In This Lesson:
Photosynthesis
(Lesson 2 of 3)
Today is Friday (!),
October 30th, 2015
Pre-Class:
Once again, what is the general chemical
equation for photosynthesis?
If you don’t know the chemical symbols, write
it in words, or draw pictures if you have to…
Today’s Agenda
• The Two [Giant] Steps of Photosynthesis
– The Light Reactions and the Dark Reaction.
• Sounds like something from Lord of the Rings.
• This may be one of the most info-heavy lesson
we’re going to do, which means I need you to do
three things:
– Stick with me.
– Use your head.
– Ask questions.
• Where is this in my book?
– Page 108 and following…
By the end of this lesson…
• You should be able to identify reactants and
products of the detailed steps of each
photosystem within the photosynthesis
reactions.
Summary of Covered Info
• Photosynthesis is made of:
– The Light Reactions (Light-Dependent)
– The Dark Reactions (Light-Independent or Calvin
Cycle)
• In the Light Reactions, the chloroplast uses an
ETC to make ATP and protons, making oxygen
as a byproduct.
• In the Calvin Cycle, the chloroplast uses the
ATP and protons to make glucose.
Now in depth…
• The first step in photosynthesis is the Light
Reactions (or Light-Dependent Reactions).
• The first step of the Light Reactions is
Photosystem II.
– Photosystem II was discovered after Photosystem I,
but was later determined to happen first. 
• Light strikes chlorophyll (pigment), and
chlorophyll’s electrons get “excited.”
– They start jumping from pigment molecules to other
molecules in the thylakoid in an electron transport
chain.
Photosystem II
• Elsewhere, two water molecules (H2O) are
split into an oxygen molecule, four H+ ions
(protons), and two extra electrons.
– The oxygen becomes waste, while the electrons
replace the lost electrons from chlorophyll.
– The protons do something cool.
Quick Summary
• This is photosynthesis.
• We’re in the first stages, known as the LightDependent Reactions.
• The first part of the Light-Dependent Reactions is
Photosystem II.
• So far, light has excited electrons and sent them
on an ETC (more to come).
• Meanwhile, water has been torn into protons,
generating replacement electrons and oxygen
waste.
About those protons…
• The electrons move between molecules,
eventually powering an H+ pump.
– The pump sends H+ ions into the thylakoid.
• The buildup of
protons inside
creates an
electrochemical
gradient.
– Like a
concentration
gradient but
with charge.
http://bioweb.wku.edu/courses/Biol120/images/non-cyclic1.jpg
Electron Transport Chain
e-
http://spellbinderspaperarts.com/blog/wp-content/uploads/2009/07/s7-015-holiday-lights.jpg
Electron Transport Chain
e-
http://bioweb.wku.edu/courses/Biol120/images/non-cyclic1.jpg
What It Looks Like
http://authors.ck12.org/wiki/images/e/e7/Bio_Thylakoid_Membrane.png
About those protons…
• The buildup of positive charge inside the
thylakoid causes the protons to “want” to
diffuse back out.
• Protons diffuse out of the cell through a
special protein. Why is it special?
– Because this is a combination carrier
protein/enzyme! It’s named ATP Synthase.
• ATP Synthase adds a phosphate group to ADP,
making ATP.
– ATP is used in the second step of photosynthesis.
Electron Transport Chain
H+
e-
http://bioweb.wku.edu/courses/Biol120/images/non-cyclic1.jpg
Pausing for summary…
• Before we go further, let’s recap:
– The first overall stage of Photosynthesis is called
the Light Reactions.
– The Light Reactions are made of two steps,
Photosystems II and I.
– Photosystem II used an electron transport chain to
create ATP and a buildup of protons as a result of
splitting water.
But, of course,
it gets more complicated…
• There is another Electron Transport Chain, still part
of photosynthesis.
• PHOTOSYSTEM I: Electron Transport Chain
– Instead of making ATP, this one makes NADPH out of
NADP+ by using those protons from before.
• NADPH is a molecule that “carries” electrons like a gift card
carries money.
• NADP+ is the empty gift card, NADPH has value, but only when
used.
– This ETC combines NADP+ and H+ into NADPH.
Pausing for summary…
• Before we go further, let’s recap:
– The first overall stage of Photosynthesis is called the
Light Reactions.
– The Light Reactions are made of two steps,
Photosystems II and I.
– Photosystem II used an electron transport chain to
create ATP and a buildup of protons as a result of
splitting water.
– Photosystem I used a different electron transport
chain and combined protons with NADP+ to make
NADPH.
– Light has been converted to chemical energy, but no
glucose has yet been made.
Summary Analogy
Step Two
No light at the end of this tunnel
• Stage 1 (Photosystems II and I) requires light;
Stage 2 does not.
• What is the ultimate goal of photosynthesis
again?
– Energy storage in the form of glucose.
• So how do we actually make glucose?
Carbon Dioxide Fixation
• The process of forming organic compounds,
(like glucose) by using CO2 is called “carbon
dioxide fixation.”
• Ever hear about structures or living things that
“sequester” carbon dioxide?
– Perhaps from this man?
http://scrapetv.com/News/News%20Pages/Science/Images/al-gore-thumbs-up.jpg
CO2 Fixation
• Carbon dioxide may
be fixed in a few
ways.
– The Calvin Cycle
– Named, once again,
for a man with one of
the goofiest names in
science:
Melvin Calvin
http://www.jergym.hiedu.cz/~canovm/biocykl/cykly/mc.jpg
The Calvin Cycle
• The whole Calvin Cycle is a little complicated.
• For now, this is what you need to know:
– Happens in the stroma.
– It’s powered by energy from ATP and NADPH.
– CO2 and other molecules are made into simple
sugars like glucose.
• Some simple sugars are made into more complex
organic molecules.
• Some simple sugars help repeat the process.
End of Stage Two
(Hooray!)
• Stuff that affects the rate of photosynthesis:
–
–
–
–
Guesses?
Amount of light.
Amount of CO2
Temperature (why?)
• Denaturation!
• Photosynthesis is only as fast as its slowest step.
– Like a relay race with a really really big guy that just can’t run that
fast.
– In other words, if any part of the long process of photosynthesis
slows down, it all slows down.
Summarizing Photosynthesis
• LIGHT REACTIONS
• Photosystem II
– Happens in the thylakoids.
– Requires light and water.
– Produces ATP.
• Photosystem I
– Happens in the thylakoids.
– Produces NADPH.
Summarizing Photosynthesis
• DARK REACTIONS
• Calvin Cycle
– Happens in the stroma.
– Requires ATP and CO2.
– Produces glucose.
Okay…deep breaths…
• I just fired off a LOT of information.
• Let’s organize it. In your notebooks, create a
table that shows the three main reactions of
photosynthesis.
– Photosystems II and I, and the Calvin Cycle.
• Also include:
– What does each step make?
– What happens in each step?
• (your own general words)
Closure
• CrashCourse – Photosynthesis
Closure
• TED: Amanda Ooten – The Simple Story of
Photosynthesis and Food