Homeostasis is a state of internal stability within the body. Homeostasis also refers to the process in which an organism maintains balance of things such as body temperature, water levels and salt levels. Many chemical reactions happen in order maintain homeostasis. Hormones must be made by breaking other molecules. Salt ions must be absorbed from the food that is eaten or stored into bones. Muscles need to produce heat in order to warm the body.
Release Energy from ATP
The vast majority of enzymes that make chemical reactions happen inside a cell use an energy molecule called adenosine triphosphate (ATP) -- “tri” means that there are three phosphate molecules on it. ATP is like a rechargeable battery. ATP can be broken into adenosine diphosphate (ADP) -- "di" means there are two phosphates -- and a single phosphate (P) molecule. When broken into ADP and P, ATP releases energy that gives enzymes the power to break or make molecules. Homeostasis is maintained by many cellular processes that require ATP. Aside from enzymes the make and break bonds, other proteins that use ATP include protein pumps that move salts across a membrane.
Vitamin D Synthesis
Vitamin D is a hormone that helps maintain calcium homeostasis; that is, proper calcium levels in the body. It needs to be made via multiple chemical reactions before it can affect homeostasis. It comes from cholesterol in the skin, which changes shape when hit by sunlight. This precursor to vitamin D then goes to the liver where it is modified. Finally, it goes to the kidneys where it is modified again to become the active form of vitamin D. The active form has a completely different structure than cholesterol, with extra chemical parts added here and there. Multiple enzymes are required to make active vitamin D, which is called 1,25-hydroxy vitamin D.
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Calcium Deposition in Bones
Calcium homeostasis also involves taking calcium out of the blood, not just absorbing it from food into the blood. Human blood cannot have too much or too little calcium, so excess calcium is stored inside bones. The process of depositing calcium ions into bone tissue is a chemical reaction that happens regularly. Calcium exists as a cation (pronounced cat-eye-on), meaning it has a positive electrical charge. In bone, calcium is stored as calcium hydroxyapatite, meaning it is bound to negatively charged molecules called phosphates. When the cell wants to take calcium out of the blood and store it in bones, bone cells spit out phosphate molecules around them, which attracts the positively charged calcium ions. The calcium binds to the phosphate and forms crystals.
Cellular Respiration to Produce Heat
When the human body gets too cold, it maintains temperature homeostasis by producing heat to warm itself. The human body can increase its internal temperature by making heat in skeletal muscle cells and brown fat cells. These cells contain many mitochondria, which are pouches within a cell that produce ATP molecules. Mitochondria make ATP by first storing up lots of hydrogen ions in one compartment, and then letting those ions naturally flow into another compartment -- like water flowing through a dam. This flow generates power that is used to form new ATP molecules. However, heat is produced when hydrogen ions flow in this way. The body warms up by telling cells to purposely cause leaks in mitochondria, so that more hydrogen ions flow. Many chemical reactions need to take place in order for this to happen. These reactions are part of what is called cellular respiration.