Photosynthetic autotrophs use light energy to form organic molecules from carbon dioxide and water. A simplified overall equation is 6CO2+6H2O→C6H12O6+6O2, with light energy required. Carbon atoms in the carbohydrate come from carbon dioxide. Photosynthesis transfers energy into chemical stores rather than creating energy or matter. Plants also respire, including in darkness.
In chloroplasts, thylakoid membranes provide a large area for light-dependent processes, while carbon fixation takes place in the stroma. Rate can be limited by light, carbon dioxide or suitable temperature. Raising one factor may have no effect when another is limiting. Read the axes and units before comparing rates; a plateau means no further measured increase across that range, not that all photosynthesis stopped.
Example 1
A plant fixes 12CO2 molecules according to the simplified equation. How many glucose molecules can form?
The ratio is 6CO2 per glucose. 612=2 glucose molecules, assuming the other requirements are met.
Example 2
Rates at light values 10,20 and 40 are 2,4 and 4 units. Find the rate gain from 20 to 40.
The gain is 4−4=0 units. Doubling light in that range did not increase the measured rate.
Watch out: Plants carry out respiration as well as photosynthesis; the processes have different roles.