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WHAT IS A CELL?
.A
cell is the very smallest unit of living matter. All
living things including plants and animals are made
up of cells. Cells are made of atoms, which are the
smallest units of matter. There are many different
kinds of cells. The two kinds you are most
likely to be familiar with are animal and
plant cells. Some of the differences
between them are that plant cells have a
cell wall and chloroplasts.
Cells all have different sizes, shapes, and
jobs to do. Did you know that the yoke from an
ostrich's egg is the biggest cell? Even the cells in a
single organism may have different shapes, sizes,
and jobs. Organisms like humans are made up of
trillions of cells. There are also one celled organisms
such as euglenas, amoebas, and bacteria.
All cells have some parts in common. One part
found in all cells is the cell membrane. The cell
membrane surrounds the cell, holds the other parts
of the cell in place, and protects the cell. Molecules
can pass in and out of the cell membrane. Inside the
membrane, all cells, except for bacterial cells,
contain a nucleus and cytoplasm. The nucleus is a
dark structure located in the middle of the cell. It
controls the cell's activities, and acts like the cell's
brain. Inside the nucleus there is DNA which
contains genetic information. The cytoplasm is a
jelly-like substance inside the cell where most of
the cell's activities take place. It's made out of
water and other chemicals. All cell parts, except
the nucleus, are located in the cytoplasm.
Like all living things, cells die. If you watched a
clock for one minute, about three billion cells in your
body die. In that same minute, about three billion
cells divided so that all the cells that died were
replaced. Dead skin cells flake off and dead cells
from internal organs will pass through and out of
the body with waste products. The length of time
cells live can vary. For example, white blood cells
only live for thirteen days, red blood cells live for
about one hundred and twenty days, liver cells live
about eighteen months, and nerve cells can live up to
one hundred years. That is a very big difference!
Basically, each cell was made from an already
existing cell. New cells are made through a process
called cell division or mitosis. One cell turns into two
cells and then two cells turn into four cells, etc. Even
humans started life with only one cell. Most of the
cells in many celled organisms use mitosis to
reproduce. The animation shows the basic steps in
mitosis which are:
1) At the start of mitosis chromosomes are in the
nucleus.
2) The chromosomes in the nucleus will then make a
copy of themselves
3) Next the cell divides.
4) Last one set of chromosomes goes to the new cell
and one set remains in the parent cell.
One of the most important tools that scientists use to
study cells is the microscope. An optical microscope
can magnify a cell up to two thousand times. If an
ant were to be magnified at two hundred thousand
times, it would be more than two and one half miles
(four kilometers) long. Electron microscopes can
magnify a cell by one million times. But even with
such large magnifications, the very detailed parts of
some cells can't be seen. If you would like to see real
photographs of cells viewed through the lens of an
electron microscope, make sure you visit the
Nanoworld Image Gallery (you'll need to register in
order to view the gallery).
Scientists also put dyes in cells to make certain parts
look bright under a microscope. Another way that
scientists use to examine cells is to separate them
into their parts by using a machine called a
centrifuge. It spins substances at really high speeds.
First scientists grind up cells in a tube, and then they
place the tube in the machine. After whirling around
in the centrifuge, the parts of the cell that weigh
more sink, while the lighter parts stay on top. That
makes it easier for the scientist to study the parts.
ANIMAL CELL
Animal cells are the cells found in animals. You are made up
of trillions of animal cells.
These are the basic parts of an animal cell:
Cell
Membra
ne: A cell
membrane
works like
a screen.
It lets the
good things in like nutrients and
lets the bad things out like
waste. It is like a sieve.
Cytoplasm: Cytoplasm is the
jelly-like part of the cell where
the action takes place. It is
where the nutrients are used.
Nucleus: The nucleus is the
brain of the cell. It controls the
cell, and tells it what to do. The
nucleus also contains the DNA
which is like a blueprint. A
blueprint is a plan that people
use when they build. A cell's
blueprint is the plan the cell uses
when it reproduces.
One of the most important ways that animal cells are different
from plant cells is that animal cells can't make their own food.
PLANT CELL
Plant cells are cells that are in plants. Plant cells are like
animal cells, but they have a cell wall and chloroplasts.
Here's a labeled picture of a plant cell. You will see all the
main parts of the cell. Listed below are definitions of those
words. Read on to learn more about photosynthesis.
Cell Membrane: A thin layer that
surrounds the cell.
Cell Wall: A rigid layer that surrounds the
cell membrane of a plant cell.
Chloroplast: (KLOR-o-plast) A cell
structure in which photosynthesis takes
place, and it contains the chloropyll.
Chlorophyll: (KLOR-a-fil) A green
substance in plant cells that helps to make
food.
Cytoplasm: (SIGH-ta-plaz-em) The
jellylike liquid in cells where activities take
place.
Nucleus: (NEW-kle-us) The dark structure
inside the cell that controls the cell's
activities and contains material such as
DNA.
Photosynthesis: (foto-SIN-thi-sis) The
process when plants use lights energy to
make food.
A very special process that takes place inside of the
chloroplasts in plant cells is called photosynthesis (fotoSIN-thi-sis). Photo means "light" and synthesis means "
putting things together." In photosynthesis, green plants
actually make their own food. That is so cool! Imagine if
we could make our own food. I wonder what we'd make?
Personally, I would make pizza. Plants seem to have the
right idea since they make sugar. This amazing process is
responsible for everything we eat. That's because animals
eat plants, and we eat the animals and the plants.
If you would like to see an actual photograph of a
chloroplast viewed through the lens of an electron
microscope, visit the Nanoworld Image Gallery (you'll
need to register in order to view the gallery).
In photosynthesis, the chloropyll (KLOR-a-fil) takes in
energy from sunlight. The energy comes in the form of a
tiny bundle known as a photon. The photon hits a
molecule of water inside the chlorophyll. The photon's
energy splits the water molecule into hydrogen and oxygen.
The hydrogen combines with carbon dioxide (which the
plant has absorbed from the air) to make sugars or
glucose. The oxygen is released back into the atmosphere
to give us more air. Did you know that the largest
suppliers of oxygen in the whole world are tiny plants
known as photoplankton? They are found in the oceans
and are really important to giving us the oxygen needed to
keep life going on our planet.
Believe it or not, a plant uses only about one-sixth of the
energy it gets from the sun to nourish itself. The rest of the
energy is stored in the glucose until it is eaten by other
animals or humans. What an amazing process!