Photosynthesis function in plants is to produce reduced carbon compounds (food) ,

Photosynthesis function in plants is to produce reduced carbon compounds (food), needed by the plant to provide: 1) energy 2) carbon used in formation of organic molecules

also has global effects: 1) photosynthesis releases O2 into the atmosphere 2) photosynthesis removes CO2 from the atmosphere

the process of photosynthesis occurs in two major phases: 1) the light reactions - light energy is converted to chemical energy in ATP and NADPH molecules 2) the carbon fixation reactions - CO2 and H2O are used to make reduced carbon compounds , using energy from ATP and NADPH

Light, and Light Absorption: - light can be described as having a) qualities of a wave (wavelength, frequency, etc.) and b) qualities of discrete particles (photons)

- the energy in light is inversely proportional to its wavelength - energy contained in one photon of light is called a quantum (quantum energy is highest for light with the shortest wavelengths)

pigments = molecules that can absorb light energy - for any light-dependent event to occur, light energy must be absorbed by a pigment - during light absorption, energy in a photon of light is transferred to an electron in the pigment - a photon can be absorbedonly if the energy it contains is equal to the energy needed to move an electron to an allowable energy state *each photon excites one, and only one, photon

Absorption spectra vary with variations in "usable" photons: 1- bacteriochlorophyll 2- chlorophyll a 3- chlorophyll b 4- phycoerythrobilin 5- b-carotene

once an electron has been excited, it will eventually release its excitation energy in one of four ways: 1) thermal deactivation 2) fluorescence 3) energy transfer (inductive resonance) 4) photochemical reaction

To study a photobiological process it is important to know: 1) What pigment is absorbing light to drive the process? 2) How much light is reaching the pigment within the plant? 3) What mix of wavelengths is reaching the pigment within the plant?

Photosynthetic pigments = chlorophylls and carotenoids - are lipids located within the chloroplast membrane Chlorophylls - two types in plants: chl a and chl b - both absorb well at a) short wavelengths b) long wavelengths - do not absorb well between 480 nm and 600 nm

Carotenoids - includes carotenes and xanthophylls - absorb light at wavelengths of 480 - 510 nm

** the combination of chlorophylls and carotenoids - are inefficient in absorption of green light - are especially poor at absorption of yellow light **however, photosynthesis will occur in light of any wavelength

both types of pigments are located in the lamellae:

- pigments and other molecules involved in the "light reactions", arranged in three kinds of large complexes: photosystem I photosystem II cytochrome complex

- each photosystem includes chlorophyll molecules carotenoid molecules associated proteins antenna pigments ( light harvesting complex) a) absorb light energy b) passes energy to other molecules reaction center molecule a) absorbs light energy b) passes excited electron to another molecule P700 and P680

PSI, PSII, and the cytochrome complex function together during the light reactions of photosynthesis Reactions of PSI: 1) P700 absorbs light energy 2) P700 passes an electron the "primary electron acceptor" (A) 3) A passes the electron into an electron transport chain 4) the electron is passed to ferredoxin 5) ferredoxin-NADP reductase passes the electron from fd to NADP+;

Net results of PSI reactions: - formation of NADPH from NADP+- requires two electrons - oxidation of P700 oxidized P700 must regain the lost electrons (be reduced)

Reactions of PSII: 1) P680 absorbs light energy 2) P680 passes electron to pheophytin 3) pheophytin passes the electron to a plastoquinone (QA) - QA passes the electron to another plastoquinone (QB) - QB combines with two H+, forming QH2 - QH2is mobile; can move away from PSII

Cytochrome complex: 1) QH2moves through membrane to the cytochrome complex; passes an electron to an Fe-S protein, then to cyt b6, then to cyt f 2) cyt f passes the electron to solubleplastocyanin (PC) 3) PC carries an electron to P700

Results of electron transport through the cytochrome complex: 1) replaces electrons lost by P700 2) contributes to forming a H+ concentration gradient across lamellae

replacement of electrons lost by P700 - requires absorption of two photons of light by PSII pigments - P680 becomes oxidized in the process - its electrons must be replaced replacement of electrons lost by P680 - oxidation of H2O - role of oxygen evolving complex (OEC)

formation of H+ concentration gradient across the lamellae - high H+ concentration in the thylakoid lumen, low H+concentration in the stroma results from: 1) release of H+ from water into the lumen 2) removal of H+ from the stroma to reduce QB 3) release of H+ into the lumen when QH2 is oxidized 4) removal of H+ from the stroma to reduce NADP+

diffusion of H+’s back to the stroma through anATP synthase - links diffusion of H+to the synthesis of ATP chemiosmosis noncyclic photophosphorylation - one example of chemiosmosis

alternate electron transport pathway under circumstances of slow NADPH usage = cyclic electron transport 1) P700 absorbs light 2) P700 passes an excited electron to A 3) A passes the electron on to Fe-S proteins; then to ferredoxin 4) ferredoxin passes the electron to the cytochrome complex and back to P700

this pathway provides: 1) replacement of electrons lost by P700 2) a proton concentration gradient that can be used in photophosphorylation cyclic photophosphorylation

Summary of the light reactions: noncyclic electron transportnoncyclic photophosphorylation cyclic electron transport cyclic photophosphorylation

products of the light reactions: NADPH ATP O2

Photosynthesis function in plants is to produce reduced carbon compounds (food) ,