Bioenergetics

Photosynthesis is the reduction of energy-poor oxidised compounds, carbon dioxide and water, into energy-rich carbohydrate using light energy absorbed by chlorophyll and other photosynthetic pigments. Its summary equation carries twelve molecules of water on the left and six on the right, so water sits on both sides, and because there is no net yield it simplifies:

$$6CO_2 + 6H_2O + \text{light energy} \xrightarrow{\text{chlorophyll}} C_6H_{12}O_6 + 6O_2$$

That is almost the exact reverse of aerobic respiration, and the two differ in one respect the paper tests directly: photosynthesis runs in daylight only while respiration goes on day and night, so at dawn and dusk the rates may briefly be equal, with the carbon dioxide released by respiration meeting exactly what the leaf fixes and no net gas exchange at all. That state is the compensation point.

The compartments decide most questions. Inside the chloroplast, the stroma holds the carbohydrate-building enzymes, and the disc-shaped sacs called thylakoids, stacked into columns called grana, carry the light reactions. Chlorophyll $a$ is the reaction-centre pigment and is bluish green, absorbing in the blue-violet and orange-red. Carotenoids are accessory pigments absorbing between $620$ and $700\ \mathrm{nm}$ and passing energy on through chlorophyll $b$ to the chlorophyll $a$ that can drive the reaction. An absorption spectrum plots what a pigment absorbs, an action spectrum plots photosynthetic rate against wavelength, and an estimate of carbon dioxide consumption or oxygen release is the second of those.