--- title: "How small is a cell?" subtitle: "Year 8 Biology · Cells · Lesson 1" --- # How small is a cell? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** For thousands of years nobody knew that every living thing is made of cells. Then, in the 1660s, someone looked at a thin slice of cork through an early microscope. :::teacher-note **In one sentence:** Students describe the scale of cells relative to organisms and familiar objects, and explain why microscopes were needed to discover and observe them (AC9S8U01, node 1). **Students should be able to say:** I can explain why cells can't be seen without a microscope. I can compare a cell's size with something familiar. I can explain why microscopes were needed to discover cells. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage :::keep-together **About how many cells are in your body?** ☐ About 37 trillion (37 000 000 000 000) ☐ About 37 thousand ☐ Just a few, very large ones ☐ About 37 million My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** About 37 trillion. A typical human cell is about 0.02 mm across, so even a speck of skin you can just see holds dozens of cells. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: too small to see :::keep-together ### Too small to see **Why can't you see your own skin cells?** ☐ Each is about 0.02 mm across, smaller than the 0.1 mm or so the eye can see ☐ They are hidden under a layer of dead skin ☐ They are see-through ::: :::teacher-note **Answer:** Each is about 0.02 mm across, smaller than the 0.1 mm or so the eye can see Your eye can just make out things about 0.1 mm across (the width of a hair). Most cells are about 2 to 10 times smaller, and bacteria about 50 times smaller. ::: :::keep-together ### Fifty in a millimetre **A cell is 0.02 mm wide. How many cells, side by side, would stretch across 1 mm?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 50 1 ÷ 0.02 = 50 cells in one millimetre. ::: :::keep-together ### Magnification **A microscope magnifies 400 times. A cell 0.02 mm wide looks how many millimetres wide?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 8 0.02 × 400 = 8 mm: big enough to see clearly. ::: :::keep-together ### With or without **Can you see it without a microscope?** Groups: Yes, No: you need a microscope - An ant (about 3 mm): {{field-15}} - A grain of sand (about 0.5 mm): {{field-15}} - A human hair's width (about 0.1 mm): {{field-15}} - A human cheek cell (about 0.05 mm): {{field-15}} - A red blood cell (about 0.008 mm): {{field-15}} - A bacterium (about 0.002 mm): {{field-15}} ::: :::teacher-note **Answer:** An ant (about 3 mm) → Yes; A grain of sand (about 0.5 mm) → Yes; A human hair's width (about 0.1 mm) → Yes; A human cheek cell (about 0.05 mm) → No: you need a microscope; A red blood cell (about 0.008 mm) → No: you need a microscope; A bacterium (about 0.002 mm) → No: you need a microscope ::: :::keep-together ### Why the 1600s **Why were cells only discovered in the 1600s?** ☐ Microscopes good enough to magnify them were only made then ☐ People weren't interested before then ☐ Living things didn't have cells before then ::: :::teacher-note **Answer:** Microscopes good enough to magnify them were only made then Robert Hooke saw tiny boxes in cork in 1665 and called them cells; Antonie van Leeuwenhoek saw living single cells soon after. New technology made new evidence possible. ::: :::typical ## Elaborate: a whole living thing in one cell :::keep-together ### Smaller still **A bacterium is about 0.002 mm long and is a whole living thing. How does it compare with one of your cells?** ☐ The same size, because all cells are the same size ☐ About ten times smaller, yet it feeds, grows and reproduces on its own ☐ Bigger, because it is a whole organism ::: :::teacher-note **Answer:** About ten times smaller, yet it feeds, grows and reproduces on its own Bacteria are among the smallest living things. Next lesson: every living thing, from bacteria to you, is made of cells. ::: ::: :::wider ## Elaborate: Hooke and Leeuwenhoek :::keep-together ### Two discoveries **Hooke called the boxes in cork 'cells' after small rooms. Leeuwenhoek's better lenses showed living things moving in pond water. What did the two discoveries together suggest?** ☐ Only plants are made of cells ☐ Cells are only found in dead things ☐ Living things are made of tiny units, and some living things are just one of them ::: :::teacher-note **Answer:** Living things are made of tiny units, and some living things are just one of them Later scientists (Schleiden, Schwann and others, in the 1800s) brought this evidence together as cell theory. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - Most cells are about 0.01 to 0.1 mm across; bacteria are smaller still. The eye can't see anything smaller than about 0.1 mm. - Your body has about 37 trillion cells. - A microscope **magnifies**: a cell 0.02 mm wide magnified 400 times looks 8 mm wide. - Cells were discovered only when microscopes were invented (Hooke, 1665; Leeuwenhoek soon after). ## Exit ticket :::keep-together **Why couldn't anyone in 1500 have known that plants are made of cells?** {{lines-3}} ::: :::teacher-note **Answer:** Cells are smaller than the eye can see (about 0.01 to 0.1 mm), and microscopes strong enough to magnify them hadn't been invented. :::