PHOTOSYNTHESIS
Section 6.2 – The Reactions of
Photosynthesis.
Pages 186-199
The Reactions of Photosynthesis
• Chlorophyll and other pigments are in
chloroplasts which capture packages
of energy called photons from sunlight.
• Photosynthesis – the captured energy
is converted into chemical energy
(glucose).
– A process made up of a series of complex
chemical reactions.
Energy Containing Molecules
Formed in Photosynthesis
1. ATP – principal energy supply molecule. It
provides an immediate source of energy
for cellular processes (such as growth and
movement).
2. NADPH – electron donor involved in
energy transfers.
3. Glucose – sugar; transport molecule;
medium term energy storage in most cells
(used to store energy for later use).
ATP
• Adenosine triphosphate (ATP):
– Used by all living cells
– Provides immediate energy for cellular functions
(ex. Synthesis of needed chemicals, transport
materials across membranes).
– Formed by the addition of an inorganic
phosphate group (Pi) to a molecule of lower
energy adenosine diphosphate (ADP).
– Same energy can be released to the cell by a
reaction that splits ATP back to ADP and Pi.
– Figure 1 on page 186
NADPH
• Nicotinamide dinucleotide phosphate
(NADP+) appears in several places
during photosynthesis.
– Accepts one hydrogen atoms and two
electrons to form NADPH.
– NADPH can donate electrons to other
molecules and becomes NADP+ again.
An Overview of Photosynthesis
•
Three distinct (but connected) stages:
1. Capturing solar energy and transferring it to
electrons.
2. Using captured solar energy to make ATP and
to transform high energy electrons to NADP+;
make NADPH, which is then used as a high
energy electron carrier molecule.
3. Using energy stored in ATP and high energy
electrons carried by NADPH to form energy
rich organic molecules, such as glucose, from
CO2.
An Overview of Photosynthesis
• Light dependent reactions:
– First 2 stages
– Directly energized by light
– Require chlorophyll and occur on the thylakoid
membranes in chloroplasts
– Light energy is transferred to carbohydrate
molecules (last stage)
• Carbon fixation:
– 3rd stage
– Incorporates CO2 into carbohydrate
– Takes place in the stroma
An Overview of Photosynthesis
• Light-independent:
– The second set of reactions in
photosynthesis and does not require solar
energy.
• Calvin cycle:
– A cyclic set of reactions occurring in the
stroma of chloroplasts that fixes the
carbon of CO2 into carbohydrate
molecules and recycles coenzymes.
Calvin Cycle
Stage 1: Capturing Solar Energy
• Photosystems: a cluster of
photosynthetic pigments embedded in
a thylakoid membrane of a chloroplast
that absorbs light energy.
– Photosystem I & II
• These are the actual molecules responsible
for capturing light energy.
Stage 1: Capturing Solar Energy
• Solar energy is captured when an
electron in a chlorophyll molecule
absorbs a photon.
– Electrons are high energy particles
present in atoms (excited).
– Electrons have low energy before a
photon hits them.
– Photon has now been converted to
chemical energy! 
Stage 1: Capturing Solar Energy
• Electron transport chain: a series (steps) of
progressively stronger electron acceptors; each
time an electron is transferred, energy is released.
• Photolysis: a chemical reaction in which a
compound is broken down by light.
– Electrons removed from the photosystem need to be
replaced so this is where they come from.
– Replacement electrons come from water.
– In photolysis, the solar energy absorbed by the cholorphyll
is used to split water into hydrogen ions (H+) and oxygen
gas.
– Occurs in the thylakoid lumen.
Stage 1: Capturing Solar Energy
• For every 2 water molecules
consumed, 4 electrons and 1 oxygen
are made.
2H2O (l) + energy  4H+ + 4e- + O2(g)
Stage 2: Electron Transfer and ATP
Synthesis
•
•
Now that we have solar energy
captured, we need to make ATP.
Two tasks:
1. The electron transport train
- Build up of charged particles
2. Oxidation-reduction reactions
- Direct transfer of electrons
The Electron Transport Chain
• Solar energy is used to excite
electrons which were removed from
water.
• This added energy lifts them up.
• Potential energy is gradually released
and electrons travel down the stairs to
their original lower level.
• Some energy is captured to make ATP
• See figure 3 on page 189.
Oxidation-Reduction Reactions
• High energy electron donor passes an
electron to a lower energy electron acceptor
– this is an oxidation-reduction or a redox
reaction.
• Oxidation: a reaction in which an atom or
molecule loses electrons. (an ox loses)
• Reduction: a reaction in which an atom or
molecule gains electrons. (red cat gains)
Oxidation-Reduction Reactions
• Electron donors such as NADPH tend
to lose electrons.
• Electron acceptors like NADP+ tend to
gain electrons.
Look at Figure 4 page 190
• animation
Key steps in electron transfer during
the light dependent reactions:
1.
2.
3.
4.
Electrons from photosystem II are transferred
along an electron transport chain and across the
thylakoid membrane to the inner surface.
Some of their energy is used to pull H+ ions
across the membrane, making a build up of a
positive charged lumen.
The electrons, which have lost most of their
energy, are then transferred to chlorophyll
molecules in the photosystem I complex, where
they absorb solar energy and reach an excited
state again.
High energy electrons from photosystem I are
transferred to NADP+ to form NADPH.
Chemiosmosis
• H+ ions were pulled across the thylakoid
membrane making a concentration gradient.
• The H+ ions are now not able to escape
from the except by special proteins
embedded in the membrane – ATP synthase
complexes.
• ATP synthase complex: sepecialized protein
complex in the thylakoid membrane that
allows H+ ions to escape from the lumen
and uses the resulting energy to generate
ATP.
Chemiosmosis
• Chemiomosis: a process for
synthesizing ATP using the energy of
an electrochemical gradient and the
ATP synthase enzyme.
Stage 3: The Calvin Cycle and
Carbon Fixation
• Carbon fixation: formation of high
energy organic molecules from CO2.
– Referred to as the Calvin Cycle to honour
the 1961 Nobel Prize winner Melvin
Calvin.
• Involves a large number of lightindependent reactions
Stage 3: The Calvin Cycle and
Carbon Fixation
Figure 6 on page 193
Stage 3: The Calvin Cycle and
Carbon Fixation
• Utilizes both ATP and high energy
electrons carried by NADPH to make
G3P (sugar used to create glucose)
• Carbon and oxygen atoms are
supplied by CO2 and hydrogen atoms
are supplied by the light dependent
reactions.
• 3 ATPs and 2 NADPHs are consumed
for every CO2 that enters the cycle.
Stage 3: The Calvin Cycle and
Carbon Fixation
• Building one simple sugar molecule
(like glucose – C6H12O6) requires
energy from 18 ATP molecules and 12
NADPH molecules.
• Water produced is < consumed. There
are 6 H2O molecules for every 1
glucose formed.
Questions
• Section 6.2 #1-7