Section 9.2
Photosynthesis: Trapping the
Sun’s Energy
p. 225 - 230
Trapping Energy From Sunlight
• Plants and other green organisms must
trap light energy from the sun to be able
to use it.
• The energy must then be stored in a way
that can be used by the cells.
• ATP is the form!
• Autotrophs trap and make energy with
photosynthesis
Trapping Energy From Sunlight
• During photosynthesis, plants use the
sun’s energy to make simple sugars.
• These sugars are then made into
complex carbohydrates, such as starch.
• Starches store energy.
Trapping Energy From Sunlight
• There are two parts to photosynthesis:
light-dependent reactions and lightindependent reactions.
• Light-dependent reactions change light
energy into chemical energy, splitting
water and releasing oxygen.
• Light-independent reactions produce
simple sugars.
Where Does Photosynthesis Take Place?
• Photosynthesis takes place in the
chloroplast .
• Chloroplasts contains pigment.
• Pigments take in specific wavelengths of
sunlight.
Where Does Photosynthesis Take Place?
• Wavelengths of sunlight transfer energy.
• The most common pigment in the
chloroplast is chlorophyll.
• Chlorophyll is the main energy-trapping
molecule in the plant.
Chlorophyll a
• Every place that
doesn’t have a letter
telling you the
element, is a carbon.
• Hydrogens are
attached to all of the
empty spots on the
carbons.
The red, yellow, and
purple pigments are
visible in the
autumn.
Why can’t you see
the colors during the
summer?
During the summer,
the plant cells
manufacture
chlorophyll, which
hides the other
pigments that are
present in leaves.
Where Does Photosynthesis Take Place?
• Chlorophyll is a plant pigment that absorbs
most wavelengths of sunlight except
green.
• Since it cannot absorb green, it reflects
green.
• This makes leaves look green!
• In fall the leaves stop producing
chlorophyll so other pigments are now
visible.
Wavelengths of light
• Light that we see and plants use for photosynthesis is a
small part of the spectrum of electromagnetic radiation,
from just below 400 nm to just above 700 nm.
• Our peak sensitivity is in the middle of this range at
about 550 nm or in the green part of the spectrum.
• Coincidentally this is the part of the spectrum which
plants do not use or "see".
• They absorb light in the blue and in the red.
Chlorophyll-a absorption spectrum
Wavelengths of light
• The sunlight is absorbed by pigments.
• Chlorophyll absorbs red and blue light and
reflects green so it appears green.
• Carotenoids are other pigments which
absorb in the blue and appear yellow.
Light-Dependent Reactions
• The first phase of photosynthesis needs
sunlight.
• This first phase takes place in the chloroplasts,
more specifically, in the thylakoid membranes:
membranes within the chloroplast that contain
enzymes.
• Energy from sunlight is trapped by chlorophyll in
this thylakoid membrane.
Light-Dependent Reactions
• When sunlight strikes the chlorophyll in
the plant’s leaves the energy from the
sunlight is transferred to electrons in the
chlorophyll.
• The electrons move from the
chlorophyll to an electron transport chain.
Chlorophyll molecules absorb
light energy and energize
electrons for producing ATP
and NADPH.
NADP is
nicotinamide adenine
dinucleotide phosphate
The H is a hydrogen ion that
gets combined with the NADP
along with two excited
electrons.
Light-Dependent Reactions
• *** The electrons move from the
chlorophyll to an electron transport chain.
• An electron transport chain is a line of
proteins embedded in a membrane along
which the electrons are passed down.
Light-Dependent Reactions
• Each protein in the line passes the
electron to the next protein.
• As the electrons pass along this line, they
lose some of their energy.
Light-Dependent Reactions
• If you filled a bucket with water and
passed it along a line of people very
quickly, some of the water would spill.
• This is similar to how the electrons lose
their energy.
Electron transport chain
What Happens to the Lost Energy?
• The energy the electrons lose can be used
to form ATP from ADP.
• Energy that is not used to form ATP can
be stored for use in the light-independent
reactions.
What Happens to the Lost Energy?
• The energy is stored in an electron
carrier called NADPH.
• NADPH carries the energy to the lightindependent reaction.
NADP
• NADP = an electron and proton “carrier”
• Carries the electrons and protons to make
energy.
This is what NADP+ looks like:
Conversion of NADP + to NADPH
• And here is a closer look at the
conversion of NADP + to NADPH
Photololysis – still in phase one , light dependent
In photolysis, a molecule of water is split to
replace electrons lost from chlorophyll.
It splits water into oxygen and hydrogen.
Light-Independent Reactions
• The second phase of photosynthesis does not
need sunlight.
• It also takes place in the chloroplasts and is called
the Calvin Cycle.
• More specifically, it takes place in the stroma.
• The stroma is the liquid-y part of the chloroplast.
Light-Independent Reactions
• The Calvin Cycle uses carbon dioxide to form
sugars.
• The sugars then become stored energy.
• You could say it is a series of reactions that
synthesize simple sugars from carbon dioxide
and hydrogen.
• It’s a big job to make sugars from carbon
dioxide!
Light-Independent Reactions
• The ATP and NADPH produced in the
light dependent reactions are used.
• The chloroplasts breaks down this large
task into very small steps.
• The end result is energy stored in the
plant as sugar.
Light-Independent Reactions
• Organisms that eat plants use these
sugars to give them energy.
• The energy is used in cellular
respiration.
The stroma in chloroplast is where the Calvin Cycle takes place.
The Calvin Cycle takes the carbon in CO2,
adds it to one molecule of RuBP, and forms
sugars through a series of reactions in the
stroma of the chloroplasts.
The NADPH and ATP produced during the
earlier light-dependent reactions are
important molecules for this series of
reactions.
Why is the Calvin cycle in plants directly
and indirectly important to animals?
The energy in the bonds of the sugars
synthesized by plants is the source of
energy for animals. Indirectly, Plants also
provide shelter and a home to many
animals
Calvin Cycle
• At the beginning, one molecule of CO2 is
added to one molecule of a five-carbon
sugar RuBP to form a six-carbon sugar.
• After a series of reactions, the sugar
breaks down to two three-carbon sugars
called phosphoglyceraldehyde, or PGAL.
Calvin Cycle
• After 3 rounds of this cycle, 6 molecules of
PGAL are produced.
• 5 PGALs are rearranged to make 3 new
RuBP molecules
Calvin Cycle
• The sixth PGAL is available to make
sugars elsewhere in the body.
• Calvin cycle takes 6 rounds to make one
glucose molecule