Light Dependent and Light independent reactions-Photosynthesis

The light independent reaction relies on sunlight energy. The reactants needed for this first stage of photosynthesis are water, light and carbon dioxide. The light independent reaction takes place in the thylakoid membranes of the granum which are located in the cholorophyll-packed chloroplasts.

Step 1-

Light is absorbed by the chlorophyll molecule and the pair of electrons within the molecule are raised to a higher energy state so much so that the electrons leave the molecule. The electrons are taken up by an electron acceptor.

Step 2-

The electrons pass through an electron transport chain via a series of oxidation-reduction reactions. At this stage, protons (positive hydrogen ions) are pumped through into the lumen of thethylakoid. This establishes a concentration gradient of protons across the membrane.

Step 3

The protons can be transported across the membrane via an enzyme on the thylakoid membrane named ATP synthase. The movement of these protons generate enough energy to combine an inorganic phosphate with ADP to form ATP, a product of this stage of photosynthesis. 

Step 4

The protons generated by the photolysis of water (where light splits a water molecule), make the second product of the light-dependent reaction. The equation of the photolysis of water is summarised below:

The hydrogen protons reduce NADP. This is important as NADPH acts as an alternative energy source.

Step 5

The electrons which have been lost from the chlorophyll need to be replaced in order for the molecule to be able to absorb light energy again. Another product of the photolysis of water are electrons and these serve as replacement electrons as they replace the electrons that have been lost by the chlorophyll molecule.

Step 6

Oxygen which is a by-product of photolysis can be diffused out of the leaf as a waste-product of photosynthesis. This is important as we need oxygen for respiration. Alternatively, oxygen can be used in the plant's own respiration purposes.

Note: Reduced NADP and ATP go on to become reactants of the light-independent reaction

The light independent reaction does not need light energy to occur. The light-independent reaction takes place in the stroma (fluid-filled sac) of the chloroplast.

Step 1

Carbon dioxide is diffused into the leaf, through the plasma membrane of the mesophyll cells, into the cytoplasm, through the chloroplast membrane, and into the stroma. 

Step 2

Carbon dioxide combines with adenine biphosphate (AbP) using an enzyme. The combination of carbon dioxide with AbP forms two molecules of glycerate 3-phosphate (GP).

Step 3:

The activated form of GP is reduced by ATP and reduced NADP, our products from the light-dependent reaction.

Step 4:

In this reaction, NADP is reformed to be reduced by accepting more hydrogen from the light-dependent reaction. ATP is also broken down into ADP and an inorganic phosphate.

Step 5

In turn, GP is reduced to form triose phosphate (TP) of which there are two molecules.

Step 6

Some of the TP molecules are converted into useful organic molecules such as glucode but most of the molecules are used to regenerate AbP (adenine biphosphate).

Cholroplasts are adapted to carry out the light-dependent reaction in the following ways:

•   Thylakoid membranes have a large surface area for the attachment of electron carriers, proteins and enzymes needed for the light-dependent reaction.

•   Proteins within the granum hold the chlorophyll in a precise way to allow for the maximum absorption of light.

The chloroplast contains all of the DNA and ribosomes needed to manufacture the proteins needed for the light-dependent reaction.