Cells require energy to perform various biological reactions, and this energy is transported through activated carriers. These carriers facilitate the coupling of energetically favorable and unfavorable reactions, allowing the energy released from one reaction to fuel another. This process, known as reaction coupling, involves spontaneous reactions with a negative change in Gibbs free energy (ΔG) providing energy to reactions that require energy input, characterized by a positive ΔG.
Activated carriers are small molecules that store energy and transfer electrons, which are essential for driving chemical reactions. These carriers contain rich covalent bonds that, when broken, release energy. A prime example of an activated carrier is Adenosine Triphosphate (ATP), often referred to as the energy currency of the cell. ATP consists of three phosphate groups, and when one of these phosphate groups is hydrolyzed, energy is released, converting ATP into Adenosine Diphosphate (ADP). This released energy can then be utilized to power unfavorable reactions, such as transporting molecules across cell membranes against their concentration gradients.
In addition to ATP, other important activated carriers include Nicotinamide Adenine Dinucleotide (NADH) and its oxidized form, NAD+. NAD+ serves as an electron carrier, storing high-energy electrons. When NAD+ gains an electron, it is reduced to NADH, which can then donate this energy to other reactions. The process of oxidation and reduction is crucial here, where the loss of electrons is termed oxidation (remembered by the phrase "LEO says GER" - Loss of Electrons is Oxidation, Gain of Electrons is Reduction).
Another activated carrier is Flavin Adenine Dinucleotide (FAD), which also plays a role in cellular respiration and photosynthesis. Both ATP and NADH are generated through these metabolic processes, highlighting their significance in energy transfer within cells. Understanding the structure and function of these activated carriers is essential for grasping how energy is managed and utilized in biological systems.