Concept 6.5 Photosynthesis, Light energy, and Chemical Energy Kimberly Javier & Kaylin Malinit

During Photosynthesis, Light Energy is Converted to Chemical Energy The energy released by catabolic pathways in all organisms (animals, plants, and prokaryotes) ultimately come from the sun. Photosynthesis - an anabolic process by which the energy of sunlight is captured and used to convert carbon dioxide (CO 2 ) and water (H 2 0) into carbohydrates (which represent as a six-carbon sugar, C 6 H 12 O 6 ) and oxygen gas (0 2 ). 6CO 2 +6H 2 0  C 6 H ,

Involves Two Pathways:  The light reactions convert light energy into chemical energy in the form of ATP(Adenosine triphosphate.) and the reduced electron carrier NADPH(Nicotinamide adenine dinucleotide phosphate).  The carbon-fixation reactions do not use light directly, but instead use the ATP and NADPH made by the light reactions, along with CO2 to produce carbohydrates  Both the light reactions and the carbon- fixation reactions stop in the dark because ATP synthesis and NADP+ reduction require light.  In plants, both pathways proceed within the chloroplast, but they occur in different parts of that organelle.  I/movie-flash.htm I/movie-flash.htm

Light energy is absorbed by chlorophyll and other pigments..  Photochemistry: - Light is a form of electromagnetic radiation. - Propagated in waves, and the amount of energy in the radiation is inversely proportional to its wavelength - (shorter wave length = greater energy) Shorter wavelengths are more energetic. Longer wave lengths are less energetic.

- -Light also behaves as particles called photons. - Photons have no mass. - Receptive molecules absorb photons in order to harvest their energy for biological processes. These receptive molecules absorb only specific wavelengths of light – photons with specific amounts of energy.  When a photon meets a molecule: 1. The photon may bounce off the molecule- scattered or reflected 2. The photon may pass through the molecule- it may be transmitted. 3. The photon may be absorbed by the molecule, adding energy to the molecule.

 In absorption, the photon disappears and its energy is absorbed by the molecule.  When the molecule acquires energy of the photon it is raised from a ground state (with lower energy) to an excited state (with higher energy)  The difference in free energy between the molecule’s excited state and its ground state is approximately equal to the free energy of the absorbed photon.  The increase in energy boosts one of the electrons within the molecule into a shell farther from its nucleus; this electron is now held less firmly, making the molecule unstable and more chemically reactive.

Photobiology  Pigments - molecules that absorb wavelengths in visible spectrum.  When a beam of white light (containing all the wavelengths of visible light) falls on a pigment, certain wavelengths are absorbed. The remaining wavelengths are scattered or transmitted and make the pigment appear colored. Ex) The pigment chlorophyll absorbed blue and red light, and we see the remaining light which is primarily green. Plotting light absorbed by a purified pigment against wavelength results is an absorption spectrum for that pigment.

 An action spectrum is a plot of the biological activity of an organism against the wavelengths of light to which it is exposed. Light Absorption results in photochemical change  Chlorophyll absorbs light  excited state (unstable situation).  Chlorophyll rapidly returns to its ground state, releasing most of absorbed energy.  Most chlorophyll molecules embedded in the thylakoid membrane, the released energy is absorbed by other, adjacent chlorophyll molecules.

 The pigments in photosynthetic organisms are arranged into energy-absorbing antenna systems, light-harvesting complexes.  They form part of a large multi-protein complex, photosystem (spans the thylakoid membrane and consists of multiple antenna systems with their associated pigment molecules, all surrounding a reaction center.

 Ground-state chlorophyll molecule at reaction center (Chl)  absorbs energy from adjacent chlorophylls and becomes excited(Chl*).  Chlorophyll returns to ground state – the reaction center converts the absorbed light energy into chemical energy.  Chlorophyll molecule absorbs sufficient energy that it gives up its excited electron to a chemical acceptor.  Chl*  acceptor  Chl + + acceptor- The reaction center chlorophyll (Chl*) loses its excited electron in a redox reaction and becomes Chl+. The chlorophyll gets oxidizes while the acceptor molecule is reduced.

Reduction leads to ATP and NADPH formation  Electrons are passed from one carrier to another in a “downhill” series of reductions and oxidations.  Thylakoid membrane has an electron transport system similar to the respiratory chain of mitochondria.  As in mitochondria, ATP is produced chemiosmotically during the process of electron transport (photophosphorylation). 

There are 2 Photosystems, each with its own reaction center:  Photosystem I - (contains the “P 700” chlorophylls at its reaction center) absorbs light energy at 7nm, and passes an excited electron to NADP +, reducing it to NADPH.  Photosystem II - (with “P 680 ” chlorophylls at its reaction center) absorbs light energy at 680 nm and produces ATP and oxidizes water molecules.

Photosystem II  After an excited chlorophyll in the reaction center (Chl*) gives up its energetic electron to reduce a chemical acceptor molecule  Chlorophyll lacks an electron and is very unstable  Strong tendency to “grab” an electron from another molecule to replace the one it lost  It is a strong oxidizing agent  Electron transport system: the energetic electrons are passed through a series of membrane-bound carriers to a final acceptor at a lower energy level

Photosystem I  An excited electron from Chl* at the reaction center reduces an acceptor  Oxidized chlorophyll (Chl + ) “grabs” an electron  Electron comes from last carrier in electron transport system of photosystem II  Links two photosystems chemically  Linked spatially  Two photosystems adjacent to one another in thylakoid membrane  Energetic electrons from photosystem I pass through several molecules and end up reducing NADP + to NADPH

 Carbon-fixation reactions- require more ATP than NADPH  Cyclic electron transport makes up for imbalance  Uses only photosystem I and produces ATP but not NADPH  Cyclic because an electron is passed from excited chlorophyll and recycles back to same chlorophyll