Discovery Science: Earth – Cells – Further Metabolic Processes

Earth Science: Cells – Further Metabolic Processes

Cell metabolism involves the transport of electrons in a multistep process utilizing oxygen (aerobic) or in a less efficient process of fermentation without the use of oxygen (anaerobic).

The cell’s metabolic processes, such as the breakdown of glucose (glycolysis) and the citric acid cycle, lead to the formation of the hydrogen-rich compounds NADH/H and FADH … In the last stage of cellular respiration—the electron transport chain- comparatively large amounts of energy are gained in the form of ATP (an energy storage molecule as well as a building block of the nucleic acids DNA and RNA).

Additional amounts of NAD* or FAD are also formed for reuse. During this process, electrons are transferred in a multistep process to oxygen molecules breathed in by the organism. The multistage reaction process over different redox systems (using various enzymes and cofactors) is important; otherwise the oxidation of H , to water would take place as an explosive reaction.

The electron transport chain occurs on membranes: within the mitochondria among eukaryotes, and on the cell’s inner membrane for prokaryotes. During some redox stages, protons are transported from the interior of the mitochondrion or cell into the space between the inner and outer membrane. This proton gradient is then used for ATP synthesis (oxidative phosphorylation).

Through the aerobic (oxygen-utilizing) breakdown of a single glucose molecule, 36 to 38 molecules of ATP are formed from ADP and phosphate.


Many bacteria, some fungi (such as yeast), and some animal and human cells are able to break down nutrients not only aerobically, but also anaerobically (without oxygen), although this breakdown is not as complete.

This process, called fermentation, begins with glycolysis, just as aerobic metabolic processes do, with pyruvate as an end product. Since no oxygen is available, however, the hydrogen of the NADH-+ H- cannot be oxidized to water as it is in the electron trans- port chain. Instead, it is transferred to intermediate reaction products, where it serves to reduce them.

In the more Complete aerobic breakdown process, the energy- poor molecules CO2 and H2O emerge as end products. With fermentation, however, the end products still contain substantial amounts of energy, such as lactic acid in lactic acid fermentation. Much less energy is thus gained from fermentation than from aerobic processes, so that, for instance, only two ATP molecules are formed from one glucose molecule through lactic acid fermentation.

Fatty acid breakdown

In addition to carbohydrates, cells can also break down other substances, such as fats, and use them to produce energy. After the splitting (hydrolysis) of a fat into its components of fatty acids and monoglycerides, the glycerides are used in glycolysis as C3-compounds, while the fatty acids are attached to coenzyme A in the mitochondrial matrix and broken down in stages (13-oxidation).

Acetyl-CoA (C2-compound) is removed during each stage of the process, until the fatty acid is completely broken down. The resulting Acetyl-CoA molecules then enter the citric acid cycle.


BIOMEMBRANES Only about eight nanometers thick, cell membranes consist of a semiliquid double layer of phospholipids. These molecules have a polar (water- soluble) “head” and two nonpolar (water-repelling) fatty acid “tails.”

Other substances are also associated with the cell membrane, such as enzyme complexes of the electron transport chain, as well as special proteins.