Photosynthesis

Absorption spectra

a plot of the amount of light energy of various wavelengths that a substance absorbs

used to determine a pigment’s wavelength

photosynthesis is dependent on the absorption of light by pigments

Action spectra

a plot of effectiveness of light energy of different wavelengths driving a chemical reaction

determined by the suspension of chloroplasts and measure the amount of o2 released by photosynthesis at different wavelengths

Engelmann’s experiment

used light microscope and glass prism to determine most effective wavelength in photosynthesis

placed algae and bacteria was only gathering around areas that most o2 was produced (site of photosynthesis)

large clusters of bacteria under red and purple wavelength

smallest cluster under green wavelength

Chlorophyll A

primary pigment

becomes oxidized during photosynthesis and donates an electron to an electron acceptor

located in the reaction centre

only absorbs blue and red light

Chlorophyll b + carotenoids

accessory pigments

only absorb blue and green light

after light absorption: transfers excitation energy to chlorophyll A

Oxygen 18 experiment

O¹⁸ is a heavier isotope of O

wanted to find out if o2 produced came from Co2 or H2O

grew plants in an environment where water had O¹⁸ but Co2 did not

O2 given off had O¹⁸: O2 came from water

Light dependent reactions

first stage of photosynthesis

occurs in thylakoid membrane

photosystem I

contains specialized chlorophyll A molecules that absorb light at a wavelength of 70nm

excites electrons to a higher level which get passed to ferredoxin then to NADP→ NADPH

gets replacement electrons from photosystem II

photosystem II: photolysis

splits h2o into electrons, protons and O2

sun→ PSII absorbs light → splits H₂O → boosts electrons → makes proton gradient→ helps make ATP → sends electrons to PSI

once electrons are boosted, they are passed along proteins like PQ in the thylakoid membrane

Z-Scheme diagram

1. Light hits PSII and boosts electrons

2. as electrons travel, they loose energy, lost energy is used to pump protons into thylakoid membrane, creating proton gradient

3. electrons arrive to PSI which boosts electrons

4. electrons are passed to ferrodoxin, then NADP+→ NADPH ( energy carrier)

Cyclic electron transport

PSI can function independently of PSII
electron transport of PSI to ferrodoxin is not followed by NADP+

instead, ferrodoxin donates electrons back to PQ

PQ is continually oxidized and moving protons across thylakoid membrane w/o PSII

results in ATP formation w/o oxidation of H2O of reduction of NADP+→ NADPH

occurs in thylakoid membrane

Light independent reactions

occurs in stroma of chloroplast

11 reactions to reduce co2 into sugar using NADPH

endergonic process

Phases of Calvin cycle

1. Carbon fixation

2. Reduction

3. Glucose synthesis

4. RuBP generation

Carbon fixation

conversion of carbon from inorganic → organic

inorganic Co2 reacts w/ RuBP→ 2 PGA

Reduction

3PGA is phosphorlyzed from hydrolysis of ATP and reduced by electrons of NADPH

reduces G3P

Glucose synthesis

6 G3P molecules produced, one is excessed

excess G3P combines with a second process (reverse of glycolysis)

RuBP generation

5 G3P molecules regenerate RuBP to restart cycle

fix another 3 molecules of Co2 to produce another G3P molecule

importance of Rubisco

is the enzyme that catalyzes the first reaction of Calvin cycle

catalyzes Co2 function in all autotrophs

provides source of organic carbon molecules

Rubisco “stupidity”

can accidentally bind to O2 instead of CO2

has low affinity for Co2

photorespiration

catalysts of O2 instead of Co2 by rubisco in RuBP which slows the Calvin cycle, consumes ATP and results in a release of carbon

C4 plants

found in hot and dry climates

minimizes photorespiration

C4 cycle

carbon fixation that plants use to increase [Co2] available for Calvin cycle reactins

co2 combines with 3-c molecules (PEP) to produce oxaloacetate

oxaloacetate reduced by malate by electrons produced by NADPH

malate enters bundle sheath cells that enters chloroplast and is oxidized to pyruvate which releases Co2

C3 vs C4

C4 has an additional energy requirement

additional ATP is easily met by cyclic light reactions

C4 has lower nitrogen demand due to less RuBP needed

CAM plants

same location, different time

metabolic pathway used mostly by succulent plants

process of CAM plants

Co2 accumulates at night and stored as malic acid

second phase happens in daylight, temp increases and stomate loses water and cuts of gaseous exchange with atmosphere

magic acid diffuses into cytosol where it is oxidized into pyruvate

pyruvate accumulates during the day and is converted back into malate at night

stomata

controlled gas exchange

small pores on leaf’s surface that can be opened and closed by guard cells

open during the day allowing Co2 to enter to be used for photosynthesis