Cells harvest energy primarily through a process called cellular respiration. This universal biochemical pathway breaks down organic molecules, such as glucose, to release energy in the form of ATP (adenosine triphosphate), the cell's primary energy currency.
Cellular Respiration: The Core Energy Harvesting Process
Cellular respiration is a series of metabolic reactions that occur in cells to convert biochemical energy from nutrients into ATP. It's essentially a controlled burning process, but instead of releasing all the energy as heat, it captures much of it in the chemical bonds of ATP. This process involves several key steps:
-
Glycolysis: Occurs in the cytoplasm and breaks down glucose into pyruvate, producing a small amount of ATP and NADH (an electron carrier).
-
Pyruvate Oxidation: Pyruvate is transported into the mitochondria (in eukaryotes) and converted to acetyl CoA, releasing carbon dioxide and producing more NADH.
-
Citric Acid Cycle (Krebs Cycle): Acetyl CoA enters the citric acid cycle, a series of reactions that further oxidize the molecule, releasing carbon dioxide, ATP, NADH, and FADH2 (another electron carrier).
-
Electron Transport Chain (ETC) and Oxidative Phosphorylation: NADH and FADH2 deliver electrons to the electron transport chain, located in the mitochondrial inner membrane (in eukaryotes). As electrons move through the chain, protons (H+) are pumped across the membrane, creating an electrochemical gradient. ATP synthase uses this gradient to generate ATP through oxidative phosphorylation. Oxygen acts as the final electron acceptor in the chain, forming water.
Summary of Cellular Respiration
The following table summarizes the key steps and outcomes of cellular respiration:
| Step | Location | Inputs | Outputs | ATP Production |
|---|---|---|---|---|
| Glycolysis | Cytoplasm | Glucose | Pyruvate, ATP, NADH | Small |
| Pyruvate Oxidation | Mitochondrial Matrix | Pyruvate | Acetyl CoA, CO2, NADH | None |
| Citric Acid Cycle | Mitochondrial Matrix | Acetyl CoA | CO2, ATP, NADH, FADH2 | Small |
| Electron Transport Chain | Mitochondrial Membrane | NADH, FADH2, O2 | H2O, ATP | Large |
Anaerobic Respiration and Fermentation
When oxygen is limited or absent, some cells can harvest energy through anaerobic respiration or fermentation. These processes are less efficient than aerobic cellular respiration, producing significantly less ATP.
-
Anaerobic Respiration: Uses an electron transport chain with a final electron acceptor other than oxygen, such as sulfate or nitrate.
-
Fermentation: Does not use an electron transport chain. It regenerates NAD+ from NADH so that glycolysis can continue, producing ATP. Common fermentation products include lactic acid and ethanol.
Other Energy Sources
While glucose is a common energy source, cells can also break down other organic molecules like:
-
Lipids (fats): Broken down through beta-oxidation to produce acetyl CoA, which then enters the citric acid cycle.
-
Proteins: Broken down into amino acids, which can be converted into intermediates that enter glycolysis or the citric acid cycle.
In conclusion, cells primarily harvest energy by breaking down organic molecules through the process of cellular respiration. This process, particularly the electron transport chain and oxidative phosphorylation, efficiently generates ATP, the energy currency of the cell. When oxygen is unavailable, cells can use less efficient processes like anaerobic respiration or fermentation.
