Chapter 10: Photosynthesis
What happens once the energy is transferred between pigments?
the original excited electron falls back to its ground state
What happens when a photon strikes an object?
the photon may be absorbed, transmitted, or reflected
Lumen
the space inside a thylakoid
How many turns of the calvin cycle is necessary?
three turns -three turns of the cycle fixes three molecules of CO2, which yields one molecule of G3P and three fully regenerated RuBP
Describe rubisco
ribulose-1,5-bisphosphate carboxylase/oxygenase -CO2-fixing enzyme -rougly cube-shaped -consists of 16 polypeptides that form 8 active sites where CO2 is fixed -very slow and inefficient
What is necessary to manufacture sugar from oxidized carbons in CO2?
strong reducing agents like NADPH
What happens to the electrons in the Calvin cycle?
the electrons in NADPH and the potential energy in ATP are used to reduce CO2 into carbohydrate
What happens to the energy that are stored in the bonds of the carbohydrates?
the stored energy can be extracted via cellular respiration (the sugars are oxidized in the mitochondria and O2 is consumed in the process of making ATP) just as animals and other eukaryotes do
What makes it possible for resonance energy to be efficiently moved between pigments?
the organization of the antenna complex
Where is PEP carboxylase commonly found?
in mesophyll cells
How is photosynthesis regulated?
in response to changes in environmental conditions
Chloroplasts and its properties
FUNCTION: site of photosynthesis PROPERTIES: 1. enclosed by an inner and outer membrane 2. interior dominated by thylakoids 3. thylakoids often occur in interconnected stacks called grana 4. fluid-filled space between the thylakoids and the inner membrane is the stroma 5. contains huge quantities of pigments
How does light affect what type of reaction the light-capturing is?
light excited electrons in chlorophyll to a high-energy state which makes it exergonic -without light, the reaction would be endergonic
Where does photorespiration take place?
part of the pathway occurs in chloroplasts, part occurs in peroxisomes and mitochondria
Photons
particles/ discrete packets of light
What type of reaction is carbon fixation?
redox -reduced by attaching it to another carbon
Bundle-sheath cells
a type of cell found around the vascular tissue (veins) of plant leaves
What determines the type of electromagnetic radiation?
wavelength determines the type of light
White light
consists of all wavelengths in the visible portion of the electromagnetic spectrum at once
What happens to the sugar that is produced by photosynthesis?
gluconeogenesis
Grana
interconnected stacks of thylakoid in a chloroplast
Day/night of CAM plants
-CAM plants will open their stomata at night because it's typically cooler and more humid; allows them to take in mass amounts of CO2 that is stored in vacuoles -during the day the stored CO2 is used
What happens in the reduction phase of the calvin cycle?
-3PGA is phosphorylated by ATP and is then reduced by electrons from NADPH -the product is G3P (glyceraldehyde-3-phosphate) -some G3P synthesized is drawn off to produce other organic molecules, like glucose
Photorespiration
-A metabolic pathway that consumes oxygen, releases carbon dioxide, generates no ATP, and decreases photosynthetic output -generally occurs on hot, dry, bright days -when stomata close and the oxygen concentration in the leaf exceeds that of carbon dioxide -low levels of CO2 and high light
What's the difference between CAM plants and C4 plants?
-CAM occurs at a different time than the Calvin cycle; CAM plants photosynthesize and grow even when their stomata are closed during the day -C4 occurs at a different place than the Calvin cycle does; concentrates carbon dioxide in cells deep inside the leaf, so stomata do not have to be fully open
What happens in the fixation phase of the calvin cycle?
-CO2 reacts with RuBP -produces two molecules of 3PGGA
Why is PQ's structure important to its function?
-PQ is a small hydrophobic molecule that can transport electrons between molecules -lipid soluble and not anchored to the thylakoid membrane, thus free to move within the thylakoid membrane
Action spectrum for photosynthesis
-graph that shows the relative rate of photosynthesis at different wavelengths -measured by O2 production -violet to blue and red regions=high O2 concentrations=define the action spectrum
What is the net effect of the electron transport of PQ to the cytochrome complex in PSII
-a large proton electrochemical gradient -results in a proton-motive force that drives protons out of the stroma and into the lumen -proton flow down the electrochemical gradient is an exergonic process that is coupled with the endergonic process of ATP synthesis -the stream of protons flow though ATP synthase, causing conformational changes in the enzyme that drive the production of ATP
How does a pigment molecule react to light? Give ex
-a pigment molecule absorbs photons of particular wavelengths -ex. if a pigment absorbs many wavelengths in the blue/green parts of the spectrum, it appears red
Where does PQ carry the electrons from photosystem II to? What does this result in?
-across the membrane to the lumen side of the thylakoid -delivers them to molecules with a higher redox potential in the *cytochrome complex* RESULT: -the potential energy released by these reactions allows PQ to pick up protons from the chromoplast stroma and drop them off in the thylakoid lumen -high concentration of protons in the lumen -stroma becomes negatively charged relative to the lumen
Noncyclic electron flow
-an electron pathway where electrons travel one-way from H2O to NADP+ -the complete path that electrons follow from PSII to PSI and how it is oriented in the thylakoid membrane
What type of reaction is photosynthesis?
-an energy demanding (endergonic) series of redox reactions -potential energy increases
Overview of what occurs in photosystem II
-contains antenna complexes and a reaction center with a pair of P680 chlorophyll molecules -antenna complex transmits resonance energy to the reaction center -absorbed light energy is used to reduce electron carriers in an ETC that produces a proton-motive force for the synthesis of ATP
Downside to C4 pathway
-for each G3P molecule generated via photosynthesis, C4 plants expend 15 ATP molecules compared to the 9 ATP molecules in C3 plants
Autotrophs
-make their own food from ions and simple molecules -ex. maples, mosses, and other photosynthetic organisms
Absorption spectrum
-measures how the wavelength of photons influences the amount of light absorbed by a pigment
Z-scheme
-model for how photosystem II and I interact -changes occur in electron potential energy as plotted on a vertical axis
Pigments
-molecules that absorb only certain wavelengths of light -other wavelengths are either reflected or transmitted/pass through -appear colored
Heterotrophs
-must obtain the sugars and many of the other macromolecules they need from other organisms -depend on the molecules produced by autotrophs
On a chemical level, how do carotenoids protect plants?
-photons are high energy -photons contain enough energy to knock electrons out of atoms and create free radicals -free radicals are unstable and can trigger reactions that disrupt and degrade molecules -carotenoids accept or stabilize unpaired electrons, which protects chlorophylls from harm
Enhancement effect
-photosynthesis increases dramatically when cells are exposed to both red and far-red light -thus, photosynthesis is more efficient when both photosystems are operating together
C4 plants
-plants that have adapted their photosynthetic process to more efficiently handle hot and dry conditions -fix carbon by using PEP carboxylase -use CO2 efficiently -mesophyll cells take up CO2 then transfer to bundle-sheath cells, where the Calvin cycle operates
What happens in the regeneration phase of the calvin cycle?
-reactions use additional ATP in the regeneration of RuBP -the rest of G3P keeps the cycle going by serving as the substrate for this phase
Why is photorespiration able to occur?
-rubisco will catalyze CO2 or O2 to RuBP
When do stomata open? When do stomata close?
-stomata are normally open during the day, when photosynthesis is occurring -stomata will close at night -if the daytime is extremely hot and dry, leaf cells may lose a great deal of water to evaporation through the stomata; they either close their opening and halt photosynthesis or risk death
Why are the sugars often combined to create sucrose?
-sucrose is water soluble and readily transported to other parts of the plant -can be delivered to rapidly growing parts of the plant to be broken down to fuel cellular respiration and growth
Carbon fixation
-the addition of carbon dioxide to an organic compound -the process converts CO2 gas into a biologically useful form -once carbon atoms are fixed, they can be used as sources of energy and as building blocks to construct the molecules found in cells -a redox reaction
What happens to the oxidized PSII reaction center?
-the electrons removed from the reaction center pigments need to be replaced -electrons come from the oxidation of water/splitting water
What happens when a chlorophyll molecule absorbs a photon?
-the photon's energy is transferred to bonds in the chlorophyll's head region -this transfer causes the electron to be excited/ bumped up to a higher energy state -higher energy state = higher potential energy -this excited state is unstable and will only last a short period of time
What happens to the reaction center pigment after the excited electron is transferred to pheophytin? Why is this a potential problem and what solves it? PSII
-the reaction center pigment becomes an incredibly strong oxidizing agent -the electron is at risk of being pulled back to the oxidizing agent -to fix this problem the electron is quickly shuttled away from the reaction center to an electron transport chain
Calvin cycle
-the reactions that reduce carbon dioxide and produce sugar -functions only if the light-capturing reactions that produce O2 are also occurring (requires products of the light-capturing reactions)
Cyclic electron flow
-uses only photosystem I and produces ATP, but not NADPH -the electrons excited in PSI are transferred back to the ETC, generating ATP through photophosphorylation instead of reducing NADP+ -coexists with noncyclic electron flow and produces additional ATP to meet the energy demand for manufacturing sugars
Why is O2 a good electron acceptor via cellular respiration?
-very electronegative that it creates a huge potential energy drop for the ETCs involved -organisms that use O2 as an electron acceptor in cellular respiration can produce much more ATP than organisms that use other electron receptors
When is photorespiration more common?
-when conditions are hot and dry, the stomata must close -CO2 and O2 transport stops -photosynthesis slows and photorespiration increases
When is starch produced?
-when photosynthesis is proceeding rapidly and sucrose is abundant -glucose molecules are polymerized -starch stored in the cells of leaves and roots -will be broken down into glucose and fed into cellular respiration or used to manufacture sucrose for transport to other parts of the plant
Resonance energy transfer
-when pigments in the antenna complex absorb photons, the energy is passed to a nearby chlorophyll molecule -energy transfer between nearby chlorophyll molecules -only possible between pigment that are able to absorb different wavelengths of photons (i.e. from those absorbing higher-energy photons to those absorbing lower-energy photons)
Explain carbon fixation in C4 plants
1. PEP carboxylase fixes CO2 to a 3-carbon molecule (phosphoenolpyruvate/ PEP) in mesophyll cells 2. the 4 carbon molecules that result are transported to bundle-sheath cells via channels (plasmodesmata) 3. the 4-carbon molecules are broken down to release CO2 molecules -bundle sheath cells are less permeable to gases, so CO2 concentrations rise and promote carbon fixation by rubisco to form 3PGA -this step initiates the Calvin cycle 4. the 3-carbon compound remaining after CO2 is released is returned to the mesophyll cell to regenerate PEP
What are the two classes of carotenoids?
1. carotenes 2. xanthophylls
What are the two major pigment classes in plant leaves?
1. chlorophylls 2. carotenoids
Carotenoid function
1. considered accessary pigment because they absorb light and pass the energy on to chlorophyll 2. absorb some pigment of light that is not absorbed by chlorophyll, which extends the range of wavelengths that can drive photosynthesis 3. protect the plant (primary function)
Explain the three phases of the Calvin Cycle
1. fixation 2. reduction 3. regeneration
What are the two distinct sets of reactions in photosynthesis?
1. one that uses light to produce O2 from H2O 2. one that converts CO2 into sugars
Steps of photosystem I
1. pigments in the antenna complex absorb photons and pass the energy to the PSI reaction center 2. electrons are excited in the reaction center chlorophyll molecules 3. the reaction center pigments are oxidized -the excited electrons are passed through a series of carriers inside the photosystem -the electrons are then passed to ferredoxin -the electrons are then passed to the enzyme NADP+ reductase 4. NADP+ reductase transfers two electrons and a proton to reduce NADP+ and form NADPH
Problems with rubisco
1. slow -plants synthesize huge amounts as an adaptation compensation for its lack of speed 2. inefficient -rubisco will catalyze the addition of either O2 or CO2 to RuBP -oxygen and carbon dioxide compete at the enzyme's active sites, which slows the rate of CO2 reduction
What happens to the electrons in the light-capturing phase of photosynthesis?
1. the electrons are promoted to a high-energy state by light 2. the electrons are transferred through a series of reactions that reduce NADP+ to NADPH 3. NADPH functions as a reducing agent 4. some energy released from these redox reactions are used to produce ATP
What happens when a photon or resonance energy from the antenna complex reaches the reaction center?
1. the energy is absorbed by one of two specialized chlorophyll molecules 2. pigment is energized 3. an excited electron is transferred from the pigment to an electron acceptor 4. acceptor becomes reduced and its potential energy increases ***electromagnetic energy from sunlight has now been transformed to chemical energy***
RuBP + CO2 yields
2 3-phosphateglycerate used in calvin cycle
Each mole of CO2 requires the energy from ____ moles of ATP and _____ moles of NADPH to fix it and reduce it to a sugar
3 moles of ATP; 2 moles of NADPH
RuBP + O2 yields
3-phosphoglyerate (used in calvin cycle) + 2-phosphoglycolate (used ATP and releases CO2)
What is the initial product of carbon fixation?
3PGA 3-phosphoglycerate
Stoichiometry of fixation phase with 3 turns
3RuBP + 3CO2 yields 6 3PGA
What is the range for visible light?
400 to 710 nm
Stoichiometry of the regeneration phase
5 G3P + 3 ATP yield 3 RuBP
Stoichiometry of the reduction phase after 3 turns
6 3PGA + 6 ATP + 6NADPH yields 5 G3P (for regeneration of RuBP) and 1 G3P (released for production of glucose)
___% of energized pigments use their excited electrons to drive photosynthesis
98%
How can photosynthesizing cells raise CO2 concentrations to make photosynthesis more efficient?
C4 pathway
C4 pathway vs C3 pathway
C4: fixing CO2 to produce four-carbon molecules -used to concentrate CO2 to reduce photorespiration in the calvin cycle while stomata are closed to prevent water loss -a three-carbon compound + CO2 treated with PEP carboxylase -can serve as an additional fixation step C3: fixing CO2 to produce three-carbon molecules -most common form of photosynthesis -RuBP + CO2 treated by rubisco
The overall reaction of photosynthesis
CO2 + H2O + light --> carbohydrate (CH2O)n + O2 n greater than or equal to three
Where is the antenna complex and reaction center located?
In the thylakoid space (chlorophyll)
What's the difference between how ATP is used in animal cells vs. plant cells?
MITOCHONDRIA: ATP is exported and fuels many different cellular processes CHLOROPLASTS: the ATP remains within the organelle and is used for the production of carbohydrate
What is the result of splitting water?
O2 as a byproduct and H+ ions that contribute to the electrochemical gradient
Oxygenic photosynthesis vs anoxygenic photosynthesis
OXYGENIC: photosynthetic organisms that oxidize water will have O2 as a by-product ANOXYGENIC: other organisms only have a single photosystem that do not oxidize water, thus do not product O2 as a by-product
Trends in a combined graph of action and absorption spectra
PEAKS = wavelengths where absorbance/photosynthetic activity is high TROUGHS = wavelengths where absorbance/photosynthetic activity is low
Splitting water
PSII -oxidation of water that supplies electrons for PSII -highly endergonic reaction -energy necessary is supplied by the harvested light energy
What is special about pheophytin?
PSII -pheophytin does not become excited by photons or resonance energy -it solely accepts excited electrons from the reaction center chlorophylls
Photophosphorylation
PSII -the synthesis of ATP in chloroplasts is initiated by the energy from light
Why is the redox reaction between pheophytin and the reaction center chlorophyll pigment so important?
PSII it is a key step in transforming light energy into chemical energy
Which compound reacts with CO2 to produce 3PGA?
RuBP ribulose bisphosphate 5 carbon compound
Describe the difference between shorter wavelengths and longer wavelengths in terms of energy
SHORTER wavelengths: more energy, more energy in blue light LONGER wavelengths: less energy, less energy in red light
Similarities between photosystem II's ETC and mitochondrial ETC
STRUCTURALLY: both contain quinones and cytochromes FUNCTIONALLY: -the redox rxns both result in protons being actively transported from one side of an internal membrane to the other -the resulting proton-motive force drives ATP production via ATP synthase
Carbohydrates contain ___oxidized/ reduced___ carbons in the form of C-C and C-H bonds
reduced
What are the chlorophylls? What are their properties?
TYPES: chlorophyll a & chlorophyll b PROPERTIES: strongly absorb in the blue and red regions, which makes plants look green (reflects) -main photosynthetic pigment
At what part of the day would there be the highest concentration of four-carbon acids in the vacuoles of CAM plants?
The highest concentration of organic acids in the vacuoles of CAM plants would be found in the morning, since these acids are made during the night and used up during the day.
What is light?
WHAT: electromagnetic radiation, a form of energy PROPERTIES: 1. wavelike 2. particle like (photon)
What is the result of photosynthesis? Why must it occur?
WHY: photosynthetic organisms cannot store the electromagnetic energy of light unless it is first converted into another form WHAT: photosynthesis converts the energy in sunlight to chemical energy in the form of C-C and C-H bonds of carbohydrate
Pheophytin
a molecule in photosystem II that accepts excited electrons from the reaction center chlorophyll and passes them to an ETC
Carotenoid properties in regards to light absorbance
absorbs in the blue and green parts, which appears yellow, orange, and red
Where does resonance energy transfer occur?
antenna complex pigments
PEP carboxylase
an enzyme in C4 plants that fixes carbon adding a three-carbon compound to CO2 and forming a 4-carbon organic acid -has a higher affinity for CO2 than rubisco, thus the stomata can be open for shorter periods of time in C4 plants
What makes up the photosystem?
antenna complex and reaction center as well as the molecules that capture and process excited electrons
Where is rubisco commonly found?
buddle-shealth cells
Examples of CAM plants
cacti and other species that routinely keep their stomata closed on hot, dry days
What do photosystem I and II produce?
chemical energy stored in ATP and NADPH
Structure of chlorophylls
chlorophyll a & chlorophyll b have similar structures: 1. long isoprenoid tail, which interacts with proteins embedded in the thykaloid membrane 2. the head with a large ring structure and magnesium atom in the middle, which is where light is absorbed
What happens if carotenoids aren't present?
chlorophyll molecules are destroyed by free radicals and photosynthesis stops
What is the most abundant pigment in the thylakoid membrane?
chlorophyll, which reflects or transmits green light
When is fluorescence (un)common?
common of isolated pigments uncommon in chloroplasts
What is done to maximize carbon fixation?
concentrations of CO2 must be higher then the O2 produced by the light reactions
CAM plants
crassulacean acid metabolism -CO2 concentrator that acts as an additional step to the Calvin cycle -generates 4-carbon organic acid in first CO2 fixation step -open their stomata at night and store CO2 -convert CO2 to sugar during the day by the calvin cycle
How does the electron potential energy change between the primary electron donor and the terminal electron acceptor?
electron donor: H2O reduced terminal electron acceptor: NADPH *the electron donor has lower potential energy than the reduced terminal electron acceptor
When can a molecule absorb a photon?
if the difference between the possible energy states is the same as the energy in the photon, the photon can be absorbed and an electron excited to a higher energy state -must be discrete/incremental energy levels
When does fluorescence occur? What is the result?
if the excited electron falls back to its ground state, the absorbed energy can be released as a combination of heat and light -results in lower energy and a longer wavelength than the original photon -occurs 2% of the time
Where does sucrose production take place?
in the cytosol
Where does starch production take place?
inside the chloroplast
Mesophyll cells
near the surface of the leaves
One turn of the Calvin cycle fixes ______ molecule of CO2
one
Where does photosynthesis take place?
only in the green portions of plants
PC
plastocyanin -a small protein that shuttles electrons originating from the cytochrome complex in PSII to the reaction center of PSI -in its reduced form, PC diffuses through the lumen of the thylakoid and donated the electron to an oxidized reaction center pigment in PSI -physical link between PSII and I -replaces electrons that are carried away from the pair of pigments (P700) in photosystem I reaction center
PQ
plastoquinone -electron carrier in the ETC that receives excited electrons from PSII (cyclic) or PSI (noncyclic) and passes them through the ETC -transports protons from the stroma to the thylakoid lumen, generating a proton-motive force
What happens to the potential energy as resonance energy is moved between pigments?
potential energy drops at each step
Photosynthesis
process by which plants and some other organisms use light energy to convert water and carbon dioxide into oxygen and high-energy carbohydrates such as sugars and starches
Electromagnetic spectrum
range of wavelengths of electromagnetic radiation
Where do redox reactions occur?
reaction center
Thin layer chromatography
separates the pigments to determine which wavelengths are absorbed
What does the Calvin Cycle depend on?
the ATP and the NADPH produced by the light-capturing reactions
What are the four possible fates of electrons in chlorophyll that are excited by photons?
the energy released from these electrons can 1. be emitted in the form of light via fluorescence 2. be given off as heat alone 3. excite an electron in a nearby pigment and induce resonance 4. be transferred to an electron acceptor in a redox reaction
What does the Z-scheme model help explain?
the enhancement effect 1. when chloroplasts are illuminated with wavelengths in the red portion of the spectrum, only PSII can run at maximum rate -the overall rate of electron flow through the Z scheme is moderate because PSI's efficiency is reduced 2. when chloroplasts are illuminated with wavelengths in the far-red portion of the spectrum, only PSI is capable of peak efficiency 3. but when both wavelength are available at the same time, the photosystems are activated and work at a maximum rate, leading to the enhanced efficiency
Stroma
the fluid-filled space between the thylakoids and the inner membrane of a chloroplast
Where does the calvin cycle take place?
the stroma of the chloroplasts
How are the two sets of reactions of photosynthesis linked together?
the two reactions (light-capturing and Calvin cycle) are linked by a series of redox reactions that starts when water is split/oxidized to form O2
What happens to the electrons in PSI after they're carried away by the reaction center?
transferred to ferredoxin, which passes electrons to an enzyme that catalyzes the reduction of NADP+ to NADPH
Gluconeogenesis
used to produce glucose from the products of the calvin cycle -glucose often combined with fructose to form a disaccharide sucrose -occurs when photosynthesis is taking place slowly
What are the wavelengths that drive photosynthesis? Why?
violet-to-blue and red photons -chlorophyll absorb these wavelengths
Fluorescence
when the electron energy produces light
