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Cell Structures - We All Defecate

In the alphabet, the basic unit is a letter. In all living things, the basic unit is a cell. The smallest living things are single cells that carry out all of the processes of life by themselves. 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. Sugar is 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 of these things that 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 the vending machines. Plants get their energy differently than 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
You may have noticed that some of your friends can make unusual body noises with unpleasant odors. That is part of getting rid of waste. Plants do the same thing but there is no odor. 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 one kind of waste that we need to get rid of is carbon dioxide which we breathe out. We have other kinds of waste. It is by these products of the cell process that make the odoriferous releases from our bodies. 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:
Does it make sense that cells all use energy and release waste?

What are some other forms of energy?

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 that we can prove how it uses energy is yeast. Yeast 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. It is this carbon dioxide waste that makes the bread rise or have holes in it.

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 timer


  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 and seal.
  7. Roll Ziploc® from top down and measure thickness of Ziploc®
  8. Set stopwatch at zero.
  9. Record measurement.
  10. Continue using your fingers to mix the yeast in the bag.
  11. Record measurement of bag every five minutes.

Sample Data Table:

Time (Minutes) Size (cm)
0 10.5
10 11
20 12
30 13
40 14

Sample Graph:

Safety concerns: Be sure to follow all glassware, heat, and chemical safety rules that are specified by your teacher in all general laboratory experiences. As with all science lab activities, the most important safety rule is to follow all teacher directions.


  1. Why did the water need to be heated?
  2. Why did you mix the bag every few minutes?
  3. What was the evidence that the yeast cells were giving off carbon dioxide as a waste product?
utah state board of education This Sci-ber Text was developed by the Utah State Board of Education and Utah educators.