Cracking the AP Biology Exam


Cellular Energetics


We’ve all heard the expression nothing is for free. The same holds true in nature. Here’s a fundamental principle of energy that it is necessary to address:

Energy cannot be created or destroyed. In other words, the sum of energy in the universe is constant.

This rule is called the first law of thermodynamics. As a result, the cell cannot take energy out of thin air. Rather, it must harvest it somewhere.

The second law of thermodynamics states that energy transfer leads to less organization. That means the universe tends toward disorder (or entropy).

As we just saw, almost everything an organism does requires energy. How, then, can the cell acquire the energy it needs without becoming a major mess? Fortunately, it’s through adenosine triphosphate (ATP).

ATP, as the name indicates, consists of a molecule of adenosine bonded to three phosphates. The great thing about ATP is that an enormous amount of energy is packed into those phosphate bonds, particularly the third bond.

When a cell needs energy, it takes one of these potential-packed molecules of ATP and splits off the third phosphate, forming adenosine diphosphate (ADP) and one loose phosphate (Pi), while releasing energy in the process:

ATP → ADP + Pi + energy

The energy released from this reaction can then be put to whatever use the cell so pleases. Of course, this doesn’t mean that the cell is above the laws of thermodynamics. But within those constraints, ATP is the best source of energy the cell has available. It is relatively neat (only one bond needs to be broken to release that energy) and relatively easy to form. As such, it’s the ideal energy currency for living things. Anywhere you look in nature, you’re bound to find ATP.


But where does all this ATP come from? It is produced in one of two ways: (1) through photosynthesis, or (2) through cellular respiration.

Photosynthesis involves the transformation of solar energy into chemical energy. Plants take carbon dioxide, water, and energy (in the form of sunlight) and use them to produce glucose. The overall reaction is:

6CO2 + 6H2O + sunlight → C6H12O6 + 6O2

We’ll hold off on a full discussion of photosynthesis until Chapter 5. For now, let’s focus on the other means of ATP production—cellular respiration.


In cellular respiration, which is performed by all organisms, ATP is produced through the breakdown of nutrients. You’ll recall from the beginning of this chapter that many organic molecules are important to cells because they are energy-rich. This is where that energy comes into play.

In the shorthand version, cellular respiration looks something like this:

C6H12O6+ 6O2 → 6CO2 + 6H2O + ATP

Notice that we’ve taken a sugar, perhaps a molecule of glucose, and combined it with oxygen and water to produce carbon dioxide, water, and energy in the form of our old friend, ATP. However, as you probably already know, the actual picture of what really happens is far more complicated.

Generally speaking, we can break cellular respiration down to two different approaches: aerobic respiration or anaerobic respiration. If ATP is made in the presence of oxygen, we call it aerobic respiration. If oxygen isn’t present, we call it anaerobic respiration. Let’s jump right in with aerobic respiration.