Category: 4. Respiration

Temperature—To a point, the higher the temperature the faster respiration occurs. At some temperature, enzymes will become inactivated, although there are thermophilic (heat-loving) organisms that do quite well in high-temperature environments. Energy from sugar is released faster as the rate of respiration increases which results in a net weight loss. Plants offset the weight loss by increasing photosynthetic production of sugar. Note that during respiration, some of the energy is lost as heat, which results in an overall increase in organism temperature—not necessarily detectible by human hands.

Water—Enzymes generally operate in the presence of water, and reduced water in a plant will reduce the rate of respiration. Seeds usually have a water content of less than 10%, while mature living cells usually are in excess of 90% water. Seeds keep better if they are kept dry as the respiration rate remains quite low. However, if a seed comes into contact with water and via imbibition swells, the respiration rate will skyrocket. The temperature could increase to the point of killing the seeds. Spontaneous combustion can occur from the respiration generated heat when a fungus or bacterium is permitted to grow on wet seeds. Kind of a neat little trivia fact to tuck away.

Oxygen—Oxygen is an important regulator of respiration. If oxygen is drastically reduced, respiration may drop off to the point of retarding growth or death. Low oxygen concentrations can lead to the onset of fermentation processes.

As a whole, from glycolysis to finish aerobic respiration yields the following:

Glycolysis:

• 4 molecules of ATP +2 molecules of NADH (which yields 4 ATP in the ETS) = 8 molecules of ATP net
• 8 molecules of ATP net – 2 ATP molecules to start the glycolysis process = 6 ATP molecules

Conversion of pyruvic acid to acetyl CoA:

• 2 molecules of NADH (yields 6 ATP in the ETS)

Krebs Cycle:

• 2 molecules of ATP + 2 molecules of FADH₂ (which yields 4-ATP in the ETS) + 6 molecules of NADH₂ (which yields 18-ATP in the ETS) = Total ATP yield: 36

The 36 resulting ATP molecules represent approximately 39% of the energy in a molecule of glucose. Compared to each other, aerobic respiration is about six times as efficient as anaerobic respiration.

Anaerobic respiration and fermentation result in a net gain of 2 ATP molecules from glycolysis. It should be noted, that the by-products of these processes, lactic acid, and alcohol, will eventually kill the organism if the products are not digested.

Glycolysis—the first step does not require oxygen gas (O₂) and takes place in the cytoplasm. The glycolytic phase is subdivided into three main steps and several smaller ones. Each step is mediated by an enzyme. A small amount of energy is released and hydrogen atoms are removed from compounds derived from glucose. The main gist of the steps are:

• The glucose molecules go through several steps and become a double phosphorylated fructose molecule.
• The 6-carbon fructose molecule is split into two 3-carbon fragments, each with a phosphate, GA3P
• Hydrogen, energy, and water are removed from the GA3P molecules leaving pyruvic acid.

Glycolysis requires two molecules of ATP to get the process started. In the processes, four ATP molecules are created, with a net gain of 2 ATP molecules at the end of glycolysis. The hydrogen ions and electrons that are released are held by an acceptor molecule called NAD—nicotinamide adenine dinucleotide. The overall end products of glycolysis is: 2-ATP molecules, 2-NADH molecules, and pyruvic acid.

The next step depends on the kind of respiration involved—aerobic, true anaerobic or fermentation. In aerobic respiration (with oxygen present), the next steps are Krebs Cycle and Electron Transport Chain.

The Krebs Cycle (or citric acid cycle)—The Krebs cycle takes place in the fluid matrix of the cristae compartments of the mitochondria. It is called the citric acid cycle because of all the intermediate acids in the cycle. The pyruvic acid product of glycolysis is restructured, some of the CO₂ is lost and becomes acetyl CoA which then dumps into the Krebs cycle. During the restructuring of pyruvic acid, a molecule of NADH is produced. The Krebs cycle removes energy, CO_2, and hydrogen from acetyl CoA via enzyme-mediated reactions of organic acids.

The hydrogen removed during the Krebs cycle is picked up by FAD and NAD acceptor molecules. The end result of the metabolizing of two acetyl CoA molecules in the Krebs cycle is two ATP molecules, oxaloacetic acid (to further drive the cycle), six NADH₂ molecules, two FADH₂ molecules, and two CO₂ molecules.

The NAD and FAD molecules and the hydrogens that they carry will be dumped into the next step in respiration in order to extract the energy from the molecules.

The Electron Transport Chain—The electron transport chain (ETC) is a bit like a bucket brigade in that the chain passes the molecules along until the job is done. Energy is released as the hydrogen and electrons from the NAD+ and FAD+ carrier molecules are dumped into the system. When the electrons reach the end of the chain they pick up oxygen and water is released. ATP is produced by oxidative phosphorylation during the action of the electron transport chain. This occurs essentially like chemiosmosis.

Respiration is the group of processes that utilizes the energy that is stored through the photosynthetic processes. The steps in respiration are small enzyme-mediated steps that release tiny amounts of immediately available energy, the energy released is usually stored in ATP molecules which allow for even more efficient use of an organism’s energy. Respiration occurs in the mitochondria and cytoplasm of cells.

There are several forms of respiration: aerobic—which requires oxygen, anaerobic—which occurs in the absence of oxygen, and fermentation—which also occurs in the absence of oxygen.

Aerobic respiration is the most common form of respiration and cannot be completed without oxygen gas. The controlled release of energy is the main event in aerobic respiration.

Certain types of bacteria and other organisms without oxygen gas carry on anaerobic respiration and fermentation. Compared to aerobic respiration the amount of energy released is quite small. The main difference between aerobic respiration and fermentation is in the way hydrogen is released and combined with other substances. Two very common forms of fermentation are summed up by the following equations:

(Equation 1)  C₆H₁₂O₆ (glucose) -> (with enzymes)-> 2C₂H₅OH  (ethyl alcohol)+ 2CO₂ (carbon dioxide) + energy (ATP)

(Equation 2)  C₆H₁₂O₆ (glucose) -> (with enzymes) -> 2C₃H₆O₃ (lactic acid) + energy (ATP)

Note the first equation is particularly valuable to the brewing industry.