Question

What are the functions of grana and stroma of chloroplast?

Answer 1

These parts of the chloroplast work together to produce glucose and oxygen from carbon dioxide and water via the light dependent and light independent reactions of photosynthesis.

A chloroplast within a plant cell is the site of photosynthesis and consists of two chlorophyll molecules (photosynthetic pigments that are responsible for the green colour of the chloroplast) that are each arranged in a photosystem (PS1 and PS2 respectively) that are embedded in the thylakoid membranes. Each chlorophyll molecule absorbs light which excites two electrons from each photosystem, causing them to leave the chlorophyll molecules. The electrons from PS2 pass along the electron transport chain (a series of electron carriers embedded in the thylakoid membranes) in a series of redox reactions causing energy to be released which is used to synthesise ATP via photophosphorylation/chemiosmosis (as H+ ions diffuse down the electrochemical gradient through the stalked particle ATP synthase, causing it to change shape and catalyse the formation of ATP from ADP and P). These electrons then replace those lost from PS1. The electrons from PS2 are replaced by electrons from photolysis; when light strikes a chloroplast, enzymes in the thylakoid space catalyse the splitting of water to form oxygen gas, H+ ions, and electrons (2H20 →2H+ + 2e- + O2). The electrons from PS1 combine with H+ ions from photolysis and NADP to form NADPH (reduced NADP). These are the light dependent reactions of photosynthesis in the chloroplast, and the NADPH and ATP produced are used in the light independent reactions. A stack of thylakoid membranes is called a granum.

The light independent reactions take place in the stroma. This is the site of carbon fixation; CO2 combines with RUBP to form GP (glycerate-3-phosphate) which is catalysed by the enzyme RUBISCO (the most abundant enzyme in the world). GP is then converted/reduced to GALP (glyceraldehyde-3-phosphate) using the NADPH and ATP from the light dependent reactions. 2 out of every 12 GALP molecules produced is used to synthesise glucose which can be used in respiration or combined in condensation polymerisation to form cellulose, giving the plant cell wall extra strength. The remaining GALP molecules are converted back into RUBP using energy from ATP.