Year 7 Science · Unit 1 · Lesson 6
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Learning Goals
1. In 1855 Virchow wrote the Latin sentence “Omnis cellula e cellula”. What does it mean in English, and which tenet of cell theory is it?
Order the discoveries
Number the discoveries from 1 (earliest) to 5 (latest). Then name the scientist who made each one.
| Step number | Discovery | Scientist |
|---|---|---|
| A Dutch cloth merchant grinds a glass bead into a lens and sees tiny living things in rainwater. | ||
| A German doctor writes that a new cell can only come from a cell that already exists. | ||
| Another German scientist studies many animal samples and finds cells in every one. | ||
| An English scientist looks at a thin slice of cork and names the tiny boxes he sees. | ||
| A German scientist studies many plant samples and concludes that all plants are made of cells. |
Which tenet?
Tenet 1: all living things are made of one or more cells. Tenet 2: the cell is the basic unit of life. Tenet 3: all cells come from existing cells.
2. For each observation, write the tenet it best supports and one reason.
(a) A graze on your knee heals over two weeks as new skin cells grow.
(b) A student checks 50 different plant and animal samples under a microscope and finds cells in every one.
(c) An amoeba is only one cell, yet it moves, feeds and grows on its own.
(d) A yeast cell in bread dough splits in two, making two yeast cells.
Cause and effect: from Virchow to the doctor
Complete the chain to show why Virchow’s rule changed what doctors do. Write in the empty boxes.
Read the data
Investigation: can cells appear out of broth?
Before 1855, some scientists believed cells could appear out of broth or other fluids on their own. A class tests this idea. They boil the same broth in eight flasks to kill any cells in it. Four are sealed straight away (A) and four are left open (B). On days 1, 3, 5 and 7 they open one sealed flask, take one open flask, and look at a drop from each under a microscope.
| Day | A flask (sealed until that day) | B flask (open to the air) |
|---|---|---|
| 1 | none | none |
| 3 | none | a few |
| 5 | none | many |
| 7 | none | very many |
Source: sample class results.
3. Name the variable the class changed, the variable they observed, and one variable they kept the same.
4. Do the results support the idea that cells appear out of broth, or Virchow’s tenet? Use both sets of flasks in your answer, and suggest where the cells in the B flasks came from.
5. Predict: if doctors still believed that cells appear out of fluid, how might our understanding of disease be different today?
6. A classmate says: “Cell theory was invented by one brilliant scientist who figured it all out.” Using evidence from this lesson, explain why this statement is incorrect.
Question 1
“Every cell comes from a cell” (1); tenet 3: cells come from existing cells (1). [2 marks]
Order the discoveries
Reading down the table as printed, the correct step numbers are 2, 5, 4, 1, 3. In order: 1 Hooke, cork; 2 Leeuwenhoek, living things in water; 3 Schleiden, plants; 4 Schwann, animals; 5 Virchow, cells from cells.
Question 2
(a) 3: new skin cells come from existing cells dividing. (b) 1: every living sample had cells. (c) 2: one cell is enough to be alive (accept 1: it is made of one cell). (d) 3: new yeast cells come from an existing cell splitting. [4 marks]
Cause and effect
Box 1: so anything wrong in a body, such as a tumour, must have started in cells that were already there. Box 2: so disease is cells behaving badly, and looking at the cells can show the cause. [2 marks]
Question 3
Changed: sealed or open to the air (1). Observed: cells seen in a drop (1). Same: the broth, boiling, days or microscope (1). [3 marks]
Question 4
Virchow’s tenet (1). The sealed A flasks never had cells, though their broth was the same (1); the B flasks’ cells most likely came from microbes in the air (1). [3 marks]
Question 5
Disease could appear from nothing (1), so doctors would not take biopsies (1). [2 marks]
Question 6
Marking guide: 1 position; up to 3 for scientists and what each added (Hooke, Leeuwenhoek, Schleiden, Schwann, Virchow); 1 building on earlier work; 1 about 200 years, as microscopes improved and evidence built up. [6 marks]
Model answer: Wrong. Hooke named cells, Leeuwenhoek saw living ones, Schleiden and Schwann found cells in plants and animals, and Virchow added cells come from cells. Each scientist built on earlier work: Leeuwenhoek’s better lens showed living cells Hooke’s could not, and Virchow corrected Schleiden’s idea that cells form from fluid. It took about 200 years because microscopes had to improve and evidence had to build up.
Common error: giving Schleiden the third tenet; he thought cells formed from fluid.