Mini-Lesson

Unity & Diversity: Cells (HL)

Cells share a deep common ancestry. This HL lesson covers A2.1 Origins of cells (HL), A2.2 Cell structure and A2.3 Viruses (HL).

You will explore how the first cells arose, compare prokaryotic and eukaryotic structure, and examine viruses and their replication.

Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you are ready.

A2.1 Origins of cells

How cells began (HL)

Life probably arose from simple molecules forming under early-Earth conditions, assembling into protocells with a boundary membrane.

The RNA world hypothesis: early life may have used RNA, which can both store information and catalyse reactions.
All life shares a last universal common ancestor (LUCA), reflected in the universal genetic code.

The endosymbiotic theory explains eukaryotic mitochondria and chloroplasts as once free-living bacteria engulfed by a host cell.

A2.2 Cell structure

Prokaryotic and eukaryotic cells

Prokaryotes: no nucleus; DNA as a loop in a nucleoid; 70S ribosomes; often a cell wall and plasmids.

Eukaryotes: a membrane-bound nucleus and organelles (mitochondria, ER, Golgi); 80S ribosomes.

magnification = image size ÷ real sizereal size = image size ÷ magnification

Calculate

Your turn — calculate

1A bacterium is 2 µm long. On a micrograph it measures 30 mm. Calculate the magnification (convert to the same unit first).

×

Hint: 30 mm = 30 000 µm; magnification = 30 000 ÷ 2.

A2.3 Viruses

Viruses (HL)

Viruses are acellular: a nucleic acid genome (DNA or RNA) inside a protein capsid, sometimes with an envelope. They are obligate intracellular parasites — they can only replicate inside a host cell.

Lytic cycle: replicate and burst the host.
Lysogenic cycle: integrate into the host genome and lie dormant.
Retroviruses (e.g. HIV) use reverse transcriptase to make DNA from their RNA.

Viruses are enormously diverse and evolve rapidly, raising debate about whether they count as living.

Calculate

Your turn — calculate

2A virus capsid image is 45 mm across at a magnification of ×900000. Calculate its real diameter in nanometres (1 mm = 1 000 000 nm).

nm

Hint: real = 45 ÷ 900 000 = 0.00005 mm, then ×1 000 000 to get nm.

Sort it

Sort each item into its group

Sort each feature by the type of biological entity.

Prokaryotic cell

Eukaryotic cell

Virus

Quick check

Question

?Evidence that mitochondria arose by endosymbiosis includes that they have...

Quick check

Question

?Viruses are described as obligate intracellular parasites because they...

Quick check

Question

?The RNA world hypothesis proposes that early life could use RNA because RNA can...

Match it

Match each item to its partner

Match each concept to its correct description.

Item

Partner

Quick check

Question

?Electron microscopes reveal far more sub-cellular detail than light microscopes because they have much greater...

Recap

The big ideas to take away

A2.1 (HL): protocells, RNA world, LUCA and endosymbiosis explain cell origins

A2.2: prokaryotes (nucleoid, 70S) vs eukaryotes (nucleus, 80S)

A2.3 (HL): viruses are acellular obligate parasites; lytic/lysogenic; retroviruses

Skills: magnification = image / real; resolution beats magnification for detail

Press Finish to see your score.

🏆

Mini-lesson complete!

⭐⭐⭐

You have worked through Theme A: Unity & Diversity — Cells for IB Diploma Biology. 🎉

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