You've likely heard someone attribute their body size to a fast or slow metabolism. But did you know there's much more to metabolism than calories burned? Metabolism includes all the chemical changes that occur as our bodies use enzymes to break down food, medicines, and biological substances as well as produce energy and materials needed for growth.

The Two Sides of Metabolism

Our bodies have many metabolic pathways, but they all fall into two main categories: catabolic and anabolic. Catabolic pathways break down complex molecules into simpler ones, usually releasing energy in the process. For example, catabolic pathways turn large carbohydrate molecules from our food into simple sugars, such as glucose. Some of the most well-known catabolic pathways then convert the simple sugar glucose into adenosine triphosphate (ATP), a molecule that cells commonly use as an energy source.

“I remember thinking in my first cellular biology class how impossibly beautiful it is that there are tiny machines in our bodies doing work,” says Morgan DeSantis, Ph.D., an assistant professor of molecular, cellular, and developmental biology at the University of Michigan in Ann Arbor. We talked with Dr. DeSantis about how her career in science almost didn’t happen, how happy she is that it did, and what she’s learning through her research on molecular machines.

Q: How did you become interested in science?

A: I wasn’t remotely interested in science in high school—I was a self-identified artist. I went to the College of Wooster in Ohio with the sole purpose of studying art and doing pottery. But one day during my freshman year, a box with all the pieces I made throughout the year fell, and everything inside broke. It’s hard to describe the emotions I felt that day, but something changed in me, and I realized pottery wasn’t for me. I couldn’t start the projects over, and I didn’t want to drop out and move back home. So, I decided to become a medical doctor.

“Turning personal grief into a relentless drive for answers and action has been a big part of my life,” says Caroline Palavicino-Maggio, Ph.D., the director of the Neurobiological Mechanisms of Aggression Laboratory at McLean Hospital in Belmont, Massachusetts. Her sister’s death played a large role in her decision to study the biological mechanisms of behavior and do advocacy work in mental wellness. We spoke with Dr. Palavicino-Maggio about what her path was to becoming a researcher, what she’s learning about the cellular basis of aggression, and why she believes a career in science is exciting.

The brain is a large and complex organ, but some very small structures guide its development. Xuecai Ge, Ph.D., an associate professor of molecular and cell biology at the University of California, Merced (UC Merced), has devoted her career to understanding one of these structures called the primary cilium. In an interview, Dr. Ge shared how her childhood experience inspired her to study science and what makes the primary cilium fascinating.

Q: How did you first become interested in science?

A: When I was a little kid, my mom was a primary care doctor, and I saw her treat patients in our community. I noticed that no matter who got a particular illness, she could use the same medicine to treat them. My little mind was amazed that the same medicine could work for so many different people! I think this early experience planted the original seed of my interest in life science.

Have you ever noticed a skin care product advertised as “microbiome friendly” and wondered what that meant? The microbiome is the collection of all the microbes—including bacteria, viruses, and fungi—that live in a specific environment, such as on the skin or in the digestive tract.

It’s a common misconception that all microbes are harmful—in truth, much of the human microbiome is made up of microbes that form beneficial symbiotic relationships with us. Microbiome-friendly skin care products don’t have antimicrobial properties that could harm the beneficial bacteria that live on our skin.

Your Microbiome and You

Every day, our cells must produce all the various molecules they need to stay alive. But the chemical reactions to create these molecules can’t occur without help—which is where enzymes come in. Enzymes are biological catalysts, meaning they speed up the rate of specific chemical reactions by reducing the amount of energy needed for the reaction to occur. Most enzymes are proteins, but some RNA molecules can also act as enzymes.

Thousands of different enzymes catalyze the vast range of reactions that take place within cells, but each enzyme typically supports one of the following types of tasks:

Throughout our immunology miniseries, we introduced the immune system and its many functions and components. Additionally, we highlighted how vaccines train your immune system, how the system can go awry, and how NIGMS-supported researchers are studying immunology and infectious diseases. Put your knowledge about the immune system to the test by taking the quiz below.

Happy Valentine's Day! In place of red roses, we hope you'll accept a bouquet of beautiful scientific images featuring rich, red hues. Be sure to click all the way through to see the festive protein flowing through your blood!

For more scientific photos, illustrations, and videos in all the colors of the rainbow, visit our image and video gallery.

It's common knowledge that too much cholesterol and other fats can lead to disease and that a healthy diet involves watching how much fatty food we eat. However, our bodies need a certain amount of fat to function—and we can't make it from scratch.

Triglycerides, cholesterol, and other essential fatty acids—the fats our bodies can't make on their own—store energy, insulate us, and protect our vital organs. They act as messengers, helping proteins do their jobs. They also start chemical reactions that help control growth, immune function, reproduction, and other aspects of basic metabolism. Fats also help the body stockpile certain nutrients. Vitamins A, D, E, and K, for example, are stored in the liver and in fatty tissues.

The cycle of making, breaking, storing, and using fats is at the core of how all animals, including humans, regulate their energy. An imbalance in any step can result in disease. For instance, having too many triglycerides in our bloodstream raises our risk of clogged arteries, which can lead to heart attack and stroke.