Chapter 5
Section II:
The Cell and Energy
Introduction
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Believe it or not, the energy comes
from the sun.
Sun is the primary source of energy.
What is photosynthesis?
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Plants and some other organisms are
autotrophs that use the sun’s energy to make
their own food.
The process by which a cell of an autotroph
captures the energy in sunlight and uses it to
make food is called photosynthesis (foh toh
sin thuh sis).
Chlorophyll and other light-absorbing chemicals
capture light energy and use it to power
photosynthesis.
In plants, it occurs in their cells’ chloroplasts.
(See Notes on Parts of a cell Chapter 4 section III)
Chloroplasts
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Chloroplasts
Photosynthesis occurs
inside chloroplasts in
the cells of plants and
some other organisms.
The Events of Photosynthesis
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Photosynthesis is a complex process
that results in chemical change.
During photosynthesis, cells capture
energy from sunlight and use it to
change carbon dioxide gas and
water into oxygen and sugars, such
as glucose.
Understanding the Equation
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Left side:
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6 molecules of
carbon dioxide
6 molecules of
water
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yields
Right Side:
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1 molecule of
glucose
6 molecules of
oxygen
An arrow, which is read as “yields,” connects the raw materials to the products.
Light energy, which is necessary for the chemical reaction to occur, appears
above the arrow.
Storing and Releasing Energy In Food
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Plant cells use some of the glucose
for food.
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cells break down the glucose molecules
to release the chemical energy the
molecules contain
Other glucose molecules are
converted to cellulose

a chemical that plays an important
role in the structure of the cell walls of
plants
Storing and Releasing Energy In Food
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When heterotrophs eat food from
plants or animals that have eaten
plants, they take in the plant’s
stored food.
Remember:
Bird eats caterpillar, which eats the leaf, that
produced its energy through the process of
photosynthesis – that uses the energy from the
sun.
Storing and Releasing Energy In Food
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The other product of photosynthesis is
oxygen
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oxygen passes out of the plant and into the
atmosphere.
Autotrophs have produced about 21 percent
oxygen in the Earth’s atmosphere.
Most living things use oxygen in the cells
of their bodies to release the energy in
food.
Storing and Releasing Energy In Food
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Cells store and use energy
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during photosynthesis, plants capture
energy from sunlight and “save” it in
the form of substances such as sugars
(glucose).
When their cells need energy, they
break down the sugars and
releasing energy.
What is Respiration?
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The process in which cells obtain
energy that they need from glucose.
During respiration, cells break down
simple food molecules such as
glucose and release the energy they
contain.
Cellular respiration occurs in the
mitochondria of an organism’s cells.
The Respiration Equation
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The raw materials for cellular respiration
are glucose and oxygen.
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Plants and other organisms that undergo
photosynthesis make their own glucose.
The glucose in animals and other organisms
comes from the food they eat.
The oxygen comes from the air or water
surrounding the organism.
Comparing Photosynthesis and
Respiration
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Photosynthesis and respiration can
be thought of as opposite
processes.
Comparing Photosynthesis and
Respiration
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During photosynthesis, carbon dioxide
and water are used to produce sugars and
oxygen.
During respiration, sugars and oxygen are
used to produce carbon dioxide and
water.
Together, they form a cycle that keeps the
levels of oxygen and carbon dioxide fairly
constant in Earth’s atmosphere.
Fermentation
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Fermentation = an energy-releasing
process that does not require
oxygen.
There are two types of fermentation:
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Alcoholic Fermentation
Lactic Acid Fermentation
Alcoholic Fermentation
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Occurs in yeast and some other
single-celled organisms.
Called alcoholic fermentation
because alcohol is one of the
products made when these
organisms break down sugars.
The carbon dioxide produced by
yeast causes dough to rise, and it
creates the air pockets you see in
bread.
Lactic Acid Fermentation
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Takes place at times in your body.
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Your muscle cells used up the oxygen
faster than it could be replaced.
Because your cells lacked oxygen, they
used the process of fermentation to
produce energy.
One by-product of this type of
fermentation is a substance known
as lactic acid.
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When lactic acid builds up, your
muscles feel weak, tired, and sore.