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Photosynthesis (The Calvin Cycle (Determining steps of Calvin Cycle…
Photosynthesis
The Calvin Cycle
Carboxylation
CO2 added to 5C sugar
RuBP
CO2 and RuBP diffuse into active site of
Rubisco
Bad enzyme
very slow
poor substrate specificity
must be compromise between rate and substrate specificity
CO2 no distinguishing features
hard to recognize by rubisco
Bio engineering: red algae
solution: lots of rubisco
Diffusion is spontaneous
6 C compound breaks into
3-PGA
Reduction
NADPH
reducing agent
increases energy within 3-PGA bonds
Reduction of 3-PGA
ATP donates phosphate to 3-PGA
NADPH transfers 2 electrons and 1 hydrogen to phosphorylated compound (releases 1 phosphate group)
2 ATP and 2 NADPH required for each molecule of CO2
Triose phospates
6 triose phosphates --> one withdrawn from CC
Regeneration
Five 3C triose phophate molecules --> three 5C RuBP molecules
input of ATP
1 glucose --> 6 times around cycle
CC does not need light directly
needs ATP and NADPH from CC
several CC enzymes regulated by cofactors activated by ETC
Determining steps of Calvin Cycle (Experiment)
supplied radioactively labeled CO2 to green alga
Initial step in CC: 5C RuBP + CO2
3-PGA = first stable product
No CO2 --> RuBP = reactant
Starch
accumulation of carbohydrates --> water enters cell by osmosis
excess carbs converted to starch
Overview
Widely distributed
Plants = major contributor
angiosperm (leaves = organs)
single-cellular bacteria
cyanobacteria
some animals (sea slugs)
Redox Reaction
CO2 molecules reduced to higher energy molecules
input of energy (sunlight)
Water = ultimate electron donor
ETC
absorbed sunlight provides energy
ATP and NADPH used in Calvin Cycle --> carbs
ETC takes place on specialized membranes
Photosynthetic bacteria: ETC in membranes within the cytoplasm
Photosynthesis in eukaryotic cells in chloroplasts --> highly folded thylakoid membranes
form flattened sacs
grouped into grana --> grana connected by membrane (encloses lumen); surrounded by stroma
Light reactions: thylakoid membrane
Fixing CO2 = stroma
NADH versus NADPH
different phosphate group
allows molecule to be easily identifiable
Light-Dependent Reactions:
Sunlight --> Chemical Bonds
Pigments
Chlorophyll
major photosynthetic pigment
Structure
Head
large ring structure with Mg at center --> allows destabilization of electrons
large number of single and double bonds in head region --> absorbs visible light
Tail
hydrocarbon
Bound to membrane proteins of thylakoid membrane
accessory pigments
orange-yellow carotenoids
absorption of broader range of visible light
Proteins + pigments = photosystems
Photosystems
Chlorophyll molecules absorbs visible light --> electron elevated to higher energy state
In solution: Energy emitted as light and heat
In plant cell: energy efficiently transferred to antennae molecule
Reaction center = two chlorophyll molecules
light energy --> chemical energy
Transfers electron to electron acceptor (oxidized)
Electrons from H2O replace those lost from oxidation
2 photosystems (raises energy of electrons): water takes increased energy to pull electrons even though it is an ideal donor
Photosystem II
electron pulled from water
With light energy, has enough reductive potential
Photosystem I
pushes electron onto NADP+
PII -->
Plastoquinone (Pq)
diffuses through membrane -->
cytochrome-b6
-->
plastocyanin (Pc)
--> PI (diffuses through thylakoid lumen)-->
ferredoxin (Fd)
(using ferredoxin-NADP+ reductase catalyzes formation of NADPH)
Movement of protons from lumen --> stroma --> synthesis of ATP
Buildup of protons in lumen
oxidation of water
protein complex between PII and PI = pump (2 electrons and 2 protons by diffusion of pq and transfer of electrons within cytochrome-b6f complex)
Cyclic electric transport
transport of 4 electrons through photosynthetic ETC not enough protons into lumen
Electrons from PI redirected from ferredoxin back into ETC (re-enter through pq)
Challenges to Photosynthetic Efficiency
Excess light energy --> damage
oxidation of lipids, proteins, and nucleic acids
reactive oxygen species
formation of O2-
major lines of defense
Antioxidants
surround entire system
detoxify reactive oxygen species (neutralize oxygen species)
Xanthophylls
slow formation of reactive oxygen species by reducing excess light energy
converts energy to heat
Photorespiration
Rubisco can use both CO2 and O2
Creates 3-PGA and 2-phosphoglycolate
fewer carbohydrates exit as 3-carbon compounds (loss of carbon)
consumes ATP (drives recycling of 2-phosphoglycolate --> 3-PGA)
Amount of solar energy used by photosynthesis
breakdown
ETC captures about 24% of sun's usable energy
4% yield of sunlight energy
Evolution of Photosynthesis
Early cells fulfilling energy requirements
Early electron donors outside of cells (Fe2+)
cyclic light-driven electron transport
Random mutations --> chlorophyll pigments
Water = electron donor
Organisms with only one photosystem must use more easily oxidized compound (H2S)
Cyanobacteria incorporated 2 photosystems (Hypotheses)
genetic material associated with one photosystem transferred to bacteria already having photosystem
genetic material associated with one photosystem underwent duplication (duplication and division)
Gaining photosynthesis through endosymbiosis
cyanobacteria took up residence inside eukaryotic cell --> evolved into chloroplasts