-process by which autotrophs trap energy from the sunlight and use it to make carbohydrates
-anabolic process which produces organic compounds (therefore building up reactions)
-it occurs in the chloroplast
Summary of the photosynthetic reaction:
6CO2 + 6H2O + light energy -----> C6H12O6 + 6O2
-actual process has many steps, each with own enzymes
2 Stages:
Light stage (requires light) - this is the "photo" part, light energy is trapped and converted into ATP and H2O is split
Dark stage (light independent) - this is the "synthesis" part, simple sugars are formed from CO2 and H+ (gained from water splitting)
Light Stage
Outline:
1. Chlorophyll traps light energy and converts it to ATP (will be the energy for the dark stage).
2. Photolysis of H2O occurs, which produces O2 and electrons and protons (H+). This occurs in the granum of the chloroplast.
3. H+ combine with NADP+ to form NADPH+H+ (provides H2 for dark stage).
Review/reminder: What is light?
-energy in packets called photons travel at different wavelengths.
-white light is a mix of the whole range from red (700nm) to violet (400nm)
-objects have colours because they absorb certain wavelengths and reflect others. Chlorophyll appears green becasue it reflects green light and absorbs the others.
-different photosynthetic pigments trap different wavelengths:
the primary pigment, chlorophyll a (bluish-green), absorbs mainly blue-violet and some red
the accessory pigments include chlorophyll b (yellowish-green, absorbs blue-violet, red), carotene (orange-yellow, absorbs blue), xanthophyll (yellow, absorbs blue-green)
-accessory pigments pass the absorbed energy on to the primary pigments, this increases effectiveness and protects the primary pigment from excessive light.
Photosystems
In the thylakoid (of the chloroplast), pigments are arranged into clusters called photosystems. These are designed to catch photons.
-each photosystem (PS) has about 250-400 pigment molecules
-energy absorbed by a pigment is passed to a neighbouring pigment until it reaches the primary electron acceptor (chlorophyll a)
- in green plants and algae, there are 2 photosystems (I and II)
-photosystem I contains carotene, chlorophyll b and chlorophyll a. P700 is the chlorophyll a that is associated with the reaction centre of this photosystem. Its name indicates that 700nm is its peak absorption spectrum.
-photosystem II contains xanthophyll, chlorophyll b and chlorophyll a. P680 is the chlorophyll a that is associated with its reaction centre. 680nm is its peak absorption.
-there are thousands of photosystems in the thylakoid membranes of just one chloroplast (and many chloroplasts in a photosynthetic plant cell)
How does photosynthetic pigment bind light energy?
- pigment molecules have conjugated double bonds, this means that every second bond is a double bond (see image below)
-the electrons in these bonds are easily "detachable", so when a light photon excites (gerjeszt) them, the energy level of one of the electrons in the bond increases, then the energy is quickly given off as heat, light, phosphorescence or it can be transferred to another pigment molecule.
-when chlorophyll a in the reaction centre gets excited, it releases an electon to an electron acceptor
-the electron acceptor is the 1st member of the electron transport chain. The electron acceptor is a special protein called a cytochrome, which is designed to carry electrons as it has a central Fe ion that can change oxidation states (2+/3+)
Z scheme of photosynthesis
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| www.mdpi.com |
At the end of the light stage the net gain is NADPH + H+, ATP
You can watch a more complete explanation at http://www.youtube.com/watch?v=hj_WKgnL6MI&feature=related
Dark Stage
-these reactions occur in the stroma
-CO2 is reduced to simple sugars (CO2 enters the leaf through the open stomata and then diffuses into mesophyll cells and then into the stroma of the chloroplasts that are found in them)
-energy (ATP) and hydrogen (NADPH + H+) are required and are obtained from the light stage reactions
The dark stage is also called the Calvin cycle (named for Melvin Calvin, who won the Nobel Prize in 1961 for mapping out the cycle)
Image from http://kvhs.nbed.nb.ca/gallant/biology/calvin_cycle.jpg
When looking at the diagram, carefully follow the carbons through.
-the final product of the cycle is glyceraldehyde-3-phosphate, which reacts further to form glucose, sucrose, starch, cellulose and other organic compounds that the plant requires.
2 other similar cycles are known in tropical and desert plants, where the need to conserve H2O is important and stomata cannot remain open for long periods
-C4 cycle in tropical plants (CO2 binds a reactive C3 molecule to form a C4 compound, which can later regenerate CO2 and enter the Calvin cycle - all this require more energy though!)
-CAM (or crassulean acid metabolism) in desert plants (stomata are only open at night, so CO2 only enters at night. It is then stored as an acid until daytime, when it can then be extracted and converted into sugars - once again, a less energy efficient process)
And just for some entertainment:
http://www.mrdurand.info/singscience.html
Chemosynthesis (a different process from photosynthesis, but needs a brief mention)
-some bacteria are capable of obtaining energy from chemical reactions - this is chemosynthesis
Examples:
Nitrifying bacteria: oxidize NH4 in soil to HNO2 (nitric acid) and then HNO3. This is extremely useful to plants.
Methane-producing bacteria: found in marshes, lake sediments and ruminants stomachs. These are anaerobic bacteria which convert CO2 and H2 to CH4
Sulfur bacteria: found in deep-sea vents. They are the basis of whole deep sea communities. They gain energy from chemical reactions carried out with the sulfur coming from the vents.
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