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General Education
Lesson 2: Cytology
Cytology is the study of cells. It comes from the Greek word "cyto" meaning cell and "logos" meaning study. Cells are the basic units of life, so studying cells is very important to understanding all biology.
Two Types of Cells
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| Image from Science | HowStuffWorks |
Every organism is made of cells, but there are two very different types of cells: prokaryotic and eukaryotic.
1. Prokaryotic Cells - are cells that do NOT contain a true nucleus (DNA floats freely in the cell)
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| Image from ThoughtCo |
What organisms have prokaryotic cells:
- Bacteria - are microscopic, single‑celled living organisms. Can be found anywhere, reproduce through binary fission (splitting into two).
- Archaebacteria (Archaea) - single‑celled microorganisms that live in extreme environments such as salty, hot, and oxygen free environment.
Its Characteristics:
- No nucleus (DNA is loose in the cytoplasm)
- Generally smaller than eukaryotic cells
- Simpler structure
- Have been around for billions of years (the first cells on Earth)
Prokaryotic cells are simpler cells without a nucleus. Bacteria are prokaryotic.
- Example: Escherichia coli (E. coli) bacteria are prokaryotic cells. Cyanobacteria are prokaryotic cells. Mycobacterium tuberculosis (causes TB) is also prokaryotic.
2. Eukaryotic Cells - are cells that contain a true nucleus where the DNA is kept.
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| Image from The Science Fellow |
What organisms have eukaryotic cells:
- Protists - mostly single‑celled organisms that live in water and are not plants, animals, or fungi.
- Fungi - organisms like molds, yeasts, and mushrooms that absorb nutrients from decaying or living matter.
- Plants - multicellular organisms that make their own food through photosynthesis and cannot move from place to place.
- Animals - multicellular organisms that eat food for energy and can move on their own.
Its Characteristics:
- Have a nucleus containing DNA
- More complex structure than prokaryotic cells
- Have organelles with specific jobs
- Larger than prokaryotic cells
- More advanced cells
Eukaryotic cells are more complex cells with a nucleus. Animals, plants, and fungi have eukaryotic cells.
- Example: Human cells are eukaryotic. Plant cells are eukaryotic. Yeast cells are eukaryotic.
Special Structures on Cells
Some cells have special structures that help them move or work in different ways.
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| Image from Britannica |
Cilia (plural; cilium is singular) - are hair-like structures on the surface of some cells. Cilia move back and forth like tiny hairs to help the cell move through liquid or to move liquid across the cell's surface.
- Example: Cells in your windpipe have cilia that move back and forth to push mucus out of your lungs. Protist organisms use cilia to swim through water.
Cilia are tiny hairs that help cells move or move things across them.
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| Image from example |
Flagellum (plural: flagella) - is a tail-like structure on some cells. A flagellum moves like a tail to help the cell swim or move through liquid.
- Example: Sperm cells have a flagellum (tail) that helps them swim. Some bacteria have a flagellum that helps them move.
A flagellum is a tail that helps a cell swim.
The Discovery of Cells
Two scientists made important discoveries about cells
Robert Hooke (1600s)
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| Image from Ask A Biologist |
Robert Hooke discovered the cell by examining cork tissue from the bark of an oak tree under a microscope. He saw that the cork was made of many tiny, empty compartments. He called these compartments "cellulae," which is Latin for "small room." This is where the word "cell" comes from. Hooke was the first person to describe and name cells.
Robert Hooke discovered cells and gave them their name because they looked like small rooms.
Robert Brown (1800s)
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| Image from Mastrid |
Robert Brown discovered the cell's nucleus. He looked at cells from orchid plants and saw a large, dark structure in the middle of each cell. He called this structure the "nucleus," which means "kernel" or "center." Brown showed that cells have a nucleus, which we now know contains the cell's DNA and controls what the cell does.
Robert Brown discovered the nucleus, which is like the control center of the cell.
Viruses: Not Living, But Important
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| Image from Elephango |
A virus is something that can be confusing because it is not quite alive, but it is not completely non-living either. Viruses are best described as inert chemicals, they do not have the characteristics of living things by themselves.
- A virus is a particle made of genetic material (DNA or RNA) surrounded by a protective protein shell.
Parts of a Virus
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| Image from Biology Online |
Capsid: The protein shell that surrounds and protects the virus's DNA or RNA. The capsid is made of many protein molecules arranged in a geometric shape.
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| Image from ThoughtCo |
DNA or RNA: The genetic material inside the capsid. This is what tells the virus how to reproduce.
A virus is like a tiny package containing genetic instructions wrapped in a protein coating.
Viruses Are Not Living
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| Image from News Medical |
- They cannot reproduce by themselves. They need to infect a living cell to make copies of themselves.
- They cannot make their own energy. They have no metabolism.
- They cannot respond to their environment.
- They are just chemicals until they infect a living cell.
Viruses Are Important
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| Image from Science Encyclopedia |
Even though viruses are not technically alive, they are very important in biology because they infect living cells and can cause disease. Some viruses cause colds and flu. Some cause more serious diseases. Understanding viruses helps scientists create vaccines and treat viral infections.
Viruses are not alive by themselves, but they can infect living cells and cause disease.
- Example: The common cold virus is a virus. The flu virus is a virus. The coronavirus (COVID-19) is a virus.
Cytology Quiz: click here
