Cell Respiration Bio.4.2.1 Analyze photosynthesis and cellular respiration in terms of how energy is stored, released, and transferred within and between these systems.

Harvesting Cell Energy Cellular respiration is the process by which cells break down organic compounds to produce ATP. Both autotrophs and heterotrophs use cellular respiration to make CO2 and water from organic compounds and O2. The products of cellular respiration are the reactants in photosynthesis; conversely, the products of photosynthesis are reactants in cellular respiration. Cellular respiration can be divided into two stages: glycolysis and aerobic respiration.

Circle of Life

Glycolysis Cellular respiration begins with glycolysis, which takes place in the cytosol of cells. During glycolysis, one six-carbon glucose molecule is split to form two three-carbon pyruvic acid molecules. A net yield of two ATP molecules is produced for every molecule of glucose that undergoes glycolysis.

Aerobic Respiration In eukaryotic cells, the processes of aerobic respiration occur in the mitochondria. Aerobic respiration only occurs if oxygen is present in the cell. The Krebs cycle occurs in the mitochondrial matrix. The electron transport chain is located in the inner membrane.

Krebs cycle In the mitochondrial matrix, the two pyruvic acids produced in glycolysis react with coenzyme A to form acetyl CoA. Then, acetyl CoA enters the Krebs cycle. It takes two turns of the Krebs cycle to process both acetyl CoA. These two turns produce four CO2 molecules, two ATP molecules, and hydrogen atoms that are used to make six NADH and two FADH2 molecules.

Electron Transport Chain The electrons from the hydrogen atoms in NADH and FADH2 are passed from molecule to molecule in the electron transport chain along the inner mitochondrial membrane. As the electrons move they lose energy. This energy is used to move protons from the matrix into the space between the inner and outer mitochondrial membranes. As protons leave the membrane space through ATP synthase ATP is produced. Oxygen combines with the electrons and protons to form water.

Energy Produced by Cell Respiration Cellular respiration can produce up to 38 ATP molecules from a single molecule of glucose. Most eukaryotic cells produce about 36 ATP molecules per molecule of glucose. Thus, cellular respiration is nearly 20 times more efficient than glycolysis alone.

Cellular Respiration

Extra Bonuses from Cell Respiration Molecules formed at different steps in glycolysis and the Krebs cycle are often used by cells to make compounds that are missing in food.