Unit One. The Study of Life
1. The Science of Biology
1.2. Properties of Life
Biology is the study of life—but what does it mean to be alive? What are the properties that define a living organism? This is not as simple a question as it seems because some of the most obvious properties of living organisms are also properties of many nonliving things—for example, complexity (a computer is complex), movement (clouds move in the sky), and response to stimulation (a soap bubble pops if you touch it). To appreciate why these three properties, so common among living things, do not help us to define life, imagine a mushroom standing next to a television: The television seems more complex than the mushroom, the picture on the television screen is moving while the mushroom just stands there, and the television responds to a remote control device while the mushroom continues to just stand there—yet it is the mushroom that is alive.
All living things share five basic properties, passed down over millions of years from the first organisms to evolve on earth: cellular organization, metabolism, homeostasis, growth and reproduction, and heredity.
1. Cellular organization. All living things are composed of one or more cells. A cell is a tiny compartment with a thin covering called a membrane. Some cells have simple interiors, while others are complexly organized, but all are able to grow and reproduce. Many organisms possess only a single cell, like the paramecia in figure 1.2; your body contains about 10-100 trillion cells (depending on how big you are)—that’s how many centimeters long a string would be wrapped around the world 1,600 times!
Figure 1.2. Cellular organization.
These paramecia are complex single-celled protists that have just ingested several yeast cells. Like these paramecia, many organisms consist of just a single cell, while others are composed of trillions of cells.
2. Metabolism. All living things use energy. Moving, growing, thinking—everything you do requires energy. Where does all this energy come from? It is captured from sunlight by plants and algae through photosynthesis. To get the energy that powers our lives, we extract it from plants or from plant-eating animals. That’s what the kingfisher is doing in figure 1.3, eating a fish that ate algae. The transfer of energy from one form to another in cells is an example of metabolism. All organisms require energy to grow, and all organisms transfer this energy from one place to another within cells using special energy-carrying molecules called ATP molecules.
Figure 1.3. Metabolism.
This kingfisher obtains the energy it needs to move, grow, and carry out its body processes by eating fish. It metabolizes this food using chemical processes that occur within cells.
3. Homeostasis. All living things maintain stable internal conditions so that their complex processes can be better coordinated. While the environment often varies a lot, organisms act to keep their interior conditions relatively constant; a process called homeostasis. Your body acts to maintain an internal temperature of 37°C (98.6°F), however hot or cold the weather might be.
4. Growth and reproduction. All living things grow and reproduce. Bacteria increase in size and simply split in two as often as every 15 minutes, while more complex organisms grow by increasing the number of cells and reproduce sexually (some, like the bristlecone pine of California, have reproduced after 4,600 years).
5. Heredity. All organisms possess a genetic system that is based on the replication and duplication of a long molecule called DNA (deoxyribonucleic acid). The information that determines what an individual organism will be like is contained in a code that is dictated by the order of the subunits making up the DNA molecule, just as the order of letters on this page determines the sense of what you are reading. Each set of instructions within the DNA is called a gene. Together, the genes determine what the organism will be like. Because DNA is faithfully copied from one generation to the next, any change in a gene is also preserved and passed on to future generations. The transmission of characteristics from parent to offspring is a process called heredity.
Key Learning Outcome 1.2. All living things possess cells that carry out metabolism, maintain stable internal conditions, reproduce themselves, and use DNA to transmit hereditary information to offspring.