Science, Tech, Math › Science The Most Important Molecules in Your Body Share Flipboard Email Print Science Chemistry Molecules Basics Chemical Laws Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated July 03, 2019 A molecule is a group of atoms bound together to perform a function. There are thousands of different molecules in the human body, all serving critical tasks. Some are compounds you can't live without (at least not for very long). Take a look at some of the most important molecules in the body. Water Water is an essential molecule for life. It needs to be replenished because it's lost through breathing, perspiring, and urinating. Boris Austin / Getty Images You can't live without water! Depending on age, gender, and health, your body is around 50-65% water. Water is a small molecule consisting of two hydrogen atoms and one oxygen atom (H2O), yet it's a key compound despite its size. Water participates in many biochemical reactions and serves as the building block of most tissue. It's used to regulate body temperature, absorb shock, flush away toxins, digest and absorb food, and lubricate joints. Water has to be replenished. Depending on temperature, humidity, and health, you can go no more than 3-7 days without water or you'll perish. The record appears to be 18 days, but the person in question (a prisoner accidentally left in a holding cell) is said to have licked condensed water from walls. Oxygen About 20% of air consists of oxygen. ZenShui/Milena Boniek / Getty Images Oxygen is a chemical element that occurs in the air as a gas composed of two oxygen atoms (O2). While the atom is found in many organic compounds, the molecule plays an essential role. It's used in many reactions, but the most critical is cellular respiration. Through this process, energy from food is converted in a form of chemical energy cells can use. The chemical reactions convert the oxygen molecule into other compounds, like carbon dioxide. So, oxygen needs to be replenished. While you can live days without water, you won't last past three minutes without air. DNA DNA codes for all the proteins in the body, not just for new cells. VICTOR HABBICK VISIONS / Getty Images DNA is the acronym for deoxyribonucleic acid. While water and oxygen are small, DNA is a large molecule or macromolecule. DNA carries the genetic information or blueprints to make new cells or even a new you if you were cloned. While you can't live without making new cells, DNA is important for another reason. It codes for every single protein the body. Proteins include hair and nails, plus enzymes, hormones, antibodies, and transport molecules. If all your DNA suddenly vanished, you'd be dead pretty much instantly. Hemoglobin Hemoglobin is a macromolecule that transports oxygen in red blood cells. INDIGO MOLECULAR IMAGES LTD / Getty Images Hemoglobin is another super-sized macromolecule that you can't live without. It's so large, red blood cells lack a nucleus so they can accommodate it. Hemoglobin consists of iron-bearing heme molecules bound to globin protein subunits. The macromolecule transports oxygen to cells. While you need oxygen to live, you wouldn't be able to use it without hemoglobin. Once hemoglobin has delivered oxygen, it binds to carbon dioxide. Essentially, the molecule also serves as a sort of intercellular garbage collector. ATP Breaking the bonds that join phosphate groups to ATP releases energy. MOLEKUUL/SCIENCE PHOTO LIBRARY / Getty Images ATP stands for adenosine triphosphate. It's an average-sized molecule, larger than oxygen or water, but much smaller than a macromolecule. ATP is the body's fuel. It's made inside organelles in cells called mitochondria. Breaking the phosphate groups off the ATP molecule releases energy in a form the body can use. Oxygen, hemoglobin, and ATP are all members of the same team. If any of the molecules is missing, the game is over. Pepsin Pepsin is a key stomach enzyme. LAGUNA DESIGN / Getty Images Pepsin is a digestive enzyme and another example of a macromolecule. An inactive form, called pepsinogen, is secreted into the stomach where the hydrochloric acid in gastric juice converts it into active pepsin. What makes this enzyme particularly important is that it's able to cleave proteins into smaller polypeptides. While the body can make some amino acids and polypeptides, others (the essential amino acids) can only be obtained from the diet. Pepsin turns protein from food into a form that can be used to build new proteins and other molecules. Cholesterol Lipoproteins are complex structures that transport cholesterol throughout the body. SPRINGER MEDIZIN / Getty Images Cholesterol gets a bad rap as an artery-clogging molecule, but it's an essential molecule that's used to make hormones. Hormones are signal molecules that control thirst, hunger, mental function, emotions, weight, and much more. Cholesterol is also used to synthesize bile, which is used to digest fats. If cholesterol suddenly departed your body, you'd be dead immediately because it's a structural component of every cell. The body actually produces some cholesterol, but so much is needed that it's supplemented from food. The body is a sort of complex biological machine, so thousands of other molecules are essential. Examples include glucose, carbon dioxide, and sodium chloride. Some of these key molecules consist of only two atoms, while more are complex macromolecules. The molecules work together via chemical reactions, so missing even one of like breaking a link in the chain of life.