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Eukaryotic and Prokaryotic Cells
Eukaryotic and Prokaryotic Cells
This lesson covers the two fundamental types of cell — eukaryotic and prokaryotic — as required by the Edexcel GCSE Biology specification (1BI0), Topic 1: Key Concepts in Biology. You need to be able to describe the structures found in both cell types and explain their functions. Understanding the differences between these cell types is essential for the exam.
What Are Cells?
All living organisms are made up of cells. The cell is the basic structural and functional unit of life. Some organisms consist of only one cell (unicellular, e.g. bacteria and some protists), while others are made of many cells (multicellular, e.g. animals and plants).
There are two main categories of cell:
- Eukaryotic cells — complex cells with a nucleus and membrane-bound organelles.
- Prokaryotic cells — simpler, smaller cells with no true nucleus.
Exam Tip: The Edexcel specification expects you to know the key differences between eukaryotic and prokaryotic cells. A comparison question is very common on Paper 1.
Eukaryotic Cells
Eukaryotic cells are found in animals, plants, fungi and protists. The word "eukaryotic" means "true nucleus" — these cells contain a nucleus enclosed by a nuclear membrane (nuclear envelope). They also contain other membrane-bound organelles such as mitochondria and, in plant cells, chloroplasts.
Eukaryotic cells are typically 10–100 μm in diameter (animal cells are usually around 10–30 μm).
Animal Cell Structure
An animal cell contains the following sub-cellular structures (organelles):
| Organelle | Structure | Function |
|---|---|---|
| Nucleus | Large, spherical, surrounded by a nuclear membrane | Contains the cell's genetic material (DNA) arranged into chromosomes. Controls cell activities and cell division. |
| Cell membrane | Thin, flexible phospholipid bilayer | Controls what enters and leaves the cell. It is partially permeable (selectively permeable). |
| Cytoplasm | Jelly-like substance filling the cell | Where most chemical reactions take place. Contains enzymes that catalyse metabolic reactions. |
| Mitochondria | Small, oval-shaped with a folded inner membrane (cristae) | The site of aerobic respiration, where energy is transferred from glucose. Produces ATP. |
| Ribosomes | Very small structures (about 20–25 nm), found free in the cytoplasm or on rough endoplasmic reticulum | The site of protein synthesis — amino acids are assembled into proteins here. |
Exam Tip: Never say mitochondria "produce energy". Energy cannot be created or destroyed. The correct phrasing is: mitochondria are the site of aerobic respiration, where energy is transferred (or released) from glucose.
Plant Cell Structure
Plant cells are also eukaryotic. They contain all of the organelles found in animal cells, plus three additional structures:
| Organelle | Structure | Function |
|---|---|---|
| Cell wall | Rigid outer layer made of cellulose | Provides structural support and protection. Prevents the cell from bursting when it takes in water by osmosis. It is freely permeable (allows all molecules through). |
| Permanent vacuole | Large, central, fluid-filled sac | Contains cell sap (a dilute solution of sugars, mineral salts and sometimes pigments). Maintains turgor pressure, keeping the cell firm. |
| Chloroplasts | Green, disc-shaped organelles containing membranes (thylakoids) | Contain the green pigment chlorophyll, which absorbs light energy for photosynthesis. Carbon dioxide and water are converted into glucose and oxygen. |
Key Points
- Animal cells have an irregular (rounded) shape. Plant cells tend to have a regular, rectangular shape due to the cell wall.
- Not all plant cells contain chloroplasts — only cells in green parts of the plant (e.g. palisade mesophyll in leaves). Root cells do not photosynthesise and lack chloroplasts.
- Mitochondria are found in both animal and plant cells because all living cells need to respire.
- Ribosomes in eukaryotic cells are 80S ribosomes (larger than prokaryotic ribosomes).
Summary Table: Animal Cells vs Plant Cells
| Feature | Animal Cell | Plant Cell |
|---|---|---|
| Nucleus | ✓ | ✓ |
| Cell membrane | ✓ | ✓ |
| Cytoplasm | ✓ | ✓ |
| Mitochondria | ✓ | ✓ |
| Ribosomes (80S) | ✓ | ✓ |
| Cell wall (cellulose) | ✗ | ✓ |
| Permanent vacuole | ✗ (may have small temporary vacuoles) | ✓ (large, central) |
| Chloroplasts | ✗ | ✓ (in green parts only) |
Prokaryotic Cells
Prokaryotic cells are found in bacteria (and archaea). The word "prokaryotic" means "before nucleus" — these cells do not have a true nucleus. Their genetic material is not enclosed in a nuclear membrane.
Prokaryotic cells are much smaller than eukaryotic cells, typically 1–5 μm in diameter — roughly one-tenth the size of a typical animal cell.
Structure of a Bacterial Cell
| Structure | Description | Function |
|---|---|---|
| Cell membrane | Phospholipid bilayer | Controls what enters and leaves the cell. |
| Cell wall | Made of peptidoglycan (not cellulose) | Provides structural support and protection. |
| Cytoplasm | Jelly-like substance | Where chemical reactions occur. |
| Ribosomes | Smaller than eukaryotic ribosomes (70S vs 80S) | Site of protein synthesis. |
| Chromosomal DNA | A single, circular loop of free DNA (not enclosed in a nucleus) | Carries the main genetic information for the cell. |
| Plasmids | Small, extra circles of DNA | Carry additional genes (e.g. antibiotic resistance genes). Can be transferred between bacteria. |
| Flagellum (plural: flagella) | A long, whip-like tail | Used for movement — rotates to propel the bacterium. Not all bacteria have flagella. |
| Capsule / slime layer | A sticky outer coating (in some bacteria) | Provides extra protection and helps the bacterium attach to surfaces. |
Key Features of Prokaryotic Cells
- No nucleus — DNA is free in the cytoplasm as a single circular chromosome.
- No membrane-bound organelles — no mitochondria, no chloroplasts, no endoplasmic reticulum.
- Plasmids are unique to prokaryotes and are important in genetic engineering and antibiotic resistance.
- Prokaryotic ribosomes are smaller (70S) than eukaryotic ribosomes (80S).
Exam Tip: Do not say prokaryotic cells have "no DNA". They do have DNA — it is just not enclosed within a nucleus. Say they have "no true nucleus" or "DNA is not enclosed in a nuclear membrane".
Eukaryotic vs Prokaryotic Cells — Comparison
| Feature | Eukaryotic Cell | Prokaryotic Cell |
|---|---|---|
| Size | Larger (10–100 μm) | Smaller (1–5 μm) |
| Nucleus | True nucleus with nuclear membrane | No true nucleus — free DNA in cytoplasm |
| DNA | Linear chromosomes inside the nucleus | Single circular DNA loop + plasmids |
| Membrane-bound organelles | Present (mitochondria, chloroplasts, etc.) | Absent |
| Ribosomes | Larger (80S) | Smaller (70S) |
| Cell wall | Present in plants (cellulose); absent in animals | Present (peptidoglycan) |
| Flagella | Some cells (e.g. sperm) | Some bacteria |
| Plasmids | Not normally present | Present |
| Examples | Animal cells, plant cells, fungi | Bacteria |
Size and Scale
Understanding the relative sizes of cells is important:
| Object | Approximate Size |
|---|---|
| Eukaryotic cell (animal) | 10–30 μm |
| Eukaryotic cell (plant) | 10–100 μm |
| Prokaryotic cell (bacterium) | 1–5 μm |
| Virus (for comparison — not a cell) | 20–300 nm |
| Ribosome (eukaryotic) | ~25 nm |
| Ribosome (prokaryotic) | ~20 nm |
Unit Conversions
You need to be confident converting between units of length:
- 1 mm = 1000 μm (millimetres to micrometres — multiply by 1000)
- 1 μm = 1000 nm (micrometres to nanometres — multiply by 1000)
- 1 mm = 1,000,000 nm (millimetres to nanometres — multiply by 1,000,000)
To convert the other way (smaller to larger), divide by 1000.
Worked Example:
A bacterial cell is 2.5 μm long. What is this in nanometres?
2.5 μm × 1000 = 2500 nm
What is 2.5 μm in millimetres?
2.5 μm ÷ 1000 = 0.0025 mm
Exam Tip: Unit conversions are tested frequently. Remember: mm → μm → nm, each step is ×1000. Going the other way (nm → μm → mm) is ÷1000. Write the conversion chain and work through it step by step.
Worked Example: Comparing Cells
Question: A student observes an animal cell that is 20 μm in diameter and a bacterium that is 2 μm in length. Calculate how many times larger the animal cell is compared to the bacterium.
Answer:
Size ratio = size of animal cell ÷ size of bacterium
Size ratio = 20 μm ÷ 2 μm = 10 times larger
Exam Tip: When comparing sizes, make sure both measurements are in the same units before dividing. Always show your working clearly.
Summary
- Eukaryotic cells (animal and plant) have a nucleus, membrane-bound organelles (mitochondria, ribosomes), and are 10–100 μm in size.
- Plant cells additionally have a cellulose cell wall, a permanent vacuole (cell sap), and chloroplasts (photosynthesis).
- Prokaryotic cells (bacteria) have no true nucleus, no membrane-bound organelles, smaller 70S ribosomes, a single circular DNA loop, plasmids, and a peptidoglycan cell wall. They are 1–5 μm in size.
- Eukaryotic ribosomes are 80S; prokaryotic ribosomes are 70S.
- Know the unit conversion chain: mm → μm → nm (×1000 each step).