3.5.1 Photosynthesis

Photosynthesis is important as it converts light energy into metabolic energy (in the form of ATP) which can be passed along the food chain. Photosynthesis can be split into three 'sections': capturing of the light energy (by chloroplast pigments such as chlorophyll), the light-dependant reaction, and the light-independant reaction.

The chemical equation is 6CO2 + 6H2O (+ light energy) = C6H12O6 + 6O2.

The light dependant reaction
In the light dependant reaction some of the light energy is converted into chemical bonds. In short, an electron flow is created causing water to split (photolysis) into protons, electrons, and oxygen. NADPH, ATP, and oxygen are produced. It occurs in thylakoids as they contain the photosynthetic pigment chlorophyll.

There are two purposes to the light dependant reaction. It captures light to... add an inorganic phosphate to ADP (making ATP), and to split water into protons and hydrogen and oxygen (photolysis). Here is the process of the light dependant reaction:

• A photon is absorbed by a chlorophyll molecule on thylakoid membrane. This chlorophyll molecule is part of photosystem 2 (Plastoquinone Oxidoreductase)
• An pair of electrons absorb this energy which raises them to a higher energy level. The electrons are now said to be excited (photoexcitation)
• The pair of electrons are so excited they leave photosystem two onto an electron carrier (the beginning of the electron transport chain). The electron carrier becomes reduced. 
• The chlorophyll molecule becomes ionised (this is known as photoionisation) and also oxidised.
• The electrons pass along a number of electron carriers (an electron transport chain) in a series of oxidation-reduction reactions located in the thylakoid membranes.
• The electrons lose energy at each stage as each carrier has a slightly lower energy level than the previous - this produces the energy needed to move them along.
• Having lost electrons, the chlorophyll molecule in photosystem wants to replace them. It splits water using light energy (photolysis) producing hydrogen ions (protons), oxygen, and electrons.
• The cytochrome complex uses a bit of the energy from the electron transport chain to pump protons into the thylakoid
• The thylakoid begins to fill up with protons from photolysis and proton pumping. A concentration gradient is established between the thylakoid and stroma (across the thylakoid membrane).
• Protons travel down this concentration gradient and diffuse across the thylakoid membrane through ATP synthase - this energy is used to form ATP from ADP + Pi.
• Since the electrons keep losing energy as they travel to each electron carrier they're now a little tired. The electrons get reenergised as they now enter photosystem 1
• The reexcited electrons move to another electron carrier
• All of the energy produced is used to form NADPH by reducing NADP (adding a proton/hydrogen ion)
• products: ATP, NADPH, and oxygen

The light independent reaction
The protons from the photolysis in the light dependant reaction are used to produce sugars (e.g glucose) and other organic molecules (e.g starch). This is known as the Calvin cycle and does not require light energy (hence light independent):

• In the stroma, CO2 is combined with RuBP (ribulose bisphosphate) with the help of a rubisco enzyme
• This new six carbon molecule is very unstable so it breaks in two producing two lots of 3-phosphoglycerate (glycerate-3-phosphate/GP) which are pretty stable
• ATP adds a Pi onto each 3GP
• NADHP adds an electron to each
• Now we have two molecules of Glyceraldehyde 3 phosphate (G3P) - also known as triose phosphate.
• Triose phosphate can be converted into useful organic substances such as glucose (for short term energy storage), cellulose (structure), or starch (or long term storage)
• Some of the G3Ps form the above (glucose/starch/cellulose) - the others go into reforming RuBP which required ATP

So that's photosynthesis. All we need to know now is a little bit about what might limit the rate of photosynthesis.

Photosynthesis is limited by a limiting factor - not by many things at once. Many things can affect the rate of photosynthesis but it is only limited by the one whose level is at a least favourable value (the one in short supply). This means that changing the levels of the other factors will not change the rate of photosynthesis if the level of the limiting factor is still low. Limiting factors of photosynthesis can include:

• Light (as light intensity increases the volume of o2 produced and co2 absorbed increases to a point where it is balanced by the volume of o2 absorbed and co2 released by respiration. At this point there will be no net exchange of gases - this is known as the compensation point)
• Carbon dioxide concentration (affects enzyme activity - particularly rubisco)
• Temperature
• Water