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Figure 1.7: An anole lizard can change color in response to its environment

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Textbook- Homeostasis: Maintaining an Internal Balance

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Textbook- Homeostasis: Maintaining an Internal Balance

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A second characteristic of life—and a second unifying principle of biology—involves a living system’s ability to maintain an internal balance. This is called homeostasis.

All organisms regulate their internal systems in response to changes in their surroundings. Have you ever noticed how your heart beats faster when you are startled? Quickly pumping blood through your body helps you respond quickly to a potentially dangerous situation.

In fact, all organisms show interesting kinds of internal regulation. Bacteria can adjust their production of certain chemicals. Plants respond to changes in humidity by opening or closing tiny holes in the underside of their leaves. And some animals can change colors to match their environment (see Figure 1.7).

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One significant characteristic of living systems is that they evolve, or change, over time. The fossils of organisms from a long time ago reveal many organisms that look different from organisms today.

Changes in populations of living things have resulted in an amazing diversity of characteristics. Changes over time can also lead to the generation of new species. All species are linked because they share a common ancestor with all other living things. Evolution represents one of the unifying principles in biology.

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A related property of all living systems is that they require energy to build their structures and to carry out their activities. Organisms typically harness light energy from the Sun or chemical energy from other organisms.

Look at the image of the blue whale in Figure 1.9. Blue whales are the largest organisms on Earth. They get the energy they need by eating krill and other organisms in the ocean.

Think about what happens when an organism dies. A dead bird, for example, loses its ability to use matter and energy to keep its body repaired and functional.

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Figure 1.8: The cell is the basic unit of living matter.

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Another common characteristic of life is organization. All living systems are highly organized forms of matter. One of the most remarkable similarities among all living things is that they are mostly made up of only a few types of atoms. These are mainly carbon, nitrogen, oxygen, hydrogen, phosphorus, and sulfur. In organisms, atoms are often held together in large, complex molecules.

The molecules of living materials are organized into complex structures known as cells. Cells are the basic structural units of living matter, but they are too small to see with the unaided eye. As Figure 1.8 illustrates, cells are bag-like structures surrounded by a barrier called a membrane.

Membranes allow cells to accumulate certain substances and exclude other substances. In this way, cells can maintain a different composition from their environment.

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Figure 1.10: DNA allows genetic information to be continually passed from one generation to the next.

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Textbook - Continuity: Reproduction and Inheritance

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Textbook - Continuity: Reproduction and Inheritance

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All living systems depend on plans that are encoded in genetic material, or DNA. The speciﬁc information in your DNA has shaped how your body has grown and developed. DNA is a long and complex molecule that stores information.

One of the most signiﬁcant characteristics that uniﬁes living systems is the universal nature of this DNA. All organisms, from bacteria to plants to humans, use DNA to provide instructions for building and maintaining their cells and bodies.

In addition, all organisms have the ability to transfer those instructions—through DNA—to the next generation during reproduction. Together, DNA and reproduction make up a ﬁfth important unifying principle of life: continuity (see Figure 1.10).

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Figure 1.11: Organisms along and in this stream interact and depend on one another. 

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Textbook - Ecology: Interactions Between Living Things

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Textbook - Ecology: Interactions Between Living Things

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Finally, all living systems on Earth interact and depend on other organisms in the web of life (refer to Figure 1.11).

Imagine a wooded area on an early summer day. Plants provide shelter and food for a variety of birds. A rabbit has created a burrow Figure 1.11 Organisms along and in this nearby and is feeding on stream interact and depend on one another. wild berries. Not far away, a fox just left her den in search of food for her young. This community of different, but connected, living systems illustrates the seventh unifying principle of biology: the interactive and interdependent nature of life.

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Image - Genetic Similarities (Continuity)

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Image - Genetic Similarities (Continuity)

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Image - Binary Fission/Asexual Reproduction in Bacteria (Continuity & Development)

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Image - Binary Fission/Asexual Reproduction in Bacteria (Continuity & Development)

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Image - Temperature regulation in animals

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Image - Temperature regulation in animals

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Image - Relationship between cape buffalo and oxpecker birds

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Image - Relationship between cape buffalo and oxpecker birds

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Image - Finch beak adaptations in the Galapagos Islands

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Image - Finch beak adaptations in the Galapagos Islands

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Note: What living and nonliving things are part of this ecosystem?

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Ecology is the science of how living things interact with each other, with their environment and how that may change in the future. Studying these interactions helps us to understand how the natural world works so we can support an environment where nature and people can thrive.

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Image - Life Cycle of Frogs (Continuity & Development)

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Image - Life Cycle of Frogs (Continuity & Development)

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Textbook - Development: Growth and Specialization

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Textbook - Development: Growth and Specialization

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The ability to develop represents the sixth unifying characteristic of living systems. Growth is an important activity for young organisms, as the body assembles new tissue. As an organism grows, the way in which the organism is organized also changes. For example, many plants begin life as small seedlings that push up through the soil. They grow into mature plants that look very different from the seedlings.

Differentiation—the way in which cells “specialize”—occurs in organisms with more than one cell. This allows the body to “divide the labor” that it must carry out. Differentiated cells, like skin cells and neurons, develop speciﬁc shapes and carry out speciﬁc functions.

Development is the sum of all the growth and differentiation that occurs over the lifetime of an organism.

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Image - Stem cell differentiation (Development)

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Image - Stem cell differentiation (Development)

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Image - Life Cycle of Frogs (Continuity & Development)

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Image - Life Cycle of Frogs (Continuity & Development)