked stages:
| Glucose is… | because… |
|---|---|
| used in respiration | it releases energy for the plant's cells |
| changed to starch and stored | starch is insoluble, so it does not upset the water balance of cells (osmosis) |
| changed to sucrose | sucrose is the form carried round the plant in the phloem |
| changed to cellulose | cellulose builds cell walls |
| combined with nitrates to make amino acids and proteins | proteins are needed for growth and enzymes |
Glucose is quickly turned into starch, so finding starch in a leaf shows that photosynthesis has taken place.
Worked example
A destarched plant with variegated leaves (green with white edges) is left in sunlight for a day. After the starch test, the green parts turn blue-black but the white edges stay orange-brown. Conclusion: the white parts have no chlorophyll, and they made no starch, so chlorophyll is needed for photosynthesis.
The same method shows the other needs: cover part of a leaf with a strip of foil and that part stays brown (light is needed); enclose a leaf in a flask with potassium hydroxide, which absorbs carbon dioxide, and it stays brown (carbon dioxide is needed).
Tip
Memory hook: "6 in, 6 in, 1 out, 6 out": 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
Exam technique: when asked why a plant is destarched, write the full reason: "so that any starch found at the end must have been made during the experiment." Just "to remove starch" loses the mark.
Try each question first, then check the worked solution.
Name the two raw materials and the two products of photosynthesis.
Solution: Raw materials: carbon dioxide and water. Products: glucose (a sugar) and oxygen. Light and chlorophyll are conditions, not raw materials, because they are not used up to build the glucose.
A strip of black paper is clipped across a destarched leaf, which is then left in sunlight for six hours. Predict the result of the starch test and explain it.
Solution: The covered strip stays orange-brown and the rest turns blue-black. No light reached the covered part, so it could not photosynthesise and made no starch. This shows light is needed.
Why is the tube of ethanol heated in a water bath instead of directly over a Bunsen flame?
Solution: Ethanol is highly flammable. A water bath heats it safely (ethanol boils at about 78 °C) without any flame touching it.
Using van Helmont's figures (tree gained about 74.7 kg; soil lost 0.057 kg), calculate the soil's contribution as a percentage of the tree's gain.
Solution: (0.057 ÷ 74.7) × 100 ≈ 0.076%. The soil supplied less than a tenth of one percent, so the tree's mass came mainly from carbon dioxide and water.
Remember
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
