Metabolic reactions sometime involve the transfer of electrons from one molecule to another. One molecule donates an electron and another molecule accepts the electron. This transfer of electrons is called oxidation-reduction or redox reaction. A redox reaction is comprised of two events. The first event happens when a molecule donates an electron. This is called oxidation. The second event happens when another molecule accepts the donated electron. This is called reduction.
The cell uses electron carrier molecules to carry electrons between areas within the cell. Think of these carrier molecules as "shuttle buses." Carrier molecules are necessary because the cytoplasm of the cell does not contain free electrons.
Two important electron carrier molecules that are used in cell metabolism are
• nicotinamide adenine dinucleotide (NAD+)
• flavine adenine dinucleotide (FAD)
• nicotinamide adenine dinucleotide phosphate (NADP+)
For example, when synthesizing ATP, NAD+ carries electrons of a hydrogen (H) atom, making NADH. FAD carries two electrons of hydrogen making FADH2. Very often electrons of hydrogen atoms are the electrons transported by the carrier molecule. NADP+ is used to reduce CO2 to carbohydrates during the Dark Phase of photosynthesis.
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