Life Processes in Plants · Lesson 2 of 6
How Leaves Make and Store Food
“Use starch tests to discover how sunlight and chlorophyll help leaves make food.”
• Explain why leaves are described as food factories. • Describe chlorophyll and the iodine test for starch. • Interpret tests on leaves exposed to sunlight or kept in darkness. • Connect green patches with detectable starch production. • Distinguish a leaf’s visible colour from the presence of chlorophyll.
Leaves as food factories
Animals obtain food by eating plants or other animals. Plants obtain their food in a different way: they make it. A leaf is usually the main place where this happens. To understand that statement, we need evidence that food is present and that particular conditions help the leaf produce it.
Many leaves are broad and flat. This shape gives a surface that can receive light. Most leaves look green because they contain chlorophyll, a green pigment that helps capture sunlight. A pigment is a substance that gives colour; here, chlorophyll has an important role in using light for food production as well.
The green pigment in plant parts that helps capture sunlight for food production.
Plants can store food as starch, a type of carbohydrate. A carbohydrate is a food substance; glucose and starch are examples. Finding starch in a leaf gives us a way to investigate food production without being able to watch the invisible chemical changes directly.
A carbohydrate in which plants can store food. Dilute iodine solution turns blue-black where enough starch is present to be detected.
The expression food factory is an analogy. It means that leaves produce food from materials supplied to them, using light. A leaf does not contain miniature kitchens, and roots do not deliver ready-made meals from the soil.
What the iodine test reveals
An iodine test is useful only if you know what the colour change means. Dilute iodine is added to a prepared leaf, and a blue-black colour indicates starch. Preparing the leaf first allows that colour change to be seen clearly.
In the teacher’s demonstration, the leaf is kept in boiling water for about five minutes to soften it. It is then placed in alcohol in a test tube. The test tube is warmed in a beaker of hot water until the leaf loses its green colour. Removing the colour prevents the green pigment from hiding the iodine result.
This is a teacher demonstration. Alcohol is highly flammable and must never be heated directly over a flame or placed near a heat source where it can ignite. The alcohol-containing tube is warmed using hot water. Students should interpret the demonstration rather than perform unsafe heating.
The decolourised leaf is placed on a plate and a few drops of dilute iodine are added. After a short wait, blue-black regions show detectable starch. Decolourisation helps us see the result; it does not itself produce starch.
Problem
Why would it be harder to interpret an iodine test on a dark green leaf that has not been decolourised?
- 1.The test depends on seeing whether iodine produces a blue-black colour.
- 2.The leaf’s existing green colour can make the new colour difficult to distinguish.
- 3.Removing the pigment improves visibility, allowing a clearer judgement about starch.
A positive iodine test shows that starch is present. By itself it does not establish when that starch formed or which condition caused it to form. Comparisons between prepared leaves are needed to investigate those questions.
Comparing sunlight and green patches
A variegated leaf has patches of different colours, often green and non-green. It gives us an especially helpful comparison: two regions of the same leaf can receive the same surrounding conditions while differing in their visible green pigment. We can also compare similar plants kept in sunlight and darkness.
Bhaskar’s investigation uses a leaf with green and non-green patches from each of two similar potted plants. One plant is kept in sunlight, while the other is kept in the dark for 36 hours. Before testing, he traces or sketches the leaves and records where the different patches are. This starting sketch matters because the leaf loses its original colour during preparation.
After the iodine test, he compares each patch with its position on the starting sketch. On the leaf from the sunlit plant, the green patches become blue-black. The non-green patches do not. On the leaf from the plant kept in darkness, neither set of patches shows the blue-black result.
| Plant condition | Green patches after iodine | Non-green patches after iodine | Interpretation |
|---|---|---|---|
| Sunlight | Blue-black | No blue-black colour | Detectable starch is associated with the green patches exposed to light |
| Dark for 36 hours | No blue-black colour | No blue-black colour | The leaf does not show detectable starch under the recorded dark condition |
Compare the green patches on the two leaves first. Both are green, but the patches from the sunlit plant show starch and those from the dark-kept plant do not. This supports the importance of light. Next compare green and non-green patches on the sunlit leaf. Their different results support the importance of sufficient chlorophyll.
Together, the observations support the conclusion that sunlight and chlorophyll are needed for food production. They are stronger than simply noticing that many healthy leaves are green, because the investigation compares conditions and uses a test for starch.
Problem
How does the patch investigation provide evidence about both sunlight and chlorophyll?
- 1.Compare green regions from the sunlit and dark-kept plants: a difference in the starch result supports the importance of sunlight.
- 2.Compare green and non-green regions on the sunlit leaf: the blue-black result is associated with green regions.
- 3.Use both comparisons together to explain the need for light and sufficient chlorophyll in starch production.
A non-green patch may contain too little chlorophyll to make enough starch for the iodine test to detect. Do not claim that every such patch contains absolutely no chlorophyll. The colour test also measures detectable starch, rather than every tiny amount that might be present.
Can a red or purple leaf make food?
A leaf’s appearance does not always reveal every pigment inside it. Red, violet, or brown leaves can contain chlorophyll even when other pigments hide its green colour. Such leaves can still carry out photosynthesis, the process of making food using light.
Some of the other pigments also assist with capturing light. An iodine test can provide evidence of starch in these coloured leaves. This does not contradict the need for chlorophyll: the leaf’s green pigment may be hidden rather than absent.
Problem
A purple leaf gives a blue-black result with iodine after appropriate preparation. Does this show that chlorophyll is unnecessary?
- 1.A blue-black result indicates starch in the prepared leaf.
- 2.Purple appearance does not establish that chlorophyll is absent; other pigments can mask the green colour.
- 3.The result is consistent with a leaf containing chlorophyll and making food, even though it does not appear green.
We can now connect the evidence. Leaves are important food-making sites; chlorophyll helps them capture sunlight; and starch gives us a detectable sign of stored food. We still need to identify the materials used to make that food and find out whether anything else is released.
Quiz
What does a blue-black iodine result on a prepared leaf indicate?
Why is the leaf decolourised before testing?
Which result best supports a role for sunlight?
Why record the patch positions before decolourising a leaf?
A red leaf can make food because it may contain which hidden pigment?
What is the most careful interpretation of a non-green patch with no blue-black result?
Practice Problems
- Explain how a broad, flat leaf and chlorophyll each help with food production.
- Describe the purpose of the iodine test and why the leaf is decolourised first.
- Outline the teacher’s starch-test demonstration, including the safety precaution about alcohol.
- Use the sunlight-versus-darkness patch results to explain the roles of sunlight and chlorophyll.
- A purple leaf and a pale non-green patch both look non-green. Explain why their appearances alone do not establish that both lack chlorophyll.
- Why does finding starch in a leaf not, by itself, prove the time at which the starch was produced?
Key Takeaways
• Leaves are the main food-making sites in many plants. • Chlorophyll helps capture sunlight, and starch is a stored carbohydrate. • Blue-black iodine colour indicates detectable starch. • Light-and-patch comparisons support the need for sunlight and chlorophyll. • Other pigments can hide chlorophyll, so coloured leaves may still make food. • Use the evidence from a test carefully and follow the teacher-demonstration safety precautions.