Cells and Cell Theory

Cells and Cell Theory

Middle School

​

Learning Objectives

• Describe the discovery of cells and the key principles of the cell theory.

• Identify the four structures common to all cells and explain their primary functions.

• Differentiate between plant and animal cells, highlighting their unique organelles.

• Explain the difference between unicellular and multicellular organisms.

Key Vocabulary

​

Cell

The most basic unit of structure and function for all known forms of life.

​

Cell Theory

The theory that all life is made of cells, which always come from other cells.

​

Organelle

A specialized structure inside a cell that is responsible for performing a very specific job.

​

Mitochondria

This organelle is the powerhouse of the cell because it generates most chemical energy.

​

Chloroplast

The organelle in plant cells where photosynthesis happens, which is how the plant makes food.

​

Unicellular Organism

A complete living organism that is made up of only one single cell, like bacteria.

​

​

Discovery of Cells and The Cell Theory

• Robert Hooke first observed 'cells' in cork using a microscope in 1665.

• Anton van Leeuwenhoek later discovered bacteria and the first human cells.

• Schleiden and Schwann stated cells are the basic building blocks of all life.

• Rudolf Virchow concluded that all cells are made from other existing cells.

​

Multiple Choice

Which scientist's contribution established the principle that all cells come from pre-existing cells?

• Rudolf Virchow

• Theodor Schwann

• Robert Hooke

• Anton van Leeuwenhoek

​

​

Seeing Inside Cells and Their Common Structures

Seeing Inside Cells

• ​The invention of the light microscope first allowed scientists to discover and see cells.

• ​​To see tiny structures inside a cell, the more powerful electron microscope is needed.

• ​Electron microscopes use a beam of electrons instead of light to reveal cell details.

Common Cell Parts

• ​The plasma membrane is the outer boundary, controlling what enters and leaves the cell.

• ​​Cytoplasm is the jelly-like substance that fills the cell and holds its internal parts.

• ​Cells have ribosomes to make proteins and DNA to hold genetic instructions for the cell.

Multiple Choice

What is the primary function of ribosomes in a cell?

• To make proteins

• To form the outer boundary of the cell

• To hold the cell's genetic instructions

• To fill the cell with a watery substance

​

​

Cell Organelles: The 'Power Plants'

Mitochondria

• These are known as the ‘powerhouses of the cell’ and are found in most eukaryotic cells.

• Their main job is to generate the chemical energy that is needed for the cell’s reactions.

• This energy is essential for the cell to grow, move, and perform all of its functions.

Chloroplasts

• Chloroplasts are special organelles that are found only within the cells of plants and some algae.

• They are the specific site where the important process of photosynthesis happens inside a plant cell.

• These organelles capture light energy from the sun to make food for the plant to survive.

Multiple Choice

Which organelle is called the 'powerhouse of the cell' because it generates chemical energy?

• Nucleus

• Mitochondria

• Ribosome

• Chloroplast

​

​

Organelles for Storage and Cleanup

Vacuoles

• Vacuoles are membrane-bound sacs that act as the storage area for the cell.

• They can store important materials for the cell, like water and food.

• Plant cells are known for having a large central vacuole for storing water.

Lysosomes

• Lysosomes are known as the cell's cleanup crew, breaking down waste materials.

• This organelle is filled with digestive enzymes that break down worn-out cell parts.

• They can also be used to destroy invading viruses and other harmful bacteria.

Multiple Choice

Which organelle's main function is to break down old cell parts and destroy invading bacteria?

• Vacuole

• Lysosome

• Chloroplast

• Mitochondria

​

​

Plant Cells vs. Animal Cells

• Plant cells have a rigid cell wall for structure and protection.

• They contain chloroplasts for photosynthesis and a large vacuole for water storage.

• Animal cells lack a cell wall, chloroplasts, and a large central vacuole.

• They have lysosomes and centrosomes, which are not found in plant cells.

Multiple Choice

What is a key structure found in plant cells but not in animal cells?

• Cytoplasm

• Plasma membrane

• Cell wall

• Mitochondria

​

​

Unicellular vs. Multicellular Organisms

Unicellular Organisms

• These living organisms are made up of only a single cell.

• This one cell is responsible for performing all of life's essential functions.

• Some examples include living things like bacteria, protozoa, and yeast.

Multicellular Organisms

• These are more complex organisms that are made up of many cells.

• Different cells in the body are specialized to do specific jobs.

• Examples of specialized cells include nerve cells, muscle cells, and brain cells.

Multiple Choice

Which of the following is an example of a unicellular organism?

• Human

• Yeast

• Muscle cell

• Plant

​

​

Cell Wall vs. Cell Membrane

Cell Wall

• It is a tough, rigid outer layer found only in plant cells.

• The cell wall provides the cell with structural support and protection.

• It also gives the plant cell a fixed, regular shape.

Cell Membrane

• This flexible barrier is found in both plant and animal cells.

• It surrounds the cell and separates its contents from the outside environment.

• It controls what substances can enter and exit the cell.

Multiple Choice

What is the main difference in function between a cell wall and a cell membrane?

• The cell wall provides rigid support, while the membrane controls passage of materials.

• The cell wall controls passage of materials, while the membrane provides rigid support.

• The cell wall is only in animals, and the membrane is only in plants.

• The cell wall makes energy, while the membrane stores water.

​

Common Misconceptions About Cells

Misconception	Correction
New cells are created from non-living matter.	All cells must arise from pre-existing, living cells.
All living things are visible to the naked eye.	Unicellular organisms, like bacteria, are microscopic.
Plant and animal cells are identical.	Plant cells have a cell wall, chloroplasts, and a large central vacuole.
All cells are the same basic, round shape.	Cells come in many shapes and sizes depending on their function.

Multiple Choice

Why was the invention of the electron microscope a more significant breakthrough for understanding cell structure than the light microscope?

• It allowed scientists to see cells for the first time.

• It allowed scientists to see extremely small structures inside the cell, like organelles.

• It was the first microscope invented.

• It used a beam of light to see living organisms.

Multiple Choice

In the context of cell theory, which of the following is considered a correct statement?

• All inanimate things are made up of cells.

• The cell is the basic unit of structure and function in all living organisms.

• All cells arise from pre-existing cells.

• Cells can be spontaneously generated from non-living matter.

Multiple Choice

A plant is not watered for a long time, causing it to wilt. Which organelle inside its cells has likely shrunk, and what primary function is compromised?

• The chloroplast, affecting photosynthesis.

• The lysosome, affecting waste removal.

• The mitochondria, affecting energy production.

• The central vacuole, affecting water storage and structural support.

Multiple Choice

If a genetic mutation caused a cell's ribosomes to become non-functional, what would be the most immediate and critical consequence for the cell?

• The cell would be unable to store water or nutrients.

• The cell would be unable to produce any new proteins.

• The cell could not generate energy from food.

• The cell would lose its shape and structure.

​

Summary

• Cells are the basic building blocks of all living things.

• All cells come from other cells and have four common parts.

• Plant cells have a cell wall and chloroplasts, but animal cells do not.

• Organisms can be made of a single cell or many cells.

Poll

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

• 1

• 2

• 3

• 4