6
Photosynthesis (Nelson text Ch6.1 and 6.2)
WHAT IS IT?
Photosynthesis is the process in which green plants store the energy of
sunlight in chemical bonds, by converting carbon dioxide and water into
organic compounds
General Equation:
6 CO2 + 6 H2O  C6H12O6 + 6 O2
WHERE DOES IT HAPPEN?
Photosynthesis takes place in the chloroplasts of plant cells. The
chloroplast is surrounded by an inner and outer membrane, and is
generally larger than a mitochondrion. The inner membrane
encloses an internal compartment, which contains many flattened
membrane-bound sacs called thylakoids. When these thylakoids
are stacked on one-another they are known as a granum (pleural =
grana). It is the thylakoid membranes that contain all the
photosynthetic pigments (chlorophyll a/b, carotenoids, etc). The
fluid surrounding the grana in the inner membrane is a gel-like
substance called the stroma.
The reactions of photosynthesis occur in two phases:
1. light-dependent reactions
(sunlight energy stored in as chemical energy)
 happen in the thylakoid membrane
2. light-independent reactions (a.k.a. “dark reactions”, Calvin Cycle)
(carbon into sugars a.k.a. carbon fixation)
 which occur in the stroma.
Photosynthesis Overview: (p187)
Do: p187 #1,2,3
Overview:
The light-dependent reactions (capturing solar energy p.188)
 are the steps of photosynthesis that convert solar energy to
chemical energy
 take place in the thylakoid membranes
 Light-absorbing pigments are embedded in clusters called
photosystems
 light energy absorbed by chlorophyll is used to energize
electrons
 the energized (“excited”) electrons are temporarily stored in
2 products:
1. ATP (via an electron transport chain and
chemiosmosis)
2. NADPH (chemical “taxi” like NAD+ in CR)
 There is no sugar produced in the light reactions!!
what was that?!
The light-independent reactions (Calvin cycle)
 produce sugar from carbon dioxide (“carbon fixation”)
 occur in the stroma
 use energy stored in ATP and NADPH from light reactions
 incorporate CO2 from the air and H+ from NADPH
to form G3P (a 3C sugar that is used to make glucose or
complex carbohydrates)
Photosynthesis Overview:
Now – a more detailed look…
Light Reactions (p188)
- The light reactions are dependent on the photo-excitation of chlorophyll
- In the thylakoid membrane, chlorophyll is organized along with other
pigments into 2 photosystems
- A photosystem is a cluster of a few hundred chlorophyll and carotenoid
molecules (types of pigment).
o This number and variety allow a photosystem to harvest light
over a larger surface and larger portion of the spectrum than
any single pigment molecule could harvest.
- Within each photosystem is one particular chlorophyll molecule that acts
as the reaction center – where the combined energy from all the light
photons absorbed elevates an electron to a higher energy level. This
electron is then transferred to an electron acceptor molecule
(reduction reaction).
- The electron acceptor traps the high-energy electron before it can return
to ground state.
-
All the other pigments are known as antenna pigments,
transmitting absorbed photons of energy to one another until it
reaches the reaction center
- The solar-powered transfer of electrons from chlorophyll to the
primary electron acceptor is the first step of the light reactions.
- There are two types of photosystems in the thylakoid membrane –
photosystem I and photosystem II
- Each photosystem has a different reaction center molecule:
o The reaction center of photosystem I is known as P700 because
this pigment is best at absorbing wavelengths of 700 nm
(far red part of the spectrum).
o The reaction center of photosystem II is known as P680
because its absorption spectrum has a peak at 680 nm
(also in the red part of the spectrum, but further to the right on
the spectrum – shorter wavelength).
Electron Flow in the Light Reactions (p188)
1. When photosystem II absorbs light, an electron excited to a higher energy
level in the reaction center chlorophyll (P680) is passed to an electron
acceptor in the thylakoid membrane
Note: the oxidized chlorophyll
p680 is now short an electron.
2. Each photo-excited electron passes from the primary electron acceptor
through a series of redox reactions in an electron transport system similar
to that in cellular respiration (involving chemiosmosis).
3. When an electron reaches the “bottom” of the electron transport chain, it
fills an electron “hole” in photosystem I that was created by its own light
energy reaction
Q: HOW IS THIS DIFFERENT FROM THE ETS IN CR?
4. The primary electron acceptor of photosystem I (P700) passes the photoexcited electrons through a series of redox reactions, eventually reducing
NADP+ to NADPH. NADPH will later provide the reducing power
required for the synthesis of sugar in the Calvin cycle.
Electron Flow in the Light Reactions
…so how does p680 in PSII
keep providing high energy
electrons for this process?
Photolysis:
An enzyme in the thylakoid lumen extracts electrons from water and
supplies them to P680, replacing each electron that the chlorophyll lost
when it absorbed the light energy. This is step that releases oxygen into
the atmosphere
2H20(l) + solar energy 4H+ + 4e- + O2(g)
Do:
p188 #4-6
p190 #7
p191 #11-13
Calvin Cycle (light-independent reactions) (p192)
Carbon dioxide is “strung together” to form high-energy organic
compounds (= carbon fixation).
Involves 3 phases:
Phase 1: Carbon Fixation
o a carbon dioxide molecule binds with a 5-carbon sugar,
ribulose biphosphate (RBP), producing a six-carbon
molecule
o the 6C molecule is so unstable it immediately splits in half to
form two 3C phosphoglycerate (PGA) molecules.
Phase 2: Reduction
o PGA gains a phosphate and an electron from ATP and NADPH,
respectively
o This reduces PGA to G3P (glyceraldehyde-3-phosphate)
o For every 3 molecules of carbon dioxide, there are
6 molecules of G3P produced. Only one of these 6 will exit
the Calvin cycle.
o The remainder are recycled to regenerate 3 molecules of RBP
Phase 3: Regeneration of the carbon dioxide acceptor (RBP)
o
5 molecules of G3P are rearranged in a series of reactions
into
3 molecules of RBP
o
This requires 3 ATP molecules
Products of Calvin cycle
(3 CO2 needed to produce one G3P)
 1 molecule of G3P
 9 molecules of ADP (from ATP)
 6 molecules of NADP+ (from NADPH)
- the G3P goes on to make carbohydrates such as glucose, sucrose
(glucose + fructose), starch etc
Questions:
1. how much CO2 is needed to produce one glucose molecule?
2. How much ATP?
3. where does the energy come from for carbon fixation?
4. where does the Calvin cycle occur?
The Calvin Cycle (Light Independent Reactions)
easy recap
Got it? (p194)
 Review Summary
 Do: #3-7