Cells and Organisms | Middle school biology | Khan Academy

• [Narrator] Hi, everyone. In this video, we are going to be talking about one of the most fascinating and complex features of life on Earth, cells. But before we do, I'd like to take us way back to when I was a little kid.

Now I know that for me, at least, I had a great time playing with toy building blocks and creating things out of them. I used blocks of different shapes and sizes to make all kinds of things like houses, cars, even spaceships and airplanes. You're probably thinking, what does this have to do with cells? Well, cells are considered the building blocks of life. Like my blocks, cells can come in a variety of shapes, and they have different features too. They can also stand alone, or they can come together to build something larger.

So let's dive deeper into this fascinating topic of cells. So all living things, or organisms, like you, me, your pet dog, the birds in the sky, the fish in the sea, all organisms are made of these basic units called cells. Take this moss plant, for example; if we zoom way in, we can see that it's actually made up of many smaller units. These units are cells. Each cell contains a flurry of activity as it works to carry out the functions that keep the organism alive. In fact, cells are the smallest unit that can carry out all the tasks necessary for life.

For instance, all cells can take up nutrients and water, get rid of waste, use energy, and interact with the things around them in their environment. These functions, plus many others that cells carry out, are what make life possible. There are many different kinds of cells. For instance, take a look at these cells that make up an onion's outer skin, or these really cool red blood cells, or even these bacterial cells. This type of bacteria is called e-coli, which you might've heard can be a safety hazard found in our food.

So cells are fascinating to look at, but we generally can't see them with our eyes alone. Most cells are so small that we need a microscope to see them. In fact, all of these pictures of cells here were taken with the aid of microscopes. And something else that I find really cool is that some organisms are made up of only a single cell, while others are made up of many cells. So organisms that are made up of one cell are called unicellular, and organisms that are made up of many cells are called multicellular. Uni telling us that there is only one cell, and multi telling us that there are many cells.

So an example of a unicellular organism would be the e-coli bacteria that I mentioned a minute ago. Each bacterium is a single cell that's responsible for taking in nutrients, reproducing, and doing all the things that it needs to stay alive. So an example of a multicellular organism would be you, me, or most any other organism that you can see with the naked eye. And the cells of multicellular organisms are often specialized to carry out specific functions.

So for example, some cells help the organism absorb nutrients, while others transmit information, or defend against bacteria and viruses. So going back to our comparison between building blocks and cells, specialized cells are like different building blocks of different sizes, shapes, and colors. And these different blocks can come together to build something larger, like this airplane here.

Similarly, cells with different structures and functions come together to make something larger, an organism like this adorable kitten right here. So to sum up cells and organisms, today, we learned that cells are the smallest unit of life. We talked about how all organisms are made up of cells and that organisms can be unicellular or multicellular.

So I hope that now you have a good understanding of cells and organisms, as well as a new perspective about building blocks, not the toys, but the cells that make up life itself.