Earth Science: Biology – Evolution – General Cell Metabolism
The cells of all living things need enough energy to carry out their life processes. Heterotrophs (organisms that feed on other living things) gain this energy from breaking down energy-rich carbohydrates, fats, and proteins.
Aerobic energy production (cellular respiration) begins with glycolysis, which takes place in the cytoplasm of the cell.
Glycolysis is a chain of reactions in which carbo-hydrates such as glucose are broken down into pyruvate without the need for oxygen.
Glycolysis
At the start of glycolysis, two molecules of ATP (see basics) each contribute a phosphate group to a glucose molecule, which is a 6-carbon sugar. This produces fructose 1,6-biphosphate. This activation makes it easier to split the sugar into two nonidentical 3-carbon molecules. They are later transformed into two identical compounds (glyceraldehyde 3-phosphate).
Both molecules are then oxidized to pyruvate, and some of the energy released through this process is transferred to ADP, forming the energy storage molecule ATP. In addition, some energy is transferred to NAD* in the form of electrons (see basics). Later, highly energy-rich ATP is formed from the hydrogen-containing compound NADH/H+
The citric acid cycle
The citric acid cycle, which is also known as the Krebs cycle, derives energy with maximum efficiency from pyruvate, the product of glycolysis, which is still quite rich in energy. It does so by breaking down the molecule completely into three molecules of CO2 (oxidative decarboxylation), which we breathe out as a waste product.
The hydrogen produced in large quantities during these oxidative steps is captured in the form of NADH/H+ or FADH2, which will later be used to produce energy (ATP synthesis). In addition, a molecule of an ATP analogue, GTP, is formed directly. There is an intermediate activation step between glycolysis and the citric acid cycle, in which the first CO2 molecule is removed, and coenzyme A is added to the resulting acetic acid.
The product, acetyl-CoA, now enters the multistep cycle, binding with a 4-carbon (C4) compound (oxaloacetate), to form citrate (a C f compound). Some oxaloacetate also emerges from this process, reacting again with a C2 compound. Among eukaryotes, the citric acid cycle takes place in the mitochondria (membrane-enclosed organelles); in prokaryotes, it occurs in the cytoplasm (gelatinous fluid that fills cells). Like glycolysis, it still does not require oxygen.
The electron transport chain
The resulting H2 -rich compounds NADH/H+ and FADH2 are now used in the last phase of cellular respiration-oxidative phosphorylation, or the electron transport chain—to store energy in the form of ATP. During this process, electrons are transferred in a cascade of reactions onto the oxygen that is inhaled by the organism.
This multistage reaction process using various redox systems is important, since otherwise the oxidation of H2 to water would occur in an explosive reaction. The electron transport chain takes place on membranes: among eukaryotes, on those of the mitochondria, and in prokaryotes, on the cell membrane.
EXPLAINING CELLULAR RESPIRATION
Among the scientists whose research significantly contributed to the understanding of cellular respiration is German biochemist Otto Heinrich Warburg (1883-1970). One of the 20th century’s greatest cell biologists, he was particularly interested in cellular respiration in cancer cells.
He received the Nobel Prize in physiology or medicine in 1931, for his discovery of the nature and mode of action of the respiratory enzyme (cytochrome oxidase).
BASICS
ATP, THE MULTIFUNCTIONAL nucleotide adenosine triphosphate, composed of adenine, ribose, and three phosphate groups, is the most important energy storage substance in cellular respiration.
The stored energy (30 kj/mol) can later be released through the removal of a phosphate group (ATP –> ADP + Pi)
NAD + (nicotinamide adenine dinucleotide) is a hydro- gen bearing coenzyme; it acts as an electron acceptor.