Exam 3 Notes (Chapters 10-16)Photosynthesis- Photosynthesis converts solar energy into chemical energy- Chloroplasts use energy from light to transform CO2 and H2O into sugar and O2o Chloroplasts are found mainly in the cells of the mesophyll (interior tissue of the leaf)- Stomata pores that CO2 enter/O2 exist from- Thylakoids flattened membranous sacs where light energy is converted to chemical energy- Photosystems large complexes of proteins and chlorophyll that capture light energy- Stroma dense interior fluid- Chloroplasts split H2O, incorporating electrons of hydrogen into sugar molecules and released O2o Their thykaloids transform light energy into chemical energy of ATP and NADPH- Photosynthesis is endergonic- 2 stages of photosynthesis: light reactions and Calvin cycleo Light reactions in thykaloids: Split H2O Release CO2 Reduce NADP+ to NADPH Generate ATP from ADP by phosphorylationo Calvin cycle in stroma: Form sugar from CO2 using ATP and NADPH Begins with carbon fixation- Leaves appear green because chlorophyll reflects/transmits green light- Absorbance spectrum graph plotting light absorption v wavelength- Action spectrum profiles relative effectiveness of different wavelengths of radiation- Chlorophyll a is main photosynthetic pigment- Chlorophyll B broadens spectrum- Carotenoids absorb excessive light that would damage chlorophyll- If illuminated, chlorophyll will fluoresce and give off light and heat- Photosystem consists of a reaction-center complex surrounded by light harvesting complexeso Light-harvesting complexes transfer the energy of photons to reaction center- Primary electron acceptor accepts excited electrons and is reduced as aresult- Transfer of an electron from chlorophyll a to the primary electron acceptor is the 1st step of light reactions- 2 photosystems in thykaloid membrane: photosystem II (P680) and photosystem I (P700)- Linear electron flow (primary pathway) produces ATP and NADPH using light energy o Photon hits a pigments and its energy is passed among pigment molecules until it exits P680, H2O is split by enzymes, O2 is released as a by-productso Electrons falling down electron transport chain creates a proton gradient across thykaloid membraneo In PS I, transferred light energy excites P700, loses an electron, then accepts an electron passed down from PS II via the electron transport chaino Electrons are then transferred to NADP+ and reduce it to NADPH, which are available for reactions of the Calvin cycle- Cyclic electron flow uses only photosystem I and only produces ATP o No oxygen is releasedo Satisfies the higher demand in the Calvin cycle- Comparison of chemiosmosis in chloroplasts and mitochondria: o Mitochondria generate ATP by chemiosmosis by transferring chemical energy from food (intermembrance space mitochondrial matrix)o Chloroplasts transform light energy into the chemical energy of ATP (thykaloid space stroma)- Calvin cycle:o Regenerates its starting material after molecules enter and leave thecycleo Creates G3P 1 G3P: 3 CO2 moleculeso Steps: Carbon fixation Reduction Regerenation of CO2 acceptor- Photorespiration in most C3 plants it produces a 2-carbon compound, releases CO2 without producing TP or sugaro Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar- C4 plants minimize the cost of photorepiration by incorporating Co2 into 4-carbon compoundso These 4-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycleo Require enzyme pep carboxlose- Some plants use CAM to fix carbono CAM plants open their stomata at night, incorporating CO2 into organic molecules, the CO2 is then used in the Calvin cycleCell Communication- Cells most often communicate with each other via chemical signals- Evolution of cell signaling:o Cells of different mating types locate each other via secreted factors specific to each typeo Signal transduction pathway a series of steps by which a signal on a cell’s surface is converted into a specific cellular responseo It converts signals on a cell’s surface into cellular responseso Pathway similarities suggest that ancestral signaling molecules evolved in prokaryotes- Cells in a multicellular organism communicate by chemical messengerso Cell junctions directly connect the cytoplasm of adjacent cells- In local signaling, animal cells may communicate by direct contact, or cell-cell recognition- Local regulators messenger molecules that travel only short distances- Hormones used for long-distance signaling- The ability of a cell to respond to a signal depends on whether or not it hasa receptor specific to that signal- 3 stages of cell signaling:o 1) Reception The binding between a signal molecule (ligand) and receptor is highly specific, most signal receptors are plasma membraneproteins Most water-soluble signal molecules bind to specific sites on receptor proteins that span the plasma membrane 3 main types of membrane receptors:- 1) G protein-coupled receptorso Largest family of cell-surface receptorso Acts as an on-off switch- 2) Receptor tyrosine kinaseso Membrane receptors that attach phosphates to tyrosineso Can trigger multiple signal transduction pathways at once- 3) Ion channel receptorso Acts as a gate when the receptor changes shapeo When a signal molecules binds as a ligand to the receptor, the gate allows specific ions through a channel in the receptor- Intracellular receptor proteins are found in the cytosol ornucleus of target cells, small or hydrophobic chemical messengers can readily cross the membrane and activate receptorso 2) Transduction Multistep pathways can amplify a signal - They can also provide more opportunities for coordination and regulation of the cellular response The molecules that relay a signal from the receptor to response are mostly proteins- The receptor activates another protein which activates another and so on, until the protein producing the response is activated- At each step the signal is transduced into a different form (usually shape change)o 3) Response Sometimes called the “output response” Cell signaling leads to regulation of transcription of cytoplasmic activities Response may occur in the cytoplasm or in the nucleus The final activated molecule in the signaling pathway may function as a transcription factor- Fine-tuning of the
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