photosynthesis summary 1. capturing energy from sunlight 2. using the energy to make atp and...

Photosynthesis Summary

1. Capturing energy from sunlight

2. using the energy to make ATP and reducing power in the form of a compound called NADPH

3. Using the ATP and NADPH to power the synthesis of organic molecules from CO2 in the air

(carbon fixation)

7

1

2

3

4

5

6

7

8

9

10

3

The Role of Soil and Water

Joseph Priestly - 1771

Priestley – 1771 – plants restore “good quality” to air

Photosynthesis has two separate reactions

·Experiments by F.F. Blackman in 1905 demonstrated that photosynthesis has two stages or steps - one is a light-dependent stage and the other is a light-independent stage - due to changes in the effectiveness of the light-independent stage with increases in temperature, Blackman concluded that this stage was controlled by enzymeshttp://vcell.ndsu.edu/animations/

photosynthesis/movie-flash.htm

The Photosynthetic Pigments

·Chlorophyll a - found in all photosynthetic eukaryotes and cyanobacteria - essential for photosynthesis in these organisms·chlorophyll b - found in vascular plants, bryophytes, green algae and euglenoid algae - it is an accessory pigment - a pigment that serves to broaden the range of light that can be used in photosynthesis - the energy the accessory pigment absorbs is transmitted to chlorophyll a·carotenoids - red, orange or yellow fat-soluble accessory pigments found in all chloroplasts and cyanobacteria - caroteniods are embedded in thylakoids as are chlorophylls - two types - carotenes and xanthophylls (xanthophylls have oxygen in their structure, carotenes don't)

CHO group in Chl b

Although they all absorb light not all pigments are created equal, some absorb light from certain wavelengths better than others. The most important pigment in plants is chlorophyll. There are actually two types of chlorophyll in plants, chlorophyll a (chl a) and chlorophyll b (chl b) (there is also a third, chlorophyll c found in some algae). Chlorophyll is composed of two parts; the first is a porphyrin ring with magnesium at its center, the second is a hyrophobic phytol tail.. The ring has many delocalized electrons that are shared between several of the C, N, and H atoms; these delocalized electrons are very important for the function of chlorophyll. The tail is a 20 carbon chain that is highly hydrophic and stabilizes the molecule in the hydrophobic core of the thylakoid membrane.Structurally, the only difference between chlorophyll a and b is the functional group indicated in green. The CH3 group is present in chl a where chl b has a CHO group. Functionally, they are very different. Chlorophyll a and b absorb differnt wavelengths better than others. For instance chl a absorbs best at 450 and 680 nm, where chl b absorbs best at 500 and 640 nm. While chlorophyll a is directly involved in the redox reactions of the light reactions, chl b functions as an accessory pigment, meaning it is not directly involved in the light reactions. Accessory pigments absorb light and pass the energy from the light to the chl a in the reaction center where the first stage of the light reactions take place. Other accessory pigments can be present such as xanthophylls and the more well known carotenoids. The most well known carotenoid is beta-carotene which absorbs different wavelengths than the chlorophylls. Click on the link here <spectra.html>to see an example of the different wavelengths of light that Chlorophyll a, chlorophyll b, and beta-carotene absorb. Think about how this will affect the wavelengths we can percieve with our eyes. Also, compare the absorbtion spectra of the different pigments with the action spectrum of photosynthesis.

Did you notice that these main pigments do not absorb green light well? Chlorophyll a and b both absorb blue light and red light best, resulting in an overall green appearance, whereas beta-carotene absorbs blue and some green light best resulting in an orange color (carotene was first derived from carrots, hence its name). During the summer, chlorophylls dominate, resulting in leaves of plants being green. In the autumn, when deciduous plants are getting ready for winter, they digest their chlorophyll resulting in the accessory pigments like carotenes becoming dominant, hence the bright red and orange colors of fall foliage

The role of pigments·A pigment is any substance that absorbs

visible light - most absorb only certain wavelengths and reflect or transmit the wavelengths they don't absorb ·Chlorophyll absorbs light primarily in the violet, blue and red wavelengths and reflects green wavelengths, and thus appears green·Absorption spectrum - the light absorption pattern of a pigment ·Action spectrum - the relative effectiveness of different wavelengths for a specific light-requiring process ·Chlorophyll is implicated as the principle pigment in photosynthesis because its absorption spectrum is the same as the action spectrum for photosynthesis

When pigments absorb light, electrons are temporarily boosted to

a higher energy level One of three things may happen to that energy:1. the energy may be dissipated as heat2. the energy may be re-emitted almost instantly as light of a longer wavelength - this is called fluorescence3. the energy may be captured by the formation of a chemical bond - as in photosynthesis

The Photosystems·The chlorophylls and other pigments are embedded

in thylakoids in discrete units called photosystems ·Each photosystem has 250 to 400 pigment molecules in two closely linked components - the reaction center-protein complex and the antenna protein complex·All pigments in the photosystem are capable of absorbing photons of light, but only one pair of those in the reaction center-protein complex can actually use the energy in a photochemical reaction·The other pigments in the antenna protein complex act like antenna to gather light and transfer that energy to the photochemically active pigments

http://vcell.ndsu.edu/animations/photosystemII/movie-flash.htm

http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120072/bio13.swf::Photosynthetic%20Electron%20Transport%20and%20ATP%20Synthesis

http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter10/animations.html#

CALVIN CYCLE

http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter39/calvin_cycle.html

Calvin Cycle - details

·The Calvin cycle begins when CO2 enters the cycle and is joined to RuBP this forms a 6 carbon compound which immediately splits into two 3 carbon compounds (the 6 carbon intermediate has never been isolated) - the 3 carbon compound is 3-phosphoglycerate (PGA)·Because each PGA has three carbons, this is sometimes also called the C3 pathway·Each full turn of the Calvin cycle begins with entry of a CO2 molecule and ends when RuBP is regenerated - it takes 6 full turns of the Calvin cycle to generate a 6 carbon sugar such as glucose·Although we usually report glucose as the product of photosynthesis, the cell usually produces either sucrose or starch as its storage products·At night, sucrose is produced from the starch and it is transported from the chloroplast to the rest of the cell

The full Calvin Cycle equation

6CO2 + 12NADPH + 12H+ + 18ATP => C6H12O6 (GLUCOSE) + 12NADP+ + 18ADP + 18 Pi + 6H2O

http://www.science.smith.edu/departments/Biology/Bio231/calvin.html

The C4 Pathway

·In some plants the first carbon compound produced through the light-independent reactions is not the 3 carbon PGA, but rather is a 4 carbon molecule oxaloacetate - plants that use this pathway are called C4 plants ·Leaves of C4 plants typically have very orderly arrangement of mesophyll around a layer of bundle sheath cells

Electronmicrographwith C4pathway shown

Why use C4 pathway?·A problem with C3 is that for all C3 plants,

photosynthesis is always accompanied by photorespiration which consumes and releases CO2 in the presence of light - it wastes carbon fixed by photosynthesis - up to 50% of carbon fixed in photosynthesis may be used in photorespiration in C3 plants as fixed carbon is reoxidized to CO2

·Photorespiration is nearly absent in C4 plants - so greatly increases their efficiency - this is because a high CO2: low O2 concentration limits photorespiration - C4 plants essentially pump CO2 into bundle sheath cells (or the products of its reduction) thus maintaining high CO2 concentration in cells where Calvin cycle will occur·Thus net photosynthetic rates are higher for C4 plants (corn, sorgham, sugarcane) than in C3 relatives (wheat, rice, rye, oats)

Why use C4 pathway?

·C4 plants evolved in tropics and are well adapted to life at high temperature, high light intensity and dry conditions - optimal temperature for C4 photosynthesis is much higher than for C3 - efficient use of CO2 allows C4 plants to keep stomata closed longer and thus they lose less water during photosynthesis than do C3 plants·C4 monocots do especially well at high temperature·C4 dicots do especially well in dry conditions

Crassulacean Acid Metabolism

·Crassulacean Acid Metabolism (CAM) has evolved independently in many plant families including the stoneworts (Crassulaceae) and cacti (Cactaceae)·Plants which carry out CAM have ability to fix CO2 in the dark (night) via the activity of PEP carboxylase - malic acid (malate) so formed is stored in the cell's vacuole - during the light (day) the malic acid is decarboxylated and CO2 is transferred to RuBP in Calvin cycle within the same cell·so CAM plants, like C4 plants, use both C4 and C3 pathways, but CAM plants separate the cycles temporally and C4 plants separate them spatially

Comparison of C4 and CAM pathways

a. water

b. carbon dioxide

light energy

gaseous oxygen

water

light-dependent reaction

light independent reaction

sugar

starch

chemical energy

http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120072/bio13.swf::Photosynthetic%20Electron%20Transport%20and%20ATP%20Synthesis

http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter39/calvin_cycle.html