Cells and tissues
Everyone’s body is made of the same basic stuff. All living things, large or small, plant or
animal, are made up of cells. Most living things are made up of one cell and they are called
unicellular organisms. Many other living things are made up of a large number of cells that form
a larger plant or animal. These living things are known as multicellular organisms. Water makes
up about two thirds of the weight of cells.
Cells are very small; most cells can only be seen through a microscope. Cells are the smallest
living units that are capable of reproducing themselves. Each cell in your body was made from
an already existing cell. All plants and animals are made up of cells. In this article, we will talk
about the cells that make up You.
All the parts of your body are made up of cells. There is no such thing as a typical cell. Your
body has many different kinds of cells. Though they might look different under a microscope,
most cells have chemical and structural features in common. In humans, there are about 200
different types of cells, and within these cells there are about 20 different types of structures or
organelles.
All cells have a membrane. Cell membranes are the outer
layers that hold the cell together. They let nutrients pass into the
cell and waste products pass out. Not everything can pass
through a cell membrane. What gets through and what doesn’t
depends on both the size of the particle trying to get in and the
size of the opening in the membrane.
Cells also have a nucleus. This is the cell’s control center. Cells
continually divide to make more cells for growth and repair in
your body. The nucleus contains the information that allows
cells to reproduce, or make more cells. Another important part
of a cell is the mitochondrion. This is the part of the cell where
food and oxygen combine to make energy.
You know that you need air to breathe. It is the oxygen in air that your body really needs. Every
cell in your body needs oxygen to help it metabolize (burn) the nutrients released from food for
energy. You also know that you need food. Food gives you energy, but oxygen is needed to
break down the food into pieces that are small enough for your cells to use This is known as
cellular respiration and it is the process of oxidizing food molecules, like glucose, to carbon
dioxide and water. The energy released is chemically trapped for use by all the energyconsuming activities of the cell. Your cells are the
energy converters for your body.
Different cells have different jobs to do. Each cell
has a size and shape that is suited to its job. Cells
that do the same job combine together to form
body tissue, such as muscle, skin, or bone tissue.
Groups of different types of cells make up the
organs in your body, such as your heart, liver, or
lungs. Each organ has its own job to do, but all
organs work together to maintain your body. A
group of different organs working together to do a job makes up a system. All the systems in
your body are like members of a team whose job it is to keep you alive and healthy.
The different types of cells in your body have different, specialized jobs to do. The specialization
of cells depends almost always on the exaggeration of properties common to cells. For
example, cells that line the intestine have extended cell membranes. This increases the amount
of surface area that is available to absorb food. Nerve cells can be very long, which makes them
efficient in sending signals from the brain to the rest of your body. Cells in heart muscle process
a lot of energy, so they have a large number of mitochondrion, the part of the cells where
energy is made.
Like all living things, cells die. The number of cells that an adult male loses per minute is
roughly 96 million. Fortunately, in that same minute, about 96 million cells divided, replacing
those that died. Just as you shed dead skin cells, dead cells from internal organs pass through
and out of the body with waste products. The length of a cell’s life can vary. For example, white
blood cells live for about thirteen days, cells in the top layer of your skin live about 30 days, red
blood cells live for about 120 days, and liver cells live about 18 months.
Kinds of cells
Prokaryotic cells
The vast majority of cells on Earth are actually prokaryotic.
Prokaryotes have no organized nucleus. Like we said, the DNA is clumped in an area but there
is no organized nucleus with a membrane.
Prokaryotes do not usually have any organelles. They will probably have ribosomes inside of
their cells, but ribosomes are not technically considered organelles. No chloroplasts. No
mitochondria. No nucleus. Not much at all.
Prokaryotes are very small. Because they don't have all of the normal cell machinery, they are
limited in size. As always in biology, there are exceptions, but generally, prokaryotes are very
small (compared to other cells). Mind you, compared to a virus they are big, but next to an
amoeba,
tiny.
There are two major kinds of prokaryotes:

Bacteria

Archaea (single-celled organisms)
As you may have read earlier in this unit, biologists now estimate that each human being carries
nearly 20 times more bacterial, or prokaryotic, cells in his or her body than human, or
eukaryotic, cells. If that statistic overwhelms you, rest assured that most of these bacteria are
trying to help, and not hurt, you. Numerically, at minimum, there are 20 times more prokaryotic
cells on Earth than there are eukaryotic cells. This is only a minimum estimate because there
are trillions of trillions of bacterial cells that are not associated with eukaryotic organisms. In
addition, all Archaea are also prokaryotic. As is the case for bacteria, it is unknown how many
Archaean cells are on Earth, but the number is sure to be astronomical. In all, eukaryotic cells
make up only a very small fraction of the total number of cells on Earth.
There are four main structures shared by all prokaryotic cells, bacterial or Archaean:
1. The plasma membrane
2. Cytoplasm
3. Ribosomes
4. Genetic material (DNA and RNA)
Some prokaryotic cells also have other structures like the cell wall, pili (singular pillus),
and flagella (singular flagellum). Each of these structures and cellular components plays a
critical role in the growth, survival, and reproduction of prokaryotic cells.
Prokaryotic Plasma Membrane
Prokaryotic cells can have multiple plasma membranes. Prokaryotes known as "gramnegative bacteria," for example, often have two plasma membranes with a space between
them known as the periplasm. As in all cells, the plasma membrane in prokaryotic cells is
responsible for controlling what gets into and out of the cell. A series of proteins stuck in the
membrane also aid prokaryotic cells in communicating with the surrounding environment.
Among other things, this communication can include sending and receiving chemical signals
from other bacteria and interacting with the cells of eukaryotic organisms during the process of
infection.
Prokaryotic Cytoplasm
The cytoplasm in prokaryotic cells is a gel-like, yet fluid, substance in which all of the other
cellular components are suspended. Jello for cells. It is very similar to the eukaryotic
cytoplasm, except that it does not contain organelles. Recently, biologists have discovered
that prokaryotic cells have a complex and functional cytoskeleton similar to that seen in
eukaryotic cells. The cytoskeleton helps prokaryotic cells divide and helps the cell maintain its
plump, round shape. As is the case in eukaryotic cells, the cytoskeleton is the framework along
which particles in the cell, including proteins, ribosomes, and small rings of DNA called
plasmids, move around. The "highway system" suspended in Jello.
Prokaryotic Ribosomes
Prokaryotic ribosomes are smaller and have a slightly different shape and composition than
those found in eukaryotic cells. Bacterial ribosomes, for instance, have about half of the amount
of ribosomal RNA (rRNA) and one third fewer ribosomal proteins than eukaryotic
ribosomes have. Despite these differences, the function of the prokaryotic ribosome is virtually
identical to the eukaryotic version. Just like in eukaryotic cells, prokaryotic ribosomes build
proteins by translating messages sent from DNA.
Prokaryotic Genetic Material
All prokaryotic cells contain large quantities of genetic material in the form of DNA and RNA.
Because prokaryotic cells, by definition, do not have a nucleus, the single large circular strand
of DNA containing most of the genes needed for cell growth, survival, and reproduction is found
in the cytoplasm.
The
DNA
tends
Transmission
to
look
like
electron
a
mess
of
string
in
micrograph
the
middle
of
the
cell:
image source
Eukaryotic Cell Structure and Function
A cell is defined as eukaryotic if it has a membrane-bound nucleus. Any organism
composed of eukaryotic cells is also considered a eukaryotic organism. Case in point: You.
Biologists do not know of any single organism on Earth that is composed of both eukaryotic and
prokaryotic cells. However, many different types of prokaryotic cells, usually bacteria, can live
inside larger eukaryotic organisms.
We humans, for example, have trillions of bacteria living in our colons, not to mention in our
mouths and stomachs and small intestines and. Despite the fact that we have gobs of
prokaryotic cells living inside and on us, humans are still categorically eukaryotic organisms.
This means that all human cells, including those found in the brain, the heart, the muscles, and
so on, are also eukaryotic.
All of the organisms we can see with the naked eye are composed of one or more eukaryotic
cells, with most having many more than one. This means that most of the organisms we are
familiar with are eukaryotic. However, most of the organisms on Earth, by number, are actually
prokaryotic.
Here are some examples of eukaryotes:

Animals

Plants


Fungi (mushrooms, etc.)
Protists (algae, plankton, etc.)
Most plants, animals, and fungi are composed of many cells and are, for that reason, aptly
classified as multicellular, while most protists consist of a single cell and are classified
as unicellular.
All eukaryotic cells have
1. A nucleus
2. Genetic material
3. A plasma membrane
4. Ribosomes
5. Cytoplasm, including the cytoskeleton
Most eukaryotic cells also have other membrane-bound internal structures called organelles.
Organelles include

Mitochondria

Golgi bodies

Lysosomes


Endoplasmic reticulum
Vesicle
The nucleus and Eukaryotic Genetic Material
The nucleus in the cell is analogous to the brain in the body. It is a control center for a cell.
Presenting, the nucleus:
The nucleus stores all the information the cell needs to grow, reproduce, and function. This
information is contained in long but thin molecules of deoxyribonucleic acid, or DNA. One of the
functions of the nucleus is to protect the cell’s DNA from damage, but that is not all that it does.
The nucleus is basically a large membranous sac.
The nucleus also contains a small round body called a nucleolus that holds nucleic acids and
proteins. The nuclear membrane has pores through which the contents of the nucleus
communicate with the rest of the cell. The nuclear membrane tightly controls what gets into the
nucleus and what gets out. This regulation of communication by the nuclear membrane has a
great effect on what a cell looks like and what it does.
Chromosomes are also located in the nucleus and are basically organized structures of DNA
and proteins. In eukaryotes, the chromosomal DNA is packaged and organized into a
condensed structure called chromatin. Chromosomes are single pieces of DNA along with
genes, proteins, and nucleotides, and chromatin is a condensed package of chromosomes that
basically allows all the necessary DNA to fit inside the nucleus.
In eukaryotic organisms, the DNA inside the nucleus is also closely associated with large
protein complexes called histones. Along with the nuclear membrane, histones help control
which messages get sent from the DNA to the rest of the cell. The information stored in DNA
gets transferred to the rest of the cell by a very elegant process—a process so common and so
important to life on Earth that it is called the central dogma of biology.
In eukaryotic cells, the first stage of this process takes place in the nucleus and consists of
specific portions of the DNA, called genes, being copied, or transcribed, into small strands of
ribonucleic acid, or RNA. RNA containing a copy, or transcript, of DNA is called messenger
RNA, or mRNA. These mRNA molecules are then physically transported out of the nucleus
through the pores (holes) in the nuclear membrane and into the cytoplasm where they are
eventually translated into proteins by ribosomes.
Therefore, the central dogma of biology is simply:
DNA
→
RNA
→
Protein
and it all starts in the nucleus! Warning: This does NOT apply to prokaryotes. You are in
eukaryote-only territory, and don't you forget it.
Most eukaryotic cells have a nucleus throughout their entire life cycles, but there are a few
notable exceptions. Human red blood cells (the good ol' RBCs), for example, get rid of their
nuclei as they mature. Rebels without a cause. Or actually, with a cause, because, with their
nuclei removed, red blood cells have more space to carry oxygen throughout the body.
Eukaryotic Plasma Membrane
The plasma membrane in eukaryotic cells is responsible for controlling what gets into and out of
the cell. A series of proteins stuck in the membrane help the cell communicate with the
surrounding environment. Among other things, this communication can include

Sending

Interacting with the cells of prokaryotic organisms during the process of infection.
and
receiving
chemical
signals
from
other
eukaryotic
cells
Keep in mind that the plasma membrane is universal to all cells, prokaryotic and eukaryotic.
Because this cellular component is so important and so common, it is addressed in greater
detail further on in the In Depth section.
Eukaryotic ribosome
Ribsomes are small cellular machines made of proteins and ribosomal RNA. All cells, both
eukaryotic and prokaryotic, have ribosomes.
Presenting, the ribosome:
Eukaryotic ribosomes are larger and have a slightly different shape and composition than those
found in prokaryotic cells. Eukaryotic ribosomes, for instance, have about twice the amount of
ribosomal RNA (rRNA) and one third more ribosomal proteins than prokaryotic ribosomes have.
Despite these differences, the function of the eukaryotic ribosome is virtually identical to the
prokaryotic version. This is a remarkable example of what we call evolutionary unity. Ribosomes
translate mRNA into protein, or the last step in the central dogma of biology described earlier. It
all comes together…
Eukaryotic Cytoplasm and Cytoskeleton
The cytoplasm in eukaryotic cells is a gel-like, yet fluid, substance in which all of the other
cellular components are suspended, including all of the organelles. The underlying structure
and function of the cytoplasm, and of the cell itself, is largely determined by the cytoskeleton, a
protein framework along which particles in the cell, including proteins, ribosomes, and
organelles, move around.
You can think of the cytoskeleton as a type of 3D "highway system" with roads running in every
direction, including up and down. The cytoplasm is the thick fluid in which the "highway system"
is suspended and through which cellular materials are transported.
Helpful tip: Whenever you see "cyto" as part of a word, think "inside the cell."
Endoplasmic Reticulum
An extensive network of channels that stretches from the nucleus to the plasma membrane
Function is to transport materials through the cell
ROUGH ER - distinguished by the ribosomes attached to it
SMOOTH ER- many functions such as production of membrane phospholipids, production of
sex hormones, breaks down harmful substances, stores calcium ions, transports liquid based
compounds, and aids liver cells in releasing glucose into the blood when needed
Golgi Apparatus
Composed of many flattened sacs called cisternae, which are stacked on top of each other
Normally located between the ER and plasma membrane
It collects, packages, modifies, and distributes materials throughout the cell
Found in high numbers in cells that produce and secrete substances
Lysosomes
Single membrane bound organelle produced by the GOLGI APPARATUS that contain strong
hydraulic enzymes which break down biological molecules
Breaks down worn out cell parts that are no longer functioning properly
Involved in breaking down materials brought into a cell VIA phagocytosis
Mitochondria
Very large, double membrane bound rod-shaped organelles found scattered in the cytoplasm
and contain their own DNA
Other membrane is smooth
Inner membrane is highly folded into cristane which increases the surgace area for cellular
respiration, which is the main function of the mitochondria
Found in high numbers in cells that have high energy needs (EX: muscle cells)
Known as the "power house" of the cell
Pili
Small hairlike projections emerging from the outside cell surface
Assists in the cell attaching to other cells and surfaces
Flagella
Hairlike structures that allow the cell to move by beating in a propeller-like motion
Help bacterium move toward nutrients, away from toxic chemicals, ETC