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Chapter 5 Answers
Energy and Life
Visual Understanding
Figure 5.8
You eat a hamburger. Salivary amylase begins to digest the carbohydrates in the bum
while you are still chewing. Pepsin works in your stomach to digest the protein, and
trypsin is active in your small intestine to break the bonds between specific amino acids.
How does the optimum pH for pepsin and trypsin reflect this chain of events?
Although the pH of your saliva is only very slightly acidic, about pH 6, the pH in your
stomach is very low, about 1 to 2. However as food passes from the stomach into the
upper portion of the small intestine chemicals are added that rapidly raise the pH to
about 6, and then, over time, to about 7 or 8. Thus pepsin, with an optimum pH of 2 to 3,
is active in breaking down large protein molecules in your stomach, and trypsin, with an
optimum pH of 6 to 8, works best in your small intestine, where it breaks bonds between
particular amino acids.
Table 5.1
ATP comes primarily from the breakdown of glucose. If your blood glucose level drops
what sorts of problems can that cause?
Your cells will not be receiving enough glucose to continue a steady supply of ATP.
Probably the first problem you will notice is being tired, your muscles not wanting to
work. However, inside your body, your cells will not be making the new proteins that you
need, activating certain molecules, bringing molecules into cells through active
transport, and moving things around cells along the microtubules. All of these will cause
problems in the long term.
Challenge Questions
Cells and Energy
Living organisms on the surface of the planet and the top layer of the ocean get their
energy from the photosynthetic plants and bacteria that capture the sun’s energy and
transform it to molecules that other organisms can use. At the bottom of some deep
ocean trenches there are entire colonies of many types of organisms. What do they use
for energy?
The heated water, filled with chemicals, that comes from the deep vents provides the
basis for chemosynthetic bacteria to capture energy. Other life forms then live on the
bacteria, and the food web grows from there.
Cell Chemistry
Why do you need enzymes to help your cells do their jobs?
The things that cells do can be done without enzymes, but it would take a very long time.
The enzymes speed things up so that foods are broken down, parts of your body are
repaired and replaced, and new things are made in a timely manner.
Enzymes
What sorts of things can keep an enzyme from doing its job?
If an enzyme is denatured then its bonds are broken and it loses its shape; then it cannot
do its job. This could happen if you had a high temperature, or if your body’s pH was
not correct in the various places where it needs to change. If something is blocking an
enzyme’s active site, or somehow changing the shape of the active site, then the substrate
molecule cannot fit.
How Cells Use Energy
After a rain your new iron fenceposts are rusting. Explain how this is an example of a
redox reaction, just like the ones that occur in your body. Now can you think of a reason
why blood coming from the lungs is bright red?
In the presence of moisture, such as rain, iron will combine with oxygen to form the
bright red compound iron oxide, which we call rust. This oxidized iron has “lost” two
electrons to the oxygen atom. In your red blood cells are molecules called hemoglobin.
At the center of every hemoglobin complex are four iron atoms. When the blood reaches
your lungs it picks up oxygen which attaches to these iron atoms, turning your blood a
brighter red color.