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Biology overview

Biology at Khan Academy explores life in all its forms, from humans to bacteria and how living things convert energy, grow, and reproduce. The course covers key biological molecules, the role of DNA, and the complexity of cells. It also touches on the intriguing world of viruses and how life continues to change and adapt through evolution and natural selection. Created by Sal Khan.

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  • old spice man green style avatar for user Andrew Xavier
    I don't understand why the HIV virus is coming out of the immune cell. Isn't the Immune cell supposed to get rid of the viruses, not make more?
    (120 votes)
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    • leaf blue style avatar for user Esther Dickey
      A virus reproduces by hijacking a cell, taking over its machinery, and forcing it to make new viruses. When the poor cell has made so many new viruses that it is completely filled with them, the viruses burst the cell's wall and escape. This kills the cell. The new viruses then float off to hijack more cells.
      Instead of hijacking just any old cell it runs into, the HIV virus specifically targets immune cells.
      (233 votes)
  • mr pants teal style avatar for user Eric Song
    Are we really "alive" if we are made up of inanimate material?
    (30 votes)
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  • blobby green style avatar for user 兆壹 罗
    The test said:"The properties shared by all living things are: They are made up of cells."
    But virus don't have cell structures and virus are living things?

    so ?
    (21 votes)
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    • purple pi teal style avatar for user Ellen Wight
      Viruses are kind of the 'zombies' of the cell world - they are only active and able to reproduce if they have infected an organism. Outside of an organism, like in the air, they can do basically nothing. The only resemblance viruses have to cells is a small amount of DNA and the ability to reproduce - they have no other purpose.
      (61 votes)
  • duskpin ultimate style avatar for user Jessica
    What exactly does he mean at ?
    (13 votes)
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  • spunky sam blue style avatar for user Mark Karikari
    is a virus a living thing?
    (12 votes)
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  • leafers seed style avatar for user jyothuv
    How did living things come to life?
    (12 votes)
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    • aqualine seedling style avatar for user SpinosaurusRex
      Earth was able to support life only after the planet had cooled enough for a rocky crust to solidify. Once that happened, water vapor from volcanoes condensed in the atmosphere, fell as rain, and collected on the Earth’s surface. Besides water vapor, volcanoes also produced gases rich in the basic ingredients of life: carbon, hydrogen, oxygen, and nitrogen. Toxic gases such as ammonia and methane were common. At this point, Earth's early atmosphere consisted entirely of these volcanic gases, and there was no free oxygen. In the primordial “soup” of the early seas, organic molecules concentrated, formed more complex molecules, and became simple cells.

      The transition from complex organic molecules to living cells could have occurred in several environments. Small, warm ponds are one possibility, but recent work has suggested that deep-sea hydrothermal vents, such as those found along mid-ocean spreading centers today, may have been the cradle of Earth's life. These environments contain the chemicals and the source of energy needed to synthesize more complex organic structures. Although scientists have not succeeded in creating life from organic molecules in the laboratory, they have reproduced many of the intermediate steps.

      So what were the first living things and when did they appear? Studies of genetic material indicate that a living group of single-celled organisms called Archaea may share many features with early life on Earth. Many Archaea now live in hot springs, deep-sea vents, saline water, and other harsh environments. If the first organisms resembled modern Archaea, they also may have lived in such places, but direct evidence for early life is controversial because it is difficult to distinguish between complex inorganic structures and simple biological ones in the geologic record. The oldest evidence for life may be 3.5-billion-year-old sedimentary structures from Australia that resemble stromatolites. Stromatolites are created today by living mats of microorganisms (mostly cyanobacteria, or blue-green algae). These primitive organisms trap thin layers of sediment with their sticky filaments and grow upward to get light for photosynthesis. Modern-day examples of stromatolites can be found in waters off Australia, the Bahamas, and Belize.

      In the Archean structures, layers similar to those seen in living stromatolites are evident, and secondary structures interpreted as simple filamentous microfossils have been recovered from the layers. The biotic origin of the structures has, however, been questioned. Both the supposed Archean stromatolites and the microfossils may have been produced by inorganic processes. Regardless, uncontested microfossils and chemical traces of life were present at least by 2.7 billion years ago. Stromatolites that were produced by microorganisms are abundant later in the Archean and throughout the Proterozoic. These sedimentary structures, formed by organic processes, provide important evidence of early life. At present, we can say with certainty that life had evolved by 2.7 billion years ago, and possibly as early as 3.5 billion years ago.
      (23 votes)
  • mr pants green style avatar for user Neil Stamatelaky
    What is the purpose of Bio- Engineering?
    (13 votes)
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  • starky sapling style avatar for user Loyalty Matters🔥❤
    Is it just me, or tbh, in some classes the window or wall is just FACINATING..yk?
    (15 votes)
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  • blobby green style avatar for user jirdehaliismail55
    What are the branches of Biology?
    (8 votes)
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  • blobby green style avatar for user Dwi Lis
    How does cell division actually create new cells from just splitting?
    (8 votes)
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Video transcript

[Voiceover] I would like to welcome you to Biology at Khan Academy. And biology, as you might know, 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 think 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.