AP Biology Chapter 10 Notes Photosynthesis I.
Overview: a. Photosynthesis: capturing light energy from the sun and converting it to chemical energy b. Autotrophs: self-­‐ feeders-­‐ organisms that can convert light energy from the sun to chemical energy i. Self sustaining ii. Organic molecules produced from CO2 iii. Ultimate source of organic compounds for all non-­‐autotrophic organisms iv. Often called producers v. Most plants, algae, some protists and some prokaryotes vi. Consume CO2 and produce O2 in the process c. Heterotrophs: live on compounds produced by autotrophs i. Unable to make their own food ii. Consumers iii. Decomposers-­‐ consume dead organisms 1. Fungi and many types of prokaryotes iv. Consume oxygen and produce CO2 in the process II.
Chapter 10.1: Photosynthesis converts light energy to the chemical energy of food. a. Chloroplasts: The sites of photosynthesis i. Chloroplasts are found in the green parts of the plant 1. Major site of photosynthesis is in the leave 2. About ½ million chloroplast / square millimeter of leaf surface 3. Found in the mesophyll (spongy and palisade) a. Mesophyll contains about 30 to 40 chloroplasts 4. About 2-­‐4 μm by 4-­‐7μm 5. Double membrane 6. Stroma: dense fluid in the chloroplast 7. Thylakoids: interconnected membranous sacs a. Stacked in columns called grana b. Contains chlorophyll 8. Thylakoid space ii. Chlorophyll: green photosynthetic pigment found in the chloroplast 1. Light energy absorbed by the chlorophyll drive photosynthesis iii. Stomata: microscopic pores by which CO2 enters the leaf and O2 exists. b.
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iv. Phloem and Xylem-­‐ transport vessels bringing water and nutrients from the soil Tracking Atoms Through Photosynthesis: i. 6CO2 + 12H2O + light energy è C6H12O6 + 6H2O The Splitting of Water: i. The oxygen given off by plants comes from water and not carbon dioxide 1. Chloroplasts splits water into hydrogen and oxygen 2. 1930-­‐ CB van Niel of Stanford University challenged the conventional theory that CO2 split into carbon and O2, then the addition of water formed the glucose molecule (CH2O-­‐ general formula for carbohydrates) a. Some bacteria use H2S instead of water (purple sulfur bacteria) leaving yellow globules of sulfur as a waste product b. Niel concluded that all photosynthetic organisms need a source of hydrogen, whether it be from H2O, H2S or some other source. i. General formula 1. CO2 + 2H2Xè|CH2O| + H2O + X 3. Scientists later confirmed Niels work by tagging the Oxygen from water and carbon dioxide with isotopes (oxygen-­‐ 18) Photosynthesis as a redox process: i. Like cellular respiration, photosynthesis is a redox process 1. In cellular respiration, energy released from sugar when electrons are transported by carriers to oxygen forming water as a by product a. Fall down process-­‐ towards the very electronegative process 2. Photosynthesis reverses the flow of the electrons a. Starts with the splitting of water by a photon i. Electrons are then transferred with H+ from water to CO2, reducing it to a sugar 1. Electrons are increasing in potential energy 2. Energy boost provided by photons The Two Stages of Photosynthesis: i. Stage one: light reactions (photo) 1. Converts solar energy to chemical energy 2. Light absorbed by chlorophyll drives a transfer of electrons and hydrogen from water to an electron acceptor called: NADP (nicotineamide adnenine dinucleotide phosphate) which temporarily stores the electrons a. NADP+ gets reduced to NADPH III.
b. Cousin to NAD+ 3. Water is split and O2 is released as a by product 4. ATP is generated in the process a. Chemiosmosis adds a phosphate group to ADP i. Photophosphorylation 5. Summary: light energy is converted to chemical energy in the form of NADPH and ATP ii. Stage two: calvin cycle (synthesis) 1. Melvin Calvin-­‐ 1940’s 2. Incorporates CO2 from the air into organic molecules a. Carbon fixation-­‐ reduction of the fixed carbon to carbohydrates by the addition of electrons b. Requires chemical energy in the form of ATP i. Generated by the light reaction 3. Sometimes referred to as the dark reaction or light independent reactions a. Occurs during daylight because it requires the reduction of NADP+ Chapter 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH: a. The nature of sunlight: i. Wavelength = the distance between the crests of electromagnetic waves ii. Electromagnetic spectrum: the range of radiation 1. Photosynthesis uses electromagnetic radiation form the visible light range: 380 – 750 nm a. Visible range because its detected by the naked eye. b. Photons: discrete particles of light c. Photosynthetic pigments i. Matter reflects, transmits or absorbs light ii. Pigments absorb light 1. Different pigments absorb different wavelengths a. Wavelengths that are absorbed disappear i. The absorbance of all pigments = black ii. The reflection of all wavelengths = white 2. The ability to absorb or reflect pigments can be measured with a spectrophotometer a. Measures the fraction of light transmitted at a each wavelength b. Absorption spectrum: a graph plotting a pigments light absorption versus wavelength iii. Three types of chlorophyll pigments: 1. Chlorophyll a-­‐ violet-­‐ blue and red light work best for photosynthesis a. Blue-­‐ green in color 2. Chlorophyll b-­‐ slightly different in structure a. Yellow-­‐ green in color 3. Carotenoids: hydrocarbons that are various shades of yellow and orange because they absorb violet and blue-­‐
green light iv. Excitation of chlorophyll by light 1. When a molecule absorbs a photon of light, one of the electrons is elevated to an orbital where it has more potential energy a. Ground state-­‐ when a pigments electrons are not excited b. Excited state-­‐ when a pigments electrons are excited d. A photosystem: A reaction center associated with light harvesting complexes. i. Photosystem: composed of a reaction center surrounding by a number of light harvesting complexes ii. Light harvesting complex consists of pigment molecules bound to particular proteins 1. Pigments: chlorophyll a and b and carotenoids iii. Reaction center: a protein complex that includes two special chlorophyll a molecules and another molecule called a primary electron acceptor. 1. Transferring an electron from the chlorophyll a to the primary electron acceptor is the first step of the light reaction. iv. Thylakoid membrane contains two types of photosystems(named in order of their discovery). 1. Photosystem II (PSII)-­‐ functions first a. P680-­‐ the reaction center in chlorophyll a of PSII 2. Photosytem I (PSI)-­‐ discovered/ named first a. P700-­‐ the reaction center in chlorophyll a of PSI 3. P680 and P700 are identical chlorophyll a molecules, but are associated with different proteins in the thylakoid membrane and affects the distribution of electrons in the chlorophyll molecule and accounts for slight differences in light absorbing properties e. Noncyclic electron flow: i. Light strikes a pigment molecule and excites one of the electrons of P680 in PSII ii. Electron is captured in primary electron acceptor iii. Enzyme splits water molecule into two electrons, two hydrogen ions and an oxygen atom 1. When P680 is missing an electron it becomes a very powerful oxidizing agent IV.
iv. Each electron is passed down an electron transport chain from PSII to PSI. 1. Electron transport chain is composed of plastoquinone(Pq), cytochrome complex, and plastcyanin (Pc) v. Electrons fall, exergonic process providing energy to produce ATP vi. Electrons get excited by a photon in PSI’s primary electron acceptor to a second electron transport chain through the protein ferrodoxin (Fd) vii. NADP+ reductase transfers electrons from Fd to NADP+ reducing it to NADPH (requires two electrons) f. Cyclic electron flow: i. Photoexcited elctrons from PSI are occasionally shunted back from ferredoxing (Fd) to chlorophyll via the cytochrome complex and plastocyanin (Pc) 1. Supplies energy for ATP 2. No NADP+ is reduced Chapter 10.3: The Calvin Cycle uses ATP and NADPH to convert CO2 to sugar: a. Calvin cycle is similar to cirtic acid cycle i. Citric acid cycle is catabolic-­‐ oxidizing glucose and releasing enegy ii. Calvin cycle is anabolic-­‐ building sugar from smaller molecules and consuming energy b. Calvin cycle does not produce a six carbon glucose directly i. It produces a 3 carbon sugars called glyceraldehyde 3-­‐ phosphate (G3P) ii. Net synthesis of one molecule of glucose, the cycle must take place three times 1. Fixing three molecules of CO2 c. Three phases of the calvin cycle: i. Carbon fixation: 1. Each molecule of carbon dioxide(3) is attached to a 5 carbon sugar named ribose biphosphate(RuBP) a. This reactionis catalyzed by the enzyme Rubisco i. RuBP carboxylase ii. Most abundant protein in the chloroplast and probably the most abundant protein on the planet 2. Product of this reaction is a six carbon intermediate molecule a. Short lived because its is unstable and will immediately split in half forming two molecules of 3-­‐ phosphoglycerate (for each CO2) ii. Reduction: V.
1. Each 3-­‐ phosphoglycerate is phosphorylated by ATP a. Produces 1,3 bisphosphoglycerate b. Each 1,3 bisphosphoglycerate gets reduced by NADPH to G3P i. A three carbon sugar ii. High potential energy iii. Also formed during glycolysis iii. Regeneration of the CO2 acceptor (RuBP) 1. Carbon skeleton of 5 G3P molecules are rearranged to 3 molecules of RuBP a. RuBP is now ready to receive CO2 again. d. Input vs. Output of Calvin cycle: i. 3 CO2 molecules enter one at a time ii. produces 3 short lived six carbon intermediate iii. cleaves into 6, 3-­‐ phosphoglycerate molecules iv. 6, 3-­‐ phosphoglycerate molecules are phosphorylated into 6, 1,3 biphosphoglycerate by 6 ATP 1. producing 6 ADP and 6 inorgainc phosphates v. 6, 1,3 phosphoglycerate molecules are reduced to 6 G3P by NADPH 1. 6 NADP+ molecules and 6 inorganic phosphates are produced vi. 1 of the 6 G3P will go onto form glucose and other organic molecules vii. 5 of the 6 G3P will go on to regenerate RuBP which are phosphorylated by 3 ATP 1. 3 ADP molecules are produced 2. 3 inorganic phosphates are also produced viii. Each RuBP cylces back to CO2 to start the cycle over Chapter 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates. a. Terrestrial life has proposed lots of problems to plant life i. Dehydration b. Photorespiration: An evolutionary relic? i. C3 plant: plants that fix CO2 via rubisco to produce G3P 1. During hot and dry conditions, the plants stomata are closed to prevent dehydration a. Very little CO2 can enter the plant and the rate of photosynthesis slows 2. Photorespiration: if CO2 is not present the plant will add O2 to the Calvin cycle a. O2 splits and a two carbon compound leaves the chloroplast releasing CO2 b. No ATP is generated, it is consumed instead c. No sugar is produced d. Siphons organic material from the Calvin cycle e. This could have been a metabolic process from long ago during times when there was more CO2 in the air and less O2. i. Rubisco retains some affinity for O2 f. A certain amount of photorespiration is inevitable given the high concentration of O2 in the atmosphere g. Drains as much as 50% of the carbon fixed by CO2 ii. C4 plants: preface the Calvin cycle with an alternate mode of carbon fixation that forms a four-­‐ carbon compound as its first product 1. Grasses, sugar cane and corn use C4 pathways 2. C4 pathway plants have unique leaf anatomy a. Bundle sheath: photosynthetic cells arranged into tightly packed sheaths around the veins of the leaf i. Calvin cycle is confined to this area b. Mesophyll cells-­‐ loosely arranged cells found between the bundle sheath and leaf surface. 3. Steps of the C4 pathway: a. PEP carboxylase (phosphoenolpyruvate carboxylase) adds CO2 to PEP forming a four carbon product (oxaloacetate) i. PEP carboxylase has a much higher affinity for CO2 than Rubisco ii. PEP carboxylase has no affinity for O2 b. The four carbon molecule (malate) is shipped to the bundle sheath cells via the plasmodesmata c. In the bundle sheath the four carbon molecule releases CO2 which enters the Calvin cycle d. Pyruvate is regenerated for conversion to PEP in the mesophyll 4. Results: a. The C4 pathways keeps the concentration of CO2 abundant in the mesophyll so rubisco can fix carbon dioxide rather than O2 b. Process is powered by ATP c. Advantages in hot and dry regions iii. CAM Plants (Crassulacean Acid Metabolism) 1. Use by succulents (water storing ) plants a. Jade plants, cacti, pineapples. b. These plants open their stomata at night and close them during the day c. Opposite of how other plants behave d. CO2 is taken in at night an converted into a variety of organic acids e. The organic acids are stored in vacuoles in the cells of the mesophyll until morning f. When the stomata close during the day, CO2 is released from the organic acids made the night before i. ATP and NADPH made from the light reaction can then be used to make sugars in the Calvin cycle iv. C3, C4 and CAM plants all use the Calvin cycle to make sugar from CO2 1. The methods by which CO2 is delivered to the Calvin cycle is different a. C3 plants – CO2 is directly brought into the Calvin cycle b. C4 plants-­‐ a four carbon molecule releases CO2 to the Calvin cycle c. CAM plants-­‐ organic acids carry CO2 to the Calvin cycle