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.
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Intro to biology

New to biology? You're in the right place! Start your biological journey here.

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.

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.

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 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.

Photosynthesis

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.

DNA as the genetic material

What exactly is DNA? This seemingly simple polymer, made up of just four different types of monomers, serves as the genetic material for all living organisms. For example, your DNA provides instructions for building and maintaining your unique body and can be passed on if you have children. Learn more about the discovery, structure, and synthesis of this remarkable molecule.

Central dogma

How does a gene in your DNA provide instructions for building a protein? In gene expression, a DNA sequence is first copied to make an RNA molecule, which is then "decoded" to build a protein. Learn more about this remarkable process, shared by all living things.

Gene regulation

You have tens of thousands of genes in your genome. Does that mean your cells express all of those genes, all the time? Not by a long shot! Even an organism as simple as a bacterium must carefully regulate gene expression, ensuring that the right genes are expressed at the right time. Learn more about the mechanisms cells use to turn genes "on" and "off."

Biotechnology

What is biotechnology? Well, if you've ever taken the antibiotic penicillin or eaten bread made with yeast, you've experienced it firsthand! Biotechnology is the use of a living organism, or some component of a living system, to make a useful product. Learn more about modern biotechnology and how it is used in research and biomedical applications.

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.

Bacteria and archaea

They're everywhere. They number in the millions of trillions of trillions. They live on every surface, in every environment, and even in your gut. That's right...I'm talking about prokaryotes! Learn more about bacteria and archaea.

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.

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!
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All content in “Human biology”

Circulatory and pulmonary systems

As humans, we really like breathing oxygen. That's because the cells in our body will die if they don't get the oxygen they need to function. This tutorial describes how gases are exchanged between blood and the atmosphere in our lungs, and how oxygen is then pumped through the body by way of blood and the circulatory system.

The kidney and nephron

How do we get unwanted substances out of our blood? Through the kidney. This tutorial goes into detail about how the kidney works.

Muscles

Without muscles, we wouldn't be able to do much of anything. This tutorial explores what muscle cells are and how they contract to let us move our bodies.