Life is beautiful! From atoms to cells, from genes to proteins, from populations to ecosystems, biology is the study of the fascinating and intricate systems that make life possible. Dive in to learn more about the many branches of biology and why they are exciting and important. Covers topics seen in a high school or first-year college biology course.
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Chemistry of life

Learn how chemistry makes life possible! From you, to your dog, to your dinner, to the global ecosystem, all living systems are made out of atoms that obey the basic rules of chemistry. Here, you can learn about the key properties of atoms, including what particles they contain, how they are organized, and how they form chemical bonds with one another.

Water, acids, and bases

Where would we be without water? Well, we probably wouldn't exist at all! Learn more about this fascinating molecule: its hydrogen bonds, its properties, and why it's a key ingredient for biological systems of all sizes. Plus, brush up on the acid-base chemistry that takes place in water (and learn how it works in your own bloodstream)!

Properties of carbon

Did you know that your body is approximately 18% carbon? Learn more about carbon and why it makes a great building block for biological molecules. We'll look at the bonding properties of carbon, the structures of hydrocarbons, different types of isomerism, and some functional groups important in biology.


There's an old saying, "You are what you eat." In some senses, this is literally true! When we eat food, we take in the large biological molecules found in the food, including carbohydrates, proteins, lipids (such as fats), and nucleic acids (such as DNA). Dive into the different types of macromolecules, what they are made up of, and how they are built up and broken down.

Structure of a cell

You, my friend, are made up of cells. Lots and lots of them. Some of them are eukaryotic (human), but many more of them are prokaryotic, thanks to the friendly bacteria of your gut, skin, and other body systems. Jump in to learn more about prokaryotic and eukaryotic cells and the complex and beautiful structures inside of them.

Membranes and transport

How do the cells in your body define their boundaries (and control what comes in or goes out)? As it turns out, cells have a sophisticated and flexible barrier, the plasma membrane, and a wide array of strategies for transporting molecules in and out. Learn more about what the membrane's made of and how different types of molecules move across it.

Energy and enzymes

You, like other living systems, are an amazing energy transformer. As you move your eyes to read these words, your body is busily converting chemical energy from your lunch into kinetic energy and thermal energy (heat). Learn more about how biological energy transfers work, as well as how protein machines called enzymes direct metabolic traffic through your cells.

Cellular respiration

How do your cells extract energy from the food that you eat? As it turns out, cells have a network of elegant metabolic pathways dedicated to just this task. Learn more about cellular respiration, fermentation, and other processes that extract energy from fuel molecules like glucose.


There's a lot of carbon in your body - in DNA, proteins, carbohydrates, fats, and other key biological molecules. How did that carbon get there? Drumroll... photosynthesis! Learn how plants and other photosynthetic organisms use light energy to make sugars from carbon dioxide and water, bringing both chemical energy and readily usable carbon into Earth’s global ecosystem.

Cell signaling

Did you know that your cells are continually "talking" to one another? Cells communicate by sending and receiving chemical signals. This molecular conversation allows the cells in your body to coordinate their activities, forming a functional, harmonious whole (a.k.a., you!). Learn more about the remarkable process of cell signaling.

Cell division

All living things, including you, are made up of cells. Find out how cells divide through mitosis and meiosis, how the cell cycle is controlled, and how problems in cell cycle control can lead to cancer.

Classical and molecular genetics

Have you ever been told that you have your mother's eyes or grandfather's nose? Learn why traits run in families and how they're passed on. We'll look at Mendel's model of inheritance, see how it's been revised and extended in the past 150 years, and get a sneak preview of the modern concept of a gene.

Evolution and the tree of life

Discover the diversity of life on Earth and the forces that shape it! In this section, you can learn about evolution, natural selection, and the tree of life, as well as basic population genetics. Special bonus: take a peek into the amazing worlds of bacteria and viruses.

Human biology

Your body is an amazing system! The human body is made up of groups of organs, called organ systems, that work together to keep the body in balance. In this section, we'll travel from the circulatory system, to the nervous system, to the immune system and beyond. Learn about the amazing biology that keeps your body ticking!

Crash Course: Biology and Ecology

Brush up on a wide range of biology topics, from biochemistry to mitosis to ecology, with fun and informative Crash Course videos!

Community explanations

Calling great explainers of the world! Help us answer frequently asked biology questions by writing clear, deep, engaging explanations. Your explanation may even be selected for an article on Khan Academy (with full credit to you)!

Ask a biologist (archive)

Archived questions from open question and answer session that ran July 28-August 2, 2015. The session is no longer open for questions, but can still be browsed. Please feel free to ask your own questions in the comments sections below other videos and questions.