5

key concepts

5.1

Cells Are the Fundamental Units of Life

5.2

Prokaryotic Cells Are the Simplest Cells

5.3

Eukaryotic Cells Contain Organelles

5.4

Extracellular Structures Have Important Roles

5.5

Eukaryotic Cells Evolved in Several Steps

Natural Sunscreen

Sunlight plays a key role in life, providing the energy that is transformed into stored chemical energy in green plants through the process of photosynthesis. When we eat food, the energy stored in its chemical bonds ultimately came from the sun. But sunlight has a nasty side effect: part of it, the highly energetic ultraviolet light, damages the genetic material, DNA. Even without knowing this chemistry, some people take measures to minimize exposure to excess sunlight, either by staying in the shade or using chemical sunscreens. But there is also a biological mechanism that has evolved to reduce the sun’s damage to organisms. In most animals, dark brown or black chemical pigments called melanins are made inside cells, the basic units of life. Melanins absorb ultraviolet light, thereby protecting DNA.

In humans, melanin is made in certain skin cells called melanocytes. Humans first evolved in Africa, and the intense sunlight near the equator acted as an evolutionary selective agent for the development of large numbers of melanocytes. As some of these dark-skinned people migrated to more northern regions where the sun is not as intense, the selection pressure for melanocytes was reduced, and over millennia, genetic changes leading to fewer melanocytes persisted in offspring, resulting in lighter skin. Exposure to intense sunlight in these people results in the production of more melanin, the familiar tanning response.

Melanocytes are specialized cells. You find them in the skin but not typically in internal organs. We will return to the process of how different cells have different functions in later chapters. But even inside the melanocyte, there is specialization. Melanin is made inside a special cell compartment called the melanosome. These compartments can be expelled from melanocytes and transferred to other skin cells, so they too become loaded with pigment. This occurs spontaneously in dark-skinned people, while in paler people it is stimulated by exposure to intense sunlight.

The observation that the chemistry of life takes place within cells, and in some cases within specialized compartments in them, is a key concept of biological science.

Are pigments compartmentalized in all types of cells?