moll's half leaf experiment lab activity

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Top 11 Experiments on Photosynthesis in Plants

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The following points highlight the top eleven experiments on photosynthesis in plants. Some of the experiments are: 1. Simple Demonstration of Photosynthesis 2. To Study the ”Primary Photochemical Reaction” of Photo­synthesis 3. To Study the “Dark Reaction” of Photosynthesis 4. To Study the Essentiality of the Factors for the Photosynthetic Process and Others.

Experiment # 1

Simple demonstration of photosynthesis:.

(a) With the help of a beaker and a funnel:

Experiment:

A large beaker of capacity 500 ml is taken and are filled two-thirds with distilled water containing 0.1 % KHCO 3 which acts as a source of CO 2 . Some fresh and healthy aquatic plants like Hydrilla are taken in a beaker and the plants are cut obliquely at their bases under water.

Cut ends are tied together with the help of a thread and are kept towards the neck of an inverted funnel in such a way that the limb of the funnel almost covers the Hydrilla plants and the stem of the funnel remains about one centimeter under the water surface.

The whole set-up is now exposed to bright light and observed from time to time. Another set-up is similarly prepared and kept under a very low light intensity.

Observation:

It is observed that evolution of bubbles from cut ends of the plants takes place in the set-up exposed to light. Little evolution of bubbles takes place in the set-up maintained in low light intensity.

In light, evolution of oxygen bubbles takes place due to photosynthesis. This is further proved by the fact that little evolution of bubbles takes place in the set-up placed in low light intensity.

(b) With the help of Wilmott’s bubbler:

The apparatus consists of a flask of capacity 500 ml fitted with a rubber cork having a central hole through which passes a glass tube. The lower end of the tube reaches the middle of the flask while its upper end forms a jet within a cylindrical cup. A graduated tube having a stopcock at one end remains inverted over the jet (Figure 27).

Experiment # 8

Determination of real and apparent photosynthesis :.

Two experimental set-ups are arranged as in Expt. 2.a (iv). Here tap water is used (which may be previously oxygenated by photosynthesis). The experimental materials should be of comparable number and weight in each case.

The initial O 2 content of water in each set-up is determined following the procedure given in the referred experi­ment. One set-up is kept in bright light for photosynthesis and the other in dark for respiration only. Alter two hours the O 2 content of water of both the set-ups is again determined.

The difference between the final and the initial content of O 2 in case of the set-up kept in light gives the excess of O 2 liberated by photo­synthesis. This amount indicates apparent photosynthesis.

Again the difference between the initial and final content of O 2 in case of the set-up kept in dark gives the amount of O 2 consumed during respiration. When this amount of O 2 is added to the amount of O 2 liberated by photosynthesis it gives the real photosynthesis.

When the net result of photosynthesis is measured and no correction is made for respiration it is called the apparent pkolosytahesis. When, on the other hand, corrections for respiration are made, the photo­synthesis thus calculated is called die real photosynthesis or true photosyn­thesis.

So to obtain a measure of true photosynthesis it is desirable to make corrections for respiration. Some of the O 2 given off in photosynthesis is used up in respiration and some of the CO 2 given off in respiration is taken up in photosynthesis.

It is therefore necessary to measure the respiration of an identical sample in dark. It is easily seen that the apparent photo­synthesis is less than the true photosynthesis by the amount of O 2 used up in respiration.

The respiration error is variable and is always present, and it thus complicates attempts to measure true photosynthesis, since the factor which influence photosynthesis are likely also to influence respiration.

Experiment # 9

To show that the entry of co, takes place through stomatal pores :.

A suitable potted plant is so selected that its leaves are dorsiventral and upper surfaces are thickly circularised. The plant is kept in dark for two days to deplete their leaves of starch. Now some leaves are smeared with Vaseline only on the lower surfaces and other leaves are smeared only on the upper surfaces.

Vaseline should be applied as a thin film so that stomatal pores are closed. The plant is then kept in light for several hours. After that period the leaves are severed and tested for starch by iodine.

It is observed that the leaves having Vaseline applied on the lower surface only do not show the presence of starch whereas the leaves in which Vaseline was applied on the upper surface shows the presence of starch by iodine.

In case of the leaves smeared with Vaseline on the lower surface starch synthesis cannot take place because CO 2 could not enter into the leaf for use in photosynthesis owing to the closure of stomatal pores by Vaseline.

Those leaves in which Vaseline has been applied on the upper surface have their stomata open on lower surface through which CO 2 could enter and allow the photosynthesis to occur resulting in starch synthesis. Thus the experiment shows that the CO 2 enters the leaves through stomatal pores of the lower surface in case of dorsiventral leaves.

Experiment # 10

To show the necessity of light for synthesis of chlorophyll in leaves :.

Some pea or gram seeds are allowed to germinate in dark as well as in light.

After 10 days the dark grown seedlings show yellow or white leaves and stems. Other seeds grown in light show normal seedlings with green leaves and stems. When etiolated plants are exposed to light these turn to green.

The experiment shows that light is necessary for chloro­phyll synthesis. In a light-mediated reaction protochlorophyll is reduced to form chlorophylla. Photoreduction of protochlorophyllide to chlorophyllide a, followed by phytol esterification to form chlorophyll a, is now thought to be the major pathway.

Experiment # 11

To show that starch synthesis in etiolated (or albino) leaves are independent of light provided sugars are present :.

Two comparable leaf samples are picked from etiolated rice or wheat seedlings grown in a dark room. Each sample is placed in a beaker of 100 to 150 ml capacity and some weight is placed on the leaves to prevent them from floating.

One sample is covered with pure water and the other with 0.1 M sucrose solution for two days in the dark room. The samples are removed from the beaker; excess liquid is blotted off, placed on moistened filter paper in petridishes and set aside in the dark room for a period of 24 hours. At the end of the experimental period the leaves from both the sets are tested for starch by iodine.

No starch is obtained in case of leaves kept in water whereas starch synthesis takes place in case of leaves kept in sucrose solution.

The experiment shows that the starch synthesis in etiolated leaves is independent of light if sugars arc supplied to the etiolated leaves.

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Moll’s half leaf experiment is performed to show that: A. Light is needed for photosynthesis B. Chlorophyll is needed for photosynthesis C. Oxygen is evolved during photosynthesis D. Carbon dioxide is needed for photosynthesis

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