Life Processes in Plants · Lesson 3 of 6
Photosynthesis: Inputs, Products, and Gas Exchange
“Connect experimental evidence into an explanation of photosynthesis and the exchange of gases through leaves.”
• Interpret evidence that carbon dioxide is needed for food production. • Explain how an investigation reveals oxygen release in light. • Describe the inputs, conditions, and products of photosynthesis. • Distinguish glucose production from starch storage. • Explain how stomata allow gas exchange between leaves and air. • Recognise that other plant parts containing chlorophyll can also photosynthesise.
Does air supply a material for making food?
We have evidence that sunlight and chlorophyll are important for food production. Water is also essential. But a plant’s surroundings contain more than water and light: its leaves are in contact with air. We can investigate whether a gas in that air supplies another material needed to make food.
Air is a mixture of gases. Carbon dioxide is one of them. To investigate its role, we need to compare a green leaf region with access to carbon dioxide against a region without that access, while keeping water and light available.
The teacher first keeps a potted green plant in darkness for two or three days. During this time, the plant uses stored food, helping remove the starch already in its leaves. This preparation is called destarching. Without it, old starch could give a positive iodine result even in a region that has made no new food during the investigation.
Preparing a plant so that previously stored starch in its leaves is used up before a starch-production investigation, commonly by keeping it in darkness for a suitable time.
Half of a chosen leaf is inserted into a wide-mouthed bottle through a split cork, and the other half remains outside. The bottle contains sodium hydroxide, also called caustic soda, which absorbs carbon dioxide from the enclosed air. The setup is sealed around the leaf and placed in sunlight for a few hours. Both leaf regions are still attached to the plant, so water can reach them.
Sodium hydroxide can cause chemical burns. Only the teacher should handle it and arrange this demonstration. Students should focus on comparing the conditions and interpreting the starch results.
| Leaf region | Water | Sunlight | Chlorophyll | Carbon dioxide | Starch-test result |
|---|---|---|---|---|---|
| Inside bottle | Available | Available | Present | Removed by sodium hydroxide | No blue-black colour |
| Outside bottle | Available | Available | Present | Available in surrounding air | Blue-black colour |
The outside region turns blue-black when tested for starch. The inside region does not. Because sodium hydroxide removes the carbon dioxide inside the bottle, the comparison supports the conclusion that carbon dioxide is needed for the leaf to make food. It is not enough simply to say that the bottle stops food production; the important changed condition is carbon dioxide availability.
Problem
A teacher skips destarching and finds some starch in the enclosed leaf half. Can that result show that carbon dioxide is unnecessary?
- 1.The iodine test detects starch present at the time of testing, including starch made earlier.
- 2.Without destarching, some starch may remain from before the leaf entered the carbon dioxide-removal setup.
- 3.That result would not establish new starch production without carbon dioxide. The preparation is needed for the comparison to answer the intended question.
Plants release oxygen while making food
A food-making process may release a substance as well as use materials. A water plant offers a way to see gas release because bubbles can move through water and be collected. Comparing light and dark conditions helps connect the observation to photosynthesis.
A water plant is placed under a funnel in a beaker of water. An inverted test tube, initially filled with water, is placed over the funnel stem. In sunlight, gas bubbles rise through the funnel and collect at the top of the inverted tube, pushing some water out. A matching setup kept in darkness provides a comparison.
In the sunlit setup, the collected gas is rich in oxygen. The demonstration described in the investigation tests the gas by showing that a lit match burns more intensely in it. This is evidence about the gas, rather than a reason for students to handle flames. The comparison supports the conclusion that oxygen is released during photosynthesis in light.
Bubbles show that gas is being released; their appearance alone does not identify the gas. The oxygen test supplies additional evidence. Any flame test belongs in a teacher-controlled demonstration.
Problem
What are the separate steps in reasoning from the water-plant setup to oxygen release?
- 1.Gas bubbles collect in the inverted tube in the light setup, showing that a gas has been released.
- 2.The gas supports a more intense flame in the teacher demonstration, indicating that it is rich in oxygen.
- 3.Together with the light-and-dark comparison, this supports oxygen release during photosynthesis.
Putting the process together
The investigations now fit into a single explanation. Plants use carbon dioxide and water to make a carbohydrate called glucose. Sunlight provides the energy, and chlorophyll helps capture that light. Oxygen is released as a product. This process is called photosynthesis.
The process in which plant parts containing chlorophyll use light energy to make glucose from carbon dioxide and water, releasing oxygen.
Glucose is a simple carbohydrate. It can be used by the plant, and some can be converted into starch for storage. This distinction explains why an iodine test can be used as evidence about food production even though the food first produced is glucose. The test detects starch, not glucose directly.
A simple carbohydrate produced during photosynthesis. Plants can use it in processes that release energy or convert some of it into stored starch.
A word equation names the materials used and the substances produced. The arrow means that the materials on the left are changed into the products on the right. Sunlight and chlorophyll are shown with the arrow because they are essential conditions, rather than additional food products.
Read the equation as a relationship, rather than a list to memorise: the plant obtains carbon dioxide from air and water through its roots, uses captured light energy to make glucose, and releases oxygen. Soil minerals support growth, but they are not shown as the carbon dioxide or glucose in this equation.
Leaves are usually the main site because they contain chlorophyll and expose a surface to light. Other green plant parts containing chlorophyll can also perform photosynthesis. The essential condition is the presence of suitable food-making tissue with chlorophyll, rather than the organ having the name leaf.
Problem
A learner says, “Photosynthesis makes starch directly because iodine turns blue-black.” How would you improve the explanation?
- 1.The word equation identifies glucose as the food carbohydrate produced during photosynthesis.
- 2.The plant can convert glucose into starch for storage.
- 3.Iodine detects that stored starch, so a positive test can support evidence of food production without directly testing glucose.
Indian plant scientist Rustom Hormusji Dastur (1896–1961) studied how factors such as water availability, temperature, and the colour of light affect photosynthesis. He headed the Botany Department at the Royal Institute of Science in Bombay, now the Institute of Science in Mumbai, from 1921 to 1935. His work illustrates how a broad process can be investigated through carefully chosen conditions.
Where gases enter and leave a leaf
A leaf must obtain carbon dioxide from its surroundings and release oxygen during photosynthesis. Its surface contains tiny pores that allow gases to pass between the leaf and the surrounding air. These pores are called stomata; a single pore is called a stoma.
Tiny pores on the surfaces of leaves that help exchange gases between the plant and its surroundings.
The teacher can prepare a thin peel from the lower surface of a leaf, such as rhoeo, money plant, onion, hibiscus, coleus, or grass. The peel is kept in a watch glass of water, transferred with forceps onto a microscope slide with a drop of water, stained with a drop of ink, and covered with a coverslip. Under the microscope, small pores become visible among the surface cells.
The microscope image is enlarged because stomata are too small to examine clearly with the unaided eye. Recognising a pore helps connect the structure to its function: it provides an opening through which gases can be exchanged. The plant’s transport system must separately supply water and move food; stomata are not the same as those internal transport tubes.
Problem
A leaf needs water and carbon dioxide. Does it obtain both through stomata?
- 1.Carbon dioxide comes from the air and can enter through leaf pores.
- 2.Water is taken up by roots and carried to the leaf through the plant’s internal transport system.
- 3.The materials reach the leaf by different routes. Stomata help gas exchange, while water transport is explained by xylem.
Oxygen release during photosynthesis does not mean that a plant never uses oxygen. Plants also respire to release energy from food. Stomata support gas exchange for plant processes; they are not valves that always move each gas in only one direction.
Quiz
Why is the plant destarched before the carbon dioxide investigation?
What does sodium hydroxide do in the enclosed-leaf setup?
Which result supports the need for carbon dioxide?
What does collecting bubbles establish before the gas is tested?
Which pair contains the inputs named in the photosynthesis equation?
Why may an iodine test show starch after photosynthesis?
What is the main function of stomata discussed here?
Which statement about photosynthesis sites is correct?
Practice Problems
- Explain the purpose of destarching and the role of sodium hydroxide in the half-leaf investigation.
- Make a condition table for the enclosed and exposed halves of a leaf. Explain the expected iodine results.
- Describe how the water-plant setup collects gas. Distinguish the observation of bubbles from evidence that the gas is rich in oxygen.
- Write the photosynthesis word equation and identify inputs, products, and essential conditions.
- Explain the relationship between glucose made during photosynthesis and starch detected with iodine.
- Describe the microscope preparation used to observe stomata and connect the pores with gas exchange.
- A green stem receives light, water, and carbon dioxide. Explain why being a stem does not rule out photosynthesis.
Key Takeaways
• Photosynthesis uses carbon dioxide and water to make glucose and release oxygen. • Sunlight supplies energy, and chlorophyll helps capture that light. • Destarching allows a starch test to give more useful evidence about new food production. • Glucose can be converted to starch for storage; iodine detects starch. • Stomata enable gas exchange at leaf surfaces. • Leaves are the main photosynthesis sites, but other chlorophyll-containing parts can also make food.