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

JosephPriestley1771

Jan Ingenhousz - 1796

• CO2 + H2O + light energy => (CH2O) + O2 – he said Oxygen came from splitting CO2

• His mechanism turned out to be incorrect

C.B. Van Niel – 1930’s

• Observed photosynthesis in purple sulfur bacteria

• 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

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

OverviewOf

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 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

• the equation to produce a molecule of glucose is:

• 6CO2 + 12NADPH + 12H+ + 18ATP => 1 Glucose + 12NADP + 6O2 + 18ADP + 18 Pi + 6H2O

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 CO2 has a higher energetic cost in C4 plants than in C3

plants – it takes 5 ATP to fix one molecule of CO2 in C4 but only 3 ATP in C3

• 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 - this is because a high CO2: low O2 concentration limits photorespiration - C4 plants essentially pump CO2 into bundle sheath cells thus maintaining high CO2 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 CO2 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 CO2

then, then close stomata during day and thus retard water loss