Are pigments compartmentalized in all types of cells?

In this chapter, you learned of several situations in which pigments are concentrated in discrete organelles: in humans, melanin is packaged in melanosomes in human skin cells; in plants, tannins are packaged in tannosomes, chlorophyll is packaged in thylakoids, other pigments are contained in chromoplasts, and anthocyanins that may attract pollinators and seed dispersers are packaged in the vacuoles in petals and fruits. You learned, too, of the remarkable ability of motor proteins to move pigment granules into the center of cells in fish and amphibians, to lighten their skin color as a defensive mechanism.

Many biological traits and adaptations are linked to pigments in organelles. For example, the distribution of melanosomes is related to eye color, genetically determined errors in the formation of melanosomes can lead to albinism, and a lack of protection against ultraviolet light. In plants, the formation of tannosomes is associated with plant defenses (see Chapter 38). Understanding the pathway by which tannosomes are formed could have beneficial applications in agriculture. For example, tannins are important to the taste of red wine and manipulating tannin concentrations can result in new flavors.

Future directions

The most common form of human cancer is skin cancer. Most skin cancers are easily treated, but a rare type, melanoma, can be lethal. As its name implies, melanoma occurs when melanocytes in the skin multiply excessively. This can happen if melanocytes are exposed to ultraviolet light from the sun in excess of the ability of their melanin pigment to absorb it. As the tumor develops, it often becomes resistant to the drugs used to treat it. Recent research indicates that one way that drug resistance develops is that the drugs get sucked up by melanosomes and stay in the compartments, preventing them from getting to their targets elsewhere in the cell. Biologists are trying to develop drugs that target melanosomes to prevent this drug sequestration.