Cell theory | Structure of a cell | Biology | Khan Academy

To us modern folk, we tend to take for granted the existence of cells

and the idea that all living things are made up of cells

that we as human beings, as living organisms, we are made up of many, many, many cells

the estimates of the cells, of the human body, are around the order of 37 trillion cells!

But, if we were to rewind, even 400 years ago to the 1600's

this wasn't so obvious.

and that is because people really didn't have the tools

to make direct observations of cells. They didn't know that these cells were around.

They didn't know that things, even like unicellular organisms, even existed!

But, all of that began to change, with Robert Hooke.

Robert Hooke, was able to leverage a primitive microscope

and this is a picture of his microscope

and when you see this, you appreciate how

at least relative, to what we have today, how primitive it was.

He had lenses here, that would provide some magnification,

but he had to use -- this right over here -- this is a flame, and he is able to channel that light

so that it gets reflected over whatever he is observing.

And in, 1665, he publishes the "Micrographia".

I am probably not pronouncing it right.

So, this is 1665, Robert Hooke publishes this

and in this, he describes and he draws a lot

of his observations, using his microscope.

He has some fascinating drawings, he was actually quite a good artist,

where he was able to draw pictures

of his observations, of things like lice, and fleas.

But, it's not just lice and fleas that he is able to observe.

He actually takes a look at some corks or what he just sees as "cork"

and when he makes that observation, he says, "Hey look, you know there are these little squares over here, and this kind of seems to be somehow kind of a basic unit of this cork. "

And he says, " Well, this kinda reminds me of these little rooms that monks live in and spend their time in, which we call cells".

So he calls these cells.

And, that is where the word comes from.

He calls them cells, in "Micrographia".

That is a cell.

Now unfortunately for us, he was a great artist

and he was really good at drawing things

and he drew a lot in his "Micrographia", but we don't have any pictures of Robert Hooke anymore.

And it's a fascinating story. Most theories are that

actually, it was Isaac Newton, who burned the only original painting, that we have of Robert Hooke.

And, that is another intriguing story, that we can talk about, at a future date.

But, what Robert Hooke — he coined these things "cells" — but he was looking at dead tissue, and actually he was

looking at the remnants of cells. And what he saw was really just the cell wall remnants.

He wasn't actually able to observe, directly, "living cells". And he didn't have enough to go on

to think , "Hey maybe, this is a building block of all life and maybe, cells themselves reproduce."

"And, all cells come from other cells. " And that doesn't start to get developed, until we get further on into history.

You go a few years later, we get to Antonie von Leeuwenhoek.

I know I'm not probably pronouncing it right.

But he was a Dutch lense crafter, and he was inspired

by Hooke's work , and he says "Hey, I can craft lenses, maybe I can use that to make better microscopes, and make better observations. "

And he was able to do that, and doing that he was able to directly observe

living cells and living unicellular organisms.

So, he was able to directly observe sperm.

He said, "Hey this thing looks like it's alive!"

He was able to directly observe protists — so these unicellular, eukaryotic...

... they have cell walls, these things that look like unicellular animals.

And he called them, actually, "animalcules".

Saying, "Hey, look, these look like these little mini-animals going on here. "

And so he started to say, well maybe this is some form of a basis of life, or at least started to say,

at this very small scale, you actually have life.

But, a fully, modern theory of the cell, doesn't really start to emerge until we get another 100-150 years, into the future.

And, we get into the early 1800's.

So, if we fast-forward, to the 1830's , these two gentlemen, Matthias Schleiden and Theodor Schwann, come into the picture.

And, they are the one's that start laying the foundation

of what we can call, "modern cell theory".

So, modern cell theory

And using their observations and kind of what they are able to deduce

they said, "Hey maybe, all living things

all life, is composed of one or more cells."

And today, we kind of take this for granted, but this wasn't obvious.

It wasn't obvious that all things, that this was somehow a building block for life, one or more cells.

And then, following that if all life is composed of one or more cells,

you can also say, that a cell, is the basic unit of life.

Cell is basic unit of life

And these are strong statements, that all life, if you were to get down small enough,

you are going to come to cells, living cells, that make up that life.

Now, this isn't our full, complete cell theory and both of these gentlemen, they knew that cells could come from other cells.

They were able to observe cells reproducing, but, it still

was an open question, "Hey, maybe some cells come from other cells? While others, maybe they somehow get spontaneously created ....

... if you have the right elements and the right amount of conditions. Maybe they somehow just emerge out of nothing. "

And, it wasn't until this kind of third tenet of modern cell theory, it wasn't until the mid 1800's that we get our third tenet of modern cell theory that gets established.

And this is this idea that all cells come from other cells.

All cells from other cells

And the real father of this idea, the one that really established this,

is this gentleman, right here, Robert Remak.

Now sometimes, the credit for this goes to, Rudolph Virchow, right over here, but it turns out that

he plagiarized Remak's work, so it's really Remak who deserves the bulk of the credit

for this tenant, that all cells come from other cells.

And once again, he wasn't the first person to say, "Hey, maybe, we will observe that some cells..

.. come from other cells." But he said, "This is a fundamental thing, all cells, this is how they actually come about they aren't just somehow, spontaneously emerged".

Now whenever, even in today's world, people

see this last thing, all cells come from other cells,

there is a natural question, " Well, there must have been a first cell or an initial set of "proto-cells ?".

And, people aren't a hundred percent sure, but when we

look back, at the evolution of life on earth, we think the first cells emerged about three and a half billion years ago.

And we are not a hundred percent sure about how they emerged, but there are some theories.

For example, we have videos on, phospholipids,

and it turns out that, phospholipids, naturally form bi-layers

and they can actually form, spherical membranes, that have phospholipid bi-layers.

So, that kinda gets you a start, so that can spontaneously form.

And, there is also, theories that maybe in the information, the machinery of cells

maybe their ancestors, if we were to go three and a half billion years or longer ago,

that it might have been self-replicating RNA molecules or maybe it was somehow, self-replicating proteins

that over time, over very large, large time scales,

were able to, start replicating themselves, have more sophisticated machinery,

the ones that had or were approaching our modern cells, were able to reproduce more,

were able to take advantage of more of the energy, the resources in an environment.

So, that we eventually get to our modern cells. But, this is a fascinating question, and is still an area of research.