Life on Earth is solar powered. The chloroplasts of plants use a process called photosynthesis to capture light energy from the sun and convert it to chemical energy stored in sugars and other organic molecules.
Leaves are the major site of photosynthesis. ◦ About half a million chloroplasts per square millimeter of leaf surface. The color of a leaf comes from chlorophyll, the green pigment in the chloroplasts. ◦ Plays an important role in the absorption of light energy during photosynthesis.
Site of photosynthesis
Chloroplasts are found mainly in mesophyll cells O 2 exits and CO2 enters through microscopic pores, stomata Veins deliver water from the roots and carry off sugar from mesophyll cells to other plant areas. A typical mesophyll cell has 30-40 chloroplasts, each about 2-4 microns by 4-7 microns long.
Each chloroplast has two membranes around a central aqueous space, the stroma. In the stroma are membranous sacs, the thylakoids. ◦ These have an internal aqueous space, the thylakoid lumen or thylakoid space. ◦ Thylakoids may be stacked into columns called grana.
Thylakoids Stroma GranumInner & Outer Membranes
Powered by light, the equation describing the net process is: 6CO 2 + 12H 2 O + light energy -> C 6 H 12 O 6 + 6O 2 + ATP O 2 given off by plants comes from H 2 O, not CO 2 . Hydrogen extracted from water is incorporated into sugar and the oxygen is released
Water is split and electrons transferred with H + from water to CO 2 , reducing it to sugar. Light boosts the potential energy of electrons as they move from water to sugar.
Photosynthesis is two processes. The light reactions convert solar energy to chemical energy (ATP & NADPH) The Calvin cycle incorporates CO2 from the atmosphere into an organic molecule and uses energy from the light reaction to reduce the new carbon piece to sugar.
While the light reactions occur at thethylakoids, the Calvin cycle occurs in the stroma.
Light-Dependent Reactions H2O O2 (in thylakoids) Depleted Energized Carriers Carriers (ADP, NADP+) (ATP, NADPH) Light-Independent ReactionsGlucose CO2+H2O (in stroma)
Light, like other electromagnetic energy, travels in rhythmic waves. The entire range of electromagnetic radiation is the electromagnetic spectrum. The most important segment for life is a narrow band between 380 to 750 nm, visible light. The energy of is inversely proportional to the wavelength: longer wavelengths have less energy than do shorter ones.
While light travels as a wave, many of its properties are those of a discrete particle, the photon. Photons with shorter wavelengths pack more energy. When light meets matter, it may be reflected, transmitted, or absorbed.
In the thylakoid are several pigments that differ in their absorption spectrum. Chlorophyll is by far the most important Chlorophyll a , the dominant pigment, absorbs best in the red and blue wavelengths, and least in the green.
Chlorophyll b , with a slightly different structure, has different absorption spectrum and funnels the energy from these wavelengths to chlorophyll a .• Other pigments with different structures have different absorption spectra.
All chlorophylls comprises a complex ring system called a porphyrin ring & a long hydrocarbon tail The tail is lipid soluble & therefore embedded in the thylakoid membrane The porphyrin ring is hydrophilic & lies on the membrane structure
The second group of pigments involved are the carotenoids Basic structure comprises two small rings linked by a long hydrocarbon chain They absorb light in the blue-violet range Two main types of carotenoids: ◦ Carotenes ◦ Xantophyll
Only chlorophyll a participates directly in the light reactions but accessory photosynthetic pigments absorb light and transfer energy to chlorophyll a . Carotenoids can funnel the energy from other wavelengths to chlorophyll a and also participate in photoprotection against excessive light.
When a molecule absorbs a photon, one of that molecule’s electrons is elevated to an orbital with more potential energy. The electron moves from its ground state to an excited state. In chlorophyll a and b , it is an electron from magnesium in the porphyrin ring that is excited.
Excited electrons are unstable. Eventually, they drop to their ground state, releasing heat energy. In the thylakoid membrane, chlorophyll is organized along with proteins and smaller organic molecules into photosystems.
A photosystem acts like a light- gathering “antenna complex” consisting of a few hundred chlorophyll a , chlorophyll b ,and carotenoid molecules.
When any antenna molecule absorbs a photon, energy is transmitted from molecule to molecule until it reaches a particular chlorophyll a molecule, the reaction center. At the reaction center is a primary electron acceptor which removes an excited electron from the reaction center chlorophyll a. This starts the light reactions.
Photosystem I has a reaction center chlorophyll, the P700 center, that has an absorption peak at 700nm. Photosystem II has a reaction center P680 with a peak at 680nm. These two photosystems work together to use light energy to generate ATP and NADPH.
Light dependent reactions Photoactivation of photosystem I andphotosystem II Photolysis of water Production of NADPH and ATP Cyclic and non-cyclic photophosphorylation
During the light reactions, there are two possible routes for electron flow: cyclic and noncyclic
Uses two photosystems, PS I (P700) and PS II (P680). ◦ Electrons energized by absorption of light and pass through electron transport chain to NADP+ ◦ Series of redox reactions ◦ ATP and NADPH formed
Noncyclic electron transport
• Photon strikes both PSI & PSII and the energy absorbed isfunneled/passed to the reaction center• PSI & PSII become photoactivated and energized electrons arereleased & accepted by primary electron acceptors (higher energylevel)•PSI & PSII are oxidized & become unstable
Both Primary Electron Acceptors are later oxidizedreleasing electrons to the Electron Transport Chain
PSII/ P680•Photoexcited electrons from PSII pass along an ETC (Quinone,cytochrome complex & Plastocyanin) before ending up at anoxidized PSI reaction center thus stabilizing the oxidized PSI.•As these electrons pass along the chain, their energy is harnessedto produce ATP (via chemiosmosis)•The mechanism is similar to the process on oxidativephosphorylation.
PSI/ P700•Excited electrons from PSI pass along ETC (Ferredoxin,NADP reductase)• These electrons are passed from the ETC to NADP+,reducing it to produce NADPH.• NADPH is a reducing agent carrying high-energy electrons tothe Calvin cycle.
Electrons from PSII flow to PSI, restoring the missingelectrons in the reaction center of P700. This however, leavesthe PSII reaction center with missing electrons; thus theoxidized PSII becomes a strong oxidizing agent (unstable)
Photolysis of Water• The splitting of water molecules (with the aid of photon) to produceelectrons, H+ & O2 2H2O ------------- 4H+ + O2 + 4e•The H+ builds up in the thylakoid lumen, causing a proton gradient(for chemiosmosis)• O2 is given off or used in respiration• Electrons are used to stabilize chlorophyll in photosystem II
The main objectives of The Non Cyclic Photophosphorylation are:• to produce ATP• to produce NADPH (reducing agent)Both will be used in the Ligt Independent Reactions
Under certain conditions, photoexcited electrons from photosystem I, can take an alternative pathway, cyclic electron flow. Excited electrons cycle from their reaction center to a primary acceptor, along an electron transport chain, and return to the oxidized P700. As electrons flow along the electron transport chain (Fd, cytochrome complexes & pC), they generate ATP by cyclic photophosphorylation.
Cyclic electron transport ◦ Electrons from photosystem I returned to photosystem I ◦ ATP produced by chemiosmosis ◦ No NADPH or O2 generated
Noncyclic photophosphorylation produces ATP and NADPH in roughly equal quantities. However, the Calvin cycle consumes more ATP than NADPH. Cyclic photophosphorylation allows the chloroplast to generate enough ATP to satisfy the higher demand for ATP in the Calvin cycle.
Chloroplasts and mitochondria generate ATP by the same mechanism: chemiosmosis. An electron transport chain pumps protons across a membrane as electrons are passed along ETC. This builds the proton-motive force in the form of an H + gradient across the membrane. ATP synthase molecules harness the proton-motive force to generate ATP as H + diffuses back across the membrane through the enzyme.
Mitochondria transfer chemical energy from food molecules to ATP and chloroplasts transform light energy into the chemical energy of ATP. The light-reaction “machinery” produces ATP and NADPH on the stroma side of the thylakoid.
The cycle spends the energy of ATP and the reducing power of electrons carried by NADPH to make the sugar. The actual sugar product of the Calvin cycle is not glucose, but a 3C sugar, glyceraldehyde-3-phosphate (PGAL/ G3P).
Each turn of the Calvin cycle fixes one CO2. For the net synthesis of one G3P molecule (3C), the cycle must take place three times, fixing three molecules of CO2. To make one glucose molecules (6C) would require six cycles and the fixation of six CO 2 molecules.
In the carbon fixation phase, each CO 2 molecule is attached to a five-carbon sugar, ribulose bisphosphate (RuBP). ◦ Catalyzed by RuBP carboxylase or rubisco. ◦ The six-carbon intermediate splits in half to form two molecules of 3-phosphoglycerate (PGA) per CO 2 .
During the carbon reduction phase, each 3-phosphoglycerate receives another phosphate group from ATP (phosphorylation) to form 1,3- bisphosphoglycerate. A pair of electrons from NADPH reduces each 1,3-bisphosphoglycerate to produce Glyceraldehyde-3-phosphate (PGAL/G3P).
If our goal was to produce one PGAL/G3P net, we would start with 3 CO 2 (3C) and three RuBP (15C). After fixation and reduction we would have six molecules of PGAL/G3P (18C). ◦ One of these six G3P (3C) is a net gain of carbohydrate. This molecule can exit the cycle to be used by the plant cell. ◦ The other five (15C) must remain in the cycle to regenerate three RuBP.
One of the major problems facing terrestrial plants is dehydration. The stomata are not only the major route for gas exchange (CO2 in and O2 out), but also for the evaporative loss of water. On hot, dry days plants close the stomata to conserve water, but this causes problems for photosynthesis.
When their stomata are closed, CO 2 levels drop as CO2 is consumed in the Calvin cycle. At the same time, O 2 levels rise as the light reaction converts light to chemical energy. In most plants (C 3 plants) initial fixation of CO 2 occurs via rubisco and results in a 3C compound, 3- phosphoglycerate. ◦ These plants include rice, wheat & soybeans.
While rubisco normally accepts CO2, when the O 2 /CO 2 ratio increases (on a hot, dry day with closed stomata), rubisco can add O 2 to RuBP. When rubisco adds O2 to RuBP, RuBP splits into a 3C compound [3- phosphoglycerate (PGA)] and a 2C compound [phosphoglycolate] in a process called photorespiration.
The two-carbon fragment is exported from the chloroplast and degraded to CO2 ◦ this process produces no ATP, nor additional organic molecules. Photorespiration can drain away as much as 50% of the carbon fixed by the Calvin cycle on a hot, dry day.
In C3 photosynthesis, RuBP is carboxylated to form a 3C compound via the activity of rubisco Other plants use C4 photosynthesis, in which phosphoenolpyruvate, or PEP, is carboxylated to form a 4C compound using the enzyme PEP carboxylase or pepco
Pepco has a much greater activity for CO 2 than rubisco does The 4C compound undergoes further modification, only to be decarboxylated (to release CO2) The CO 2 released is then captured by rubisco and drawn into the Calvin cycle Since an organic compound (the 4C compound) is donating the CO2, the concentration of CO 2 relative to O 2 is increased, and the photorespiration is minimized
Plants that adapted to warmer environments have evolved 2 principal ways that use the C4 pathway to deal with this problem ◦ Conduct C4 photosynthesis in the mesophyll cells ◦ The Calvin cycle in the bundle sheath cells Sugercane and corn, use this pathway.
There are 2 distinct types of photosynthetic cells: • 1). Bundle-sheath cells • 2). Mesophyll cells This distinctive arrangement is called Kranz anatomy. In C 4 leaf, bundle sheath cells and mesophyll cells contain chloroplasts. Mesophyll cells are arranged concentrically around the bundle sheath cells.
C3 and C4 plant structure compared
• C4 plants, mesophyll cells incorporate•In Decarboxylation of the organic molecules release CO2CO2 into organic molecules & transport themto•the bundle-sheath cells then use rubisco to The bundle-sheath cells start the Calvin cycle with the abundant supply of CO2
The enzyme, Pepco, adds CO 2 to phosphoenolpyruvate (PEP) to form oxaloacetetate (OAA). Oxalacetate is in turn converted into the intermediate malate, which is transported to an adjacent bundle- sheath cell.
Inside the bundle-sheath cell, malate is decarboxylated to produce pyruvate, releasing CO 2 The bundle-sheath cells then use rubisco to start the Calvin cycle with an abundant supply of CO 2 . Pyruvate returns to the mesophyll cell where it is phosphorylated by ATP to become phosphoenolpyruvate PEP, thus completing the cycle
Summaryof the C4pathway
C3 C4CO2 fixation Occurs once Occurs twice, first in mesophyll cells, then in bundle sheath cellsCO2 acceptor RuBP, a 5C Mesophyll cells Bundle sheath compound PEP, a 3C cells compound RuBPCO2 fixing enzyme RuBP carboxylase PEP carboxylase RuBP which is very carboxylase efficientFirst product of A C3 acid, G3P A C4 acid, OxaloacetatephotosynthesisPhotorespiration Occurs ; therefore Is inhibited by high CO2 O2 is an inhibitor of concentration. Therefore photosynthesis atmospheric O2 not an inhibitor of photosynthesisEfficiency Less efficient than More efficient than C3 plants. C4 plants. Yields Yields usually much higher usually much lower
A second strategy to minimize photorespiration is found in succulent plants, cacti, pineapples, and several other plant families. These plants, known as CAM plants for crassulacean acid metabolism (CAM), open stomata during the night and close them during the day. Temperatures are typically lower at night and humidity is higher.
During the night, these plants fix CO 2 into organic acids in mesophyll cells using the C4 pathway. These organic compound accumulate throughout the night (in the vacuole) and are decarboxylated during the day During the day, the light reactions supply ATP and NADPH to the Calvin cycle and CO 2 is released from the organic acids.
Both C4 and CAM plants add CO 2 into organic intermediates before it enters the Calvin cycle. In C4 plants, carbon fixation and the Calvin cycle are spatially separated. In CAM plants, carbon fixation and the Calvin cycle are temporally separated. Both eventually use the Calvin cycle to incorporate light energy into the production of sugar.
Types of photosynthesis CO2 CO2 CO2 night C C4 Mesophyll 4 cell RuBP Bundle day CO2 CO2 sheath Calvin cell cycle Calvin Calvin PGA cycle cycle (C3) PGAL Mesophyll cell PGAL PGALCO2 fixation in a C3 plant CO2 fixation in a C4 plant CO2 fixation in a CAM plant