# 2: Energy

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Living organisms are made up of cells, and cells contain many biochemical components such as proteins, lipids, and carbohydrates. But, living cells are not random collections of these molecules. They are extraordinarily organized or "ordered". By contrast, in the nonliving world, there is a universal tendency to increasing disorder. Maintaining and creating order in cells takes the input of energy. Without energy, life is not possible. It is therefore important that we consider energy first in our attempt to understand biochemistry. Where does energy come from? Photosynthetic organisms can capture energy from the sun, converting it to chemical forms usable by cells. Heterotrophic organisms like ourselves get our energy from the food we eat. How do we extract the energy from the food we eat?

• 2.1: Oxidative Energy
• 2.2: Oxidation vs Reduction in Metabolism
Catabolic processes are often oxidative in nature and energy releasing. Some, but not all of that energy is captured as ATP.  Not all of the energy is captured as ATP, and the rest is released as heat and it is for this reason that we get hot when we exercise. By contrast, synthesizing large molecules from smaller ones (for example, making proteins from amino acids) is referred to as anabolism. Anabolic processes are often reductive in nature and require energy input.
• 2.3: Energy Coupling
The addition of phosphate to a sugar is a common reaction that occurs in a cell. By itself, this process is not very energetically favorable (that is, it needs an input of energy to occur). Cells overcome this energy obstacle by using ATP to “drive” the reaction. The energy needed to drive reactions is harvested in very controlled conditions in the confines of an enzyme. This involves a process called ‘coupling’.
• 2.4: Entropy and Energy
• 2.5: Gibbs Free Energy
• 2.6: Cellular Phosphorylations
Formation of triphosphates is essential to meet the cell’s immediate energy needs for synthesis, motion, and signaling. In a given day, an average human being uses more than their body weight in triphosphates. Since triphosphates are the “currency” that meet immediate needs of the cell, it is important to understand how triphosphates are made. There are three phosphorylation mechanisms – 1) substrate level; 2) oxidative; and 3) photophosphorylation. We consider them here individually.
• 2.7: Energy Efficiency
• 2.8: Metabolic Controls of Energy
• 2.9: Molecular Backups for Muscles
• 2.10: Summary

This page titled 2: Energy is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Kevin Ahern & Indira Rajagopal via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.