3

3.1 Atoms

3.2 Electrons Determine What Atoms Are Like

3.3 Isotopes

3.4 Molecules

3.7 Forming Macromolecules

3.8 Carbohydrates

3.9 Lipids

3.10 Proteins

3.11 Nucleic Acids

3.5 Hydrogen Bonds Give Water Unique Properties

3.6 Water Ionizes

3.12 Origin of Life

3.13 How Cells Arose

3.14 Has Life Evolved Elsewhere?

3.15 Evolution's Critics

Animations
(Animation Requirements)

• Atomic Structure
• Covalent bond
• Ionic bond
• DNA Structure

Explorations

Cell Chemistry: Thermodynamics
This interactive exercise allows you to explore the way in which reaction conditions affect how an enzyme catalyzes a chemical reaction, focusing on the key roles of enzyme concentration, temperature, and pH.

Mad Cow Disease. We are accustomed to thinking of proteins as enzymes and structural macromolecules. It has come as something of a shock to discover that a protein can be responsible for infectious disease. Called "prions," these very stable proteins are misfolded versions of normal brain proteins that have the unfortunate ability to induce others of their kind to similarly misfold. A chain reaction of misfolding results, eventually destroying the brain. An epidemic spread among cows by feeding them protein supplements prepared from infected animals, and the prions responsible for mad cow disease have now spread to humans who have eaten infected cows.

1. Mad cows and prions.
2. Do prions threaten our blood supply?
3. The growing epidemic of mad cow disease.
4. Mad deer disease.
5. The mad cow disease epidemic spreads to Europe.

The Search for Extraterrestrial Life
The universe has 10²⁰ (100,000,000,000,000,000,000) stars similar to our sun. We don't know how many of these stars have planets, but it seems increasingly likely that many do. Astronomers now estimate that as many as 10% of stars have planetary systems. If only 1 in 10,000 of these planets is the right size and at the right distance from its star as earth, the "life experiment" will have been repeated a million billion times. It does not seem likely that we are alone. It is even possible that other planets orbiting our own sun have harbored life. Mars once was covered by oceans, although it is dry now. Mars meteorites have been claimed to exhibit fossil microbacteria, although the evidence is sketchy at best. Europa, one of the many moons of the large planet Jupiter, is covered with vast seas of liquid water beneath a thin skin of ice. The conditions on Europa now are far less hostile to life than the conditions that existed in the oceans of the primitive earth. In coming decades, satellite missions are scheduled to land on Europa, drill through the ice, and look for life.

Unraveling the Mystery of How Geckos Defy Gravity
Geckos exhibit amazing climbing abilities, strolling up walls and across ceilings. How do geckos perform this gripping feat? What force prevents gravity from dropping the gecko to the ground? Tracking down the answer, Kellar Autumn of Lewis & Clark College in Portland, Oregon, and Robert Full of the University of California, Berkeley, took a closer look at gecko feet. Geckos have rows of tiny hairs called setae on the bottoms of their feet. When you look at these hairs under the microscope (each only one-tenth the diameter of a human hair), the end of each seta is divided into 400 to 1,000 fine projections called spatulae. There are about half a million setae on each foot.

Autumn and Full put together an interdisciplinary team of scientists and set out to see if the setae were the adhesive structures responsible for the gecko's gripping abilities, by measuring the adhesive force produced by a single seta. To do this, they had to overcome two significant experimental challenges: isolating a single seta (which had never been done before) and accurately measuring such a very small force.