Photosysnthesis
Light - Responsible for life
What is light? A small portion of the energy available from the electromagnetic spectrum.

Electromagnetic radiation from radio waves to x-ray waves
Wave & particle nature of EM radiation
From photosynthesis standpoint particle nature of light is most important
All radiation carries energy that is proportional to the frequency of the radiation and inversely proportional to the wavelength

Light sources and quantifying light energy

energy - wave nature
quantity - particle nature

 
Illumination (energy)- light in relation to what the human eye sees
energy = total energy delivered
1 langley = 1 cal cm -2

about 2.00 cal cm-2 min-1 hits the earth's atmosphere - 700cal/cm2 /day
illumination = foot candle = lummens ft-2

lux = lumens m-2

lumen = flux through a unit solid angle from a uniform point source of one candle

candle = 1/60 of the intensity of one cm2 of a blackbody radiator at the temperature of solidification of platinum (2042° K)

brightest day 100,000 lux = 0.6 cal/cm2 min-1 in the visible spectra of chlorophyll action

 
Quantity of light

particle nature - imagine that light, instead of a continuous wave, has discrete packets or bundles of energy (photons)

photons have energy but no mass
6.02 x1023 - Avogadros number

The atomic weight of each element contains 6.02 x1023 atoms

6.02 x1023 photons = mole of photons = 1 Einstein
 
 
What is most important in photosynthesis?
Quantity of light in 400-700 nm (visible range)
5% photens below 400 nm                 striking Atmosphere
28% photens between 400-700 nm
67% photens above 740 nm
 
2% photens below 400 nm
45% photns between 400-700 nm                at earth's surface
53% photens above 740 nm

 
 
 
 
 

What happens to the absorbed light energy?

Light energy is absorbed by individual pigments, but is not used immediately by these pigments for energy conversion. Instead, the light energy is transferred to chlorophylls that are in a special protein environment where the actual energy conversion event occurs:

the light energy is used to transfer an electron to a neighboring pigment. Pigments and protein involved with this actual primary electron transfer event together are called the reaction center.

A large number of pigment molecules (100-5000), collectively referred to as antenna, "harvest" light and transfer the light energy to the same reaction center. The purpose is to maintain a high rate of electron transfer in the reaction center, even at lower light intensities.

Many antenna pigments transfer their light energy to a single reaction center by having this energy "hop" to another antenna pigment, and yet to another, etc., until the energy is "trapped" in the reaction center. Each step of this energy transfer must be very efficient to avoid a large loss in the overall transfer process, and the association of the various pigments with proteins ensures that transfer efficiencies are high by having appropriate pigments close to each other, and by having an appropriate molecular geometry of the pigments with respect to each other.

In many systems the size of the photosynthetic antenna is flexible, and photosynthetic organisms growing at low light (in the shade, for example) generally will have a larger number of antenna pigments per reaction center than those growing at higher light intensity.

However, at high light intensities (for example, in full sunlight) the amount of light that is absorbed by plants exceeds the capacity of electron transfer initiated by reaction centers. Plants have developed means to convert some of the absorbed light energy to heat rather than to use the absorbed light necessarily for photosynthesis. However, in particular the first part of photosynthetic electron transfer in plants is rather sensitive to overly high rates of electron transfer, and part of the photosynthetic electron transport chain may be shut down when the light intensity is too high; this phenomenon is known as photoinhibition.
 
How does CO2 become part of organic compounds?
 
C-4 pathway
 
Bundle sheath cells in C-4 plants contains almost all of the RUBP while the outlying mesophyll cells contain all or most of the PEP carboxylase

-CO compensation point -

  for C-4 plants - 0-5 PPM

  for C-3 plants - 50-100 PPM

PS and crop production of light energy absorbed > 95% is lost as heat only 5% results in PS.