Lecture 12 photosynthesis Flashcards
Define photosynthesis
A metabolic process by which energy of sunlight is captures and used to convert CO2 and H2O into carbohydrate sugars and oxygen gas
What three aspects of photosynthesis were established by early in the 19th century?
- Water for photosynthesis comes primarily from soil via the roots
- Gaseous exchange occurs through the stomata
- Light is necessary for oxygen and glucose production
Is photosynthesis the reverse of cellular respiration?
NO
What method was used to determine the source of oxygen produced in photosynthesis determined experimentally?
Radioisotope 18-O
1 group given water with oxygen isotope and CO2 with regular oxygen and second group reverse
What happened after radioisotopes were supplied to the two different groups?
Oxygen gas produced was taken and analyzed
What was the results and conclusion of the radioisotope experiment to determine the source of oxygen produced in photosynthesis?
18-O oxygen gas was produced by plants given the 18-O labelled water only
Therefore water is the source of O2 produced by photosynthesis
What is the revised, balanced photosynthesis equation?
6CO2 + 12H2O –> C6H12O6 + 6O2 + 6H20
What are the two pathways of photosynthesis?
Light reactions, light independent reactions
What are the light reactions?
Driven by light energy
Converts light energy into chemical energy in the form ATP and NADPH + H+
What are the light independent reactions?
Do not use light directly
Use ATP, NADPH +H+ and CO2 to produce sugars
What are the three forms of the light independent reaction?
Calvin cycle
C4
Crassulacean acid metabolism
Where do the light and light independent reactions occur?
Within the chloroplasts
How are the light and light independent reactions linked?
Exchange of ATP, ADP, NAD, NADP
Rate of each reaction depends on the rate of the other
How does light (electromagnetic radiation) act?
As both a particle and a wave- discrete packages called photons
What does absorption of a photon do to a pigment molecule?
Excites it
What else can happen when a photon meets a pigment?
- Bounce off (scattered/reflected)
- Pass through (transmitted)
What happens when a photon is absorbed according to the first law of thermodynamics?
The molecule acquires the energy of that photon, raising it from ground state to excited state
Why is there some difference between the energy of a photon and the subsequent energy of the pigment molecule?
Lost as entropy
What does the difference in free energy after absorbing a photon do to a pigment molecule in an excited state?
The energy boosts one of the electrons in the molecule to the furthest shell from the nucleus
The molecule is more chemically reactive.
What are pigments?
Molecules that absorb wavelengths in the visible spectrum
How do absorbed wavelengths correlate with biological activity?
Specific wavelengths are absorbed by each type of molecule
What can be plotted based on the activity of an organism at each wavelength of the spectrum?
An action spectrum
What are the two predominant types of chlorophyll used in photosynthesis?
Chlorophyll a and b
How do chlorophyll a and b differ?
Molecular structure
What is the structure of chlorophyll and and b?
Complex ring structure with magnesium atom in the centre and a hydrocarbon tail (which anchors)
Where is chlorophyll attached?
The thylakoid membrane of the chloroplast
How is chlorophyll attached to the thylakoid membrane?
Hydrocarbon tail is attached to a peripheral location anchoring the molecule to integral proteins in the thylakoid
What wavelengths do chlorophyll absorb?
Red and blue wavelengths
What are accessory pigments?
Pigments that absorb intermediate wavelengths and transfer energy to chlorophyll
Name two accessory pigments.
Carotenoids,
Phycobilins
What wavelengths do carotenoids absorb? How do they appear?
Blue and blue-green wavelengths (appear deep yellow)
Give an example of a carotenoid.
Beta carotene
Where are phycobilins found?
Red algae and cyanobacteria
What wavelength do phycobilins absorb?
Absorb yellow, yellow-green, orange wavelengths
When does light absorption not result in photochemical change?
Molecule returns to ground state, some energy is given off as heat or light energy (fluorescence)
What happens when light is given off as heat or light?
No chemical work is done- no chemical changes/biological functions
What happens when light is not given off as heat or light energy?
The molecule passes its energy to another molecule if nearby, in the right orientation and with the appropriate structure
How are pigments in photosynthetic organisms arranged?
Antenna systems
What are antenna systems?
Pigments are packed such that the excitation from an abosorbed photon can be passed from one pigment molecule to another
Energy from an absorbed photon is passed from pigments that absorb ______ energy, ______ wavelengths to pigments that absorb _____ energy, ______ wavelengths.
Higher energy, shorter wavelengths to lower energy, longer wavelengths.
What is the name of the molecule that absorbs the longest wavelength and where the excitation ends up?
The reaction carrier
What does the reaction carrier do?
Converts the energy into chemical energy
How does the reaction carrier convert the energy into chemical energy?
Becoming chemically oxidized- positively charged
In plants, what is the reaction carrier always?
Chlorophyll a
How does excited chlorophyll act in the reaction center?
As a reducing agent
What are the two main roles of chlorophll a in photosynthesis?
To absorb light energy
To transform it into chemical energy (electrons) and transfer those electrons
How does chlorophyll act as a reducing agent?
By donating an electron to a stable electron acceptor in a redox reaction
How does reduction lead to electron transport?
The oxidizing agent that was reduced by excited chlorophyll is the first in the chain of electron transport carriers
Where are the electron transport carriers for photosynthesis?
The thylakoid membrane
What is electron transport?
A series of oxidation-reduction reactions
What is the final electron acceptor in the electron transport chain in the thylakoid membrane?
NADP+
What happens when NADP+ accepts an electron?
It is reduced to NADPH + H+ (a reduced coenzyme)
What does NADP+ stand for?
Nicotinamide adenine dinucleotide phosphate
How is the structure of NADP+ different to NAD+ seen in respiration?
The addition of a phosphate group attached to the ribose
How is the function of NADP+ different to NAD+?
NAD+ is used in catabolism
NADPH+ is used anabolic reactions
What are the two different systems for electron transport in photosynthesis?
Nonclyclic electron transport
Cyclic electron transport
What does noncyclic electron transport produce?
NADPH +H+ and ATP
What does cyclic electron transport produce?
Only ATP
What is a photosystem?
A complex of pigments which capture light energy to energise reactions
What is the first step in the electron transport chain of photosytem II?
The P680 chlorophyll molecule in the reaction centre of photosystem II absorbs light and becomes Chl*
What does non-cyclic electrons transport require?
Two different photosystems
What does photosystem II do?
Use light energy to oxidize water molecules, producing electrons, protons and water
What does photosystem I do?
Uses light energy to reduce NADP+ to NADPH +H+
Why is the chlorophll a molecule in the reaction centre for photosystem II called P680?
Because it absorbs light maximally at 680nm
What is the name of the chlorophll molecule in the reaction centre of photosystem I?
P700
Which photosystem requires light that is slightly more energetic?
Photosystem II (absorbs shorter wavelengths)
What is the Z scheme model?
The interaction between photosystem II and I due to the path of the electrons when placed along an axis of rising energy level
What is required to keep noncyclic electron transport going?
Both photosystems must be constantly absorbing light to boost electrons to higher energy shells
What happens in photosystem II when chlorophyll lacks an electron?
It takes electrons from water, resulting in O2, H+ and e-
Where do electrons from P680 pass to?
The primary electron acceptor- the first carrier in the electron train
What happens to P680 when it passes its electrons to the primary electron acceptor?
It becomes P680+
How does P680+ become reudced?
Electrons from oxidation of water reduce it
What happens to electrons from Photosystem II?
They pass through a series of exergonic reactions in the electron transport chain
What are the exergonic reactions in the electron transport chain coupled do?
Proton pumping across the thylakoid membrane
What is the name of the proton pumping across thylakoid membranes?
Chemiosmotic pumping
What does chemiosmotic pumping create?
A proton gradient that produces energy for ATP synthesis
What is the first step in photosystem I?
The reaction centre containing P700 becomes excited (P700*)
What does the P700* reduce?
Ferredoxin (an oxidizing agent)
How does P700+ return to its ground state?
By accepting electrons passed through the electron transport chain from photosystem II.
What happens to the electrons from ferrodoxin?
They reduce NADP+ to NADPH +H+
What organisms use cyclic electron transport?
Organisms where the ratio of NADPH +H+ to NADP+ is high
Why is cyclic electron transport cyclic?
Electron passed from an excited chlorophyll molecule at the outset cycles back to the same chlorophll molecule at the end
What is the first step in cyclic electron transport?
P700 absorbs a photon and becomes P700*
What is the second step in cyclic electron transport, after P700* is formed?
P700* reduces oxidised ferrodoxin in an exergonic reaction
What happens after ferrodoxin is formed?
It passes its electron to another oxidising agent, plastoquinone (PQ)
What happens when plastoquinone is being reduced?
It pumps 2H+ back across the thlakoid membrane
How does reduced plastoquinone pass its electrons to the electron transport chain?
By way of plastocyanin (PC)
What is the final step in the cyclic electron transport chain?
the electron passes back to P700+
The cyclic electron transport chain is a series of _____ reactions, each ______.
redox
exergonic
How is released energy from the cyclic electron transport chain stored?
In a proton gradient used to form ATP
Define photophosphorylation.
The light driven production of ATP from ADP + Pi in the chloroplast
Protons move from the ______ to the ______ of the thylakoid to create a proton gradient.
Stroma (interior matrix of chloroplast)
to the lumen of the thylakoid
How do protons move back into the stroma?
They diffuse out of the thylakoid through protein channels called ATP synthase
How is ATP synthase in animals and plants?
Orientation (in animals, protons diffuse back into mitochondrial matrix)
Where are most enzymes that catalyze CO2 fixation found?
Dissolved in the stroma of the chloroplasts
Why does the light independent reaction/CO2 fixation only occur in the presence of light?
It depends on ATP and NADPH+ produced in light dependent reactions which are not stockpiled
How were the steps of the calvin cycle revealed experimentally?
Radioisotope labelling
What radioisotope was used to discover the steps of the calvin cycle?
C14 labelled CO2
What are the properties of carbon 14?
Emits radiation but is behaves the same chemically (not distinguished by enzymes)
What did Calvin and his colleagues perform their experiments on?
Unicellular green algae, Chlorella
What was the first step Calvin and his colleagues did to reveal the stages of the Calvin cycle?
Expose Chlorella to 14CO2 for 30 seconds
What happened after Chlorella was exposed to carbon-14 CO2 for 30 seconds?
The cells were killed and their organic compounds extracted
How were different components of Chlorella extracted?
Paper chromatography
What is the method of paper chromatography that was used to separate the Chlorella extract?
Dissolve extract in alcohol, applied to a sheet of filter paper, hydrogen bonds formed with cellulose, paper put into phenol-water (solvent) and crept up by capillary action
How were the results of paper chromatography of Chlorella extract observed?
Exposing X-ray film to the filter paper to reveal positions of the radioactive compounds
What was wrong with the results of the Chlorella extract after the 30 second exposure time?
Many compounds, including monosaccharides and amino acids contained the radioisotope
How long was Chlorella exposed to the carbon-14 CO2 in the second experiment?
3 seconds
What was the result of the second experiment?
Only one compound was labelled
Which compound was labelled in the second experiment?
3-phosphoglycerate
How did Calvin and his colleagues discover the series of compounds that make up the carbon cycle that fixes the CO2?
Tracing steps with successive, increasingly long exposures
Why was the first observed compound 3-phosphoglycerate?
The initial step adds CO2 to ribulose 1,5-bisphosphate to make intermediate 6 carbon compound which is quickly broken down into three carbon compound
What enzyme catalyzes the fixation of CO2 and ribulose-1,5-bisphosphate?
Ribulose bisphosphate carboxylase/oxygenase (rubisco)
Fun fact about rubisco
Most abundant protein in the world
50% of all the protein in every plant leaf
What three processes make up the Calvin cycle?
Fixation of CO2
Reduction of 3PG
Regeneration of CO2 acceptor RuBP
Where does the calvin cycle take place?
In the stroma of the chloroplasts
How much G3P goes where?
5/6 recycled into RuBP, remaining 1/6 has two fates
What are the two fates of the remaining 1/6 G3P?
1/3 becomes starch and stored in chloroplast
2/3 converted in cytosol into sucrose and transported elsewhere
What is sucrose made of?
Fructose and glucose
What happens after RuBP and CO2 have combined and formed two molecules of 3PG?
3PG is reduced to G3P in a two step reaction involving ATP and NADPH+ +H+
What does G3P stand for?
Glyceraldehyde 3-phosphate
Other than using ATP and NADPH made through photophosphorylation, how else is the light reaction and CO2 fixation pathway connected?
- Light induced pH changes in the stroma
- Light induced electron flow
What does light induced pH changes in the stroma do?
Activates some enzymes in the calvin cycle
How is the pH of the stroma changed by light?
The pumping of H+ into the thylakoids from the stroma raises pH of stroma from 7 to 8 which activates rubisco
What does light induced electron flow do?
Reduces disulfide bonds to activate 4 calvin cycle enzymes.
How is electron flow induced by light?
Ferrodoxin is reduced in PSI, some electrons passed to protein thioredoxin, passes electrons to 4 enzymes in CO2 fixation pathway
What happens when electrons are passed to the 4 enzymes in the CO2 fixation pathway?
Disulfide bridges near active sites broken when reduced, change in 3D shape activates these enzymes
What is a major limitation of rubisco?
Its tendency to react with oxygen
What is it called when rubisco reacts with O2 instead of CO2?
Photorespiration
What does photorespiration do?
Lowers the rate of CO2 fixation
What is the product of RuBP + O2?
Phosphoglycolate + 3PG
What have some plants done to partially recover some carbon channeled away from the calvin cycle?
Phosphoglycolate forms glycolate which diffuses into peroxisomes
What happens in the peroxisome?
Series of reactions
Glycolate–> glycine
What happens after glycine has formed?
It diffuses into the mitochondria
What happens to glycine in the mitochondria?
2glycine –> glycerate + CO2
Photorespiration reduces net carbon fixed by the Calvin cycle by __%
25
How much affinity does rubisco have for CO2 compared to O2?
10x
What happens with rubisco when relative concentrations of CO2 and O2 vary?
Rubisco acts as an oxygenase when O2 is relatively abundant etc.
Under what conditions is photorespiration more likely to occur?
High temperatures
Why is photorespiration more likely to occur in high temperatures?
Hot dry day, stomata close to prevent water loss, this prevents gas exchange, CO2 concentration falls, O2 concentration increases.
Give examples of plants whose palisade mesophyll cells just below the surface of the leaf are full of chloroplasts that contain abundant rubisco.
Roses
Wheat
Rice
Why are these plants called C3 plants?
The first product of CO2 fixation in these plants is 3PG
Give an example of C4 plants.
Corn, sugarcane, tropical grasses
What happens when C4 plants close their stomata on a hot day?
Rate of photosynthesis does not fall
Photorespiration does not occur
How do C4 plants keep the ratio of CO2 to O2 high?
By making 4 carbon compound as the first product of CO2 fixation
What is the 4 carbon compound made as the first step of CO2 fixation in C4 plants called?
Oxaloacetate
What is the name of the CO2 fixing enxyme in C4 plants?
Phosphoenolpyruvate (PEP)
What are the 2 advantages of PEP over rubisco?
- No oxygenase activity
- Fixes CO2 even at very low [CO2]
What is the difference between crassulacean acid metabolism and C4 plants?
Both make 4 carbon compounds
CAM reactions are separated by time rather than space
In c4 plants, molecules are synthesized in mesophyll cells and transported to…
bundle sheath cells for the calvin cycle
What happens to CAM plants during the night?
Stomata open, CO2 is fixed to oxaloacetate, which is converted to malic acid
What happens to CAM plants during the day?
Stomata close
Malic acid is shipped to chloroplasts
Decarboxylation supplies CO2 for calvin cycle
Light reactions supply ATP and NADPH
Why is partitioning of G3P important?
It can be used as part of the glycolysis pathway and converted into pyruvate or enter a pathway of glycolysis reserve (gluconeogenesis)
