3.4: Cellular Energy

In topic 3.2, we learned that enzymes can both break apart and join substrates. However, as with most things, it's much harder to build than destroy. Topic 3.4 explains the energy cells need to join molecules.

Vocab List

• Metabolism
  • Energy
• Endergonic reaction
• Exergonic reaction
• ATP (Adenosine triphosphate)
  • ADP (Adenosine diphosphate)
• Energy coupling
• Thermodynamics
  • 1st Law
  • 2nd Law
  • Entropy
• Free energy

Written Explanation

Energy:

An organism's metabolism is the entirety of the chemical reactions happening within it. For the most part, metabolism is facilitated by enzymes.

As mentioned in the introduction to this lesson, chemical reactions that create bonds (join molecules) require energy. Since organisms are constantly building, repairing, and growing themselves, they need a constant input of energy to create these new structures. In biology (and in other sciences), energy is simply the ability to do work, such as catalysis and active transport.

Thermodynamics

The study of systems of energy is called thermodynamics. Thermodynamics also provides a few simple rules to help explain why certain systems of energy act the way they do. The 1st Law of Thermodynamics states that energy can neither be created or destroyed. Energy can only change state and be transferred from one place to another. When a plant photosynthesizes, it converts energy from the sun to chemical energy (in the form of bonds in glucose).

The 2nd Law of Thermodynamics states that energy has a tendency to spread out to less energy-dense areas (heat moves from hot to cold areas). All work a cell can do is a result of differences in energy, so another way to think of this law is that over time, the amount of usable energy always decreases (the energy-density of different regions gets smaller). Entropy is another word for this concept: energy always diffuses, disorder increases.

Free energy:

Free energy is the amount of energy that is available to be used by a system. You can also think of free energy as "potential" or "stored" energy. Changes in free energy are notated as ΔG. No reaction is perfect, so usable energy is always lost with reactions (entropy). In most cases this "lost" energy is converted to heat energy.

Now for a quick recap of the two types of reactions:

Exergonic reactions	Endergonic reactions
Release energy	Store energy
Free energy after an exergonic reaction	Free energy after an endergonic reaction
ΔG < 0 The product has less energy than the substrates	ΔG > 0 The product has more energy than the substrates

Explanations of the graphs in the table above:

• Exergonic reaction graph: It took some amount of energy to trigger the reaction, which in turn lost some energy. This energy could be lost as heat, or stored through a future endergonic reaction. By breaking a bond, we have a net loss of usable energy.
• Endergonic reaction graph: It took some amount of energy to trigger the reaction (activation energy), which in turn stored a portion of that used energy. By creating a bond, we have a net gain of usable (stored) energy.

How the cell uses/stores energy:

All cells use the same molecule to store energy. This molecule is adenosine triphosphate (aka ATP). The reason ATP is so useful is that it has three phosphate groups in a chain at its end. By snapping off one of these phosphate groups (exergonic reactions), cells release a small amount of energy that can be used for other cellular activities. The result of this snapping is a molecule of adenosine diphosphate (ADP) - di meaning that it has two phosphate groups. When cells want to store energy (endergonic reactions), they can reconnect ADP with its removed phosphate group to get back ATP.

Energy coupling:

The 1st law of thermodynamics is embodied in the concept of energy coupling. Energy coupling is the idea that endergonic and exergonic reactions fuel each other. The released energy from an exergonic reaction can be used to store energy in an endergonic reaction. Then, that stored energy can be released in another exergonic reaction, keeping the cycle going. It's important to remember that in each of these steps, some energy is lost as heat, so an outside input of energy will continue to be required.