SAT Subject Test Biology: Espresso for Cells -- ATP and Energy

The energy source for every cell on earth — from your SAT-busting brain cells to the dumbest bacteria — is a tiny molecule that you may have never heard of before: ATP. Every ATP molecule contains a small amount of energy that a cell can use to perform some action, like pumping in tasty nutrients or pumping out nasty waste products.

Getting energy

Since the energy in one ATP (adenosine triphosphate) molecule is so small, a single cell goes through millions and millions of them every day to accomplish the huge variety of tasks involved in staying alive. Luckily, ATPs are recyclable and can be recharged with energy and used again and again. But this recharging process isn't free (nothing is in this harsh world), so cells always need to have a source of energy coming in from outside in order to recharge their ATPs. There are two main outside energy sources that different cells can use: food and light. Cells that use food as their outside energy source are called heterotrophs. Hetero means other and troph refers to eating, so heterotrophs like us eat other organisms in order to steal their energy. These include all animals and fungi, and many varieties of bacteria and protists. Cells that use light are called autotrophs. Since auto means self, these cells feed themselves. They use light energy to make their own food, then use that food to recharge their ATPs! Autotrophs include all plants, and many other varieties of bacteria and protists.

Storing and releasing energy

Unfortunately, we need to deal with a little bit of technical jargon and chemistry here — not because without it you won't understand how cells use energy, but because you just may get asked about it on the SAT Subject Test. So, here we go. ATP stands for adenosine triphosphate. As the name suggests, this molecule is made of an adenosine molecule with three phosphates attached. When the molecule is energized and ready to go, the third phosphate is attached. When the molecule gives up its energy, the third phosphate is removed. Recharging of ATP means re-attaching its third phosphate.

ATP reaction could be tested on the SAT Subject Test in the following manner.

Which of the following molecules is the immediate energy source for all cellular processes?

1. O₂
2. H₂O
3. sugar
4. ATP
5. DNA

All of the molecules above (except DNA) are involved in aerobic cellular respiration, but only sugar and ATP are used as energy sources, so you can eliminate (A), (B), and (E). The important word to notice in this question is "immediate" because it points you to the correct answer, which is ATP, answer choice (D). Sugars are the original fuel molecules, but they go through the whole long process of aerobic cellular respiration in which their energy is stored in ATP molecules. Then, when the cell needs to actually use energy to do something, it gets the energy from ATPs.

Questions 1 and 2 refer to the following groups of organisms.

1. Animals
2. Plants
3. Fungi
4. Bacteria
5. Protists

1. Which of the above groups contain autotrophs?

1. I only
2. I and II only
3. II and III only
4. II, III, and IV only
5. II, IV, and V only

Autotrophs are organisms that can perform photosynthesis to make their own sugars. Animals obviously don't do this, so you can cross any answer that contains I in it, answer choices (A) and (B). Fungi are always decomposers, so cross answers (C) and (D) because they contain III. This leaves you with the correct answer (E), which contains groups with at least a few species that can perform photosynthesis.

2. Which of the above groups contain heterotrophs?

1. I only
2. I and III only
3. all except II
4. I and V only
5. II and IV only

The only kingdom that contains only autotrophs is the plant kingdom; all of the other kingdoms contain heterotroph species, so the answer is (C).