Photosynthesis Chapter 10
Photosynthesis as a Redox Process • Respiration is an exergonic redox process • Energy is released from the oxidation of sugar • The electrons associated with the sugar’s hydrogens, lose potential energy as the carriers transport them to oxygen
Photosynthesis is an endergonic redox process • What molecule is being reduced? • Carbon dioxide • What is the source of energy? • Light • The light boosts the energy of the electrons as they are moved from water to sugar • Water is split, electrons are transferred from the water to carbon dioxide reducing it into sugar
Summary Equation • What is the equation for photosynthesis? • 6CO2 + 6H2O ---> C6H12O6 + 6O2 • How do the light reactions capture solar energy? • Photosynthetic pigments • There are many different pigments that are associated with the light reactions of photosynthesis
Chlorophyll a • What happens during photosynthesis? • Light energy is converted into chemical energy. • How is this done? • The energy of the light is “passed” to the weak electron, making it “mighty” • What molecule is capable of passing the light energy to an electron? • Only chlorophyll a
Photosynthetic Activity • In order to determine the wavelengths of light that are the best for photosynthesis the following experiment was performed.
Chlorophyll a • Remember it is the chlorophyll a that passes the energy of light to the electrons • Let’s go back and look at the absorption spectrum for chlorophyll a
If the chlorophyll a is the only pigment that actually passes the light energy to electrons, then how is it that there is photosynthesis at 500 nm of light?
Why do the leaves turn color in the Fall? • What causes the leaves to be green to begin with? • Chlorophyll a and b • The colors that we see are a reflection of the wavelengths of light that the pigments cannot absorb • Why do leaves turn colors in the fall? • Why do the leaves fall off of the trees, and die soon after the color change?
The transfer of energy to the electrons and from the electrons to the primary electron acceptor is the first step of the light reactions • There are two Photosystems I and II • How does the chlorophyll a differ in each? • It differs only in the different proteins with which it associates
Cyclic Photophosphorylation • Electrons are used to pump protons into the thylakoid space • As the ions diffuse out through ATP synthase the energy lost is used to generate ATP • What is produced during cyclic electron flow? • ATP
Where does Photosystem I get its electrons? • From Photosystem II • Where does Photosystem II get its electrons? • From water • What is the product of cyclic photophosphorylation? • ATP • What is the product of non-cyclic ? • ATP and NADPH
Calvin Cycle • Uses ATP and NADPH from light reactions to convert CO2 to sugar • 3 molecules of CO2 are fixed into organic material • One molecule of G3P (glyceraldehyde-3-phosphate) is produced for every 3 turns of the Calvin cycle
Phase 1: Carbon fixation • Incorporation of 3 CO2 molecules, one at a time, to a five carbon sugar named RuBP • Enzyme that catalyzes the reaction is rubisco
Phase 2: Reduction • A series of changes to RuBP results in 6 molecules of G3P • One molecule of G3P exit the plant cell • The other 5 molecules of G3P are recycled to regenerate the three molecules RuBP that are available for Phase 1: Carbon fixation
Phase 3: Regeneration of the CO2 acceptor (RuBP) • Five molecules of G3P are rearranged into three molecules of RuBP • 3 molecules of ATP are spent and the cycle continues
Evolution • What is the biggest problem that organisms that live on land must conquer? • Water loss • How do plants combat water loss?
What happens to the guard cells when the plant is losing excessive water?
Transpiration • Water evaporates out of the stomata • When the transpiration rate is too extreme, then the plant will close the stomata • The guard cell become flaccid, and stomata close • Besides removing water from the plant, what other purpose is served by the stomata? • Gas exchange
What environmental conditions will induce stomata closing? • Hot, dry, sunny days • As the stomata close what gas accumulates in the spongy mesophyll? • Oxygen, since the light reactions are still occurring • Rubisco, (RuBP carboxylase), is the enzyme that fixes CO2 into RuBP • Rubisco can accept O2 as well as CO2 at its active site
As concentrations of O2 accumulate in the leaf, rubisco accepts oxygen in place of carbon dioxide • This process is called photorespiration
How can you reconcile the fact that rubisco can accept either CO2 or O2? • Rubisco evolved before there was an accumulation of O2 in the atmosphere • Due to the abundance of O2 in the atmosphere a certain amount of photorespiration is inevitable • What is a C3 plant?
C4 plants • How have C4 plants solved the photorespiration problem?
CAM plants • The CAM plants are desert plants such as cacti. • How do these plants solve the photorespiration problem?
B? A? C?