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 parenchyma - photosynthetic  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 transport- chemiosmosis  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

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 photorespi ration

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 leaf- corn 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 CO % 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