Cells

Cells

Middle School

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Learning Objectives

• Define the cell as the most basic unit of all living things.

• Differentiate between prokaryotic cells and more complex eukaryotic cells.

• Distinguish between unicellular organisms and multicellular organisms.

• Identify the key historical discoveries that led to the modern cell theory.

Key Vocabulary

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Cell

The cell is the basic structural and functional unit of all known living organisms.

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Microscope

A microscope is an instrument used to see objects that are too small for the naked eye.

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Prokaryote

A prokaryote is a unicellular organism that lacks a distinct nucleus and other organelles.

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Eukaryote

A eukaryote is an organism with cells that have a nucleus enclosed within membranes.

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Unicellular

A unicellular organism consists of a single cell that performs all of its vital functions.

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Multicellular

A multicellular organism consists of many specialized cells that perform a variety of different functions.

Key Vocabulary

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Organelle

A specialized subunit within a cell that has a specific function.

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Nucleus

An organelle in eukaryotic cells that contains the genetic material.

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DNA

The molecule that carries the genetic instructions for all known organisms.

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The Discovery of Cells

Robert Hooke

• In 1665, Robert Hooke used a microscope to observe a thin slice of cork.

• He saw tiny, box-like compartments which he decided to name ‘cells’.

• What he actually saw were the cell walls of dead plant cells.

Antonie van Leeuwenhoek

• In 1670, Antonie van Leeuwenhoek looked at pond water through his microscope.

• He discovered tiny, living organisms that he called ‘animalcules’ or little animals.

• These were some of the first observations of single-celled organisms.

Multiple Choice

What tool was essential for both Robert Hooke and Antonie van Leeuwenhoek to make their discoveries about cells?

• A microscope

• A telescope

• A magnifying glass

• A ruler

Multiple Choice

What is a key difference between what Robert Hooke observed in cork and what Antonie van Leeuwenhoek observed in pond water?

• Hooke observed living organisms, while Leeuwenhoek observed non-living structures.

• Hooke observed the walls of dead plant cells, while Leeuwenhoek observed living, single-celled organisms.

• Hooke discovered animals, while Leeuwenhoek discovered plants.

• Hooke observed pond water, while Leeuwenhoek observed cork.

Multiple Choice

If Antonie van Leeuwenhoek had used his microscope to look at a thin slice of cork instead of pond water, what would he most likely have observed?

• He would have seen tiny, living 'animalcules'.

• He would have seen box-like compartments from dead cell walls.

• He would have seen nothing because his microscope was different.

• He would have seen the same organisms he saw in pond water.

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Two Main Types of Cells

Prokaryotes

• These simple cells do not have a nucleus or other membrane-bound organelles.

• Their genetic material, or DNA, floats freely inside the cell’s cytoplasm.

• Bacteria are common examples of prokaryotes and have a cell membrane and ribosomes.

Eukaryotes

• These are more complex cells that have a nucleus to store their DNA.

• They have various other membrane-bound organelles, such as mitochondria.

• The cells of plants, animals, protists, and fungi are all eukaryotic.

Multiple Choice

What is the main structural difference between prokaryotic and eukaryotic cells?

• Eukaryotic cells have a nucleus, while prokaryotic cells do not.

• Prokaryotic cells are always larger than eukaryotic cells.

• Eukaryotic cells have cytoplasm, while prokaryotic cells do not.

• Prokaryotic cells contain mitochondria, while eukaryotic cells do not.

Multiple Choice

What is the relationship between a prokaryote's simple structure and the location of its DNA?

• It explains why their DNA is not contained in a nucleus.

• It shows that they are more complex than eukaryotic cells.

• It means they do not have genetic material.

• It proves they must have a cell membrane.

Multiple Choice

A scientist discovers a new organism. Its cells contain a nucleus, mitochondria, and ribosomes. How should this organism's cells be classified?

• As a eukaryote, because it has a nucleus and other membrane-bound organelles.

• As a prokaryote, because it has ribosomes.

• As a eukaryote, because it is a type of bacteria.

• As a prokaryote, because it has a cell membrane.

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One Cell or Many?

Unicellular Organisms

• These living things are made of just one single cell.

• This single cell must perform all the functions necessary for life.

• Common examples of unicellular organisms include bacteria and euglena.

Multicellular Organisms

• These living things are made of many different types of cells.

• In multicellular organisms, different cells have very specific functions.

• For example, humans have nerve cells, muscle cells, and blood cells.

Multiple Choice

What is the fundamental difference between a unicellular and a multicellular organism?

• One is made of a single cell, while the other is made of many cells.

• One lives in water, while the other lives on land.

• One has a nucleus, while the other does not.

• One can move, while the other cannot.

Multiple Choice

How does the way life functions are carried out differ between unicellular and multicellular organisms?

• A single cell performs all functions in a unicellular organism, while different cells have specific functions in a multicellular organism.

• Unicellular organisms do not need energy, while multicellular organisms do.

• Unicellular organisms are all bacteria, while multicellular organisms are all animals.

• Unicellular organisms cannot reproduce, while multicellular organisms can.

Multiple Choice

If a complex multicellular organism, like a human, were made of only one type of cell instead of many specialized types, what would be the most likely result?

• The organism would be much smaller than other humans.

• The organism would not be able to perform all the complex functions necessary for life.

• The organism would be able to live in more extreme environments.

• The organism would have to consume less food to survive.

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Common Misconceptions

Misconception	Correction
All cells are round and look the same.	Cells have different shapes and sizes depending on their job.
All living things are big enough to see.	Many living things, like bacteria, are microscopic and have only one cell.
Prokaryotic cells are completely empty inside.	Prokaryotic cells contain cytoplasm, ribosomes, and genetic material.

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Summary

• The cell is the basic unit of life; organisms are unicellular or multicellular.

• All cells are either prokaryotic (no nucleus) or eukaryotic (with a nucleus).

• All cells have a cell membrane, cytoplasm, and DNA; eukaryotes have organelles.

• In multicellular organisms, cells specialize to perform different functions.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

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