Photosynthesis

provides (either directly or indirectly) the I. Introduction All creatures depend on the sun b/c it provides (either directly or indirectly) the free NRG that powers life on earth Autotrophs: organisms that use light NRG (i.e. sun) & inorganic carbon (CO2) to live & to produce organic compounds (i.e. sugars) (e.g. plants) Heterotrophs: organisms that depend upon autotrophs for food and NRG b/c they can’t produce their own (e.g. animals)

green plants, algae, and some bacteria PHOTOSYNTHESIS – The process in which green plants, algae, and some bacteria convert solar NRG into chemical NRG. The overall process can be summarized according to the following equation: 6 CO2 + 12 H2O + Light  C6H12O6 + 6 O2 + 6 H2O raw materials glucose by-products

II. Requirements of Photosynthesis Light 1. Provides NRG for the process to occur 2. Photon – unit of light NRG that travels in specific wavelengths Electromagnetic spectrum – a scale that depicts a range of solar radiation based on wavelength. --Gamma rays: shortest wavelength highest NRG --Radio waves: longest wavelength lowest NRG --Photosynthesis utilizes the portion of the spectrum known as visible light

Pigments 1. Chlorophyll: the green pigment in plants that absorbs or reflects light  Two main kinds: Chlorophyll a & b a. absorbs violet, blue, orange, red b. reflects what? 1. Chlorophyll a: blue-green 2. Chlorophyll b: green 2. Accessory pigments:  Carotenoids – pigments that absorb & transfer additional NRG to chlorophyll (absorb green/blue; reflect orange/red).

3. Spectrophotometer – measures the amt. of light that passes thru a sample of pigments a. Absorption spectrum - % of light absorbed at each wavelength b. Action spectrum – this will indicate O2 production and thus photosynthetic activity

Absorption Spectrum

Chloroplast – a double membrane-bound organelle that contains the chlorophyll a. Thylakoid – a mem-bound sac within the inner membrane where chlorophyll is stored b. Grana – stacks of thylakoids (used in light rxns) c. Stroma – the enzyme-rich fluid b/n the grana & inner membrane (used in dark rxns)

CO2 & H2O 1. CO2 (from atmosphere) supplies C and O for glucose 2. H2O supplies H for glucose, O is released as a gas

Process of Photosynthesis (Overview) Light Reactions – “light capture” 1. Also called:  Light-dependent reactions  photophosphorylation 2. Takes place in the thylakoid 3. Three pathways:  Fluorescence  Cyclic photophosphorylation(includes ETC)  Non-cyclic photophosphorylation (i.e. photosystems I & II; includes E.T.C.) 4. Purpose:  uses light & water to make ATP and O2  also produces NADPH for dark rxns

Dark Reactions 1. Also called:  Light-independent reactions  Calvin-Benson Cycle  “CO2 Fixation” 2. Takes place in the stroma 3. Uses the products of light rxns (i.e. ATP and NADPH) to reduce CO2 and make glucose

Light Reactions – “Photophosphorylation” Activation of chlorophyll 1. Light strikes chlorophyll & becomes “excited” 2. e- “jumps” to a higher NRG level 3. If nothing further happens, the e- falls back to ground state and the NRG that was gained is emitted as light;  this is called Fluorescence  this NRG cannot be used by the plant

Keep in Mind: In order for the plant to use the NRG from the excited e-, the NRG must be transferred to another molecule. The e- can follow 1 of 2 pathways: a. Cyclic photophosphorylation b. Non-cyclic photophosphorylation

B. Cyclic Photophosphorylation Chlorophyll absorbs light 2. e- becomes excited & jumps to a higher NRG level 3. e- returns to chlorophyll by traveling down E.T.C. (produces ATP) No oxygen produced b/c no e- deficit in chlorophyll (thus no need to split water)

C. Non-cyclic Photophosphorylation Results in the synthesis of ATP and NADPH + H+ The production involves two components: a. Photosystem II b. Photosystem I } each has pigments and is } activated by a diff. wavelength

3. Steps: a. PS II absorbs light (680 nm)  most reactive b. 2e- become excited & jump to higher NRG level  the chlorophyll now has a 2e- deficit c. The deficit is paid by taking 2e- from H2O, thus H2O is split (the O is released) H20  2e- + 2H+ + ½ O2 The excited 2e- travel down E.T.C.  NRG is released to form ATP e. The 2e- reach the bottom & enters PS I light absorbed (700 nm), 2e- excited; jump up g. excited 2e- react w/ NADP+ and 2 H+ to form: 2e- + NADP+ + 2H+  NADPH + H+

http://highered.mcgraw-hill.com/olc/dl/120072/bio12.swf http://www.sumanasinc.com/webcontent/animations/content/harvestinglight.html So why cyclic over non-cyclic? Sometimes an organism has all the reductive power (NADPH) that it needs to synthesize new carbon skeletons, but still needs ATP to power other activities in the chloroplast. Many bacteria can shut off PS2, allowing the production of ATP in the absence of glucose  this is called cyclic photophosphorylation

ATP Production (The Chemiosmotic Model) -- As e- moves down the E.T.C. b/n PS II and PS I, H+ ions will be pumped from the stroma into the interior of the thylakoid. (proton motive force) They will collect along the membrane. This creates an electrochemical gradient (where the interior of the thylakoid is more acidic than the outside). Thus, H+ ions return to the stroma by diffusing across the the thylakoid membrane thru ATP Synthase “channels”. This action activates the synthase to catalyze the formation of ATP. http://highered.mcgraw-hill.com/olc/dl/120072/bio13.swf

Summary of Light Reactions

Dark Reactions Introduction 1. Uses the products of the light rxns (i.e. ATP and NADPH + H+) to reduce CO2 & produce glucose

2. Steps: a. CO2 is “fixed” to a 5-C molecule called RuBP (Ribulose Biphosphate). b. This forms a 6-C molecule c. This 6-C molecule is split into 2, 3-C molecules called 3-phosphoglyceric acid (3-PGA) PGA reduced to phosphoglyceraldehyde (PGAL)  this is where the NADPH and ATP from light rxns are used ONE molecule of PGAL leaves to make glucose. The other 5 molecules are used to regenerate RuBP through a series of reactions This occurs 5 more times in order to form a 6-C compound  glucose

http://www.uic.edu/classes/bios/bios100/lectures/calvin.htm

http://www.mhhe.com/biosci/bio_animations/02_MH_Photosynthesis_Web/index.html

C. Modes of Photosynthesis C3 plants – wheat, oats, rice --Calvin cycle fixes CO2 directly (takes place in mesophyll cells) 2. C4 plants – sugarcane, corn --work best in mild climates --CO2 is fixed by forming C4 molecule prior to cycle (takes place in sheath cells)

CAM plants – flowering plants --live in warm, arid regions --Photosynthesis is minimal b/c of limited fixation of CO2 at night