Layman D. Biology Demystified: A Self-Teaching Guide
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CHAPTER 4 Chemicals: The Tiniest Blocks 67
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Fig. 4.6 An overview of photosynthesis.
bonds can readily be broken down by glycolysis with the help of enzymes,
releasing lots of free energy to make ATP. During glycolysis, one 6-carbon
glucose molecule is broken down into two 3-carbon pyruvic (pie-ROO-vik)
acid molecules. Overall, as shown in the figure, 2 ATP molecules are produced.
In anaerobic (an-er-OH-bik) catabolism of a glucose molecule, this 6-car-
bon compound is broken down ‘‘without’’ (an) any oxygen from the ‘‘air’’
(aer) being present. During such conditions, the simple process of glucose
decomposing into 2 pyruvic acid molecules (that is, glycolysis) is quickly
followed by the conversion of the pyruvic acids into lactic (LAK-tik) acid
molecules, as shown in the figure overleaf.
Lactic acid is named for its occurrence in sour ‘‘milk’’ (lact). But it is also
produced by muscle and other body cells during anaerobic catabolism of
glucose. When a human being runs very rapidly, for instance, the fibers
(cells) in the leg muscles cannot get oxygen fast enough to allow them to
utilize it for metabolism. Hence, glucose is broken down by anaerobic cata-
bolism. Excess lactic acid molecules quickly accumulate, and they irritate local
nerve endings within the muscle fibers. This creates leg cramping and pain.
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PART 2 Universal Building Blocks of Life
68
When enough O
2
is present within the cell, however, glycolysis is followed
by respiration (res-pir-AY-shun). Respiration literally translates from Latin as
the ‘‘process of breathing’’ (spir), which is appropriate when it applies to the
entire human or animal body. In the biology of individual heterotroph cells,
however, respiration has a far different meaning. It is cellular respiration, not
the process of breathing, itself, that is directly tied to cell metabolism. Cellular
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4, Disorder
respiration is the breakdown or catabolism of organic molecules (such as
carbohydrates, lipids, or proteins) within the cell, and in the presence of oxy-
gen. Cellular respiration is therefore often called aerobic (air-OH-bik) respira-
tion. This is because the process involves the breakdown of organic molecules
within cells when oxygen from the ‘‘air’’ (aero-) is present.
Cellular respiration (aerobic respiration within cells) most often involves
the catabolism of glucose or other simple sugars. Cellular respiration, then,
starts up where the process of glycolysis stops. Glycolysis always provides the
cell with a net of 2 ATP molecules/1 glucose molecule catabolized. Aerobic
respiration provides the cell with many, many more.
To discuss respiration, we must go where it occurs – within the mitochon-
drion. You may recall (from Chapter 3) that the mitochondria (my-toe-
KAHN-dree-uh) are the organelles where the cell’s aerobic (oxygen-using)
metabolism takes place. The mitochondria are literally ‘‘thread’’ (mito-)
‘‘granules’’ (chondr). This is evident from an examination of Figure 4.7.
Some mitochondria are long and slender, like threads. Others are short
and rounded, much like tiny granules. But whatever their shape, these mito-
chondria receive the two pyruvic acids produced from glycolysis.
Each pyruvic acid then enters the Krebs cycle. Named after the German
biochemist, Hans Krebs, the Krebs cycle is a repeating cycle of aerobic reac-
tions that break down the pyruvic acids produced by glycolysis. The enzymes
that run the reactions of the Krebs cycle are located in the middle cavity of each
mitochondrion. The Krebs cycle rotates twice (once for each of the pyruvic
acids fed into it). Along the way, a net total of 2 ATPs, several CO
2
molecules,
and a number of hydrogen-carrier molecules result. The hydrogen-carrier mole-
cules, produced by the Krebs cycle then move onto the electron transport
system.
The electron transport system is located along the cristae (KRIS-tee), the
inner ‘‘crests’’ or ‘‘ridges’’ of the mitochondrion. The large molecules of the
electron transport system, as their name indicates, carry high-energy elec-
trons from H atoms down to lower and lower energy levels, releasing con-
siderable amounts of energy along the way. A net total of 34 more ATPs are
produced from the electron transport process. Finally, the transported elec-
trons, now depleted of most of their former energy, are transferred to an
oxygen atom. The oxygen atom then combines with two H
þ
ions, thereby
creating water (H
2
O).
The overall equation for cellular (aerobic) respiration, using glucose as the
fuel molecule, is:
C
6
H
12
O
6
þ 6O
2
! 6CO
2
þ 6H
2
O þ 36 ATP
Glucose Oxygen Carbon dioxide Water Free energy
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PART 2 Universal Building Blocks of Life
70
CHAPTER 4 Chemicals: The Tiniest Blocks 71
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Fig. 4.7 An overview of cellular respiration (aerobic respiration).
Capsulizing this section, we have:
Glycolysis alone (anaerobic conditions) ! 2 ATPs/glucose
Glycolysis þ cellular respiration (aerobic conditions) ! 38 ATPs/glucose
SUMMARY
Let us now put together what we have learned about metabolism in plants
(autotrophs) versus animals (heterotrophs). Photosynthesis uses CO
2
and
H
2
O, but gives off O
2
. Cellular respiration consumes the O
2
given off during
photosynthesis, and gives off CO
2
. These CO
2
molecules are then picked up
by plant cells and consumed during photosynthesis. By this exchange of
chemicals, photosynthesis feeds cellular respiration what it needs to proceed,
while cellular respiration returns the favor. And so the Chemical Balance of
Life between plants and animals, autotrophs and heterotrophs, goes on and
on and on, generating and maintaining Earth’s biosphere for hundreds of
thousands of years.
Quiz
Refer to the text in this chapter if necessary. A good score is at least 8 correct
answers out of these 10 questions. The answers are listed in the back of this
book.
1. Chemical bonds are:
(a) Ways in which Biological Disorder is promoted
(b) The means by which plants grow without need for any energy
(c) Chemical linkages that serve as temporary storers of potential
energy
(d) Always due to a net loss of outermost electrons between different
atoms
2. The four major types of chemical body-builders identified in this
chapter are:
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PART 2 Universal Building Blocks of Life
72
2, Web
(a) Proteins, lipids, glucose, and benzene
(b) Lipids, fats, proteins, and carbohydrates
(c) Nucleic acids, inorganic salts, proteins, and lipids
(d) Carbohydrates, proteins, lipids, and nucleic acids
3. DNA and RNA molecules:
(a) Are nucleic acids found in all living organisms
(b) May help build proteins only in certain plants and fungi
(c) Are both located as sections of genes within cell nuclei
(d) Neither help nor hinder the synthesis of cell proteins
4. Kinetic energy:
(a) Represents free energy that is immediately available to do work
(b) Is basically the same thing as potential energy
(c) Is seldom useful for animals, just for green plants
(d) Is the form of energy locked up in the foods humans eat
5. Anabolism and catabolism differ in that:
(a) Anabolism is synthesis, whereas catabolism is breakdown
(b) Catabolism is synthesis, whereas anabolism is breakdown
(c) One involves only organic molecules; the other, only inorganic
molecules
(d) Anabolism involves splitting of ATP, but catabolism never does
6. The overall equation for photosynthesis reveals that:
(a) Both oxygen and carbon dioxide are consumed, while H
2
Ois
released
(b) Glucose and CO
2
are the ultimate products
(c) Sunlight provides the energy to produce carbon dioxide
(d) Carbon dioxide and water are the inputs, glucose and oxygen the
outputs
7. The biosphere:
(a) Counterbalances the oxygen consumption of plants with carbon
dioxide released from animals
(b) Represents the portion of planet Earth supporting living organisms
within ecological systems
(c) Has no need for photosynthesis, as long as the sun is still shining
(d) Utilizes the blue-green algae as its sole free-energy source
8. In heterotrophs:
(a) Organic foodstuffs are consumed to obtain fuel molecules for ATP
production
(b) Glycolysis is always the sole source of free energy for metabolism
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(c) Energy from sunlight is utilized to agitate the electrons in
chlorophyll
(d) ATP is split by ATPase enzyme, thereby consuming too much free
energy
9. Under anaerobic conditions, 1 molecule of eaten glucose:
(a) Ultimately results in 2 molecules of lactic acid
(b) Immediately enters the Krebs cycle
(c) Results in a total of 38 ATP molecules released for cell work
(d) Provides free electrons for the cristae
10. These cellular reactions occur only within the mitochondria:
(a) Photosynthesis by activation of chlorophyll
(b) Both glycolysis and aerobic respiration
(c) Aerobic respiration, Krebs cycle, and electron transport system
(d) Calvin and Krebs cycles in heterotrophs
The Giraffe ORDER TABLE for Chapter 4
(Key Text Facts About Biological Order Within An Organism)
1. ____________________________________________________________
2. ____________________________________________________________
3. ____________________________________________________________
4. ____________________________________________________________
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PART 2 Universal Building Blocks of Life
74
The Dead Giraffe DISORDER TABLE for Chapter 4
(Key Text Facts About Biological Disorder Within An Organism)
1. ____________________________________________________________
2. ____________________________________________________________
3. ____________________________________________________________
4. ____________________________________________________________
The Spider Web ORDER TABLE for Chapter 4
(Key Text Facts About Biological Order Beyond the Individual Organism)
1. ____________________________________________________________
2. ____________________________________________________________
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76
CHAPTER
5
Cells: The ‘‘Little
Chambers’’ in Plants
and Animals
We have already encountered cells (Chapter 2) as Level V within the Pyramid
of Life. Figure 5.1 reminds us that this is the so-called ‘‘Life-line,’’ meaning
that the cell represents the lowest living level of biological organization.
Review of this figure also reveals that Level I (subatomic particles), II
(atoms), III (molecules), and IV (organelles), are included within the cellular
level. Cellular literally ‘‘pertains to a cell or little cell.’’
The actual word, cell, in turn, comes from Latin and means ‘‘a chamber.’’
This terminology arises from the early observation of dead cork under the
microscope. Cork is actually the lightweight outer bark of the cork oak tree.
When viewed through the microscope, dead cork is shown to consist of a
complex latticework of hollow chambers with stiff walls. Each dark, hollow
chamber in cork bark, then, is a type of non-living cell. But in general,
however, the cell is defined as a living, tiny chamber that is surrounded by
a thin membrane and contains various organelles.
1, Order
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