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- [Voiceover] I would like to welcome you to Biology at Khan Academy. And biology, as you might now, is the study of life. And I can't really imagine anything more interesting than the study of life. And when I say "life," I'm not just talking about us, human beings. I'm talking about all animals. I'm talking about plants. I'm talking about bacteria. And it really is fascinating. How do we start off with inanimate molecules and atoms? You know, this right here is a molecule of DNA. How do we start with things like that, and we get the complexity of living things? And you might be saying, well, what makes something living? Well, living things convert energy from one form to another. They use that energy to grow. They use that energy to change. And I guess growth is a form of change. They use that energy to reproduce. And these are all, in and of themselves, fascinating questions. How do they do this? You know, we look around us. How do we, you know, eat a muffin? And how does that allow us to move around and think and do all the things we do? Where did the energy from that muffin come from? How are we similar to a plant or an insect? And we are eerily or strangely similar to these things. We actually have a lot more in common with, you know, that tree outside your window, or that insect, that bee, that might be buzzing around, than you realize. Even with the bacteria that you can only even see at a microscopic level. In fact, we have so much bacteria as part of what makes us, us. So these are fascinating questions. How did life even emerge? And so over the course of what you see in Biology on Khan Academy, we're going to answer these fundamental, fascinating questions. We're going to think about things like energy and the role of energy in life. We're going to thing about important molecules in biology. And perhaps most importantly, DNA and its role in reproduction and containing information. And we're going to study cells, which are the basic building block of life. And as we'll see, even though we view cells as these super, super small, small things, cells in and of themselves are incredibly complex. And if you compare them to an atomic scale, they're quite large. In fact, this entire blue background that I have there, that's the surface of an immune cell. And what you see here emerging from it, these little yellow things. These are HIV viruses, emerging from an immune cell. So even though you imagine cells as these very, very small microscopic things, this incredible complexity. Even viruses. Viruses are one of these fascinating things that kind of are right on the edge between life and nonlife. They definitely reproduce, and they definitely evolve. But they don't necessarily have a metabolism. We'll learn a lot more about that. They don't necessarily use energy and growth in the same way that we would associate with life. And then perhaps one of the biggest questions of all is how did life come about? And we will study that as we look at evolution and natural selection. So welcome to Khan Academy's Biology section. I think you're going to find it fascinating. You're going to realize that biology, in some way, is the most complex of the sciences. And in a lot of ways, the one that we understand the least. It's going to be built on top of chemistry, which in turn is built on top of physics, which in turn is built on top of mathematics. And biology is one of our Frankly, even in the last hundred years, we're just starting to scratch the surface of understanding it. But what's really exciting is where the field of biology is going. As we understand things at a deeper level, at a molecular level, we're going to start thinking about how can we even do things like engineer biology, or affect the world around us? It's going to raise all sorts of fascinating and deep and ethical questions. So, hopefully you enjoy this. Biology is one of the most, arguably, maybe the most fascinating subject of all. I don't want to offend the chemists and the physicists out there. I actually find those quite fascinating as well. But we're going to answer, or attempt to start to answer, some of the most fundamental questions of our existence.