Energy Transformation: Photosynthesis and Cellular Respiration

Exam Tip: Remember that while the light-dependent reactions require photons, they do not directly use $CO_2$. Instead, they provide the "fuel" (ATP and NADPH) needed for the next stage.

ATP Production via Chemiosmosis

As the ETC moves electrons, the resulting proton gradient creates a "proton motive force." These protons seek to move back across the membrane to areas of lower concentration. They can only do this through a specialized enzyme called ATP synthase. As protons flow through this enzyme, the mechanical energy is used to phosphorylate ADP into ATP. This process of using a proton gradient to drive ATP synthesis is called chemiosmosis.

The Calvin Cycle and Carbon Fixation

The second stage of photosynthesis occurs in the stroma, the fluid-filled space surrounding the thylakoids. This stage is often called the light-independent reactions because it uses the ATP and NADPH produced earlier to build sugars.

The cycle begins with carbon fixation, where inorganic $CO_2$ is attached to a 5-carbon sugar called ribulose bisphosphate (RuBP). This reaction is catalyzed by the enzyme RuBisCO. Through a series of reduction reactions using ATP and NADPH, the cycle produces a 3-carbon sugar called glyceraldehyde-3-phosphate (G3P). While some G3P leaves the cycle to become glucose or starch, the rest must be used to regenerate RuBP so the cycle can continue.

Cellular Respiration: Extracting Energy

While photosynthesis builds sugars, cellular respiration breaks them down to produce ATP. This is a catabolic process that can occur with or without oxygen.

Aerobic Respiration: The Citric Acid Cycle

In the presence of oxygen, cells perform aerobic respiration. After glycolysis (the initial breakdown of glucose into pyruvate in the cytoplasm), the derivatives of pyruvate enter the mitochondria.

The citric acid cycle (also known as the Krebs cycle) processes these derivatives to produce high-energy electron carriers: NADH and FADH$_2$. These carriers then donate their electrons to a respiratory electron transport chain located in the mitochondrial inner membrane. This triggers chemiosmosis, leading to a massive production of ATP.

Anaerobic Conditions and Fermentation

If oxygen is unavailable, the electron transport chain cannot function because there is no oxygen to act as the final electron acceptor. This causes a backup: the supply of $NAD^+$ and $FAD$ becomes depleted because they are all stuck in their reduced forms (NADH and FADH$_2$). Without $NAD^+$, even glycolysis—the cell's most basic energy pathway—will stop.

To prevent this, cells use fermentation. Fermentation is not a way to make large amounts of ATP; rather, its primary purpose is to regenerate $NAD^+$ so that glycolysis can continue to produce a small, steady amount of ATP.

In fermentation, pyruvate (or a derivative) acts as an electron acceptor. It accepts electrons from NADH, becoming reduced to molecules like lactate (in animals) or ethanol (in yeast).

Worked Example: The Barnacle Scenario Imagine a barnacle that closes its shell during low tide, entering an anaerobic state. Since it cannot use oxygen for the citric acid cycle or the ETC, it survives by relying on glycolysis. To keep glycolysis running, the barnacle uses fermentation to turn its NADH back into $NAD^+$, allowing it to continue producing small amounts of ATP.

Summary Table of Key Processes