Sunlight is used by green plants in photosynthesis, but it is also used by animals in the synthesis of Vitamin D. Are there any similarities between the two processes and how is the light energy actually used?
We don't get it from the sun, it's synthesized.
Humans can get it...
- via nutrition.
- via synthesis in the skin, which depends on UV radiation. Sun is the major source of it (the radiation, not the vitamin), and synthesis in the skin the major source of the vitamin. However, it needs further modification in the liver or kidney to become bioactive. UV radiation is necessary and directly causing the opening of a cyclic molecule structure, changing a cholesterol to a previtamin D3.
For more details see chapter 'Biosynthesis' at https://en.wikipedia.org/wiki/Vitamin_D
Despite major overall differences, there are some basic similarities in the two processes:
- Light energy excites an electron to a higher energy level.
- A covalent bond is broken as the electron moves elsewhere. This process is called ‘photolysis’ and is purely chemical.
The difference is in what happens to the excited electron and the extent of involvement of cellular proteins and lipids:
- In Vitamin D synthesis the electron just moves ‘spontaneously’ to another bond in the new molecule that is formed. That is the end of the matter and enzymes are not involved in this reaction.
- In photosynthesis the light is first captured by photoreceptors and funnelled to the site of photolysis. After photolysis, the co-operation of many proteins in a complex process within a membrane-bond system ensures that the excited electron is transferred to NAD+, forming NADH, the reducing power for then subsequent conversion of carbon dioxide to phosphoglycerate. (An electrochemical membrane potential is also generated, allowing ADP to be converted to ATP, the hydrolysis of which is used for formation of the C-C bonds of the phosphoglycerate.)
Vitamin D synthesis
The photolysis step in Vitamin D synthesis is shown below:
The precursor molecule is formed from cholesterol in a series of enzymic reactions and the Vitamin D3 produced (actually resulting from a spontaneous rearrangement of the initial product molecule) may be metabolized further in enzymic processes (see Berg et al. for more detail). However the photolysis of the bond indicated in red is purely chemical.
One may ask why no other electrons are raised to a higher energy level and the corresponding bonds broken. Other electrons may conceivably be excited, but in the absence of a suitable reaction pathway to a compound of lower thermodynamic free energy the electron will fall back to its original energy level, liberating heat. In the case of 7-dehydrocholesterol the co-ordinated double-bond system provides a suitable reaction pathway to a product of lower free energy. One can think of light (of appropriate wavelength) providing the activation energy of the reaction (overcoming the energy barrier resulting from the higher free energy of the reaction intermediate).
The reactions of photosynthesis that utilize light energy (the so-called ‘light reactions’) involve two complex photosystems that are described in standard texts, e.g. Berg et al.. The simplified diagram deals only with the overall photolytic process.
The molecule undergoing photolysis here is water — the two H–O bonds are broken and oxygen is produced. However in this case the product is incidental to enabling the electron to reduce NAD+. (Chemically oxidation is removal of electrons, and reduction their addition.) As mentioned above, this complex series of reactions also results in an electrochemical gradient within the chloroplast. The movement of hydrogen ions through an ATP synthase in the thylakoid membrane of the chloroplast stroma converts ADP to ATP (in a similar manner to that of oxidative phosphorylation).
The result of the photolysis in this case is the provision of molecular reducing power and ‘energy’ that will be used to convert 1-C carbon dioxide to the 3-C reduced sugar in a separate series of reactions that do not themselves involve light energy (these are referred to as ‘dark reactions’, although they obviously do not have to take place in the dark).
Although one often hears loose reference to light energy being used to ‘make’ Vitamin D or sugars, this can be quite misleading. I would suggest that it is better to express what the light energy is achieving in chemical terms and then place this in the context of the relevant synthetic process.