
Hand out penny candy to the students.
Inquire as to what is in the candy. The
students will identify sugar. Ask them if candy
gives them an energy boost. If yes, ask them
to explain how that happens. They will
probably say that they digest the sugar. Shift
the discussion to plants. Ask how plants get
their energy. They might know about the
production of food through photosynthesis
but not how manufactured food is used.

1 Describe cellular respiration.

2 Describe the processes of cellular
respiration.

3 Identify factors that affect cellular
respiration.
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_ aerobic respiration
_ anabolism
_ anaerobic respiration
_ catabolism
_ cellular respiration
_ chemiosmosis
_ citric acid cycle
_ cytosol
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_ electron transport system (ETS)
_ fermentation
_ glycolysis
_ metabolism
_ mitochondria
_ oxidation
_ redox reactions
_ reduction

I. All living plant cells respire and
use the energy released to live
and function. Cellular respiration
is the process in which chemical
energy stored in certain foods is
converted to ATP, or high-energy
compounds.

A. The cells that make up organisms are at
constant work producing materials for
growth, reproduction, movement, and
maintenance of life. All these chemical
processes are called metabolism. Converting
substances to make ATP and other energyrich molecules is one vital process. Metabolic
processes entail both the combining and the
splitting of molecules.

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1. The assembly of complex molecules from
simple molecules is a process called anabolism.
Anabolism is the process by which plants store
energy during photosynthesis, produce new
materials for cellular activities, and grow.
2. The breakdown of larger molecules into
smaller molecules is an element of metabolism
known as catabolism. An example of a catabolic
reaction is the breakdown of carbohydrates
manufactured during photosynthesis to release
ATP.

B. The most common catabolic reaction in
plants involves the breakdown of glucose.
This process requires oxygen and is known as
aerobic respiration. The process of
respiration takes place in complex organelles
known as mitochondria.
1. Aerobic respiration is essentially the reverse of
photosynthesis. In aerobic respiration, sugars
made in photosynthesis are broken down into
energy-rich molecules through a long sequence of
reactions.
 2. Oxygen and water are critical ingredients to the
reaction.
 3. In the process, chemical energy is released
when the molecular bonds of the sugar molecules
are broken. The extracted energy, ATP, drives a
variety of chemical reactions important in the
growth and development of the plant. Byproducts
of the reaction are carbon dioxide and water.

C. Some cells perform a less efficient form
of respiration called anaerobic respiration.
Anaerobic respiration does not require
oxygen. Cells that rely on anaerobic
respiration are found in soil or water
where oxygen is in short supply.
 1. Some bacteria and fungi adapted to
anaerobic respiration convert energy by
fermentation. Fermentation is a type of
cellular respiration that produces ethyl
alcohol or lactic acid and is anaerobic.
 2. Humans recognize the value of
fermentation in the making of silage,
beer, and wine.


D. Plant growth takes place primarily at night
when photosynthesis is shut down. It is fueled
by aerobic respiration. With signals from
hormones, enzymes or chemical activators are
produced. Each enzyme has one specific job.
With split-second timing, they break down
sugars and recombine them with nitrogen and
other minerals. Many complex molecules are
produced, including starches; pectin to bind
cells; lignin, which is a tough, durable
substance; cellulose; lipids; proteins; pigments;
hormones; vitamins; and alkaloids and tannins,
materials that protect plants from pests and
diseases.

II. Aerobic respiration of glucose involves four primary
stages.

A. The first stage of aerobic respiration is glycolysis.
Glycolysis is the splitting of sugar. A specific enzyme
is the catalyst for this reaction that involves about ten
steps. Glycolysis takes place in the cytosol, the fluid of
a cell in which organelles are suspended. Six-carbon
glucose molecules are split into two, three-carbon
pyruvate molecules with a net profit of two ATP and
two NADH molecules. Glycolysis does not require
oxygen.

B. Stage two involves the formation of acetyl
coenzyme A (acetyl CoA). Pyruvate molecules
enter the mitochondria. Each pyruvate
molecule becomes a two-carbon molecule,
acetate, through oxidation. The acetate
combines with coenzyme A to form acetyl
CoA. In the process, carbon dioxide is
removed, and hydrogen atoms are
contributed to the production of NADH.

C. The third stage is the citric acid cycle, also
known as the Kreb’s cycle. It consists of eight
steps that take place in the mitochondria. Acetyl
CoA combines with a four-carbon molecule to
form a six-carbon citrate molecule. In a series of
events, the citrate reforms a four-carbon
molecule. With each turn of the cycle, one ATP
and two carbon dioxide molecules are released.
NADH and FADH2 are also produced. The cycle
must turn twice to process one glucose
molecule.

D. Throughout the first three steps, hydrogen
atoms were transferred to hydrogen
acceptors NAD and FAD, forming NADH and
FADH2. In the final stage of aerobic
respiration, these reduced compounds enter
the electron transport system.

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1. The electron transport system (ETS) is a series
of chemical reactions by which electrons are
passed from one acceptor molecule to another.
Oxygen is the final hydrogen acceptor. During
this pathway, ATP is synthesized by the process
of chemiosmosis.
2. Chemiosmosis is the process in which
hydrogen atoms cross the thylakoid membrane
and travel down a protein gradient, producing
ATP. This fourth stage sees the production of 32
ATP molecules. Water is also made and released.

E. Energy in cells is transferred by the flow of protons
and electrons.

1. A chemical reaction in which a substance loses
electrons is called oxidation. Substances gain
electrons through the chemical process of reduction.
Electrons released during oxidation cannot exist
independently. Therefore, every oxidation reaction is
accompanied by a reduction reaction. These
oxidation-reduction reactions are called redox
reactions. Oxidation-reduction reactions are crucial to
photosynthesis, cellular respiration, and other
chemical reactions that occur in cells.

2. During aerobic respiration, glucose or
other forms of carbohydrates are broken
down in the presence of water and oxygen.
The products are energy in the form of ATP,
carbon dioxide, and water. In aerobic
respiration, glucose is oxidized, and oxygen is
reduced. In the multi-step reaction, hydrogen
is transferred from the glucose molecule to
oxygen. Energy from the hydrogen electrons
is used for ATP synthesis.

III. Nearly all energy used to
maintain life originates from
the sun. Plants convert the
solar energy to chemical
energy through
photosynthesis. Plants and
animals then release the
chemical energy for their use
through respiration. Various
factors influence respiration.

A. Respiration increases as
temperatures rise and decreases
as temperatures drop.

B. Oxygen is required for aerobic
respiration to occur. If levels of
oxygen are reduced in the
atmosphere, respiration slows.

C. Soils saturated with water
lack oxygen. In the absence
of oxygen, respiration does
not occur. This explains why
root cells and, ultimately,
root systems die in
waterlogged soil.

D. Under low light conditions, fewer
carbohydrates are produced by plants. Low
levels of carbohydrates, in turn, mean low
rates of respiration.

E. The stage of growth and the age of the
plant influences the rate of respiration
needed to maintain life processes. Young
actively growing plants tend to have a higher
rate of respiration.

F. Water is essential for cellular
respiration as well as
photosynthesis. Without water,
enzyme activity grinds to a halt.
The plant essentially stops
growing. The lack of available
water during drought conditions
can cause respiration to stop.

1. What is cellular respiration?

2. What are the processes of cellular
respiration?

3. What factors affect cellular respiration?