Overview of Photosynthesis: Videos & Practice Problems

Photosynthesis consists of two main stages: the light-dependent reactions and the light-independent reactions (Calvin cycle). The light-dependent reactions occur in the thylakoid membranes of the chloroplasts, where light energy is absorbed by chlorophyll and converted into chemical energy in the form of ATP and NADPH. These reactions also produce oxygen as a byproduct. The Calvin cycle, which takes place in the stroma of the chloroplasts, uses the ATP and NADPH generated in the light-dependent reactions to fix carbon dioxide (CO₂) into a 3-carbon sugar. This sugar can then be used to form glucose and other carbohydrates.

In light-dependent reactions, light energy is absorbed by chlorophyll, exciting electrons to a higher energy state. These high-energy electrons travel through the electron transport chain (ETC) in the thylakoid membrane. As electrons move through the ETC, they drive the pumping of protons (H⁺) into the thylakoid lumen, creating a proton gradient. This gradient powers ATP synthase to produce ATP from ADP and inorganic phosphate (Pi) in a process called photophosphorylation. Simultaneously, the electrons reduce NADP⁺ to NADPH, which is used in the Calvin cycle for carbon fixation.

Chlorophyll is a pigment located in the thylakoid membranes of chloroplasts that plays a crucial role in photosynthesis. It absorbs light energy, primarily in the blue and red wavelengths, and converts it into chemical energy. When chlorophyll absorbs light, its electrons become excited and are transferred to the electron transport chain, initiating the light-dependent reactions. This process ultimately leads to the production of ATP and NADPH, which are essential for the Calvin cycle to synthesize sugars from CO₂.

The Calvin cycle, also known as the light-independent reactions or carbon fixation, occurs in the stroma of chloroplasts. It uses ATP and NADPH produced in the light-dependent reactions to convert CO₂ into a 3-carbon sugar, glyceraldehyde-3-phosphate (G3P). The cycle involves three main phases: carbon fixation, reduction, and regeneration of the starting molecule, ribulose-1,5-bisphosphate (RuBP). For every three molecules of CO₂ fixed, the cycle produces one G3P, which can be used to form glucose and other carbohydrates.

Oxygen is produced during the light-dependent reactions of photosynthesis. When chlorophyll absorbs light energy, it excites electrons that are then transferred to the electron transport chain. To replace these lost electrons, water molecules (H₂O) are split in a process called photolysis. This splitting of water releases electrons, protons (H⁺), and oxygen (O₂). The oxygen is released as a byproduct and diffuses out of the chloroplast, eventually exiting the plant through the stomata.