Photosynthesis
Section 5-2
Photosynthetic Organisms
Use the Energy in Sunlight
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Plants capture energy from sunlight
Most of the time when you trace your food
back to its origin, you end up with plants
Plants, algae, and some bacteria capture
about 1% of the energy in the sunlight that
reaches Earth and convert it to chemical
energy through the process of
photosynthesis.
Photosynthesis
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The process that provides energy for
almost all life.
Has three stages
OVERALL CHEMICAL FORMULA
3 CO2 + 3 H2O → C3H6O3 + 3 O2
3 Stages of Photosynthesis
Stage 1:
Energy is captured from sunlight.
Stage 2:
Light energy is converted to chemical
energy, which is temporarily stored in
ATP and the energy carrier molecule
NADPH.
3 Stages of Photosynthesis (con’t)
Stage 3:
The chemical energy stored in ATP and
NADPH powers the formation of
organic compounds, using CO2
Photosynthesis
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Occurs in the chloroplast of plant cells
and algae and in the cell membrane of
certain bacteria
Plants use the organic compounds
they make during photosynthesis to
carry out their life processes.
Stage One of Photosynthesis
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Light energy is absorbed
These reactions are often called “light
reactions” or “light-dependent
reactions” because the absorption of
light has to occur for the reactions to
proceed
Light energy is used to make energystoring compounds
Stage One of Photosynthesis
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Pigments absorb different wavelengths of light.
Pigments – light-absorbing substances that
absorb only certain wavelengths
Chlorophyll – primary pigment involved in
photosynthesis. Absorbs mostly blue and red
light. Reflects green and yellow. This is what
makes a plant look green.
Two types of chlorophyll – Chlorophyll A and
Chlorophyll B
Stage One of Photosynthesis
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Carotenoids – pigments that produce
yellow and orange fall leaf colors.
Produces the colors of many fruits,
vegetables, and flowers.
Absorb different wavelengths of light
from those absorbed by chlorophyll
Stage One of Photosynthesis
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By using chlorophyll and carotenoids,
plants absorb more light energy during
photosynthesis.
Pigments are located in the
chloroplasts of leaf cells.
Thylakoids – clusters of pigments are
embedded in the membranes of diskshaped structures called thylakoids
Stage One of Photosynthesis
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When light strikes thylakoid, energy is
transferred to electrons in chlorophyll
and other pigments.
This energy transfer causes electrons
to jump to a higher energy level.
These electrons are said to be
“excited”.
This is how plants first capture energy
from sunlight.
Stage One of Photosynthesis
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Excited electrons jump from chlorophyll
molecules to other nearby molecules in the
thylakoid membrane, where the electrons are
used to power the second stage of
photosynthesis.
The excited electrons that leave chlorophyll
molecules must be replaced by other
electrons. Replacement electrons come from
water molecules. The water molecules are
split by an enzyme inside the thylakoid.
Stage One of Photosynthesis
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Water molecules split – chlorophyll molecules
take electrons from hydrogen atoms leaving
hydrogen ions.
Remaining oxygen atoms from disassembled
water molecules combine to form oxygen gas.
Stage Two of Photosynthesis
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In this stage, light is converted to chemical
energy.
Electron Transport Chains – series of
molecules through which excited electrons are
passed along a thylakoid membrane
Excited electrons leave chlorophyll molecules
are used to produce new molecules that
temporarily store chemical energy, including
ATP.
Stage Two of Photosynthesis
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An excited electron jumps to a nearby
molecule in a thylakoid membrane.
The electron is passed through a series of
molecules along the thylakoid membrane like a
ball being passed down a line of people.
Figure 5-8 p. 100
Stage Two of Photosynthesis –
Action of Electron Transport Chains
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One type of ETC contains a protein that acts
as a membrane pump.
Excited electrons lose some of their energy
as they pass through this protein.
Energy lost by electrons used to pump
hydrogen ions (H+) into the thylakoid.
Recall… H+ produced when water
molecules are split inside the thylakoid.
Stage Two of Photosynthesis –
Action of Electron Transport Chains
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H+ concentration higher on inside of thylakoid
– produces concentration gradient across
thylakoid membrane.
Results in H+ having the tendency to diffuse
back out of thylakoid down their concentration
gradient
Occurs through specialized carrier proteins –
unique – function as enzyme and carrier
protein/ion channel
Stage Two of Photosynthesis –
Action of Electron Transport Chains
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As H+ ions pass through channel portion –
protein catalyzes a reaction in which a
phosphate group is added to ADP – makes
ATP.
Movement of H+ across thylakoid membrane
through carrier proteins provides energy to
make ATP used to power the third stage of
photosynthesis
Stage Two of Photosynthesis –
Action of Electron Transport Chains
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A second ETC provides energy used to make
NADPH
NADPH – an electron carrier that provides the
high-energy electrons needed to make carbonhydrogen bonds in third stage of
photosynthesis
Excited electrons combine with H+ as well as
an electron acceptor called NADP+ forming
NADPH.
Stage Two of Photosynthesis –
Light-Dependent Reactions
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Pigment molecules in thylakoids of chloroplast absorb
light energy.
Electrons in pigments excited by light and move
through ETC in thylakoid
These electrons replaced by electrons from water
molecules, which are split by an enzyme
Oxygen atoms from water molecules combine to form
oxygen gas
H+ ions accumulate inside thylakoids, setting up a
concentration gradient that provides the energy to
make ATP and NADPH.
Stage Three of Photosynthesis
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Carbon atoms from CO2 in the atmosphere are
used to make organic compounds which store
chemical energy.
Carbon dioxide fixation – transfer of CO2 to
organic compounds
The reactions that “fix” CO2 often called “dark
reactions” or light-independent reactions.
Stage Three of Photosynthesis –
Calvin Cycle
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Calvin Cycle – series of enzyme-assisted
chemical reactions that produce a 3-carbon
sugar. Figure 5-9 p. 102
STEP 1: Each molecule of CO2 is added to a
5-carbon compound by an enzyme
STEP 2: The 6-carbon compound splits into 2
3-carbon compounds. Phosphate groups from
ATP and electrons from NADPH are added to
the 3-carbon compounds. Forms a 3-carbon
sugar.
Stage Three of Photosynthesis –
Calvin Cycle
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STEP 3: 1 3-carbon sugar is used to make an
organic compound
STEP 4: The other 3-carbon sugar is used to
regenerate the 3-carbon compound used in the
beginning of the cycle
The reactions are cyclic because they recycle
the 5-carbon compound.
3 CO2 molecules must enter the cycle to make
3-carbon molecules (6 are made)
Energy is supplied by 6NADPH and 9 ATP
molecules.
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http://www.science.smith.edu/departm
ents/Biology/Bio231/calvin.html
http://www.phschool.com/science/biolo
gy_place/biocoach/photosynth/calvin1.
html
http://www.emc.maricopa.edu/faculty/f
arabee/BIOBK/BioBookPS.html
Factors that Affect
Photosynthesis
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Light – the rate of photosynthesis increases as light
intensity increases until all pigments are being used.
Then the rate of photosynthesis levels off.
Most efficient within a range of temperatures.
Like all metabolic processes, involves many enzymeassisted chemical reactions. Enzymes only react
properly within certain temperature ranges.
CO2 concentration – once certain concentration of
CO2 is present, cannot proceed any faster.