Professor Dave here, let’s talk about animal cells.
We now know the basics about prokaryotic cells, but cells get much more complicated than this.
Human beings and all other animals are made of eukaryotic cells, which contain a number
of components called organelles that are not present in prokaryotic cells, some of which
resemble individual organisms unto themselves.
Because they are more complex, we know that eukaryotic cells evolved from prokaryotic
cells.
Endosymbiotic theory proposes that billions of years ago there were many different types
of bacteria floating around, all of which were unicellular.
Some of these species of bacteria acquired unique functions, like the ability to perform
photosynthesis, or breathe oxygen.
It is believed that larger host cells then enveloped some of these smaller bacteria,
and instead of digesting them, a mutual dependency arose, each relying on the other for some
crucial function.
The smaller cell received protection, while the larger one reaped the benefits of some
kind of energy-producing process.
This is where the name endosymbiotic comes from.
Endo means inside, and symbiosis refers to organisms living and working together.
These smaller bacteria grew more specialized over millions of years of evolution, and eventually
became some of the organelles we find within a single eukaryotic cell.
So what are these organelles exactly?
To answer that, let’s first make the distinction between plant cells and animal cells.
These are both examples of eukaryotic cells, but they differ in the precise components
within.
Let’s first take a look at animal cells, which are the kind inside you and me, since
humans are considered part of the animal kingdom.
The first thing we want to point out is that while prokaryotic cells have their genetic
information just floating in the middle of the cell, eukaryotic cells store their genetic
information inside a nucleus.
This nucleus has its own membrane, which unlike the cell membrane is actually a double membrane,
and this nuclear envelope only lets certain things in or out by passing through tiny holes
called pores.
In this way, it keeps all the genetic material, or chromatin, separate from the cytoplasm
of the cell.
Within the nucleus is a smaller area called the nucleolus where ribosomal RNA is synthesized,
as well as the parts of the ribosomes themselves.
The large and small subunits of the ribosome are formed, which then exit the nucleus and
assemble in the cytoplasm.
Some of these ribosomes can be found floating around in the cytosol, but some are stuck
to the surface of the nucleus or other organelles like barnacles on a boat.
Ribosomes, which are present in both prokaryotic and eukaryotic cells, are made of ribosomal
RNA and proteins, and these in turn synthesize all the proteins the cell needs to perform
its various functions.
Other components of the cell come together to make up the endomembrane system.
This is a set of membrane bound regions, which consists of the nuclear envelope, the rough
and smooth endoplasmic reticulum, Golgi apparatus, and lysosomes.
The rough endoplasmic reticulum is a network of membranes that enclose the ER lumen, which
is the stuff inside, consisting of sacs called cisternae.
It’s called the rough endoplasmic reticulum because its surface is studded with ribosomes,
giving it a rough appearance.
These ribosomes synthesize polypeptides that are inserted directly into the ER lumen, where
they are typically folded and modified by enzymes in the ER membrane, and sometimes
affixed with a carbohydrate to make glycoproteins.
These are often intended for secretion from the cell, so they gather into a vesicle, which
is like a bubble that buds from the ER membrane and then makes its way to the cell membrane,
thus releasing its contents to the extracellular space.
The rough ER also synthesizes phospholipids that can be delivered to other parts of the
endomembrane system, and it takes proteins synthesized by the ribosomes that will be
cell membrane proteins, embeds them in its own membrane, and then delivers them via similar
transport vesicles.
The rough ER is connected to the smooth ER, which is different in that it is not studded
with ribosomes.
It also has a wider variety of functions, like synthesis, metabolism, and storage of
calcium ions to be used for signaling.
The smooth ER produces phospholipids and steroids like sex hormones.
Enzymes in the smooth ER can detoxify drugs and poisons, by adding hydroxyl groups to
enhance water solubility and flush them from the body.
Next in the endomembrane system is the Golgi apparatus.
Most transport vesicles, like the ones going from the ER to the cell membrane, stop off
here first.
The things inside the vesicles, like proteins, are modified and stored, and later sent to
where they need to go.
The Golgi apparatus consists of a series of stacks, and there is a cis face, usually facing
the ER, where vesicles arrive, and a trans face, where vesicles leave.
In between, molecules will be modified, with sugars removed or substituted, and generally
prepared for their eventual function.
This is all done piecewise as the components move from one cisterna to the next, as each
of these sections has its own set of enzymes.
Before sending things off, products are given molecular tags like phosphate groups, which
is kind of like an address on a piece of mail to target things for various parts of the
cell, and some vesicles also have certain molecules on their surfaces that can be recognized
by specific organelles, which helps target vesicle delivery.
Another organelle is the lysosome.
This is a sac containing an environment that is much more acidic than the rest of the cell,
and it holds enzymes that digest large molecules via hydrolysis, which is the breakdown of
a compound due to reaction with a water molecule.
When food enters a cell through a vesicle, by a process called phagocytosis, multiple
vesicles fuse to form a vacuole.
Then, a lysosome will fuse with the vacuole and break apart the big chunks of food into
smaller bits.
Lysosomes can also digest damaged organelles that need to be dismantled in a process called
autophagy.
This allows the cell to maintain optimal functionality.
There are a few organelles that are not part of the endomembrane system, and a crucial
one is the mitochondrion.
There are hundreds of mitochondria in most eukaryotic cells, and these are the sites
that carry out cellular respiration, which generates most of the energy the cell needs
to go about its business.
Cellular respiration occurs in three steps, glycolysis, the citric acid cycle, and oxidative
phosphorylation, and you can visit the three-part series in my biochemistry tutorials for more
details on how cellular respiration works.
Beyond this, mitochondria also have two membranes.
The outer membrane is smooth, but the inner membrane makes many folds called cristae.
The area in between the two membranes is the intermembrane space, while the innermost part
is called the mitochondrial matrix.
The matrix contains circular DNA molecules called mitochondrial DNA, and ribosomes, as
well as many of the enzymes that carry out cellular respiration, while some of these
enzymes can be found in the inner membrane itself.
Another organelle that also has metabolic function is the peroxisome.
This contains enzymes that perform oxidative processes, which make hydrogen peroxide as
a byproduct.
These work with mitochondria by breaking down fatty acids into smaller components that are
ready for cellular respiration.
That’s all the organelles, so what else is in there?
Well the cellular components are typically tethered to something called the cytoskeleton.
This is a network of fibers that stretch throughout the cytoplasm of the cell and keep everything
nice and organized.
These fibers consist of hollow microtubules made of tubulin dimers, microfilaments made
of two intertwined strands of actin, and intermediate filaments made of coiled keratins.
Beyond organization, these also determine and maintain the shape and structural integrity
of the cell.
Sometimes vesicles utilize motor proteins to deliver their contents to their destinations
by moving along microtubules, like from the ER to the Golgi apparatus or from the Golgi
apparatus to the cell membrane.
This is a process that requires ATP to power, and two components move sequentially along
the microtubule in an action that looks a lot like walking.
In animal cells, microtubules grow outwards from a centrosome, which is located near the
nucleus, and consists of a pair of centrioles, which have microtubules arranged in a ring,
nine sets of three to be exact.
Now that we know a bit about the structure and function of each organelle, we can almost
view the cell as a tiny factory, with each component serving a specific purpose.
The plasma membrane is like the security guard at the gate, deciding what can go in or out.
Once inside, we see the cytoskeleton, which is the system of girders that provides structure,
and the cytoplasm, which is the factory floor.
The nucleus is the central office where the boss sits, giving out orders from the genetic
instructions.
The endoplasmic reticulum is the assembly line, and the ribosomes are the little factory
workers, building all the proteins the cell needs.
The Golgi apparatus is the sorting and shipping center, the lysosomes are the maintenance
and disposal workers, while the mitochondria are the furnace or power plant that powers
the whole show.
Of course this is just an analogy, none of these organelles have any sentience, but it
illustrates how a cell is much greater than the sum of its parts, a living system with
components that cooperate for survival.
In addition, after learning a bit about organelle structure and function, endosymbiotic theory
should become quite compelling.
It is now clear that membrane-enclosed organelles like the nucleus, endoplasmic reticulum, and
Golgi apparatus, probably originated from deep folds in the plasma membrane, whereas
mitochondria, as well as chloroplasts, which we will get to next, must have begun as completely
separate organisms, since they have their own DNA and membranes that are different from
the cell membrane.
All of these diverse structures eventually came to serve as compartments for different
cellular activities, which over an immense time span resulted in things as complex as
the plant and animal cells on earth today.
We still have more to learn about cells, and we are just scratching the surface of their
complexity.
But we’ve now gone from biomolecules, to prokaryotic cells, to eukaryotic cells, so
let’s learn about another type of eukaryotic cell next.