Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Capturing Solar Energy: Photosynthesis (a) (b) internal leaf structure mesophyll cells (c) chloroplast in mesophyll cell outer membrane inner membrane thylakoid stroma vein channel interconnecting thylakoids stoma chloroplasts Photosynthesis Complex series of chemical reactions involving a transition in forms of energy Photosynthesis Uses light energy to make food process by which some organisms can make organic compounds from inorganic compounds uses energy from the sun Photosynthesis Light energy captured and stored as chemical potential energy in the covalent bonds of carbohydrate molecules 6 CO2 + 6 H2O + light C6H12O6 + 6 O2 Life depends on photosynthesis A. Foundation of energy for most ecosystems B. Source of oxygen C. Key component of the carbon cycle The mechanism of photosynthesis Solar energy and light The electromagnetic spectrum Pigment molecules absorb some wavelengths of light and reflect others • Chlorophyll—a green photosynthetic pigment associated with the thylakoid membranes of chloroplasts (a) (b) chlorophyll carotenoids phycocyanin (c) The mechanism of photosynthesis Chloroplasts are the sites of photosynthesis Have a membrane system within internal space (stroma) Arranged in disk-shaped sacks (thylakoids) The thylakoids contain light-harvesting photosynthetic pigments & enzymes Internal membranes define space (lumen) that is separate from the rest of the stroma chloroplast thylakoids reaction center electron transport system light- harvesting complex The mechanism of photosynthesis Photosynthesis occurs in two steps 1. Light-dependent reactions • a. Provides the energy necessary to fix carbon • b. Occurs in the thylakoid membranes • c. Generates ATP • d. Photolysis—light, electrons and water 7 8 electron transport system 3 6 5 reaction center 2 4 energy to drive 1 synthesis reaction center photosystem I 9 photosystem II ELECTRON TRANSPORT STEPS Step 1: Light energy excites electrons in chlorophyll “a” molecules of photosystem 2 (ps) Step 2: These electrons move to a primary electron acceptor. Step 3: Electrons are transferred along an electron transport chain in thylakoid membrane, losing energy as they move. Step 4: Light excites electrons in chlorophyll “a” molecules in photosystem 1. As these electrons move to another primary electron acceptor, they are replaced by electrons from photosystem 2. Step 5: The electrons from photsystem 1 are transferred along a second electron transport chain. At the end of this chain they combine with NADP and H to make NADPH FYI: PHOTOSYSTEM 1 WAS DISCOVERED BEFORE 2, BUT IT GOES AFTER 2 IN THIS PROCESS! Light-Dependent Reactions What happens during light reactions? Chlorophyll in plants (Photosystem I [PS I]) Absorbs blue & red light and reflects back the green light Blue & red light energy boosts electrons to higher energy levels Electrons are passed to electron transport molecule in the thylakoid Can only be passed when highly energized Light-Dependent Reactions What happens during light reactions? During transport of electrons from PS II to PS I Some energy is harnessed to produce ATP Eventually, chlorophyll from PS II is oxidized, in other words: Gets replacement electrons from water. Light-Dependent Reactions Energy of light has thus been captured in two forms: The synthesis of NADPH from NADP+ Proton gradient across the thylakoid membrane Cannot be used directly to make food Must first be converted to ATP by chloroplast ATP synthase The mechanism of photosynthesis Energy carriers ATP and NADPH transport energy from the light reactions to the dark reactions. energy from sunlight Light-dependent reactions occur in thylakoids. Light-independent reactions (C3cycle) occur in stroma. The mechanism of photosynthesis 2. Light-independent reactions a. Uses energy of the light-dependent reaction to make sugar from CO2 b. Occurs in the stroma Light-Independent Reactions Steps in Light-Independent Reactions: CO2 joins with RuBP forming an unstable 6-C molecule Breaks into two 3-C PGA molecules This first step in Calvin cycle is catalyzed by enzyme Called ribulose biphosphate carboxylase (Rubisco) 1 3 Carbon fixation combines CO2 with RuBP. RuBP regeneration uses energy and 10 G3P. 2 G3P synthesis uses energy. 2 G3P available for synthesis of organic molecules. Keep track of the carbons as you follow the illustration around. Six molecules of RuBP ( react with 6 molecules of CO2 and 6 molecules of H2O to form 12 molecules of PGA. The energy of 12 ATPs and the electrons and hydrogens of 12 NADPHs are used to convert the 12 PGA molecules to 12 G3Ps. (G3P: glyceraldehyde 3-phosphate) Energy from 6 ATPs is used to rearrange 10 G3Ps into 6 RuBPs, completing one turn of the C3 cycle. The remaining 2 G3P molecules are further processed into glucose. Photosynthesis Summary •Light Dependent Reaction –Light + chlorophyll --> ATP + NADPH + (O2 as waste) •Light Independent Reaction (Calvin Cycle) –CO2 + ATP + NADPH --> glucose Alternative pathways in Photosynthesis Calvin cycle is most common pathway for carbon fixation Plant species that fix carbon this way are known as C3 plants. Other methods of carbon fixation -Generally found in plants that evolved in hot dry climates. -water loss occurs when stomata open for gas exchange. - Closed stomata means LOW CO2 and high O2. This inhibits carbon fixation. C4 Pathway - enables plants to fix carbon into a four carbon compound. - C4 plants have an enzyme that can fix carbon even when )2 is high and CO2 is low. - species include corn, sugar cane, and crab grass. C4 Pathway - enables plants to fix carbon into a four carbon compound. - C4 plants have an enzyme that can fix carbon even when O2 is high and CO2 is low. - species include corn, sugar cane, and crab grass. CAM pathway - CAM plants open stomata at night and close them during the day - Opposite of what other plants do). - CO2 is fixed at night into organic compounds. During the day CO2 is released from these compounds and enters the Calvin cycle. - grow more slowly than other plants due to stomata being open at night (temps are lower). - species include cactuses and pineapples. CAM pathway - CAM plants open stomata at night and close them during the day -Opposite of what other plants do). - CO2 is fixed at night into organic compounds. During the day CO2 is released from these compounds and enters the Calvin cycle. - grow more slowly than other plants due to stomata being open at night (temps are lower). - species include cactuses and pineapples.