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1.3 Eukaryotic and Prokaryotic Cells

GCSE Biology · AQA · Chapter 1: Cell Structure and Transport

A free GCSE Biology lesson for AQA (8461) from OBA School: how bacterial cells differ from animal, plant and fungal cells, and comparing sizes using orders of magnitude. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe the features of eukaryotic and prokaryotic cells
  • Label the structures of a bacterial cell and say what each one does
  • Explain how the genetic material of a bacterium is arranged
  • Compare the sizes of cells using orders of magnitude

Key words

  • Eukaryotic cell: A cell whose genetic material is enclosed in a nucleus. Animal, plant, fungal and protist cells are eukaryotic.
  • Prokaryotic cell: A cell with no nucleus. Its genetic material is free in the cytoplasm. Bacteria are prokaryotes.
  • Plasmid: A small ring of DNA found in many bacteria, in addition to the main loop. It often carries useful genes, such as antibiotic resistance.
  • Flagellum (plural flagella): A long whip-like structure that some bacteria spin to move.
  • Order of magnitude: A factor of 10. Something 100 times bigger is two orders of magnitude bigger (10²).

Two kinds of cell

Every living cell has a cell membrane, cytoplasm and genetic material. Cells are sorted into two groups by where the genetic material is kept.

  • Examples; Eukaryotic cells animal, plant, fungal (e.g. yeast) and protist cells; Prokaryotic cells bacteria
  • Genetic material; Eukaryotic cells in chromosomes, enclosed in a nucleus; Prokaryotic cells a single loop of DNA free in the cytoplasm, plus plasmids
  • Size; Eukaryotic cells usually 10 to 100 µm; Prokaryotic cells much smaller, usually 0.5 to 5 µm
  • Cell wall; Eukaryotic cells plants and algae (cellulose); fungi (chitin); none in animals; Prokaryotic cells yes, but not made of cellulose
  • Mitochondria, chloroplasts; Eukaryotic cells yes (chloroplasts in plants and algae only); Prokaryotic cells no
  • Ribosomes; Eukaryotic cells yes; Prokaryotic cells yes (smaller ones)

Key fact: The word prokaryotic means “before nucleus”. If you see a nucleus, the cell is eukaryotic.

Inside a bacterial cell

  • Cell wall; Function Supports the cell and stops it bursting. Not made of cellulose.
  • Cell membrane; Function Controls what enters and leaves the cell.
  • Cytoplasm; Function Where the cell’s chemical reactions happen.
  • Single loop of DNA; Function Carries most of the genes that control the cell. Not in a nucleus.
  • Plasmids; Function Small extra rings of DNA. Can be passed between bacteria.
  • Ribosomes; Function Make proteins.
  • Flagellum; Function Spins to move the bacterium (only some bacteria).

Exam tip: Bacteria have no mitochondria. Respiration happens in their cytoplasm and on their cell membrane.

Comparing sizes: orders of magnitude

To compare two sizes, put them in the same unit and divide. Then find the nearest power of 10. Each power of 10 is one order of magnitude.

Worked example: orders of magnitude

A bacterium is 1 µm long. A plant cell is 0.1 mm long.
Same units: 0.1 mm = 100 µm.
100 ÷ 1 = 100 = 10²

Answer: the plant cell is 2 orders of magnitude bigger

Worked example: real size from a micrograph

In an electron micrograph at ×20 000, a bacterium is 36 mm long.
Real size = 36 ÷ 20 000 = 0.0018 mm = 1.8 µm
In metres: 1.8 × 10⁻⁶ m

Answer: 1.8 µm

Common mistakes

  • Saying bacteria have a nucleus. Their DNA is free in the cytoplasm.
  • Saying bacterial cell walls are made of cellulose. Only plant and algal cell walls are cellulose.
  • Thinking yeast is a prokaryote. Yeast is a fungus, so its cells are eukaryotic.
  • Mixing up plasmids (small extra DNA rings) with the main loop of DNA.
  • Dividing sizes in different units when working out how many times bigger one cell is.

Revision questions

Try these, then check your answers below each one.

Question 1. Which of these is a prokaryotic cell? (Choose from: Bacterium, Yeast cell, Onion cell, Red blood cell.)

Answer: Bacterium

Working: Bacteria are prokaryotes. Yeast, onion and blood cells are eukaryotic (yeast is a fungus).

Question 2. True or false? Prokaryotic cells contain mitochondria.

Answer: False

Working: False. Bacteria have no mitochondria, chloroplasts or nucleus.

Question 3. A bacterium is 2 µm long. A cheek cell is 60 µm across. How many times bigger is the cheek cell?

Answer: 30

Working: 60 ÷ 2 = 30 times bigger.

Related lessons

Continue in the full course

This lesson is section 1.3 (AGCBH0103) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.

 

 

 

 

 

 

 

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