Cracking the AP Biology Exam

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The Chemistry of Life

THE HETEROTROPH HYPOTHESIS

We’ve just seen the organic compounds that are essential for life. But where did they come from in the first place? This is still a hotly debated topic among scientists. Most scientists believe that the earliest precursors of life arose from nonliving matter (basically gases) in the primitive oceans of the earth. But this theory didn’t take shape until the 1920s. Two scientists, Oparin and Haldane, proposed that the primitive atmosphere contained the following gases: methane (CH4), ammonia (NH3), hydrogen (H2), and water (H2O). Interestingly enough, there was almost no free oxygen (O2) in this early atmosphere. They believed that these gases collided, producing chemical reactions that eventually led to the organic molecules we know today.

This theory didn’t receive any substantial support until 1953. In that year, Stanley Miller and Harold Urey simulated the conditions of primitive Earth in a laboratory. They put the gases theorized to be abundant in the early atmosphere into a flask, struck them with electrical charges in order to mimic lightning, and organic compounds similar to amino acids appeared!

But how do we make the leap from simple organic molecules to more complex compounds and life as we know it? Since no one was around to witness the process, no one knows for sure how (or when) it occurred. Complex organic compounds (such as proteins) must have formed via dehydration synthesis. Simple cells then used organic molecules as their source of food. Over time, simple cells evolved into complex cells.

Now let’s throw in a few new terms. Living organisms that rely on organic molecules for food are called heterotrophs (or consumers). For example, we’re heterotrophs. Eventually, some organisms (such as plants) found a way to make their own food. These organisms are calledautotrophs (or producers).