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Intro to Biochemistry > 28 - Photosynthesis: Light Reactions and Dark Reactions > Flashcards

28 - Photosynthesis: Light Reactions and Dark Reactions Flashcards

1
Q

What is the chemical equation for photosynthesis? (just list what goes on either side of the equation with correct stoichiometry)

A

In

  • Energy
  • 6 CO2
  • 6 H2O

Out

  • 1 C6H12O6 (glucose)
  • 6 O2
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2
Q

What 4 process do both photosynthesis and respiration rely on?

A
  1. Both rely on the transfer of electrons
  2. Both generate a proton-motive force that drives the synthesis of ATP
  3. The ATP synthase and other components in plants and animals share features and overall organization
  4. Both processes are carried out in a spatially defined manner within double membraned-organelles
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3
Q

Where does photosynthesis occur?

A

In the chloroplasts

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4
Q

What are the 7 components of chloroplasts? (just able to recognize)

A
  • Outer membrane
  • stromal lamella
  • Inner membrane
  • Intermembrane compartment
  • Granum
  • Stroma
  • Thylakoid compartment
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5
Q

True or false, mitochondria have reactions that involve light and reactions that are light independent?

A

True. Light and dark reactions occur in the chloroplast, but they are distinct.

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6
Q

Where do light reactions occur in chloroplasts?

A

Within the thylakoid membrane

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7
Q

Where do light independent (dark) reactions occur in chloroplasts?

A

In the stroma

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8
Q

What are light reactions of chloroplasts?

A

Light energy driving the synthesis of NADPH and ATP, H2O is oxidized to O2

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9
Q

What are the light-independent reactions of chloroplasts (dark)?

A

The NADPH and ATP (made from light reactions) are used to make sugar from CO2

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10
Q

What are chlorophylls?

A

Pigments of light reaction.
The overall structure of chlorophyll is similar to the heme groups of hemoglobin and cytochromes, but with a cyclopentanone ring fused to pyrrole ring III and a Mg ion where Fe ions ar found in hemes and cytochromes

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11
Q

What is chlrorophyll a?

A

Chlorophyll a absorbs light energy and it is light harvesting

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12
Q

What is chlorophyll b?

A

It is only a light harvesting pigment

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13
Q

What are 2 light absorbing pigments that aren’t chlorophyll?

A
  • Carotenoids (eg. lutein and B-carotene harvest light)

- Pheophytin has a different role we’ll see later

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14
Q

Why are plants green? (it’s not just because they have chlorophyll, go deeper)

A

Chlorophyll a and b absorb red and blue light, which is why plants are predominantly green, it is the colour that they are not absorbing

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15
Q

Once chlorophylls gain energy, what do they do with it?

A

They transfer energy from photons to neighbouring chlorophylls until the energy is released or captured by a reaction centre (P700 chlorophyll a is a very strong reducing agent that acts as a reaction centre)

This means that light capturing molecules are light harvestors, they have no direct role in photosynthesis

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16
Q

What is photosystem II?

A

It is a key centre carrying out a part of the light reaction in photosynthesis. PSII is located in the stacked grana membrane region of chloroplasts, which has little or no contact with the stroma. PSII contains many protein subunits and it contains a key chlorophyll dimer: P680

17
Q

What does P680 do in photosynthesis?

A

P680 within photosystem II is a very strong oxidizing agent. It can capture electrons from water, resulting in oxygen as a product. The electrons are transferred to plastoquinone, which functions in the same way as ubiquinone in animals.

At the same time, cytochrome bf transfers protons to the thylakoid lumen

P680 absorbs light at 680 nm

18
Q

What transfers eccited electrons in photosynthesis to electron acceptors and eventually plastoquinone (PQ)?

A

P680 of Photosystem II

19
Q

What happens when energy is transferred to P680 in PSII (photosystem II)?

A

The newly energized electron is ejected and donated to pheophytin and then on to plastiquinone

20
Q

What is photosystem I in chloroplasts?

A

Photosystem I (PSI) is the other two key centres carrying out the light reactions. PSI is located in the individual (unstacked) stroma lamella, where stromal NADPH is available.

It contains the P700 chlorophyll, a reaction centre that generates high energy electrons in response to light energy. These electrons travel to ferredoxin, which is reduced as a result.

The flavoprotein ferredoxin-NADP+ reductase collects two electrons to become fully reduced and then in turn reduces NADP+ to NADPH in turn

21
Q

What does photosystem I do?

A

PSI energizes and transfers electrons that are donated to NADP+.

It is a large membrane spanning multisubunit complex with two special chlorophyll a molecules in the reaction centre inside the PsaAB dimer

22
Q

What happens when electrons are transferred within PSI (photosystem I)?

A

They are taken from plastocyanin and delivered to ferredoxin

23
Q

How is NADPH produced in photosystem I electron transport?

A

Ferredoxin donates its electron to ferredoxin-NADP oxidoreductase

To produce NADPH, two electrons are needed

24
Q

Is the formation of NADPH in photosystem I a light or dark reaction?

A

A light reaction

25
Q

NADP+ accepts what to become NADPH?

A

A proton and two electrons, in photosystem I, electrons are from ferredoxin and ferredoxin-NADP oxidoreductase transfers them to NADP+ to form NADPH

26
Q

What is the final electron acceptor in electron transport from light reactions moving along PSI and PSII?

A

NADP+

27
Q

What happens in cyclic electron transport in chloroplasts?

A

Reduced ferredoxin donates its electrons to plastoquinone, which then passes them to cytochrome bf complex, plastocyanin and eventually P700.

No NADPH is produced, but it provides the transfer of additional protons across the thylakoid membrane, leading to additional ATP synthesis

28
Q

True or false, a proton gradient is made in chloroplasts which drives ATP synthase?

A

True. The chloroplast has a CF1CF0 complex, which is an ATP synthase.

Like the mitochondrial ATP synthase it is a multisubunit enzyme that catalyzes the phosphorylation of ADP to ATP. and this reaction is drived by the movement of protons down their concentration gradient across a membrane (chemiosmosis)

29
Q

The F1 portion of chloroplast ATP synthase extends into what part of the chloroplast?

A

The ATP synthase is embedded in the stroma membrane and the F1 subunit extends into the stroma

30
Q

Where do products of photosynthesis go?

A

O2 (to the atmosphere)
NADPH (to dark reaction)
ATP (to dark reaction)

31
Q

What is the Calvin cycle? Where does it take place.

A

The light-independent (dark) reaction inside chloroplasts where CO2 is incorporated into a carbohydrate. This takes place in the stroma as well.

Even though it is called a dark reaction, these usually occur in the day when ATP and NADPH is being made.

Overall it’s using ATP and NADPH to generate hexose

32
Q

What is the rate limiting step of the calvin cycle?

A

CO2 reacting with a C5 sugar (ribulose-1, 5BP) to form two phosphoglycerates (PG)

33
Q

What are the steps of the calvin cycle?

A
  1. CO2 reacts with a C5 sugar (ribulose-1, 5BP) to form two 3-phosphoglycerates (rate limiting step)
  2. 3-phosphoglycerate reacts to glyceraldehyde-3-phosphate (GAP), this requires 2 ATP and 2 NADPH per two molecuels of 3-PG
  3. 2 GAPs can be used to make 1 hexose

Overall: ATP and NADPH are used to generate hexose

34
Q

What enzyme catalyzes the conversion of ribulose-1,5-bisphosphate to two molecules of glycerate-3-phosphate in the Calvin cycle?

A

Ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCo)

Rubisco is likely the most abundant enzyme on earth

35
Q

What happens to CO2 in the calvin cycle?

A

It is fixed into organic molecules (GAP), for every three molecules of CO2 that are fixed there is a net gain of one glyceraldehyde-3-phosphate ((GAP)

36
Q

How many NADPH and ATP are needed for every glucose molecule produced?

A

12 NADPH

18 ATP

Intro to Biochemistry (28 decks)

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