Essay on Studying Life

Lecture Notes by Ian Craine to accompany
Life: The Science of Biology, Ninth Edition
Sadava • Hillis • Heller • Berenbaum

Chapter 1: Studying Life

Introduction
• Biologists’ goals are to understand how organisms function and to use that knowledge to help solve problems.
• For example, University of California at Berkeley professor Tyrone Hayes applied the scientific method in his studies on why amphibian populations around the world are in serious decline. • Hayes found that a common pesticide, atrazine, used around the world, can significantly disrupt male frog sexual development at levels previously considered to be safe. • Levels as low as 0.1 ppb had a dramatic effect on tadpole development, leading to feminization of males. • The U.S. Environmental Protection Agency considers levels of 10 to 20 ppb to be of no concern, but at 25 ppb adult male frogs had a tenfold reduction in testosterone and did not produce sperm. • Like other biologists, Hayes made observations, generated predictions based on them, and then conducted experiments to test the hypotheses.
• Some conclusions of this work could have profound implications not only for amphibians but also for humans.

1.1 What Is Biology?
• Biology is the scientific study of living things.
• Biologists define “living things” as all the diverse organisms descended from a single-celled ancestor that evolved almost 4 billion years ago.
• Living organisms: • Consist of one or more cells • Contain genetic information • Use genetic information to reproduce themselves • Are genetically related and have evolved • Can convert molecules obtained from their environment into new biological molecules • Can extract energy from the environment and use it to do biological work • Can regulate their internal environment
• Life is incredibly diverse and complex, and not all organisms display all these characteristics of life all the time.
• Viruses are particularly hard to define as living because they do not carry out physiological functions on their own, yet contain genetic material that evolves, and they probably evolved from cellular organisms. Based on this, do you think viruses are alive?
• Evolution is a central theme of biology.
• Through differential survival and reproduction, living systems evolve and become adapted to Earth’s many environments.
• The process of evolution has produced the enormous diversity that we see on Earth today. (See Figure 1.1.) Cells are the basic unit of life • Some organisms are unicellular (See Figure 1.1A–C), whereas others are multicellular (See Figure 1.1D–G) and consist of specialized cells that fulfill different functions. • Discovery of cells was made possible by the invention of the microscope in the 1590s by Dutch spectacle makers Zaccharias and Hans Janssen. • The first biologists to improve and apply this technology to living organisms were Antony van Leeuwenhoek and Robert Hooke in the late 1600s. • Hooke coined the term cells to describe the repeated structures he saw in cork tissue. (See Figure 1.2.) • The cell theory, first formulated by Matthias Schleiden and Theodor Schwann, in 1838, states: • Cells are the basic structural and physiological units of all living organisms. • Cells are both distinct entities and building blocks of more complex organisms. • The origin of cells was unknown but thought to arise from nonliving materials by spontaneous generation, like crystal formation in a liquid. • Louis Pasteur proved through experiments, in 1859, that life must be present to generate new life. (See Figure 4.7.) • Modern cell theory states: • All cells come from preexisting cells. • All cells are similar in chemical composition. • Most of the chemical reactions in life occur within cells. • Complete sets of genetic information are replicated and passed on during cell division. • Viruses lack cellular structure but remain dependent on cellular organisms.

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