1
Q
What is photosynthesis?
A
· Photosynthesis is a biochemical process for building carbohydrates using sunlight and carbon dioxide taken from the air.
- Photosynthesis is the major entry point for energy into biological systems.
- The carbohydrates are used as staring points for the synthesis of other molecules and as a means of storing energy that can be converted into ATP through cellular respiration.
- It is the source of all the food we eat and the oxygen that we breathe, as well as fuels for heating and transportation.
- Photosynthesis is widely distributed.
While plants are the primary organism we think of when we think of photosynthesis, there are prokaryotic organisms that also conduct photosynthesis.
2
Q
Metabolic Classification of Organisms
A
Organisms can harvest energy from the sun or from chemical compounds.
- Depending on how an organism harvests energy, it is classified as a phototroph (obtains energy from the sun) or a chemotroph (obtains energy from chemical compounds).
- The most common phototrophs are plants, which obtain energy from the sun and use it to convert carbon dioxide and water into sugar and oxygen.
- The most common chemotrophs are animals, which obtain energy by breaking down organic compounds acquired from ingesting other organisms into carbon dioxide and water.
3
Q
What are autotrophs?
A
Organisms that make required organic (food) molecules from inorganic sources such as CO2 and water; self-feeding
4
Q
What are heterotrophs?
A
Consumers and decomposers which need a source of organic (food) molecules to survive
5
Q
What are photoautotrophs?
A
Autotrophs that use light as the energy source to make organic molecules by photosynthesis
6
Q
Who are the primary producers of the earth?
A
Photoautotrophs
7
Q
Photosynthetic organisms?
A
· Convert sunlight energy into chemical energy
· Use energy to assemble complex organic molecules from inorganic raw materials
· The organic molecules are then used as energy sources (but also used as energy source by other organisms
8
Q
What is the energy flow?
A
· The Sun is the ultimate source of energy for most organisms.
Photosynthesis
· Captures energy of sunlight
· Converts it to chemical energy of complex organic molecules
Respiration
Extracts the potential energy from such molecules, and converts it into chemical energy in the form of ATP that can be used to drive most relations of the cell.
9
Q
What are the two stages of photosynthesis?
A
Light reactions and Calvin cycle
- Photosynthesis begins with the absorption of light by protein-pigment complexes known as photosystems.
- Photosystems use absorbed light energy to drive redox reactions and thereby set the photosynthetic electron transport chain in motion.
- The movement of electrons through this chain is used to drive the synthesis of ATP and NADPH.
- ATP and NADPH are the energy sources needed to synthesize carbohydrates from CO2 in the Calvin cycle.
10
Q
Where do the two stages of photosynthesis occur?
A
in the chloroplasts of photoautotrophic eukaryotes (plants and algae) as well as in photosynthetic bacteria.
11
Q
What is photosynthesis dependent on?
A
Dependent, NADPH = NADH + FADH2
HIGH ENERGY E- CARRIER
12
Q
Light reactions?
A
light energy absorbed by the pigment
molecules are transformed into ATP and NADPH; O2 that is produced as a result of the oxidation of water is released as a by-product
13
Q
Calvin cycle?
A
NADPH and ATP produced during the light reactions provide energy and reducing power to fix carbon from CO2 and convert it into
carbohydrates
14
Q
What type of reaction is photosynthesis?
A
redox reaction
15
Q
How does the energy from sunlight become incorporated into chemical bonds?
A
During photosynthesis CO2 mlcs are reduced to form higher energy carbohydrate mlcs
This requires an input of energy. This energy comes from sunlight and the transfer of electrons from an electron donor (ultimate e- donor is water)
energy from sunlight is used to produce ATP and electron donor molecules capable of reducing CO2.
The electron donor in these reactions is water.
The oxidation of water results in the production of electrons, protons, and O2.
The electrons and protons are incorporated into the carbohydrate product, and O2 is a by-product.
The oxidation of water is linked with the reduction of carbon dioxide through a series of redox reactions making up the photosynthetic electron transport chain.
16
Q
Where does photosynthesis take place in eukaryotes?
A
in the chloroplasts
17
Q
What are some qualities of chloroplasts?
A
- Outer and inner membranes
- Intermembrane compartment
· Aqueous environment within the inner membrane is the stroma
· Thylakoid membranes are a complex of flattened internal membrane compartments - Stacks of membranes called grana
- Tubular lamellae connections between the grana
Compartment enclosed by thylakoids called the thylakoid lumen
- Intermembrane compartment
18
Q
What are Mesophyll cells?
A
the primary photosynthesis cells
19
Q
What is the structure of chloroplasts?
A
· Surrounded by two membranes: outer and inner membranes
- Separated by intermembrane space
- Chloroplasts are enclosed by a double membrane.
- Filling much of the center of the chloroplast is a third, highly folded membrane known as the thylakoid membrane.
- The photosynthetic electron transport chain is located on the thylakoid membrane.
- Sunlight is captured and transformed into chemical energy by the photosynthetic electron transport chain in the thylakoid membrane.
- Thylakoid membranes form structures that resemble flattened sacs, which are grouped into stacks called grana.
- Within the grana is the lumen, and the area outside is the stroma.
- While photosynthetic organisms are described as autotrophic because they can form carbohydrates, they also require a constant supply of ATP to meet each cell’s energy requirements.
- Although ATP is produced within chloroplasts, only carbohydrates are exported outside of the chloroplasts.
- This helps explain why plants still must conduct cellular respiration in mitochondria to produce ATP.
20
Q
What is the main aqueous compartment of chloroplasts and where it is located?
A
The main aqueous compartment is called the stroma
- Location of carbohydrate synthesis (Calvin cycle)
21
Q
What are thylakoid membranes?
A
- Location of photosynthetic pigments and electron transport chain
- A complex of flattened, closed sacs
- Stacks of membranes called grana
- Tubular lamellae connect the grana
The soluble compartment enclosed by thylakoids called the thylakoid lumen
22
Q
What are light and the electromagnetic spectrum?
A
· Light is the ultimate source of energy, sustaining virtually all organisms.
· The Sun converts matter to energy, releasing it as electromagnetic radiation.
· The range of wavelengths of electromagnetic radiation is called the electromagnetic spectrum.
23
Q
What is light?
A
· Light is defined as the portion of the electromagnetic spectrum that humans can detect with their eyes
· Light behaves like a wave and like particles of energy (photons), and thus can be understood as a wave of photons.
· Electromagnetic spectrum
· Forms of radiant energy that differ in wavelength (horizontal distance between crests of successive waves
24
Q
What is the relationship between visible light and photons?
A
· Visible light has wavelengths between about 700 nm (red light) and 400 nm (blue light)
· We see the entire spectrum combined together as white light
· The amount of energy in a unit of light (photon) is inversely proportional to wavelength
- the shorter the wavelength, the greater the energy of the photon
25
How does light interact with matter?
· When photons of light hit an object, 1 of 3 things can happen. The photon can be:
- reflected
- transmitted
- absorbed
· To be used as energy, light must be absorbed - the energy of the photon is transferred to an electron within a molecule.
· The energy transfer switches the electron from a grounded state to an excited state.
The absorption of a photon by a molecule results in the energy being transferred to an electron. This causes the energy to move to a higher-energy, excited state.
26
What are pigments?
molecules that absorb photons of specific wavelengths
27
What is the Critical light absorption feature?
a region where carbon atoms are covalently bonded to each other with alternating single and double bonds (conjugated system
28
How do pigments absorb photons?
· Differences in the arrangement of conjugated systems and different chemical structure explain why each type of pigment absorbs light of only certain wavelengths.
· A pigment’s color is the result of photons of light it does not absorb.
29
What is the structure of some common pigments?
Chlorophyll a, photosynthesis; 11-cis-retinal, vision; indigo, dye; phycoerythrobilin, red photosynthetic pigment found in red algae; carmine, scale pigment found in some insects; beta-carotene, an orange accessory photosynthetic pigment. A common feature of all these pigments that is critical for light absorption is the presence of a conjugated system of double/single carbon bonds (shown in red for beta-carotene).
30
Why is a t-shirt red?
Pigment molecules bound to the fabric of the shirt absorb blue, green, and yellow photons of light. Red photons are not absorbed and are instead transmitted through the shirt or are reflected.
31
What is the major photosynthetic pigment?
chlorophyll
1. It appears green because it is poor at absorbing green wavelengths.
2. Chlorophyll has a large, light-absorbing head containing magnesium at its center and a long hydrocarbon tail that allows the pigment to be anchored in the lipid membrane.
3. Chlorophyll molecules are bound by their tail region to integral membrane proteins in the thylakoid membrane.
These complexes are called photosystems, which are the functional and structural units that absorb light energy and use it to drive electron transport.
32
What is the fate of an excited-state electron?
Light reactions depend on the absorption of light energy by pigment molecules in the thylakoid membrane.
3 possibilities:
The energy released as heat or as the light of a longer wavelength (fluorescence), the electron returns to the ground state
Energy from excited electrons in one pigment molecule is transferred to a neighbouring pigment molecule by inductive resonance. Transfer excites the second pigment and the first pigment returns to the ground state
Excited-state electron itself is transferred to nearby electron-accepting molecule, the primary acceptor
33
How do Chlorophyll and Carotenoid work together?
* Pigment molecules used in photosynthesis. (a) Chlorophylls a and b, differ only in the side group attached at the X. (b) An example of a carotenoid. In both (a) and (b), the light-absorbing electrons are distributed among the bonds shaded in orange.
In photosynthesis, light is absorbed by green pigments called chlorophylls and yellow-orange pigments called carotenoids
34
How do carotenoids and chlorophylls drive the reactions of photosynthesis?
· Light absorbed by carotenoids and chlorophylls, acting in combination, drives the reactions of photosynthesis.
· The amount of light of different wavelengths that is absorbed by a pigment is its absorption spectrum.
· The effectiveness of light of each wavelength in driving photosynthesis produces a graph called the action spectrum of photosynthesis.
35
What was Engelmann’s Experiment (1883)?
· Theodor Engelmann used a glass prism to break light into a spectrum of colours—cast across a microscope slide with a strand of algae and aerobic bacteria.
· Bacteria grew best where algae released oxygen in greatest quantity —in areas of blue, violet, and red light.
· Engelmann constructed an action spectrum for wavelengths of light, showing the effects of each colour on photosynthesis.
36
Light-dependant reactions?
capture of light energy by pigment molecules and energy used to synthesize both ATP and NADPH
37
Light-independent reactions?
electrons carried by NADPH and ATP energy used to convert CO2 from inorganic to organic form
38
How does photosynthesis begin?
1. Photosynthesis begins with the absorption of light by protein-pigment complexes known as photosystems.
2. Photosystems use absorbed light energy to drive redox reactions and thereby set the photosynthetic electron transport chain in motion.
3. The movement of electrons through this chain is used to drive the synthesis of ATP and NADPH.
4. ATP and NADPH are the energy sources needed to synthesize carbohydrates from CO2 in the Calvin cycle.
39
What is each photosystem composed of?
a large antenna complex of pigments that surrounds a central reaction center
40
How do pigments work in photosystems?
A group of pigment-proteins form an antenna complex that surrounds a reaction centre. Light energy absorbed anywhere in the antenna complex is transferred to a special chlorophyll a molecule in the reaction centre. The absorbed light is converted to chemical energy when an excited electron from the chlorophyll a is transferred to a primary acceptor, also in the reaction centre. High-energy electrons are passed out of the photosystem to the electron transport system. The yellow arrow shows the migration of energy from one pigment to the other, and the blue arrows show movement of electrons.
41
How does light energy work in photosystems?
· Light energy absorbed by photosynthetic pigments has to be converted somehow into chemical energy (ATP and NADPH) so that it can be used to drive the second stage of photosynthesis - - the light-independent reactions. In order to do this, the pigments are arranged into photosystems, and the energy that they absorb is used to initiate a series of redox reactions, which again involve an electron transport chain. Some similarities, and some differences as compared to the mitochondrial ETC: One outcome is the production of ATP. Another outcome is the production of the high-energy carrier, NADPH (different from mito ETC which gets its electrons from NADH).
42
What are the major components of a photosystem?
43
What are photosystems?
· Photosystems composed of many pigments and proteins
Special chlorophyll a molecules in the reaction centers can be oxidized (photoreduction)
44
What is photosystem ll?
· Special chlorophyll molecules in the reaction center are called P680 (P = pigment; 680 denotes wavelength of max absorbance for the reaction center)
ll comes first because made first
e- in P680 raised from ground state to excited state àP680*
· P680* oxidized to P680+ by primary e- acceptor Pheophytin (Pheo)
· Pheo transfers e- to PQ
· Shuttles e- to cytochrome complex
· P680 is reformed when P680+ gains an e- from oxidation of H2O
Water splitting complex
45
What is photosystem l?
· Specialized chlorophyll a molecules in the reaction center are called P700
46
Traits of photosystems?
- e- in P680 raised from ground state to excited state à P680*
- P680* oxidized to P680+ by primary e- acceptor Pheophytin (Pheo)
- Pheo transfers e- to PQ
- shuttles e- over to cytochrome complex
- P680 is reformed when P680+ gains an e- from oxidation of H2O
- water splitting complex
47
The two photosystems?
· Takes a lot of energy to pull e-s from water, the energy from a single photosystem isn’t enough o both pull e-s from water and produce an e- donor capable of reducing NADP
- solution is to use 2 photosystems in series
- energy supplied by first photosystem allows e-s to be pulled from water
energy from second photosystem allows e-s to be transferred to NADP+
48
What is photosynthetic Electron Transport
and ATP Synthesis?
· Electrons released by oxidation of the reaction centre chlorophylls of photosystem II (P680) are passed along an electron transport chain
· This photosynthetic electron transport chain (the light reactions) uses energy of light absorbed by photosystem II and photosystem I to generate reducing power in the form of NADPH (high energy electron carrier), and forms a H+ gradient that can be used to generate ATP.
49
What are the steps of linear electron transport?
Oxidation of P680
Oxidation-reduction of the plastoquinone pool
Electron transfer from the cytochrome complex and shuttling by plastocyanin
Oxidation-reduction of P700
Electron transfer to NADP+ by ferredoxin
50
Linear Electron Transport 1 Oxidation of P680 in Photosystem II
Absorption of light by PS II results in formation of P680*
· P680* gets oxidized to P680+ by the primary e- acceptor Pheophytin (Pheo)
51
Linear Electron Transport 2
Oxidation-reduction of Plastoquinone Pool
· The Electron given up by chlorophyll to Pheo is transferred to plastoquinone (PQ); the electron “hole” in P680 chlorophyll is replaced by the oxidation of water
· PQ also takes a H+ from stroma, migrates through lipid bilayer
· Donates e- to cytochrome b6/f complex
· Releases H+ into lumen
52
Linear Electron Transport 3
Electron transfer from cytochrome complex to PC
· The cytochrome complex transfers electrons to plastocyanin (PC)
· PC shuttles e- to Photosystem
53
Linear Electron Transport 4
Oxidation-Reduction of P700 in PS
· Absorption of light by PS I results in formation of P700*
· Oxidation of P700* to P700+ by the primary e- acceptor of PS I
· e- “hole” in P700+ chlorophyll is replaced by e- donated by PC
54
Linear Electron Transport 5
Electron Transfer to NADP+ by Ferredoxin
· e- from PSI is transferred to ferredoxin (iron-sulfur protein)
· e- transferred to NADP+ (Final e- acceptor) via Ferredoxin
· NADP+ gets reduced to NADPH by NADP+ reductase
55
What is the Chemiosmotic Synthesis of ATP
· Proton-motive force established across thylakoid membrane used to synthesize ATP by chemiosmosis and ATP Synthase
· H+ flow from thylakoid lumen into stroma drives synthesis of ATP
Proton gradient not generated by:
1. proton pumping, but by reduction-oxidation of PQ pool, which results in movement of H+ across thylakoid
2. Oxidation of water in the lumen increases the proton concentration
Consumption of protons during the reduction of NADP+ to NADPH in the stroma
56
Components of thylakoid membrane?
· The components of the thylakoid membrane organized according to their energy level. This is also referred to as the Z scheme. When photosystem II absorbs a photon of light, an electron within the reaction centre chlorophyll of P680 gets excited to a higher energy level (P680*). This results in spontaneous electron transport to photosystem I. However, the energy level of NADP+ is greater than that of P700. This energy difference is overcome by photosystem I absorbing a photon of light, producing P700*. Thus, two photons of light, one absorbed by photosystem II and another absorbed by photosystem I, are required to overcome the energy difference between H2O and NADP+.
-if you follow the flow of e-s from water through both photosystems and on to NADP+ can see large increase in energy as e-s pass through each of two photosystems
1. As an electron is transferred from water in photosystem II, its energy level is initially high, but as the electron travels through the electron transport chain, its energy level decreases.
2. It takes a second input of light energy in photosystem I to raise the electron energy level high enough so that it can be used to reduce NADP+.
3. The energy trajectory resembles a “Z” and is sometimes referred to as a Z scheme.
- decrease in energy is exergonic rx-one direction, if went in opposite direction would require input of energy
57
Electronegativity?
PSI energizes e-s with a second input of energy, used to reduce NADP
- During respiration, electrons flow spontaneously from NADH to oxygen. The photosynthetic electron transport chain is essentially the opposite process: electrons flow from water to NADP+. This requires the input of energy. Electron flow is spontaneous, but only after the absorption of light energy. The chlorophyll molecules of the P680 reaction centre are only high enough energy (and therefore able to be oxidized) after absorption of light energy. Likewise with the chlorophyll’s of the P700 reaction centre. They can only be oxidized and donate their electrons to NADP+ after the absorption of light energy.
58
Energy Levels in Thylakoid Membrane?
59
How many photons need to be absorbed by the photosynthetic apparatus to produce a single molecule of O2?
2H2O → 4H+ + 4e- + O2
60
How many electrons does the reduction of oxygen to water require?
· Recall that the reduction of oxygen to water requires 4 electrons. Likewise, to form oxygen from the oxidation of water also requires 4 electrons, and therefore 2 molecules of water.
· To move one electron through the chain requires the absorption of 2 photons - - one each by PSII and PSI. But to produce a single molecule of oxygen, 4 electrons are required, and therefore 4 photons must be absorbed by PSII and another 4 by PSI - - - and therefore a total of 8 photons are required to produce one molecule of oxygen.
61
How do we get 1 electron through the ETC from photosystem ll?
· To get 1 electron through the electron transport chain from photosystem II to NADP+ takes 2 photons of light (i.e. 1 photon absorbed by each photosystem)
· Therefore, for 4 electrons (or one molecule of O2), a total of 8 photons of light need to be absorbed -- 4 by each photosystem
62
What is cyclic electron transport?
· Electrons move in a circular pathway from photosystem I through ferredoxin back to the plastoquinone pool, through the cytochrome complex and plastocyanin and then back to photosystem I. Photosystem II does not operate in cyclic electron transport. The pathway generates proton pumping and thus leads to ATP production but does not result in the synthesis of NADPH.
· Cyclic electron transport increases the production of ATP.
· Cyclic electron transport generates ATP in the absence of NADPH
· Electron flow from photosystem I to ferredoxin is not always followed by electron donation to the NADP+ reductase complex
· Instead, reduced ferredoxin can donate electrons back to the plastoquinone pool, which gets continually reduced and oxidized
· Energy absorbed from light is therefore converted into chemical energy of ATP but not NADPH as a result of cyclic electron transport
· Calvin cycle requires more ATP than NADPH
· Additional ATP provided by cyclic electron transport & chemiosmosis
63
What is the Calvin cycle?
he Calvin cycle is a metabolic pathway that reduces CO2 and converts it into organic substances. It is an anabolic, endergonic process.
► NADPH provides electrons and hydrogen
► ATP provides additional energy
· Carbon fixation involves capturing CO2 molecules with the key enzyme Rubisco (RuBP carboxylase/oxygenase)
· During fixtion, CO2 is absorbed from the air and is added to a 5-carbon molecule, ribulose-1,5-bisphosphate (RuBP).
· This reaction is catalyzed by the enzyme ribulose bisphosphate carboxylase oxygenase (rubisco).
· Rubisco is a very slow enzyme and must be produced in very large amounts in a cell.
· The 6-carbon molecule formed is immediately broken down into two 3-carbon molecules of 3-phosphoglycerate (3-PGA)
· How many “turns” of the Calvin Cycle are required to provide enough fixed carbon for the synthesis of one molecule of glucose?
· Discovered by Melvin Calvin, who won the Nobel Prize for the discovery.
· It’s a cycle involving 11 enzyme-catalyzed steps.
64
What are the 3 stages of the Calvin cycle?
fixation, regeneration and reduction
An overview of the three phases of the Calvin cycle. The figure tracks the carbon atoms (dark grey balls) during three turns of the cycle. For every three molecules of CO2 that are fixed, one molecule of the three-carbon sugar G3P is synthesized.
65
What is Glyceraldehyde 3-Phosphate (G3P)?
MOST IMPORTANT ORGANIC MOLECULE
· Starting point for the synthesis of many organic molecules:
► Glucose
► Sucrose
▪ Disaccharide of glucose linked to fructose
▪ Main form for photosynthetic products that circulate among cells
► Starch
► Cellulose
► Amino acids
► Fatty acids and lipids
► Proteins
► Nucleic acids
· G3P can be transported OUT of the chloroplast by an antiporter called the triose phosphate/phosphate transporter, where things like sucrose can be made…
PRODUCTS NOT JUST USED AS A SOURCE OF ENERGY
66
What is Rubisco?
· Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase): Most abundant protein on Earth
· Provides the source of organic molecules for most of the world’s organisms
· 100 billion tons of CO2 are converted into carbohydrates each year
· Represents 50% or more of the total protein in leaves of higher plants
· (a) The functional enzyme is composed of a total of 16 subunits: 8 large subunits (LSU) (shown in white and grey) and 8 small subunits (SSU) (shown in orange and blue). The synthesis of Rubisco (b) requires coordinated gene expression of two genomes. Each LSU is synthesized in the stroma of the chloroplast following the transcription of a gene coded by the chloroplast chromosome. The gene that encodes the SSU is found in the nucleus, with SSU monomers being synthesized by cytosolic ribosomes before being imported into the chloroplast.
- first enzyme of Calvin cycle
67
What is rubisco composed of?
· Composed of:
- 8 Large Subunits (LSU)
▪ chloroplast genome
- 8 Small Subunits (SSU)
▪ nuclear genome
· Requires coordinated gene expression of two genomes, and post- translational import of the SSU from cytosol to chloroplast stroma
68
What are rubisco's carboxylase and oxygenase activities?
· Compared with the usual carboxylase activity of the Calvin cycle, the oxygenase activity results in a net loss of carbon by the plant. Because oxygenase activity consumes O2 and releases CO2, it is also called photorespiration.
Called photorespiration b/c it occurs in the light, and consumes oxygen and releases CO2 - which is similar to cellular or oxygenic respiration, BUT Photorespiration should NOT be confused with cellular (oxygenic) respiration, which also occurs in the mitochondria of plant cells.
69
What are Carbon-Concentrating Mechanisms in Algae?
· Aquatic environments
· Concentration of CO2 dissolved in water is well below what is needed to saturate the Rubisco active site
· Experimentally, addition of CO2 to phytoplankton does not usually lead to increase in rate of photosynthesis
· Algae pump carbon dioxide into their cells
70
Who has carbon-concentrating mechanisms?
Many aquatic photoautotrophs (e.g., algae) can increase their intracellular carbon dioxide concentrations through a mechanism that involves an ATP-dependent bicarbonate (HCO3−) pump on the plasma membrane. The bicarbonate is rapidly converted in the cytosol to CO2 by the enzyme carbonic anhydrase.
Suggests that aquatic photoautotrophs have a carbon-concentrating mechanism
71
What is the dilemma of plants in hot and dry climates?
· Terrestrial plants, especially those in hot dry climates, face problems of photorespiration and water loss
· Leaf surface is covered with waxy cuticle to prevent water loss; but also prevents rapid diffusion of gases into leaf
· Stomata regulate gas exchange
· Dilemma of plants in hot dry climates: Need to open stomata to let in CO2, but need to keep them closed to conserve water
Stomata. (a) Micrograph of a leaf surface showing the presence of pores called stomata (singular, stoma). (b) Each stoma is formed from two guard cells that control the opening and closing of the pore. This controls the movement of gases into and out of the plant and water loss.
72
What increases photorespiration?
High temperatures
73
What happens to the solubility of CO2 AND O2 when temperature increases?
Solubility of both CO2 and O2 decreases (at different rates) as the temperature increases; but the solubility of CO2 decreases faster, so the ratio of CO2:O2 goes up (relatively more O2 than CO2 at high temps)…which means the oxygenase reaction of Rubisco becomes more prevalent, and photorespiration becomes more of a problem.
74
What is the C4 Photosynthesis?
· Some plants have evolved the C4 pathway to increase the concentration of CO2 relative to O2 near Rubisco so that photorespiration is minimized
· In the C4 cycle, CO2 is combined with a 3-carbon molecule, phosphoenolpyruvate (PEP), to produce a 4-carbon intermediate
· The Calvin cycle is called the C3 cycle because a 3-carbon intermediate is produced first (3PGA)
· The enzyme PEP carboxylase that binds CO2 does not have any oxygenase activity
· Later, when O2 concentrations are low, the 4-carbon molecule is oxidized to release CO2
CO2 then enters the Calvin cycle by binding to Rubisco
75
How do C4 and the Calvin cycle correlate?
· The C4 cycle and its integration with the Calvin cycle. Each turn of the cycle delivers one molecule of CO2 to the Calvin cycle. This process is energy dependent, consuming ATP.
· C4 plants spatially separate the C4 pathway and the Calvin cycle
76
In which locations is C4 present?
· Some C4 plants run the C4 and Calvin cycles in different cells (spatial separation of the two pathways)
· CO2 is captured by PEP carboxylase in mesophyll cells close to the surface of leaves, since PEP carboxylase is not affected by high O2 concentrations
· The 4-carbon intermediate is then transported to bundle sheath cells deeper in the leaf where CO2 is released
There, O2 is less abundant and CO2 is more concentrated, so Rubisco’s oxygenase activity is minimized (less photorespiration)
77
What happens in Crassulacean Acid Metabolism (CAM)?
· Some C4 plants run the C4 and Calvin cycles at different times (instead of in different cells) to avoid photorespiration: temporal separation
· In crassulacean acid metabolism (CAM), certain cacti and succulent desert plants capture CO2 at night with the C4 cycle, but run the Calvin cycle during the day when sunlight is available (and therefore more ATP & NADPH are being generated)
· CAM plants open their leaf stomata during the night to capture CO2 using the C4 cycle
· 4-carbon intermediates like malate accumulate and are stored in the large cell vacuole
· During the day, plants close their stomata to conserve water and malate is oxidized to release CO2 inside the chloroplasts
· CO2 concentrations are high inside the chloroplast and O2 concentrations are relatively low, so photorespiration is minimize
· CAM plants temporally separate the C4 pathway and the Calvin cycle
78
How do the C4 and CAM Pathways correlate?
· Two alternative processes of carbon fixation to minimize photorespiration. In each case, carbon fixation produces the four-carbon oxaloacetate, which is processed to generate the CO2 that feeds into the Calvin (C3 ) cycle. (a) In C4 plants, carbon fixation and the Calvin cycle occur in different cell types: carbon fixation by the C4 pathway takes place in mesophyll cells, while the Calvin cycle takes place in bundle sheath cells. (b) In CAM plants, carbon fixation and the Calvin cycle occur at different times in mesophyll cells: carbon fixation by the C4 pathway takes place at night, while the Calvin cycle takes place during the day.
C4 plants spatially separate the Ca pathway and the Calvin cycle
CAM plants temporally separate the Capathway and the Calvin cycle
79
Photosynthesis and cellular respiration?
Schematic diagrams of the process of photosynthesis (left) and cellular respiration (right). Cellular respiration is shown upside down with respect to the direction of reactions to help illustrate the similarities of the process with photosynthesis.
79
Photosynthesis and cellular respiration?
Schematic diagrams of the process of photosynthesis (left) and cellular respiration (right). Cellular respiration is shown upside down with respect to the direction of reactions to help illustrate the similarities of the process with photosynthesis.
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