3.3 Cellular Energy

All living things need a steady supply of energy. Plants and other producers capture it from light (3.4), and consumers get it from the molecules in their food (3.5).

The first law of thermodynamics says energy can't be created or destroyed, only converted from one form to another. A cell doesn't make energy. It converts it, for example from light to chemical energy in sugar, or from sugar to ATP to motion.

The second law says that every energy conversion loses some energy as heat, and that the total disorder of the universe increases. Living things are highly ordered, so they might seem to break this law. They don't. An organism stays ordered only by taking in energy and releasing heat and disorder into its surroundings. The order inside is paid for by more disorder outside, so the universe as a whole still becomes more disordered.

To stay organized and keep its processes running, an organism must take in more energy than it loses. If its energy input drops too low, or if it loses its internal order (for example, if its membranes break down or its pathways stop), it dies.

Some cellular processes release energy, such as breaking down glucose. Others require energy, such as building proteins, pumping ions and contracting muscles. Cells pair, or couple, them so the energy released by one powers the other.

ATP (adenosine triphosphate) is the usual go-between. Hydrolyzing ATP into ADP and an inorganic phosphate (Pi) releases energy. Often, the phosphate is transferred to another molecule, such as a substrate or a protein like the sodium-potassium pump (2.8). That phosphorylated molecule now has more energy or a new shape, so it can do something it couldn't before. Then cells rebuild ATP from ADP and Pi using energy from respiration or photosynthesis. A cell recycles its ATP constantly.

Cells rarely do a big job in one step. They use metabolic pathways: a series of reactions, each catalyzed by its own enzyme, where the product of one step becomes the reactant (substrate) of the next. A → B → C → D.

Going step by step lets energy be released or captured in small, controlled amounts. Burning glucose in a flame releases its energy all at once as heat and light. Cellular respiration releases the same total energy in many small steps, so much of it can be captured in ATP instead of being wasted. Steps also give the cell many places to regulate a pathway, for example by feedback inhibition (3.2).

Some core metabolic pathways are found in organisms from all three domains of life: Bacteria, Archaea and Eukarya. Glycolysis (3.5) and the use of an electron transport chain and ATP synthase to make ATP are examples. The simplest explanation is that all three domains inherited these pathways from a common ancestor long ago, which is evidence for common ancestry (7.7).

You need to understand energy conceptually, but you won't be asked to use the Gibbs free energy equation on the AP exam.