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Chapter 7 Plants and photosynthesis Major Plant organs Leaves – photosynthesis, gas exchange, water movement Stem – transport, made of xylem (water) and phloem (sugar) Roots- absorb water and minerals, anchor plant Flowers- reproduction Fruit/seeds – dispersion /reproduction Leaf Structures Cuticle- prevents water loss Epidermis Spongy and palisade parenchymaphotosynthetic Vascular bundles- move water and sugar Stomata/guard cells- allow for water and gas exchange Stem structures Xylem – moves water from roots to leaves via transpiration. Active transport of minerals, osmosis, cohesion and adhesion are all involved. Water evaporates constantly from stomata Phloem moves from source (makes or stores sugar) to sink (uses or stores sugar) as sap by Translocation Photosynthesis Overview Two phases Light dependent reactions in thylakoid membranes of chloroplasts Light independent reactions or Calvin Cycle in stroma of chloroplasts Photosynthesis Light dependent reactions Convert light energy to energy stored in ATP and NADPH using chlorophyll Reactants- Water Products- ATP, NADPH, and Oxygen from the splitting of water (photolysis) Photophosphorylation, chemiosmosis Chlorophyll Needed to absorb visible light Acts as a catalyst When it absorbs light electrons are excited Visible Spectrum chromatography Calvin Cycle Uses ATP and NADPH from light reactions to energize CO2 molecules and link them together to form a glucose Process- carbon fixation Reactants- CO2, ATP, NADPH Product – Glucose Light reactions details Light reactions occur in the thylakoid membrane Chlorophyll molecules are arranged into clusters called photosystems Chlorophyll a and b antenna pigments surround a chlorophyll a reaction center Antenna pigments absorb the sun’s energy and pass it to the reaction center Photosystems Two kinds arranged in sequence along the thylakoid membrane Photosystem II: P680 Photosystem I:P700 Each is associated with an electron transport chain Sequence of steps in the light reactions Antenna pigments absorb the sun’s energy and pass it off to the reaction center of the photosystems An electron from the reaction center becomes excited and moves to a higher energy level The excited electron is captured by the first protein in the electron transport chain Sequence continued The electron then “falls” down the etc and loses energy. As the electron falls down the electron transport chain H+ ions are pumped by active transportchemiosmosis ETChains make two products H+ ion gradients that drive ATP production NADPH Sequence continued H+ ions rush through ATP synthase to make ATP Water is split in photolysis to replace electrons lost from photosystems This creates oxygen as a waste Calvin cycle Occurs in stroma Enzyme called rubisco combines CO2 with molecules of RuBP Energy from NADPH and ATP is used to energize the molecules . Make NADP+ and ADP I molecule called G3P is produced and RuBP is regenerated 2 G3Ps make 1 glucose NADP+ and ADP are recycled back to the light reactions ration Calvin cycle adaptations C3 plants- Use normal un-modified Calvin Cycle Dry weather causes stomates to close and plants cannot take up CO2. when CO2 levels are low rubisco will try to fix O2 instead of CO2- photorespiration- useless C4 and CAM plants are adapted to hot dry climates C4 and CAM plants C4- keep stomata closed to reduce water loss, converts CO2 into a 4 carbon product and shuttles it deep into the leafcorn and sugar cane CAM- pineapple, cacti, succulents – takes in CO2 at night when it is cool, stores CO2 as an acid. Stomates are closed all day Greenhouse effect CO2 in our atmosphere traps radiant heat from the sun. excess CO2 is removed by plants. Without it surface temp would be -18 C Excess CO2 and other greenhouse gases trap too much heat. In 1850 CO2- 0.03% of atmosphere Today CO2 – 30% of atmosphere Development, use of fossil fuels, cutting down trees all increase CO2 in atmosphere Global climate change Melting of polar ice Weather changes that may effect agriculture, spread of tropical disease like malaria Widespread drought
