Cells are the Starting Point
All living organisms on Earth are divided in pieces
called cells. There are smaller pieces to cells that
include proteins and organelles. There are also
larger pieces called tissues and systems. Cells are
small compartments that hold all of the biological
equipment necessary to keep an organism alive and
successful on Earth.
A main purpose of a cell is to organize. Cells hold a
variety of pieces and each cell has a different set of
functions. It is easier for an organism to grow and
survive when cells are present. If you were only made
of one cell, you would only be able to grow to a certain size. You don't find single cells that are as large
as a cow. Also, if you were only one cell you couldn't have a nervous system, no muscles for movement,
and using the internet would be out of the question. The trillions of cells in your body make your life
possible.
One Name, Many Types
There are many types of cells. In biology class, you will usually work with plant-like cells and animallike cells. We say animal-like because an animal type of cell could be anything from a tiny
microorganism to a nerve cell in your brain. Plant cells are easier to identify because they have a
protective structure called a cell wall made of cellulose. Plants have the wall; animals do not. Plants
also have organelles like the chloroplast (the things that make them green) or large water-filled vacuoles.
We said that there are many types of cells. Cells are unique to each type of organism. Humans may
have hundreds of types of cells. Some cells are used to carry oxygen (O 2) through the blood (red blood
cells) and others might be specific to the heart. If you look at very simple organisms, you will discover
cells that have no defined nucleus (prokaryotes) and other cells that have hundreds of nuclei
(multinucleated). The thing they all have in common is that they are compartments surrounded by
some type of membrane.
Cell Membranes
We have been talking about cells being a unit of organization in
biology. Let's look at the cell membrane and see how that
membrane keeps all of the pieces inside. When you think about a
membrane, imagine it is like a big plastic bag with some tiny
holes. That bag holds all of the cell pieces and fluids inside the cell
and keeps any nasty things outside the cell. The holes are there to
let some things move in and out of the cell.
Flexible Containers
The cell membrane is not one solid piece. Everything in life is made
of smaller pieces and a membrane is no different. Compounds called proteins and phospholipids
make up most of the cell membrane. The phospholipids make the basic bag. The proteins are found
around the holes and help move molecules in and out of the cell.
Scientists describe the organization of the phospholipids and
proteins with the fluid mosaic model. That model shows that the
phospholipids are in a shape like a head and a tail. The heads like
water (hydrophilic) and the tails do not like water (hydrophobic).
The tails bump up against each other and the heads are out facing
the watery area surrounding the cell. The two layers of cells are
called the bilayer.
Ingrained in the Membrane
What about the membrane proteins? Scientists have shown that the proteins float in that bilayer. Some of
them are found on the inside of the cell and some on the outside. Other proteins cross the bilayer with
one end outside of the cell and one end inside. Those proteins that cross the layer are very important in
the active transport of ions and small molecules.
Many Membranes
As you learn more about the organelles inside of the cell, you will find that most have a membrane.
They do not have the same chemical makeup as the cell membrane. Each membrane is unique to the
organelle. The membrane that surrounds a lysosome is different from the membrane around the
endoplasmic reticulum. They are both different from the cell membrane.
Some organelles have two membranes. A mitochondrion has an outer and inner membrane. The outer
membrane contains the mitochondrion parts. The inner molecule holds digestive enzymes that break
down food. While we talk about membranes all the time, you should remember they all use a basic
phospholipid bilayer, but have many other different parts.
Cell Nucleus - Commanding the Cell
The cell nucleus acts like the brain of the cell. It helps control eating, movement, and reproduction. If
it happens in a cell, chances are the nucleus knows about it. The nucleus is not always in the center
of the cell. It will be a big dark spot somewhere in the middle of all of the cytoplasm (cytosol). You
probably won't find it near the edge of a cell because that might be a dangerous place for the nucleus
to be. If you don't remember, the cytoplasm is the fluid that fills cells.
Life Before a Nucleus
Not all cells have a nucleus. Biology breaks cell types into eukaryotic (those with a defined nucleus)
and prokaryotic (those with no defined nucleus). You may have heard of chromatin and DNA. You
don't need a nucleus to have DNA. If you don't have a defined nucleus, your DNA is probably floating
around the cell in a region called the nucleoid. A defined nucleus that holds the genetic code is an
advanced feature in a cell.
Important Materials in the Envelope
The things that make a eukaryotic cell are a defined nucleus and other
organelles. The nuclear envelope surrounds the nucleus and all of its
contents. The nuclear envelope is a membrane similar to the cell
membrane around the whole cell. There are pores and spaces for RNA
and proteins to pass through while the
nuclear envelope keeps all of the
chromatin and nucleolus inside.
When the cell is in a resting state there is something called chromatin
in the nucleus. Chromatin is made of DNA, RNA, and nuclear proteins.
DNA and RNA are the nucleic acids inside of the cell. When the cell is going to divide, the chromatin
becomes very compact. It condenses. When the chromatin comes together, you can see the
chromosomes. You will also find the nucleolus inside of the nucleus. When you look through a
microscope, it looks like a nucleus inside of the nucleus. It is made of RNA and protein. It does not
have much DNA at all.
Mitochondria - Turning on the Powerhouse
Mitochondria are known as the powerhouses of the cell. They are organelles that act like a digestive
system that takes in nutrients, breaks them down, and creates energy for the cell. The process of
creating cell energy is known as cellular respiration. Most of the chemical reactions involved in
cellular respiration happen in the mitochondria. A mitochondrion is shaped perfectly to maximize its
efforts.
Mitochondria are very small organelles. You might find
cells with several thousand mitochondria. The number
depends on what the cell needs to do. If the purpose of
the cell is to transmit nerve impulses, there will be fewer
mitochondria than in a muscle cell that needs loads of
energy. If the cell feels it is not getting enough energy to
survive, more mitochondria can be created. Sometimes
they can even grow, move, and combine with other
mitochondria, depending on the cell's needs.
Mitochondria Structure
Mitochondria have two membranes (not one as in other
organelles). The outer membrane covers the organelle
and contains it. The inner membrane folds over many times (cristae). That folding increases the
surface area inside the organelle. Many of the chemical reactions happen on the inner membrane of
the mitochondria. The increased surface area allows the small organelle to do as much work as
possible. If you have more room to work, you can get more work done. Similar surface area strategies
are used by microvilli in your intestinal cells. The fluid inside of the mitochondria is called the matrix.
Using Oxygen to Release Energy
How are mitochondria used in cellular respiration? The matrix is filled with water (H 2O) and proteins
(enzymes). Those proteins take food molecules and combine them with oxygen (O 2). The mitochondria
are the only place in the cell where oxygen can be combined with the food molecules. After the oxygen
is added, the material can be digested. They are working organelles that keep the cell full of energy.
A mitochondrion may also be involved in controlling the concentration of calcium (Ca) within the cell.
Chloroplasts - Show me
the Green
Chloroplasts are the food producers of the
cell. They are only found in plant cells and
some protists. Animal cells do not have
chloroplasts. Every green plant you see is
working to convert the energy of the sun into
sugars. Plants are the basis of all life on
Earth. They create sugars, and the
byproduct of that process is the oxygen that
we breathe. That process happens in the
chloroplast. Mitochondria work in the
opposite direction and break down the
sugars and nutrients that the cell receives.
Special Structures
We'll hit the high points for the structure of a chloroplast.
Two membranes contain and protect the inner parts of the
chloroplast. The stroma is an area inside of the chloroplast
where reactions occur and starches (sugars) are created.
One thylakoid stack is called a granum. The thylakoids
have chlorophyll molecules on their surface. That
chlorophyll uses sunlight to create sugars. The stacks of
sacs are connected by stromal
lamellae. The lamellae act like
the skeleton of the chloroplast, keeping all of the sacs a safe distance
from each other and maximizing the efficiency of the organelle.
Making Food
The purpose of the chloroplast is to make sugars and starches. They use
a process called photosynthesis to get the job done. Photosynthesis is
the process of a plant taking energy from the Sun and creating sugars.
When the energy from the Sun hits a chloroplast, chlorophyll uses that
energy to combine carbon dioxide (CO2) and water (H2O). The molecular
reactions create sugar and oxygen (O2). Plants and animals then use the
sugars (glucose) for food and energy. Animals also use the oxygen to breathe.
Different Chlorophyll Molecules
We said that chlorophyll molecules sit on the outside of the thylakoid sacs. Not all chlorophyll is the
same. Three types of chlorophyll can complete photosynthesis. There are even molecules other than
chlorophyll that are photosynthetic. One day you might hear about carotenoids, phycocyanin
(bacteria), phycoerythrin (algae), and fucoxanthin (brown algae). While those compounds might
complete photosynthesis, they are not all green or the same structure as chlorophyll.
Looking at Cell Functions
All cells have a purpose. If they don't do anything productive, they
are not needed anymore. In the big picture, a cell's purpose is
much more important than acting as small organizational pieces.
They had their purpose long before they started working together
in groups and building more advanced organisms. When alone, a
cell's main purpose is to survive.
Even if you were a single cell, you would have a purpose. You
would have to survive. You would be moving around (probably in a
liquid) and just trying to stay alive. You would have all of your
pieces inside of you. If you were missing a piece you needed to
survive, you would die. Scientists call those pieces organelles. Organelles are groups of complex
molecules that help a cell survive.
All Cells are not Created Equal
In the same way that cells survive in different ways; all cells have
different types and amounts of organelles. The larger a cell
becomes the more organelles it will need. It makes sense if you
think about it. If you are a big cell, you will need to eat more than a
little cell. You will also need to convert that food into energy. A
larger cell would need to eat more and may wind up having more
mitochondria to process that food into energy.
While they might have a purpose, more advanced cells have a
difficult time surviving on their own. A cell from your brain could not survive in a Petri dish for long. It
doesn't have the right pieces to live on its own. It does have the ability to transmit electrical systems
around your body. An amoeba could survive in a dish forever, thrive, and reproduce. On the other
hand, that amoeba will never help you transmit electrical impulses. The brain cell is far more
advanced and has specific abilities and organelles. Simpler cells have a better chance of surviving on
their own while complex cells can accomplish tasks that are more advanced.