Biology

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.
Community Questions

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)!

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.

Macromolecules

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.

Cellular and molecular biology

All living things, including you, are made up of cells. Find out how cells divide through mitosis and meiosis, as well as how they extract energy from sugars through cellular respiration (and harvest light energy through photosynthesis)!

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.

Heredity and evolution

Learn about evolution and genetics! Here, you can dive deeper into evolution, natural selection, and the tree of life, as well as Mendelian genetics and population genetics. Special bonus: explore genetics at the molecular level, learning about DNA, RNA, transcription, and translation.

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!

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.
Biology: Questions

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