Explain the factors affecting photosynthesis.

(N/A) Photosynthesis is influenced by both internal and external factors.
Internal factors: These include the number,size,age,and orientation of leaves,mesophyll cells,chloroplasts,internal $CO_2$ concentration,and the amount of chlorophyll.
Blackman's Law of Limiting Factors: If a chemical process is affected by more than one factor,its rate is determined by the factor nearest to its minimal value. This factor directly limits the process if its quantity is changed.
External Factors:
$1$. Light: There is a linear relationship between incident light and $CO_2$ fixation at low light intensities. At higher intensities,the rate plateaus as other factors become limiting. Light saturation occurs at $10\%$ of full sunlight. Excessive light can cause chlorophyll breakdown.
$2$. Carbon dioxide concentration: $CO_2$ is a major limiting factor. Atmospheric concentration is low $(0.03-0.04\%)$. $C_3$ and $C_4$ plants respond differently; $C_4$ plants saturate at $360 \mu l L^{-1}$,while $C_3$ plants saturate beyond $450 \mu l L^{-1}$.
$3$. Temperature: Dark reactions are enzymatic and temperature-sensitive. $C_4$ plants have a higher temperature optimum compared to $C_3$ plants. Tropical plants generally have higher temperature optima than temperate plants.
$4$. Water: Water stress causes stomata to close,reducing $CO_2$ availability. It also causes leaf wilting,reducing surface area and metabolic activity.