Download Key Terms Prokaryote Nucleus Organelle Cytoplasm Eukaryote Cell

Biology B4.a B4.c (part 1 only)
Key Terms
Prokaryote
Nucleus
Organelle
Cytoplasm
Eukaryote
Cell (plasma) membrane
Cell wall
Virus
Unicellular
Multicellular
The Classification of Living Things
There are lots of different types of living things on our planet. Every living thing on the planet
can be classified into two groups based on the cells that make them up. Cells are the basic units
of life, and all living things are composed of cells. Although all cells have some things in
common, each cell type also has unique characteristics that make them easy to identify. Let’s
focus on these unique characteristics.
Prokaryotes
Our first cell type is called a prokaryotic cell or prokaryote (pro-kar-e-oat). These cells are
unique because they are only found in the smallest organisms on the planet, bacteria. Scientists
believe that these bacterial cells (prokaryotes) were the very first life forms on Earth. That
means that these cells have been on the planet longer than any other living thing. These
prokaryotic cells are very simple and small-they are made of only one cell (unicellular).
Prokaryotes do not have any membrane-bound organelles (cell parts). That means that bacteria
don’t have a nucleus (PRO-NO). Their DNA just floats around in the cytoplasm (the fluid
inside the cell).
Look at the example of a prokaryote below
Eukaryotes
If bacteria are the only living examples of prokaryotes, everything other living thing on the
planet must be composed of eukaryotic cells. Eukaryotes (U-kar-e-oats) are much bigger than
prokaryotic cells. Some eukaryotic cells are even large enough to be seen without a microscope!
Eukaryotic cells are complex. They have lots of different compartments inside of them called
membrane-bound organelles. These are located in the cytoplasm. The most important organelle
Biology B4.a B4.c (part 1 only)
in a eukaryotic cell is its nucleus, which holds its DNA. This is the control center of the cell.
This means eukaryotes can grow much larger than prokaryotic cells, and control lots of unique
cellular activities all at once. Eukaryotes also have a cell membrane. Some, like plants, have a
cell wall, but animal cells do not. All multicellular (made of more than one cell) organisms are
eukaryotes, including you! You are EU-karyote!
Look at the example of a eukaryotic cell below. It is much more complex than the prokaryote
above. This is an animal cell—it does not have a cell wall.
Viruses
Viruses are not generally considered to be alive because they are not cells. We study them in Biology
because they are found in living organisms. Viruses have DNA (or sometimes RNA), but do not have a
cell membrane, or any organelles. They also do not have cytoplasm. They are very small (smaller than
prokaryotes even). Look at the examples of different virus shapes.
Biology B4.a B4.c (part 1 only)
Use the facts from the reading to complete this chart.
QUESTION
PROKARYOTES
EUKARYOTES
VIRUSES
Does it have a nucleus?
Does it have membrane-bound organelles?
Does it have a cell membrane?
Does it have DNA?
Is it small, smallest or biggest in size of the
three?
Imagine in the future that you are an astrobiologist (astrobiologists are scientists who look for life in
space) on a mission to another planet. You have discovered something that may or may not be alive,
may or may not be prokaryotic or eukaryotic.
1. What information could you gather and analyze about its structure to determine the answer?
2. How can you justify (support) your answer?
Use the information from your chart and the reading to complete the Venn Diagram.
Biology B4.a B4.c (part 1 only)
Comparing and Contrasting Prokaryotes and Eukaryotes
PROKARYOTES
BOTH
EUKARYOTES