Glycolysis and Cellular Respiration
Glycolysis is a process that occurs in heterotrophs1 that converts food into energy and begins cellular respiration. Glycolysis is a catabolic process, meaning it breaks things down. It begins with a standard glucose molecule in the presence of ADP and a phosphate. Using enzymes, the glucose is broken down into pyruvic acid. The energy released also turns the ADP and the phosphate into ATP. As you can see from the diagram from one glucose molecule we gain two pyruvic acids and two ATP molecules.
Without oxygen pyruvic acid builds up in the cell, irritating nerve endings and ultimately causing pain to an animal. However, when oxygen is present the pyruvic acid undergoes aerobic respiration. Oxygen is brought to the cells via blood cells and ultimately from the lungs. In single celled animals, the oxygen may be ambient in its environment. In any case, with the oxygen, the pyruvic acid is transported to the mitochondria of the cell.
The pyruvic acid then goes through the Krebs cycle (discovered by a German scientist named Hans Krebs2). It is a repetitive cycle of actions involving oxygen that breaks down the pyruvic acid. The result of the Krebs cycle is two ATP molecules, carbon dioxide (which is given back to the blood cells to be transported from the body), and several hydrogen-carrier molecules (containing high-energy hydrogen atoms).
The hydrogen carriers are then brought into the electron transport system (still within the mitochondria - along the cristae or ridges). The hydrogen then is brought to lower levels of energy, and in the process a considerable amount of ATP is produced (34 more ATP to be exact). When the energy from the hydrogen atom is completely depleted, the hydrogen is sloughed off to oxygen atoms to form water.
The equation for aerobic respiration (or cellular respiration) is:
C6H12O6 + 6O2 -> 6CO2 + 6 H2O + 36 ATP
You can see that when oxygen is present much more energy can be created through cellular respiration than through glycolysis alone. In any case, cellular respiration is part of a life cycle of gasses that keeps our planet's air in balance. The process of photosynthesis requires carbon dioxide to create energy for plants with the help of the sun, but has a byproduct of oxygen. Cellular respiration uses that oxygen as well as glucose produced in plants to create energy, but as a byproduct gives off the carbon dioxide so essential for plants.
1. Heterotrophs are living things that consume other plants or creatures to obtain most of the glucose that fuels their movement and growth.
2. Biography of Krebs