U3 AOS2 Topic 1: C3 Photosynthesis Introduction

Photosynthesis is the process in which food is produced by using the energy from sunlight. The organisms which carry out photosynthesis to make their own food are called as photoautotrophs. These organisms convert light energy from sun into chemical energy which is stored in food.

Plants are photoautotrophs as they make their own food in the form of glucose by using sunlight. The glucose produced by photosynthesis can be used for following purposes:

·        Used in cellular respiration to produce energy in the form of ATP. This energy can be used for different activities inside the cell.

·        Stored in the form of starch.

·        Used to make more complex molecules such as cellulose.

Photosynthesis can be described as follows:

Carbon dioxide and water react in the presence of sunlight to produce glucose and oxygen

Photosynthesis is not a single step reaction as shown in above equation but instead it is a complex series of biochemical reactions. Photosynthesis can be broken into two stages as:

·        Light dependent stage

·        Light independent stage

Both these stages occur inside the chloroplast of plant cells.

Light dependent stage:

The first stage involves splitting of water into oxygen and hydrogen in the presence of sunlight. This reaction occurs inside the thylakoid membranes of chloroplast where chlorophyll pigments are present. This reaction produces high energy coenzymes called as NADPH and ATP which can be further utilized in second stage of photosynthesis. Light energy from sun is used to excite the electrons of chlorophyll which in turn pumps H+ into thylakoid membranes. Water donates its electron to chlorophyll to fulfill the lost electron and then splits into oxygen and H+. Oxygen diffuses out of stomata whereas; H+ is used to produce coenzymes NADPH and ATP.

Light Independent stage:

This stage occurs inside the stroma of chloroplasts. Glucose is produced by using CO². Unlike light dependent stage, light independent stage does not use energy from sunlight but instead it uses energy from coenzyme NADPH and ATP produced in light dependent stage. CO² enters the cyclic reactions to produce glucose where water is also produced. The cyclic reaction is called as Calvin cycle. The NADPH and ATP are converted into NADP⁺ and ADP which can be reutilized in first stage of photosynthesis.