photosynthesis overview
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Photosynthesis overview. Ch 10. What is the equation for photosynthesis?. 6CO 2 + 6H 2 O + light energy C 6 H 12 O 6 + 6O 2. What does each part do for the plant?. Epidermis – protects leaf tissues Cuticle – waxy covering, resists water, dirt - PowerPoint PPT PresentationTRANSCRIPT
Photosynthesis overviewCh 10
What is the equation for photosynthesis?
6CO2 + 6H2O + light energy C6H12O6 + 6O2
What does each part do for the plant? Epidermis – protects leaf tissues Cuticle – waxy covering, resists water,
dirt Stoma- pore used for gaseous exchange
(water, CO2, O2) (stomata – plural) guard cell – open & close stomata
Vein – vascular tissue Xylem – moves water & minerals from roots to leaf Phloem – moves sap from photosynthesis to other
parts of plant palisade layer (mesophyll) – dense upper middle
layer of dicot leaf, where photosynthesis takes place
spongy mesophyll – lower middle layer of dicot leaf, has lots of air spaces, where photosynthesis takes place
ChloroplastInner membrane
Stroma
Thylakoid(thylakoid space inside)
Outer membraneGranum (stack of thylakoids)
2 stages of photosynthesisStage Where it
occursReactants Products
Light dependent reactions
Thylakoid membrane in chloroplast
H2O, light
ADP,NADP+
ATP, NADPHO2 (waste)
Calvin cycle(light independent reactions)
Stroma CO2ATPNADPH
2 G3P to make C6H12O6
ADPNADP+
Tracking atoms in photosynthesis
- Redox reaction: H2O is oxidized, CO2 is reduced- Endergonic reaction – requires energy- Experiments done with heavy water to determine where oxygen comes from – it comes from water
Capturing energy from light
Plant PigmentsPigments – a substance that
absorbs lightPlant pigments absorb light in the
blue/violet and red region of the spectrum
Plant pigments reflect light in the yellow/green region of the spectrum
Absorbance vs. Action spectrum
Absorbance spectra = range of wavelengths absorbed by a particular pigment
Chlorophyll A & B
Chlorophyll a – main photosynthetic pigment
Accessory pigments: help broaden spectrumChlorophyll bCarotenoids
Action spectra = range of wavelengths capable of driving a particular biological process
Chlorophyll excited by light
When pigment gets excited by light it goes from ground state to excited state
When electron goes back to ground state, it gives off energy
Englemann’s experiment- first action spectrum- 1883- Used filamentous
algae- Put on flat surface
w/water & CO2- Solution of aerobic
bacteriaLooked to see where
bacteria built up
Photosystem Light gathering complex –
various pigment molecules bound to proteins
Reaction center – has special chlorophyll A associated with a primary electron acceptor
Photon excites pigments – they transfer electrons until they get to the chlorophyll a in reaction center
Non cyclic photophosphorylation
Cyclic Phosphorylation
Cyclic photophosphorylation Alternate electron path Photosystem 1 transfers electrons back
to the ETC from PS II. This generates ATP through photophosphorylation, rather than NADPH.
- some bacteria only have PS 1, some plants have cells that only have PS1
Calvin Cycle(equations for 3 turns of cycle)
1) Fixation of carbon dioxide: CO2 reacts with RuBP, produces 2 molecules of 3 PGA. (catalyzed by rubisco) 3 RuBP + 3 CO2 6 3PGA2) Reduction of 3PG to form glyceraldehyde-3-phosphate, or G3P 6 3PGA + 6 ATP + 6 NADPH (5 G3P to step 3, 1 G3P yield)
3) Regeneration of the CO2 acceptor, RuBP, ribulose biphosphate 5 G3P + 3 ATP 3 RuBP
Rubisco is the key enzyme
One complete turn of the Calvin Cycle (with one CO2): 1 CO2 + 2 NADPH + 3 ATP (CH2O) + 2 NADP+ + 3 ADP + 3 Pi
For each turn, one CO2 is converted into one (CH2O) unit
It takes 3 turns to produce one net G3P It takes 6 turns to produce one 6 carbon
carbohydrate, such as glucose
Calvin cycle animation http://www.science.smith.edu/departme
nts/Biology/Bio231/calvin.html
Rubiscoslow: Catalyzes 3 reactions per second vs.
thousands per second for other enzymes Inefficient: catalyzes addition of either
CO2 or oxygen.
CO2 and oxygen compete at the enzyme’s active sites.
•In most plants (C3 plants), • initial fixation of CO2 (via rubisco), •forms a three-carbon compound (3-phosphoglycerate)
•In photorespiration, •rubisco adds O2 instead of CO2 in the Calvin cycle, •producing a two-carbon compound
Photorespiration
The reaction of oxygen and RuBP results in photorespiration, which consumes ATP.
Photorespiration consumes energy and releases fixed CO2 , so it “undoes”
photosynthesis.
Photorespiration consumes O2 and organic fuel, and releases CO2 without producing ATP or sugar
Photorespiration
Alternate pathways for photosynthesis• Problem - Dehydration is a problem• This can result in trade-offs with
photosynthesis
• On hot, dry days, plants close stomata, - conserves H2O
• - limits photosynthesis• - reduces access to CO2 • - causes O2 to build up
• These conditions favor an apparently wasteful process called photorespiration
• Photorespiration • may be an evolutionary relic because rubisco first
evolved at a time when the atmosphere had far less O2 and more CO2
• limits damaging products of light reactions that build up in the absence of the Calvin cycle
• - a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle
C4 plantsC4 plants minimize the cost of
photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells
This step requires the enzyme PEP carboxylase
PEP carboxylase has a higher affinity for CO2 than rubisco does; it can fix CO2 even when CO2 concentrations are low
These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle
CAM plantsOther plants also use PEP carboxylase to fix and
accumulate CO2.Some plants, including succulents, use
crassulacean acid metabolism (CAM) to fix carbon
CAM plants open their stomata at night, incorporating CO2 into organic acids
(oxaloacetate – 4 C, then converted to malic acid)Stomata close during the day, and CO2 is released
from organic acids and used in the Calvin cycle (malic acid goes to chloroplasts)
Plants can make everything they need from CO2, H2O, sulfate, phosphate & ammonium.- Some G3P can enter glycolysis cycle & be converted to pyruvate- Some G3P can enter gluconeogenesis pathway and form 6 carbon sugars, and then sucrose
Chemiosmosis in Mitochondria vs. chloroplast