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BMB 402 > Chapter 12: Photosynthesis > Flashcards

Chapter 12: Photosynthesis Flashcards

1
Q

What is photosynthesis?

A

production of energy from the sun
- recycles CO2 and O2

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2
Q

What is photooxidation?

A

transfer of e- from chlorophyll to an e- acceptor

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3
Q

Photosynthetic electron transport system

A

-light reactions
- depends on carbohydrates for metabolic fuel for mitochondrial respiration
Consist of:
-PS1
-PS2
-cytochrome b6f

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4
Q

Calvin Cycle

A

-dark reactions
- most active when ATP and NADPH are high
- produces triose phosphates used to make hexose sugars
- rubisco catalyzes the first step of carbon fixation

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5
Q

What does the photosynthetic electron transport system do?

A
  • converts solar energy into chemical energy and carbohydrate sugar precursors
  • occurs in 5 steps
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6
Q

Steps of the photosynthetic electron transport system

A

Step 1
- 4 photons are absorbed by a chlorophyll molecule, water is oxidized to O2, 4H+ are released and contribute to PMF
Step 2
- e- transport via carrier molecules, 8H+ are translocated for every 4e- donated
Step 3
- photon absorption by PSI, NADPH is produced
Step 4
- chloroplast ATP synthase produces ATP, 12H+ are used to produce 3 ATP, however only 8H+ are translocated
Step 5
- ATP and NADPH are used in enzymes in the Calvin cycle to drive carbon fixation
- 3 CO2 are converted to 1 glyceraldehyde-3-phosphate

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7
Q

What is a chloroplast?

A

photosynthetic machinery in eukaryotic cells
- contain their own DNA
- carry out protein synthesis
have 3 membranes:
- outer
-inner
-thylakoid

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8
Q

What wavelengths of light do chloroplasts absorb?

A

they absorb maximum amounts of light at 400 and 700 nm

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9
Q

Describe the interior structure of chloroplasts (not including membranes)

A

Chloroplasts contain light-absorbing pigments and alternating double bonds

They are associated with chromophore proteins that participate in energy transfer reactions

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10
Q

What are the mobile electron carriers of photosynthesis?

A

pheophytin
plastoquinone
plastocyanin

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11
Q

Describe PS2

A

This is the P680 reaction center that absorbs light energy at 680 nm and oxidizes water to oxygen

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12
Q

What can inhibit PS2?

A

3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)

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13
Q

Describe PS1

A

This is the P700 reaction center that absorbs light energy at 700 nm. It is a large protein complex embedded in the chloroplast membrane and it generates NADPH for carbohydrate synthesis.

Fe-S cluster in the final electron acceptor to ferroredoxin in this complex.

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14
Q

What is paraquat?

A

A potent herbicide that accepts electrons from PS1 and donates them to oxygen, blocking NADPH production.

This process generates superoxide anion (O2-) and hydrogen peroxide (H202), reactive oxygen species that can harm the plant.

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15
Q

What are light harvesting complexes (LHCs)?

A

“Solar panels” that capture light energy for photooxidation in reaction center complexes. They are proteins containing chormophores that participate in energy transfer reactions.

LHCs are most abundant in the thylakoid membrane and there are two types:
- LHC1
- LHC2 (outnumbers LHC1, major light gathering antenna in photosynthetic membrane)

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16
Q

Describe the Z scheme

A

This is a series of photosystems that require energy input from light absorption at PS2 and PS1.

Photon absorption by PS2 results in electron flow from water to plastocyanin, translocation H+ (creates gradient)

Photon absorption by PS1 provides energy to reduce NADP+ to NADPH (generates reducing power)

Plastocyanin transports one electron from cytochrome b6f to the PS1 reaction center

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17
Q

What is the order of electron carriers in the Z scheme?

A
  1. Pheophytin (pheo)
  2. Plastoquinone (PQ)
  3. Cytochrome b6f
  4. Plastocyanin (PC)
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18
Q

What must be available to replace the electron lost by photooxidation in each reaction center complex?

A

a reductant

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19
Q

What is anoxogenic photosynthesis?

A

A photosynthetic process used by some bacteria under anaerobic conditions that sees hydrogen sulfide (H2S) used instead of water. Therefore, oxygen is not produced.

20
Q

What is the PQ Cycle?

A

A process analogous to the Q cycle in the mitochondria. 8 H+ are translocated in the thylakoid lumen.

21
Q

What are the components of chloroplast ATP synthase?

A
  • CF nought (membrane bound F nought complex)
  • CF1 (catalytic F1 complex)

Has the same binding change mechanism as mitochondrial ATP synthase and still requires a proton gradient

22
Q

What occurs in cyclic photophosphorylation?

A

PS1 transfers its electrons to the PQ cycle, which passes them to cytochrome 6fb, causing protons to be transported to the lumen. These protons flow through ATP synthase and generate ATP. Since PS2 is bypassed, no NADPH or O2 is produced.

23
Q

How can the rate of photooxidation be regulated?

A
  • level of light harvesting needs to be regulated, can exceed the capacity of photooxidation to capture electrons. When this happens ROS is produced and photo-induced damage can occur
  • light harvesting can be regulated by controlling physical distribution of LHC2 in the thylakoid membrane
24
Q

Where in the thylakoid does the H+ gradient form and where does it flow to?

A

The gradient forms in the thylakoid space (interior) and flows through ATP synthase into the stroma.

25
Q

What is the net reaction of the Calvin Cycle?

A

3CO2 + 9ATP + 6NADPH + 6H+ —> C3H6O3-phosphate + 9ADP + 8Pi + 6NADP+ + 3H20

26
Q

What controls the Calvin Cylce?

A

light

27
Q

What does the Calvin Cycle generate, and what can these products be used for?

A

Generates:
- 3-phosphoglycerate
- glyceraldehyde-3-phosphate (G3P)
- dihydroxyacetone phosphate

These metabolites can be used to make
- fructose-1,6-BP
- fructose-6-P

28
Q

What are the three stages of the Calvin Cycle?

A
  1. Fixation: Synthesis of a C6 molecule from Rubisco, which is cleaved into two molecules of 3-phosphoglycerate
  2. Reduction: 3-phosphoglycerate is reduced to G3P (ATP and NADPH are reactants)
  3. Regeneration: Ribulose-5-phosphate is resynthesized in “carbon shuffle” reactions
29
Q

How is light involved in the regulation of the Calvin Cycle?

A

Since photophosphorylation and NADPH production by the photosynthetic electron transport system is shut down in the dark, it is crucial that the Calvin Cycle be active only in the light.

30
Q

Besides light, what else regulates the Calvin Cycle?

A

pH and Mg2+ regulation can activate certain Calvin Cycle enzymes

31
Q

How is thioredoxin reduction involved with Calvin Cycle enzymes?

A

In PS1, photon absorption leads to reduced ferroredoxin, which then donates e- to ferredoxin-thioredoxin reductase. Ferrodoxin-thioredoxin reducatase then reduces thioredoxin, which reduces and activates Calvin Cycle enzymes.

Essentially, reduced thioredoxin keeps the Calvin Cycle enzymes in the active state.

32
Q

What is the Glycolate Pathway?

A

This is a pathway that occurs in plants and sees 2-phosphoglycerate converted to glycolate in the stroma.
- Glycolate is exported to plant cell peroxisomes where it is oxidized to glyoxylate.
- Transamination of glyoxlate in peroxisomes produces glycine, which is transported to the mitochondria
- CO2 is produced and O2 is consumed

This pathway is also called the C2 pathway

33
Q

What are the Glycolate Pathway reactions known as?

A

Photorespiration - CO2 is produced and O2 is consumed

34
Q

What is photorespiration?

A

A light dependent process where oxygen is consumed and CO2 is produced. It is a multistep process initiated by ribulose bisphosphate carboxylase, which produces glycolate-2-phosphate and then glycerate-3-phosphate (not the same as G3P)

Photorespiration reactions compete with carboxylation, reducing the efficiency of photosynthesis.

35
Q

What conditions increase photorespiration?

A

Higher temperatures and increased oxygen concentration sees increased photorespiration, which reduces plant growth.

Plants that grow under these conditions have evolved ways to minimize photorespiration.

36
Q

Describe the C4 pathway in plants.

A
  • found in tropical plants
  • one cell type is required for CO2 uptake, and another for rubisco-mediated carboxylation
  • captures CO2 in the form of oxaloacetate in mesophyll cells and delivers it to bundle sheath cells as malate, another C4 intermediate
  • bundle sheath cells have lower O2 levels and PS2 is absent, so O2 is not generated there and photorespiration is minimized
  • carbon fixation in mesophyll cells, Calvin Cycle in bundle sheath cells
37
Q

Describe the CAM pathway in plants

A
  • Crassulacean Acid Metabolism
  • in desert succulent plants
  • Captures CO2 at night (open stomata to reduce water loss during the day) in the form of malate
  • Stomata are closed during the day, keeping O2 levels low, and malate provides CO2 for carbon fixation for the Calvin Cycle
38
Q

Which structure in the chloroplast is essential to energy production?

A

thylakoid membrane

39
Q

The change of pH in the stroma (pH = 8) and the thylakoid membrane (pH = 5) result in a ….?

A

proton gradient and ATP synthesis

40
Q

The leaves of the Amazon water lily are up to 6 feet in diameter. How would this affect light absorption?

A

Light absorption would be increased, but it would also depend on how many leaves are present

41
Q

The Z scheme of photosynthetic electron transport is most similar to which process?

A

electron transport system

42
Q

Cacti are found in dry environments such as a desert. They would use _____ pathway to fix CO2.

A

CAM

43
Q

What does the Glyoxylate Pathway do?

A
  • converts lipids into carbohydrates
  • is most active in seedlings
  • provides a mechanism for fats stored in seeds to be converted to sucrose
  • metabolic pathways that converts acetyl CoA molecules into succinate, which serves as the carbon source for glucose biosynthesis
44
Q

What happens after the Glyoxylate Cycle occurs?

A
  • succinate is converted into fumerate in the mitochondria by the Citrate Cycle
  • fumerate is hydrated to form malate
  • malate is converted to oxaloacetate, or exported to the cytosol, and coverted to oxaloacetate by cystolic malate dehydrogenase
  • cystolic OAA is converted to phosphoenolpyruvate, which leads to the formation of glucose via gluconeogenesis
45
Q

What happens to the lipid stores in seeds over time?

A

They decrease

BMB 402 (14 decks)

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