Biology

Review for the AP Biology exam by working through practice problems with Sal!

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.

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)!

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), and use them to power our cells and build our bodies. Dive into the different types of macromolecules, what they are made up of, and how they are built up and broken down.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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."

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.

Did you know that a frog starts out as a single cell? In fact, so do dogs, elephants, and even human beings! Learn how multicellular organisms end up with many different cell types, organized into the tissues and organs of the body.

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.

Viruses: Are they alive? Are they dead? Are they...undead? Learn more about these fascinating (and clinically important) particles that occupy a "gray area" between living and non-living things.

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.

Our planet has an amazing history spanning about 4.5 billion years! Learn more about how Earth formed, the early history of life on Earth, and the origins of humans.

Why are polar bears found only in the Arctic? Why does mildew grow in your shower and not (hopefully) in your sock drawer? Learn how ecologists study the interactions between organisms and their environment, and how these interactions affect where, and in what numbers, different types of organisms are found.

From penguins to palm trees to bacteria, our planet is host to an awe-inspiring diversity of organisms. Dive deep into Earth's breathtaking biodiversity, learn why this biodiversity is in peril today, and discover what you can do to protect the species and ecosystems that support human life on Earth. This content is provided by the California Academy of Sciences.

What do a dog begging for food, and ant leaving a chemical trail to food, and a human baby learning language all have in common? These are all examples of behaviors. Behavior is a crucial way that animals interact with each other and with their environment. Learn more about types of animal behaviors and how behavior is shaped by genes and environment.

How is your body organized? How do your organ systems work together to keep you running? What keeps your blood at the right pH or your body at the right temperature? Learn more about tissues, organs, and organ systems and the control mechanisms that keep internal conditions steady in humans and other animals.

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!

Plants may seem less interactive than animals. But they actually have many ways of responding to their environment, changing their growth or physiology to make the best use of available resources. Learn more about plants and their response to environmental cues.

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

Do you enjoy your biology class, like nature, have an interest in medicine, or simply feel lots of curiosity about living things? If so, you may be thinking about focusing on or majoring in biology. Learn more about what it's like to be a biologist by hearing from professionals in a wide range of fields, from primatology to personalized genetics to high school teaching!

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)!