In this video we discuss the structure of a typical human cell, the components of a cell, and the functions of many of the organelles of a cell. Notes (partial) The …
The major components of
a typical human cell.
The main components of a typical human
cell include, the plasma membrane,
the cytoplasm, including the organelles
and cytoskeleton, and the nucleus.
The plasma membrane is the outer boundry of
a cell. It is composed mainly of a double
layer of phospholipid molecules that are
held together by chemical attractions.
These phospholipid molecules have
a water soluble head and lipid or
fat soluble tails, so they form a
bilayer that acts like a fence,
allowing lipid soluble molecules
to pass through easily and
preventing water soluble molecules
from passing through easily.
The cytoskeleton is the internal
framework of a cell as it
provides structural support and
allows the movement of materials.
The cytoskeleton is made up
of fibers that form a kind
of scaffolding, which are
made up of protein molecules.
There are 3 different types
of fibers, microfilaments,
intermediate filaments
and microtubules.
Microfilaments are the
smallest of the three, and
they are made of twisted
strands of proteins.
They can be pulled together to shorten
the cell, such as in muscle contraction.
Intermediate filaments are also twisted
strands of proteins that provide
much of the framework support in a
cell as they hold the cell together.
Microtubules are made up of proteins that
take on a spiral shape to from a hollow
cylinder. They help
maintain the cells shape,
and move some of the
organelles within a cell.
The centrosome is typically
found near the nucleus and it
basically builds and breaks
apart microtubules in the cell.
It consists of centrioles that are
perpendicular to each other, which
have nine bundles of microtubules
with 3 tubules in each bundle.
The cytoplasm consists of the
cytosol, which is a watery
solution also referred to
as intracellular fluid.
It has a syrup like consistency and
contains a high water content. The
cytoplasm also houses the tiny
organs, or organelles in the cell.
The first organelle we are going to
look at is the endoplasmic reticulum.
There are two types of endoplasmic
reticulum, the rough ER and smooth ER.
The rough ER extends out from
the nucleus and has ribsomes
attached to its membrane
giving it a rough appearance.
These ribsomes produce proteins
or polypeptide chains,
which are either embedded
into the membrane of the ER,
or enter the lumen, or
cavity of the rough ER.
Here they are modified, packaged and
stored until they are released.
They are transported by
enclosed membrane sacs called
transport vesicles that pinch
off from the smooth ER.
These vesicles transport the
proteins to another organelle
called the Golgi Apparatus
for further modification.
The smooth ER is a membranous organelle
that does not have any ribosomes.
The smooth ER contains enzymes
that alter polypeptides,
synthesize lipids and
carbohydrates and destroy toxins.
Most of the phospholipids
and cholesterol that form
cell membranes are synthesized
in the smooth ER.
Ribosomes can be found
attatched to the rough ER,
or scattered freely
throughout the cytoplasm.
They are a non membranous structure
comprised of large and small subunits.
Ribosomes are protein factories,
the two subunits come
together as transfer RNA brings
amino acids to the ribosome,
and messenger RNA provides
the code of how to
assemble the amino acids
into a polypeptide chain.
The golgi apparatus is a
membranous organelle that’s
main function is to modify
package and store proteins.
Transport vesicles, delivering
proteins from the ER,
fuse with the first cistern
of the golgi apparatus.
Here they release the proteins
which are modified by enzymes.
Modification of the molecules
continues as it moves
through the golgi apparatus
by way of the golgi vesicles,
some of this modification
may include the
addition of carbohydrates
to form glycoproteins.
After modification is complete they
are pinched off in another vesicle
called a secretory vesicle where many
are sent to the cell's plasma membrane.
At the plasma membrane, some of the
components of the vesicle are released
outside the cell, and some vesicles become
part of the plasma membrane itself.
Lysosomes are membranous
vesicles that pinch off from the
golgi apparatus, and they
contain digestive enzymes.
Lysosomes are like garbage bags,
picking up cellular waste, or
picking up defective organelles,
and recycling the contents.
Lysosomal enzymes also help a cell destroy
bacteria and viruses engulfed by the cell.
Proteasomes are cylindrical
protein destroying
organelles found
throughout the cytoplasm.
They are responsible for breaking
down abnormal and misfolded
proteins, and normal proteins that
are no longer needed by the cell.
A targeted protein is
marked by having a
separate protein called
ubiquitin bound to it.
The ubiquitin pulls the
protein into the proteasome
where many of its peptide
bonds are broken.
The resulting smaller peptide
chains and amino acids
are then released to be
recycled by the cell.
Peroxisomes are membranous vesicles
that are pinched off from the rough ER.
Peroxisomes break down long chain
fatty acids and amino acids,
and also contain the enzyme that
that decomposes the harmful
hydrogen peroxide into water
and oxygen inside the cell.
Mitochondria, which are found in the
cytoplasm, are the power plants of cells,
as they can take energy from food molecules
and use it to build ATP molecules.
ATP being the energy currency of
cells needed to do cellular work.
Mitochondria have an inner
and outer membrane, and vary
in numbers depending on the
amount of work a cell does.
For instance liver cells contain 1000 or
more mitochondria, and frequent aerobic
exercise increases the number of
mitochondria in skeletal muscle cells.
The nucleus is the largest of all cell
structures, and it has two membranes that
form a nuclear envelope, that, along with
nuclear pores enclose the nucleoplasm.
The nuclear pores allow certain
molecules into and out of the nucleus.
Inside the nucleus is the nucleolus,
which is comprised of protein and RNA.
The nucleolus is responsible for producing
the large and small subunits of ribosomes.
The nucleus also contains
DNA, and in non dividing
cells appear as tiny bunches
and are called chromatin.
Some cells have extensions,
such as microvilli,
which are important in
nutrient absorption,
cilia, which push substances
along the surface of a cell,
and flagella, which help
to propel an entire cell.
We will cover these in
depth in a later video.
And that be the basics on the
components of a typical human cell.

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