ESSENTIAL KNOWLEDGE 2A2 PHOTOSYNTHESIS
Feb 23, 2016
ESSENTIAL KNOWLEDGE 2A2PHOTOSYNTHESIS
How the photosystems collect light energy and convert it to chemical energy.
How photosystem II produces ATP by chemiosmosis in the light reactions.
How the double membrane structure of the chloroplasts enable their function in chemiosmosis.
How photosystem I produces NADPH in the light reactions.
How the Calvin cycle uses the products of the light reactions to synthesize sugar (G3P).
The causes of and metabolic costs of photorespiration.
The evolutionary adaptations of C4 and CAM plants that enable them to efficiently perform the Calvin cycle in hot, dry habitats.
The commonalities and distinctions between photosynthesis in chloroplasts and aerobic respiration in mitochondria.
light-dependent reactions
Calvin cycle light-independent
reactions photosystem photosystem II photosystem I carbon fixation absorption spectrum pigments
chlorophyll chlorophyll a chlorophyll b action spectrum carotenoid NADPH cyclic
photophosphorylation noncyclic
photophosphorylation
Calvin cycle Rubisco RuBP glyceraldehyde-3-
phosphate C3 photosynthesis photorespiration C4 photosynthesis crassulacean acid
metabolism (CAM)
PHOTOSYNTHESIS
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PHOTOSYNTHESIS OVERVIEW Energy for all life on Earth ultimately
comes from photosynthesis
6CO2 + 12H2O C6H12O6 + 6H2O + 6O2
Oxygenic photosynthesis is carried out by Cyanobacteria 7 groups of algae All land plants – chloroplasts
CHLOROPLAST Thylakoid membrane – internal membrane
Contains chlorophyll and other photosynthetic pigments
Pigments clustered into photosystems Grana – stacks of flattened sacs of
thylakoid membrane Stroma lamella – connect grana Stroma – semiliquid surrounding thylakoid
membranes
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STAGES Light-dependent reactions
Require light1. Capture energy from sunlight2. Make ATP and reduce NADP+ to NADPH
Carbon fixation reactions or light-independent reactions Does not require light3. Use ATP and NADPH to synthesize organic
molecules from CO2
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DISCOVERY OF PHOTOSYNTHESIS Jan Baptista van Helmont (1580–1644)
Demonstrated that the substance of the plant was not produced only from the soil
Joseph Priestly (1733–1804) Living vegetation adds something to the air
Jan Ingen-Housz (1730–1799) Proposed plants carry out a process that
uses sunlight to split carbon dioxide into carbon and oxygen (O2 gas)
F.F. Blackman (1866–1947) Came to the
startling conclusion that photosynthesis is in fact a multistage process, only one portion of which uses light directly
Light versus dark reactions
Enzymes involved
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C. B. van Niel (1897–1985) Found purple sulfur bacteria do not
release O2 but accumulate sulfur Proposed general formula for
photosynthesis CO2 + 2 H2A + light energy → (CH2O) + H2O +
2 A Later researchers found O2 produced
comes from water Robin Hill (1899–1991)
Demonstrated Niel was right that light energy could be harvested and used in a reduction reaction
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PIGMENTS Molecules that absorb light energy in the
visible range Light is a form of energy Photon – particle of light
Acts as a discrete bundle of energy Energy content of a photon is inversely
proportional to the wavelength of the light Photoelectric effect – removal of an
electron from a molecule by light
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ABSORPTION SPECTRUM When a photon strikes a molecule, its
energy is either Lost as heat Absorbed by the electrons of the molecule
Boosts electrons into higher energy level Absorption spectrum – range and
efficiency of photons molecule is capable of absorbing
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Organisms have evolved a variety of different pigments
Only two general types are used in green plant photosynthesis Chlorophylls Carotenoids
In some organisms, other molecules also absorb light energy
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CHLOROPHYLLS Chlorophyll a
Main pigment in plants and cyanobacteria Only pigment that can act directly to
convert light energy to chemical energy Absorbs violet-blue and red light
Chlorophyll b Accessory pigment or secondary pigment
absorbing light wavelengths that chlorophyll a does not absorb
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PIGMENTS
Structure of chlorophyll
porphyrin ring Complex ring
structure with alternating double and single bonds
Magnesium ion at the center of the ring
Photons excite electrons in the ring
Electrons are shuttled away from the ring
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Action spectrum Relative effectiveness of different
wavelengths of light in promoting photosynthesis
Corresponds to the absorption spectrum for chlorophylls
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Carotenoids Carbon rings linked
to chains with alternating single and double bonds
Can absorb photons with a wide range of energies
Also scavenge free radicals – antioxidant
Protective role Phycobiloproteins
Important in low-light ocean areas
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PHOTOSYSTEM ORGANIZATION Antenna complex
Hundreds of accessory pigment molecules Gather photons and feed the captured light
energy to the reaction center Reaction center
1 or more chlorophyll a molecules Passes excited electrons out of the
photosystem
ANTENNA COMPLEX Also called light-harvesting complex Captures photons from sunlight and
channels them to the reaction center chlorophylls
In chloroplasts, light-harvesting complexes consist of a web of chlorophyll molecules linked together and held tightly in the thylakoid membrane by a matrix of proteins
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REACTION CENTER Transmembrane protein–pigment
complex When a chlorophyll in the reaction
center absorbs a photon of light, an electron is excited to a higher energy level
Light-energized electron can be transferred to the primary electron acceptor, reducing it
Oxidized chlorophyll then fills its electron “hole” by oxidizing a donor molecule
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LIGHT-DEPENDENT REACTIONS1. Primary photoevent
Photon of light is captured by a pigment molecule2. Charge separation
Energy is transferred to the reaction center; an excited electron is transferred to an acceptor molecule
3. Electron transport Electrons move through carriers to reduce NADP+
4. Chemiosmosis Produces ATP
Capt
ure
of li
ght e
nerg
y
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In sulfur bacteria, only one photosystem is used
Generates ATP via electron transport Anoxygenic photosynthesis Excited electron passed to electron
transport chain Generates a proton gradient for ATP
synthesis
CYCLIC PHOTOPHOSPHORYLATION
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CHLOROPLASTS HAVE TWO CONNECTED PHOTOSYSTEMS
Oxygenic photosynthesis Photosystem I (P700)
Functions like sulfur bacteria Photosystem II (P680)
Can generate an oxidation potential high enough to oxidize water
Working together, the two photosystems carry out a noncyclic transfer of electrons that is used to generate both ATP and NADPH
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Photosystem I transfers electrons ultimately to NADP+, producing NADPH
Electrons lost from photosystem I are replaced by electrons from photosystem II
Photosystem II oxidizes water to replace the electrons transferred to photosystem I
2 photosystems connected by cytochrome/ b6-f complex
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NONCYCLIC PHOTOPHOSPHORYLATION Plants use photosystems II and I in
series to produce both ATP and NADPH Path of electrons not a circle Photosystems replenished with
electrons obtained by splitting water Z diagram
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PHOTOSYSTEM II Resembles the reaction center of purple
bacteria Core of 10 transmembrane protein subunits
with electron transfer components and two P680 chlorophyll molecules
Reaction center differs from purple bacteria in that it also contains four manganese atoms Essential for the oxidation of water
b6-f complex Proton pump embedded in thylakoid membrane
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PHOTOSYSTEM I Reaction center consists of a core
transmembrane complex consisting of 12 to 14 protein subunits with two bound P700 chlorophyll molecules
Photosystem I accepts an electron from plastocyanin into the “hole” created by the exit of a light-energized electron
Passes electrons to NADP+ to form NADPH
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CHEMIOSMOSIS Electrochemical gradient can be used
to synthesize ATP Chloroplast has ATP synthase enzymes
in the thylakoid membrane Allows protons back into stroma
Stroma also contains enzymes that catalyze the reactions of carbon fixation – the Calvin cycle reactions
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PRODUCTION OF ADDITIONAL ATP Noncyclic photophosphorylation generates
NADPH ATP
Building organic molecules takes more energy than that alone
Cyclic photophosphorylation used to produce additional ATP Short-circuit photosystem I to make a larger
proton gradient to make more ATP
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CARBON FIXATION – CALVIN CYCLE To build carbohydrates cells use Energy
ATP from light-dependent reactions Cyclic and noncyclic photophosphorylation Drives endergonic reaction
Reduction potential NADPH from photosystem I Source of protons and energetic electrons
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CALVIN CYCLE Named after Melvin Calvin (1911–1997) Also called C3 photosynthesis Key step is attachment of CO2 to RuBP
to form PGA Uses enzyme ribulose bisphosphate
carboxylase/oxygenase or rubisco
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3 PHASES1. Carbon fixation
RuBP + CO2 → PGA2. Reduction
PGA is reduced to G3P3. Regeneration of RuBP
PGA is used to regenerate RuBP 3 turns incorporate enough carbon to
produce a new G3P 6 turns incorporate enough carbon for 1
glucose
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OUTPUT OF CALVIN CYCLE Glucose is not a direct product of the
Calvin cycle G3P is a 3 carbon sugar
Used to form sucrose Major transport sugar in plants Disaccharide made of fructose and glucose
Used to make starch Insoluble glucose polymer Stored for later use
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ENERGY CYCLE Photosynthesis uses the products of
respiration as starting substrates Respiration uses the products of
photosynthesis as starting substrates Production of glucose from G3P even uses
part of the ancient glycolytic pathway, run in reverse
Principal proteins involved in electron transport and ATP production in plants are evolutionarily related to those in mitochondria
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PHOTORESPIRATION Rubisco has 2 enzymatic activities
Carboxylation Addition of CO2 to RuBP Favored under normal conditions
Photorespiration Oxidation of RuBP by the addition of O2 Favored when stoma are closed in hot
conditions Creates low-CO2 and high-O2
CO2 and O2 compete for the active site on RuBP
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TYPES OF PHOTOSYNTHESIS C3
Plants that fix carbon using only C3 photosynthesis (the Calvin cycle)
C4 and CAM Add CO2 to PEP to form 4 carbon molecule Use PEP carboxylase Greater affinity for CO2, no oxidase activity C4 – spatial solution CAM – temporal solution
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C4 PLANTS Corn, sugarcane, sorghum, and a number of
other grasses Initially fix carbon using PEP carboxylase in
mesophyll cells Produces oxaloacetate, converted to malate,
transported to bundle-sheath cells Within the bundle-sheath cells, malate is
decarboxylated to produce pyruvate and CO2 Carbon fixation then by rubisco and the
Calvin cycle
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C4 pathway, although it overcomes the problems of photorespiration, does have a cost
To produce a single glucose requires 12 additional ATP compared with the Calvin cycle alone
C4 photosynthesis is advantageous in hot dry climates where photorespiration would remove more than half of the carbon fixed by the usual C3 pathway alone
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CAM PLANTS Many succulent (water-storing) plants,
such as cacti, pineapples, and some members of about two dozen other plant groups
Stomata open during the night and close during the day Reverse of that in most plants
Fix CO2 using PEP carboxylase during the night and store in vacuole
When stomata closed during the day, organic acids are decarboxylated to yield high levels of CO2
High levels of CO2 drive the Calvin cycle and minimize photorespiration
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COMPARE C4 AND CAM Both use both C3 and C4 pathways C4 – two pathways occur in different
cells CAM – C4 pathway at night and the C3
pathway during the day
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