Download Objectives Compare and contrast how autotrophs and heterotrophs

Objectives
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Compare and contrast how autotrophs and heterotrophs obtain food.
Explain how cellular respiration harvests the energy in food.
Key Terms
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autotroph
photosynthesis
producer
heterotroph
consumer
cellular respiration
Look up on a clear day, and you will see the sun burning brightly almost 150 million
kilometers away. A tiny fraction of the light energy radiated from the sun reaches
Earth's surface. And a tiny fraction of that sunlight powers most of the planet's life.
This is true because certain organisms can convert the energy of sunlight to the
chemical energy in food—sugars and other organic molecules.
Obtaining Food
All organisms need food for energy and building materials. Biologists classify
organisms according to how they obtain food.
Autotrophs An organism such as a plant that makes its own food is called an
autotroph, which means "self-feeder" in Greek. Starting with inorganic molecules,
autotrophs make organic molecules. For example, plants use the sun's energy to
convert water and carbon dioxide into sugars. This process is called photosynthesis
(from the Greek photo- meaning "light," and synthesis meaning "making
something"). You will read more about photosynthesis in Chapter 8.
Autotrophs are also called producers because they produce the organic molecules
that serve as food for the organisms in their ecosystem. On land, plants are the major
producers. In oceans, lakes, and streams, producers include algae and certain
photosynthetic bacteria.
Heterotrophs Organisms that cannot make their own food, such as humans, are called
heterotrophs, meaning "other eaters." Heterotrophs, also called consumers, must
obtain food by eating producers or other consumers. Heterotrophs depend on
producers to supply energy and materials for life and growth. Since most producers
depend on sunlight as their energy source, you could say that life on Earth is solarpowered.
Harvesting the Energy in Food
As you have read, plants and certain other producers use light energy to make
organic molecules. These organic molecules are a source of energy and building
materials for organisms.
Many organisms, including both producers and consumers harvest the energy stored
in foods through cellular respiration. Cellular respiration is a chemical process that
uses oxygen to convert the chemical energy stored in organic molecules into another
form of chemical energy—a molecule called adenosine triphosphate (ATP). Cells in
plants and animals then use ATP as their main energy supply. You will read about
cellular respiration and ATP in more detail later in this chapter.
The processes of photosynthesis and cellular respiration recycle a common set of
chemicals: water, carbon dioxide, oxygen, and organic compounds such as glucose.
The diagram in Figure 7-3 visually summarizes this chemical recycling.
Figure 7-3
The products of photosynthesis are the
chemical ingredients for cellular respiration,
while the products of cellular respiration are the
chemical ingredients for photosynthesis.
Water and carbon dioxide are the raw ingredients for photosynthesis. Plants use
energy from sunlight to rearrange the atoms of water and carbon dioxide, producing
glucose and oxygen. Oxygen is used by both plant and animal cells during cellular
respiration to release the energy stored in glucose. The released energy enables cells
to produce ATP. Cellular respiration also produces carbon dioxide and water. The
result is a continual cycling of these chemical ingredients.
Objectives
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Distinguish between kinetic and potential energy.
Explain what chemical energy is and how cells release it from food.
Define calories and kilocalories as units of energy.
Key Terms
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kinetic energy
potential energy
thermal energy
chemical energy
calorie
If you've ever felt tired or groggy from being hungry, you probably know that you
need food for energy. But how does your body use the energy stored in food? To
understand this, you first need to know some basic facts about energy.
Introduction to Energy
Energy is the ability to perform work. In the physical science sense of the word,
work is performed whenever an object is moved against an opposing force. For
example, your leg muscles do work when you climb the steps to the top of a water
slide—your legs move your body against the opposing force of gravity.
The two basic forms of energy are kinetic energy and potential energy. While you
climb the stairs, you have kinetic energy, the energy of motion. Anything that is
moving has kinetic energy. In fact, the word kinetic comes from a Greek word
meaning "motion." Once you reach the top of the stairs and are standing still, you
have low kinetic energy. Where has the energy gone? Although it is not possible to
destroy or create energy, energy can be converted from one form to another. By
climbing the stairs, your body converted kinetic energy to potential energy. Potential
energy is energy that is stored due to an object's position or arrangement. As you
climb higher against the force of gravity, your body gains potential energy due to its
position—its higher location. The potential energy is converted back to kinetic
energy as you move down the slide.
What becomes of the energy once you have splashed into the pool and come to a
stop? As you go down the slide, your body collides with air and water molecules,
increasing their motion. And when you splash into the pool, the rest of your motion
is transferred to the water. The air and water molecules transfer their motion in
random directions as they collide again and again. This type of kinetic energy—
random molecular motion—is called thermal energy. (Thermal energy that is
transferred from a warmer object to a cooler one is referred to as heat.) Overall, your
trip down the slide converted potential energy into the directed kinetic energy of
your sliding body, and then into the random kinetic energy of molecular motion
(thermal energy). You cannot retrieve this thermal energy and put it to work again.
So, to climb the stairs again, you need a fresh supply of energy. That energy is
provided by food.
Chemical Energy
How do the organic compounds in food provide energy for a climb up a water slide?
Just like the molecules in gasoline and other fuels, these organic compounds have a
form of potential energy called chemical energy. In the case of chemical energy, the
potential to perform work is due to the arrangement of the atoms within the
molecules. Put another way, chemical energy depends on the structure of molecules.
Organic molecules such as the carbohydrates, fats, and proteins you learned about in
Chapter 5 have structures that make them especially rich in chemical energy (Figure
7-5).
Figure 7-5
The stored chemical energy of foods such as peanuts can
be released through cellular respiration.
Figure 7-6 compares potential energy due to position and chemical structure
(chemical energy). In the case of potential energy due to position, the potential
energy is converted to kinetic energy in the forms of motion and thermal energy
(which is released to the surroundings as heat). In the case of chemical energy, the
rearrangement of atoms during chemical reactions releases the potential energy. This
energy is then available for work such as contracting a muscle.
Figure 7-6
Potential energy can be converted to other types of energy.
Putting Chemical Energy to Work
The organic molecules in food are high in chemical energy, just as the organic
molecules in gasoline are. Cells and automobile engines make chemical energy
available for work through similar processes. In both cases, a complex molecule is
broken into smaller molecules that have less chemical energy than the original
substance (Figure 7-7).
Figure 7-7
Both engines and cells use oxygen to convert the potential energy in complex
molecules to energy that can be used for work.
An automobile engine is called an internal combustion (burning) engine. This type of
engine mixes oxygen with gasoline in a very fast chemical reaction that results in the
molecules of gasoline breaking down. The reaction releases thermal energy as heat,
which is then used to power the car. The main waste products emitted from the
automobile's exhaust pipe are carbon dioxide and water. Only about 25 percent of the
energy from the gasoline is converted into the car's kinetic energy (motion). The rest
is lost to the surroundings as heat, which explains why it gets very hot under the
hood of a running car.
Within your cells, organic molecules such as glucose also react with oxygen in the
process of cellular respiration. And similar to an automobile engine, working cells
produce carbon dioxide and water as their "exhaust." Fortunately, the "burning" is
much slower in your cells than in an automobile engine. Your cells are also more
efficient than automobile engines—they convert about 40 percent of the energy from
food into useful work. The other 60 percent of the energy is converted to thermal
energy, which is lost from your body in the form of heat.
The heat generated by cellular respiration is not completely wasted. Retaining some
of this heat enables your body to maintain a constant temperature, even when the
surrounding air is cold. When you are sitting still in class, you radiate about as much
heat as a 100-watt lightbulb. You've probably experienced the discomfort of this heat
while sitting in a closed room crowded with other "human lightbulbs." When you
exercise, your cells increase the rate of cellular respiration. This is why you feel
warm after exercise such as running or inline skating. Excess heat is lost through
sweating and other cooling methods, much as a car's radiator keeps the engine from
overheating.
Calories: Units of Energy
You have probably heard the term calorie used to refer to food or exercise. A calorie
is the amount of energy required to raise the temperature of 1 gram (g) of water by 1
degree Celsius (°C). However, a calorie is such a tiny unit of energy that it is not
very practical for measuring the energy content of food. Instead, people usually
express the energy in food in kilocalories. One kilocalorie (kcal) equals 1,000
calories. The "calories" shown on a food label are actually kilocalories.
You can actually measure the energy content of a food such as a peanut in the
laboratory. First you dry the peanut and burn it under an insulated container of water.
Burning the peanut converts its stored chemical energy to thermal energy, releasing
heat. By measuring the increase in water temperature and using the definition of a
calorie, you can calculate the number of calories in the peanut. One peanut has about
5,000 calories, or 5 kcal. That is enough chemical energy to raise the temperature of
1 kilogram (1,000 g) of water by 5°C. Of course, your cells don't burn molecules
from peanuts or other foods with a flame. Cells use enzymes to break down organic
molecules through the more controlled process of cellular respiration. As a result, the
released energy is easier to manage for work. As shown in Figure 7-8, just a handful
of peanuts provides enough fuel to power an hour-long walk.
Objectives
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Describe the structure of ATP and how it stores energy.
Give examples of work that cells perform.
Summarize the ATP cycle.
Key Term
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ATP
It's a good thing that food doesn't fuel your cells by burning like the torched peanut
described in Concept 7.2. In fact, the carbohydrates, fats, and proteins obtained from
food do not drive work in your cells in any direct way. The chemical energy stored in
these compounds must first be converted to energy stored in another molecule.
How ATP Packs Energy
As you read in Chapter 6, ATP stands for adenosine triphosphate. The "adenosine"
part consists of a nitrogen-containing compound called adenine and a five-carbon
sugar called ribose (Figure 7-9). The triphosphate "tail" consists of three phosphate
groups. The tail is the "business" end of ATP—it is the source of energy used for
most cellular work.
Figure 7-9
An ATP molecule contains potential energy, much like a compressed spring.
When a phosphate group is pulled away during a chemical reaction, energy is
released.
Each phosphate group is negatively charged. Because like charges repel, the
crowding of negative charge in the ATP tail contributes to the potential energy stored
in ATP. You can compare this to storing energy by compressing a spring. The tightly
coiled spring has potential energy. When the compressed spring relaxes, its potential
energy is released. The spring's kinetic energy can be used to perform work such as
pushing a block attached to one end of the spring.
The phosphate bonds are symbolized by springs in Figure 7-9. When ATP is
involved in a chemical reaction that breaks one or both of these phosphate bonds,
potential energy is released. In most cases of cellular work, only one phosphate
group is lost from ATP. Then the tail of the molecule has only two phosphate groups
left. The resulting molecule is called adenosine diphosphate, or ADP.
ATP and Cellular Work
During a chemical reaction that breaks one of ATP's bonds, the phosphate group is
transferred from ATP to another molecule. Specific enzymes enable this transfer to
occur. The molecule that accepts the phosphate undergoes a change, driving the
work.
Your cells perform three main types of work: chemical work, mechanical work, and
transport work (Figure 7-10). An example of chemical work is building large
molecules such as proteins. ATP provides the energy for the dehydration synthesis
reaction that links amino acids together. An example of mechanical work is the
contraction of a muscle. In your muscle cells, ATP transfers phosphate groups to
certain proteins. These proteins change shape, starting a chain of events which cause
muscle cells to contract. An example of transport work is pumping solutes such as
ions across a cellular membrane. Again, the transfer of a phosphate group from ATP
causes the receiving membrane protein to change shape, enabling ions to pass
through.
Figure 7-10
The energy in ATP drives three main types of
cellular work.
The ATP Cycle
ATP is continuously converted to ADP as your cells do work. Fortunately, ATP is
"recyclable." For example, ATP can be restored from ADP by adding a third
phosphate group (Figure 7-11). Like compressing a spring, adding the phosphate
group requires energy. The source of this energy is the organic molecules from food.
Thus, ATP operates in a cycle within your cells. Work consumes ATP, which is then
regenerated from ADP and phosphate.
Figure 7-11
ATP is constantly recycled in your cells.
The ATP cycle churns at an astonishing pace. A working muscle cell recycles all of
its ATP molecules about once each minute. That's 10 million ATP molecules spent
and regenerated per second! The next concept focuses on how your cells keep pace
with this incredible demand for ATP.
Objectives
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Relate breathing and cellular respiration.
Summarize the cellular respiration equation.
Tell how "falling" electrons are a source of energy.
Explain the role of electron transport chains.
Key Terms
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aerobic
electron transport chain
You have read that cells, like automobile engines, use oxygen in the process of
breaking down fuel. The cell's living version of internal combustion is cellular
respiration. Cellular respiration converts the energy stored in food to energy stored in
ATP. But how is oxygen involved?
Relationship of Cellular Respiration to Breathing
Cellular respiration is an aerobic process, meaning that it requires oxygen. You have
probably heard the word respiration used to describe breathing. Although breathing
for a whole organism is not the same as cellular respiration, the two processes are
related (Figure 7-12).
Figure 7-12
Breathing supports cellular respiration by providing
the body with oxygen and removing carbon dioxide.
During cellular respiration, a cell exchanges two gases with its surroundings. The
cell takes in oxygen and releases carbon dioxide. Your bloodstream delivers oxygen
to cells and carries away carbon dioxide. The process of breathing results in the
exchange of these gases between your blood and the outside air. This exchange takes
place in tiny air sacs in your lungs. Oxygen in the air you inhale diffuses from your
lungs across the lining of the air sacs and into your bloodstream. The carbon dioxide
diffuses from your blood across the air sacs' lining and into your lungs. From there it
is exhaled.
Overall Equation for Cellular Respiration
Glucose is a common fuel for cellular respiration. Figure 7-13 shows the overall
equation of what happens to glucose during cellular respiration. The series of arrows
indicates that cellular respiration consists of many chemical steps, not just a single
chemical reaction.
Figure 7-13
In cellular respiration, the atoms in glucose and oxygen are
rearranged, forming carbon dioxide and water. The cell uses the
energy released to produce ATP.
Cellular respiration's main function is to generate ATP for cellular work. In fact, the
process can produce up to 38 ATP molecules for each glucose molecule consumed.
Notice that cellular respiration also transfers hydrogen and carbon atoms from
glucose to oxygen atoms, thus forming water and carbon dioxide. Now, take a closer
look at how these events release energy.
"Falling" Electrons as an Energy Source
Why does the process of cellular respiration release energy? For an analogy, recall
the water slide. At the top, your potential energy is high. As you are pulled down the
slide by the force of gravity, the potential energy is converted to kinetic energy.
Similarly, an atom's positively charged nucleus exerts an electrical "pull" on
negatively charged electrons. When an electron "falls" toward the nucleus, potential
energy is released.
Here is how "falling" electrons relate to cellular respiration. Oxygen attracts
electrons very strongly, similar to how gravity pulls objects downhill. In fact, oxygen
is sometimes called an "electron grabber." In contrast to oxygen, carbon and
hydrogen atoms exert much less pull on electrons. A sugar molecule has several
carbon-hydrogen bonds. During cellular respiration, the carbon and hydrogen atoms
change partners and bond with oxygen atoms instead. The carbon-hydrogen bonds
are replaced by carbon-oxygen and hydrogen-oxygen bonds. As the electrons of
these bonds "fall" toward oxygen, energy is released. If you burn sugar in a test tube,
this reaction happens very quickly, releasing energy in the form of heat and light
(Figure 7-14). In your cells, however, the "burning" happens in controlled steps.
Some of the released energy is used to generate ATP molecules instead of being
converted to heat and light.
Figure 7-14
When sugar is burned, oxygen
atoms pull electrons from carbon
and hydrogen, forming new
chemical bonds. Burning releases
energy in the form of heat and light.
Electron Transport Chains
Compared with burning, cellular respiration is a more controlled fall of electrons—
more like a step-by-step "walk" of electrons down an energy staircase. Instead of
releasing energy in a burst of flame, cellular respiration unlocks the energy in
glucose in small amounts that cells can put to productive use—the formation of ATP
molecules.
In contrast to burning, where oxygen reacts directly with glucose, cellular respiration
involves breaking down glucose in several steps. Oxygen only enters as an electron
acceptor in the final electron transfer. During the breakdown, molecules called
electron carriers accept many of the high-energy electrons from the glucose
molecule. The electron carriers pass the electrons on to other carriers in a series of
transfers called an electron transport chain (Figure 7-15). Each carrier holds the
electrons more strongly than the carrier before it. At the end of the chain, oxygen—
the electron grabber—pulls electrons from the final carrier molecule and joins them
with hydrogen ions, forming water.
Figure 7-15
An electron transport chain is like a staircase—as
electrons move down each step in the chain, a
small amount of energy is released.
As electrons undergo each transfer in the chain, they release a little energy. The cell
has a mechanism that traps this released energy and uses it to make ATP. You will
read about these events in more detail in Concept 7.5.
Objectives
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Describe the structure of a mitochondrion.
Summarize the three stages of cellular respiration and identify where ATP is
made.
Key Terms
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metabolism
glycolysis
Krebs cycle
ATP synthase
While the above sentence summarizes the outcome of cellular respiration, the
process actually consists of more than two dozen chemical reactions. Many of the
reactions take place in specialized organelles—mitochondria.
Structure of Mitochondria
Mitochondria (singular, mitochondrion) are found in almost all eukaryotic cells. A
mitochondrion's structure is key to its role in cellular respiration. An envelope of two
membranes encloses the mitochondrion (Figure 7-16). There is a space between the
outer and inner membranes. The highly folded inner membrane encloses a thick fluid
called the matrix. Many enzymes and other molecules involved in cellular respiration
are built into the inner membrane. The complex folding pattern of this membrane
allows for many sites where these reactions can occur. This maximizes the
mitochondrion's ATP production.
Figure 7-16
Mitochondria are the sites of two stages of cellular respiration.
A Road Map for Cellular Respiration
Cellular respiration is one type of chemical process that takes place in cells. All
together, a cell's chemical processes make up the cell's metabolism. Because cellular
respiration consists of a series of reactions, it is referred to as a metabolic pathway. A
specific enzyme catalyzes (speeds up) each reaction in a metabolic pathway.
Figure 7-16 is a simplified "road map" of cellular respiration. You can use the
diagram to follow glucose through the metabolic pathway of cellular respiration. The
three main stages are color-coded: glycolysis (green), the Krebs cycle (purple), and
electron transport and ATP synthase (gold). The road map also shows where in your
cells each stage occurs.
Stage I: Glycolysis
The first stage in breaking down a glucose molecule, called glycolysis, takes place
outside the mitochondria in the cytoplasm of the cell. The word glycolysis means
"splitting of sugar." Figure 7-17 illustrates this process. In the figure, note that
glucose is shown as a chain of six carbon atoms (represented by gray balls) rather
than as a ring. This allows you to focus on the number of carbon atoms in each
molecule.
Figure 7-17
A cell invests two ATP molecules to break down glucose. The products of
glycolysis are two pyruvic acid molecules, two NADH molecules and four ATP
molecules.
Using two ATP molecules as an initial "investment," the cell splits a six-carbon
glucose molecule in half. The result is two three-carbon molecules, each with one
phosphate group. Each three-carbon molecule then transfers electrons and hydrogen
ions to a carrier molecule called NAD+. Accepting two electrons and one hydrogen
ion converts the NAD+ to a compound called NADH. (The yellow dots in the NADH
symbols represent electrons.) The next step is the "payback" on the ATP
investment—four new ATP molecules are produced, a net gain of two ATP
molecules.
In summary, the original glucose molecule has been converted to two molecules of a
substance called pyruvic acid. Two ATP molecules have been spent, and four ATP
molecules have been produced. The pyruvic acid molecules still hold most of the
energy of the original glucose molecule.
Stage 2: The Krebs Cycle
This stage is named for the biochemist Hans Krebs, who figured out the steps of the
process in the 1930s. The Krebs cycle finishes the breakdown of pyruvic acid
molecules to carbon dioxide, releasing more energy in the process. The enzymes for
the Krebs cycle are dissolved in the fluid matrix within a mitchondrion's inner
membrane.
Recall that glycolysis takes place outside the mitochondrion and produces two
pyruvic acid molecules. These pyruvic acid molecules do not themselves take part in
the Krebs cycle. Instead, after diffusing into the mitochondrion, each three-carbon
pyruvic acid molecule loses a molecule of carbon dioxide. The resulting molecule is
then converted to a two-carbon compound called acetyl coenzyme A, or acetyl CoA.
This acetyl CoA molecule then enters the Krebs cycle, as shown in Figure 7-18. In
the Krebs cycle, each acetyl CoA molecule joins a four-carbon acceptor molecule.
The reactions in the Krebs cycle produce two more carbon dioxide molecules and
one ATP molecule per acetyl CoA molecule. However, NADH and another electron
carrier called FADH2 trap most of the energy. At the end of the Krebs cycle, the
four-carbon acceptor molecule has been regenerated and the cycle can continue.
Figure 7-18
Since glycolysis splits glucose into two pyruvic acid molecules, the Krebs cycle
turns twice for each glucose molecule.
As you have read, glycolysis produces two pyruvic acid molecules from one glucose
molecule. Each pyruvic acid molecule is converted to one acetyl CoA molecule.
Since each turn of the Krebs cycle breaks down one acetyl CoA molecule, the cycle
actually turns twice for each glucose molecule, producing a total of four carbon
dioxide molecules and two ATP molecules.
Stage 3: Electron Transport Chain and ATP Synthase Action
The final stage of cellular respiration occurs in the inner membranes of
mitochondria. This stage has two parts: an electron transport chain and ATP
production by ATP synthase.
First, the carrier molecule NADH transfers electrons from the original glucose
molecule to an electron transport chain (Figure 7-19). As you read in Concept 7.4,
electrons move to carriers that attract them more strongly. In this way the electrons
move from carrier to carrier within the inner membrane of the mitochondria,
eventually being "pulled" to oxygen at the end of the chain. There the oxygen and
electrons combine with hydrogen ions, forming water.
Each transfer in the chain releases a small amount of energy. This energy is used to
pump hydrogen ions across the membrane from where they are less concentrated to
where they are more concentrated. This pumping action stores potential energy in
much the same way as a dam stores potential energy by holding back water.
The energy stored by a dam can be harnessed to do work (such as generating
electricity) when the water is allowed to rush downhill, turning giant wheels called
turbines. Similarly, your mitochondria have protein structures called ATP synthases
that act like miniature turbines. Hydrogen ions pumped by electron transport rush
back "downhill" through the ATP synthase. The ATP synthase uses the energy from
the flow of H+ ions to convert ADP to ATP. This process can generate up to 34 ATP
molecules per original glucose molecule.
Figure 7-19
Hydrogen ions pumped by electron transport rush back across the membrane. In
doing so they cause ATP synthase "turbines" to spin, like the rushing water at a
hydroelectric dam spins turbines that generate electricity.
Adding Up the ATP Molecules
When taking cellular respiration apart to see how all its metabolic machinery works,
it's easy to forget the overall function. The result of cellular respiration is to generate
ATP for cellular work. A cell can convert the energy of one glucose molecule to as
many as 38 molecules of ATP (Figure 7-20).
Figure 7-20
The three stages of cellular respiration together produce as many as 38 ATPs for
each molecule of glucose that enters the pathway.
Glycolysis produces four ATP molecules, but recall that it requires two ATP
molecules as an initial energy investment. So the result is a net gain of two ATP
molecules. The Krebs cycle produces two more ATP molecules (one for each threecarbon pyruvic acid molecule). And finally, the ATP synthase turbines produce
about 34 more molecules of ATP.
Notice that most ATP production occurs after glycolysis and requires oxygen.
Without oxygen, most of your cells would be unable to produce much ATP. As a
result, you cannot survive for long without a fresh supply of oxygen.
Objectives
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Explain how fermentation in muscle cells is different from cellular
respiration.
Give examples of products that depend on fermentation in microorganisms.
Key Terms
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fermentation
anaerobic
When you walk down the street, your lungs supply your cells with oxygen at a rate
that keeps pace with ATP demand. But what happens when you sprint to catch a bus?
Your leg muscles are forced to work without enough oxygen because you are
spending ATP more quickly than your lungs and bloodstream can deliver oxygen to
your muscles for cellular respiration. Fortunately, some of your cells can produce
ATP and continue working for short periods without oxygen.
Fermentation in Human Muscle Cells
If you exercise for a certain amount of time, your muscles must regenerate ATP.
Normally, the cells can produce ATP through cellular respiration. But when you
sprint, your lungs and bloodstream can't supply oxygen fast enough to meet your
muscles' need for ATP. In such situations, your muscle cells use another process,
called fermentation, that makes ATP without using oxygen. Cellular respiration still
continues, but it is not the main source of ATP while fermentation is occurring.
Fermentation makes ATP entirely from glycolysis, the same process that is the first
stage of cellular respiration. Note in Figure 7-21 that glycolysis does not use oxygen.
Figure 7-21
When little oxygen is available in muscle cells, fermentation allows glycolysis to
continue.
As you read in Concept 7.5, glycolysis directly produces a net of two molecules of
ATP from each molecule of glucose it consumes. Remember that glycolysis
produces 4 ATP but that 2 ATP molecules are required to power this stage, yielding
a net of 2 ATP. This may not seem very efficient compared to the 38 molecules of
ATP generated during all of cellular respiration. However, by burning enough
glucose, fermentation can regenerate enough ATP molecules for short bursts of
activity such as a sprint to catch the bus.
Fermentation in muscle cells produces a waste product called lactic acid. The
temporary buildup of lactic acid in muscle cells contributes to the fatigue you feel
during and after a long run or a set of push-ups. Your body consumes oxygen as it
converts the lactic acid back to pyruvic acid. You restore your oxygen supply by
breathing heavily for several minutes after you stop exercising.
Fermentation in Microorganisms
Like your muscle cells, yeast (a microscopic fungus) is capable of both cellular
respiration and fermentation. When yeast cells are kept in an anaerobic
environment—an environment without oxygen—they are forced to ferment sugar
and other foods. In contrast to fermentation in your muscle cells, fermentation in
yeast produces alcohol, instead of lactic acid, as a waste product (Figure 7-23). This
fermentation reaction, called alcoholic fermentation, also releases carbon dioxide.
For thousands of years, humans have put yeast to work producing alcoholic
beverages such as beer and wine. The carbon dioxide is what makes champagne and
beer bubbly. In another example of "taming" microbes, the carbon dioxide bubbles
from baker's yeast make bread rise.
Figure 7-23
Fermentation in yeast produces ethyl alcohol. The carbon dioxide that is released
during fermentation creates bubbles and pockets that make bread rise. The
alcohol evaporates during baking.
There are also fungi and bacteria that produce lactic acid during fermentation, just as
your muscle cells do. Humans use these microbes to transform milk into cheese and
yogurt. The sharpness or sour flavor of yogurt and some cheeses is mainly due to
lactic acid. Similar kinds of microbial fermentation turn soybeans into soy sauce and
cabbage into sauerkraut.
Yeast cells and muscle cells are versatile in their ability to harvest energy by either
respiration or fermentation. In contrast, some bacteria found in still ponds or deep in
the soil are actually poisoned if they come into contact with oxygen. These bacteria
generate all of their ATP by fermentation. If you had to do that—though you don't
and you can't—you would have to consume almost 20 times more food than normal.
Oxygen enables you to get the most energy from your food. In the next chapter,
you'll learn about the original source of this energy—photosynthesis.