Photosynthesis: Energy from the Sun

Identifying Photosynthetic Reactants and Products

Reactants needed for photosynthesis:

H2O, & CO2,

Products of photosynthesis:

carbohydrates and O2

Energy driving reaction:

Light

6 CO2 + 12 H2O C6H12O6 + 6 O2 + 6 H2O

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 CO2 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 O2

e- come in from H2O 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 P700 (max = 700nm)

Photosystem II (PS II) Initiates non-cyclic e- transport Splits H2O to produce e-, H+, and O2. reaction center chlorophyll a is P680 (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- plastoquinoneFd – ferredoxinCyt – cytochrome complexPC - plastocyanin

Mn4

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 CO2

3 sec reaction

30 sec reaction

The Calvin–Benson Cycle: A fixation with carbon

Initial reaction adds one CO2 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 CO2 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 CO2,

Conversion of fixed CO2 into Gyceraldehyde-3P

Uses ATP and NADPH

Regeneration of the CO2 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 CO2 to RuBP.

Rubisco as an oxygenase Adds O2 to RuBP.

These two reactions compete with each other.

Reaction with O2, reduces the rate of CO2 fixation

Oxygenase reaction occurs when CO2 levels are very low and the O2 levels are very high

Rubisco binds CO2 with a

O2 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 + O2 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

CM

P

Overcoming Photorespiration

C3 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, O2 builds up, and photorespiration occurs.

Overcoming Photorespiration

C4 plants have two enzymes for CO2 fixation in different chloroplasts, in different locations in the leaf.

PEP carboxylase is present in the mesophyll cells. It fixes CO2 to 3-C phosphoenolpyruvate (PEP) to form 4-C oxaloacetate.

PEP carboxylase does not have oxygenase activity. It fixes CO2 even when the level of CO2 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 CO2 and regenerating the PEP.

The process pumps up the concentration around rubisco to start the Calvin-Benson cycle.

Figure 8.16 Leaf Anatomy of C3 and C4 Plants

Figure 8.17 (b) The Anatomy and Biochemistry of C4 Carbon Fixation

OAA Pyruvate

Figure 8.17 (a) The Anatomy and Biochemistry of C4 Carbon Fixation

Photorespiration and Its Consequences

CAM plants use PEP carboxylase to fix and accumulate CO2 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, CO2 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 CO2 for the Calvin–Benson cycle.

Figure 8.18 Metabolic Interactions in a Plant Cell