Photosynthesis
Updated September 21, 2011

The wave theory of light explains the absorption and action spectra of photosynthesis

1. Leaves are the principal photosynthetic organ of most plants.

Diagram	Cross-section of Ligustrum, sp., i.e. privet

1. Visible light consists of electromagnetic waves, which have specific wavelengths and frequencies.

2. Visible light constitutes a very small part of the electromagnetic spectrum:

3. Visible light includes those electromagnetic waves whose wavelengths are from 380 nm (violet) to 760 nm (red).

4. Both chlorophyll a and chlorophyll b have absorption peaks in the blue range (400-500 nm) and in the orange-red range (650-750 nm).

5. These peaks are also found in the action spectrum of photosynthesis, in which the growth rates of living plants exposed to different wavelengths of light are measured.

The photon theory of light explains how light drives the light reactions to split water and produce ATP molecules from ADP + P.

1. Chlorophyll molecules consist of a photon-absorbing porphyrin ring, anchored in place by a long hydrocarbon tail.

2. Chlorophyll and accessory pigment molecules are arranged in a specific order along the membranes of the thylakoid membranes, and they absorb photons of light.

3. By absorption of light, photolysis occurs, in which water is split into 2 electrons, 2 protons and oxygen. Oxygen is released as a waste product.

4. Electrons, ultimately flow to electron acceptor NADP+. Meanwhile, protons are forced into thylakoid interior space to form a proton gradient. Protons are allowed to flow through ATP synthase to generate molecules of ATP. Protons are eventually bonded to NADP to form NADPH.

Video clip of photosynthesis light phase:
http://www.youtube.com/watch?v=v590JJV96lc&feature=related

ATP drives endergonic reactions of carbon fixation

1. 12 NADPH + 18 ATP + 6 CO₂ è C₆H₁₂O₆ + 12 NADP⁺ + 18 ADP + 18 P + 6 H₂O

Video clip of Calvin cycle, AKA dark reactions:
http://www.youtube.com/watch?v=mHU27qYJNU0 or

http://www.youtube.com/watch?v=E_XQR800AgM&feature=related

or

http://www.youtube.com/watch?v=Z03i3mscbwc&feature=related

Water enters the plant via the roots, while carbon dioxide enters via the stomata:

Plants living in xeric environments, where water is scarce, have an alternative means of fixing carbon, involving both enzyme pathways and morphological adaptations.

1. Such plants are called C₄ plants.

2. The alternative enzyme pathway involves bonding carbon dioxide to phosphoenolpyruvate (PEP) to form oxaloacetate. Oxaloacetate is then shuttled to bundle sheath cells, which are not found in typical C₃ plants.