Step 1 – Glycolysis

The first step in the process of cellular respiration is glycolysis. Glycolysis (Latin glyco- “sweet” and Greek -lysis “divide, cut apart”) consists of breaking a single glucose molecule (C₆H₁₂O₆) into two pyruvic acid molecules (C₃H₆O₃). It occurs in the cytoplasm and does not require mitochondria. The process does require energy, in the form of two ATP molecules, but then creates four ATP molecules for a net gain of two ATPs per glucose. The process also releases four protons and four electrons which are captured by NAD⁺ and carried to the electron transfer chain as 2 NADH + 2 H⁺.

The details of glycolysis, for those who are interested, are shown in this diagram. If you are interested in where the 2 ATPs per glucose molecule go, or where the 2 hydrogen atoms carried by NAD come from, or where the 4 ATPs per glucose molecule come from, this is for you. Note that the 6 carbon glucose is split into two 3-carbon molecules, so everything on the right and bottom of the diagram is multiplied by two.

If you just want to get the big picture and not get caught up in the details, you can safely ignore them (which is why there is a blue box around this image). The next image is for those people who want the big picture without the details.

Remember from Unit 2 that a hydrogen atom consists of a proton (H⁺) and an electron (e^–). Thus, there are a net of 2 ATP molecules, 4 hydrogen atoms (4 H⁺ and 4 e^–), and 2 pyruvate molecules produced by glycolysis. We have a use for the hydrogen atoms, protons, and electrons, but we will discuss that use later.

Glycolysis has two stages,

• an energy investment phase that consumes two ATP molecules per glucose;
• an energy payoff phase that produces four H⁺, four e^–, and four ATP molecules per glucose.

The net gain of energy-containing molecules is shown in the bottom box of this figure.

Aerobic vs Anaerobic Respiration

All we have done so far is split a 6-carbon glucose into two 3-carbon pyruvic acids (we also picked up two ATPs and saved some protons and electrons for later). The next step is formation of acetyl coenzyme A.

We earlier described coenzyme A (CoA) as a shovel that holds a single 2-carbon molecule. Problem is, we now have two 3-carbon pyruvic acids.

How do we “get rid” of one carbon atom from each pyruvic acid (leaving two 2-carbon atoms), so they will fit into the CoA shovel? In the process of cellular respiration, we get rid of carbons by attaching one carbon atom to two oxygen atoms to make carbon dioxide (C + O₂ → CO₂), which is then delivered to the lungs to be exhaled. In other words, oxygen is required to continue the cellular respiration process past glycolysis.

If plenty of oxygen is available, aerobic (with air) respiration can take place—the process continues through formation of acetyl coenzyme A, the citric acid cycle, and the electron transport chain—and yields about 30 ATP molecules per glucose molecule.

If oxygen is absent or in short supply, only anaerobic (without air) respiration can occur. With no way to get rid of carbons, the two pyruvic acids are converted to two lactic acids (also 3-carbon molecules) and the entire process stops—with a yield of only 2 ATPs per glucose.

The process of converting pyruvic acid to lactic acid in the absence of oxygen (i.e. anaerobic conditions) is called fermentation. In some eukaryotes, though not typically in humans, fermentation results in the formation of ethanol and carbon dioxide. This is economically important, for example, in yeast fermenting sugar to make carbon dioxide bubbles in bread or beer. In bread, ethanol produced by this process evaporates in the oven but humans who make beer or sparkling wine prefer the ethanol to remain in solution along with the carbonation.

When lactic acid builds up in cells, as a result of continued anaerobic respiration, the cell becomes more acidic (cellular pH decreases) and the cell’s metabolic processes are rendered even less efficient, a condition called lactic acidosis. This lactic acid buildup can also cause muscle pain and interfere with muscle strength during exercise.