Chapter 20: Photosynthesis
Plants are literally formed ‘out of thin air’. While a small portion of their (dry weight) mass comes from the soil, approximately 98% comes from the atmosphere. A typical serving of potatoes is 6 oz = 170 grams. Since potatoes are roughly 80% water a serving of potatoes is about 34 grams of dry weight. This weight is nearly pure starch which has a chemical formula (C6 H12 O6). Carbon atoms represent 40% of the weight of starch (or of any hexose or hexose polymer) so that 34 grams of potato has 13.6 grams of carbon = 1.13 mols of carbon. This has been derived from the air which is roughly 400 ppm carbon dioxide. In order to acquire the carbon needed for a serving of potatoes the plant has to extract carbon from approximately 67 thousand liters of air. This photosynthetic ability is even more remarkable because it involves a transfer of the energy of light (electromagnetic radiation) into chemical energy present in the carbohydrate that is produced. The synthesized hexoses have considerably more energy than the raw materials (carbon dioxide and water) used to produce them.
TOPICS
- Overview
- The light dependent reactions
- The carbon dioxide assimilation reactions
- Photosynthetic constraints: the photosynthesis/transpiration compromise
- C4 photosynthesis
- CAM photosynthesis
Overview
Photosynthesis produces carbohydrates. The name reveals what they were originally considered: hydrated carbons (i.e., water added to carbon), but research has revealed that a more accurate description of carbohydrates is reduced carbon dioxide. This is illustrated in the overall equation for photosynthesis:
CO2 + H2O —> (CH2 O)n + O2
Hydrogen is added to the carbon dioxide, i.e., it is reduced, and hydrogen is being removed from oxygen, i.e., it is being oxidized. Note that, as in the case for cellular respiration, the overall equation is a summary of the net effect of multiple reactions taking place simultaneously. Specifically in photosynthesis, carbon dioxide does NOT react with water. Instead, both water and carbon dioxide are consumed in a group of reactions that ultimately produces oxygen and carbohydrates.
The overall reaction as written is exceptional because oxygen is a highly electronegative atom, one that attracts hydrogens strongly, much more strongly than the carbon atom that the hydrogens are transferred to. For this reason, the reaction is ‘uphill’ and unlikely to occur, while the reaction in the opposite direction is much more likely to occur, with carbohydrates being oxidized by oxygen to produce carbon dioxide and water (see the previous chapter). The last chapter illustrated that cellular respiration (carbohydrate oxidation) is ‘driven’ by the electronegativity of oxygen; this electronegativity ‘pulls’ electrons through the inner mitochondrial membrane, ultimately uniting them with oxygen. Thus a significant question concerning photosynthesis is what pulls electrons away from the oxygen of a water molecule—what has a st