1. In biological systems:
* NADH and NADPH: These coenzymes are important electron carriers in cellular respiration and other metabolic processes. They carry hydrogen atoms (specifically, electrons and protons) from one molecule to another.
* FADH2: Similar to NADH and NADPH, FADH2 is another electron carrier that plays a crucial role in cellular respiration.
* Other enzymes: Many enzymes use hydrogen ions (H+) in their catalytic activities. However, these enzymes don't act as specific "carriers" in the same way as NADH or FADH2.
2. In chemical reactions:
* Metal hydrides: Certain metals can form compounds called hydrides, where they bond directly with hydrogen atoms. These hydrides can be used as a means to store and transport hydrogen.
* Organic molecules: Hydrogen can be stored in chemical bonds within organic molecules like hydrocarbons (e.g., methane, propane) or alcohols (e.g., methanol, ethanol).
3. In the atmosphere:
* Hydrogen gas (H2): This is the most common form of hydrogen in the atmosphere, though it's present in very small amounts.
Therefore, the "carrier" of hydrogen depends on the specific process or environment you're considering. It's more accurate to say that hydrogen is transported through various mechanisms rather than a single specific carrier molecule.
