Post on 14-Dec-2015
"Without this flow of energy from the sun, channeled largely through the eukaryotic cells, the pace of life on this planet would swiftly diminish and then, following the inexorable second law of thermodynamics would virtually cease altogether." Raven, P. H. et al. 1992.
I. INTRODUCTIONCHAPTER 7
PHOTOSYNTHESIS
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PHOTOSYNTHESISOVERVIEW
Photosynthesis - summary CARBON DIOXIDE + WATER + LIGHT CARBOHYDRATE + OXYGEN
6 CO2 + 6 H2O C6H12O6 + 6 O2 + H2O
• This reaction captures energy from sunlight• Light energy is used to reduce carbon (removes hydrogen
from water and adds it to carbon)• Energy is stored in plant tissues as starch, sugars and in
cellulose (wood, coal, gas, oil)• Stored energy can be released by burning, or by metabolism
IMPORTANCE OF PHOTOSYNTHESIS
•AUTOTROPHS (Can make own food, i.e., can manufacture organic compounds from inorganic compounds)•HETEROTROPHS (Eat others to get organic compounds)
PHOTOSYNTHETIC ORGANISMS ARE AT THE BASE OF THE FOOD CHAIN
Photosynthesis is the source of Oxygen in the atmosphere
Earth's atmosphere is a layer of gases surrounding the planet and retained by the Earth's gravity.
78% nitrogen 20.95% oxygen 0.93% argon 0.038% carbon dioxide (up from 0.028%) a variable amount (average around 1%) of water vapor
Oxygen is reactive with inorganic compounds and does not stay in the atmosphere. Thus, our current atmosphere is a balance of oxygen produced by plants and loss to geological processes
THE NATURE OF LIGHT
• THE VISIBLE SPECTRUM CAN BE SEEN WHEN WHITE LIGHT IS PASSED THROUGH A PRISM.
• THE SPECTRUM IS SEEN AS DIFFERENT
COLORS OF LIGHT
Spectrum
A “SPECTRUM” IS A RANGE OF WAVELENGTHS OF ELECTROMAGNETIC RADIATION
One wavelength
Ultraviolet light, X-rays and gamma rays have too much energy for biological systems. They break bonds, knock electrons out of orbit and generally cause cell damage. We are protected from UV by the ozone layer
Infrared light is heat and has too low an energy level to be used for photosynthesis
• Pigments are molecules that absorb light. We see them as being the color of the light that is reflected
• In order to trap light, light must be absorbed. (If light is reflected, it has no value)
Pigments - absorption of light
PIGMENTS
T. W. Engelmann (1883)
• Designed an elegant experiment with spirogyra, an
algae with a single long coiled chloroplast. • He combined the algae with an oxygen-requiring
bacterium and exposed the algae to light coming
through a prism. • He showed that the bacteria clustered in the blue
and red areas suggesting that these areas were
producing oxygen.
T. W. Englemann’s experiment 1882
Spirogyra
•Chlorophyll a •Chlorophyll b• Chlorophyll c
(replaces Chlorophyll b in some algae)•Carotenoids
•Carotene•Xanthophylls
•Phycobilins (photosynthetic bacteria) •Phycoerythrin•Phycocyanin
PIGMENTS OF PLANTSPIGMENTS OF PLANTSPIGMENTS OF PLANTSPIGMENTS OF PLANTS
Chlorophyll a
CarotenoidsCarotenoids•Absorb maximally between 460 and 550 nm, absorb in the blue range and therefore appear red, orange and yellow
Carotenoids
The third major class of pigments, the phycobilins, are found in the cyanobacteria and red algae only
REACTIONS OF PHOTOSYNTHESIS
A. LIGHT vs DARK REACTIONS
F. F. Blackman (1905)
English Plant Physiologist: Concluded that there were two reactions, one of which was light dependent (photochemical reactions) and one of which wastemperature dependent (biochemical reactions or dark reactions)
Stroma
Grana (stacked granum thylakoids)
Stroma thylakoids
Plant cell wall
Chloroplast double membrane
Light reactions take place in the grana thylakoids
Light reactions
Closeup of grana and stroma
Stack of granum
thylakoids
Stroma
A stack of folded membranes is referred to as a granum. The stroma is a watery matrix
Light reactions
Dark or biochemical rxns
REACTIONS OF PHOTOSYNTHESIS
B. LIGHT REACTION
Light-trapping pigments float in the thylakoid membranes
The pigment aggregates have been given the name antenna complex because it acts as an antenna to trap light and transfer the absorbed energy
Antenna complex = Chlorophyll a, chlorophyll b and carotenoids
e-
When light has the correct energy level, it is absorbed by an electron. The electron must move to a new orbital whose energy level corresponds to the electron’s new energy load
Photon
Absorption of light
e-
The excited state is unstable. The atom can become stable if the electron drops back into lower orbit. When this happens the excess energy may be released as visible light (fluorescence) or heat (infared light)
Heat or light
Absorption of light
Alternately, the electron can move to a more stable orbital on an entirely different atom. This electron transfer is the critical step in photosynthesis
Absorption of light
Reaction center: Within the antenna complex, there is a special pair of chlorophyll A molecules and associated proteins called a reaction center
Pigment complexes
Function of the reaction center: accepts electrons transferred from the antenna complex and boosts them into higher orbit where they can be transferred to another electron acceptor molecule
Pigment complexes
There are two different types of reaction center/antenna complexes in the thylakoid membranes (light reaction).
• The reaction center of Photosystem I absorbs maximally at 700 nm. The reaction center is called P700
• The reaction center of Photosystem II absorbs maximally at 680 nm. The reaction center is called P680
Energy depiction of Photosynthesis, the “Z” scheme
Photophosphorylation = LIGHT driven production of ATP 1. Electrons are passed along a series of electron carriers
When electrons reach transmembrane pumps, protons are pumped into the lumen of the thylakoid membrane
This proton gradient drivessynthesis of ATP into the stroma (cytoplasm) of thechloroplast
2.Similar in principle to oxidative phosphorylation in mitochondria
Photophosphorylation
• Requires light• Independent of temperature (an indication that enzymes are not involved)
• Light is used to make ATP and to reduce electron carrier molecules
Light reactions - summary
Produce 3 ATPsReduce 2 molecules of NADP+ to NADPHOxygen is produced from splitting water
Summary of light reactionEight photons used in the light (photochemical) reaction
CYCLIC ELECTRON FLOWCYCLIC ELECTRON FLOW
In photosynthetic prokaryotes, only part of the reaction is present (photosystem 1). This is the older system. Green plants have added a second system to increase energy capture. •No water is split•No NADPH is made
a. Cyclic phosphorylation alone may have existed for a billion years in bacteria who used sulfur as an electron donor but modern plants use water as a source of electrons and that required a major change because the energy in P700 isn’t high enough to remove an electron from water.b. Plants overcame this problem by grafting on a second, more powerful photosystem which could harvest shorter, higher energy wavelengths of light (Z scheme)
from
ATP sy
nthas
e
REACTIONS OF PHOTOSYNTHESIS
B. DARK REACTIONS
PHOTOSYNTHESIS IS COMPOSED OF TWO SETS OF REACTIONS
LIGHT DEPENDENT = PHOTOCHEMICALTEMPERATURE INDEPENDENTLIGHT DEPENDENT
DARK REACTIONS = CARBON FIXATION RXNS = BIOCHEMICAL RXNS
TEMPERATURE DEPENDENTLIGHT INDEPENDENT
Berkeley scientist, Melvin Calvin (1950) was the first to elucidate the steps in the dark reaction of photosynthesis
Dark dependent stage• Occurs in the light but does not require light• Temperature dependent or sensitive reactions. Reactions
increase up to 30C then decrease. This temperature profile is an indication that enzymes are involved
• Energy products of the light reaction are used to reduce carbon from CO2 to a sugar (carbon fixation)
RUBISCO
Ribulose-1,5-bisphosphate or RuBP8 large subunits (gene in chloroplast)8 small subunits (gene in nucleus)
Most abundant protein on earth
PGAL is an intermediate in respiration during the oxidation of glucose to pyruvate
Sugar
C3 PLANTS
PLANTS WHICH USE ONLY THE CALVIN CYCLE TO FIX CO2 INTO SUGARS ARE CALLED C3 PLANTS (BECAUSE A THREE-CARBON MOLECULE IS THE FIRST STABLE MOLECULE IN THE CYCLE)
MOST PLANT SPECIES ARE C3 PLANTS CEREALSPEANUTSCOTTONSOYBEANSMOST TREESMOST LAWN GRASSES
REACTIONS OF PHOTOSYNTHESIS
Photorespiration
•When CO2 levels in the leaf get too low, RUBISCO starts binding oxygen instead of CO2
•Happens when stomates close to prevent water loss. (This prevents CO2 diffusion into the leaf)•Forces another pathway O2 + RuBP PGA + phosphoglycolic acid
•Reactions called photorespiration because they occur in the light, use oxygen and release CO2
•Unlike normal respiration, it produces no ATP•It is a wasteful salvage pathway
PHOTOSYNTHESIS IS NOT PERFECT:PHOTORESPIRATION
RUBISCO
•RUBISCO binds CO2and O2 in the same site
O2
CO2
•If oxygen is bound, the Calvin cycle cannot take place
•Instead, photorespiration takes place
waste
d
REACTIONS OF PHOTOSYNTHESIS
The C4 pathway
• Some plants have developed mechanisms to bypass the RUBISCO carboxylase/oxygenase problem
• These plants are called C4 plants because they fix carbon into a 4-carbon sugar
• Plants out-compete C3 plants in hot dry weather only.
C4 PLANTS
C4 carbon fixation has evolved on up to 40 independent occasions in different groups of plants, making it an example of convergent evolution. Plants which use C4 metabolism include sugarcane, maize, sorghum, finger millet, amaranth, and switchgrass. C4 plants arose around 25 to 32 million years ago
Calvin cycle Malate
C4 plants have a specific kind of leaf anatomy. Carbon is fixed into malate in mesophyll cells. The Calvin cycle occurs in bundle sheath cells that surround the vascular system (few thylakoid grana and protected from oxygen). Thus, the two processes are separated in space.
Bundle sheath cellChloroplasts lack grana
Mesophyll cell with granna
Calvin cycle-note few grana
Malate
Plasmodesmata
REACTIONS OF PHOTOSYNTHESIS
CAM Plants
Crassulacean acid metabolismSeparates CO2 intake and the Calvin cycle by time rather than by space
Crassula ovata – Jade Plant
Crassulacean acid metabolism
The majority of plants possessing Crassulacean Acid Metabolism are either epiphytes (e.g. orchids, bromeliads) or succulent xerophytes (e.g. cacti, cactoid Euphorbias).
16,000 species (7% of plants). Most are angiosperms but includes some gymnosperms, ferns and quillworts
REACTIONS OF PHOTOSYNTHESIS
Carbon Cycle
Global Warming
Another important carbon “sink” is the arctic tundra which is now decomposing and releasing CO2 as the arctic thaws.
Carbon Dioxide in the atmosphere
GREENHOUSE EFFECT – HUMANS ARE RELEASING STORED CARBON WHICH INCREASES ATMOSPHERIC CO2
The present level is higher than at any time during the last 800 thousand years,[6] and likely higher than in the past 20 million years.[7]
6.Amos, Jonathan (2006-09-04). "Deep ice tells long climate story". BBC News. http://news.bbc.co.uk/2/hi/science/nature/5314592.stm. Retrieved 2010-04-28.7. Climate Change 2001: The Scientific Basis
TRENDS•TOTAL GHG EMMISIONS INCREASED 14.2% SINCE 1990
•DOMINANT GAS EMITTED IS CO2, MOSTLY FROM FOSSIL FUEL
COMBUSTION (oxidation of sugars produced by fossil plants)
SummerNOAA CO2 tracker
WinterNOAA CO2 tracker
Red=high CO2 emissionsBlue = low CO2 emissions
END
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