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Biology 2 > Lecture 12 photosynthesis > Flashcards

Lecture 12 photosynthesis Flashcards

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1
Q

Define photosynthesis

A

A metabolic process by which energy of sunlight is captures and used to convert CO2 and H2O into carbohydrate sugars and oxygen gas

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2
Q

What three aspects of photosynthesis were established by early in the 19th century?

A
  • Water for photosynthesis comes primarily from soil via the roots
  • Gaseous exchange occurs through the stomata
  • Light is necessary for oxygen and glucose production
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3
Q

Is photosynthesis the reverse of cellular respiration?

A

NO

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4
Q

What method was used to determine the source of oxygen produced in photosynthesis determined experimentally?

A

Radioisotope 18-O

1 group given water with oxygen isotope and CO2 with regular oxygen and second group reverse

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5
Q

What happened after radioisotopes were supplied to the two different groups?

A

Oxygen gas produced was taken and analyzed

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6
Q

What was the results and conclusion of the radioisotope experiment to determine the source of oxygen produced in photosynthesis?

A

18-O oxygen gas was produced by plants given the 18-O labelled water only
Therefore water is the source of O2 produced by photosynthesis

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7
Q

What is the revised, balanced photosynthesis equation?

A

6CO2 + 12H2O –> C6H12O6 + 6O2 + 6H20

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8
Q

What are the two pathways of photosynthesis?

A

Light reactions, light independent reactions

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9
Q

What are the light reactions?

A

Driven by light energy

Converts light energy into chemical energy in the form ATP and NADPH + H+

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10
Q

What are the light independent reactions?

A

Do not use light directly

Use ATP, NADPH +H+ and CO2 to produce sugars

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11
Q

What are the three forms of the light independent reaction?

A

Calvin cycle
C4
Crassulacean acid metabolism

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12
Q

Where do the light and light independent reactions occur?

A

Within the chloroplasts

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13
Q

How are the light and light independent reactions linked?

A

Exchange of ATP, ADP, NAD, NADP

Rate of each reaction depends on the rate of the other

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14
Q

How does light (electromagnetic radiation) act?

A

As both a particle and a wave- discrete packages called photons

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15
Q

What does absorption of a photon do to a pigment molecule?

A

Excites it

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16
Q

What else can happen when a photon meets a pigment?

A
  • Bounce off (scattered/reflected)

- Pass through (transmitted)

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17
Q

What happens when a photon is absorbed according to the first law of thermodynamics?

A

The molecule acquires the energy of that photon, raising it from ground state to excited state

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18
Q

Why is there some difference between the energy of a photon and the subsequent energy of the pigment molecule?

A

Lost as entropy

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19
Q

What does the difference in free energy after absorbing a photon do to a pigment molecule in an excited state?

A

The energy boosts one of the electrons in the molecule to the furthest shell from the nucleus
The molecule is more chemically reactive.

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20
Q

What are pigments?

A

Molecules that absorb wavelengths in the visible spectrum

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21
Q

How do absorbed wavelengths correlate with biological activity?

A

Specific wavelengths are absorbed by each type of molecule

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22
Q

What can be plotted based on the activity of an organism at each wavelength of the spectrum?

A

An action spectrum

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23
Q

What are the two predominant types of chlorophyll used in photosynthesis?

A

Chlorophyll a and b

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24
Q

How do chlorophyll a and b differ?

A

Molecular structure

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25
Q

What is the structure of chlorophyll and and b?

A

Complex ring structure with magnesium atom in the centre and a hydrocarbon tail (which anchors)

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26
Q

Where is chlorophyll attached?

A

The thylakoid membrane of the chloroplast

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27
Q

How is chlorophyll attached to the thylakoid membrane?

A

Hydrocarbon tail is attached to a peripheral location anchoring the molecule to integral proteins in the thylakoid

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28
Q

What wavelengths do chlorophyll absorb?

A

Red and blue wavelengths

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29
Q

What are accessory pigments?

A

Pigments that absorb intermediate wavelengths and transfer energy to chlorophyll

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30
Q

Name two accessory pigments.

A

Carotenoids,

Phycobilins

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31
Q

What wavelengths do carotenoids absorb? How do they appear?

A

Blue and blue-green wavelengths (appear deep yellow)

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32
Q

Give an example of a carotenoid.

A

Beta carotene

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33
Q

Where are phycobilins found?

A

Red algae and cyanobacteria

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34
Q

What wavelength do phycobilins absorb?

A

Absorb yellow, yellow-green, orange wavelengths

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35
Q

When does light absorption not result in photochemical change?

A

Molecule returns to ground state, some energy is given off as heat or light energy (fluorescence)

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36
Q

What happens when light is given off as heat or light?

A

No chemical work is done- no chemical changes/biological functions

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37
Q

What happens when light is not given off as heat or light energy?

A

The molecule passes its energy to another molecule if nearby, in the right orientation and with the appropriate structure

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38
Q

How are pigments in photosynthetic organisms arranged?

A

Antenna systems

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39
Q

What are antenna systems?

A

Pigments are packed such that the excitation from an abosorbed photon can be passed from one pigment molecule to another

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40
Q

Energy from an absorbed photon is passed from pigments that absorb ______ energy, ______ wavelengths to pigments that absorb _____ energy, ______ wavelengths.

A

Higher energy, shorter wavelengths to lower energy, longer wavelengths.

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41
Q

What is the name of the molecule that absorbs the longest wavelength and where the excitation ends up?

A

The reaction carrier

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42
Q

What does the reaction carrier do?

A

Converts the energy into chemical energy

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43
Q

How does the reaction carrier convert the energy into chemical energy?

A

Becoming chemically oxidized- positively charged

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44
Q

In plants, what is the reaction carrier always?

A

Chlorophyll a

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45
Q

How does excited chlorophyll act in the reaction center?

A

As a reducing agent

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46
Q

What are the two main roles of chlorophll a in photosynthesis?

A

To absorb light energy

To transform it into chemical energy (electrons) and transfer those electrons

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47
Q

How does chlorophyll act as a reducing agent?

A

By donating an electron to a stable electron acceptor in a redox reaction

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48
Q

How does reduction lead to electron transport?

A

The oxidizing agent that was reduced by excited chlorophyll is the first in the chain of electron transport carriers

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49
Q

Where are the electron transport carriers for photosynthesis?

A

The thylakoid membrane

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50
Q

What is electron transport?

A

A series of oxidation-reduction reactions

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51
Q

What is the final electron acceptor in the electron transport chain in the thylakoid membrane?

A

NADP+

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52
Q

What happens when NADP+ accepts an electron?

A

It is reduced to NADPH + H+ (a reduced coenzyme)

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53
Q

What does NADP+ stand for?

A

Nicotinamide adenine dinucleotide phosphate

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54
Q

How is the structure of NADP+ different to NAD+ seen in respiration?

A

The addition of a phosphate group attached to the ribose

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55
Q

How is the function of NADP+ different to NAD+?

A

NAD+ is used in catabolism

NADPH+ is used anabolic reactions

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56
Q

What are the two different systems for electron transport in photosynthesis?

A

Nonclyclic electron transport

Cyclic electron transport

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57
Q

What does noncyclic electron transport produce?

A

NADPH +H+ and ATP

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58
Q

What does cyclic electron transport produce?

A

Only ATP

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59
Q

What is a photosystem?

A

A complex of pigments which capture light energy to energise reactions

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60
Q

What is the first step in the electron transport chain of photosytem II?

A

The P680 chlorophyll molecule in the reaction centre of photosystem II absorbs light and becomes Chl*

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61
Q

What does non-cyclic electrons transport require?

A

Two different photosystems

62
Q

What does photosystem II do?

A

Use light energy to oxidize water molecules, producing electrons, protons and water

63
Q

What does photosystem I do?

A

Uses light energy to reduce NADP+ to NADPH +H+

64
Q

Why is the chlorophll a molecule in the reaction centre for photosystem II called P680?

A

Because it absorbs light maximally at 680nm

65
Q

What is the name of the chlorophll molecule in the reaction centre of photosystem I?

A

P700

66
Q

Which photosystem requires light that is slightly more energetic?

A

Photosystem II (absorbs shorter wavelengths)

67
Q

What is the Z scheme model?

A

The interaction between photosystem II and I due to the path of the electrons when placed along an axis of rising energy level

68
Q

What is required to keep noncyclic electron transport going?

A

Both photosystems must be constantly absorbing light to boost electrons to higher energy shells

69
Q

What happens in photosystem II when chlorophyll lacks an electron?

A

It takes electrons from water, resulting in O2, H+ and e-

70
Q

Where do electrons from P680 pass to?

A

The primary electron acceptor- the first carrier in the electron train

71
Q

What happens to P680 when it passes its electrons to the primary electron acceptor?

A

It becomes P680+

72
Q

How does P680+ become reudced?

A

Electrons from oxidation of water reduce it

73
Q

What happens to electrons from Photosystem II?

A

They pass through a series of exergonic reactions in the electron transport chain

74
Q

What are the exergonic reactions in the electron transport chain coupled do?

A

Proton pumping across the thylakoid membrane

75
Q

What is the name of the proton pumping across thylakoid membranes?

A

Chemiosmotic pumping

76
Q

What does chemiosmotic pumping create?

A

A proton gradient that produces energy for ATP synthesis

77
Q

What is the first step in photosystem I?

A

The reaction centre containing P700 becomes excited (P700*)

78
Q

What does the P700* reduce?

A

Ferredoxin (an oxidizing agent)

79
Q

How does P700+ return to its ground state?

A

By accepting electrons passed through the electron transport chain from photosystem II.

80
Q

What happens to the electrons from ferrodoxin?

A

They reduce NADP+ to NADPH +H+

81
Q

What organisms use cyclic electron transport?

A

Organisms where the ratio of NADPH +H+ to NADP+ is high

82
Q

Why is cyclic electron transport cyclic?

A

Electron passed from an excited chlorophyll molecule at the outset cycles back to the same chlorophll molecule at the end

83
Q

What is the first step in cyclic electron transport?

A

P700 absorbs a photon and becomes P700*

84
Q

What is the second step in cyclic electron transport, after P700* is formed?

A

P700* reduces oxidised ferrodoxin in an exergonic reaction

85
Q

What happens after ferrodoxin is formed?

A

It passes its electron to another oxidising agent, plastoquinone (PQ)

86
Q

What happens when plastoquinone is being reduced?

A

It pumps 2H+ back across the thlakoid membrane

87
Q

How does reduced plastoquinone pass its electrons to the electron transport chain?

A

By way of plastocyanin (PC)

88
Q

What is the final step in the cyclic electron transport chain?

A

the electron passes back to P700+

89
Q

The cyclic electron transport chain is a series of _____ reactions, each ______.

A

redox

exergonic

90
Q

How is released energy from the cyclic electron transport chain stored?

A

In a proton gradient used to form ATP

91
Q

Define photophosphorylation.

A

The light driven production of ATP from ADP + Pi in the chloroplast

92
Q

Protons move from the ______ to the ______ of the thylakoid to create a proton gradient.

A

Stroma (interior matrix of chloroplast)

to the lumen of the thylakoid

93
Q

How do protons move back into the stroma?

A

They diffuse out of the thylakoid through protein channels called ATP synthase

94
Q

How is ATP synthase in animals and plants?

A

Orientation (in animals, protons diffuse back into mitochondrial matrix)

95
Q

Where are most enzymes that catalyze CO2 fixation found?

A

Dissolved in the stroma of the chloroplasts

96
Q

Why does the light independent reaction/CO2 fixation only occur in the presence of light?

A

It depends on ATP and NADPH+ produced in light dependent reactions which are not stockpiled

97
Q

How were the steps of the calvin cycle revealed experimentally?

A

Radioisotope labelling

98
Q

What radioisotope was used to discover the steps of the calvin cycle?

A

C14 labelled CO2

99
Q

What are the properties of carbon 14?

A

Emits radiation but is behaves the same chemically (not distinguished by enzymes)

100
Q

What did Calvin and his colleagues perform their experiments on?

A

Unicellular green algae, Chlorella

101
Q

What was the first step Calvin and his colleagues did to reveal the stages of the Calvin cycle?

A

Expose Chlorella to 14CO2 for 30 seconds

102
Q

What happened after Chlorella was exposed to carbon-14 CO2 for 30 seconds?

A

The cells were killed and their organic compounds extracted

103
Q

How were different components of Chlorella extracted?

A

Paper chromatography

104
Q

What is the method of paper chromatography that was used to separate the Chlorella extract?

A

Dissolve extract in alcohol, applied to a sheet of filter paper, hydrogen bonds formed with cellulose, paper put into phenol-water (solvent) and crept up by capillary action

105
Q

How were the results of paper chromatography of Chlorella extract observed?

A

Exposing X-ray film to the filter paper to reveal positions of the radioactive compounds

106
Q

What was wrong with the results of the Chlorella extract after the 30 second exposure time?

A

Many compounds, including monosaccharides and amino acids contained the radioisotope

107
Q

How long was Chlorella exposed to the carbon-14 CO2 in the second experiment?

A

3 seconds

108
Q

What was the result of the second experiment?

A

Only one compound was labelled

109
Q

Which compound was labelled in the second experiment?

A

3-phosphoglycerate

110
Q

How did Calvin and his colleagues discover the series of compounds that make up the carbon cycle that fixes the CO2?

A

Tracing steps with successive, increasingly long exposures

111
Q

Why was the first observed compound 3-phosphoglycerate?

A

The initial step adds CO2 to ribulose 1,5-bisphosphate to make intermediate 6 carbon compound which is quickly broken down into three carbon compound

112
Q

What enzyme catalyzes the fixation of CO2 and ribulose-1,5-bisphosphate?

A

Ribulose bisphosphate carboxylase/oxygenase (rubisco)

113
Q

Fun fact about rubisco

A

Most abundant protein in the world

50% of all the protein in every plant leaf

114
Q

What three processes make up the Calvin cycle?

A

Fixation of CO2
Reduction of 3PG
Regeneration of CO2 acceptor RuBP

115
Q

Where does the calvin cycle take place?

A

In the stroma of the chloroplasts

116
Q

How much G3P goes where?

A

5/6 recycled into RuBP, remaining 1/6 has two fates

117
Q

What are the two fates of the remaining 1/6 G3P?

A

1/3 becomes starch and stored in chloroplast

2/3 converted in cytosol into sucrose and transported elsewhere

118
Q

What is sucrose made of?

A

Fructose and glucose

119
Q

What happens after RuBP and CO2 have combined and formed two molecules of 3PG?

A

3PG is reduced to G3P in a two step reaction involving ATP and NADPH+ +H+

120
Q

What does G3P stand for?

A

Glyceraldehyde 3-phosphate

121
Q

Other than using ATP and NADPH made through photophosphorylation, how else is the light reaction and CO2 fixation pathway connected?

A
  • Light induced pH changes in the stroma

- Light induced electron flow

122
Q

What does light induced pH changes in the stroma do?

A

Activates some enzymes in the calvin cycle

123
Q

How is the pH of the stroma changed by light?

A

The pumping of H+ into the thylakoids from the stroma raises pH of stroma from 7 to 8 which activates rubisco

124
Q

What does light induced electron flow do?

A

Reduces disulfide bonds to activate 4 calvin cycle enzymes.

125
Q

How is electron flow induced by light?

A

Ferrodoxin is reduced in PSI, some electrons passed to protein thioredoxin, passes electrons to 4 enzymes in CO2 fixation pathway

126
Q

What happens when electrons are passed to the 4 enzymes in the CO2 fixation pathway?

A

Disulfide bridges near active sites broken when reduced, change in 3D shape activates these enzymes

127
Q

What is a major limitation of rubisco?

A

Its tendency to react with oxygen

128
Q

What is it called when rubisco reacts with O2 instead of CO2?

A

Photorespiration

129
Q

What does photorespiration do?

A

Lowers the rate of CO2 fixation

130
Q

What is the product of RuBP + O2?

A

Phosphoglycolate + 3PG

131
Q

What have some plants done to partially recover some carbon channeled away from the calvin cycle?

A

Phosphoglycolate forms glycolate which diffuses into peroxisomes

132
Q

What happens in the peroxisome?

A

Series of reactions

Glycolate–> glycine

133
Q

What happens after glycine has formed?

A

It diffuses into the mitochondria

134
Q

What happens to glycine in the mitochondria?

A

2glycine –> glycerate + CO2

135
Q

Photorespiration reduces net carbon fixed by the Calvin cycle by __%

A

25

136
Q

How much affinity does rubisco have for CO2 compared to O2?

A

10x

137
Q

What happens with rubisco when relative concentrations of CO2 and O2 vary?

A

Rubisco acts as an oxygenase when O2 is relatively abundant etc.

138
Q

Under what conditions is photorespiration more likely to occur?

A

High temperatures

139
Q

Why is photorespiration more likely to occur in high temperatures?

A

Hot dry day, stomata close to prevent water loss, this prevents gas exchange, CO2 concentration falls, O2 concentration increases.

140
Q

Give examples of plants whose palisade mesophyll cells just below the surface of the leaf are full of chloroplasts that contain abundant rubisco.

A

Roses
Wheat
Rice

141
Q

Why are these plants called C3 plants?

A

The first product of CO2 fixation in these plants is 3PG

142
Q

Give an example of C4 plants.

A

Corn, sugarcane, tropical grasses

143
Q

What happens when C4 plants close their stomata on a hot day?

A

Rate of photosynthesis does not fall

Photorespiration does not occur

144
Q

How do C4 plants keep the ratio of CO2 to O2 high?

A

By making 4 carbon compound as the first product of CO2 fixation

145
Q

What is the 4 carbon compound made as the first step of CO2 fixation in C4 plants called?

A

Oxaloacetate

146
Q

What is the name of the CO2 fixing enxyme in C4 plants?

A

Phosphoenolpyruvate (PEP)

147
Q

What are the 2 advantages of PEP over rubisco?

A
  • No oxygenase activity

- Fixes CO2 even at very low [CO2]

148
Q

What is the difference between crassulacean acid metabolism and C4 plants?

A

Both make 4 carbon compounds

CAM reactions are separated by time rather than space

149
Q

In c4 plants, molecules are synthesized in mesophyll cells and transported to…

A

bundle sheath cells for the calvin cycle

150
Q

What happens to CAM plants during the night?

A

Stomata open, CO2 is fixed to oxaloacetate, which is converted to malic acid

151
Q

What happens to CAM plants during the day?

A

Stomata close
Malic acid is shipped to chloroplasts
Decarboxylation supplies CO2 for calvin cycle
Light reactions supply ATP and NADPH

152
Q

Why is partitioning of G3P important?

A

It can be used as part of the glycolysis pathway and converted into pyruvate or enter a pathway of glycolysis reserve (gluconeogenesis)

Biology 2 (14 decks)

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