Experiment to Show that Carbon Dioxide is Necessary for Photosynthesis

1. We take a potted plant having long and narrow leaves and place it in a completely dark place for; about three days to destarch its leaves.

2. Take a glass bottle having a wide mouth and put some potassium hydroxide solution (KOH solution! in it. (This potassium hydroxide solution is to absorb the carbon dioxide gas from the air present in the glass bottle so that no carbon dioxide is left in the air inside the glass bottle).

3. Take a rubber cork which fits tightly into the mouth of the glass bottle and cut it into two halves.

4. Put a destarched leaf of the potted plant (while it is still attached to the plant), in-between the two halves of the cut cork and then fit the cork in the mouth of the glass bottle. The; upper half of the leaf should remain outside the glass bottle and only the lower half of the leaf should: be inside the glass bottle.

5. The potted plant (with its one destarched leaf half inside the glass bottle containing potassium hydroxide solution) is kept in sunlight for 3 to 4 days. During this period, the upper half of the leaf (which is outside the glass bottle) gets carbon dioxide from the air but the lower half of the leaf (which is inside the glass bottle) does not get any carbon dioxide. This is because all the carbon dioxide of the air present in the glass bottle has been absorbed by potassium hydroxide solution. And no fresh air can come into the closed glass bottle.

6. Pluck the leaf from the plant and take it out from the glass bottle. Remove the green coloured chlorophyll from the leaf by boiling it in alcohol. In this way, we get a decolourised leaf.

7. Wash the decolourised leaf with water to remove any chlorophyll which may be sticking to it.

8. Pour iodine solution over the colourless leaf and observe the change in colour of the leaf.

9. We will find that the lower half part of the leaf (which was inside the glass bottle having no carbon dioxide around it), does not turn blue-black on adding iodine solution showing that no starch is present in this lower half of the leaf. From this observation we conclude that the photosynthesis to make starch in the leaf does not take place without carbon dioxide.

10. The upper half part of the leaf (which was outside the glass bottle, having carbon dioxide around it) turns blue-black on adding iodine solution showing that starch is present in this upper half of the leaf. From this observation we conclude that photosynthesis (to make starch) takes place in the presence of carbon dioxide. In other words, carbon dioxide is necessary for the process of photosynthesis to take place.

Raw Materials for Photosynthesis :

The preparation of carbohydrates (food) by plants by the process of photosynthesis requires two materials (or substances): carbon dioxide, and water. Thus, the raw materials for photosynthesis are:

(i) Carbon dioxide, and

(ii) Water.

We will now describe how these two raw materials become available to plants for photosynthesis.

1. How the Plants Obtain Carbon Dioxide :

There are a large number of tiny pores called stomata on the surface of the leaves of plants (The singular of stomata is stoma). The green plants take carbon dioxide from air for photosynthesis. The carbon dioxide gas enters the leaves of the plant through the stomata present on their surface.

Each stomata pore (or stoma) is surrounded by a pair of guard cells. The opening and closing of stomata pores is controlled by the guard cells. When water flows into the guard cells, they swell, become curved and cause the pore to open. On the other hand, when the guard cells lose water, they shrink, become straight and close the stomata pore.

A large amount of water is also lost from the cells of the plant leaves through open stomatal pores. So, when the plant does not need carbon dioxide and wants to conserve water, the stomatal pores are closed. The oxygen gas produced during photosynthesis also goes out through the stomatal pores of the leaves. Please note that in addition to leaves, the stomata are also present in the green stems (or shoots) of a plant.

So, the green stems (or shoots) of a plant also carry out photosynthesis. It is clear from the above discussion that stomata allow the movement of gases in and out of plant cells. In other words, the gaseous exchange in plants takes place through the stomata in leaves (and other green parts).

Please note that in most broad-leaved plants, the stomata occur only in the lower surface of the leaf but in narrow-leaved plants, the stomata are equally distributed on both the sides of the leaf. Another point to be noted is that the aquatic plants (or water plants) use the carbon dioxide gas dissolved in water for carrying out photosynthesis.

2. How the Plants Obtain Water for Photosynthesis:

The water required by the plants for photosynthesis is absorbed by the roots of the plants from the soil through the process of osmosis. The water absorbed by the roots of the plants is transported upward through the xylem vessels to the leaves where it reaches the photosynthetic cells and utilised in photosynthesis.

The two raw materials, carbon dioxide and water, are required by the plants to prepare energy foods called carbohydrates (such as glucose and starch). But the plants also need other raw materials such as nitrogen, phosphorus, iron and magnesium, etc., for building their body.

The plants take materials like nitrogen, phosphorus, iron and magnesium, etc., from the soil. For example, nitrogen is an essential element used by the plants to make proteins and other compounds.

The plants take up nitrogen from the soil in the form of inorganic salts called nitrates (or nitrites), or in the form of organic compounds which are produced by bacteria from the atmospheric nitrogen.

Site of Photosynthesis: Chloroplasts

Chloroplasts are the organelles in the cells of green plants which contain chlorophyll and where photosynthesis takes place. Thus, photosynthesis occurs in the organelles called chloroplasts present in the photosynthetic cells (or mesophyll cells) of green plants. In other words, the site of photosynthesis in a: cell of the leaf are chloroplasts. Chloroplasts can be seen easily by using a light microscope. In a cross-section of a leaf, chloroplasts can be seen as numerous disc-like organelles in the photosynthetic cells (or mesophyll cells) of the palisade tissue just below the upper epidermis.

In the structure of a leaf we can see that the middle layers in the leaf (palisade layer and spongy layer) contain photosynthetic cells called mesophyll cells. These cells contain more chlorophyll than other plant cells. A typical photosynthetic cell (or mesophyll cell) of a green leaf may contain 100 or more tiny chloroplasts in it, and a whole leaf may contain many thousands of photosynthetic cells.

Carbon dioxide needed for photosynthesis enters from the air into the leaf through the stomata in its surface, and then diffuses into the mesophyll cells and reaches the chloroplasts. Water iscarried to the leaf by xylem vessels and passes into the mesophyll cells by diffusion and reaches the chloroplasts. There is a thin, waxy protective layer called cuticle above and below a leaf which helps to reduce the loss of water from the leaf.