T H E   G R E E N H O U S E

Photosynthesis
  - Introduction
  - The Leaf Structure
  - The Process
  - Pigments
  - Photosynthesis Rate
  - Photosynthesis Test
  - Links

The Process of Photosynthesis

It is important to remember that O2 comes from water, not CO2.
Photosynthesis occurs in 2 phases – the light and dark reaction.

The Light Reaction

Light "harvesting" - In chloroplasts, there are thylakoids, which have

Pigment molecules around them. These pigment molecules funnel light photons to the chlorophyll a molecules in the reaction centre. As the chlorophyll molecule absorbs this energy, its electrons move to a higher energy level – creating an ‘excited’ chlorophyll molecule. It can now pass this energy onto the chlorophyll molecule next to it, thus exciting it, and making itself normal again.

Electrons are being transport through these 2 centres:

Photosystem I (PS I)

Red light(700nm)

Photosystem II(PS II)

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Orange light(680nm)

Election Transport -

This transfer of elections results in ATP and NADPH being formed. PS I is responsible for the creation of NADPH.

PS I

When light is absorbed at PS I, an election is displaced and transferred to an election acceptor. This acceptor transfers the electron to NADP, reducing it to NADPH. But if the chlorophyll at PS I keeps loosing electrons, it will need to replace them. PS II does this.
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PS II

When light hits the chlorophyll at PS II, an electron is displaced and is sent, via electron carriers, to PS I. Thus giving PS I its lost electron back. But now

the PS II has lost an electron, how does it get it back? The photolysis of water is the answer. Water is split and an electron donated to PS II. Oxygen is given off too – this being 1 of the products of photosynthesis.

ATP is also formed by the transfer of electrons, and is made by ADP and an inorganic phosphate. The flow of electrons from PS II to PS I creates hydrogen ions(H+), and a steep concentraion gradient. The passage of H+ out of the thylakoids, provides the energy for ATP to be synthesised in the presence of ATPase. This is called non-cyclic photophosphorylation. The H+ ions also provide the hydrogen for reducing NADP to NADPH.

 

So the light reaction creates energy by phosphoralyation – producing ATP, and by adding electrons to NADP to create NADPH.

The Dark Reaction

This reaction occurs in the stroma of the chloroplast. This stage is mainly concerned with the synthesizing of carbohydrates from CO2. This occurs in a number of steps, with different enzymes along the way.

The Calvin Cycle -

Firstly CO2 is combined with RuBP(a 5 carbon organic compound). The enzyme RuBP carboxylase is needed here.This "fixes" the CO2.

Thus a 6 carbon mocule is created, which is very unstable, so it splits immediately into 2 molecules (3 carbons each), of glycerate 3-phosphate(GP).

GP is then reduced to GALP, a 3 carbon sugar. This reduction used hydrogen from NADPH. Energy for this comes from NADPH and ATP. The 3 carbon sugar is built up to a 6 carbon sugar, which can be converted to starch.

Some of the 3C sugar isn’t converted into 6C, but goes into the regeneration of RuBP, for the continuation of this cyclic process.

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Thinkquest Team 22016
Andrew Griffiths
Tim Sindle
Ben Harper