|Photosynthesis||The process by which green plants prepare their food.|
|Light Reaction||Takes place in presence of light.|
|Dark Reaction||Does not need light, but it does not mean that dark reaction cannot take place in presence of light.|
Green plants are called autortophs because they prepare their own food. They make food in the presence of sunlight. Sunlight provides energy. Carbon dioxide and water are the raw materials. Chloroplast is the site where food is made. Photosynthesis generally takes place in leaves. It also happens in other parts which are green.
In this section, you will learn about the basic concept of photosynthesis. You will learn about key steps of photosynthesis and raw materials required for photosynthesis. You will also learn about various experiments which can be conducted to show if photosynthesis took place in a plant. Moreover, you will learn about significance of photosynthesis.
Photosynthesis: The process by which green plants prepare food is called photosynthesis. During this process, the solar energy is converted into chemical energy and carbohydrates are formed. Green leaves are the main sites of photosynthesis. The green portion of the plant contains a pigment chloroplast, which contains chlorophyll. The whole process of photosynthesis can be shown by following equation:
6CO2 + 6H2O ⇨ C6H12O6 + 6O2
Dark reaction does not mean that it happens during darkness. It just means that dark reaction does not need light. In some desert plants, carbon dioxide is taken up at night. This is utilised to make an intermediary, which is later utilised to make carbohydrate.
How do raw materials for photosynthesis become available to the plant?
Fig Ref: NCERT Textbook Class 10 Science
This figure shows cross-section of leaf. The upper epidermis is continuus while lower epidermis has gaps due to presence of stomata. It is the lower epidermis which faces the sunlight. You can see small dots in cells of mesophyll. These dots are chloroplasts. The cells of mesophyll, thus, are the sites for photosynthesis.
In previous classes, and in this class too, you will read about numerous experiments to show photosynthesis took place in a plant. There is a long list of such experiements and hence it can be confusing to remember them. I will try to explain these experiments in simplest possible way.
You have read that once carbohydrate is made in leaves, it is changed into starch so that it can be stored. So, all the experiments related to photosynthesis try to show the presence of starch. Presence of starch implies that photosynthesis took place in a particular plant. You may recall that when starch reacts with iodine, it makes a complex which is dark blue in colour. This test it done in all these experiments to show the presence of starch.
In these experiments, two samples are tested. Factor of photosynthesis is ruled out in one sample, and this factor is allowed to remain in another sample. After that, leaves from both the samples is boiled in alcohol over water bath. This results in denaturation of leaf, i.e. removal of chlorophyll from leaves. After that, the boiled leaf is washed and then iodine drops are poured over it. Formation of dark blue colour indicates the presence of starch.
You can look for pictures of variegated leaves for example. A leaf which has spots of many colours apart from green is called variegated leaf. Spots with colours other than green do not have chloroplast in them. So, photosynthesis does not happen in those portions of leaf. The green portions will turn dark blue in this experiment, showing photosynthesis took place in green portions.
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