Chapter 8: Photosynthesis
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Energy and Life
Energy – the ability to do work
No energy = no life
Thermodynamics is the study of the flow and
transformation of energy in the universe.
Laws of Thermodynamics
First law — energy can be converted from one form to
another, but it cannot be created nor destroyed.
Second law — energy cannot be converted without the
loss of usable energy (thermal energy/heat).
Example – food chains.
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Energy and Life
Autotrophs and Heterotrophs
1. Autotrophs – organisms that make their own food
from energy from the sun or other sources
- Known as producers
2. Heterotroph – organisms that need to ingest food
to obtain energy.
- Known as consumers
3. All organisms have to release the energy in sugars
and other compounds to live.
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Energy and Life
Metabolism - All of the chemical reactions in a cell.
Photosynthesis —light energy from the Sun is
converted to chemical energy for use by the cell.
Cellular respiration —organic molecules are broken
down to release energy for use by the cell
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Energy and Life
Chemical Energy and ATP
ATP – Adenosine triphosphate – energy for the cell
Made of adenine, ribose, and three phosphates.
ATP releases energy when the bond between the
second and third phosphate groups is broken, forming
a molecule called adenosine diphosphate (ADP) and a
free phosphate group.
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Energy and Life
Chemical Energy and ATP
Storing Energy – When bonds are formed, energy is
stored.
a. ADP – Adenosine diphosphate is similar to ATP, but
with two phosphates instead of three.
b. Energy is stored when a phosphate is
added to ADP
ADP
ATP
Energy
Adenosine diphosphate (ADP) + Phosphate
Partially
charged
battery
Energy
Adenosine triphosphate (ATP)
Fully
charged
battery
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Energy and Life
Releasing Energy
a. Energy is released when bonds are broken.
b.
When a phosphate is removed from ATP, energy is released
c. As many as two phosphates can be removed from ATP.
remove one phosphate = ADP
(adenosine diphosphate)
remove two phosphates = AMP
(adenosine monophosphate)
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Energy and Life
Using Biochemical Energy
1. Cells use ATP for active transport, to
move organelles in the cell, and to synthesize
proteins and nucleic acids
2. Cells do not keep large amounts of ATP in
the cell. The cell can regenerate ATP from
glucose, as needed.
3. ATP is great for transferring energy, but
not for storing it.
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Photosynthesis: An Overview
Photosynthesis – the process by which plants use sunlight
to convert water and carbon dioxide into sugar and starches
A. Investigating Photosynthesis
1. Van Helmont’s Experiment – wanted to know if plants
grow from taking material out of the soil.
Describe the experiment
- Concluded that trees gain most of their mass from water.
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2. Priestley’s Experiment – finds that plants
release a substance that keeps a candle
burning - oxygen
Describe the experiment
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3. Ingenhousz’s Experiment– concludes that plants need
sunlight to produce oxygen.
Describe the experiment
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Photosynthesis: An Overview
These early investigations and the work of other
scientists led to the discovery that in the presence of
light, plants transform carbon dioxide and water
into carbohydrates and release oxygen in the
process.
The Photosynthesis Equation
6 CO2
+ 6 H2O ----> C6H12O6 + 6 O2
carbon dioxide + water  sugar and oxygen
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Photosynthesis
Light Absorption and pigments
1. Photosynthesis requires light
- mixture of
wavelengths =
ROY G BIV
2. Pigments – light
absorbing molecules in the
chloroplast that are
organized into photosystems
a. chlorophyll – principal
pigment that absorbs
light in
the blue-violet and red regions
of the visible spectrum and
not the green region
1). chlorophyll a
2). chlorophyll b
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Photosynthesis
Light Absorption and pigments
b. carotenoids – accessory pigments
such as carotene that absorbs other
wavelengths of light = reflect orange
light
c. Energy absorbed by the chlorophyll
molecules is transferred directly to the
electrons in the chlorophyll raising their
energy levels.
d. It is these high energy electrons
that make photosynthesis work.
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Reactions of Photosynthesis
Inside a Chloroplast
1. Photosynthesis takes place inside the chloroplast.
a. thylakoid – saclike photosynthetic membranes where
chlorophyll and other pigments are found
 Site of light dependent reactions
- photosystems – light collecting units in the thylakoid
membrane
b. granum – a stack of thylakoids
c. stroma – space outside the thylakoid membrane
 Site of Calvin cycle or Light Independent Reactions
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http://upload.wikimedia.org/wikipedia/commons/d/da/Photosystems.png
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Reactions of Photosynthesis
Photosynthesis occurs in 2 phases:
1. Light-dependent reactions – in the thylakoid
2. Light-independent reactions – in the stroma
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Reactions of Photosynthesis
The light dependent reactions
- The absorption of light is the first step in
photosynthesis.
- Chloroplasts capture light energy in the
thylakoids.
- These reactions produce oxygen gas and
convert NADP+ and ADP into NADPH and ATP.
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Reactions of Photosynthesis
The light dependent reactions
1. Light energy excites electrons in photosystem II and also
causes a water molecule to split.
- Releasing an electron into the electron transport
system, H+ into the thylakoid space, and O2 as a
waste product.
2. The excited electrons move from photosystem II
3. To an electron-acceptor molecule in the thylakoid membrane.
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Reactions of Photosynthesis
The light dependent reactions
4. The electron-acceptor molecule transfers the electrons along a
series of electron-carriers to photosystem I.
- This process is called electron transport and the carrier
molecules are known as the electron transport chain or
ETC.
- As the electrons move down the chain they lose energy.
This energy is used to transport protons (H+) from the
stroma into the thylakoid space.
5. Photosystem I transfers the electrons
6. To a protein called ferrodoxin.
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Reactions of Photosynthesis
The light dependent reactions
7. Ferrodoxin transfers the electrons to the electron carrier
NADP+
8. Forming the energy-storing molecule NADPH.
Light strikes the electrons in PS I causing them to become high
energy electrons that move down a second ETC until they reach
NADP+ and form NADPH
NADP+ (nicotinamide adenine dinucleotide phosphate) is an
electron carrier that is converted into NADPH when it accepts its
electrons and a proton(H+).
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Reactions of Photosynthesis
The light dependent reactions
NADPH WILL THEN GET USED IN THE CALVIN CYCLE
This accumulation of H+ in the thylakoid space causes a
difference in charge across the membrane. It is this
difference that provides the energy needed to make ATP.
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Reactions of Photosynthesis
The light dependent reactions
ATP synthase is a protein that moves the H+ ions
back to the stroma from the thylakoid space and
uses their energy to convert ADP into ATP.
Photosystem II
Hydrogen
Ion Movement
ATP synthase
Inner
Thylakoid
Space
Thylakoid
Membrane
Stroma
Electron
Transport Chain
Photosystem I
ATP Formation
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Reactions of Photosynthesis
The light dependent reactions
The light-dependent reactions produce two
high energy compounds: ATP and NADPH
that will provide the energy needed for the
Calvin cycle.
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Reactions of Photosynthesis
The light-independent reactions or the Calvin Cycle
In the second phase of photosynthesis, called the Calvin
cycle, energy is stored in organic molecules such as glucose.
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The Calvin Cycle or Light-Independent
Reactions
1. The Calvin Cycle uses ATP and NADPH from the light reactions to
produce high-energy sugars.
2.These reactions do not require light and occur in the stroma and again
consist of a series of steps summarized in figure 8-11.
3.The Calvin cycle uses six molecules of carbon dioxide to produce a single
6-carbon sugar molecule
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STEP 1
Calvin Cycle
 Phosphoglycerate (3-phosphoglycerate or 3PG) is
the first compound formed by the addition of CO2
to a 5-carbon acceptor (Rubisco). The resulting 6carbon compound is broken into two molecules.
CO2 + 5-carbon acceptor → [6-carbon
intermediate] → two phosphoglycerate.
Calvin Cycle
 In the next step a phosphate group from ATP is added to each
molecule of phosphoglycerate (3PG) to form an intermediate
called 1,3-diphosphoglycerate.
STEP 2
 In the next step a pair of electrons donated by NADPH reduces
1,3-diphosphoglycerate to form glyceraldehyde phosphate.
 Glyceraldehyde phosphate is a 3-carbon sugar, not the 6-carbon
sugar glucose generally identified as the end product of
photosynthesis.
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Calvin Cycle
STEP 3
Glyceraldehyde phosphate leaves the Calvin cycle and is converted to glucose
and stored as starch in the stroma of the chloroplast or used in other reactions
in the cytoplasm to make sucrose for transporting to other parts of the plant.
The regeneration of the initial 5-carbon acceptor in the Calvin cycle avoids
wasteful reactions that use large amounts of ATP and NADPH and allows
continuous CO2 fixation. In order to regenerate the 5-carbon acceptor the cycle
runs three times
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Calvin Cycle
 The ATP and NADPH come from the light dependent reactions, the
inorganic phosphate (Pi), the adenosine diphosphate (ADP) and
nicotinimide adenosine diphosphate are recycled to the light
reactions
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Two Three- Carbon molecules leave
to make sugars or other stuff
The 5 carbon molecule is regenerated
Energy
C is
C used.
C C C
12 ATP
12 NADPH
6 ATP
C C C
C C C
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Carbon dioxide enters the Calvin Cycle
Energy is used
Factors Affecting Photosynthesis
1. Many factors affect the rate at
which photosynthesis can
occur.
2. These factors include:
a. Water – can slow or stop
b. Temperature – enzymes
work best at 0 to 35.0 C,
slow or stop
c. Light intensity – reaches a
plateau but varies with the
plant
d. CO2 – reaches a plateau
but again varies with the
plant
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