Photosynthesis

1. Photosynthesis

2. How do plants grow? Van Helmont - 1648

3. Joseph Priestley

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

5. Jan Ingenhousz – 1796 – plants only restore good quality to air in the presence of light

6. Water is source of oxygen released during photosynthesis Van Niel was studying the activities of photosynthetic bacteria - he found that purple sulfur bacteria reduce carbon to carbohydrates but do not release oxygen; instead the purple sulfur bacteria use hydrogen sulfide in their photosynthesis - so for them the reaction is as follows: CO2 + 2H2S + light energy =>(CH2O) + H2O + 2S Van Niel then generalized this to the following reaction for all photosynthetic activity CO2 + 2H2A + light energy=>(CH2O) + H2O +2A C.B. van Niel 1930’s

7. 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 enzymes

8. 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

10. 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)

12. 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 heat 2. the energy may be re-emitted almost instantly as light of a longer wavelength - this is called fluorescence 3. the energy may be captured by the formation of a chemical bond - as in photosynthesis

13. Overview of Photosynthesis

15. 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

16. The Photosystems • There are two different kinds of photosystems – • Photosystem I - has chlorophyll a - has an optimum absorption peak of 700 nanometers of light - the chlorophyll a is called P700 because of this activity • Photosystem II - has special chlorophyll a active at 680 nanometers - the P680 chlorophyll a • In general the two photosystems work together simultaneously and continuously - however, photosystem I can work independently

18. Overview of Photosynthesis

21. 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

22. The full Calvin Cycle equation 6CO2 + 12NADPH + 12H+ + 18ATP => C6H12O6 (GLUCOSE) + 12NADP+ + 18ADP + 18 Pi + 6H2O

23. 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

24. Electron micrograph with C4 pathway shown

26. 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)

27. 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

28. 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

30. Comparison of C4 and CAM pathways