bio lesson 10

chloroplasts and mitochondria form an energy circle

-photosynthesis uses prod of respiration (CO2 and H2O) as starting substrates

-respiration uses products of photosynthesis (organic molecules and oxygen) as starting substrates

chloroplasts

-triple membrane structure

-contain an outer membrane which interacts w/ cytosol and an inner membrane that encloses an internal compartment (stroma)

-stroma contains thylakoid disks that are stacked in columns called grana. thylakoid membrane separate the stroma from the interior of the thylakoid (lumen)

-thylakoid membrane contains chlorophyll that allow chloroplasts to capture light energy as well as protein complexes that convert light to chem energy

light dependent reactions

*use photosystems to capture energy from sunlight and use that energy to prod ATP and reduce the electron carrier NADP+ to NADPH

-require light

-occur in thylakoid

-prod O2 as byprod

-essential to respiration

light independent reactions

-carbon fixation and calvin cycle

-doesn’t require light

-occur in stroma

-use ATP and NADPH to synthesize organic molecules from CO2 through carbon fixation

photons

particle of light that acts as discrete bundle of energy

pigments absorb photons of visible light

-energy is inversely proportional to the wavelength of light

-thus, a photon of blue light (short wavelength) will have more energy than a photon of red light at the longer wavelength end

photoelectric effect

-if harnessed correctly the energy contained in light is able to remove electrons from some molecules

-when a photon strikes a molecule w the correct amount of energy then the molecule will absorb the photon and raise an electron to a higher energy level, leading to excited electrons

-during photosynthesis, the pigments in the chloroplasts act as photoelectric devices (absorb sunlight, excite electrons, and transfer them to an electron carrier)

chlorophyll a

-main pigment in plants

-absorbs violet-blue and red light

-only pigment that can directly convert light energy to chem energy

chlorophyll b

-accessory pigment

-absorbs blue and green light

-adds to range of absorbed light

-also function as anti-oxidants

photosystems

-light is captured by photosystems (pigments/proteins attached to the surface of a photosynthetic membrane

-consists of 2 linked components, an antenna complex and a reaction center

-in plants, H2O is the electron donor: it replaces electrons that reaction center donated to electron acceptor

antenna complex

hundreds of accessory pigment molecules gather photons and feed the captured light energy to the reaction center

reaction center

one or more chlorophyll a molecules passes excited electrons out of photosystem to drive ATP/organic molecule synthesis

chloroplasts have 2 linked photosystems

-2 photosystems carry out oxygenic photosynthesis (oxygen generating) through phosphorylation

-prod of NADPH from NADP+ and ATP from ADP

diagram of the light dependent reactions

*proton gradient is noly created by the activity of b6f complex, but also bc of the creation of protons through oxidation of water in P1 and loss of protons on the stromal side by the generation of NADPH

-proton gradient= potential energy that can be converted to chem energy (ATP) driven by ATP synthase

ATP is prod via chemiosmosis

-as the protons move down their concentration gradient from the thylakoid space to the stroma, ATP is synthesized from stroma from ATP and inorganic phosphate

-ATP synthase uses proton gradient to provide energy for ATP synthesis (like the ATP synthase in the mitochondrial inner membrane)

non cyclic phosphorylation

generates NADPH+ and ATP but building organic molecules requires more energy that this process can create

cyclic phosphorylation

-cyclic phosphorylation allows cells to producse additional ATP by “short circuiting” P1 to create larger proton gradient

-cells switch btwn noncyclic and cyclic phosphorylation as needed

calvin cycle

-biochem pathway that allows for carbon fixation

1. uses ATP as energy source

2. uses NADPH as source for protons and electrons

3. converts inorganic CO2 into organic carbs

4. also called C3 photosynthesis

-uses prod of light dependent reactions, ATP, and protons/electrons from NADPH

3 phases of CC

-carbon fixation

1. key step: attachment of CO2 to 5C sugar, ribulose 1-5 biphosphate (RuBP). subsequently used to generate 2 molecules of PGA

2. uses enzyme rubisco

-reduction

1. PGA reduced to G3P

-regeneration of RuBP

1. G3P used to regenerate RuBP

*input of 3 CO2 molecules prod G3P

*6 turns make enough C for a glucose molecule

glucose prod

-glucose is NOT a direct prod of CC (G3P is)

-2 molecules of G3P leave CC for every 6 molecules of CO2 fixed by CC

-each G3P contains 3 C (from 6 CO2)

-2 G3P used to prod 1 glucose in reactions in cytoplasm (2 G3P prod 1 6-C glucose)

-new glucose may be converted to sucrose/starch

photorespiration

-another metabolic reaction that the enzyme rubisco participates in

-rubisco has 2 enzymatic activities:

1. carboxylation

2. oxidation

-CO2 and O2 compete for active site on rubisco

-problem for C3 plants

carboxylation

-leads to carbon fixation

-addition of CO2 to RuBP

-favored under normal conditions

oxidation

-leads to photorespiration (reverses carbon fixation)

-addition of O2 to RuBP leads to CO2 release

-favored in hot/dry conditions (low CO2 and high O2 conditions)

climate influences photorespiration

-under normal conditions at 25% C, 20% of fixed carbon is lost to phosphorylation (bc rate of carboxylation is 4x that of oxidation)

-loss becomes greater as temp inc under hot, arid conditions

C4 and CAM pathways minimize photorespiration

-some plants have evolved to capture CO2 by another mechanism (C4 and CAM)

-add CO2 to pep carboxalase to form a 4-C molecule

1. use pep instead of rubisco

2. has a greater affinity for CO2 and no oxidase activity

-C4 plants use a spatial solution to separate C3 and C4 pathways to different cells

-CAM plants use a time based solution (temporarily sep C4 reactions which occur in the night from C3 reactions that occur during the day)

the C4 plants

-corn, sorghum, sugarcane

-fix carbon internally using pep in mesophyll cells

-malate is transported to bundle sheath cells, where pyruvate and CO is prod

-CO2 enters CC, carbon fixation by rubisco and CC

-pyruvate returns to mesophyll cells to reform pep and restart the cycle

-although C4 path reduces CO2 loss due to photorespiration, C4 pathway has a cost

1. requires 12 extra ATP compared to CC to make 1 glucose

CAM pathway

-many succulent plants like cacti

-stomata open during night (cool temp) and close during day

1. reverse of most plants

-fix CO2 using PEP during the night and store in vacuole

-when stomata close during the day, CO2 is released from vacuoles

-high levels of CO2 drive CC and minimize photorespiration