Biology 12

82 MHR • Unit 1 Metabolic Processes

pyruvate — to make ATP. The organisms that use

aerobic cellular respiration also rely, to a certain

extent, on the anaerobic process of lactic acid

fermentation. The fermentation of lactic acid can

continue to provide muscles with energy in ATP

molecules when oxygen is not available.

In the next section, you will learn that green

plants also use aerobic cellular respiration to

convert pyruvate to ATP molecules. However,

unlike animals, plants must first manufacture

glucose through photosynthesis. Although often

considered the reverse of aerobic cellular

respiration, photosynthesis involves many different

enzymes and metabolic pathways to produce

glucose. You will study photosynthesis in some

detail in the next section.

SECTION REVIEW

1. Explain how enzymes within the mitochondrion
   catalyze metabolic reactions that involve the products
   of glycolysis.


2. How many turns of the citric acid cycle are
   required to catabolize one molecule of glucose?


3. Draw a diagram that shows three possible
   reaction pathways for pyruvate following glycolysis.
   Identify the main products of each reaction.


4. Under what conditions does fermentation take
   place in an animal muscle cell? Explain.


5. The conversion of pyruvate to lactic acid does
   not produce any ATP. How, then, does this reaction
   contribute to the production of energy by a cell?


6. Working with a partner or in a small group, use
   an analogy to explain the role of the mitochondrion in
   cellular respiration. Outline how various details in the
   analogy relate to various components of respiration in
   the mitochondrion.


7. In your local grocery store, compare the prices
   of foods that are rich in fat with those rich in
   carbohydrates. What general pattern can you see?
   How could you explain this pattern in terms of cellular
   metabolism?


8. A baker wishes to make a loaf of bread.
   According to the recipe, she should first prepare a
   yeast culture by mixing some dried yeast with warm
   water and a little sugar. The other ingredients are
   added to this mixture later.
   (a)Draw a diagram that illustrates the process of
   respiration taking place in the yeast cells. Why is
   the yeast necessary for the bread to rise?
   (b)If a strain of yeast existed that employed lactic
   acid fermentation, could this yeast be used in
   place of ordinary baker’s yeast? Explain.
   9. A strain of cells undergoes a mutation that
   increases the permeability of the inner mitochondrial
   membrane to hydrogen ions.
   (a)What effect would you expect this mutation to
   have on the process of cellular respiration?
   (b)Assuming the mutant cells can survive, how might
   the metabolic requirements of these cells differ
   from those of a non-mutant strain of the same
   variety?


9. The “four-minute mile” is often cited as an
   example of the limit of physical performance. That is,
   no matter how much athletes train, there will always
   be a limit to their endurance.
   (a)Why does this limit exist?
   (b)Design an experiment that you could conduct to
   test your hypothesis.


10. Crash diets that focus on highly regimented
    eating routines often produce yo-yo syndrome, in
    which weight lost is quickly regained. In many cases,
    dieters gain back more weight than they lost. How
    does an understanding of basal metabolic rate
    explain this? What kind of advice would help
    someone in this situation?

As you have learned, different types of cells can use

different types of cellular respiration. What experiment

could you perform to distinguish between yeast and

muscle cells in an oxygen-free environment?

UNIT INVESTIGATION PREP

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