Van Helmont’s willow growth experiment – early 1600’s

Joseph Priestley 1771

Jan Ingenhousz CO 2 + H 2 O + light energy => (CH 2 O) + O 2 – he said Oxygen came from splitting CO 2 His mechanism turned out to be incorrect

C.B. Van Niel – 1930’s Observed photosynthesis in purple sulfur bacteria CO 2 + 2H 2 S + light energy => (CH 2 O) + H 2 O + 2S Van Niel then generalized this to the following reaction for all photosynthetic activity CO 2 + 2H 2 A + light energy => (CH 2 O) + H 2 O + 2A

The Most Important Equation in Biology

A Really Important Equation

Light and Dark 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 rate of the light-independent stage with increases in temperature, Blackman concluded that this stage was controlled by enzymes We shall see that the first, light-dependent stage of photosynthesis uses light energy to form ATP from ADP and to reduce electron carrier molecules, especially NADP+ to NADPH – so here energy is captured In the light-independent reaction, the energy from the ATP and NADPH is used to build organic carbon molecules - and this is the process of carbon fixation

Light Spectrums Absorption spectrum - the light absorption pattern of a pigment Action spectrum - the relative effectiveness of different wavelengths for a specific light- requiring process - such as photosynthesis, flowering or phototropism

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

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 Carotenoids - red, orange or yellow fat-soluble accessory pigments found in all chloroplasts and cyanobacteria - caroteniods are embedded in thylakoids along with chlorophylls Two types of carotenoids - carotenes and xanthophylls

Overview Of Photosynthesis

Melvin Calvin 1940s Worked out the carbon- fixation pathway – now named for him Won Nobel Prize in 1961

Calvin Cycle Summary Each full turn of the Calvin cycle begins with entry of a CO 2 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 the equation to produce a molecule of glucose is: 6CO NADPH + 12H+ + 18ATP => 1 Glucose + 12NADP + 6O ADP + 18 Pi + 6H 2 O

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 Leaves of C4 plants typically have very orderly arrangement of mesophyll around a layer of bundle sheath cells

Location of C4 Pathway

Why Use C4 Pathway? Fixation of CO 2 has a higher energetic cost in C4 plants than in C3 plants – it takes 5 ATP to fix one molecule of CO 2 in C4 but only 3 ATP in C3 For all C3 plants photosynthesis is always accompanied by photorespiration which consumes and releases CO 2 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 CO 2 Photorespiration is nearly absent in C4 plants - this is because a high CO 2 : low O 2 concentration limits photorespiration - C4 plants essentially pump CO 2 into bundle sheath cells thus maintaining high CO 2 concentration in cells where Calvin cycle will occur Thus net photosynthetic rates for C4 plants (corn, sorgham, sugarcane) are higher than in C3 relatives (wheat, rice, rye, oats)

CAM – 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 CO 2 in the dark (night) 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 CAM plants typically open stomata at night and take in CO 2 then, then close stomata during day and thus retard water loss