Module 8: Photosynthesis Flashcards
What is photosynthesis?
the conversion of light energy to chemical energy
- photosynthetic organisms build carbohydrates using sunlight and CO2 from the air
- it is a major entry point of energy into biological systems
Supporting Photosynthesis
- photosynthetic organisms in a wide range of environments can carry out photosynthesis, if sunlight is available
- ex: in the ocean photosynthesis occurs in the surface layer about 100m deep, the photic zone
- food webs are supported by the biomass of photosynthetic organisms
What type of reaction is photosynthesis, what does it do and require?
its a redox reaction
- CO2 is reduced to form high energy carbohydrate molecules
- requires energy in the form of sunlight
What are the two stages of photosynthesis?
- Light capture
- Carbon fixation
What is the electron donor in the reaction in photosynthesis?
H2O
The oxidation of water results in the production of what?
Electrons
Protons
O2
Oxidation of water and reduction of CO2 are linked through…
the photosynthetic electron transport train
In photosynthetic eukaryotes both stages take place where?
in the chloroplast
Chloroplasts are _____ & ______
semi-autonomous
self replicating
Makeup of the chloroplast
- bound by 2 membranes separated by a narrow space (outer and inner membrane)
- center of chloroplast highly folded flattened membrane sacs, thylakoid
- space inside thylakoid membrane is a fluid filled space, lumen
- Orderly stacks of thylakoids, grana
- Space surrounding thylakoids, stroma
- carbon fixation occurs here
Photosynthesis balanced chemical reaction
Energy + 6 CO2 + 12 H2O –(light)–> C6H12O6(glucose) + 6 O2 + 6 H2O
Oxidation: 12 H2O to 6 O2
Reduction: 6 CO2 to C6H12O6(glucose)
What are the 2 series of reactions?
- Light-dependent reaction:
- sunlight energy converted into chemical energy, occurs in the thylakoid membranes
- products are ATP and NADPH - Light-independent reaction:
- ATP & NADPH used to synthesize carbohydrates
- occurs in the stroma
How is light absorbed?
energy from the sun is a form of electromagnetic radiation, travels in photons
- when a photon becomes absorbed, the compound is converted to a higher energy state, an excited state
What are the three ways to re-establish ground state?
- Dissipate energy as heat
- Re-emit energy in a longer wavelength, fluorescence
- Transfer energy to another molecules, this is what happens with photosynthetic pigments
What are pigments?
pigments are molecules that contain a chromophore, a chemical group capable of absorbing light of wavelengths
Leaves efficiently absorb what?
the leaf of a green plant efficiently absorbs light energy over most of the spectrum
Why green in plants?
because the pigment chlorophyll is poor at absorbing green wavelengths
Chlorophyll
-major light capturing molecules
- absorb light of blue and red wavelengths
- reflects green wavelengths
What are the two parts of chlorophyll?
Porphyrin ring
- light absorption
- specific atom in center of the ring is Mg
- different side groups on the ring, give different types of chlorophyll
Phytol side chain
- insertion of chlorophyll in lipid bilayer, thylakoid membrane
- hydrocarbon side chain
Accessory Pigments
found in thylakoid membrane
- can absorb light from regions of the visible spectrum that are poorly absorbed by chlorophyll
– increases efficiency of light absorption
– protects photosynthetic components from damage
What happens when chlorophyl absorbs light?
one of its electrons is elevated to a higher energy state
What happens if a chlorophyl is beside another chlorophyll?
energy can be transferred
- the energy is passed from chlorophyl to chlorophyll until it reaches a pair of chlorophyll molecules, the reaction center
the Antenna chlorophylls do what?
transfer energy, not electrons
Reaction-center chlorophyll
a specific chlorophyll capable of transferring electrons to an electron donor
Before the reaction center can take in more light energy it has do to what?
it must first acquire an electron from an electron donor since it lost one electron
- electron comes from H2O
Role of Photosystems
- photosynthesis begins with the absorption of light by protein-pigment complexes, i.e. photosystems
Absorbed light drives what reactions?
redox reactions
What are the 2 photosystems?
photosystem ll or PSll
photosystem l or PSl
How are the two photosystems connected? What do they do?
connected by the photosynthetic electron transport chain
- drives the formation of ATP and NADPH
- these are energy sources needed to synthesize carbohydrates from CO2 in the Calvin cycle
What does each photosystem have?
a reaction center chlorophyll
How do the electrons and energy move?
the electrons move from water to PSll
- initial high energy level, but this drops as it goes through the ETC
- more light energy coming in PSl raises the electron energy high enough to be used to reduce NADP+
- the energy trajectory resembles a “z”, called the Z scheme
Flow of electrons
a. between H2O and PSll
b. between PSll and PSl (ETC)
c. between PSl and NADP+
As electrons flow along the Z pathway…
H+ ions are moved from stroma to inner compartment of thylakoids, sets up a proton gradient
- proton concentration increases in lumen of thylakoid and decreases in stroma
ATP Production in the Chloroplast
- the thylakoid membrane has an ATP synthase
-
the ATP production in the chloroplast remains in the chloroplast
– it is used in carbon fixation reactions - plants also have a mitochondria to produce ATP needed elsewhere in the cell
– because ATP produced in chloroplast stays in chloroplast
ETC in Plants
electrons pass between PSll and PSl through the cytochrome b6f complex
Plastiquinnone
carries electrons from PSll to the cytochrome b6f complex
Plastocyanin
carries electrons from the cytochrome b6f complex to the PSl by diffusing through the thylakoid lumen
Formation of ATP in plant cell
results from electrons moving through PSll and PSl
- proton gradient drives ATP formation, enzyme is ATP synthase
- protons pass through the enzyme ATP synthase and ATP is generated
The Calvin cycle take a ___ energy carbon compound (_) and converts it into a _____ carbon compound
low potential
CO2
high energy
What does the Calvin Cycle require?
NADP and ATP
15 chemicsl reactions that synthesize carbohydrates from CO2
What are the three stages of the Calvin cycle?
- Carboxylation
- Reduction
- Regeneration
Step 1 Calvin cycle
Carboxylation
- CO2 from the air is combined with RuBP to form a 6-carbon molecule that is catalyzed by rubisco
- the 6-carbon molecule is then broken down into two-3 carbon molecules of PGA
Step 2 Calvin cycle
Reduction
- the potential energy of PGA needs to be increased, it must be reduced
- the reduction occurs in two steps
1. the PGA is phosphorylated by ATP, produce triose phosphates
2. the triose phosphate molecules are then reduced by NADPH and exported out of the chloroplast
Step 3 Calvin cycle
Regeneration
- for every six triose phosphate molecules that are produced, only one can be withdrawn from the Calvin cycle
- this is because RuBP (a 5-carbon molecule) needs to be regenerated using the other triose phosphates
- that regeneration requires energy from ATP
- regeneration produces three 5-C RuBP molecules to be used in the carboxylation portion of the Calvin cycle
Excess carbohydrates in the Calvin cycle
excess carbohydrates produced in the Calvin cycle are converted into storage, STARCH
- the formation of starch granules during the day is a source of carbohydrates that can be used during the night when there is no sunlight
Evolution of Photosynthesis
- early photosynthetic bacteria had only a single photosystem, cannot capture enough energy with one photosystem
Evolution of two photosystems
two possibilities
1. Horizontal gene transfer
2. gene duplication and divergence of one of the genes
Photosynthetic Eukaryotes
- the first organisms to use water as the electron donor were the cyanobacteria
- the endosymbiotic theory proposes that a cyanobacterium was engulfed by a eukaryotic cell
- over time the cyanobacterium lost the ability to love outside the host, thus it became the organelle we know as the chloroplast
