C3 plants are simply the plants we have been looking at already in photosynthesis. In C3 plants, CO2 is fixed from the atmosphere into the 3 carbon compound GP using the RuBisCO enzyme. Remember the problem here; above 25OC the RuBisCO starts to prefer oxygen over CO2 fixing (the CO2 and O2 are competing), reducing the rate of photosynthesis. C4 plants avoid this problem by having a more efficient way to fix the CO2.
Essentially, the CO2 is used in another biochemical pathway using pyruvate (3C) which is converted eventually to oxaloacetate (4C), and eventually to the compound malate (4C). This allows a ‘store’ of CO2 to be built up that can be released directly into the Calvin cycle. In these higher concentrations the CO2 is more readily absorbed by the RuBisCO in preference to O2.
The problem is the CO2 has to be fixed twice, which requires more energy than a C3 pathway. C3 plants require 18 molecules of ATP to synthesise 1 molecule of glucose; C4 plants require around 30 ATP per molecule. It turns out to be a balancing act – C4 plants are found in places where there is a higher temperature that would favour RuBisCO taking O2, for example the tropics. C3 plants are found in more temperate regions. C4 plants are also more water efficient, doing better in dry conditions.
Despite being only around 3% of all terrestrial plants, C4 plants are responsible for around 30% of terrestrial carbon fixation.
It’s photosynthesis time folks, just what you’ve been waiting for over those long summer weeks. It’s not really that bad, a couple of biochemical pathways, chemicals you’re already familiar with and a bit of our old favourite energy.
Since we’ve been through respiration, you should now be getting familiar with the concept of energy being transferred, or passed from one thing to another (like with the electrons and coenzymes along the ETC in the cristae). You could view organisms with the underlying idea that they are all about getting usable energy, that is transferring energy from a source to a form that can be useful to the organism. Remember that energy is used by the organism to do work – muscle movement, making new molecules, thermal energy and so on. Photosynthesis fits into this idea; we will see how the underlying process involves energy transfer.