Biology: Biochemistry and Plant Physiology - Photosynthesis (Light-Dependent Reactions, Calvin Cycle, Leaf Structure, and Limiting Factors)

Photosynthesis is the biochemical process by which photoautotrophs (like plants) convert light energy into chemical energy stored in glucose: $6CO_{2} + 6H_{2}O \xrightarrow{\text{light, chlorophyll}} C_{6}H_{12}O_{6} + 6O_{2}$

The process occurs in the Chloroplast, which contains the pigment chlorophyll. It is divided into two main stages: the Light-Dependent Reactions and the Light-Independent Reactions (Calvin Cycle).

Light-Dependent Reactions: These occur in the thylakoid membranes. Light energy is absorbed to split water molecules ($H_{2}O$) in a process called photolysis, releasing oxygen ($O_{2}$), protons ($H^{+}$), and electrons to produce energy carriers $ATP$ and $NADPH$.

Light-Independent Reactions (Calvin Cycle): These occur in the stroma. This stage does not require direct light but uses the $ATP$ and $NADPH$ from the light-dependent stage to 'fix' carbon dioxide ($CO_{2}$) into the organic molecule glucose ($C_{6}H_{12}O_{6}$).

Leaf Structure Adaptations: The Palisade Mesophyll is packed with chloroplasts for maximum light absorption. The Spongy Mesophyll has air spaces for gas exchange ($CO_{2}$ in, $O_{2}$ out). Stomata (controlled by guard cells) regulate the entry of $CO_{2}$ and loss of water vapor.

Limiting Factors: The rate of photosynthesis is limited by the factor in shortest supply. Key factors include light intensity, $CO_{2}$ concentration, and temperature. Temperature affects the rate because the Calvin Cycle is controlled by enzymes; if the temperature is too high, enzymes denature.

Inverse Square Law: Light intensity ($I$) is inversely proportional to the square of the distance ($d$) from the light source: $I \propto \frac{1}{d^{2}}$.

The glucose produced is used for cellular respiration, stored as starch, or converted into cellulose for cell walls and proteins for growth.