Why You Live ? The Strange Science of Life and Energy

This lesson explores Why You Live by explaining how life constantly fights against entropy, how cells use energy to stay alive, and why molecules like ATP are essential for keeping the universe in motion through living beings.

Science Explained

 

At this very second, you are on a narrow ledge between life and death. You probably don’t feel it, but there is an incredible amount of activity happening inside you, and this activity can never stop. To be alive is to constantly fight against stopping. Imagine yourself as a slinky falling down an escalator that is moving upward. The falling represents the self-replicating processes of your cells. The escalator represents the laws of physics pushing everything forward. Being alive means being in motion, but never actually arriving anywhere. If you ever reach the top of the escalator, the falling stops—and you are dead forever. Somewhat unsettlingly, the universe wants you to reach the top. So the question becomes: how do you avoid that, and why are you alive in the first place?

All life is based on the cell. A cell is a tiny piece of the dead universe that separated itself so it could do its own thing for a while. When this separation breaks down, the cell dies and rejoins the rest of the dead universe. Unfortunately, the universe prefers things to be boring and stable. It tries to eliminate excitement and difference. This principle is called entropy, and it is a fundamental rule of the universe. Living things, however, are inherently exciting and disordered.

Inside a single cell are millions of proteins and countless simpler molecules like water. Thousands of complex, self-replicating processes happen hundreds of thousands of times every second. To stay alive, a cell must constantly work to avoid entropy and maintain its separation from the outside world. Cells do this by controlling the concentration of molecules, keeping the inside different from the outside, and actively pumping excess material away. All of this work requires energy. Energy is the ability to do work, create movement, and cause change. Energy cannot be created or destroyed. The total amount of energy in the universe never changes. We don’t know why—it simply works that way. So one of the biggest challenges for early life was figuring out how to obtain usable energy.

Early cells used simple chemical reactions, but eventually discovered the ultimate energy-transfer molecule: adenosine triphosphate, or ATP. ATP is uniquely suited for accepting, storing briefly, and releasing energy when a cell needs it. When a cell needs energy—for example, to pump out molecules or repair microscopic machinery—it breaks down ATP and uses the released chemical energy to do work. This is why living beings are able to function at all. Every living thing on Earth—plants, fungi, bacteria, and animals—uses ATP or something very similar. Without ATP, there would be no life on Earth, and possibly nowhere else in the universe.

Early life forms missed out on the greatest energy source available: the sun. The sun releases massive amounts of electromagnetic energy, but this energy is raw and unusable without processing. It needed to be refined. After hundreds of millions of years of evolution, a cell figured out how to absorb sunlight and convert it into chemical energy. This process is called photosynthesis. Light energy is transformed into chemical energy stored in ATP. Some cells improved this system by producing glucose, a high-energy sugar that is easy to break down. Glucose became so useful that other cells decided to stop making energy themselves and instead consume cells that already did.

Eventually, one cell swallowed another but did not destroy it. Instead, they formed a permanent partnership. This moment changed Earth forever. The swallowed cell became the mitochondria, the powerhouse of the cell. This single event is the origin of all complex life. Every animal—from blue whales to dinosaurs to humans—can trace its existence back to this moment. Mitochondria specialize in producing ATP by breaking down sugar using oxygen, releasing energy, carbon dioxide, water, and heat. This division of labor gave cells far more energy than ever before, opening the door to multicellular life.

Eventually, cells formed communities, then tissues, then organisms. Today, you are made of trillions of cells, each producing ATP constantly to keep you alive. If this process stops—even for a few minutes—you die. You might wonder why we don’t store ATP the way we store fat. The reason is that ATP is excellent for energy transfer, but terrible for energy storage. It holds very little energy compared to its mass. Every day, your body produces and consumes roughly your own body weight in ATP. ATP must be made continuously, because even storing enough for a few minutes is nearly impossible.

This is the strange story of the molecule that allows you to exist as something different from the dead universe—a slinky on an escalator, constantly moving to avoid stopping. Life depends on producing fuel while already moving, like driving a car at full speed while making fuel in the trunk from scraps on the road. This began billions of years ago, when pieces of the dead universe briefly became something alive. From the first cells to you watching this now, the motion has never stopped. One day, you will return to the rest of the universe. But until then, you get to do what life does best—making the universe more interesting.