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Advanced Cell Bio > Chapter 14- Photosynthesis > Flashcards

Chapter 14- Photosynthesis Flashcards

1
Q

Photosynthesis

A

Uses light as a source of energy for growth- this is carried out by plants and phototrophic bacteria

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2
Q

Characteristics of phototrophic cells

A
  1. All cells: light energy drives phosphorylation of ADP to ATP (photophosphorylation)
  2. Some cells: light drives transfer of e- from H2O to NADP+ (forming NADPH). Reduction of NADP+ is concurrent with oxidation of H2O to O2
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3
Q

Oxygenic phototrophs

A

Produce O2 gas as a byproduct. This is because H2O is an electron donor for the reduction of NADP+ to NADPH. Observed in Cyanobacteria (widely distributed), Prochloron, Prochlorothrix, Prochlorococcus. Eukaryotic plant cells are oxygenic, which is why they are so good for the planet

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4
Q

Anoxygenic phototrophs

A

Do not produce O2 since oxygen gas will not be released as a byproduct. Compounds such as inorganic sulfur or H2 are e- donors. These cells are usually anaerobic. Observed in purple photosynthetic bacteria, green photosynthetic bacteria, Heliobacterium

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5
Q

2 types of phototrophic cells

A

Oxygenic and anoxygenic

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6
Q

Types of photosynthetic pigments (2)

A
  1. Reaction center pigments
  2. Light-harvesting pigments (antennae)
    These pigments are necessary for bacteria and chloroplasts to carry out photosynthesis
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7
Q

Reaction center pigments

A

Utilize light energy for the production of ATP & NADPH. These pigments include chlorophyll (primarily for chloroplasts) and bacteriochlorophyll (bacteria)

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8
Q

Light-harvesting pigments (antennae)

A

These pigments are important for initially taking in the sunlight. They absorb light of different wavelengths and funnel that light energy to the reaction center pigments. These pigments include carotenoids, phycobilins, chlorophylls

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9
Q

Chlorophyll structure

A

Contains a hydrocarbon tail, similar to fatty acid structure. Therefore, this tail region is hydrophobic, allowing chlorophyll to be inserted into membranes

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10
Q

Photosystem

A

Multi-protein/pigment complex that catalyzes conversion of light energy to cellular energy. Encompasses all of the machinery used to carry out photosynthesis. Consists of a light-harvesting complex and a reaction center

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11
Q

Light-harvesting complex (antenna)

A

This complex is called the antenna because it initially brings the sunlight into the reaction center. Consists of numerous protein complexes bound to several hundred chlorophylls. Includes carotenoids (accessory pigment), which protects chlorophylls from oxidation. Helps collect light of other wavelengths

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12
Q

Photochemical reaction center (RC)

A

Protein/pigment complex into which energy from the antenna feeds. As energy moves into the reaction center, it will excite an electron on a chlorophyll into a high energy state. That electron is transferred within the reaction center (from chlorophyll to chlorophyll) until it is passed to a special pair of chlorophylls- these chlorophylls are in close proximity to electron carriers like quinones. The special chlorophylls are an irreversible trap that transfers e- to a carrier and then to e- transport chain reactions then occur

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13
Q

Photosystem mechanism (7 steps)

A
  1. Light energy is funneled into the reaction center and excites a chlorophyll in the antenna
  2. The energy is passed from chlorophyll to chlorophyll within the light harvesting complex- the energy is transferred through resonance energy transfer, like a domino effect
  3. The energy reaches the reaction center and excites an electron on a chlorophyll to high energy.
  4. The high energy electron travels from chlorophyll to chlorophyll until it reaches the special pair of chlorophylls
  5. The special pair of chlorophylls transfer the electron to an acceptor, such as quinone
  6. The quinone transports the electron along the chain
  7. The lost chlorophyll electron in the reaction center is replaced by electrons from water or it is returned (cyclic)
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14
Q

“Special” pair of chlorophylls structure

A

Come together to act as a “trap” for the electron. They are in close proximity to the electron carrier quinone that will take the electron out of the reaction center

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15
Q

Purple photosynthetic bacteria

A

Gram negative bacteria, obtain their purple color from pigments. They have a light-harvesting complex that contains the pigments Bacteriochlorophyll and carotenoids. This complex is located within the membrane, in close proximity to the reaction center. A bacteriochlorophyll dimer in the reaction center
results in high energy electrons.
Electrons are transferred by quinones to the cytochrome bc1 complex. As the electrons are transferred, they provide energy for the bc1 complex to pump H+ so the proton gradient necessary for ATP synthase will be created. The electron is not replaced by water, it is returned back to bacteriochlorophyll through cyclic e- transport

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16
Q

Light harvesting complex of purple bacteria- mechanism (5)

A
  1. Some purple photosynthetic bacteria have 2 light harvesting complexes. They will be in close proximity to the reaction center regardless of if there are one or two.
  2. Harvests light energy and funnels it to the reaction center where the electron is excited and moves from chlorophyll to chlorophyll until it is picked up by quinones
  3. The electron is brought to cytochrome bc1, which will pump protons. The pump gains energy from the movement of the electron
  4. Protons will build up and be used by ATP synthase
  5. Cytochrome C picks up the electron and takes it back to the reaction center it was lost from
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17
Q

Cyclic electron transfer

A

When an electron is transported back to replace itself in the reaction center. The electron movement is so quick that the electron is not lost from the reaction center for very long

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18
Q

Intramembranous networks

A

Common in phototrophic cells- internal membranes are found around organelles. When there is more membrane, there is greater surface area, and more ATP can be produced

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19
Q

Green sulfur bacteria

A

Gram negative bacteria which contain a light-harvesting complex- also has Bacteriochlorophyll-protein-carotenoids pigments. There is much less protein than in purple bacteria. The complex is arranged into special structures called chlorosomes. Similar process to purple bacteria, although there are some differences

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20
Q

Differences between green sulfur and purple sulfur bacteria (2)

A
  1. Iron-sulfur molecules accept e- instead of quinones
  2. NADP+ is reduced (creates NADPH)
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21
Q

Chlorosome

A

The structure of the light harvesting complex in green sulfur bacteria- light is absorbed by this chlorosome

22
Q

Green sulfur bacteria photosynthesis (6 steps)

A
  1. Light absorbed by the chlorosome
  2. Light energy is transferred to the reaction center, which excites an electron in the bacteriochlorophyll
  3. e- accepted by chlorophyll derivative
  4. e- transferred by 3 iron-sulfur molecules (electron carriers) to ferredoxin NADP+ reductase
  5. The electron is accepted by NADP+, forming NADPH- this a non-cyclic movement of electrons
  6. The lost electron is replaced with water, making it oxygenic
23
Q

Structure of a chlorosome

A

The chlorosome is a light harvesting complex that is proximal to the membrane. Lots of protein-pigment complexes make up the chlorosome. It is surrounded by a galactolipid layer that contains some protein, which creates a covering around it. Bacteriochlorophyll C is in the chlorosome, and it absorbs light within the chlorosome. This light energy is transmitted to bacteriochlorophyll A in the baseplate. The baseplate connects the chlorosome to the reaction center, and it funnels the energy from the chlorosome to the reaction center- light energy goes to bacteriochlorophyll in the reaction center. It excites the electron on the bacteriochlorophyll

24
Q

Intramembranous systems in green sulfur bacteria

A

There are intramembranous systems located in the chlorosomes, which increases surface area and helps to increase photosynthesis

25
Q

Cyanobacteria

A

Gram negative bacteria with a blue-green (cyan) appearance. Previously known as blue-green algae, although they are not actually algae. They are one of the most widespread phototropic bacteria on the planet. Responsible for converting earth’s early reducing atmosphere to an oxidizing atmosphere. They essentially created the O2 atmosphere and were responsible for the emergence of aerobic life

26
Q

Thylakoids- cyanobacteria

A

Flattened, disc-like sacs found in the cytoplasm- they form concentric rings around the cytoplasm. Their purpose is to increase the available surface area for photosynthesis so there can be more photosynthetic bacteria. Extensive, intramembranous network (3rd membrane). Many thylakoids are interconnected- their lumen is called the thylakoid space

27
Q

Plastids

A

A family of specialized plant organelles- chloroplasts are the most prominent member

28
Q

Chloroplasts

A

Carry out photosynthesis in exactly the same way as cyanobacteria. They are organelles that are much larger than mitochondria. They have a double membrane- a permeable outer membrane and a less permeable inner membrane. There is also an intermembrane space between the 2 membranes. They do not have cristae like mitochondria do, but they have thylakoids that are present in the stroma

29
Q

Stroma

A

The lumen/interior of the chloroplast, analogous to the mitochondrial matrix

30
Q

Do plant cells have mitochondria?

A

Yes, they have mitochondria in addition to chloroplasts. Mitochondria are necessary in the absence of sunlight

31
Q

Thylakoids- chloroplasts

A

Thylakoid membranes are found within the chloroplasts. They are flattened, disc-like sacs in the stroma. The lumen connecting the different thylakoids is called the thylakoid space. They also exist to increase the surface area for photosynthetic machinery. Many thylakoids are interconnected and form stacks called grana (instead of concentric rings as in cyanobacteria)

32
Q

Photosynthesis of plant cells and cyanobacteria

A

Cyanobacteria and plant cells carry out photosynthesis in the exact same way. This process is a combination of the light reactions of purple photosynthetic bacteria and green sulfur photosynthetic bacteria, combined in series. There are 2 photosystems- photosystem 2 is activated first, then photosystem 1 (which is actually activated second).

33
Q

Photosystem 2

A

Activated first in cyanobacteria and plant cell photosynthesis, essentially contains the photosynthetic machinery of purple sulfur bacteria

34
Q

Photosystem 1

A

Activated second in cyanobacteria and plant cell photosynthesis, essentially contains the photosynthetic machinery of the green sulfur bacteria

35
Q

Differences in the photosynthesis of cyanobacteria and plant cells than the photosynthesis of other systems (3)

A
  1. Two serial light reactions
  2. e- flow is completely non-cyclic
  3. H2O replenishes the lost e- in the chlorophyll. Results in the release of O2, making the process oxygenic
36
Q

Phycobilisomes

A

The structure of the arrangement of photosystem 2 in cyanobacteria photosynthesis. This structure arranges the pigments (Phycobiliproteins) in a stacked manner. Phycoerythrin is the outermost pigment, which absorbs shorter wavelengths of light, phycocyanin is the middle pigment that absorbs slightly longer wavelengths of light, and allophycocyanin is the innermost pigment that absorbs the longest wavelengths of light and funnels the energy into the reaction center. These pigments in the light harvesting complex are meant to cover a range of wavelengths of light

37
Q

Photosystem 1 structure

A

Photosystem 1 has its own light harvesting complex that is not based on phycobilisomes. Contains chlorophyll a, carotenoids, & proteins. The light harvesting complex is embedded in the thylakoid membrane and is in close proximity to the reaction center.

38
Q

Process of photosynthesis in PS2 of cyanobacteria and plant cells (6)

A
  1. In photosystem 2, light is absorbed by the Phycobilisome
  2. The Phycobilisome funnels the light energy into the photosystem 2 reaction center, exciting an electron
  3. Electrons are passed chlorophyll to chlorophyll until they get to the special pair of chlorophylls. e- from special chlorophyll pair are passed to e- acceptor (quinone)
  4. Quinones transfer e- to cytochrome b6-f complex
  5. e- transferred to another carrier- plastocyanin (copper containing protein)
  6. Then, photosystem 1 reactions begin
39
Q

Replacement of lost electrons- photosystem 2

A

When light energy is transferred into the photosystem 2 reaction center, it excites an electron on a chlorophyll, and the electron is passed from chlorophyll to chlorophyll. Oxygens from 2 water molecules cluster manganese in the reaction center. As chlorophyll e- are excited & removed they are replaced w/ e- from H2O, and these oxygens will donate the electron that was lost from the chlorophyll. As soon as 4 electrons are removed from the water molecules, oxygen gas can be released. 4 photons of light are a necessary reactant for the reaction

40
Q

Cytochrome b6-f complex

A

This complex is necessary for cyanobacteria and plant cell photosynthesis. Once the electron is transferred from quinone to this complex, the complex can use its energy to pump protons into the extracellular environment. This proton gradient will be used by ATP synthase. Analogous to cytochrome BC1 in the ETC.

41
Q

Process of photosynthesis in PS1 of cyanobacteria and plant cells (5)

A
  1. Light is absorbed by the PS1 light harvesting complex
  2. Light energy is transferred to the PS1 reaction center
  3. Creates a high energy electron that is lost from a chlorophyll- replaced by plastocyanin electron
  4. The high energy electron is transferred to ferredoxin (iron-sulfur center)
  5. Ferredoxin transfers the electron to ferredoxin NADP+ reductase creating NADPH
42
Q

What replaces the lost electron in PS1?

A

The electron that was moving along from PS2, carried by plastocyanin

43
Q

2 groups of photosynthesis reactions

A
  1. Photosynthetic e–transfer reactions (light reactions)
  2. Light-independent reactions (dark reactions)
44
Q

Photosynthetic electron transfer reactions (light reactions)

A

Includes all of the reactions discussed so far in the lecture

45
Q

Light-independent reactions

A

Also called the dark reactions, although that isn’t really an accurate name because they don’t occur exclusively in the dark. These reactions require light indirectly- they need the ATP and NADPH from the light reactions. They are used to create stored energy and carbon sources for the cell

46
Q

Carbon-fixation reactions

A

A type of “dark reaction” where ATP and NADPH provide the energy and reducing power used to convert carbon dioxide gas into carbohydrates. Carbohydrates are used as a source of organic molecules and energy for growth. The Calvin cycle is the reaction that occurs

47
Q

Calvin cycle

A

The reaction that occurs for carbon fixation. Requires 3 ATP and 2 NADPH for each carbon dioxide molecule.

48
Q

First reaction of the Calvin cycle (4 steps)

A
  1. Carbon dioxide combines with the 5 carbon ribulose 1,5-bisphosphate and water
  2. Ribulose 1,5 bisphosphate is hydrolyzed when the carbon is added from carbon dioxide
  3. It form a temporary structure that splits and yields 2 molecules of a 3 carbon compound called 3-phosphoglycerate
  4. Normally this would be an unfavorable reaction, but coupling to hydrolysis makes this energetically favorable
49
Q

Calvin cycle

A
  1. 3-phosphoglycerate is formed through ribulose 1,5 bisphosphate hydrolysis
  2. ATP provides energy for the cycle, and NADPH provides reducing power to make 1,3 bisphosphoglycerate and obtain the end product
  3. 3 molecules of carbon dioxide need to be fixed to yield 1 molecule of glyceraldehyde 3 phosphate (the end product)
50
Q

Fates of glyceraldehyde 3-phosphate (3)

A
  1. Converted to glucose-1-phosphate through reverse glycolysis
  2. Converted to ADP-glucose- a sugar nucleotide and an immediate precursor to starch
  3. Starch is formed- serves as a reserve of carbohydrates
51
Q

Why do photosynthetic organisms need to have a reserve of carbohydrates?

A

Starch needs to be broken down in the absence of light to support metabolic needs. Therefore, starch is produced and stored during periods of excess photosynthetic capacity

52
Q
A

Advanced Cell Bio (17 decks)

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