Photosynthesis: Energy from the Sun

Identifying Photosynthetic Reactants and Products  Reactants needed for photosynthesis:  H 2 O, & CO 2,  Products of photosynthesis:  carbohydrates and O 2  Energy driving reaction:  Light  6 CO H 2 O  C 6 H 12 O O H 2 O

The Two Pathways of Photosynthesis: An Overview  Photosynthesis occurs in the chloroplasts of plant cells  Photosynthesis can be divided into two pathways:  The light reaction is driven by light energy captured by chlorophyll  Light energy transformed to chemical energy  ATP and NADPH + H +.  The Calvin–Benson cycle uses ATP, NADPH + H +, and CO 2 to produce sugars.  Carbon fixation

Chloroplast Structure

Photosynthesis in the Chloroplast

The Electromagnetic Radiation: Wave-Particle Duality  Electromagnetic radiation comes in discrete packets called photons  Photons behave as particles and as waves  Particles – mass and impart energy through collisions  Waves – interfere positively and negatively with each other  Photonic energy  Wavelength ( )  1/energy  Frequency  1/  Frequency  energy

The Interactions of Photons and Molecules  Transmission  Photon passes through molecule without interacting  Absorption  Photonic energy transferred to molecule  Molecules absorb photons of discrete energies (wavelengths) and transmit photons of other energies  Molecules that absorb visible wavelengths are called pigments or chromophores

The Interactions of Light and Pigments  Plotting the absorption by the compound as a function of wavelength results in an absorption spectrum.  If absorption results in a measurable activity, plotting the effectiveness of the light as a function of wavelength is called an action spectrum.

Absorption of Photonic Energy  Electrons in high enough exited states can move from molecule to molecule  Essentially an electric current

Light Absorbing Pigments for Photosynthesis  Primary chromophores  chlorophyll a and chlorophyll b.  Absorption max in blue and red wavelengths  Accessory pigments  Carotenoids (xanthophylls) & phycobillins  Absorption maxima between the red and blue wavelengths

Figure 8.7 The Molecular Structure of Chlorophyll

The Interactions of Light and Pigments  molecule enters an excited state when it absorbs a photon.  excited state is unstable, and the molecule may return to the ground state.  When this happens, some of the absorbed energy is given off as heat and the rest is given off as light energy, or fluorescence.  molecule may pass some of the absorbed energy to other molecules

The Interactions of Light and Pigments  Pigments in photosynthetic organisms are arranged into antenna systems.  The excitation energy is passed to the reaction center of the antenna complex.  In plants, the pigment molecule in the reaction center is always a molecule of chlorophyll a.

Figure 8.8 Energy Transfer and Electron Transport

The Light Reactions: Photophosphorylation  Excited chlorophyll (Chl*) in the reaction center acts as a reducing agent and participates in a redox reaction  Chl* can react with an oxidizing agent in a reaction such as: Chl* + PQ  Chl + + PQ –  PQ - passes the e - to a series of carriers in the thylakoid membrane  The e - carriers pump H + into the thylakoid space  The e - is ultimately donated to NADP to generate NADPH + H +  The H + gradient is used to synthesize ATP by ATPases in the thylakoid membrane and is called photophosphorylation

Electron Transport, Reductions, and Photophosphorylation  There are two different systems for transport of electrons in photosynthesis.  Noncyclic electron transport produces NADPH + H + and ATP and O 2  e - come in from H 2 O and leave on NADPH  Cyclic electron transport produces only ATP  e - come from chl and are returned to chl

The Light Reactions: Photophosphorylation  Photosystems  light-driven molecular units consisting of chlorophylls and accessory pigments bound to proteins in energy-absorbing antenna systems  Photosystem I (PS I)  Alone carries out cyclic electron transport  In combo with PS II, - non-cyclic transport  reaction center chlorophyll a is P 700 ( max = 700nm)  Photosystem II (PS II)  Initiates non-cyclic e - transport  Splits H 2 O to produce e -, H +, and O 2.  reaction center chlorophyll a is P 680 ( max = 680nm)  To keep noncyclic electron transport going, both photosystems must constantly be absorbing light

Figure 8. 9 Noncyclic Electron Transport Uses Two Photosystems  Coupled PS II and PS I is the arrangement found in all most all photosynthetic organisms – cyanobacteria to redwoods

Photosynthetic Machinery PQ- plastoquinone Fd – ferredoxin Cyt – cytochrome complex PC - plastocyanin Mn 4

Photosynthetic Machinery and Grana

The Calvin–Benson Cycle: When carbon breaks, we fix it  Calvin-Benson cycle reactions occur in the stroma  Requires the ATP and NADPH + H + produced in the light reactions and these can not be “stockpiled”.  Thus, the Calvin-Benson reactions require light indirectly but take place only in the presence of light.

Figure 8.12 Tracing the Pathway of CO 2 3 sec reaction 30 sec reaction

The Calvin–Benson Cycle: A fixation with carbon  Initial reaction adds one CO 2 to ribulose 1,5-bisphosphate (RuBP; a pentose)  The intermediate hexose is unstable and breaks down to form two molecules of 3-phosphoglycerate (a triose)  fixation of CO 2 is catalyzed by ribulose bisphosphate carboxylase/oxygenase - a.k.a. rubisco.  Rubisco is the most abundant protein in the world.

The Calvin–Benson Cycle:  Fixation of CO 2,  Conversion of fixed CO 2 into Gyceraldehyde-3P  Uses ATP and NADPH  Regeneration of the CO 2 acceptor RuBP  Uses ATP

Regeneration of RuBP in the Calvin-Bensen Cycle

Figure 8.13 The Calvin-Benson Cycle

The Calvin–Benson Cycle  The end product of the cycle is glyceraldehyde 3- phosphate, G3P.  There are two fates for the G3P:  One-third ends up as starch, which is stored in the chloroplast and serves as a source of glucose.  Two-thirds is converted to the disaccharide sucrose, which is transported to other organs.

Importance of The Calvin–Benson Cycle  The products are the energy yield from sunlight converted to carbohydrates  Most of the energy is released by glycolysis and cellular respiration by the plant itself.  Some of the carbon of glucose becomes part of amino acids, lipids, and nucleic acids.  Some of the stored energy is consumed by heterotrophs, where glycolysis and respiration release the stored energy.

Photorespiration  Rubisco as a carboxylase,  adds CO 2 to RuBP.  Rubisco as an oxygenase  Adds O 2 to RuBP.  These two reactions compete with each other.  Reaction with O 2, reduces the rate of CO 2 fixation  Oxygenase reaction occurs when CO 2 levels are very low and the O 2 levels are very high  Rubisco binds CO 2 with a  O 2 levels become very high when stomata are closed to prevent water loss (when the weather is hot and dry).

Reaction Pathways Compensating for Photorespiration  RuBP + O 2  phosphoglycolate + 3PG  glycolate transported into  glycolate converted to glycine in peroxisome  glycine converted to serine in mitochondria  serine converted to glycerate in peroxisome  glycerate reenters C-B cycle in chloroplast

Figure 8.15 Organelles of Photorespiration C M P

Overcoming Photorespiration  C 3 plants have a layer of mesophyll cells below the leaf surface.  Mesophyll cells are full of chloroplasts and rubisco.  On hot days the stomata close, O 2 builds up, and photorespiration occurs.

Overcoming Photorespiration  C 4 plants have two enzymes for CO 2 fixation in different chloroplasts, in different locations in the leaf.  PEP carboxylase is present in the mesophyll cells. It fixes CO 2 to 3-C phosphoenolpyruvate (PEP) to form 4-C oxaloacetate.  PEP carboxylase does not have oxygenase activity. It fixes CO 2 even when the level of CO 2 is extremely low.  The oxaloacetate diffuses into the bundle sheath cells in the interior of the leaf which contain abundant rubisco.  The oxaloacetate loses one C, forming CO 2 and regenerating the PEP.  The process pumps up the concentration around rubisco to start the Calvin-Benson cycle. 

Figure 8.16 Leaf Anatomy of C 3 and C 4 Plants

Figure 8.17 (b) The Anatomy and Biochemistry of C 4 Carbon Fixation OAAPyruvate

Figure 8.17 (a) The Anatomy and Biochemistry of C 4 Carbon Fixation

Photorespiration and Its Consequences  CAM plants use PEP carboxylase to fix and accumulate CO 2 while their stomata are closed.  These plants conserve water by keeping stomata closed during the daylight hours and opening them at night.  In CAM plants, CO 2 is fixed in the mesophyll cells to form oxaloacetate, which is then converted to malic acid.  The fixation occurs during the night, when less water is lost through the open stomata.  During the day, the malic acid moves to the chloroplast, where decarboxylation supplies CO 2 for the Calvin–Benson cycle.

Figure 8.18 Metabolic Interactions in a Plant Cell