SC.6.L.14.4: Compare and contrast the structure and function of major organelles…

Cells: The Smallest Living Things Grades 7–8

How small is a cell, how does water get in, what's inside, and why are cells so tiny? Four lessons with potatoes, strawberries and gelatin that work with a microscope or without one.

• How big is a cell? 50–60 min

  Students measure tiny things they can see, count how many fit across a millimetre, and build a scale ladder from a hand down to a single cell. With a microscope or a phone and a drop of water, they look at real onion cells and estimate their size.

• How do things get in and out of a cell? Two sessions: 45 min, then 30 min the next day

  Students weigh potato strips, soak them in plain and salty water, and measure how their mass and stiffness change. They use the results to explain diffusion and osmosis, and why a wilted salad goes limp.

• Can you pull DNA out of a strawberry? 50–60 min

  Students mash strawberries with dish soap and salt, filter the juice, and watch a cloudy white tangle of DNA appear when the teacher layers cold rubbing alcohol on top. They explain what each ingredient did to the cell, and test what happens without soap.

• Why are you made of many small cells, not one big one? 50–60 min, with cubes soaking for at least an hour

  Students soak gelatin cubes of 1, 2 and 3 cm in food colouring, cut them open and measure how far the colour got in. They calculate surface area to volume and explain why cells stay small.

Printable practice for SC.6.L.14.4

• Cells: The Smallest Living Things workbook included with the unit

  3 practice pages, 3 readings, 2 English-support pages, 1 word wall, 1 unit review, 1 unit test, with answer keys.

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