A level Biology exam revision resources written by A level Examiners





 

Photosynthesis is the process that converts inorganic molecules into organic molecules such as glucose. The organic molecules formed can be used as a source of energy or for forming other more complex molecules. Because organisms that photosynthesise can create their own respiratory substrates they are said to be autotrophic. This essentially means they do not need a food source. Humans by contrast are heterotrophic, they need to ingest organic molecules from a biological source to use as their respiratory substrates.

Photosynthesis is essentially the reverse of respiration. The overall summary equation is usually written as:

carbon dioxide + water (+ light energy) glucose + oxygen

6CO2 + 6H2O (+light energy) C6H12O6 + 6O2

This equation is a simplification of a process that contains a series of reactions. The reactions in photosynthesis can be grouped into two stages, these are the:   Exam Advice

  • Light Dependent reactions
  • Light Independent reactions

All the reactions of photosynthesis take place in the chloroplasts of green plants

Light dependent reactions: Overview

The light-dependent reactions use light energy to split water and make some ATP. The hydrogen atoms released from water are used to reduce coenzymes and the oxygen is given off as a gas. The reactions take place within the thylakoid membranes (grana and lamellae) of chloroplasts. Many of the reactions are similar to those in the electron transport chain.



H2O
Chlorophyll
O2

NADP
Light
Reduced NADP

ADP
ATP

Pi

Light independent reactions: Overview   Exam Advice

Although these reactions take place both when there is light and when there is no light because they require the products of the light dependent reactions they soon stop in the dark.



Reduced NADP
Calvin cycle enzymes
NADP

ATP
ADP

CO2
Pi

RuBP
C6H12O6

RuBP



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