Transcriber: Andrea McDonough Reviewer: Bedirhan Cinar I like to think of life science as one big pyramid. Let's call the pyramid, "biological organization".

Let's work our way from very small and specific concepts to very large and complex topics. So let's start at the tip of the pyramid,

with something so small, it can't be seen by the human eye, a single cell. A cell by itself is the most basic component of life.

A single cell, which is capable of living on its own, is called a unicellular organism, organisms like bacteria or protists.

These organisms are everywhere you look, you just can't see them. But, when cells join together,

with more than one cell like itself to perform a similar function, it's no longer a unicellular organism.

It's then called a tissue. No, not that kind of tissue. There are four types of tissues found in the human body:

connective, muscle, nervous,

and epithelial. Bone tissue is a type of connective tissue and it's not flimsy like a Kleenex at all.

Blood is also an example of the same cells working together to perform the same job, but it's a liquid,

again, not like a Kleenex at all. No matter what the consistency is, a tissue is a group of the same cells,

working together to perform a similar function. Any time there are different tissues working together, we then call it an organ.

That's the next level of the pyramid. Animals aren't the only ones with organs; plants have organs too.

When we find multiple different organs working together to perform the same job, we then call it an organ system.

Take the digestive system, for example. It's made up of a mouth, esophagus,

stomach, pancreas, liver,

gall bladder, small intestine, and large intestine,

also known as your colon. Although each individual organ in this organ system does a very different job,

together, these organs work to achieve the one goal of taking all the nutrients out of the food we eat and getting rid of what we don't need.

An organ system, by definition, can only do one job, like get the nutrients from our food,

send and receive electrical signals, or exchange oxygen for carbon dioxide. Regardless of their individual function,

all organ systems rely on one another to do the vital jobs which they can't do themselves. If one organ system doesn't work,

the others will shut down, too. An organism is a complicated, living thing, which requires the proper functioning

of multiple organ systems to maintain stability. This stability is called homeostasis. When all organ systems are working together

and maintaining homeostasis, then it results in a happy, living organism. Organisms are incredibly diverse

and can be as simple as a bacteria or as complex as you. When a group of the same organisms,

also known as a species, live together in the same environment, we then call it a population.

Often there are several populations all living in one environment. That is what we call a community.

Each time you look out the window, you're seeing a community. When we look at multiple communities,

along with how all of the organisms interact with the physical environment, then we refer to it as an ecosystem.

When you look at all the ecosystems of Earth together, you're looking at the Earth.

The Earth is a big, round circle of life. That's why we call it a biosphere - bio, meaning life

and sphere, meaning circle. And there you go! When you break it down to each level,

a very complex concept and phrase becomes easy to understand. All life starts with a single cell.

When cells join together, they are called a tissue. A group of different tissues are called an organ,

and a group of different organs are an organ system. A group of organ systems working together make up an organism,

and a group of the same organisms are a population. Combining several different populations together give us a community.

And several different communities in a large area is an ecosystem. And every ecosystem together

makes up our biosphere. And that, my friends, is biological organization, the pyramid of life.