Photosynthesis
How Plants Convert Solar Energy
into Chemical Energy (Glucose)
Leaves are organs that carry
out photosynthesis.
Go through the activity: E-Life 8.1 p.2-5
Leaf Cross-section:
Lilac Leaf Cross Section
Plant Cell
The Inside of a Chloroplast
Thylakoid space
Stroma
Outer membrane
Inner membrane
Thylakoid
Grana
Outer membrane
• Protective covering
allows only small
molecules to
diffuse through
(semi-permeable)
Inner membrane
• Forms border for
stroma and
regulates passage
of materials in and
out of the
chloroplasts
Stroma
• Dense fluid within
the chloroplast that
lies inside the
membranes, it is
the site of
conversion of CO2
to Sugar
Grana
• Dense layered
stacks of thylakoid
sacs
Thylakoid membrane
• Internal membrane
system consisting
of a flattened sac
Thylakoid Space
• Serves as site that
converts light
energy to chemical
energy inside the
thylakoid
membrane
The Chemical Formula
of Photosynthesis
6CO2 + 6H2O
Carbon
dioxide
Water
C6H12O6 + 6O2
Glucose
Oxygen
Photosynthesis consists of Two
major Reactions
Light Reactions Overview
• Occurs on thylakoid
membrane
• Light energy is absorbed
by chlorophyll found in
Photosystems I & II
• Water is split releasing O2
• NADPH & ATP are formed
after series of reactions
Light Dependent Reactions take place on
the membrane surface of the Thylakoid
http://dendro.cnre.vt.edu/forestbiology/photosynthesis.swf
http://www.science.smith.edu/departments/Biology/Bio231/ltrxn.html
Step 1: Photosystem II (PS II)
• The pigments in PS II absorb light
energy and excites electrons
which increases their energy level.
• The electrons travel to the electron
transport chain.
• The electrons come from broken
up water molecules that create 2
electrons, 2 H+ and 1 oxygen.
• The oxygen is released so we can
breathe it and the H+ are released
into the thylakoid membrane
Step 2: Electron Transport
Chain (ETC)
• The excited high energy
electrons from PS II travel
through the electron
transport chain to reach
PS I
• The electron’s energy is
used by the molecules in
the ETC to transport H+
ions from the Stroma to
the inner thylakoid space.
Step 3: Photosystem I (PS I)
• The pigments in PS I
use light to reenergize
the electrons
• A molecule NADP+
picks up the high
energy electrons and
H+ ions to then
become NADPH
Step 4: Hydrogen Ion
Movement
• During step 3 when the electrons
leave the ETC to go to the
NADP+, H+ ions are pumped
across the thylakoid membrane.
• As this continues, the Thylakoid
spaces fill up with H+ and
becomes Positively charged
while the thylakoid membrane is
negatively charged.
• This charge difference provides
the energy to make ATP
Step 5: ATP formation
• A protein called ATP
synthase on the thylakoid
membrane allows H+ ions to
pass through the membrane
since they cannot cross on
their own.
• As H+ passes through, the
protein rotates like a turbine
building up energy, this
energy allows the ADP and a
Phosphate group join
together to produce ATP
Overview of Light reactions
• Reactants (what goes in): Water, ADP,
and NADP+
• Products (what comes out): Oxygen, ATP,
and NADPH
– ATP and NADPH move on to the Calvin Cycle
Light Reactions Children’s Story
• Compose a children’s story about the
travels of an electron through the light
reactions.
• Be sure to represent all the elements of
the light reactions in your story.
• Illustrate a scene from your story which
will best represent the activity during the
light reactions.
• Attach your story to your illustration.
The Calvin Cycle
1. Occurs in the Stroma
2. Uses 6CO2 from the
atmosphere
3. Energy is provided from
ATP and NADPH (from Light
Reactions)
4. After 2 cycles, glucose is
made.
Step 1: Carbon Fixation
• 6 carbon dioxide
(CO2) molecules
enter the cycle from
the atmosphere.
• CO2 combines with
6, 5-carbon
molecules and
produces 12, 3carbon molecules
Step 2: Reduction
• Energy from
ATP and high
energy electrons
NADPH are
used to convert
the 12 3-carbon
molecules into
higher energy
forms.
Step 3: Release of one G3P
• Two of the 12 3Carbon molecules
leave the cycle to
produce things like
sugars, lipids,
amino acids, and
other compounds
needed for the
plants metabolism
and growth.
Step 4: Regeneration of RuBP
• The remaining ten 3carbons are converted
back into six 5-carbon
molecules.
• These molecules will
combine with 6 new CO2
molecules to begin the
next cycle.
• Remember: the glucose is
not made until after the
second cycle!
Photosynthesis Essential
Questions
1. Name the two most important parts of a
chloroplast. Which parts of
photosynthesis occurs in each structure?
2. What are the reactants and products of
the Light Reactions? The Calvin Cycle?
3. Explain how the Calvin Cycle is indirectly
dependent on light.
4. Explain how ATP synthase is a protein
channel and an enzyme.
PhotoFinish Alterations
• Fill 2 beakers with 80 mL sodium
bicarbonate solution
• Once air spaces are vacuumed, pour
syringe solution and leaf disks into each
beakers (10 disks each).
• Make sure both sets are ready to start at
the same time.
• May use stop watch feature on phone.
PhotoFinish Cleanup
• Discard solution down drain—Do NOT let
leaf disks down drain!!
• Discard used spinach and disks in trash
• Rinse out beakers; return to side
• Separate syringes, clean out hole
punches; return to side
• Unplug lamp & dry off area