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Table of Contents


Photosynthesis

Photosynthesis is the process in which plants make their own food using the energy in light and CO2 and H2O.


Light hits cholroplasts and excites electrons, splitting H2O into O2 and H+. The oxygen gas is released through the stomata. The H+ create a difference of charge on each side of the membrane, and the ions diffuse through the membrane creating ATP.

Here in the Chloroplasts of plant cells is where all the action happens.


Stages of Photosynthesis

Photosynthesis is a two stage process. First the Light Reactions, and requires the direct energy of light to make energy carrier molecules that are used in the second process. The Dark Reactions occur when the products of the Light Reaction are used to form covalent bonds of carbohydrates. The Dark Reactions can occur in the dark, if the energy carriers from the light process are present. Recent evidence suggests that a major enzyme of the Dark Reaction is indirectly stimulated by light, so the term Dark Reaction isn't the best name. The Light Reactions occur in the grana and the Dark Reactions take place in the stroma of the chloroplasts.

The first stable product of the Calvin Cycle (Dark Reactions) is phosphoglycerate (PGA), a 3-C chemical. The energy from ATP and NADPH energy carriers generated by the Photosystems is used to attach phosphates to the PGA. Eventually there are 12 molecules of glyceraldehyde phosphate (GAP, also known as PGAL, a 3-C), two of which are removed from the cycle to make a glucose. The remaining GAP molecules are converted by ATP energy to reform 6 RuBP molecules, and thus start the cycle again. Remember the complexity of life, each reaction in this process, as in Kreb's Cycle, is catalyzed by a different reaction-specific enzyme.


The Nature of Light and its Properties

White light is separated into the different colors (wavelengths) of light by passing it through a prism(learned in 8th grade science). The order of colors is determined by the wavelength of light. Visible light is one small part of the electromagnetic spectrum. The longer the wavelength of visible light, the more red the color. Likewise the shorter wavelengths are towards the Violet side of the spectrum. Wavelengths longer than red are referred to as infrared, while those shorter than violet are ultraviolet.

Light behaves both as a wave and a particle. Wave properties of light include the bending of the wave path when passing from one material (medium) into another (i.e. the prism, rainbows, pencil in a glass-of-water, etc.). The particle properties are demonstrated by the photoelectric effect. Zinc exposed to ultraviolet light becomes positively charged because light energy forces electrons from the zinc. These electrons can create an electrical current. Sodium, potassium and selenium have critical wavelengths in the visible light range. The critical wavelength is the maximum wavelength of light (visible or invisible) that creates a photoelectric effect. In 1905 Albert Einstein developed a theory that light was composed of particles, photons, whose energy is inversely proportional to the wavelength of the light. So light has properties explainable by the wave model and by the particle model.



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By Brad Mace, Scott Ernst, Mark Traynham, and Jaime Erice.
Last Updated September 25, 1997.
Some other web pages were used for researching photosythensis and respiration.