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Energy Metabolism: Photosynthesis (Energy and Reducing Power (Energized…
Energy Metabolism: Photosynthesis
Energy and Reducing Power
Energized pigments
Cannot move across membranes
Large molecules, not mobile
Too energetic; would react with too many things in the cell
Energetic intermediates: ATP, GTP
ADP --> ATP methods
Substrate-level phosphorylation - high-energy phosphate groups are forced onto ADP
Oxidative phosphorylation - last stage of cellular respiration, phosphate group added to ADP
Photophosphorylation - involves light energy in photosynthesis
Redox Reactions
Oxidation
Oxidation Reaction - increases the positive charge on an atom
Oxidized Compounds - tend to contain a great deal of oxygen
Oxidation State - the number of electrons added to or removed from a molecule during a redox reaction
Oxidizing Agent - an electron carrier that is not carrying electrons
Oxidize - to raise the oxidation state of a molecule by removing an electron from it
Reduction
Reduced Compounds - contain a great deal of hydrogen
Reducing Power - The ability of an electron carrier to force electrons onto another compound
Reduction Reaction - reduces the positive charge on an atom
Reducing Agent - An electron carrier that is carrying electrons
Reduce - to lower the oxidation state of a molecule by adding an electron to it
Cytochromes - small electron carriers that contain iron
Iron cycles between 2+ and 3+ oxidation state
in thylakoid membranes
Plastoquinones - a class of lipid-soluable electron carriers
long hydrocarbon tails, hydrophobic
Plastocyanin - a copper-containing electron carrier
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Copper cycles between 1+ and 2+ oxidation state
in chloroplast membrane
Photosynthesis - use of light energy to create carbohydrates
Stroma Reactions, C3 Reactions
RuBP reacts with CO2 and breaks into 2 molecules of 3-phoshoglycerate
this is done by the enzyme RuBP carboxylase (RUBISCO)
RUBISCO is one of the largest and most complex enzymes, 8 subunits making it the most abundant protein on Earth
Heterotrophs would starve without RUBISCO
ATP gives phosphate up, creates 1,3-diphosphoglycerate
NADPH reduces it to 3-phosphoglyceraldehyde
Plants use this to build sugars, fats, amino acids, and nucleic acids and convert it to RuBP
Anabolic Metabolism - using C3 product to form larger molecules needed in the cell
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Intermediate-term storage: glucose, sucrose, can be moved from cell to cell, last for weeks/months
Gluconeogenesis - synthesis of glucose
3-phosphoglyceraldehyde --> dihydroxyacetone phosphate* --> fructose-1,6-biphosphate -P-> fructose-6-phosphate -->glucose-6-phosphate -P->glucose
Steps:
Glucose-6-phosphate makes amylose, amylopectin, and cellulose
Amylose and amylopectin make starch. Amylose has ~20,000 glucose residues, amylopectin has ~1 million.
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Long-term storage: starch, made of glucose, lasts for years...and lipid, can synthesize rapidly and be stored in large quantities
Starch degradation: amylase in saliva helps in animals
Plants store starch in their chloroplasts and convert it into glucose during the night, then moved to cytosol and phloem as sucrose
Short-term storage: ATP, NADH
Light-Dependent Reactions
Electromagnetic radiation spectrum
Radiation can be quanta, waves, or photons
Short waves = lots of energy in quanta (UV, X-ray, etc)
Long waves = little energy (microwaves, radio, etc)
Plants use visible light (350 - 760nm) for photosynthesis
Photosynthetic pigments transfer absorbed light energy into electrons to be used for chemical reactions
chlorophyll a
absorbs red and blue light, letting most radiation pass through
Absorption spectrum - shows wavelengths that are most strongly absorbed by a pigment
Action spectrum - shows which wavelengths are most effective at powering photochemical processes
chlorophyll b
and carotenoids aid photosynthesis by broadening the action spectrum
Extra pigments will transfer (resonance) their energy to
chlorophyll a
Photosystem II
Plastocyanin donates an electron to
chlorophyll a
of photosystem I reaction center
Cytochrome b6/f complex donates electron to plastocyanin
Plastoquinone donates an electron to ctyo b6/f
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Q donates an electron to plastoquinone
Phaeophytin donates an electron to Q
Phaeophytin obtains an electron from
chlorophyll a
of photosystem II reaction center, P680
P680
chlorophyll a
gets electrons from water
Photosystem I
P700 - chlorophyll pair at the reaction center absorbs red light, electron is excited and absorbed by Fx
Fx passes electron to ferredoxin located in thylakoid membrane
Ferredoxin passes electron to enzyme; ferredoxin-NADP+ reducatase which reduces NADP+ to NADPH
Formula - 6CO2+H2O-->C6H12O6+6O2
Electron source is water, energy source is light
Internal Factors
C4 Metabolism- caused by a lack of CO2 in the air, so plants will store what CO2 there is to avoid oxygen intake
Because, RuBP carboxylase binds to oxygen
Oxygen is broken down into one molecule of 3-phosphoglycerate and one molecule of phosphoglycolate
Phosphoglycolate will move out of the chloropast into a perioxisome where it will be broken down into glycine, serine, and CO2
Called Photorespiration, very exergonic, takes up 30% of all ATP and NADPH produced by chloroplast to break down this toxic molecule
Occurs in leaves with Kranz anatomy; chlorophyllous sheath of cells, mesophyll cells with the enzyme PEP
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PEP carboxylase - has high affinity for CO2, never picks up oxygen
Acts as the iniital carboxylating enzyme
PEP is carboxylated, forms oxaloacetate, reduced to malate/other acids
Used more in plants found in warm, dry climates
Examples: corn, sugarcane, sorghum, grasses
Crassulacean Acid Metabolism (CAM) - accumulates and stores CO2 during the night while stomata are open and releases it during the day when stomata are closed
improves conservation of water while permitting photosynthesis
Like C4...PEP is carboxylated, forms oxaloacetate, reduced to malate/other acids
Opening stomata at night helps with conserving water, but bad for photosynthesis
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Stomata closed during day allows for the built up malate and acids to break down into CO2 to be used for C3 metabolism
Advantageous for plants in hot climates where water is scarce, examples; cacti, lilies, orchids
Environmental Factors
Leaf Structure
palisade parenchyma above and spongy mesophyll below
Good at absorbing carbon dioxide, but not conserving water
Desert plants need packed leaf cells without intercelluar spaces
Small internal surface area slows water evaporation
Photosynthesis is slower due to slower at dissolving carbon dioxide from air
Water
Stomata are closed during the night to preserve water
If soil is dry, plant will keep stomata closed during the day, too
Metabolic adaptations occur - C4 and CAM metabolism
Light
Quantity of Light - light intensity or brightness
Photosynthesis is faster on brighter days, more sunlight
Light Compensation Point - the level of illumination at which photosynthetic fixation of carbon dioxide matches respiratory loss
1/4 - 1/2 of ordinary sunlight is all most leaves can use
As light intensity increases, lower leaves can photosynthesize, thus entire plant photosynthesis increases
Too much light = adaptations
thick layer of dead trichomes, plant hairs
Heavy coating of wax
Duration of Sunlight - number of hours per day that sunlight is available
More light = more photosynthesis
Too much light, build up of starch, plants wait until night to convert to glucose
Photosynthesis can stop is plant has reached maximum photosynthesis
Near equator = 12 hours long per day
Near poles = 24 hours long per day
Summer = longer days, brighter light, photosynthesis > respiration
Winter = shorter days, dim light, photosynthesis about equal to respiration, rely on stored nutrients
Quality of Sunlight - the colors or wavelengths it contains
Canopy and tops of trees absorb most of the red and blue light, which is needed for chlorophyll
Shrubs and plants underneath the canopy or on the ground can receive less crucial red and blue light
Need for accessory pigments like carotenoids and
chlorophyll b
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Sunlight is pure white, contains entire visible spectrum
