Organization of multicellular organisms

- [Instructor] In this video, we're gonna take a journey in life and we're gonna start with the smallest scale of life that is indisputably life, and that is the cell. Now, the reason why I qualified that a little bit is some people debate whether viruses are living or not, 'cause they have certain aspects. Viruses can reproduce, they do have genetic information but they need other living forms to reproduce, in particular they need other cells. But even though we imagined cells to be these very small microscopic things, they are in and of themselves almost an entire world, and we go into depth into that into other videos in Khan Academy. But the fact that every cell in your body, except for a few like red blood cells have all of your genetic information in there. All of those 3 billion base pairs of DNA that make you a human being in and of itself is mind-boggling. But then the fact that the cell specializes so that not all cells are the same even though they have that same genetic information, they somehow know what type of cell to be that's even more interesting. So we could start at the most basic building block in your body or really any organism's body and that's specialized cells. So what you're seeing here is a big cluster of neurons which are dyed here in the red, and I believe these blues show their actual nuclei where they have their genetic information. And then dyed in green, you have what are called neuroglia cells which are other types of cells that are inside the human brain mainly to support neurons. Most of what we believe is thought occurs through triggering neurons, which then trigger other neurons and form cascades of these electro-chemical signals which we're just starting to understand. But this is just one little small fraction of a human brain. A human brain, for example we'll have on the order of 80 to 90 billion neurons. And for every one of those neurons depending on what part of the brain you're talking about, you're talking about five to 10 neuroglia cells. So you're talking about many hundreds of billions of cells just in one human brain. But then if we were to zoom out a little bit and you take a bunch of these specialized cells working together or at least near each other, you have tissue. And so as I said before, this is a zoomed in view of neural tissue in particular of brain tissue. And then if you zoom out a little bit more, the tissue makes up organs. And if we're thinking about neural tissue like this, we can imagine that it makes up the brain which is an organ. And then organs build up to systems. And right over here, you have a picture of the nervous system of which the brain is apart. You also have the spinal cord, and then you also have all of the nerves that go throughout the body. So we have a system. And then you put all of the systems together and you get the actual organism, which of course you can somewhat visualize right over here where you can see all of these different organs and organ systems put together to create who we are. And just to connect to the organism with the cells, that basic building block of life. If you are a average size human being, you likely have 30 to 40 trillion cells in your body. And if that isn't mind blowing enough and it is just an estimate, it's estimated that there's as many as 100 trillion bacteria in your body. And so even though you think you are just "an individual" you are a universe of living things in these complex systems. And it's an interesting question of why and right over here. We know that organisms interact with each other. We know that they interact with their environment just as each of our nerve cells might not appreciate that they are one of 86 billion in dissension mind, maybe we ourselves as organisms don't appreciate that we too are building blocks of maybe something even larger.