The Fundamental Unit of Life Critical Thinking Questions

In a nut shell:

Cellular respiration is the process of oxidizing food molecules, like glucose, to carbon dioxide and water.





Cellular respiration is the process by which food molecules react with oxygen and are broken down to carbon dioxide and water with a net gain of captured energy in the form of ATP molecules. Therefore, it is the conversion of CHEMICAL energy of ORGANIC molecules to METABOLICALLY USABLE energy within living cells. Most of the process occurs within MITOCHONDRIA in EUKARYOTIC cells. All organisms utilize the processes of cellular respiration to provide energy for cellular maintenance and for the production of starting materials for the biosynthesis of needed compounds.

Cellular respiration occurs 24 HOURS/DAY in all organisms. Respiration is sometimes called BIOLOGICAL OXIDATION and thus may be compared to BURNING. Although both are OXIDATION reactions, the manner in which the energy is RELEASED differs in the two processes. In both, the energy released is obtained from CHEMICAL BONDS. CARBON DIOXIDE may be a WASTE PRODUCT in both. In BURNING there is an UNCONTROLLED, rapid release of energy with accompanying HIGH TEMPERATURES, but in CELLULAR RESPIRATION energy is released in discrete amounts due to ENZYME CONTROL of the process. Cellular respiration is a series of ENZYMATIC REACTIONS, and biological combustion cannot take place ANY FASTER than the controlling enzymes will permit. In BURNING most of the energy is released in the form of HEAT and LIGHT, but in cellular respiration most of the energy is used to create NEW CHEMICAL BONDS and only a relatively small amount of heat energy is liberated (2nd Law of Thermodynamics).

Biological oxidation is NOT necessarily the direct action of oxygen on a substance. It may mean the REMOVAL OF some ELECTRONS resulting in the formation of HYDROGEN IONS (H+) which are passed along an assembly line of CARRIER MOLECULES. When the hydrogen ions and the electrons reach oxygen, they combine with it to form water.

All of the enzymes involved in these transfers and all of the carrier molecules are found in the mitochondria, some of them exclusively. Many of the reaction require a COENZYME which acts in concert with the enzyme. The mitochondria are also rich in coenzymes.

Concurrently with all these transfers and oxidations, another process is going on. The energy released by oxidations is not transformed into heat but into a form that can be used by the cells. Transformation consists of making energy-rich compounds of ADENOSINE TRIPHOSPHATE or ATP. Most of the energy required by the cell is provided in this way and most of it is found in the mitochondria. Some of the energy of ATP is used in the mitochondria but most of the ATP is immediately transferred to the cytoplasm to power the other activities of the cell.

The most common form of cellular respiration (AEROBIC) proceeds through three stages: GLYCOLYSIS, KREBS or CITRIC ACID CYCLE, and OXIDATIVE PHOSPHORYLATION (ELECTRON TRANSPORT CHAIN).

The cellular process used for the initial breakdown of 6-carbon sugars is called GLYCOLYSIS and occurs in the CYTOSOL of the cell. The first stage involves the breakdown of one glucose molecule (6 carbons) into two molecules of the 3-carbon sugar GLYCERALDEHYDE3PHOSPHATE (G3P) or PHOSPHOGLYCERALDEHYDE, commonly called PGAL. This reaction sequence requires the ENERGY OF ACTIVATION of two ATP molecules. In the second stage of glycolysis the PGAL is converted to another 3-carbon compound PYRUVATE or PYRUVIC ACID. This process is coupled to the formation of four molecules of ATP per molecule of glucose. This series of reactions results in a NET GAIN of TWO ATP molecules and two pairs of HYDROGEN ATOMS.

NET REACTION

C6H12O6 + 2 ATP 2(C3H4O3) + 4 ATP + 4 H

After its formation in the cytosol, pyruvate is transferred into the mitochondria where it is ultimately metabolized to carbon dioxide and water. The COST of the TRANSFER across the mitochondrial membrane is 2 ATP molecules. The final breakdown is accomplished by a series of reactions known as the KREBS CITRIC ACID CYCLE. Before beginning the cycle, pyruvate is converted into a 2-carbon fragment, known as ACETYL, with loss of carbon dioxide and 2 hydrogen atoms. The ACETYL fragment is carried into the cycle by COENZYME A or CoA. Once inside the cycle the CoA releases the Acetyl and returns to pick up additional acetyl fragments.

Any organic molecule that contains bond energy can be used as a fuel in cellular respiration. The common stage for almost all fuel molecules is the 2-carbon acetyl. The manner in which the acetyl stage is reached differs for different types of fuels.

KREBS CITRIC ACID CYCLE

This regenerating cycle is composed of 4-, 5-, and 6-carbon ORGANIC ACIDS. Prior to entering the cycle, the 3-carbon pyruvate is DECARBOXYLATED (carbon is removed) to give a 2-carbon ACETATE fragment. This fragment is activated by combining with COENZYME A to become Acetyl Coenzyme A. The carbon is given off as CARBON DIOXIDE and can be used as one measure of the rate of cellular respiration. The acetate fragment combines with a 4-carbon molecule in the cycle (OXALOACETIC ACID) to form the 6-carbon CITRIC ACID. As the cycle proceeds, CO2 is given off to produce a 5-carbon organic acid which subsequently loses another CO2 to become a 4-carbon organic acid. After a series of conversions and oxidations, the original 4-carbon oxaloacetic acid molecule is regenerated and is ready to combine with another acetate fragment and the cycle begins again.

ELECTRON TRANSPORT

Electron transport, also sometimes called OXIDATIVE PHOSPHORYLATION, is the process by which electrons are passed from the oxidation of Krebs Cycle organic acids to a series of electron acceptors (NAD, FAD or FMN, coenzyme Q, Cytochrome B, Cytochrome C, Cytochrome A, and Cytochrome A3). In this chain reaction, energy is transferred from the electron transport chain during the coupling of Pi (inorganic phosphate) to ADP to form ATP. This addition of phosphate to a molecule is called PHOSPHORYLATION.

If a pair of electrons is passed along the entire length of the electron transport chain, ATP is made in 3 separate places. Finally, after the electrons have lost most of their energy, they are transferred to molecular oxygen to produce H2O. The consequences of electron transport are to synthesize ATP and deliver protons (H+) and electrons (-) to oxygen forming water. This is the only point at which oxygen is required by aerobic organisms.

Glycolysis

C6H12O6 + 2 ATP 4 ATP + 2(C3H4O3) + 4 H

Krebs Cycle

2(C3H4O3) + 6 H2O 6 CO2 + 20 H

Electron Transport Chain

24 H + 6 O2 12 H2O + 36 ATP

Net Reaction

C6H12O6 + 6 O2 6 CO2 + 6 H2O + 38 ATP

The cell would lake the ability to reproduce.