In this lesson students recognise the relative sizes of cells, develop an understanding of cell theory, learn how to make a scientific drawing of a cell and relate food-handling regulations to cell theory.
Ask students what a ‘cell’ is. Collate their ideas onto the whiteboard.
Provide each student with a piece of paper. Ask them if their paper is as big as a cell. Ask students to rip up their paper into the size of a single cell. Compare students’ results. Ask why they made their paper the size they did. Ask students to use a ruler to make a small square of paper 1 mm x 1 mm. Suggest to students that 100 animal cells or 10 000 bacterial cells can fit on top of that 1 mm x 1 mm piece of paper.
- Suggest to students that cells can be different sizes and shapes. Demonstrate this by using different building blocks. (Flat blocks are good for covering the surface of a structure; fat blocks are used for filling in a space.) Inform students that different types of cells can be used for different purposes. Use the building blocks to make different objects (eg an animal and a flower).
- Ask students to draw around the different cell shapes on the worksheets Cells online worksheet: cell size 1 and Cells online worksheet: cell size 2.
Teacher note: Ask students to keep these worksheets, as they will be used again in lesson 4.
- Ask students to choose one of the cells and make a scientific drawing of that cell.
- Remind students to draw a large diagram (one-quarter of a page) using a sharp, grey lead pencil. The diagram should have a title and the magnitude, or display the relative size of the cell. The diagram should also be dated. Students should draw what they see, not what they expect to see. Few cells are perfectly round or square.
- Watch the animated video The wacky history of cell theory.
- Ask students to draw up a timeline in their books and use the worksheet History of cells timeline to fill in the timeline.
- Ask students to consider the significance of the discovery of the microscope and what they think was known about cells before the microscope.
- Students complete the worksheet Recipe for mice.
Discuss with students how scientists may have disagreed with the recipe for mice in the past. How would a scientist test this theory? How do they do it today? Ask students to research how Francesco Redi tested the theory of spontaneous generation and provided evidence for cell theory.