One of the most fundamental divisions among all living things is based upon where an organism derives its energy. As we know, energy is critical for life. It fuels all actions in a living thing, from movement to sensory processing, to cell repair and growth. And while all organisms must undergo some processes that require energy—not all move and process stimuli, but all living things grow and repair— the source of that energy is a great dividing line amongst living things, dividing the
heterotrophs (who get their food from outside of themselves) from the
autotrophs (who make their own food). The distinction is a relatively clear one most of the time. The question boils down to whether an organism can make its own energy (usually using the sun—see the plant below) or it needs to take in the energy from the world at large (by consuming some food source—see tiger to the left). While it may at first appear to be a simple matter of plant versus animal, it is actually a little more complex than that. There is a plethora of microorganisms (like bacteria, fungi, and protozoans) that are either autotrophic
(usually photosynthetic—using the sun to make sugars for food) or heterotrophic (taking in an energy source from their environment).
Some living things are extremely specific about the type of food source they require in order to fuel the reactions for life. This requirement is not unlike the engines in our cars…we cannot “feed” our cars water, soda, or lemonade if we want them to run properly. Other organisms, on the other hand, will take in whatever is available (think of a vulture or a rat!) in order to obtain the energy needed to fuel their body’s functions.
Once and organism has the energy it needs, the sugar must to be converted into a form that is useable by that organism— typically into a molecule called ATP (adenosine triphosphate). ATP is the molecule responsible for all activity within the cell. In most organisms the conversion from raw energy (sugar) into usable energy (ATP) is called respiration. Don’t confuse the respiration that involves us breathing with the respiration that produces ATP. Keep in mind, though, that in vertebrates, respiration is related to breathing because breathing provides the needed oxygen and helps remove the byproduct carbon dioxide. External
respiration is breathing while aerobic cellular respiration is the conversion of sugar into a usable form in the presence of oxygen.
Some organisms, however, do not require oxygen at all to break down sugar into ATP; these undergo a process known as fermentation. Fermentation requires an anaerobic environment (no oxygen) and produces byproducts that are different from those produced during cellular respiration. One of these byproducts is alcohol, a major ingredient in beer.
Beer is produced by yeast that is undergoing fermentation. Many organisms can do either process, depending on the availability of oxygen or other nutrients. If the required materials are not present, this affects an organism’s ability to grow and sometimes to survive. Factors that can affect an organism’s metabolic pathway include temperature, sugar availability, and the presence or absence of oxygen. All of the processes in an organism, from creating molecules to breaking them down, are collectively known as that organism’s metabolism.

You will be demonstrating the differences in the metabolic rate of yeast at a variety of temperatures and with a variety of energy sources.
 2 3-packs of yeast (rapid-rise—You will need 6 total!)
 3 Styrofoam or plastic cups
 Water
 Thermometer (from your Kit)
 3-17 to 20 oz soda or water bottles (empty and clean!)
 1 T Salt
 1 T Sugar
 1 T “Equal” or “Splenda” or any other sugar substitute (use 3 or 4 packets—pick these up at the store or
“borrow” from your local coffee house or restaurant)
 3 small balloons (just large enough to fit over the mouth of the soda bottles)
Part 1—Yeast and Temperature
1.
Label your cups: a.
Room temp b.
Hot c.
Cold
2.
Fill your “Room temp” cup with about 100 ml of tap water. It should measure about 35º C.
3.
Fill your “Hot” cup with about 100 ml of warm water. Heat water in the microwave at 15 second intervals until it measures about 95 ºC.
4.
Fill your “Cold” cup with about 100 ml of cool water. Add a few ice cubes until it measures about 15 ºC.
5.
Once your cups are full and the water is at about the correct temperatures, add a packet of yeast (Use rapid rise, available in the baking section of the grocery store) to each.
6.
Make an initial observation and record on the Answer Sheet what you see again every 5 minutes for 30 minutes.
7.
You may start Part 2 while Part 1 is reacting, just don’t forget your observations!
Part 2—Yeast and Energy Source
1.
Label your three bottles: a.
Salt b.
Sugar c.
Sugar substitute
2.
Fill each of the bottles with 100 ml water at 35º C.
3.
Add a packet of yeast to each of the three bottles.
4.
Add a tablespoon of salt to the “Salt bottle”, a tablespoon of sugar to your “Sugar” bottle, and a tablespoon of sugar substitute to the “Sugar Substitute” bottle.
5.
Swirl the bottles to be sure the yeast has access to the energy source.
6.
Attach the balloons to the tops of the bottles.
7.
Make an initial observation and record on the Answer Sheet what you see again every 5 minutes for 30 minutes.