You are made up of cells. So are dogs and cats and trees and fish. Cells are the
basic building blocks of life. The word cell was first coined by the biologist
Robert Hooke when he was looking at a piece of cork under a microscope. The cork
was divided into sections that looked like monastic cells, so Hooke called the
sections he saw “cells”.
have many functions, but each has a function in common, which is to help keep an
organism alive. There are many different kinds of cells.
Each organism has its own kind of cells. Human cells are different from cow cells or tree cells or frog cells or turnip cells.
And even within a specific organism, there are many different kinds of cells.
Every human, for example, is made up of about 100 trillion cells -- brain cells and bone cells and stomach cells and many other kinds of cells.
For just one example of the specialization of cells, we can study skin cells --
they are flat and waterproof and are constantly being created in the lower
epidermis (the lower layer of the outer layer of skin). Then they migrate to the
upper epidermis (the outer layer of skin) where they flake off and die. Here are
some other examples: Skeletal muscle cells function to provide movement for
animals. Liver cells have many functions – one important one is to detoxify
harmful chemicals in the body. Red blood cells function to transport oxygen from
the lungs to the rest of the body. White blood cells help to protect us against
There are several ways to classify cells, but the main division is between eukaryotic and prokaryotic cells.
Eukaryotic vs. Prokaryotic Cells
There are big differences between eukaryotic and prokaryotic cells, but they do have some things in common:
- Both have DNA as their genetic material.
- Both are encased in membranes. The membranes of Prokaryotic cells and of
some eukaryotic cells are surrounded by a strong cell wall.
- Both have a similar basic metabolism (the word metabolism refers to
the formation and break down of chemicals in the body).
The major difference is that eukaryotes have a nucleus and prokaryotes do not.
Also, eukaryotic cells contain a wide variety of organelles (tiny "organs" or structures inside the
cell) that perform various functions. Some of the most important organelles are mitochondria, chloroplasts, lysosomes and ribosomes -- which we'll tell you about in our next section of this lesson.
But first, here is the basic structure of an eukaryotic cell:
- The membrane (the outside or coating)
- The nucleus (the place where
the DNA is stored and from where instructions are sent to the rest of the cell
for the production of proteins. The nucleus has its own membrane)
- The cytoplasm or cytosol, which contains
everything between the membrane
and the nucleus (including organelles)
Eukaryotic cells are, on average, ten times as large as prokaryotic cells.
Their DNA is more complex and is formed into chromosomes.
And their organelles enable them to do more complex jobs.
3.5 Billion Years Ago
Many scientists think that prokaryotic cells arose about 3.5 billion years ago
-- and that they were the first life forms on Earth; they believe this because
they have found fossils of prokaryotic cells that are 3.5 billion years old.
After about 2 billion years, aggregates of prokaryotic cells evolved into
Today, the only prokaryotes we know of are bacteria (like the bacteria that give
you strep throat and the bacteria in our digestive systems that help us to
digest our food) and cyanobacteria (blue-green algae). To learn more about
prokaryotic cells, go to http://www.biology4kids.com/files/micro_prokaryote.html
All other organisms are formed of cells that are eukaryotes. Different
eukaryotes have different characteristics. Let’s begin by looking at the
structure of plant cells.
vs. Animal Cells
As we said, most plants are made up of eukaryotic cells.
But eukaryotic plant cells are different from eukaryotic animal cells. And the biggest difference is the cell membrane.
In animals (like us) the membrane is the actual outside of the cell.
But plants have something extra -- a cell wall that goes all the way
around the outside of the membrane, shown with the brown shading in the diagram
Remember, the main difference between plant cells and animals cells is this -- plant cells have a cell wall and animal cells don't.
This cell wall is made of cellulose.
Cellulose is important in human nutrition since it provides fiber (roughage), which keeps your digestive system going. That's why it's important to eat a diet rich in plant material like fruits, whole grains and vegetables.
from the California Department of Education
Brody, David: The Science Class You Wish You Had
Crichton, Michael: The Andromeda Strain (fiction, but deeply
involved with the activity of cells)
Additional YouTube Video Instruction
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for Students, Parents and Teachers
Now let's do Practice
Exercise 1-1 (top).
Next Page: Nucleus, Mitochondria, Chloroplasts (top)