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Chapter 6:
Cell Energy: Photosynthesis
and Respiration
Section 1:
Photosynthesis: Capturing and
Converting Energy
Photosynthesis
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In the process of photosynthesis,
plants convert the energy of sunlight
into the energy in the chemical bonds
of carbohydrates – sugars, and
starches
Put more simply, plants use the energy of
sunlight to produce carbohydrates in a
process called photosynthesis
Photosynthesis
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An understanding of photosynthesis was developed from
studies of plant growth
Dutch physician Jan Van Helmont devised an experiment
to determine how plant growth actually works
 Found the mass of a pot of dry soil and a small seedling
 Planted the seedling in the pot of soil
 Watered regularly for 5 years
 Gained about 75 kilograms
 Mass of soil was unchanged
 Concluded that most of the mass must come from water
because that is all he added to the pot
Photosynthesis
Although Van Helmont did not realize
it, carbon dioxide in the air made a
major contribution to the mass of his
tree
 It is the carbon in carbon dioxide that
is used to make carbohydrates in
photosynthesis
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Photosynthesis
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Almost 100 years after Van Helmont’s experiment,
Joseph Priestley performed an experiment that
would give another insight into the process of
photosynthesis
 Took a candle, placed a glass jar over it, and
watched as the flame gradually died out
 Something in the air was necessary to keep a
candle burning
 When that substance was used up, the candle
went out
 oxygen
Photosynthesis
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Priestley then placed a spring of mint under the
jar and allowed a few days to pass, the candle
could be relighted and would remain lighted for
awhile
The mint had produced the substance required
for burning
 oxygen
Later, Dutch scientist Jan Ingenhousz showed
that this only occurred when the plant was
exposed to light
Requirements for Photosynthesis
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These experiments reveal that in the
presence of light, plants transform carbon
dioxide and water into carbohydrates and
release oxygen
Usually produces the sugar glucose
6CO2 + 6H2O light C6H12O6 + 6O2
Sunlight
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Nearly all organisms on Earth depend on the sun for
energy
Autotroph – organisms that are able to use a source
of energy, such as sunlight, to produce food directly
from simple inorganic substances in the environment
Heterotroph – organisms that obtain energy from the
foods they eat
The sun bathes the Earth in a steady stream of light
 We see colorless “white” light but it is actually a
mixture of different wavelengths of light
 Visible spectrum
Pigments

Process of photosynthesis begins when light is
absorbed by pigments in the plant cell
 Colored substances that absorb or reflect light
 Principal pigment in green plants is chlorophyll
 Absorbs red and blue light but does not
absorb light in the middle region of the
spectrum very well
 These wavelengths are reflected
Energy-Storing Compounds

In a green plant, the energy of sunlight is
transferred to electrons, raising them to a higher
energy level
 The electrons belong to the pigment
chlorophyll
 High-energy electrons are trapped in
chemical bonds
 Two ways in which energy of sunlight is
trapped in chemical bonds
Energy-Storing Compounds

First way sunlight is trapped in chemical bonds
 Simpler of the two
 A pair of high-energy electrons are passed directly to an
electron carrier
 A molecule that can accept a pair of electrons and later
transfer them along with most of their energy to another
compound
 Plants use the electron carrier NADP+
 When NADP+ accepts a pair of high-energy electrons,
it is converted to NADPH
 ONE WAY IN WHICH SOME OF THE ENERGY
OF SUNLIGHT CAN BE TRAPPED IN CHEMICAL
FORM
Energy-Storing Compounds

Second way sunlight is trapped in chemical
bonds
 Involves adenosine triphosphate (ATP)
 Consists of adenine, a 5-carbon sugar
called ribose, and three phosphate
groups
 During photosynthesis, green plants
produce ATP, which is an energy-storing
compound used by every living cell
•As one might suspect,
there are 3 phosphate
groups.
•There is a high E bond
between the 2nd and 3rd
P group.
•When cells need E this
high E bond is broken and
E is released. It’s not ATP
anymore. What is the
new molecule formed?? ADP
•Notice the other two components of the the ATP molecule.
Adenine and Ribose
•E from the food a cell takes in is used to
convert ADP back to ATP.
ADP + phosphate
ATP by the enzyme
ATP synthetase
Chapter 6:
Cell Energy: Photosynthesis and
Respiration
Section 2:
Photosynthesis: The Light and Dark
Reactions
Photosynthesis: The Light and Dark
Reactions

The production of NADPH and ATP requires
sunlight
 Light reactions – the energy of sunlight is
captured and used to make energy-storing
compounds
 Another set of reactions called the dark
reactions uses the energy stored in NADPH
and ATP to produce glucose
 Do not require light
 However, they can and do occur in the
light also
The Light Reactions

Photosynthesis takes place in the chloroplast
 Within the chloroplast are saclike
photosynthetic membranes that contain
chlorophyll
 Light reactions take place in these membranes
 Can be divided into four basic processes: light
absorption, electron transport, oxygen
production, and ATP formation
Light Absorption

Photosynthetic membranes contain clusters of pigment
molecules, or photosystems, that are able to capture the
energy of sunlight
 Two photosystems in plants
 Photosystem I
 Photosystem II
 Each contains several hundred chlorophyll
molecules as well as other accessory pigments
 Absorb light in the regions of the spectrum
where chlorophyll does not
Light Absorption
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After light energy is absorbed by one of the
pigment molecules in a photosystem, the energy
is passed from one pigment molecule to the next
until it reaches a special pair of chlorophyll
molecules in the reaction center of the
photosystem
 In the reaction center, high-energy electrons
are released and are passed to the first of
many electron carriers
Electron Transport

High-energy electrons are transferred along a
series of electron carriers
 Electron transportthe electron carriers
themselves are known as the electron
transport chain
 At the end of the chain, the electrons are
passed to NADP+, converting it to
NADPH
Oxygen Production

The photosynthetic membrane contains a system that
provides new electrons to chlorophyll to replace the
ones that wound up in NADPH
 Four electrons are removed from two water
molecules
 4 H+ ions
 2 O atoms
 Form a single molecule of oxygen gas
 Released into the air
ATP Formation
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H+ ions are released inside the
photosynthetic membrane as well as being
pumped across the membrane
The inside of the membrane fills up with H+
ions
Makes the outside negatively charged and
the inside positively charged
 Forms ATP
A Summary of the Light Reactions
Use water, ADP, NADP+
 Produce O2, ATP and NADPH
 The dark reactions will convert these
energy-storing molecules to a more
convenient form
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The Dark Reactions
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Light does not play a role in the dark reactions
The series of chemical changes that make up
the dark reactions is critical to living things
Carbon dioxide is used to make organic
compounds
The dark reactions form a cycle called the
Calvin cycle
The Calvin Cycle
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5 carbon sugar (C5) combines with CO2 to
form two 3 carbon compounds (C3)
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Relatively slow
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Uses the enzyme rubisco to speed up the process
Using ATP and NADPH, the 3 carbon
compounds are converted to PGAL
(phosphoglyceraldehyde)
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6 turns of the cycle to make one molecule of
glucose
Chapter 6:
Cell Energy: Photosynthesis and
Respiration
Section 3:
Glycolysis and Respiration
Glycolysis – Breaking Down Glucose
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C6H12O6 + 6O2  6CO2 + 6H2O
Gives off 3811 calories
Glycolysis takes place in the cytoplasm
of a cell
In glycolysis, a series of enzymes
catalyzes chemical reactions that
change glucose, one step at a time,
into different molecules
Respiration
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If oxygen is available, respiration can take place
Aerobic process
Respiration is the process that involves oxygen
and breaks down food molecules to release
energy
 Uses the pyruvic acid formed in glycolysis
 Often used as a synonym for breathing
 Takes place in the cell’s mitochondria
The Krebs Cycle
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First set of reactions in respiration
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Krebs cycle
2 carbon atoms added (from the breakdown of
pyruvic acid)
 2 carbon atoms removed (in 2 molecules of CO2)
 3 molecules of NAD+ converted to NADH
 1 molecule of FAD converted to FADH2
 1 molecule of GDP converted to GTP
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Electron Transport in the Mitochondrion
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High energy electrons from NADH and FADH2 are
passed to electron transport enzymes in the
mitochondrion
Form an ETC along which electrons are passed
Enzyme at the end of the chain combines e- from
ETC, H+ ions from fluid inside the cell, and O2 to form
H2O
Oxygen is the final electron acceptor in respiration
 Is essential for obtaining energy from both NADH
and FADH2
ATP Formation
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Electron transport involves the movement of hydrogen
ions
As enzymes accept electrons, they pump a hydrogen
from the inside to the outside
This movement powers the formation of ATP
 On average, the movement of a pair of electrons
down the ETC produces enough energy to form 3
ATP from ADP
More H+ ions outside
 This imbalance supplies the energy to make ATP from
ADP
The Totals
 Glycolysis
and respiration
together produce a total of 36
ATP molecules
Obtaining Energy From Food
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Complex carbohydrates are broken down into
simple sugars that are then converted into
glucose
 The pathways we have discussed can be
used to produce energy
 The cell can generate chemical energy in
the form of ATP from just about any source
Breathing and Respiration
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Final acceptor for all electrons in respiration
is oxygen
Without oxygen, electron transport cannot
operate, Krebs cycle stops, and ATP
production stops
With each breath we take, air flows into our
lungs
 Oxygen has a critical role to play in the
mitochondria of every cell
Energy in Balance

Photosynthesis and respiration
can be thought of as opposite
processes
 Photosynthesis deposits energy
 Respiration withdraws energy
Chapter 6:
Cell Energy: Photosynthesis and
Respiration
Section 4:
Fermentation
Fermentation
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Fermentation is a process that enables cells to carry
out energy production in the absence of oxygen
 Breakdown of glucose and release of energy in which
organic substances are the final electron acceptors
Fermentation is anaerobic—it does not require oxygen
Fermentation enables cells to carry out energy
production in the absence of oxygen
 Produces 2 ATP
Lactic Acid Fermentation
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In many cells, the pyruvic acid that accumulates as a
result of glycolysis can be converted to lactic acid
Lactic acid fermentation
 Pyruvic acid + NADH  lactic acid + NAD+
 Lactic acid is produced in muscles during rapid
exercise when the body cannot supply enough
oxygen to tissues to produce all of the ATP that is
required
 Causes a burning, painful sensation
 Large muscles quickly run out of oxygen
 Muscle cells begin to rapidly produce ATP by
fermentation
Alcoholic Fermentation
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Another type of fermentation occurs in yeasts and a
few other microorganisms
Pyruvic acid is broken down to produce a 2 carbon
alcohol and carbon dioxide
Alcoholic fermentation
 Pyruvic acid + NADH  alcohol + CO2 + NAD+
Alcoholic Fermentation
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Particularly important to bakers and brewers
 Causes dough to rise and forms bubbles in beer and
wine
 To brewers, alcohol is a welcomed byproduct of
fermentation
 However, it is not desirable from a yeast cell’s
point of view
 Alcohol is toxic
 When the level of alcohol reaches about 12
percent, yeast cells die
 Thus alcoholic beverages must be processed if
higher concentrations of alcohol are desired