A Tour of the Cell: Crash Course Biology #23

Summary

This script from Crash Course Biology explores the history and evolution of cell theory. It begins with Aristotle's belief in spontaneous generation, moves through Robert Hooke's discovery of cells, and discusses the contributions of Schwann, Schleiden, and Virchow to classical cell theory. The differences between prokaryotic and eukaryotic cells are highlighted, along with the unique structures and functions of plant and animal cells. The script also delves into endosymbiosis and the work of Lynn Margulis, concluding with modern cell theory and the importance of scientific inquiry.

Takeaways

• 📚 Ancient Greek philosophers, including Aristotle, believed in 'spontaneous generation', the idea that life could arise from nonliving matter.
• 🐭 The concept of spontaneous generation was used to explain phenomena like mice appearing in grain silos, thought to arise from nonliving materials.
• 🧬 Modern science has disproven spontaneous generation, establishing that life is composed of cells, the basic building blocks of life.
• 🔬 Robert Hooke's improvements to the microscope in 1665 led to the discovery of 'cells' in cork, coining the term we use today.
• 🌿 The mid-1800s saw the proposal by Schwann and Schleiden that all organisms are composed of cells, forming the basis of cell theory.
• 🔬 Dr. Rudolf Virchow contributed to cell theory with the assertion that all cells come from pre-existing cells, a principle known as 'Omnis cellula e cellula'.
• 🔬 The distinction between prokaryotic and eukaryotic cells was understood in the 1900s, with eukaryotes being more complex and having a nucleus.
• 🌳 Eukaryotic cells, like those in plants, have additional structures like cell walls, large vacuoles, and chloroplasts for energy conversion.
• 🏙️ Animal cells, being eukaryotic, contain organelles like the nucleus, ribosomes, endoplasmic reticulum, Golgi apparatus, and mitochondria, each with specific functions.
• 🌱 The theory of endosymbiosis, proposed by Lynn Margulis, suggests that mitochondria and chloroplasts originated from symbiotic prokaryotic cells.
• 🔬 Modern cell theory expands on classical cell theory by incorporating concepts of energy flow, similarities in cell structure among species, and genetic information transfer during cell division.
• 🔬 Scientific advancements, including improved microscope technology, have been crucial in the evolution of our understanding of cells and their structures.

Q & A

What was the concept of 'spontaneous generation' that Aristotle and others believed in?

Spontaneous generation was the belief that nonliving matter could spontaneously create life. It was used to explain phenomena such as the appearance of mice in a grain silo or the formation of babies from semen and menstrual blood.

How did the understanding of life's origins change with the discovery of cells?

The discovery of cells by Robert Hooke in 1665 changed the understanding of life's origins by showing that life is made up of tiny, individual building blocks called cells, rather than arising spontaneously from inorganic material.

What are the three cornerstones of classical cell theory?

The three cornerstones of classical cell theory are: 1) All organisms are made from cells, 2) The cell is the basic building block of life, and 3) All cells come from pre-existing cells that have multiplied.

What are the key differences between prokaryotic and eukaryotic cells?

Prokaryotic cells lack a defined nucleus and have their genetic material in the cytoplasm, while eukaryotic cells have a defined nucleus enclosed by a nuclear membrane where genetic material is stored.

How did the understanding of cells evolve with the discovery of organelles?

The discovery of organelles within eukaryotic cells, such as the nucleus, mitochondria, and chloroplasts, showed that cells have specialized internal structures that perform unique functions, leading to a more complex understanding of cellular processes.

What is the significance of the endoplasmic reticulum (ER) in a cell?

The endoplasmic reticulum (ER) is significant as it has two sections: the rough ER, which is involved in protein synthesis, and the smooth ER, which is involved in lipid synthesis and detoxification processes.

What is the function of the Golgi apparatus in a cell?

The Golgi apparatus functions as a protein sorting facility and a part of the endomembrane system. It packages proteins into vesicles, which are then transported to various parts of the cell.

What is the role of mitochondria in a cell?

Mitochondria are the 'power plants' of the cell, where energy for all of life's cellular processes is produced by breaking down molecules like glucose through cellular respiration.

What is the concept of endosymbiosis and how does it relate to the origin of eukaryotic cells?

Endosymbiosis is the concept that mitochondria and chloroplasts originated from prokaryotic cells that merged in a symbiotic relationship with a larger cell. This theory suggests that these organelles were once free-living bacteria that became incorporated into eukaryotic cells.

What are the three additional ideas proposed by modern cell theory?

Modern cell theory adds three central ideas to classical cell theory: 1) Energy flows within cells, 2) Similar species have similar cells, and 3) Cells divide and pass along their genetic information to new cells.

Why is the surface-to-volume ratio important in determining cell size?

The surface-to-volume ratio is important because it limits cell size. As cell size increases, the volume increases faster than the surface area, which could limit the cell's ability to exchange materials and maintain homeostasis.

Outlines

00:00

🔬 The Misconception of Spontaneous Generation and Discovery of Cells

This paragraph delves into the historical misunderstanding of life's origins, where Aristotle and others believed in 'spontaneous generation'—the idea that life could arise from nonliving matter. It discusses how Aristotle's theories about semen 'enlivening' menstrual blood to create babies were closer to the truth than the 'Unified Stork Theory,' but still incorrect. The paragraph introduces Dr. Sammy, an entomologist, and the concept of cells as the fundamental building blocks of life, discovered by Robert Hooke in 1665. It emphasizes the importance of scientific tools and the iterative nature of scientific progress, leading to the understanding that all organisms are composed of cells, a theory further developed by Schwann, Schleiden, and Virchow.

05:02

🌿 Exploring the Eukaryotic Cell: Structure and Function

The second paragraph provides an in-depth exploration of eukaryotic cells, highlighting the differences between prokaryotic and eukaryotic cells. It explains the presence of a defined nucleus in eukaryotic cells, where genetic material is neatly stored within a nuclear membrane, contrasting with prokaryotic cells that lack a nucleus and have their DNA floating in the cytoplasm. The paragraph describes the unique structures and functions of plant cells, including cell walls for support, central vacuoles for storage and structural support, and chloroplasts for energy conversion. It also introduces the concept of organelles within cells, each with a specific function, and likens a cell to a city with each organelle performing a civic duty. The tour of the eukaryotic animal cell is presented as a 'Thought Bubble' journey, explaining the roles of the cell membrane, cytoskeleton, nucleus, ribosomes, endoplasmic reticulum, Golgi apparatus, and mitochondria.

10:06

🌱 The Evolution of Eukaryotic Cells and Endosymbiosis Theory

This paragraph discusses the evolutionary origins of eukaryotic cells, suggesting that single-celled prokaryotes were among the first forms of life, and that eukaryotic cells evolved from them around 2.7 billion years ago. It introduces the endosymbiosis theory, proposed by Dr. Lynn Margulis in the 1960s, which hypothesizes that mitochondria and chloroplasts originated from prokaryotic cells that merged in an endosymbiotic relationship. The paragraph emphasizes the importance of scientific advancements, such as improved microscope technology, in providing evidence for such hypotheses. It also touches on modern cell theory, which expands upon classical cell theory with additional concepts like energy flow within cells, similarities in cells of similar species, and the genetic information transfer during cell division.

Keywords

💡Spontaneous Generation

Spontaneous generation is an outdated theory that suggested life could arise directly from nonliving matter without the involvement of parents. In the script, it is mentioned as the belief that mice could arise from nonliving materials like bricks and grain, and that semen could 'enliven' menstrual blood to create babies, illustrating the lack of understanding of life's origins in ancient times.

💡Cells

Cells are the basic structural and functional units of life, composed of organelles that perform various functions necessary for life. The script explains that life is made up of cells, which are the building blocks of all organisms, and that understanding cells is crucial to understanding the origins and functions of life.

💡Robert Hooke

Robert Hooke was a physicist who, in 1665, discovered and named 'cells' after observing the tiny pores in a slice of cork that resembled the cells of monasteries. His work laid the foundation for the study of cells and microscopic life, as mentioned in the script.

💡Classical Cell Theory

Classical cell theory is a collection of principles that describe the basic nature of cells and their role in all living organisms. The script outlines that this theory was established in the mid-1800s, suggesting that all organisms are composed of cells and that the cell is the basic unit of life.

💡Prokaryotic Cells

Prokaryotic cells are a type of cell that lacks a nucleus and other membrane-bound organelles. The script mentions that these cells, such as bacteria, were among the first forms of life and that eukaryotic cells evolved from them.

💡Eukaryotic Cells

Eukaryotic cells are cells that contain a nucleus and other membrane-bound organelles. The script explains that these cells, which include those of plants, animals, and fungi, are more complex than prokaryotic cells and are characterized by their compartmentalization.

💡Nucleus

The nucleus is a membrane-bound organelle found in eukaryotic cells that contains the cell's genetic material. The script describes the nucleus as the central location where genetic information is stored and managed within a eukaryotic cell.

💡Cytoplasm

Cytoplasm is the jelly-like substance within cells, excluding the nucleus, where many cellular processes occur. In the script, it is mentioned as the main compartment of the cell where the DNA of prokaryotic cells is located, and where organelles perform various functions in eukaryotic cells.

💡Endosymbiosis

Endosymbiosis is a biological theory that explains the origin of eukaryotic cells, suggesting that mitochondria and chloroplasts originated from prokaryotic cells that entered into a symbiotic relationship with a host cell. The script credits Dr. Lynn Margulis for her work on this theory and its acceptance in the scientific community.

💡Mitochondria

Mitochondria are organelles found in eukaryotic cells that produce energy through a process called cellular respiration. The script describes them as the 'power plants' of the cell and discusses their possible origin from an endosymbiotic relationship.

💡Chloroplasts

Chloroplasts are organelles found in plant cells and some protists that conduct photosynthesis, converting sunlight into chemical energy. The script mentions chloroplasts as unique to plants and having a similar origin to mitochondria through endosymbiosis.

Highlights

Aristotle and the concept of spontaneous generation, which was the belief that nonliving matter could spontaneously create life.

The discovery of the cell by Robert Hooke in 1665, who named them after the cells in monasteries.

The development of the cell theory by Dr. Theodor Schwann and Matthias Schleiden, proposing that all organisms are made from cells.

Dr. Rudolf Virchow's contribution to cell theory in 1855, stating that all cells come from pre-existing cells.

The identification of two major cell types: prokaryotic and eukaryotic, with eukaryotic cells having a defined nucleus.

The unique property of eukaryotic cells to have organelles, specialized structures within the cell for specific functions.

The role of the cell membrane in the cell, acting as a barrier and facilitating transport.

The function of the cytoskeleton in maintaining cell shape and facilitating movement of cellular components.

The nucleus as the control center of the cell, responsible for genetic information.

The process of protein synthesis in the ribosomes and the endoplasmic reticulum.

The role of the Golgi apparatus in modifying, sorting, and packaging proteins for secretion.

Mitochondria as the powerhouse of the cell, producing energy through the breakdown of molecules.

The concept of endosymbiosis, suggesting that mitochondria and chloroplasts originated from symbiotic relationships.

Dr. Lynn Margulis's hypothesis on endosymbiosis and its acceptance in the scientific community.

The evolution of cell theory into modern cell theory, incorporating new concepts like energy flow and cell division.

The importance of the surface-to-volume ratio in limiting cell size and the adaptations that allow for larger cells.

The historical progression of scientific understanding of cells, from ancient Greek philosophy to modern discoveries.

Transcripts

Back when Aristotle was around,

people didn’t know much about the origins of life.

A lot of folks, including the famous Greek philosopher himself,

thought that nonliving matter could, just, create life.

They called it "spontaneous generation."

And they used it to explain all sorts of things.

Like, if someone was storing grain in a silo

and happened to find mice that hadn’t been there the day before,

well, they assumed the mice arose spontaneously from the nonliving

bricks, mud, and some bits of grain.

Aristotle even suggested that semen had a unique property that allowed it

to "enliven menstrual blood," and that’s how babies were made.

And sure, it’s closer to the truth than the "Unified Stork Theory"

that your parents came up with

– but still definitely wrong.

These days, we know that life doesn’t arise spontaneously from inorganic material,

it’s actually made of tiny, individual building blocks called cells.

But it took a lot of science to get there,

science that relied on all of the work that came before it.

Hi! I'm Dr. Sammy, your friendly neighborhood entomologist,

and this is Crash Course Biology.

Now ready your mind for one of the most complex theme songs ever constructed.

[THEME MUSIC]

We actually knew about the  cell a few hundred years 

before spontaneous generation had been debunked,

thanks to a physicist named Robert Hooke.

In 1665, around the same time that Isaac Newton was thinking about gravitational forces,

Hooke was focused on science at a smaller scale

- a microscopic scale actually.

Having made some tweaks to an existing microscope,

Hooke discovered something astonishing.

Looking at a slice of cork  under his improved scope,

he was amazed by the tiny pores he saw.

To him, they looked like the little rooms in monasteries that monks lived in,

which are called "cells."

And, well, the name stuck.

If Aristotle had a microscope,

we probably wouldn’t have gotten hung up on spontaneous generation for so long.

But, that’s how scientific advancement works,

it’s all about having the right tools for the right task

and being able to iterate, or “build on to”, the work of past scientists.

In the mid-1800s, Hooke’s sketches of the 

microscopic world led a  couple of other scientists,

physiologist Dr. Theodor Schwann and botanist Matthias Schleiden,

to propose that all organisms are made from cells, 

and that the cell is the  basic building block of life.

Then, in 1855, the work continued when Dr. Rudolf Virchow added his own proposal:

all cells come from pre-existing cells that have multiplied.

These three ideas became the cornerstones of what we now call classical cell theory.

It wasn’t until the 1900s that we figured out the key differences

between the two major cell types prokaryotic and eukaryotic,

thanks in part to more advanced microscopes.

You’re probably more familiar with the eukaryotic variety

because those are the cells that make up most of the living things

you see every day like bees, trees, and people.

And unless you’re using a microscope, 

you’re unlikely to see any  prokaryotes like E. coli

– a type of bacteria that can cause some nasty infections.

Although we humans do owe a lot to prokaryotes,

since they form itty-bitty communities in our guts and on our skin,

helping us digest our dinner and even ward off infection.

The best evidence so far  suggests that single-celled 

prokaryotes were among the first forms of life,

and that our eukaryotic cells evolved from them about 2.7 billion years ago.

So they’re like your great, great, great, great, well, you get it, you’re related.

There are a few important differences between the two types of cells.

Our eukaryotic cells have a defined nucleus, usually near the center of the cell.

Prokaryotic cells don’t.

In fact, their name means “pre-nucleus.”

The nucleus is where the genetic material is stored in a eukaryote,

packed up neatly within the nuclear membrane, a double-layered shell that surrounds the nucleus.

Most prokaryotes have their single, circular piece of DNA

just kinda hanging out in the main compartment of the cell with everything else,

in a water-based jelly called the cytoplasm.

So, prokaryotic cells kind of look like they packed their suitcase in a hurry,

while eukaryotes made sure to bubble-wrap their cellular accessories

and tuck them away in special compartments.

This compartmentalization lets eukaryotes develop

more complex, coordinated cellular processes than prokaryotes.

Like, take plants for example.

Not only do the eukaryotic plant cells run some complex processes,

but they also do some extra work to support the unique structure of plant life.

For starters, much like the buffalo wings I had for lunch, plants are boneless.

To make up for that, each cell membrane is braced by a surrounding cell wall

that helps plants maintain their structure.

This thick barrier consists  of structural molecules, 

including carbohydrates and proteins.

Plants also have a large  central vacuole, or cavity,

that stores a lot of water and chemicals the plant needs.

The central vacuole also provides additional structural support alongside the cell wall.

And then there’s the chloroplast, which converts sunlight into an energy the plant can use.

Both a plant’s vacuole and its chloroplast are membrane-enclosed structures called organelles.

In the same way our heart pumps blood and our lungs exchange gases,

these little mini-organs inside a cell each serve their own unique function.

In a way, a cell is like a city,

with each organelle performing its own civic duty 

to keep the city-cell  working in a coordinated way.

Let’s say goodbye to the plant cells for now

and go on a Thought Bubble tour of the eukaryotic animal cell.

Welcome to Cellular City.

Here at the entrance to the city,

we find a barrier of biomolecules called lipids surrounding the cell.

The barrier is the cell membrane.

It’s studded with proteins, some of which open doors allowing us access.

We have a very efficient transportation department here in Cellular City.

The highways are made up of the protein filaments of the cytoskeleton.

They help move cargo, and let the cell maintain—or change—its shape as needed.

They’re quite dynamic, too.

See how they assemble at one end while disassembling on the other.

As we approach the center of the cell you’ll catch a glimpse of City Hall, the nucleus.

Here, important genetic messages get sent out in the form of the nucleic acid RNA.

Ah, please, no flash photography.

Once the instructions reach the ribosomes over in the cytoplasm,

they turn the instructions from City Hall into a protein that helps make action happen in the body.

Now if you look to your right,

you’ll see that sometimes the nucleus dumps the RNA

right into these buildings of the endomembrane system,

where they enter the endoplasmic reticulum.

We just call it the ER around here, and it has two sections: the rough and the smooth.

The rough ER, as you can see,

is dotted with ribosomes of its own, so it makes proteins, same as the cytoplasm.

But here in the ER, the cell can make more complex proteins with modifications.

The Smooth ER is the lipid manufacturing plant, where new pieces of the cell membrane are made,

along with those famous message-carrying lipids called hormones –

don’t worry, they’ll be around at the end of the tour for autographs.

These cellular products are shipped out of the ER in a vesicle –

a fluid-filled structure that buds from the smooth ER’s lipid membrane

carrying cargo to the rest of the cell.

Those vesicles need to make a quick pit stop at the Golgi apparatus.

Part manufacturing plant, part protein sorting facility,

the Golgi is another member of the endomembrane system

which packages proteins into  vesicles, like this one.

And finally, we end our tour at the mitochondria, the city’s power plants,

where energy for all of life’s cellular processes is produced

by breaking down the right molecules at the right times.

All this while making versatile chemicals that can be used as building blocks

in other areas of the city.

And that ends our tour for today.

Buh-bye now, Buh-bye. Buh-bye now, Buh-bye.

Thanks, Thought Bubble!

We gotta give that tour guide a raise, they were great!

Anyway, how amazing that so many complicated things are happening inside a single, microscopic cell.

You might have noticed that our tour of the animal cell

didn’t include a stopover in the chloroplast.

That’s because the chloroplast  is unique to plants.

While animals don’t have chloroplasts, plants do have mitochondria –

double the power production for our leafy friends.

I’m not jealous…

okay, I’m a little jealous.

So while we non-plants  might have chloroplast envy,

both of these organelles are really special

– and really strange –

when you consider their origin.

One day, 1.5 billion years or so ago,

we think a small bacterium found its way inside of a larger bacterium,

whether on purpose or by accident who can say,

but this new living arrangement actually worked well for both bacteria.

These days we call this arrangement endosymbiosis.

Both mitochondria and chloroplasts might have arose from this kind of

symbiotic relationship that worked so well that it became permanent,

and it allowed them to co-evolve into the eukaryotic cells we know today.

In other words, we think  that’s what led to the cells

that allow bugs, bananas, and bears to exist.

Scientists had long believed that this could be the case,

but it wasn’t until the 1960s that the idea of endosymbiosis

as the origin for eukaryotic cells really took off,

thanks to evolutionary biologist and zoologist Dr. Lynn Margulis.

Margulis, drawing on the work of many scientists before her,

hypothesized that both mitochondria and chloroplasts

were descended from the remnants of prokaryotic cells

that had merged in an endosymbiotic relationship.

But what she had that earlier scientists didn't have were the right tools

thanks to advancements in microscope technology,

which helped her present experimental evidence to back up the hypothesis.

Further proof that science  doesn’t happen in a void;

it builds and grows and changes

as different people pick it up across generations.

People who are in the right place, at the right time, with the right tools.

And speaking of continuing the work that came before you,

remember classical cell theory?

Well, we call it "classical" for a reason.

Today, we have modern cell theory.

Modern cell theory builds on its classical counterpart

in the same way Margulis built on the work of her predecessors,

by adding three more central ideas:

energy flows within cells,

similar species have similar cells,

and cells divide and pass along their genetic information to new cells.

And, just like classical  cell theory wasn’t possible 

without the right advancements in science,

neither was modern cell theory, which also relied on advancements in microscopy.

We’ve come a long way since the old-school microscopes of Robert Hooke’s day.

Of course, the reason it takes such powerful microscopes to observe cells,

is because the vast majority of cells are really, really small.

As cell size increases,

the volume inside the cell increases faster than the surface area along the membrane,

and the surface-to-volume ratio puts a limit on cell size.

If a cell got too big, there wouldn’t be enough membrane to support all of its processes.

That’s why we’re not made up of just a couple really big cells.

Some cells dodge the size limit by changing their shape,

taking the form of rods, or spiky balls that increase their surface-to-volume ratio.

Like Caulerpa taxifolia, a single-celled organism that can grow larger than a human arm.

It’s plant-shaped, with these cool fronds that increase surface area.

There’s also more than one nucleus to support this giant water-dwelling thing.

So even though most cells are really small, nature is always full of surprises.

From Hooke to Margulis, to, well, us,

the history of the cell is a long chain of scientific iteration and experimentation.

As far back as the world of  ancient Greek philosophy,

folks were asking one of the most important questions in science.

It’s a simple question, only four words long, 

but it’s one that can help  us understand cell theory,

invent new microscopes, and figure out new ways of thinking:

“why did that happen?”

By asking that simple question,

we’ve been able to discover not only the existence of the cell and its major types.

But we’ve also learned about the busy, bustling internal structures of the organelles,

and hypothesized how cells adapted over billions of years

to become the shapes of life we see today.

Next time, we’ll check out the cattywampus membrane that holds them together.

I’ll see you then.

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