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In the alphabet the basic unit is a letter. In living things the basic unit is a cell. The smallest living things that carry out all of the processes of life by themselves are cells. Two of the basic functions of cells are getting energy and getting rid of waste.

Getting Energy

When you are getting a little sleepy during the day, what do you do? Many people grab a candy bar or a soda. Why? One reason is they both contain sugar. We call this energy. Animals obtain energy by eating things. Anything that animals eat is digested and converted to sugars, the basic source of energy. Whatever you eat for energy gets converted to sugar. Now, we also need other things, like proteins, fats, vitamins, minerals, and fiber. All the things we eat and need are actually used at the level of a cell. Each cell in your body needs energy, and when you eat foods your body breaks it down into smaller and smaller pieces until your cells can bring them into your body through the process of osmosis. This entire process of getting food and breaking it down and your cells using the energy is called digestion and respiration.

Plants also need energy, but you don't often meet a sunflower getting a candy bar at a vending machine. Plants get their energy differently from animals. Plants have a special chemical in their cells called chlorophyll. This chemical, along with a cell organelle called a chloroplast, allows plants to use carbon dioxide, energy from sunlight, and water to make sugar. This process is called photosynthesis.

Getting rid of waste

All organisms have natural processes for getting rid of waste. When plants go through the process of photosynthesis, they give off oxygen as part of their waste. We use oxygen as we go through the process of respiration and get rid of is carbon dioxide by breathing out. Each cell in our body goes through these processes of obtaining energy and releasing waste. Our circulatory system helps move cell wastes so they can be removed by our kidneys and removed from our bodies.

Discussion ideas

Describe how cells use energy and release waste?

Can you think of any living things that don’t use energy?


At this point you should remember that the cell is an important part of how organisms stay alive. One organism we can observe using energy is yeast, which extracts energy from sugar and produces carbon dioxide (CO2) gas as a waste product.

Have you ever baked bread? Did you ever wonder what it is that makes the bread rise? It's the yeast. Now ... what is it that makes the yeast grow? It's the sugar. Yeast cells extract the energy from the sugar. In the process the cells produce carbon dioxide waste. This carbon dioxide waste makes the bread rise, leaving small holes throughout.


Materials List:

  • Heat source
  • Balance
  • Weighing paper
  • Beaker (to heat water)
  • Thermometer
  • String
  • Scissors
  • Metric Ruler
  • Ziploc Sandwich Bag
  • 2 grams of yeast (room temperature)
  • 10 grams of sugar
  • 50 mL warm (38 degrees Celsius) water
  • 50 mL graduated cylinder
  • Stop watch, clock, or watch
Safety concerns: iconiconicon Be sure to keep all glassware, heat, and chemical safety rules. As with all science lab activities, the most important safety rule is to follow all teacher directions.


  1. Heat water to 38 degrees Celsius
  2. Measure yeast and place in Ziploc® bag
  3. Measure sugar and place in Ziploc® bag
  4. Measure water and place in Ziploc® bag
  5. Mix yeast, water and sugar with fingers in bottom of Ziploc®.
  6. Push air out of Ziploc® bag an seal
  7. Roll Ziploc® from top down and measure thickness of ziploc
  8. Start stopwatch at zero ( watch clock carefully so timing is accurate.)
  9. Record measurements.
  10. Continue using your fingers to mix the yeast mixture in bag.
  11. Record measurement of bag every 5 minutes

Click to see sample data and a sample graph.


  1. Why did the water need to be heated?
  2. Why did you mix the bag every few minutes?
  3. What was your evidence that the yeast cells were giving off carbon dioxide as a waste product?

Review science safety rules here.

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Teachers should view the Teacher Site Map to relate Sci-ber text and the USOE 7th grade science core.

Updated October 24, 2008 by: Glen Westbroek

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