Photosynthesis

    One of the most significant and perhaps the most familiar chemical reactions on earth is photosynthesis.  Photosynthesis is the process by which plants, some bacteria, and some protists use the sun's energy to produce sugar.  The general equation for photosynthesis is: 

6CO₂ + 6H₂O + solar energy <-->C₆H₁₂O₆ + 6O₂ 

    _{Photosynthesis occurs in a specialized cellular organelle called the chloroplast.  Chloroplasts consists of grana within a fluid called the stroma.  Inside the grana the sun's light energy is captured by  photopigments.  Chlorophyll a (Chl a) is the key photopigment for these reactions; other accessory pigments collect solar energy and transfer it to Chl a.  Different photopigments also absorb light of different wavelengths.  For more on pigments see chlorophyll and accessory pigments.}

      During the light reactions of photosynthesis, some of the sun's light energy is used to split water molecules (photolysis) which releases H⁺ ions and oxygen.  The remaining energy is passed into the stoma.  For more information on the light reactions see The Light Reactions.   Glucose is then formed when hydrogen is added to carbon dioxide using the energy captured during the light reactions.  The enzyme responsible for carbon fixation in photosynthesis is RubisCO.  (see The Dark Reactions).   RubisCO is the most abundant enzyme on earth. When exposed to high concentrations of oxygen instead of carbon dioxide, it will recognize O₂ as the substrate instead of CO₂ and in the presence of light will begin a series of reactions called photorespiration instead of carbon fixation. The plant creates phosphoglycolate, a nearly useless compound instead of a complex sugar.  As the balance of CO₂ and O₂ in the air changes as temperature increases,  plants living in warm climates adapt to this situation by closing the pores in their leaves (called stoma - see picture) to prevent withering from water loss.  

    As a result, CO₂ concentrations decrease and O₂ concentrations increase inside the plant.   A solution to this problem was the evolution of the C4 pathway.  This mechanism ensures that the concentration of CO₂ at the site of RubisCO is high, so that only photosynthesis will take place.  Pathways that involve only 3-carbon sugars are known as C3 pathways.  See Other Approaches to Energy Production

   Seventy-five percent of all photosynthesis on earth occur in the ocean among tiny organisms called phytoplanktons.   Forty percent of Earth's carbon dioxide is processed by phytoplankton called diatoms. The temperature, light intensity, carbon dioxide supply as well as availability of certain metals and nutrients, affect how efficiently these organisms converts raw materials into food.  It is important to understand the mechanism that are responsible for carbon cycling.  Current estimates cannot account for all the carbon cycling on earth.  The oceans are a logical location to continue the search.

   Back to Main Page