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Unit: CELLS
Biology
Mr. Heenan
Chapter 8
Photosynthesis
8-1 Energy and Life
• Energy is the ability to do work. Living things
depend on energy, whether it’s for sports or even
when you sleep.
• Your cells are busy using energy to build new
proteins and amino acids.
Autotrophs and Heterotrophs
Where does the energy that living things need
come from? You probably say that it is food.
• Originally, the energy in most foods comes from
the sun.
• Plants and some other types of organisms are
able to use light energy from the sun to
produce food.
• Autotrophs are organisms which make their
own food. Examples: plants and algae
• Heterotrophs are organisms which obtain
their energy from the food they consume.
Example: animals
• To live, all organisms, including plants, must
release the energy in sugars and other
compounds.
Chemical Energy and ATP
• Energy comes in many forms, including light,
heat, and electricity. Energy can be stored in
chemical compounds too.
Example:
• When you light a candle, the wax melts, soaks
into the wick and is burned, releasing energy in
the form of light and heat.
• As the candle burns, high-energy chemical
bonds between carbon and hydrogen atoms in
the wax are broken.
• The high-energy bonds are replaced by lowenergy bonds between the atoms and oxygen.
• The energy of a candle flame is released from
electrons. When the electrons in those bonds are
shifted from higher energy levels to lower energy
levels, the extra energy is released as heat and
light.
• Living things use chemical fuels as well.
• Adenosine triphosphate (ATP) is one of the
principle chemical compounds that cells use to
store and release energy.
• ATP consists of adenine, a 5-carbon sugar
called ribose, and three phosphate groups.
• Those three phosphate groups are the key to
ATP’s ability to store and release energy.
Adenine
Ribose
3 Phosphate groups
Storing energy
• Adenosine diphosphate (ADP) is almost like
ATP, but only has two phosphate groups instead
of three.
• When a cell has energy available, it can store
small amounts of it by adding a phosphate
group to ADP producing ATP.
Adenosine Diphosphate
(ADP) + Phosphate
Partially
charged
battery
Adenosine Triphosphate (ATP)
Fully
charged
battery
Releasing Energy
• By breaking the chemical bond between the
second and third phosphates, energy is
released.
• Energy can be released by the cell as needed, by
the cell subtracting that third phosphate group.
• The characteristics of ATP make it exceptionally
useful as the basic energy source of all cells.
Releasing Energy
Energy stored in ATP is released by breaking the
chemical bond between the second and third
phosphates.
2 Phosphate groups
P
ADP
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Cells use the energy provided by ATP for cellular activities
such as active transport.
Many cell membranes have a sodium-potassium pump,
which is a membrane protein that pumps sodium ions
(Na+) out of the cell and pumps potassium ions (K+)
into it.
ATP provides the energy needed to keep the pump
working, and maintaining a carefully regulated balance
of ions on both sides of the cell membrane.
ATP also provides energy for motor proteins that move
organelles throughout the cell.
ATP also powers:
• Synthesis of proteins and nucleic acids
• Responses to chemical signals at the cell
surface.
Example: Lightning bug’s light comes from an
enzyme powered by ATP.
• Most cells have only a small amount of ATP. ATP
is a great molecule for transferring energy,
but it is not a good one for storing large amounts
of energy over a long term.
• A glucose molecule stores more than 90 times
the chemical energy of a molecule of ATP.
• It is more efficient for cells to keep only a small
supply of ATP on hand.
• Cells can regenerate ATP from ADP as needed
by using the energy in foods like glucose.
8-2 Photosynthesis: An Overview
• In the process of photosynthesis, plants use
the energy of sunlight to convert water and
carbon dioxide into high-energy
carbohydrates (sugars and starches) and
oxygen, as a waste product.
Investigating Photosynthesis
Centuries ago, research began with a simple
question:
When a tiny seedling grows into a tall tree with a
mass of several tons, where does the tree’s
increase in mass come from?
From the soil?
From the water?
From the air?
Van Helmont’s Experiment
• (1600s) Belgian physician Jan van Helmont
devised an experiment to find out whether
plants grew by taking in material out of the soil.
• Helmont planted a seedling in a pot of soil and
watered it regularly.
• After 5 years, the seeding, had grown into a
small tree and had gained 75 kg.
• He noticed that the mass of the soil was almost
unchanged, so he concluded that most of the
mass was gained by adding the water. Water
was the only thing that he added.
• Van Helmont’s experiment accounted for the
“-hydrate,” or water, portion of the
carbohydrate produced by photosynthesis.
• He didn’t realize that carbon dioxide in the air
made a major contribution to the mass of the
tree. The carbon in carbon dioxide is used to
make sugars and other carbohydrates in
photosynthesis.
Priestley’s Experiment
• More than 100 years after van Helmont’s
experiment, the English minister Joseph
Priestley performed an experiment that would
give another insight into the process of
photosynthesis.
• Priestley 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 the
flame burning.
• He placed a live sprig of mint under the jar and
allowed a few days to pass. The candle was lit
again and would remain lit for a while.
• The mint was producing the required substance
for burning (oxygen).
Jan Ingenhousz
• Dutch scientist, Jan Ingenhousz, showed that
the effect observed by Priestley occurred only
when the plant was exposed to light. So, light
was necessary for plants to produce oxygen.
• The experiments performed by van Helmont,
Priestly, and Ingenhousz led to work by other
scientists who finally discovered that in the
presence of light, plants transform carbon
dioxide and water into carbohydrates, and they
also release oxygen.
The Photosynthesis Equation
• Because photosynthesis usually produces 6carbon sugars as the final product. The overall
equation is shown as follows:
6 CO2 + 6 H2O  C6H12O6 + 6 O2
Carbon Dioxide
+ Water

Sugars
+
Oxygen
• Photosynthesis uses the energy of sunlight to
convert water and carbon dioxide into highenergy sugars and oxygen.
• Plants use these sugars to produce complex
carbohydrates such as starches. The plants
obtain carbon dioxide from the air or water in
which they grow (pg. 206 fig 8-4).
PHOTOSYNTHESIS
Light energy
H2O
Light-Dependent
Reactions
(thylakoids)
ADP
+
NADP
Sugar
O2
ATP
NADPH
Calvin Cycle
(stroma)
CO2
+
H20
Light and Pigment
• In addition to water and carbon dioxide,
photosynthesis requires light and chlorophyll,
a pigment in chloroplasts.
• Energy travels from the sun to Earth as light.
Sunlight which our eyes perceive as “white light”
is actually a mixture of different wavelengths
of light.
• Many of these wavelengths that we see make up
what we call the visible spectrum. We see
different wavelengths as different colors.
• Plants gather the sun’s energy with lightabsorbing molecules called pigments.
• The plant’s primary pigment is chlorophyll.
There are two main types: Chlorophyll a and
Chlorophyll b.
• Chlorophyll absorbs light very well in the blueviolet and red-regions of the visible spectrum.
• It does not absorb light well in the green region.
Green light is reflected by leaves, which is why
plants looks green.
• Plants also contain red and orange pigments,
such as carotene, that absorb light in other
regions of the spectrum.
• Chlorophyll a – absorbs light mostly in the
blue-violet and red regions of the visible
spectrum.
• Chlorophyll b – absorbs light mostly in the blue
and red regions of the visible spectrum.
• Light is a form of energy, so any compound that
absorbs light also absorbs the energy of that
light.
• When chlorophyll absorbs light, much of the
energy is transferred directly to electrons in
the chlorophyll molecule.
• This raises the energy level of those electrons,
and it is those high-energy electrons which
make photosynthesis work.
8-3 The Reactions of Photosynthesis
Inside the Chloroplast
• Plants and other photosynthetic
eukaryotes use photosynthesis to
make carbohydrates.
• Photosynthesis takes place inside
chloroplasts.
Chloroplast
Single thylakoid
Granum
Photosystems
H2O
CO2
Light
NADP+
ADP + P
Lightdependent
reactions
Calvin
Calvin
cycle
Cycle
Chloroplast
O2
Sugars
• Chloroplasts contain saclike photosynthetic
membranes called thylakoids.
• Thylakoids are arranged in stacks called grana
(singular: granum).
• Proteins in the thylakoid membranes organize
chlorophyll and other pigments into clusters
known as photosystems.
• The photosystems are the light-collecting units
of the chloroplast.
Scientists describe the reactions of
photosynthesis in two parts:
1. Light-dependent reactions
2. Light-independent reactions (The Calvin
Cycle)
Light-dependent reactions take place within the
thylakoid membranes and the Calvin cycle
takes place in the stroma, the region outside
the thylakoid membranes.
Electron Carriers
• When sunlight excites electrons in chlorophyll,
the electrons gain a great deal of energy.
• These high-energy electrons require a special
carrier.
• A carrier molecule is a compound that can
accept a pair of high energy electrons and
transfer them along with most of their energy
to another molecule.
• This process is called electron transport, and the
electron carriers are known as the electron
transport chain.
• One of these carriers is a compound called
NADP+ (nicotinamide adenine dinucleotide
phosphate). NADP+ accepts and holds two highenergy electrons along with a hydrogen (H+) ion.
• This converts the NADP+ into NADPH, and the
energy of sunlight can be trapped in chemical
form.
• The NADPH can carry high-energy electrons by
light absorption in chlorophyll to chemical
reactions elsewhere in the cell. These electrons
build molecules the cells need, like glucose
(carbohydrates).
Light-dependent Reactions (require light)
• Light-dependent reactions produce oxygen gas
and convert ADP and NADP+ into energy
carriers ATP and NADPH.
• The reactions take place within the thylakoid
membranes of chloroplast.
A. Photosynthesis starts in Photosystem II
(because it was discovered after Photosystem I).
•
The light energy is absorbed by electrons,
increasing their energy level.
•
The electrons are passed on to the electron
transport chain. The electrons don’t run out, but
are replaced by the water (H2O) molecules.
•
Enzymes on the inner surface of the thylakoid
membranes break-up each water molecule into
two electrons, two H+ ions, and one oxygen
atom.
• The two electrons replace the two electrons lost
to the electron transport chain.
• The oxygen is released into the air and the (H+)
hydrogen ions are released inside the thylakoid
membrane.
B. High-energy electrons move through the
electron transport chain from photosystem II to
photosystem I.
• Energy from the electrons is used by the
molecules in the electron transport chain to
transport H+ ions from the stroma into the
inner thylakoid space.
C. Pigments in photosystem I use energy from
light to re energize the electrons.
• NADP+ then picks up these high-energy
electrons, along with H+ ions, at the outer
surface of the thylakoid membrane, plus an H+
ion, and becomes NADPH.
D. As electrons are passed from chlorophyll to
NADP+, more hydrogen ions are pumped
across the membrane.
•
The inside of the membrane fills with
positively charged hydrogen ions.
•
Outside the thylakoid membrane is
negatively charged and the difference in
charges across the membrane provides the
energy to make ATP.
E. Hydrogen (H+) ions cannot cross the
membrane directly, but needs a transmembrane
protein, ATP synthase, to transfer the H+ ions.
• The H+ ions passing through ATP synthase
causes the ATP synthase to spin like a turbine.
• As it rotates, ATP synthase binds ADP and a
phosphate group together to produce ATP.
• The electron transport chain produces both
high-energy electrons and ATP.
Summary:
• Light-dependent reactions use water, ADP,
NADP+ to produce two high-energy
compounds: ATP and NADPH.
• These compounds provide the energy to build
energy-containing sugars from low-energy
compounds.
Photosynthesis begins when pigments in photosystem
II absorb light, increasing their energy level.
Photosystem II
These high-energy electrons are passed on to the
electron transport chain.
Photosystem II
High-energy
electron
Electron
carriers
Enzymes on the thylakoid membrane break water
molecules into:
Photosystem II
2H2O
High-energy
electron
Electron
carriers
 hydrogen ions
 oxygen atoms
 energized electrons
Photosystem II
+ O2
2H2O
High-energy
electron
Electron
carriers
The energized electrons from water replace the
high-energy electrons that chlorophyll lost to the
electron transport chain.
Photosystem II
+ O2
2H2O
High-energy
electron
As plants remove electrons from water, oxygen is
left behind and is released into the air.
Photosystem II
+ O2
2H2O
High-energy
electron
The hydrogen ions left behind when water is
broken apart are released inside the thylakoid
membrane.
Photosystem II
+ O2
2H2O
High-energy
electron
Energy from the electrons is used to transport H+
ions from the stroma into the inner thylakoid
space.
Photosystem II
+ O2
2H2O
High-energy electrons move through the electron
transport chain from photosystem II to
photosystem I.
Photosystem II
+ O2
2H2O
Photosystem I
Pigments in photosystem I use energy from
light to re-energize the electrons.
+ O2
2H2O
Photosystem I
NADP+ then picks up these high-energy electrons,
along with H+ ions, and becomes NADPH.
+ O2
2H2O
2 NADP+
2
2
NADPH
As electrons are passed from chlorophyll to
NADP+, more H+ ions are pumped across the
membrane.
+ O2
2H2O
2 NADP+
2
2
NADPH
Soon, the inside of the membrane fills up with
positively charged hydrogen ions, which makes
the outside of the membrane negatively charged.
+ O2
2H2O
2 NADP+
2
2
NADPH
The difference in charges across the membrane
provides the energy to make ATP
+ O2
2H2O
2 NADP+
2
2
NADPH
H+ ions cannot cross the membrane directly.
ATP synthase
+ O2
2H2O
2 NADP+
2
2
NADPH
The cell membrane contains a protein called ATP
synthase that allows H+ ions to pass through it
ATP synthase
+ O2
2H2O
2 NADP+
2
2
NADPH
As H+ ions pass through ATP synthase, the protein
rotates.
ATP synthase
+ O2
2H2O
2 NADP+
2
2
NADPH
As it rotates, ATP synthase binds ADP and a
phosphate group together to produce ATP.
ATP synthase
+ O2
2H2O
ADP
2 NADP+
2
2
NADPH
Because of this system, light-dependent electron
transport produces not only high-energy electrons
but ATP as well.
ATP synthase
+ O2
2H2O
ADP
2 NADP+
2
2
NADPH
The Calvin Cycle
• The ATP and NADPH formed by the lightdependent reactions contain an abundance of
chemical energy, but they are not stable enough
to store that energy for more than a few minutes.
• The Calvin Cycle- uses ATP and NADPH from
the light-dependent reactions to produce highenergy sugars.
• Plants use the energy that ATP and NADPH
contain to build high-energy compounds that
can be stored for a long time.
• The Calvin cycle, named after Melvin Calvin
(American scientist), does not require light so
the reactions are called light-independent
reactions.
A. Six carbon dioxide (CO2) molecules enter
the cycle from the atmosphere.
•
The CO2 molecules combine with six 5-carbon
molecules.
•
The result is twelve 3-carbon molecules.
B. The twelve 3-carbon molecules are then
converted into higher-energy forms.
• The energy for the conversion comes from ATP
and high-energy electrons from NADPH.
C. Two of the twelve 3-carbon molecules are
removed from the cycle.
• The plant cell uses these molecules to produce
sugars, lipids, amino acids, and other
compounds needed for plant metabolism and
growth.
D. The remaining ten 3-carbon molecules are
converted back into six 5-carbon molecules.
• These molecules combine with 6 new CO2
molecules to begin the next cycle.
Summary:
• The Calvin Cycle uses 6 molecules of CO2 to
produce a single 6-carbon sugar molecule.
• Photosynthesis pulls CO2 out of the atmosphere
and turns it into energy-rich sugars.
• The plant uses the sugars to meet its energy
needs and to build more complex
macromolecules, such as cellulose, that it
needs for growth and development.
Six carbon dioxide molecules enter the cycle from the
atmosphere and combine with six 5-carbon molecules.
CO2 Enters the Cycle
The result is twelve 3-carbon molecules, which are
then converted into higher-energy forms.
The energy for this conversion comes from ATP and
high-energy electrons from NADPH.
Energy Input
12
12 ADP
12 NADPH
12 NADP+
Two of twelve 3-carbon molecules are removed from
the cycle.
Energy Input
12
12 ADP
12 NADPH
12 NADP+
The molecules are used to produce sugars, lipids,
amino acids and other compounds.
12
12 ADP
12 NADPH
12 NADP+
6-Carbon sugar
produced
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Sugars andCopyright
other
compounds
The 10 remaining 3-carbon molecules are converted
back into six 5-carbon molecules, which are used to
begin the next cycle.
12
12 ADP
6 ADP
12 NADPH
6
12 NADP+
5-Carbon Molecules
Regenerated
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Sugars andCopyright
other
compounds
• When other organisms eat plants, they can also
use the energy stored in carbohydrates.
• Light-dependent reactions trap the energy of
sunlight in chemical form.
• Light-independent reactions use that chemical
energy to produce high-energy sugars from CO2
and water…giving off oxygen.
Factors Affecting Photosynthesis
• Water- A shortage of water can slow or even
stop photosynthesis.
• Plants that live in dry conditions, such as desert
plants and conifers, have a waxy coating on
their leaves to reduce water loss.
• Temperature- Photosynthesis depends on
enzymes that function best at 0oC and 35oC.
• Temperatures above and below this range may
damage the enzymes, slowing down or
stopping photosynthesis.
• Intensity of Light- The intensity of light
affects the rate of photosynthesis.
• Increasing light intensity increases the rate of
photosynthesis, but levels off at a certain point.
• That maximum rate of photosynthesis depends
on the type of plant.