PHOTOSYNTHESIS AND ENERGY
Photo – light
Synthesis – build
6CO2 + 12 H2O + energy → C6H12O6 + 6O2 + 6 H2O
Light is used to convert low energy reactants into high energy products
Photosynthesis can be split into two sets of reactions:
1. Light-dependent reaction
2. Light-independent reaction
- a.k.a the Carbon fixation reaction/Calvin-Benson Cycle
1. The Light-Dependent Reaction
Overview
− Light energy is converted into chemical energy and stored as ATP
− Light splits H2O into H+ and O and electrons
− O2 is released into atmosphere
− H+ combines with NADP+ to form NADPH
The Path of Electrons in the Light-Dependent Reaction **Diagram**
- Light absorbing pigments are found in clusters called photosystems. There are two
photosystems, photosystem I (PSI) and photosystem II (PSII).
Step 1
- Light is captured by the pigments of PSII and they pass the energy on to a special chlorophyll
a molecule called the reaction centre. This pushes an electron within the photosystem to a higher
energy level.
- The electron is then passed onto an electron-acceptor (the acceptor is now reduced).
- PSII is now oxidized and needs the electron replaced.
- Solar energy is used in photolysis to split water, releasing electrons that can be used by PSII.
Hydrogen ions and oxygen atoms are also released. O2 is released into atmosphere; the H+ ions
will be used later.
Step 2
- The energized electron is transferred from the electron-acceptor along a series of electroncarrying molecules – the electron transport system (ETS).
- The electrons release a little bit of energy with each transfer. This energy is stored in a
temporary hydrogen ion concentration gradient across the thylakoid membrane.
- The energy from this gradient will be used to make ATP and help fuel the light-independent
reactions.
- Ultimately, the ETS transports the electrons to PSI.
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Step 3
- Meanwhile, light has been absorbed by PSI and an electron in its reaction centre has been
excited.
- The electron is then passed onto an electron-acceptor and the electron is replaced by an
incoming electron from PSII.
Step 4
- The electrons are excited again and transferred along an ETS, except instead of ATP being
formed, NADP+ is reduced to NADPH (H+ is from hydrolyzed water).
- The reducing power of NADPH will be used in the light-independent reactions.
- NADPH moves out of the thylakoid membrane to the stroma.
Making ATP: Chemiosmosis ** Diagram**
- During Step 2 above, the energy released by the electrons traveling down the ETS was used to
build a H+
concentration gradient.
- The gradient exists because H+ are pumped from the stroma surrounding the thylakoid into the
thylakoid lumen.
- They can’t diffuse back across (membrane is impermeable to charged molecules) so they use
an enzyme called ATP synthase to cross.
- As the H+ moves down its concentration gradient, energy is released (analogy ~ a dam and
water) that is linked with the bonding of a free phosphate group to an ADP molecule, producing
ATP.
Light-dependent Reaction Questions (Answer on a separate piece of paper)
1. Describe or sketch what happens to electrons in the electron transport system.
2. How does NADP+ get converted to NADPH? Where in the cell does this occur?
3. How are electrons replaced in phsotosystem I and what is the source of the replacement
electrons?
4. Explain why hydrogen ions cannot diffuse out of the thylakoid space.
5. What is ATP synthase and what is its significance?
6. What two events are linked in chemiosmosis?
7. What are the products of photolysis? Where do they end up?
8. What are the products of the light-dependent reaction? What is their destination? What
will they be used for?
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