Chapter 10 bio photosynthesis.docx

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BIOL 1130
Dr.Marcus Freeman

Chapter 10 – Energy Metabolism: Photosynthesis Lecture 19 • sunlight maintains and increases the orderly of life by two methods 1) directly: photosynthesis, which produces complex organic compounds 2) indirectly: respiration of organic compounds, or another organism eats it • photoautotrophs - gather energy directly from light and use it to assimilate small inorganic molecules into their own tissues. - all green plants, few bacteria undergo photosynthesis - must build all of their own molecules using CO2, water, nitrates, sulfates •heterotrophs - take in organic molecules and respire them and obtain energy in them - include all animals and all parasitic plants, all fungi, and nonphotosynthetic parasitic plants • the way these organisms get nutrients influence the bodies and metabolism of plants and animals • Plants can have both photoautotrophic and heterotrophic tissue - chlorophyllous leaves and stems are photo. - wood and flowers are hetero. and survive by carbohydrates by phloem •Photosynthesis is a complex process by which CO2 is concerted to carbohydrate - light is converted to chemical energy as it is captured by plant pigments - energized pigments can only enter 2 chemical reactions - energy carries transfer energy from pigment to endergonic reactions (ATP, NADPH) • Photosynthetic reactions produce ATP - ATP’s phosphate bonds carry enough energy to force almost any reaction to proceed - Each ATP molecule is recycled and reused repeatedly, thousands of times per second • ADP can be phosphorylated to ATP by 3 methods: (all occur in plants): 1. Photophosphorylation – photosynthesis; chloroplasts only 2. Substrate-level phosphorylation – respiration; cystol – animals, fungi 3. Oxidative phosphorylation – respiration; mitochondria • 21% oxygen in atmosphere, therefore, many compounds are found in their oxidized form - oxidized: atoms don’t carry as many electrons as it could - reduced: electrons are added to an atom • To convert CO2 to carbohydrates a plant needs both energy and reducing power • Moving and handling electrons requires small molecules that are semistable and mobile: - Nicotinamide adenine dinucleotide (NAD+) - Nicotinamide adenine dinucleotide phosphate (NADP+) oxidizing agents—they oxidize • The process forms NADH and NADPH, two strong reducing agents the material they react with • The tendency to accept or donate electrons varies and is known as a molecule’s redox potential Other electron carriers include: • Cytochromes are intrinsic membrane proteins - They contain a cofactor, heme, which holds an iron atoms - They carry electrons only between sites that are extremely close together within a membrane • Plastoquinones, like cytochromes, transport electrons over short distances within a membrane • Plastocyanin is small protein that carries electrons on a copper atoms - it is loosely associated with chloroplast membranes Photosynthesis • Water and CO2 are abundant, cheap, and diffuse into the plant automatically - they are also very stable and contain little chemical energy, so large amounts of energy can be put into them • despite being energy rich, carbohydrates are stable and chemically unreactive • both the reactants and the products of photosynthesis are nontoxic 6CH₂ + 6H₂O + energy -> C₆H₁₂O₆ + 6O₂ • In photosynthesis, the carbon of CO2 is reduced end energy is supplied to it, to form carbohydrates - electron source is water, and energy source is light • Water and light, however, do not act on CO2 directly - light-dependent reactions create ATP and NADPH - stroma reactions is where ATP and NADPH interact with CO2 and produce carbohydrates • Light is one small segment of the electromagnetic radiation spectrum • light can be treated as sets of photos (quanta) or sets of waves - short wavelengths (λ) have high energy - longer wavelengths have little • Pigments are any material that absorbs certain wavelengths • When a pigment absorbs light energy, an electron is raised to an orbital of higher energy level - the electron goes from ground state to an excited state - the electron can fluoresce and return to its original orbit - the electron can move to a more stable orbital on an entirely different atom • Absorption spectrum is the wavelengths of light a pigment absorbs – chlorophyll-a absorbs red & blue • Action spectrum shows which wavelengths are most effective at powering a photochemical process • Accessory pigments are molecules that strongly absorb wavelengths not absorbed by chlorophyll-a - most common accessory pigment in land plants: chlorophyll-b and carotenoids • Absorbed energy passes to chlorophyll-a via resonance - chlorophyll-b is very efficient at passing energy - carotenoids are poor and are important in absorbing excess light the protecting the chlorophylls • To control the very reactive electron of an excited chlorophyll-a, all the working components are packed into a granule, a photosynthetic unit - units with little chlorophyll-b are photosystem I (PSI) - units with chlorophyll-b is present at levels almost equal to chlorophyll-a are photosystem II (PSII) Photosystem I • When light strikes any pigment of a PSI array, energy is transferred to the reaction center - at the reaction center is a pair of special molecules of chlorophyll called P700 (absorb red light) Photosystem II • PSII is responsible for the reduction of P700 (absorbs red light) , allowing PSI to work repeatedly • First, a molecule of plastocyanin donates an electron to the chlorophyll-a of the PSI reaction center • It receives its new electron from the cytochrome b6/f complex, which in turn gets an electron from a molecule of plastoquinone •Plastoquinone receives electrons from another carrier, Q (quoinon), which in turn receives electrons from phaeophytin • Phaeophytin obtains another electron when a chlorophyll a molecule absorbs light and is activated • This chlorophyll-a is the reaction center of PSII and has the name P680 • Electrons PSII donates come from water: electrons are stripped off , the protons used, and oxygens discarded • Both photosystems work together Summary • electrons are passed from water to the P680s in PSII • their energy is boosted by light • they move through an electron transport chain to P700 in PSI • their energy is boosted by light again • they pass through a short second electron transport chain to NADP+ • this reduces it to NADPH Light-Dependent Reactions: ATP • NADPH now places electrons onto the carbon of CO2 in the stroma reactions • the stroma reactions are highly endergonic and must be driven by being coupled to the exergonic splitting of ATP - chemiosmosis phosphorylation generates the ATP - possible due to the structure of the chloroplast • the thylakoid membrane, thought contiguous, is organized into 2 regions: Stroma Lamella and Grana • create a compartment, known as the thylakoid lumen • the thylakoid lumen is critical to the production of ATP • the reactions that break down water, and thus produce protons and oxygen, are located on the lumen side of the thylakoid membrane • the impermeability of the thylakoid allows the accumulation of H+ and creates a proton motive force - the proton flow is channeled through an enzyme complex - ATP synthetase - ATP synthetase phosphorylated ADP to ATP • When electrons flow smoothly from water to NADPH, the process is called noncyclic election transport • there is too little ATP produces this way relative to the amount of NADPH produced • after electrons reach ferredoxin in photosystem I, they can be transferred to the plastoquinones of photosystem II instead of being used to make NADPH - the plasoquinones carry the electrons along just as though they has gotten them from Q Cyclic Electron Transport • they use their energy to pump more protons into the thylakoid lumen • with this cyclic electron transport, chloroplasts make the extra ATP needed for the stroma reactions Stroma (light-independent) Reactions • Converstion of carbon dioxide to carbohydrates occurs in the stroma reactions - also called the Calvin/Benson cycle, or the C3 cycle • in the first step is carbon fixation - an acceptor molecule (RuBP) reacts with a molecule of carbon dioxide - new molecule breaks into 2 identical molecules each containing 3 phosphoglycerate (PGA) • the enzyme that carries out this reaction is RuBP carboxylase (RUBISCO) • RUBISCO is one of the largest most complex enzymes known - a quaternary enxyome with 8 small and 8 large protein - makes up 30% protein in a leaf, most abundant protein on earth - critical to food production, without it heterotrophs would starve Stroma Rendtions: Reduction • in the 2 step, ATP donates a high-energy phosphate group to the 3-phosphoglycerate, converting it to 1,3- diphosphoglycerate - then NADPH reuces 1,3-diphosphoglycerate to 3-phosphoglyceraldehyde (PGAL) • the carbon is now both reduces and energized • the rest of the reactions are complex and involve the regeneration of RuBP - some PGAL can be taken out of chloroplast and used by cell Anabolic metabolism • 3-phosphoglyceraldehyde is a very versatile molecule • the synthetic pathways of polysaccharides and fats are important because NADPH and ATP cannot be stored for even a short time Synthesis of Polysaccharides • the anabolic synthesis of glucose is gluconeogenesis - some PGAL exported to cytoplasm, some is converted to dihydroxyacetone phosphate - a molecule of each condenses together to form the sugar fructose-1 - this loses a phosphate to become fructose-6-phosphate - part of fructose-6-phosphate is rearranged, converting it to a glucose-6-phosphate • fructose-6-phosphate& glucose-6-phospate are versatile and enter many metabolic pathways Environmental and Internal Factors: Light • photosynthesis is affected by the environment in many ways - Quality of sunlight – the colours or wavelengths it contains - Quantity of sunlight – light intensity or brightness • the light compensation point is the level of light at which photosynthesis matches respiration - plants grow for a long time in conditions below the light compensation point respire faster than they photosynthesize and starve to death • light can become too intense, and some plants in bright environments have developed adaptations to reflect light, such as thick trichomes or heavy cuticle • the duration of light refers to the number of hours per day that sunlight is available • when winter days are short and sunlight is weak, most plants can survive by stored nutrients • in many plants, longer days cause greater amounts of photosynthesis, in others, chloroplast become so full of starch that photosynthesis stops, even though light is present - at night, starch is converted into sugar - the sugar is transported out of chloroplasts and can be used for growth or stored - by morning, leaf chloroplasts can resume photosynthesis Environmental and Internal Factors: Leaf • In hot, dry habitats, plant lead cells are packed closely without intercellular space or has cylindrical leaves which reduces external surface area • this causes photosynthesis and growth to slow down because carbon dioxide absorption slows Environmental and Internal Factors: Water • the balance between water loss and photosynthesis is critical - most plants keep their stomata open during the day, if water stressed stomata will close - this prevents entry of CO2 and reduces photosynthetic activity Environmental and Internal Factors: C4 • an important factor for plants is the amount of H20 lost for each molecule of CO2 absorbed • in C4 metabolism, C02 is absorbed, moved, and concentrated – oxygen is kept away from RUBP • occurs in leaves with Kranz anatomy (VB have sheath of chlorophyllous cells) •Mesophyll cells contain the enzyme PEP carboxylase which as no affinity for oxygen • the bundle sheath chloroplasts primarily carry out cyclic electron transport - without noncyclic electron transport, there is no breakdown of water or production of oxygen • photorespiration increases with temperature, so the selective advantage of C4 metabolism depends on the environment - under warm, dry conditions, C4 metabolism has a strong selective advantage over C3 metabolism Environmental and Internal Factors: CAM • CAM (Crassulacean Acid metabolism) is a second metabolic adaptation that improves conservation of water while permitting photosynthesis • CAM is almost identical to the in io plants, EXCEPT - Malate is not transported, in effect storing CO2 (only occurs at night when stomata is open) • Stomata are closed during the hottest periods and open only at night when it is cool to reduces transpiration - In the daytime, when stomata close, malate or other acids break down, releasing CO2 for C3 metabolism •CAM is not particularly efficient - The total amount of carbon dioxide is so small that it may be entirely used after just a few hours of sunlight - CAM is selectively advantageous in a hot, very dry climate where survival rather than rapid growth is most important - CAM plants have other metabolic and structural adaptations for water conservation Chapter 11: Energy Metabolism, Respiration Lecture 20 • Energy stores in complex organic molecules must be released for use by cells - even photosynthetic cells need to get energy when light is not available • respiration is the process that breaks down the complex carbon compounds into simpler molecules - Simultaneously generates the ATP used to power other metabolic processes -Carbon is oxidized as electrons are removed by NAD+, which is converted to NADH in the process - NADH is oxidized to generate more ATP - Also, respiration provides many of the intermediate compounds used in several anabolic pathways • NADH carries electrons to an ETC - Deposits them onto oxygen, reducing it and attracting protons to form water - Energy is conserved as high-energy phosphate bonding orbitals of ATP Types of Respiration • 2 types of respiration: aerobic and anaerobic aerobic – requires oxygen as the terminal electron acceptor, animals and plants anaerobic – respiration without oxygen, often called fermentation • Facultative aerobes (fungi and certain tissues in animals and some plants) can switch depending if oxygen is present or not. (animals can but not for very long) Anaerobic Respiration • Glucose is broken down during anaerobic respiration by glycolysis or the Embden-Meyerhoff pathway - Substrate-level phosphorylation generates ATP using the enzyme phosphoglycerate kinase - It removes phosphate groups from substrates • Once pyruvate is formed and no oxygen is present, all the energy possible has been removed - From each molecule of glucose, there is a net production of two ATPs • To continue glycolysis NADH must be oxidized to NAD+. - Reducing power can be used for anabolic reactions • Far more NADH than needed, alternate acceptors for electrons - Animal tissue = pyruvate - Plant tissue = acetaldehyde = ethanol • Although the method is far from ideal, it does allow certain organisms to survive in particular environments - Keeps glycolysis going but regeneration the NAD+ - Organisms survive on the two ATP from glycolysis Facultative Anaerobes – Rice Seeds • Under flood conditions, rice seeds, facultative anaerobes, have an advantage over strictly aerobic weeds. • When flooding subsides, oxygen is available and non-rice seeds can germinate • The larger rice can outcompeted the weeds. Aerobic Respiration • Oxygen is usually abundant, inexpensive and non-toxic - It acts as the terminal electron acceptor •Aerobic respiration consists of three parts: - Glycolysis - The citric acid cycle - Oxidative phosphorylation in an electron transport chain Aerobic Respiration: Glycolysis • Glycolysis by the Embden-Meyerhoff pathway to pyruvate. - occurs just as in anaerobic respiration, - occurs In the cytosol - Products are ATP and NADH - With oxygen present, electron acceptors of the ETC oxidize NADH to NAD+ • Pyruvate can now be used in various metabolic pathways, but it often breaks down to generate ATP in the citric acid cycle - also called the Krebs cycle or the tricarboxylic acid cycle Ae
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