The Two Phases of Photosynthesis

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1 : light reactions & carbon fixation Global Importance of by green plants and algae provides nearly all of the energy and organic carbon required by living organisms. provides all of the oxygen required by aerobic organisms. The fossil fuels needed for human activity were produced by ancient photosynthetic organisms. is the largest scale biosynthetic process on Earth. Photosynthetic organisms remove approximately 200 billion tons of carbon, per year from the atmosphere. This is about 10% of the total CO 2 in the atmosphere. In energy terms, this is roughly 0.1% of the incident visible sunlight that reaches the Earth every year. Virtually all of the CO 2 "fixed" by plants is returned to the atmosphere by microbial, plant, and animal metabolism, and by biomass combustion. The Two Phases of Light Reactions (Z Scheme) Photolysis (H 2 O 2H / 2 O 2 ) Photophosphorylation (ADP + P ATP) NADP Reduction (NADP + H + NADPH) Dark Reactions (PCR,C3 or Calvin-Benson Cycle) Carbon Dioxide Fixation (CO 2 C 6 H 12 O 6 ) Overall reaction Some basic concepts on Radiation 6H 2 O + 6CO 2 C 6 H 12 O 6 + 6O 2

2 Radiation quality (or color, for visible light) is a function of its wavelength (or frequency) distribution The symbol λ is often used for wavelength Radiation quantity is measured in one of three ways, depending on the application: Note these two charts are arrayed in opposite directions one by increasing wavelength / decreasing energy and the other by increasing frequency/ increasing energy 1. Quantum measurements (numbers of photons) 2. Radiometric measurements (amount of energy) 3. Photometric measurements (light intensity, based on human perception) The amount of radiation is expressed as fluence (also known as density; quantity per area), rate (also known as flux; quantity per time) or fluence rate (also known as flux density; amount per area per time) Parameter Term Energy units Quantity per area Quantity per time Quantity per area per time Quantum units fluence J m -2 μmol m -2 rate (or flux) fluence rate (or flux density) J s -1 (watt) μmol s -1 W m -2 μmol m -2 s-1 For studies of photosynthesis and photomorphogenesis, the quantity of radiation is usually measured in quantum units (quantum flux density; quantum fluence rate): μmol m -2 s -1 PPFD = photosynthetically active photon flux density PAR = photosynthetically active radiation ( nm)

3 Spectral Quality The energy of a photon is proportional to its frequency and inversely proportional to wavelength: Energy per photon (joules) E = hν = hc/λ visible = nm, about 45% of incident insolation solar IR = nm, about 46% of incident UV = nm, about 9% of incident Planck s constant: 6.63 x joules s Frequency (s -1 ) Speed of light 3 X 10 8 m s -1 wavelength (in meters) All of the irradiance that impinges on an object is either reflected, transmitted or absorbed Reflectance, Transmission and Absorption Albedo = reflected / absorbed radiation. Emission = loss of energy that was absorbed. Fluorescence = another form of energy loss after bsorption) REFLECTED ABSORBED TRANSMITTED

4 Pigments: Molecules that absorb radiation in the visible wavelengths. The color of pigments results from the visible wavelengths that are not absorbed. Absorbed light energy causes electrons in pigment molecules to jump to higher energy levels (excited) Higher plants have two classes of pigments that are involved in energy capture and transfer for photosynthesis: chlorophylls (a and b). There is also a chlorophyll c in non-vascular plants) carotenoids there is a whole family of carotenoids; not all of them are involved in photosynthesis) Other pigments that are not involved in photosynthetic energy capture but serve photoprotective or photosensory roles: anthocyanins and phytochrome,. Chlorophylls and carotenoids are in chloroplasts Chlorophyll b Rings A,B,C and D form a porphyrin ring Anthocyanins are in the vacuole Phytochrome in the nuclear envelope? Function follows structure: The ring structure, with alternating double bonds and Mg atom at the middle leads to allowable excited energy states at relatively low levels (most proteins absorb highenergy uv light; pigments absorb lower energy visible light because of the resonance resulting from the molecular structure). The ring is hydrophilic. The phytol tail is hydrophobic; it is lipid soluble (cell membranes are made of lipids)

5 Carotenoids Chlorophyll a Chlorophyll b Β-carotene Phycoerythrin Chlorophyll c The carotenoids include: βcarotene, which is involved in light harvesting and Xanthophylls, which are involved in photoprotection. (Disruption of the photoprotection activity has been studied for development of possible herbicides) Anthocyanins are watersoluble pigments that do not participate in the reactions of photosynthesis. These are the pigments that give red color to plants. Chloroplasts are filled with thylakoid membranes that form stacks of grana They strongly absorb uv radiation in addition to visible light and are thought to have two primary functions attraction of insects and protection from uv radiation.

6 The membranes separate the inside lumen from the outside stroma. This structural organization is critical to the functions involved in energy capture. Most of the enzymes, pigments and electron carriers involved in light harvesting are imbedded in the thylakoid membranes Note the lipid bi-layer structure of the membrane OVERVIEW. The light reactions 1. Light harvesting 2. Water splitting (electron source) The dark reactions 3. Electron transport and photophosphorylation 4. Carbon fixation 5.Consumption; Transport At this juncture there is a transformation from flow of photons to flow of electrons

7 Energy from the excited state can be transferred to: 1. Chlorophyll fluorescence 2. Heat (= non photochemical quenching ) 3. (= photochemical quenching) of chlorophyll of chlorophyll The first step of photochemical electron transport ( photochemical quenching ) is the excitation of a chlorophyll reaction center, which then passes an electron to an acceptor molecule ( A ) The electrons from the first chlorophyll reaction center (PSII, or P680) are passed along a chain of acceptors until they reach another reaction center (PSI, or P700). Then this reaction center is then energetically primed to be excited by another photon absorption, and the electrons are finally passed along to other, higher energy acceptors (weak oxidants) This is the z-scheme, or photosynthetic electron transport This is where new electrons come into the cycle

8 Another perspective on the same process

9 The electrons from the electron transport chain are ultimately accepted by NADP+, which reduces this molecule to NADPH (an important source of reducing power for carbon fixation) The splitting of water results in an accumulation of protons in the thylakoid lumen. As electrons move along the electron transport chain, protons are also transported from the strma to the lumen. The result is an electrochemical gradient and across the membrane (lower ph in the lumen). Like a battery, this gradient is a form of potential energy, and is used to drive ATP production from ADP. Another perspective on the same process Photophosphorylation: adding a phosphate group onto adenosine diphosphate (ADP) to form ATP using energy from light.

10 A large class of herbicides operate by interrupting the photosynthetic electron transport chain: Turning to C assimilation steps of photosynthesis Carbon fixation Also known as : Dark reaction PCR : Photosynthetic carbon reduction cycle Calvin-Benson Cycle

11 3 steps of the Calvin-Benson cycle: Carboxylation Reduction Regeneration Rubisco activase is an enzyme that converts Rubisco from an inactive to an active form

12 Alternative mechanisms of carbon fixation have evolved in hot, arid climates One of the major problems facing terrestrial plants is dehydration. At times, solutions to this problem conflict with other metabolic processes, especially photosynthesis. The stomata are not only the major route for gas exchange (CO 2 in and O 2 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. In most plants (C 3 plants) initial fixation of CO 2 occurs via rubisco and results in a three-carbon compound, 3-phosphoglycerate. These plants include rice, wheat, and soybeans. When their stomata are closed on a hot, dry day, CO 2 levels drop as CO 2 is consumed in the Calvin cycle. At the same time, O 2 levels rise as the light reaction converts light to chemical energy. While rubisco normally accepts CO 2, when the O 2 /CO 2 ratio increases (on a hot, dry day with closed stomata), rubisco can add O 2 to RuBP. Photorespiration is initiated when RUBISCO acts as an oxygenase instead of a carboxylase: Calvin cycle Photorespiration The phosphoglycolate is recycled to produce more 3PGA, consuming energy and releasing more CO 2

13 Summary points of photorespiration: 1. The energy used to produce the RuBP acceptor is wasted 2. It takes even more energy to recycle the 2- carbon glycollate 3. High light, low CO 2 and high O 2 promote photorespiration 4. C 3 plants can lose up to ¼ of their newly fixed carbon through photorespiration 5. Photorespiration may have a role in reducing photoinhibition and photooxidation Certain plant species have evolved alternate modes of carbon fixation to minimize photorespiration. C4 and CAM The C 4 plants fix CO 2 first in a four-carbon compound. Several thousand plants, including sugercane and corn, use this pathway. In C 4 plants, mesophyll cells incorporate CO 2 into organic molecules. The key enzyme, phosphoenolpyruvate carboxylase, adds CO 2 to phosphoenolpyruvate (PEP) to form oxaloacetetate. PEP carboxylase has a very high affinity for CO 2 and can fix CO 2 efficiently when rubisco cannot - on hot, dry days with the stomata closed. The mesophyll cells pump these fourcarbon compounds into bundle-sheath cells. The bundle sheath cells strip a carbon, as CO 2, from the four-carbon compound and return the three-carbon remainder to the mesophyll cells. The bundle sheath cells then uses rubisco to start the Calvin cycle with an abundant supply of CO 2.

14 Kranz anatomy C 4 plants have specialized Kranz anatomy that separates the initial carboxylation by PEP carboxylase from the Calvin cycle, and concentrates CO2 around Rubisco.

15 Photorespiration does not occur in C 4 plants because they concentrate CO 2 around RUBISCO. CO 2 out competes O 2 for the active sites on the RUBISCO enzyme 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 a variety of organic acids in mesophyll cells. During the day, the light reactions supply ATP and NADPH to the Calvin cycle and CO 2 is released from the organic acids. Both C 4 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.

16 Assignment reading SYNTHESIS OF STARCH AND SUCROSE Pages Light quality (wavelength). Light is absorbed by pigment molecules. Chloroplasts contain pigments (chlorophyll a, chlorophyll b, and some carotenoids) that absorb mostly the red and blue portions of the visible wavelengths spectrum and reflect the green portion. Blue and red wavelengths are more photosynthetically active than green. In green plants, light is collected primarily by chlorophylls, pigments that absorb light at a wavelength below 480 nm and between 550 and 700

17 Blue and red wavelengths are more photosynthetically active than green. Light intensity (the amount of incident light energy absorbed by the leaf). The effect varies with different plants: Sun-loving plants: Some species require high light intensities to grow well (corn, potatoes, sugarcane). Shade-loving plants: Some plant species that do not grow well in high light intensities (dense shade of the forest floor, useful as house ornamentals). Intermediate: Other plant species grow well in moderately intense light. Light intensity plants. Light intensity distinctly affects the size and shape of leaves: Darker The leaves of a given plant species grow thinner and larger in area at the low light intensity than leaves grown at the high light intensity. Thinner The leaves of plants grown in high light intensities tend to be darker green than those grown in low light intensities.

18 Light compensation point and light saturation point. Light compensation point and light saturation point. Light compensation point is a light intensity at which photosynthesis and respiration rates are equal and net gas exchange is zero (CO2 is still exchanged). More photosynthesis than respiration Light saturation point: the plant is said to be light saturated when further increases in light intensity increase photosynthesis little or not at all. More respiration than photosynthesis Light intensity increasing Light compensation point of sun plants is higher than shade plants. Light saturation point: the plant is said to be light saturated when further increases in light intensity increase photosynthesis little or not at all. Light saturation point is variable and depends on CO2 concentration. The light intensity at which saturation occurs increases as the CO2 concentration surrounding the plant raises. At very high light intensities, the rate at which CO2 is available to the plant could limit the photosynthetic rate.

19 Light saturation point. Light saturation point increases as the CO2 concentration surrounding the plant raises. Light Saturation point More photosynthesis than respiration More respiration than photosynthesis Light intensity increasing Carbon Dioxide The carbon dioxide compensation point is the carbon dioxide concentration at equilibrium such that the amount of carbon dioxide evolved in respiration exactly equals the amount consumed in photosynthesis. Increasing the CO2 concentration in a closed system, such as a sealed greenhouse to about 0.10 % approximately doubles the photosynthetic rate of certain crops Applications of organic matter in the form of crop residues or green manure crops to the soil tends to increase CO2 levels in the atmosphere above the soil. Increasing the CO2 concentration in a closed system, such as a sealed greenhouse to about 0.10 % approximately doubles the photosynthetic rate of certain crops. In greenhouses, the use of horizontal airflow fans (HAFs) increases airflow around plants and exposes the leaves to a constant supply of CO2. The use of HAFs may eliminate the need for supplemental CO2 in most greenhouses.

20 Carbon dioxide compensation. Heat. If light and CO2 are not limiting, the rate of photosynthetic activity approximately doubles for each 10 C increase in temperature for many plant species in the temperate climates. Temperature affects all biochemical reactions of photosynthesis, so it is not surprising that the responses to temperature are complex. Heat. Water. Under conditions of drought (low soil moisture and hot, drying winds), plants often lose water through transpiration faster than their roots can absorb it. This causes: The stomata to close and the leaves to wilt temporarily. When this occurs, the exchange of CO2 and O2 is restricted. Resulting in a dramatic drop in photosynthesis.

21 Water. Water. Excessive soil moisture sometimes creates an anaerobic condition (lack of oxygen) around the roots. This causes: Reducing root respiration and mineral uptake and transport of water and minerals to the leaves. Thus indirectly depressing photosynthesis in the leaves. Plant development and the Source-Sink relationship. When the leaf reaches full expansion, it is often called a source leaf because the carbohydrate synthesized in that leaf is in excess of local requirements and is exported to other parts of the plant that are actively growing. These sites of active growth and metabolism are often called sink tissues. (Roots and reproductive organs). In many plant species, the photosynthesis rates in all the mature leaves begin to decline drastically when the plants flower and envelop fruits and seeds. (Proteins in the leaves, and specially photosynthetic enzyme proteins in the chloroplasts, are degraded to amino acids as the leaves begin the process called senescence. The amino acids in the leaves are transported to the developing seeds).