Organization of multicellular organisms | High school biology | Khan Academy

In this video, we're going to take a journey in life and we're going to 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. Because 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 imagine cells to be these very, very small microscopic things, they are in and of themselves almost an entire world. We go into depth into that in 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 it, all of those three billion base pairs of DNA that make you a human being, 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. I believe these blues show their actual nuclei where they have their genetic information. Then dyed in green, you have what are called neuroglial 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 electrochemical 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, will 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 ten neuroglial 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.

As I said before, this is a zoomed-in view of neural tissue, in particular of brain tissue. Then if you zoom out a little bit more, the tissue makes up organs. If we're thinking about neural tissue like this, we can imagine that it makes up the brain, which is an organ.

Then organs build up to systems, and right over here, you have a picture of the nervous system, of which the brain is a part. 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.

Just to connect the organism with the cells, that basic building block of life, if you are an average-sized human being, you likely have 30 to 40 trillion cells in your body. If that isn't mind-blowing enough—and it is just an estimate—it's estimated that there's as many as a hundred trillion bacteria in your body.

So even though you think you are just, quote, an individual, you are a universe of living things in these complex systems. It's an interesting question of why. 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 the sentient mind, maybe we ourselves, as organisms, don't appreciate that we too are building blocks of maybe something even larger.