III. Sunlight as an Energy Source

1. Properties of Light
1. Photons, originating from nuclear fusion in the core of the sun, reach Earth in different wavelengths (l ) ranging from gamma rays to radio waves (the electromagnetic spectrum).
2. Photoautotrophs absorb photons with l ’s between 400—750 h m
3. Ý Energy ® Ý Frequency ® ß Wavelength (l )
2. Pigments— "The Molecular quarterback"
1. Pigments are the bridge between sunlight and photosynthetic activity.

IV. The Rainbow Catchers

1. The Chemical Basis of Color
1. Electrons in pigments absorb photons of specific energies, which correspond to specific colors of light.
2. If the quantity of energy of an incoming photon matches the energy level required to boost an electron to a higher energy level, that wavelength will be absorbed; photons that are a mismatch will be transmitted (reflected) in the color visible to an observer.
2. On the Variety of Photosynthetic Pigments
1. Chlorophylls are the main pigments in all but one group of photoautotrophs.
1. Chlorophyll a (green) is the main pigment inside chloroplasts.
2. Chlorophyll b (bluish-green) occurs in plants, green algae, and photoautotrophic bacteria.
2. Carotenoid pigments absorb blue-violet wavelengths but reflect yellow, orange, and red.
3. Anthocyanins are pigments in many flowers.
4. Phycobilins are the red and blue pigments of the red algae and cyanobacteria.
3. What Happens to the Absorbed Energy?
1. A photosystem is a cluster of 200 to 300 light-absorbing pigments located in the thylakoid membranes.
2. The pigments "harvest" photon energy from sunlight.
1. Absorbed photons of energy boost electrons to a higher level.
2. The electrons quickly return to the lower level and release energy.
3. Chlorophylls located in the photosystem’s reaction center trap released energy.
4. The trapped energy is then used to transfer a chlorophyll electron to an acceptor molecule.
3. About Those Roving Pigments
1. Carotenoids originate in photoautotrophs and move up the food chain (flamingos eat snails who eat algae)
2. Beta-carotene molecules are split to form vitamin A (retinol), the precursor of visual pigments used in the flamingo’s eyes.

V. The Light-Dependent Reactions

1. Three events occur:
1. Pigments absorb sunlight energy and give up excited electrons.
2. Electron and hydrogen transfers lead to ATP and NADPH formation.
3. The pigments that gave up the electrons in the first place get electron replacements.
2. The ATP-Producing Machinery
1. The chloroplast’s thylakoid membrane incorporates the light-harvesting photosystems, from which electrons are picked up and transferred to an adjacent electron transport system.
2. Electron transport systems are organized sequences of enzymes and other proteins bound in a cell membrane
1. Electrons expelled from a chlorophyll molecule go through one or two electron transport systems in the thylakoid membranes.
2. As the electron passes from one molecule to another in each system, phosphate is added to ADP to form ATP (phosphorylation).
3. Cyclic Pathway of ATP Formation
1. In the cyclic pathway of ATP formation, excited electrons leave the P700 reaction center, pass through an electron transport system, and then return to the original photosystem I.
2. Energy associated with the electron flow drives the formation of ATP from ADP.
3. The cyclic pathway is probably the oldest means of ATP production, being used by early bacteria.



4. Non-Cyclic Pathway of ATP Formation
1. The non-cyclic pathway of ATP formation transfers electrons through two photosystems and two electron transport systems (ETS) in the thylakoid membranes.
2. The pathway begins when chlorophyll P680 in photosystem II absorbs energy.
1. Boosted electron moves through a transport system that releases energy for ADP + Pi ––> ATP.
2. Electron fills "hole" left by electron boost in P700 of photosystem I.
3. Electron from photolysis of water fills "electron hole" left in P680 and produces oxygen byproduct.
3. Pathway continues when chlorophyll P700 in photosystem I absorbs energy.
1. Energy hole is filled by electron from P680.
2. Boosted electron from P700 passes to acceptor, then ETS; it finally joins NADP to form NADPH (which along with ATP can be used in synthesis of organic compounds).
4. The Legacy–A New Atmosphere
1. Oxygen is a by-product of the non-cyclic pathway.
2. Beginning about 1.5 billion years ago, large amounts of oxygen began accumulating in the atmosphere, which up to that time had been oxygen-free.

VI. A Closer Look at ATP Formation in Chloroplasts

Light Dependant Reactions

1. Hydrogen ions from photolysis of water accumulate inside the thylakoid compartment of chloroplasts to set up concentration and electric gradients.
1. Oxygen atoms from photolysis combine to form O₂, which is released into the atmosphere.

1. As the hydrogen ions flow out through channels into the stroma, enzyme action links Pi to ADP to form ATP.



1. This mechanism is called the chemiosmotic theory of ATP formation.

Light-Independent Reactions

1. These reactions are the "synthesis" of photosynthesis.
1. The participants and their roles in the synthesis of carbohydrate are:
1. ATP, which provides energy;
2. NADPH, which provides hydrogen atoms and electrons;
3. Atmospheric air, which provides CO₂ (the source of carbon and oxygen).

VII. Fixing Carbon–So Near, Yet So Far

1. C4 Plants
1. Plants in hot, dry environments close their stomata to conserve water but in so doing retard carbon dioxide entry and permit oxygen buildup inside the leaves.
2. Thus, oxygen–not carbon dioxide–becomes attached to RuBP to yield one PGA (instead of two) and one phosphoglycolate (not useful); this non-productive process is called photorespiration.
3. To overcome this fate, crabgrass, sugarcane, corn, and other plants fix carbon twice (in mesophyll cells then in bundle-sheath cells) to produce oxaloacetate (a four-carbon, hence C4) compound, which can then donate the carbon dioxide to the Calvin-Benson cycle.



 

 

 

 

 

 

 

 

2. CAM Plants
1. Succulents, such as cacti, open their stomata and fix CO2 only at night, storing the intermediate product for use in photosynthesis the next day.
2. These plants are known as CAM plants because, unlike C4 species, they do not fix carbon in separate cells but at different times in the same cell.