In this lecture, I'm going to talk to you
about what a mammalian cell is.
My name is Geoff Meyer.
In this lecture, I'm only going to introduce you
to some of the general features of a cell
and then in a later lecture, I will describe some of the structural components in a cell
and what those components actually do
in terms of their function,
and then look at some more specialized
features of some cells in the body.
This will serve as a very meaningful background
before you look at more histology
when you examine other cells and the basic tissues
and then the human organ systems.
Well, let's first of all define what a cell is.
It is the basic structural, functional, and
reproductive unit of all multicellular organisms.
In other words, it is the basic unit of life.
Let me just summarize generally what
some of their body functions are
that help us to live and survive every day of our life.
Firstly, protection. We have cells
in our body that protect us.
For instance, the skin is a cellular lining that
protects us from the external environment.
It waterproofs our body, protects it
from excessive ultraviolet light.
We have cells that also are involved with
ingesting, taking in materials
and those materials, when taken in by the cell, are digested or broken down into meaningful products
where they can then be used to provide
nutrients or generate energy
that is essential for most of our
daily functions and processes.
Cells also absorb other metabolites that are
critical for other parts of our body functions.
Once all these components are absorbed by the cell
and converted into useful products,
then they can be broken down
and eliminated by wastes.
The cell can get rid of waste products as our body does.
We are involved in movement every day of our life.
It requires skeletal muscles to help our body move,
to walk, to run, to lift and carry things,
but also, we have cells in our body that affect other types of movement.
For instance, movement of blood through our heart, blood through our blood vessels,
and food products through our
alimentary system or our gut,
urine through our urinary system, and so on.
We also are involved with communicating.
We're the most communicating being on this earth.
We use communication every day of our lives.
We have cells in our bodies to help us communicate,
to help us see, to help us speak,
to help us listen, to help us smile.
All those sorts of activities that require communication require cells in our body to become very specialized.
And communication also involves our
nervous system most importantly,
where cells generate impulses that help us to innervate our skeletal muscles and move,
or we have cells also that's part of the
neuroendocrine system and the endocrine system
that are chemical messengers. They
secrete chemical messengers that can
target various cells in our body or communicate with those cells and bring about different functions.
And of course, we can reproduce. We can reproduce us as human beings but also,
almost all cells in our body can reproduce
and replace cells that die
because most cells in our body have a limited lifespan, so we need to replace them at various intervals.
And lastly, our cells are involved with necrosis, breaking down of cells and finally,
the process of aging and then death.
There are two sorts of cells.
There are the prokaryotes which include bacteria,
And there are the eukaryotes which include human cells.
I'm only briefly going to describe the prokaryote and then the rest of these lectures refer to the eukaryotes
or cells that are typical of the human
species and other animal species as well.
Prokaryotes are just a single cell. They're tiny.
They're between about 0.2 µm and about 0.5 µm so they're very, very, tiny cells.
They're unicellular, one single cell, and they're surrounded by a cell membrane, a cell wall
and a capsule, and this cell wall or capsule and membranes serve for
the rigidity of the cell wall, the strength, its protection.
It's protection not just from mechanical
stresses it may bear upon,
but also protection from the entry of perhaps large macromolecules which could be very toxic
to the inside components of the cell and
also protecting itself from loss of water,
keeping it hydrated. It's a very important structure,
the cell wall, capsule, and the membrane.
Prokaryote means without a nut, kernel, or a seed.
That's how the early histologists described or named these cells because the prokaryote was without a seed
or without a nucleus. There is no nucleus in a prokaryote. Instead, the single DNA molecule is coiled
and housed in a certain area within this cell,
in an area called the nucleoid, or sometimes,
individual strands of DNA can be found out in the cytoplasmic area, and they're called plasmids.
Reproduction of these unicellular organisms
is by a process of binary fission
whereby the DNA just replicates and then
the cell splits in two. It is an asexual reproduction.
On the other hand, the eukaryotes are far more complex. They're bigger.
They're large compared to the prokaryotes. These eukaryotes contain chromosomes,
chromosomes on where the DNA becomes very compact and condensed, ready for cell division.
The DNA is housed within a nucleus and that
nucleus is a feature of all eukaryotes,
or at least most mammalian cells have a nucleus,
and I'll talk about that in a later lecture.
Eukaryotes contain organelles. Organelles are membrane-bound structures within the cell,
each having a very specific structure or characteristic and also each having a specific function.
And the eukaryote can divide by mitosis, or if the cell is involved with becoming a germ cell,
for instance, a spermatozoa in the male
or a ovum in the female gamete,
then those cells are produced by a process called meiosis.
The eukaryote also has a cytoskeleton.
That cytoskeleton helps to reinforce the cell wall.
It also helps to stabilize positions of all the organelles within the cell,
and it also is involved with helping those organelles move about the cell.
The prokaryote does not have a cytoskeleton. It doesn't need to have one because of the rather rigid cell wall,
so it doesn't need the support that the eukaryotes need for their cell wall which is a lot more delicate.