● All animals including `color{violet}"human beings"` depend on `color{violet}"plants"` for their food.

● `color{violet}"Green plants"`, in fact, have to make or rather `color{violet}"synthesise the food"` they need and all other organisms `color{violet}"depend on them"` for their needs.

● `color{violet}"Green plants"` carry out `color{Bown}"photosynthesis’"` a physico-chemical process by which they use `color{violet}"light energy"` to drive the synthesis of `color{violet}"organic compounds"`.

● Ultimately, `color{violet}"all living forms"` on earth depend on `color{violet}"sunlight for energy"`.

● The use of `color{violet}"energy from sunlight"` by plants doing photosynthesis is the `color{violet}"basis of life"` on earth.

● `color{violet}"Photosynthesis"` is important due to `color{violet}"two reasons"`:

`star` It is the `color{violet}"primary source"` of all `color{violet}"food"` on earth.

`star` It is also responsible for the `color{violet}"release of oxygen"` into the atmosphere by `color{violet}"green plants"`.

● Some `color{Brown}"simple experiments"` done in the earlier classes have shown that `color{Brown}"chlorophyll"` (green pigment of the leaf), `color{violet}"light"` and `color{violet}("CO_2")` are required for photosynthesis to occur.

● The experiment may have been carried out to look for `color{BBrown}"starch formation"` in two leaves – a `color{violet}"variegated leaf"` or a leaf that was `color{violet}"partially covered"` with black paper, and one that was `color{violet}"exposed to light"`.

● On testing these `color{violet}"leaves for starch"` it was clear that `color{violet}"photosynthesis"` occurred only in the `color{violet}"green parts"` of the leaves in the `color{violet}"presence of light"`.

● Another experiment that may have been carried out is the `color{Brown}"half-leaf experiment"`, where a part of a leaf is `color{violet}"enclosed in a test tube"` containing some `color{violet}"KOH soaked cotton"` (which absorbs `CO_2`), while the other half is
`color{violet}"exposed to air"`.

● The setup is then `color{violet}"placed in light"` for some time.

● On `color{violet}"testing for starch later"` in the two halves of the leaf, it is found that the `color{violet}"exposed part"` of the leaf `color{violet}"tested positive"` for starch while the portion that was in the `color{violet}"tube, tested negative"`.

● This showed that `CO_2` was required for `color{violet}"photosynthesis"`.