Photosynthesis Flashcards
(95 cards)
1
Q
Photosynthesis
A
Is the process that converts solar energy into chemical energy
• Directly or indirectly, photosynthesis nourishes
almost the entire living world
2
Q
Autotrophs
A
Sustain themselves without eating anything derived from other organisms
• Autotrophs are the producers of the biosphere,
producing organic molecules from CO2 and other
inorganic molecules
• Almost all plants are photoautotrophs, using the
energy of sunlight to make organic molecules.
3
Q
Chloroplasts are structurally similar to and likely
evolved from photosynthetic bacteria
A
The structural organization of these cells allows for
the chemical reactions of photosynthesis
4
Q
The green color of plants is from chlorophyll, the green pigment within chloroplasts
A
Chloroplasts are found mainly in cells of the
mesophyll, the interior tissue of the leaf
Each mesophyll cell contains 30–40 chloroplasts
5
Q
The chlorophyll is in the membranes of
thylakoids (connected sacs in the chloroplast);
thylakoids may be stacked in columns called
grana
A
Chloroplasts also contain stroma, a dense
interior fluid
6
Q
The Two Stages of Photosynthesis
A
Photosynthesis consists of the light reactions (the
photo part) and Calvin cycle (the synthesis part)
7
Q
The light reactions (in the thylakoids)
A
– Split H2O
– Release O2
– Reduce NADP+ to NADPH
– Generate ATP from ADP by photophosphorylation
8
Q
Calvin cycle
A
(in the stroma) forms sugar from CO2, using ATP and NADPH
The Calvin cycle begins with carbon fixation,
incorporating CO2 into organic molecules
9
Q
The Nature of Sunlight
A
- Light is a form of electromagnetic energy, also called electromagnetic radiation
- Like other electromagnetic energy, light travels in rhythmic waves
- Wavelength is the distance between crests of waves
- Wavelength determines the type of electromagnetic energy
- The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation
- Visible light consists of wavelengths (including those that drive photosynthesis) that produce colors we can see
- Light also behaves as though it consists of discrete particles, called photons
10
Q
Mesophyll tissue:
A
in plant anatomy, photosynthetic parenchyma cells that lie between the upper and lower epidermis layers of a leaf
11
Q
An action spectrum depicts the magnitude of a response of a biological system to light, as a function of wavelength.
A
For example, an action spectrum for photosynthesis can be constructedfrom measurements of oxygen evolution at different wavelengths
12
Q
Photosystem I preferentially absorbs far-red light of
wavelengths greater than 680 nm;
A
Photosystem I produces a strong reductant, capable of reducing NADP+, and a weak oxidant.
Photosystem I preferentially absorbs far-red light of
wavelengths greater than 680 nm
13
Q
photosystem II preferentially absorbs red light of 680 nm and is driven very poorly by far-red light.
A
Photosystem II produces a very strong oxidant, capable of oxidizing water, and a weaker reductant than the one produced by photosystem I.
photosystem II preferentially absorbs red light of 680 nm and is driven very poorly by far-red light.
14
Q
The carbon reduction reactions, which are catalyzed by water-soluble enzymes, take place in the stroma….
A
…the region of the chloroplast outside the thylakoids.
15
Q
Stroma lamellae
A
(site of PSI)
16
Q
Grana lamellae
A
stack of thylakoids and site of PSII
17
Q
Chlorophyll “a” is the main photosynthetic pigment
Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis
A
Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll
18
Q
In PSII, the oxidation of two water molecules produces four electrons, four protons, and a single O2
A
2H2O —oxidization—> 4H + O2
19
Q
Photosystem II oxidizes water to O2 in the thylakoid
lumen and in the process releases protons into the
lumen
A
Cytochrome b6 f receives electrons from PSII and
delivers them to PSI. It also transports additional
protons into the lumen from the stroma
20
Q
pheophytin transfers electrons to the
acceptors QA and QB, which are plastoquinones. (4) The cytochrome b6 f complex transfers electrons to plastocyanin (PC),
A
The acceptor of electrons from P700* (A0) is
thought to be a chlorophyll, and the next acceptor (A1) is a quinone
21
Q
Photosystem I reduces NADP+ to NADPH in the
stroma by the action of ferredoxin (Fd) and the flavoprotein ferredoxin–NADP reductase (FNR).
A
ATP synthase produces ATP as protons diffuse back through it from the lumen into the stroma.
22
Q
Energy Is Captured When an Excited Chlorophyll
Reduces an Electron Acceptor Molecule
A
the function of light is to excite a specialized
chlorophyll in the reaction center, either by direct
absorption or, more frequently, via energy transfer from an antenna pigment.
23
Q
This excitation process can be envisioned as the promotion of an electron from the highest-energy filled orbital of the chlorophyll to the lowest-energy unfilled orbital. The electron in the upper orbital is only loosely bound to the chlorophyll and is easily lost if a molecule that can accept the electron is nearby
A
The first reaction that converts electron energy into
chemical energy—that is, the primary photochemical event—is the transfer of an electron from the excited state of a chlorophyll in the reaction center to an acceptor molecule.
24
Q
Immediately after the photochemical event, the reaction center chlorophyll is in an oxidized state (electron deficient, or positively charged) and the nearby electron acceptor molcule is reduced. The
system is now at a critical juncture
A
If the acceptor molecule donates its electron back
to the reaction center chlorophyll, the system will be returned to the state that existed before the light excitation, and all the absorbed energy will be converted into heat.
25
An absorption spectrum
is a graph plotting a pigment’s light absorption versus wavelength
26
The absorption spectrum of chlorophyll a suggests that violet blue and red light work best for photosynthesis
An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process
27
The action spectrum of photosynthesis was
first demonstrated in 1883 by Theodor W.
Engelmann
In his experiment, he exposed different
segments of a filamentous alga to different
wavelengths
Areas receiving wavelengths favorable to
photosynthesis produced excess O2
He used the growth of aerobic bacteria
clustered along the alga as a measure of
O2 production
28
The essence of photosynthetic energy storage is thus the initial transfer of an electron from an excited chlorophyll to an acceptor molecule, followed by a very rapid series of secondary chemical reactions that separate the positive and
negative charges.
These secondary reactions separate the charges to opposite sides of the thylakoid membrane in approximately 200 picoseconds
29
Water is oxidized according to the following chemical reaction
2 H2O → O2 + 4 H+ + 4 e–
This equation indicates that four electrons are removed from two water molecules, generating an oxygen molecule and four hydrogen ions
30
The quantum yield of photosynthesis (Ø) is defined as follows:
Ø = (Number of photochemical products) / (Total number of quanta absorbed)
31
The value of Φ for a particular process can range from 0
(if that process is never involved in the decay of the excited state) to 1.0 (if that process always deactivates the excited state). The sum of the quantum yields of all possible processes is 1.0.
32
A photosystem consists of a reaction-center
complex (a type of protein complex) surrounded by
light-harvesting complexes
The light-harvesting complexes (pigment
molecules bound to proteins) transfer the energy of
photons to the reaction center
A primary electron acceptor in the reaction center
accepts excited electrons and is reduced as a result
Solar-powered transfer of an electron from a
chlorophyll a molecule to the primary electron
acceptor is the first step of the light reactions
33
During the light reactions, there are two possible
routes for electron flow: cyclic and linear
Linear electron flow, the primary pathway, involves
both photosystems and produces ATP and NADPH
using light energy
A photon hits a pigment and its energy is passed
among pigment molecules until it excites P680
An excited electron from P680 is transferred to the
primary electron acceptor (we now call it P680+)
34
P680+ is a very strong oxidizing agent
H2O is split by enzymes, and the electrons are
transferred from the hydrogen atoms to P680+, thus
reducing it to P680
O2 is released as a by-product of this reaction
Each electron “falls” down an electron transport
chain from the primary electron acceptor of PS II to
PS I
35
How is ATP made in the light reactions?
In the light reaction, when electrons are transferred from Photosystem 2 to Photosystem 1, it goes through an Electron Transport Chain (ETC). This ETC pumps protons into the thykaloid. Those protons diffuse out of the thykaloid through ATP synthase which energizes a phosphate group to bond to ADP. This creates ATP.
36
In all eukaryotic photosynthetic organisms that contain both chlorophyll a and chlorophyll b, the most abundant antenna proteins are members of a large family of structurally related proteins
Antenna systems function to deliver energy efficiently to the reaction centers with which they are associated
37
Some of these proteins are associated primarily
with photosystem II and are called light-harvesting
complex II (LHCII) proteins; others are associated with photosystem I and are called LHCI proteins.
These antenna complexes are also known as chlorophyll a/b antenna proteins
38
Funneling of excitation from the antenna system
toward the reaction center. (A) The excited-state energy of pigments increases with distance from the reaction center; that is, pigments closer to the reaction center are lower in energy than those farther from the reaction center.
This energy gradient ensures that excitation transfer toward the reaction center is energetically favorable and that excitation transfer back out to the peripheral portions of the antenna is energetically unfavorable
39
Light absorbed by carotenoids or chlorophyll b in the LHC proteins is rapidly transferred to chlorophyll a and then to other antenna pigments that are intimately associated with the reaction center
..
40
Summary of the experiment carried out by
Jagendorf and coworkers. Isolated chloroplast thylakoids kept previously at pH 8 were equilibrated in an acid medium at pH 4. The thylakoids were then transferred to a buffer at pH 8 that contained ADP and Pi.
The proton gradient generated by this manipulation provided a driving force for ATP synthesis in the absence of light. This experiment verified a prediction of the chemiosmotic theory stating that a chemical potential across a membrane can provide energy for ATP synthesis.
41
Chemiosmosis is the movement of ions across a selectively permeable membrane, down their electrochemical gradient.
More specifically, it relates to the generation of ATP by the movement of hydrogen ions across a membrane during cellular respiration.
42
A Comparison of Chemiosmosis in Chloroplasts and Mitochondria
Chloroplasts and mitochondria generate ATP by
chemiosmosis, but use different sources of energy
Mitochondria transfer chemical energy from food to
ATP; chloroplasts transform light energy into the
chemical energy of ATP
Spatial organization of chemiosmosis differs between chloroplasts and mitochondria but also
shows similarities
43
In mitochondria, protons are pumped to the
intermembrane space and drive ATP synthesis
as they diffuse back into the mitochondrial
matrix
In chloroplasts, protons are pumped into the
thylakoid space and drive ATP synthesis as
they diffuse back into the stroma
ATP and NADPH are produced on the side
facing the stroma, where the Calvin cycle takes
place
In summary, light reactions generate ATP and
increase the potential energy of electrons by
moving them from H2O to NADPH
44
The Calvin cycle uses the chemical energy
| of ATP and NADPH to reduce CO2 to sugar
The Calvin cycle, like the citric acid cycle,
regenerates its starting material after molecules
enter and leave the cycle
The cycle builds sugar from smaller molecules by
using ATP and the reducing power of electrons
carried by NADPH
45
Carbon enters the cycle as CO2 and
leaves as a sugar named glyceraldehyde 3-phospate (G3P)
For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO2
The Calvin cycle has three phases
– Carbon fixation (catalyzed by rubisco)
– Reduction
– Regeneration of the CO2 acceptor
(RuBP)
CRR
46
In the Calvin cycle, CO2 and water
from the environment are enzymatically combined with a five-carbon acceptor molecule to generate two molecules of a three-carbon intermediate.
This intermediate (3-phosphoglycerate) is reduced to carbohydrate by use of the ATP and NADPH generated photochemically
47
1. Carboxylation:
Carboxylation in chemistry is a chemical reaction in which a carboxylic acid group is introduced in a substrate.
Carboxylation of the CO2 acceptor ribulose-1,5-bisphosphate, forming two molecules of 3-phosphoglycerate the first stable intermediate of the Calvin cycle
48
2. Reduction:
Reduction of 3-phosphoglycerate, forming gyceraldehyde-3-phosphate, a carbohydrate
49
3. Regeneration:
3. Regeneration of the CO2 acceptor ribulose-1,5-bisphosphate from glyceraldehyde-3-phosphate
50
Alternative mechanisms of carbon fixation have evolved in hot, arid climates
Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis
On hot, dry days, plants close stomata, which conserves H2O but also limits photosynthesis
The closing of stomata reduces access to CO2 and causes O2 to build up
These conditions favor an apparently
wasteful process called photorespiration
51
Photorespiration:
In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound (3-phosphoglycerate)
In photorespiration, rubisco adds O2 instead of CO2 in the Calvin cycle, producing a two-carbon compound
Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar
52
Ribulose-bisphosphate carboxylase/oxygenase
(RuBisCO) is one of Nature's most remarkable
proteins
It is the only known protein capable of adding
atmospheric carbon dioxide (CO2) directly to a
sugar chain and is responsible for the CO2
fixation of plants.
The enzyme can also act with dioxygen (O2) in a
competing reaction.
The enzyme from plants and algae exists as a
complex consisting of eight large and eight small
subunits, where each dimer binds a Mg2+ ion
crucial for the catalytic activity
53
C4 Plants
C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells
This step requires the enzyme PEP carboxylase
PEP carboxylase has a higher affinity for CO2
than rubisco does; it can fix CO2 even when CO2
concentrations are low
These four-carbon compounds are exported to
bundle-sheath cells, where they release CO2
that is then used in the Calvin cycle
54
The C4 photosynthetic carbon cycle is an elaborated addition to the C3 photosynthetic pathway.
It evolved as an adaptation to high light intensities, high temperatures, and dryness. Therefore, C4 plants dominate grassland floras and biomass production in the warmer climates of the tropical and subtropical regions
55
CAM Plants
Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon
CAM plants open their stomata at night, incorporating CO2 into organic acids
Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle
56
Irradiance = (A) × cosine a
..
57
Photosynthetically active radiation (PAR)
Less than 5% of incoming energy from sun converted into carbohydrates
* Half of this energy is outside PAR
* 15% reflected or transmitted (green)
* Of 85% of PAR absorbed, much lost as heat (large amt) and fluorescence (small amt.)
* Thus, less than 5% converted
58
Leaf movement in sun-tracking plants
Solar tracking
• Keeps leaves perpendicular to sun rays
• Asymmetric swelling of pulvinis
A pulvinus is a joint-like thickening at the base of a plant leaf or leaflet that facilitates growth-independence
59
P-Proteins
Deposition of P-Protein and Callose Seals Off
| Damaged Sieve Elements
60
Companion Cells Aid the Highly Specialized
| Sieve Elements
Each sieve tube element is associated with one or more companion cells. The division of a single mother cell forms the sieve tube element
and the companion cell.
61
Sap in the phloem is not translocated exclusively in either an upward or a downward direction, and translocation in the phloem is not defined with respect to gravity. Rather, sap is translocated from areas of supply, called sources, to areas of metabolism or storage, called sinks
Sources include any exporting organs, typically mature leaves, that are capable of producing photosynthate in excess of their own needs. The term photosynthate refers to products of photosynthesis
62
MATERIALS TRANSLOCATED IN THE
PHLOEM: SUCROSE, AMINO ACIDS,
HORMONES, AND SOME INORGANIC IONS
...
63
Symplastic or Apoplastic Loading
We have seen that solutes (mainly sugars) in source leaves must move from the photosynthesizing cells in the mesophyll to the veins
Two distinct methods can be employed by plants to move sugars into the phloem. Symplastic loading involves the movement of sugars through the plasmodesmata from one cell to another. Apoplastic loading involves the movement of sugars from the apoplast (the extracellular cell wall space) across the plasma membrane and into the cell. This movement of sugar against a concentration gradient is accomplished by sugar transporters in the plasma membrane such as SUC2
64
SUC2
SUC2 is a phloem specific plasma membrane sucrose transporter localized to the plasma membranes of sieve elements and/or companion cells depending on species and plays a role in apoplastic sucrose loading
65
The pressure-flow model
states that a flow of solution in the sieve
elements is driven by an osmotically generated pressure gradient between source and sink (ΔYp). The pressure gradient is established as a consequence of phloem loading at the source and phloem unloading at the sink.
66
At the receiving end of the translocation pathway,
phloem unloading leads to a lower sugar concentration in the sieve elements, generating a higher (more positive) solute potential in the sieve elements of sink tissues. As the water potential of the phloem rises above that of the xylem, water tends to leave the phloem in response to the water
potential gradient, causing a decrease in turgor pressure in the sieve elements of the sink
..
67
Sucrose Uptake in the Apoplastic Pathway
| Requires Metabolic Energy
..
68
Plants have many biotic and abiotic obstacles to overcome
First line of defense is cuticle
Second line of defense is periderm
69
* Cutin
* Waxes
* Suberin
All plant parts exposed to the atmosphere are coated with layers of lipid material that reduce water loss and help block the entry of pathogenic
fungi and bacteria
70
Cutin
Cutin is found on most aboveground parts;
Cutin is a macromolecule, a polymer consisting of many long-chain fatty acids that are attached to each other by ester linkages, creating a rigid three-dimensional network.
71
suberin
suberin is present on underground parts, woody stems, and healed wounds.
72
Waxes
Waxes are associated with both cutin and suberin.
Waxes are not macromolecules, but complex mixtures of long-chain acyl lipids that are extremely hydrophobic.
73
Cutin,Waxes, and Suberin Are Made Up of
| Hydrophobic Compounds
Prevent water loss and protect against direct
| penetration
74
Secondary Metabolites:
NOT lipids, proteins, carbohydrates or n.a.’s
particular secondary metabolites are often found in only one plant species or related group of species, whereas primary metabolites are found throughout the plant kingdom.
Important ecological functions: [PAS]
1. Prevent predation and microbial degradation
2. Attractants -- pollination and seed dispersal
3. Compete out other plants and important in plant
symbiotic relationships
1. Fungi
2. Bacteria
75
Plant secondary metabolites can be divided into three chemically distinct groups:
terpenes,
phenolics,
nitrogen-containing compounds
76
Terpenes (terpenoids)
– Most abundant, most diverse, insoluble in water
– Synthesized from AcetylCoA or its intermediates
in glycolysis
Terpenes Are Formed by the Fusion of Five-
Carbon Isoprene Units
77
Two distinct pathways for terpene biosynthesis
1. Mevalonic acid pathway -- 3 acetyl CoA joined together to form mevalonic acid
2. Methylerythritol phospate pathway (MEP)
1. Confined to plastsids
78
Some Terpenes Have Roles in Growth and
| Development
the gibberellins, an important group of plant hormones, are diterpenes. Sterols are triterpene derivatives that are essential
components of cell membranes, which they stabilize by interacting with phospholipids
The red, orange, and yellow carotenoids are tetraterpenes that function as accessory pigments in photosynthesis and protect photosynthetic tissues from photooxidation
Terpenes are toxins and feeding deterrents to many plantfeeding insects and mammals; thus they appear to play important defensive roles in the plant kingdom
79
Azadirachtin -- terpene from neem trees
serve as powerful feeding deterrents to
| insects.
80
limonene (A) and menthol (B).
These two well-known monoterpenes serve as defenses against insects and other organisms that feed on these plants.
81
PHENOLIC COMPOUNDS
Plants produce a large variety of secondary products that contain a phenol group—a hydroxyl functional group on an aromatic ring
The Release of Phenolics into the Soil
May Limit the Growth of Other Plants
82
Plant phenolics are synthesized in several different ways
...
83
NITROGEN-CONTAINING COMPOUNDS
Alarge variety of plant secondary metabolites have nitrogen in their structure. Included in this category are such well-known antiherbivore defenses as alkaloids and cyanogenic glycosides, which are of considerable interest because of their toxicity to humans and their medicinal properties. Most nitrogenous secondary metabolites are
biosynthesized from common amino acids.
84
Biological nitrogen fixation
1. Rare, extremely energy consuming conversion
because of stability of triply bonded N2
2. Produces fixed N which can be directly
assimilated into N containing biomolecules
85
System of Nitrogen fixation
1. Symbiosis: sucrose from host plant
2. Association: Roots exudates from the host plant
3. Free Living: Heterotrophs (plant residue) /Autotrophs )photosynthesis
86
How nitrogen enters biological pathways
PATHWAY 1
N + Glutamete + ATP ------> Glutamine (ADP +Pi) = Amino acid/proteins/purines
87
PATHWAY 2
N + alpha-ketoglutarate -----GDH----> glutamate = amino acids/proteins
88
A growing population must eat!
* Combined nitrogen is the most common limiting nutrient in agriculture
* Estimated that 90% of population will live in tropical and subtropical areas where (protein-rich) plant sources contribute 80% of total caloric intake.
* In 1910 humans consumed 10% of total carbon fixed by photosynthesis, by 2030 it is predicted that 80% will be used by humans.
89
Rhizobium-legume symbioses
Fixed nitrogen (ammonia) Fixed carbon (malate, sucrose)
90
Nitrogen Fixation Requires Anaerobic Conditions
Because oxygen irreversibly inactivates the nitrogenase
enzymes involved in nitrogen fixation, nitrogen must be
fixed under anaerobic conditions
91
Free-living bacteria that are capable of fixing nitrogen are aerobic, facultative, or anaerobic
For anaerobic nitrogen-fixing bacteria, oxygen does
| not pose a problem, because it is absent in their habitat. These anaerobic organisms can be either photosynthetic
92
Grasses can also develop symbiotic relationships with
nitrogen-fixing organisms, but in these associations root
nodules are not produced. Instead, the nitrogen-fixing bacteria
seem to colonize plant tissues or anchor to the root
surfaces, mainly around the elongation zone and the root
hairs
Legumes and actinorhizal plants regulate gas permeability
in their nodules, maintaining a level of oxygen
within the nodule that can support respiration but is sufficiently
low to avoid inactivation of the nitrogenase
93
Nodules contain an oxygen-binding heme protein called leghemoglobin
Establishing Symbiosis Requires an
| Exchange of Signa
94
```
The reaction catalyzed
by nitrogenase. Ferredoxin
reduces the Fe protein. Binding
and hydrolysis of ATP to the Fe
protein is thought to cause a conformational
change of the Fe protein
that facilitates the redox reactions.
The Fe protein reduces the
MoFe protein, and the MoFe protein
reduces the N2. (After Dixon
and Wheeler 1986, and Buchanan
et al. 2000.)
```
..
95
The Nodulation Process
• Chemical recognition of roots and Rhizobium
• Root hair curling
• Formation of infection thread
• Invasion of roots by Rhizobia
• Cortical cell divisions and formation of
nodule tissue
• Bacteria fix nitrogen which is transferred to
plant cells in exchange for fixed carbon
