Biology PHOTOSYNTHESIS

### KEY TOPICS

star Introduction
star Experiment for Chlorophyll Requirement
star Experiment for CO_2 Requirement

### INTRODUCTION

● 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".

### EXPERIMENT FOR CHLOROPHYLL REQUIREMENT

● 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".

### EXPERIMENT FOR CO_2 REQUIREMENT

● 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".