Photosynthesis is a process used by plants to convert light, CO2 and water into complex carbhoydrates to fuel growth. This process is carried out within the chloroplasts of a cell by chlorophyll (a green photosynthetic pigment), and to a lesser extent by carotenoids (yellow, orange or red pigments). Carotenoids however, cannot transfer sunlight energy directly to the photoysnthetic pathway, but instead pass their absorbed energy to chlorophyll. For this reason they are called accessory pigments.
Chlorophyll pigments contain a porphyrin ring (a stable, ring-shaped molecule around which electrons are free to migrate). Because the electrons move freely, the ring has the potential to gain or lose electrons readily. This is the method by which chlorophyll captures the energy from any light source. As free electrons (energy) are collected from the sun, they are passed along from chlorophyll to other molecules which manufacture sugars. These sugars then fuel the growth of the plant.
One of the most important factors that influences photosynthetic rates is leaf temperature. The hotter you go, the faster photosynthesis occurs up until a certain point. After this point the excess heat causes stress in the form of increased transpiration (water loss) from the plant, resulting in slower growth rates.
C3 plants are the most efficient in cool, wet climates, while C4 plants are most efficient in hot, sunny climates. CAM plants evolved in arid conditions, and typically are not grown indoors under grow lights. The most common plants grow indoors are C4, which have an optimal temperature between 86°F and 100°F. How much heat a plant can tolerate is also highly dependent on CO2.
CO2 to a plant is like Oxygen to a human. Increased oxygen levels in our blood allows us to run faster, jump higher and perform longer. The same is true for plants and CO2.
CO2 is taken in by a plant through tiny pores in the leaves called stomates. The atmospheric concentration of CO2 is about 300ppm (parts per million); however as you increase CO2 levels the rate of photosynthesis also increases. The increase in growth is likewise tied to temperature. By keeping temperatures in the ideal range and enriching the grow environment with CO2, maximum photosynthetic rates can be achieved.
Under normal conditions the stomata of a plant undergo a process called transpiration (similar to human respiration). During transpiration CO2 is absorbed while O2 and water are released. When plants are exposed to a stressful environment (too hot or too dry), the stomata close to prevent water loss.
When the stomata are all closed the plant cannot absorb CO2, and photosynthesis comes to a halt. It is therefore important to keep ambient humidity levels within a tolerable range (40-70%). This keeps stomata function normal allowing for continuous photosynthesis during hours when light is absorbed.
The most important factor in photosynthesis is light quality. Without light there is no energy for chlorophyll to absorb, and without energy you don't have photosynthesis. As light intensity increases, the rate of photosynthesis will generally increase until another limiting factor kicks in (too much heat, not enough CO2, etc).
Once the light intensity plateau is reached, supplying additional light intensity does not increase photosynthetic rates (similarly to how temperature after a certain time starts decreasing photosynthetic rates). Instead too much light can result in damage to the chloroplasts, causing a steep decline in photosynthesis or even death.