Cellular Respiration

Big Picture

Cellular respiration is a fundamental metabolic process that transfers energy from organic molecules, such as glucose, into ATP, the cell’s primary energy currency. In aerobic respiration, oxygen acts as the final electron acceptor in the electron transport chain (ETC), combining with electrons and hydrogen ions to form water. Some energy is released as heat during these processes.

• Cellular respiration enables cells to extract energy efficiently from nutrients.

• Oxygen is essential for aerobic respiration, facilitating the completion of the ETC.

Major Stages and Locations

Cellular respiration consists of several distinct stages, each occurring in specific cellular compartments:

• Glycolysis: Occurs in the cytosol. Glucose is split into two pyruvate molecules. Net gain: 2 ATP and 2 NADH per glucose. Oxygen is not directly required.

• Pyruvate Oxidation: In the mitochondrial matrix of eukaryotes, pyruvate is converted to acetyl-CoA, releasing CO2 and generating NADH.

• Citric Acid (Krebs) Cycle: In the mitochondrial matrix, acetyl-CoA is oxidized, releasing CO2 and producing NADH, FADH2, and a small amount of ATP.

• Oxidative Phosphorylation: At the inner mitochondrial membrane, electron transport builds an H+ gradient, and ATP synthase uses that gradient to make most ATP.

Electron Carriers and Redox

Electron carriers play a crucial role in cellular respiration by shuttling high-energy electrons:

• NAD+ and FAD accept high-energy electrons and become NADH and FADH2. These reduced carriers deliver electrons to the ETC.

• As electrons pass along ETC proteins, released energy powers proton pumping from the matrix into the intermembrane space.

• NADH generally supports more ATP production than FADH2 because its electrons enter earlier in the chain.

Chemiosmosis and ATP Synthase

The process of chemiosmosis is central to ATP production:

• The intermembrane space becomes relatively high in H+ and the matrix relatively low in H+. This electrochemical gradient stores potential energy.

• H+ flows back into the matrix through ATP synthase, driving ADP + inorganic phosphate (Pi) to form ATP. This coupling is called chemiosmosis.

• Inner membrane folds (cristae) increase surface area for ETC proteins and ATP synthase. Similar gradient-driven ATP synthesis occurs in chloroplasts, though the energy source and compartments differ.

When Oxygen is Unavailable

Without oxygen, electron flow through the aerobic ETC slows or stops, proton pumping and oxidative phosphorylation decline:

• Fermentation regenerates NAD+ from NADH, allowing glycolysis to continue producing a small amount of ATP.

• Lactic acid fermentation can occur in muscle cells and some bacteria; alcoholic fermentation produces ethanol and CO2 in yeast.

Predicting Effects of Changes

Various factors can affect cellular respiration efficiency:

• ETC inhibitor: NADH may accumulate, NAD+ may decrease, and ATP output falls because electron flow and proton pumping are reduced.

• Proton leak/uncoupling: The H+ gradient weakens; less ATP is made through ATP synthase and more energy may be released as heat (as in brown fat).

• More cristae: Greater inner-membrane area can support more ETC and ATP synthase proteins.

• Blocked glycolysis or pyruvate oxidation: Less fuel reaches downstream pathways; cells may rely more heavily on alternative metabolic routes.

Key Equations

• ATP Formation:

• Overall Cellular Respiration:

Example

During intense exercise, muscle cells may switch to lactic acid fermentation when oxygen is scarce, producing ATP and lactic acid.

Additional info: The notes have been expanded for clarity and completeness, including definitions, examples, and equations to ensure a self-contained study guide.