Cell Theory and Types of Cells

HSC Biology | Free Study Notes

This topic matters because it explains why cells are the basic unit of life and helps you compare the two main cell types you need to know, prokaryotic and eukaryotic cells.

In this lesson

• what cell theory states

• what prokaryotic cells are

• what eukaryotic cells are

• key similarities between all cells

• the main differences between prokaryotic and eukaryotic cells

What is cell theory?

Cell theory is a scientific idea that explains the basic role of cells in living things.

The three main points of cell theory

• All living things are made of one or more cells.

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

• All cells come from pre-existing cells.

These points are important because they link all organisms, from bacteria to humans, to the same basic biological unit, the cell.

Why cell theory matters

Cell theory helps explain why:

• unicellular organisms can carry out all life processes in one cell

• multicellular organisms depend on many specialised cells working together

• growth and repair happen through cell division

What are prokaryotic cells?

Prokaryotic cells are simple cells that do not have a nucleus.

Key features of prokaryotic cells

• usually smaller than eukaryotic cells

• no membrane-bound nucleus

• DNA is found free in the cytoplasm

• no membrane-bound organelles

• usually have circular DNA

• examples include bacteria and archaea

What this means in practice

Because prokaryotic cells do not have membrane-bound organelles, their internal structure is simpler. They still carry out all the processes needed for life, but they do so without the same level of compartmentalisation seen in eukaryotic cells.

What are eukaryotic cells?

Eukaryotic cells are more complex cells that do have a nucleus.

Key features of eukaryotic cells

• usually larger than prokaryotic cells

• DNA is enclosed in a nucleus

• contain membrane-bound organelles

• often have linear DNA

• examples include plant, animal, fungal and protist cells

Common organelles in eukaryotic cells

You do not need every detail here, but you should know that eukaryotic cells may contain organelles such as:

• nucleus

• mitochondria

• ribosomes

• endoplasmic reticulum

• Golgi apparatus

Plant cells also have structures such as:

• chloroplasts

• a large central vacuole

• a cell wall

Similarities between prokaryotic and eukaryotic cells

Even though they are different, both cell types share some basic features because both are cells.

Features both have

• cell membrane

• cytoplasm

• ribosomes

• DNA

• 
  

These shared features show that all cells must:

• separate themselves from the external environment

• contain genetic information

• make proteins

• carry out life processes

Differences between prokaryotic and eukaryotic cells

Main structural differences

Feature	Prokaryotic cells	Eukaryotic cells
Nucleus	No nucleus	Nucleus present
DNA	Free in cytoplasm, usually circular	In nucleus, usually linear
Membrane-bound organelles	Absent	Present
Size	Smaller	Larger
Complexity	Simpler	More complex

A common syllabus-level difference is that prokaryotic DNA is circular and found in the cytoplasm, while eukaryotic DNA is linear and found in the nucleus.

Why these differences matter

The main advantage of eukaryotic cells is compartmentalisation. Membrane-bound organelles allow different processes to happen in specific parts of the cell, which makes the cell more specialised and efficient.

Prokaryotic cells are simpler, but they are still very successful organisms and can reproduce and survive in many environments.

Cell theory and cell types together

Cell theory tells us that all living things are made of cells. Studying prokaryotic and eukaryotic cells then helps us see that cells can be simple or complex, but all still carry out the essential functions of life.

That is why this topic sits near the start of Module 1. It gives you the foundation for later work on membranes, transport, enzymes and cellular processes.

Worked example

Exam-style question

Explain two differences and one similarity between prokaryotic and eukaryotic cells.

Worked answer

A similarity is that both prokaryotic and eukaryotic cells contain DNA and ribosomes.One difference is that prokaryotic cells do not have a nucleus, while eukaryotic cells do.A second difference is that prokaryotic cells do not contain membrane-bound organelles, while eukaryotic cells contain organelles such as mitochondria.

Why this works

This answer:

• gives exactly what the question asks for

• uses clear biological terms

• makes direct comparisons

Common mistakes

• Saying prokaryotic cells have no DNA. They do have DNA, it is just not enclosed in a nucleus.

• Saying all cells have a cell wall. Animal cells do not.

• Confusing ribosomes with membrane-bound organelles. Ribosomes are not membrane-bound.

• Forgetting that both prokaryotic and eukaryotic cells have a cell membrane and cytoplasm.

• Writing that viruses are cells. Viruses are not cells.

Quick quiz

1. State the three parts of cell theory.

2. What is the main difference between a prokaryotic cell and a eukaryotic cell?

3. Name two structures found in both prokaryotic and eukaryotic cells.

4. Where is DNA found in a prokaryotic cell?

5. Give one example of a eukaryotic organism.

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