Living things run on batteries.

Major Players in the Development of the Photosynthesis Equation Van Helmont Priestly Ingenhousz

PHOTOSYNTHESIS process which light energy is used in the synthesis of organic molecules

PHOTOSYNTHESIS

Photosynthesis + + H2O CO2 Energy Calvin Cycle Chloroplast O2 C6H12O6 ATP and NADPH2 Which splits water Light is Adsorbed By Chlorophyll Calvin Cycle ADP NADP Chloroplast Used Energy and is recycled. O2 + C6H12O6 Light Reaction Dark Reaction

Chlorophyll, a green pigment, allows plants to absorb light energy Chlorophyll, a green pigment, allows plants to absorb light energy. Energy absorption, however, must be consistent with allowable (basal to excited state) electron transitions within the chlorophyll molecule (click 1). Because these transitions are not continuous, a plant obtains energy only at certain frequencies of light. Energy insufficient to reach an excited state is not absorbed (click 1). Similarly, energy that drives an electron past one energy level but is insufficient to reach a second is not absorbed (click 1). To be absorbed, the energy must be sufficient to reach only allowable energy states (click 1). This simple rule of quantum physics is all you need to know to understand an absorption spectra of chlorophyll (click 1). 2 300 700 600 500 400 Chlorophyll b Absorption Intensity Chlorophyll a excited states 1 ground state

Higher plants have two photocenters, P680 and P700, so designated by the wavelength that gives maximum absorption or O2 evolution. Assume you wish to design a photosystem in a plant. You need a membrane-bound enclosure with a hollow center (click 1). Next, you need a photocenter that serves as an electron source (click 1). To replace the departing electron this center must be capable of extracting electrons from H2O, which results in O2 (click 1). To preserve energy you need a cytochrome system that pumps protons (click 1) and an electron carrier to bring electrons to the cytochromes (click 1). Since the electron’s ultimate destination is NADP+, you need another photosystem to reduce NADP+ to NADPH (click 1). This center must boost the electron to a higher reduction potential. You will need another carrier to bring electrons to this center (click 1). Finally, the protons that are pumped in to the hollow lumen can be used to drive the synthesis of ATP. For that you need an ATP synthase complex (click 1) Q QH2 PS II Fd Cyt bf NADP+ PSI Stroma PC NADPH + H+ O2 2H2O 4H+ Lumen CFo CF1 ADP + Pi ATP

Before we leave the light reaction, there is just one more important point to consider. The light reaction is supplying the plant with ATP and NADPH. It is important that these two energy sources be balanced. To accomplish this, PSI has a switch that shunts excited electrons back to cytochrome bf. The switch is controlled by the ratio of NADP+/NADPH. To understand the regulation, consider what happens when NADP+ is high (click 1). This means there is plenty of NADP+ to take electrons from ferrodoxin (click 1). But, if NADP+ is low (because NADPH is high), the electron are shunted back to cytochrome bf to make more ATP (click 1). This is how chloroplasts maintain energy balance. NADP+ NADPH Stroma Cyt bf PSI Fd Lumen PS II O2 2H2O 4H+ Q QH2 PC CFo CF1 NADP+ NADPH ADP + Pi ATP ATP

Test Your Understanding Look at the absorption spectrum of chlorophyll. Can you can see absorption bands representing the 2 photocenters? Yes, but the wavelengths are not exactly at 680 or 700 nm. There are two peaks in and around the near infrared region. Obviously the major absorbing factors in the center at these wavelengths are chlorophyll a and b molecules. The presence of proteins, which are also part of the center will alter the position of the peaks. Why is there only one water-splitting center? Both centers lose electrons. Removing the electron from P680 (PSII) makes the chlorophyll a very powerful oxidant. The power is demonstrated by the ability to pull electrons away from a H2O molecule. P700 (PSI) has the luxury of having an electron from PSII fill the void, and hence water is not needed. Assume one mole of photons struck the P680 center at two wavelengths, 250nm and 700nm. Which  will give more energy to the plant, and how much more? 250nm will give more. A mole of photons at 250nm is equivalent to 479 kJ. One mole of photons at 700 nm is equivalent to 171 kJ. At 250 nm , 308 more kJ strike the plant. This computes out to be almost 3 times more energy at the lower wave length of light. See Voet “solutions”, p SP-12

Chl oroplast Anatomy

Comparison of Mitochondria and Chloroplasts Both have a large amount of internal membrane surface area. Both have their own ribosomes. Both have their own genomes. Both produce a large amount of ATP. Both derive energy for ATP synthesis from H+ pumps.

The mitochondrial genome (in humans) is about 16,000 nucleotides long. The chloroplast genome is about 10x the size of the mitochondrial genome.