However, the leaves are the major site of photosynthesis for most plants. There are about half a million chloroplasts per square millimeter of leaf surface. The color of a leaf comes from chlorophyll, the green pigment in the chloroplasts. Chlorophyll plays an important role in the absorption of light energy during photosynthesis. Chloroplasts are found mainly in mesophyll cells forming the tissues in the interior of the leaf. O2 exits and CO2 enters the leaf through microscopic pores called stomata in the leaf. Veins deliver water from the roots and carry off sugar from mesophyll cells to nonphotosynthetic areas of the plant. A typical mesophyll cell has 30–40 chloroplasts, each about 2–4 microns by 4–7 microns long. Each chloroplast has two membranes around a central aqueous space, the stroma. In the stroma is an elaborate system of interconnected membranous sacs, the thylakoids. The interior of the thylakoids forms another compartment, the thylakoid space. Thylakoids may be stacked into columns called grana.
Chlorophyll is located in the thylakoids.
Photosynthetic prokaryotes lack chloroplasts.
Their photosynthetic membranes arise from infolded regions of the plasma membranes, folded in a manner similar to the thylakoid membranes of chloroplasts. Evidence that chloroplasts split water molecules enabled researchers to track atoms through photosynthesis.
Powered by light, the green parts of plants produce organic compounds and O2 from CO2 and H2O. The equation describing the process of photosynthesis is:
6CO2 + 12H2O + light energy --> C6H12O6 + 6O2+ 6H2O
C6H12O6 is glucose.
Water appears on both sides of the equation because 12 molecules of water are consumed, and 6 molecules are newly formed during photosynthesis. We can simplify the equation by showing only the net consumption of water: 6CO2 + 6H2O + light energy --> C6H12O6 + 6O2
The overall chemical change during photosynthesis is the reverse of cellular respiration. In its simplest possible form: CO2 + H2O + light energy --> [CH2O] + O2 [CH2O] represents the general formula for a sugar.
One of the first clues to the mechanism of photosynthesis came from the discovery that the O2 given off by plants comes from H2O, not CO2. Before the 1930s, the prevailing hypothesis was that photosynthesis split carbon dioxide and then added water to the carbon: Step 1: CO2 --> C + O2
Step 2: C + H2O --> CH2O
C. B. van Niel challenged this hypothesis.
In the bacteria that he was studying, hydrogen sulfide (H2S), not water, is used in photosynthesis. These bacteria produce yellow globules of sulfur as a waste, rather than oxygen. Van Niel proposed this chemical equation for photosynthesis in sulfur bacteria: CO2 + 2H2S --> [CH2O] + H2O + 2S
He generalized this idea and applied it to plants, proposing this reaction for their photosynthesis: CO2 + 2H2O --> [CH2O] + H2O + O2
Thus, van Niel hypothesized that plants split water as a source of electrons from hydrogen atoms, releasing oxygen as a byproduct. Other scientists confirmed van Niel’s hypothesis twenty years later. They used 18O, a heavy isotope, as a tracer.
They could label either C18O2 or H218O.
They found that the 18O label only appeared in the oxygen produced in photosynthesis when water was the source of the tracer. Hydrogen extracted from water is incorporated into sugar, and oxygen is released to the atmosphere (where it can be used in respiration). Photosynthesis is a redox reaction.
It reverses the direction of electron flow in respiration.
Water is split and electrons transferred with H+ from water to CO2, reducing it to sugar. Because the electrons increase in potential energy as they move from water to sugar, the process requires energy. The energy boost is provided by light.
Here is a preview of the two stages of photosynthesis.
Photosynthesis is two processes, each with multiple stages.
The light reactions (photo) convert solar energy to chemical...