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Chapter 10
Photosynthesis
Modes of Nutrition• Heterotrophs – obtain organic
compounds by consuming other organisms (animals)
• Photoautotrophs – build organic compounds with light energy (plants)
• Chemoautotrophs – use inorganic substances (sulfur, ammonia) as an energy source to make organic compounds (some bacteria)
Photosynthesis
NET:
But really..
Opposite of Cellular Respiration!
• Photosynthetic prokaryotes do not have chloroplasts. Chlorophyll is in their plasma membrane.
Contains chlorophyll
Light Dependent: Closer Look
• Sunlight is electromagnetic energy– Wave-like properties and Particle-like
properties– Waves Wavelengths– Visible wavelengths = 380nm-750nm
Pigments: Substances that absorb visible light
Chlorophyll a is main pigment
Accessory Pigments:-Chlorophyll b (yellow-green-Carotenoids (oranges, yellows)
So, why do leaves look green?
Lab #4 – Plant Pigments
What wavelengths are absorbed
Effectiveness of wavelengths = activity
• Doesn’t exactly match due to accessory pigments (transfer energy to Chlor. a)
Light Dependent: Photosystems Light harvesting units of the thylakoid membrane Composed mainly of protein and pigment antenna
complexes Antenna pigment molecules are struck by photons Energy is passed to reaction centers Excited e- from chlorophyll is trapped by a primary e-
acceptor
Light Dependent: Photosystems• 2 photosystems in thylakoid membrane– Photosystem II• chlorophyll a• P680 = absorbs 680nm ~ red light
– Photosystem I• chlorophyll b• P700 = absorbs 700nm ~ red light
Light Dependent = The ETC
• ETC uses light energy to produce– ATP & NADPH
• go to Calvin cycle
• PS II absorbs light– excited electron passes from chlorophyll to “primary
electron acceptor”– need to replace electron in chlorophyll– enzyme extracts electrons from H2O & supplies
them to chlorophyll• splits H2O• O combines with another O to form O2
• O2 released to atmosphere• and we breathe easier!
Fill inStroma (fluid)
Thylakoid (disk)
Lab #4 - Photosynthesis
• DPIP was used to replace NADP+• DPIP accepted e- (reduced = RIG)• Turned from Blue Clear• More light could pass through
cuvette• Transmittance % increased
Light Dependent: Photophosphorylation
• Using light energy to add a P to ADP• Two types:– Non-Cyclic– Cyclic
Non-Cyclic Photophosphorylation
• Light reactions elevate electrons in 2 steps (PS II & PS I) – PS II generates
energy as ATP
– PS I generates reducing power as NADPH
– NADPH used in Calvin
Cycle (light independent)
Cyclic Photophosphorylation
• Involves only PS1
• Generates ATP but no NADPH or O2
• Supplements the ATP supply required for the Calvin Cycle
Light Independent: A Closer Look
• AKA The Calvin Cycle• In stroma• Uses ATP and NADPH to convert CO2 to
sugar• Makes a 3-C sugar• Needs 3 CO2 to make 1 Glucose
(C6H12O6)
• Uses 18 ATP (endergonic) and 12 NADPH to make 1 Glucose
G3P
• Glyceraldehyde-3-P – end product of Calvin cycle– energy rich 3 carbon sugar
“C3 photosynthesis”
• G3P is an important intermediate• G3P glucose carbohydrates
lipids phospholipids, fats, waxes
amino acids proteins nucleic acids DNA, RNA
RuBisCo• Enzyme which fixes carbon from air– ribulose bisphosphate carboxylase – CO2 + 5-C sugar (RuBP) is broken down by
RuBisCo into 3-C sugars–most abundant enzyme
Types of Plants• C3 - most plants, produce G3P – Ex: rice, wheat, soybeans– Occurs in a single chloroplast– CO2 + RuBp (5-C) = 6-C split into G3Ps
Glucose
Ruh-Roh…
• Hot, dry days– Stomata close to conserve water– CO2 is depleted
– O2 builds up from light reactions
• RuBisCo– when O2 concentration is high• RuBisCo bonds O to RuBP• O2 is a competitive substrate
• oxidation of RuBP• breakdown sugars
CALLED PHOTORESPIRATION
!
Photorespiration
• Consumes oxygen• Makes carbon dioxide• Produces no ATP• Decreases photosynthetic output
(decreases organic molecules used in Calvin Cycle)
SOLUTIONS!
• Plants living in hot, arid climates have evolved different modes of carbon fixation– C4 and CAM plants
• C4 – turn CO2 into a 4-C compound
– Ex: corn, sugarcane– Favored in hot, arid
environments– 2 chloroplasts– Mesophyll = CO2 is
fixed into a 4-C– Bundle Sheath Cells
(cells surrounding veins) = Calvin Cycle
– Facilitates production of CO2 to combat photorespiration
FYI: the PEP Carboxylase has a much greater affinity for CO2 than RuBisCo at higher temps
• CAM Plants– Ex: Cacti,
Pineapple– Very arid
environments– At night: Stomata
open, take in CO2
– Store a 4-C compound in vacuole
– During Day: Light Rxns supply ATP and NADPH, Uses stored CO2 to complete Calvin Cycle
Photosynthesis Summary
• Light reactions– produced ATP– produced NADPH– consumed H2O– produced O2 as byproduct
• Calvin cycle– consumed CO2
– produced G3P (sugar)– regenerated ADP– regenerated NADP
Interdependence in Nature
REDOX rxns
Cell Respiration• Exergonic
– Energy is RELEASED from the oxidation of sugar
• Electrons are transported to OXYGEN = Water– Oxygen pulls e -s down to
produce concentration gradient of H +
– H+ is pumped through ATP Synthase ADP + Pi = ATP
Photosynthesis• Endergonic
– Energy is REQUIRED– Light is the energy
source that moves e-
• Water is split, electrons are moved from water to carbon dioxide– CO2 is reduced
– Makes sugar (glucose)
Photosynthesis Animation
• http://dendro.cnre.vt.edu/forestbiology/photosynthesis.swf