Photosynthesis Ch.6
(6-1) Photosynthesis Biochemical pathway that converts light E into chemical E (organic cmpds/ carbs)
Obtaining E Autotrophs: use E from sun to make food Plants, algae, some bacteria Heterotrophs: get E from food they consume Humans, birds
Overview 2 step process: Light Rxn: light E is converted to chemical E & temporarily stored in ATP & carrier molecule NADPH Calvin Cycle (Dark Rxn): organic cmpds formed using CO2 & stored chemical E
Capturing Light E Light rxns begin w/ absorption of light in chloroplasts Inside a chloroplast are thylakoids stacked to form grana surrounded by stroma
Light & Pigments White light strikes an object & it’s colors are reflected or absorbed Pigments: cmpds that absorb light Colors not absorbed will be reflected back & seen
Light E from the Sun
Chloroplast Pigments Thlyakoid membranes have pigments that absorb light E Chlorophyll a: impt. in light rxns Accessory pigments: assist in capturing light E Chlorophyll b Carotenoids
Why so many pigments?
Light Rxns Light E excites pigment e- Photosystem 1 & 2 e- donated to e- transport chain (ETC) Moves protons (H+) into thylakoid Makes NADPH
Importance of H2O e- replacement in PS2 needs to occur or photosynthesis stops e- provided by H2O
Chemiosmosis Making ATP using a chemical conc. gradient of H+ ions (from H2O) across the thylakoid membrane via the protein ATP synthase
(6-2) Calvin Cycle Plants use E stored in ATP & NADPH during light rxns to produce organic cmpds (sugars) Not dependent on light! Carbon fixation: C-atoms from CO2 bonded into sugars
Carbon Fixation CO2 diffuses into stroma, combines with RuBP, & splits CO2 diffuses into stroma, combines with RuBP, & splits 1 new molecule uses E (ATP & NADPH) to leave the cycle to make carbs 1 new molecule gets converted back to RuBP & restarts cycle
Stomata Small pores on the undersides of leaves that allow gas exchange When stomata are partially closed H2O loss can be dec., but CO2 levels also dec., which inhibits C-fixation
CO2 in; H2O & O2 out
Alternative Pathways C4: fix carbon into a 4-C cmpd Partially close stomata during hottest part of day Ex: corn, sugar cane, crab grass CAM: operates at night Close stomata during the day Ex: cacti, pineapples
Rate of Photosynthesis Affected by: Light intensity CO2 level Temp.