Lecture 12b Flashcards by Aislinn Costa

what membranes can light pass through?

plasma membrane
chloroplast membrane
thylakoid membrane

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in chloroplasts where does the light reactions occur?

light reactions of photosynthesis occur in the thylakoids

light energy -> ATP and NADPH

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what are the four light reactions ?

photosystem II and I (PSII and PSI)
electron transport chain 1 and 2
ATP synthase
NADP+ reductase

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what are the two components that make up photosystems ?

A) **light harvesting complexes:
- contain pigment molecules (antennae) which surround the reaction center complex
B) a reaction center complex:
- contains a specialized pair of chlorophyll a pigments

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light harvesting complexes

what happens once the electron is transfered through inductive resonance (dance floor) and is at the reaction center?

once at the reaction center, a special pair of chlorophyll a tranfers an excited electron to the primary electron acceptor

here the excited electron is transferred

electron transfer only occurs in the reaction center between chlorophyll a molecules and a primary electron acceptor

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what does the reaction center contain?

a special pair of chlorophyll a molecules
a primary electron acceptor

the chlorophyll a molecules use the energy from light to boost one of their electrons to a higher energy level and to transfer it to a different molecule = the primary electron acceptor

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what are the similarities and differences between PSII and PSI?

both are in the thylakoid membrane
both participate in the light reactions of photosynthesis
*PSII functions first *
they **differ in their reaction centers **
each has a specific primary electron acceptor next to a special pair of chlorophyll a molecules associated with *specific proteins *

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what is the reaction center of PSII?

P680
- its special chorophyll a pigment pair has optimal light absorbance at wavelength 680 nm

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what is the reaction center of PSI?

P700
- its special chlorophyll a pigment pair has optimal light absorbance at wavelength 700 nm

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what causes the difference in absorbance between PSII and PSI

due to the proteins associated with chlorophyll molecules because the chlorophyll molecules themselves are identical

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what are the consequences/results of P680 primary electron acceptor being in an excited state and left with an oxidized pair of chlorophyll a molecules?

water is split by an enzyme to replace the electrons taken from chlorophyll a molecules in the photosystem II reaction center
oxygen is generated as a product
H+ is released into the thylakoid space
the electrons are taken by chlorophyll a one at a time and therefore remain in the reaction center.

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what is the first ETC chain of PSII made of?

plastoquinone (Pq)
a series of cytochromes (Cyt)- this stage creates H+ gradient
plastocynanin (Pc)
P700 in PSI

ETC1 creates a proton gradient in the thylakoid membrane which drives chemiosmosis and synthesizes ATP

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what happens in the PSI reactions?

the excited electron is transferred from P700 to the primary acceptor of PSI following which it gets transferred to the second ETC (ETC2).
ferredoxin (Fd) accepts the electrons from the primary acceptor and transfers them to NADP+, generating NADPH
catalyzed by the enzyme NADPH reductase
No H+ gradient is made, so no ATP is made
the electrons from ETC1 are transferred to P700 to regenerate their initial reduced state.

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what is NADP+?

NADP+= nicotinamide adenine dinucleotide phosphate
- its a coenzyme just like NAD+ is in cellular respiration. The main difference with NAD+ is the presence of an extra phosphate group.
- NADP+ serves as an electron carrier from the light reactions to the calvin cycle in photosynthesis.

think of the added p for phosphorylation

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what are the two possible flow of electrons through photosystems?

1) linear electron flow:
PSII -> ETC I -> PSI -> ETC 2
- makes both ATP and NADPH
- makes O2
2) cyclic electron flow
- only go through PSI and uses ETC1 instead of ETC2
- makes ATP (no NADPH)
- no oxygen made

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explains what happens in Linear electron flow

PSII produces high energy electrons which are transferred to ETC1
- the resulting oxidized chlorophyll a pair is reduced by an electron from water, making O2 in the process
- as the electrons move down ETC1 the energy is released and used to create a H+ gradient used for ATP synthesis
- at end of ETC1 electrons are transferred to the reaction center in photosystem I
- there, light energy boosts their energy level which is used to generate NADPH following ETC2

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what is cyclic electron flow?

only uses PSI and short circuitrs the rest of the steps:
- transports electrons through the ETC1 and from there back P700 in PS1.
- there is no production of NADPH and no release of O2
- there is however the production of ATP
- ferredoxin (Fd) brings the electrons to the cytochrome complex of ETC1 rather than to NADPH+ reductase.

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what happens when the calvin cycle runs low on ATP?

the NADPH will accumulate as the Calvin cycle slows down.

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which does the calvin cycle consume more of? ATP or NADPH?

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consumes more ATP than NADPH

the calvin cycle can run with no sunlight ( can run in the dark ) because of the excess ATP, but once it runs out of ATP it needs sunlight to function again

what is photophosphorylation?

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the conversion of energy from a light-excited electron into the phosphate bond of a ATP molecule

what are the three steps of photophosphorylation?

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1) when light energy boosts the energy state of an electron, the electron is transferred to a primary electron acceptor in a photosystem. The electron is then transferred to an ETC
2) transport through the ETC lowers the energy level of electrons. This released energy is used to create a H+ gradient across the thylakoid membrane ( high concentration of H+ in the thylakoid space/lumen )
3) chemiosmosis by ATP synthase: the proton graident drives ATP synthesis.

what happens in ETC 1 of photophosphorylation of ATP?

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the ETC 1 pumps H+ across the thylakoid membrane from the stroma into the thylakoid space, against its concentration gradient
as a result, H+ accumulates in the thylakoid space

H+ diffuses down its concentration gradient back into the stroma through ATP synthase found within the thylakoid membrane

where are the catalytic knobs of ATP sythase located in chloroplasts?

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located on the stroma side of thylakoids

where is ATP synthesized making it available for the calvin cycle?

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stroma

# review what are the two types of ATP synthesis?

ATP synthesis in the mitochondria (1) and within chloroplasts (2)

how does carbon enter the calvin cycle (dark / light-independent pathways) and how does carbon exit?

carbon enters in the form of **CO2** and exists in the form of **sugar**

what does the calvin cycle use for energy and consume to make sugar?

used ATP as an energy source and consumes NADPH to make sugar

# true or false the calvin cycle is anabolic ## Footnote anabolic= building of complex molecules from simpler ones in order to undergo metabolism

true

what is the starting material of the calvin cyle? | hint= same as the krebs cycle

uses **RuBP** ( 5-carbon sugar) that is regenerated after some molecules enter the cycle and others exit the cycle ## Footnote RuBP is the product created which then is used to start the next cycle

what is the sugar produced by the calvin cycle?

G3P ( glyceraldehyde-3-phosphate ) not glucose itself | 3 carbons = three turns of the calvin cycle produces G3P

what are the three parts that the calvin cycle is divided into?

1) carbon fixation 2) reduction 3) renergation of the CO2 acceptor

# carbon fixation what is the enzyme used in phase 1 of calvin cycle

the enxyme **rubisco** - rubisco is the most abundant protein in chloroplasts and on earth - RuBP is the entering molecule of this cycle(5C sugar)

what is the direct competitor of rubisco's active site?

O2 is the direct competitor

# carbon fixation what is the intermediate of phase 1 of calvin cycle

6 carbon intermediate that is short lived as it is so energetically unstable that it will split in half immediately ## Footnote each co2 fixed leads to 3-PGA ( 3-phosphoglycerate )

# carbon fixation what do we rely on photosynthesis to do?

remove the excess atomospheric CO2

# reduction what gets phosphorylated in phase two of calvin cycle

each molecule of 3-PGA gets phosphorylated into 1,3-biphosphoglycerate using ATP

# reduction why is NADPH oxidized in phase 2 of calvin cycle?

NADPH is oxidized to reduce 1,3-bisphosphoglycerate into G3P ## Footnote *remember that G3P is the intermediate in glycolysis therefore can be used for gluconeogenesis or carbon fixation (making sugar in plants)

# reduction how many G3P leave the calvin cycle in phase 2?

only 1 molecule of G3P leaves the calvin cycle and the other 5 molecules remain inside and are used in the next phase

what happens to the G3P molecule that leaves the calvin cycle at the end of the reduction phase?

1) 50% used as fuel for cellular respiration in plant cell mitochondria ( converted into sucrose and exported to other non-photosynthetic parts of the plant for this purpose 2) used to synthesize all major organic molecule in plants such as carbohydrates, lipids and proteins - a large quantity is used to synthesize cellulose 3) the excess is converted into glucose and used to make starch (stores in chloroplasts, roots, tubers, seeds and fruits)

# reduction in phase 2 of the calvin cycle, how much of CO2, ATP and NADPH is used and how much of G3P is made?

- used 3 CO2 - used 6 ATP - used 6 NADPH - made 6 G3P ( only net gain of 1 G3P)

# regeneration of the CO2 acceptor what is the main step of phase 3 of the calvin cycle?

last phase is to renegerate RuBP so that the cycle can start over.

# phase 3 what is the CO2 acceptor?

RuBP its the acceptor since each entering CO2 combines with a RuBP molecule to initiate the cycle

# regeneration of the CO2 acceptor in phase 3 of the calvin cycle, how much ATP is used?

3 ATP used in a series of catalyzed reactions to convert the remaining five G3P molecules into 3 molecules of RuBP so that the cycle can continue ## Footnote we get 3 RuBP from 5 G3P (3C)

in all three phases of the calvin cycle, how much ATP and NADPH is used?

1) carbon fixation ( phase 1)= 0 ATP and 0 NADPH 2) reduction (phase 2) = 6 ATP and 6 NADPH 3) regeneration of RuBP (phase 3) = 3 ATP and 0 NADPH | for 3 molecules of CO2 ## Footnote more ATP is required (9 ATP) than NADPH (6 NADPH) although in light reactions as much ATP is made as NADPH - this is why ATP will be depleted at a faster rate than NADPH

what happens at the very end of the calvin cycle? | what gets returned?

ADP and Pi are returned to the light reactions where they are converted back to ATP by chemiosmosis NADP+ is also returned to the thylakoid where it gets reduced as part of the light reactions

Lecture 12b Flashcards

(45 cards)