Light Reaction & Calvin Cycle
Objectives:
•
How do pigments like chlorophyll work to capture light energy?
•
What happens to water in the light reaction?
•
What are photosystems? What is their role in photosynthesis?
•
How are ATP and NADPH generated in the light reaction?
•
What are the reactants and products of the light reaction and
the calvin cycle?
•
Where does each reaction take place?
ATP = chemical energy
Photosynthesis
NADPH – carries Hydrogen
(NADP + H)
+
H2 O
CO2
Energy
Which splits
water
ATP and
NADPH2
Light is Adsorbed
By
Chlorophyll
Calvin Cycle
ADP
NADP
Chloroplast
O2
Light Reaction
Used Energy and is
recycled.
+
C6H12O6
“Dark” Reaction
Visible Light and the Electromagnetic
Spectrum
• Electromagnetic Energy = Energy that travels as a WAVE.
• Electromagnetic Spectrum= menu of energy that travels as a wave
• The shorter the wavelength, the greater the energy….and viceversa.
• VISIBLE LIGHT = Wavelength absorbed by plants to make food.
Light Used in Photosynthesis
• Chlorophyll pigments can absorb only some wavelengths of the
visible light from the electromagnetic spectrum.
• Visible light consists of the following colors or wavelengths in order of
increasing wavelengths / decreasing energy:
– Violet Blue  Green  Yellow  Orange Red.
• Chloroplasts absorb mainly blue-violet and red-orange lights.
• Green light is reflected and transmitted by green plants – hence, they
appear green.
Chlorophyll absorbs blue-violets and
red-orange for photosynthesis.
Green light is reflected; That’s why
plants appear green.
Redox Rxns (Reducing / Oxidation Reactions)
Being “reduced” = accepting electrons (usually
accompanied by H ions); electrons are negative;
accepting them makes recipient “more” negative
(hence reduced)
Example: NADP+ is reduced when it accepts electrons.
NADP+ + 2e + H+  NADPH
Being Oxidized: Lose electrons (usually when reacting
with oxygen); become more +
Example: NADH is oxidized when it donates electrons.
NADH  NAD+ + 2 e +H+
Oxidation – Reduction Reactions (Redox) occur
together.
Reduction
6 CO2 + 6 H2O  C6H12O6 + 6 O2
Oxidiation
• CO2 is reduced as it accepts electrons and hydrogen
ions.
• H2O is oxidized. It donates electrons and loses
hydrogen ions.
Light Reaction
Where: Thylakoid membrane of chloroplast.
What: Light energy is absorbed by chlorophyll.
Energy is used to SPLIT H2O molecules into:
1. H+ ions and electrons (carried by NADPH)
2. O atoms (released as O2 gas)
3. electrons (go through electron
transport chain to make ATP and NADPH)
Light Reaction:
1. Making ATP
& NADPH (used
in Calvin cycle to
make sugar)
2. Release O2
(from H2O)
Photosystems: Cluster of hundreds of chlorophyll
molecules in thylakoid membrane.
Where Light Reaction starts.
Light-Dependent Reactions
Photon of light absorbed by pigments in photosystem II (P680); excited electrons pass to primary electron acceptor;
Electrons of PS680 replaced by electrons from water (H2O  H+,1/2 O2, and electrons)
Photo excited electrons from P680 passed thru. Series of ETC proteins (exergonic; releases energy); energy fr. “falling”
electrons used to PUMP H+ ions into thylakoid space (against gradient); Build up of H+ ions used to make ATP in ATP
synthase; photo-excited electrons from photosystem II (P680) replaces excited electrons at photosystem I (P700);
Excited electrons from photosystem I are used to reduce NADP to NADPH. NADPH is a high energy molecule that will
bring electrons and hydrogens to CO2 to make sugar.
Chemiosmosis: Use H+ gradient across membrane to
phosphorylate ADP ATP
High [H+] in thylakoid space.
Active transport/ Used energy released from “excited”
electrons “falling” to ground state in ETC to pump H+ ions
against gradient.
H+ ions move down gradient to stroma through ATP
synthase (transport membrane coupled w/
phosphorylation of ADP
Light-Dependent Reaction Summary
1. Light is absorbed by chlorophyll in PS II (P680)
2. An electron is excited and captured by the primary electron
acceptor
3. Water is split to supply electrons to PSII
4. The leftover oxygen atoms pair up to form a molecule of O2
5. The electrons released from PSII are passed to PS I by an
electron transport chain
6. This “fall” of electrons down the chain provides energy to pump
H+ ions across the membrane which will then power the
production of ATP via chemiosmosis. (photophosphorylation)
7. Meanwhile, light excites an electron of chlorophyll at PS I/ The
primary electron acceptor captures this and an electron from the
bottom of the ETC replaces the lost electron.
8. The excited electron of PS I is passed to NADP+, reducing it to
NADPH
8.3: Calvin Cycle
Where: Stroma of chloroplast.
What: Use ATP and NADPH from light rxns to MAKE GLUCOSE from CO2.
Inputs:
1. CO2 (Stomate); 2. ATP fr. Light Rxn; 3. NADPH fr. Light Rxn
Outputs
1. Glucose is made (used for energy; stored as starch; used to make
cellulose)
2. ADP + P (fr. ATP)  back to light rxn
3. NADP+ (fr. NADPH)  back to light rxn
Calvin Cycle: Carbon fixation = making organic
carbon molecules from inorganic carbon.
Light
Reaction
• In Thylakoid
Membrane
Calvin
Cycle
• Stroma
• Use light to split
water into H+, O,
and electrons.
• Use ATP and
NADPH to make
GLUCOSE from
CO2
• Make ATP and
NADPH to be used
in Calvin Cycle.
• Light Independent.