Anaerobic Respiration

The Anaerobic Pathway Glycolysis allows organisms to obtain energy from nutrients in the absence of oxygen. However, step 6 (G3P to BPB) of the glycolytic pathway reduces NAD to NADH. If glycolysis continues without a mechanism to oxidize NADH back into NAD, step 6 will be blocked and glycolysis will come to a halt. Organisms have evolved several ways of recycling NAD allowing glycolysis to continue when oxygen is not available.

Fermentation There are two different ways in which fermentation occurs: One method involves transferring the hydrogen atoms of NADH to certain organic molecules instead of the electron transport chain. This process is called fermentation. Bacteria have evolved dozens of different forms of fermentation, but eukaryotes primarily use two methods: ethanol fermentation and lactate (lactic acid) fermentation.

This diagram shows where fermentation would start if there is a lack aerobic conditions. Glycolysis occurs due to it’s ability to create energy anaerobically (with out oxygen) and fermentation would occur at the end of the Glycolytic pathway.  There are then two different means in which energy can be produced depending on the organism.

Ethanol Fermentation Yeast when under anaerobic conditions converts glucose to pyruvic acid (via the glycolysis pathway). NADH passes its hydrogen atoms to acetaldehyde, a compound formed when a carbon dioxide molecule is removed from pyruvate which forms ethanol. This process allows NAD to be recycled and glycolysis to continue and occurs in the cytoplasm. The two ATP molecules produced satisfy the organism’s energy needs, and the ethanol and carbon dioxide are released as waste products. *Note: *Yeast is a eukaryotic micro-organism classified as a fungi

As shown above, glucose is converted into 2 Pyruvate molecules, which is further converted to 2 Acetaldehyde molcules where 2 Carbon Dioxide molecules and finally 2 Ethanol are released. In order for this entire process to occur NAD+ must be converted to NADH and 2 H+ ions (vice versa) or this process will not occur.\

Applications of Fermentatio Bread is leavened by mixing live yeast cells with starches and water. The yeast cells ferment the glucose from the starch and release carbon dioxide and ethanol. Small bubbles of carbon dioxide gas cause the bread to rise (or leaven) and the ethanol evaporates away when the bread is baked. In beer and wine making, yeast cells ferment the sugars found in carbohydrate-rich fruit juices, such as grape juice. The mixture bubbles as the yeast cells release carbon dioxide gas and ethanol during fermentation. The percentage of Ethanol in the solution stabilizes when the increase in Ethanol kills the remaining yeast cells.

Lactate Fermentation Occurs when NADH reduces pyruvate directly to form lactate as an end product. If one glucose molecule goes through glycolysis, 2 net ATP and 2 pyruvate molecules are produced and 2 NAD+ molecules are consumed.

2 NADH molecules and 2 H+ come and reduce the 2 pyruvate molecules in the cytoplasm of the cell, forming 2 lactate molecules and 2 NAD+. The 2 NAD+ molecules are then reused in glycolysis, enabling the cell to produce ATP even in the absence of oxygen.

What happens with the Lactate that is produced? The accumulation of lactate molecules in the blood and in muscle tissue causes stiffness, soreness, and fatigue. Lactate is transported through the bloodstream from the muscles to the liver. When vigorous exercise ceases, lactate is oxidized back to pyruvate,which then goes through the Krebs cycle and oxidative phosphorylation.

VO2 and Lactate Threshold? Lactate fermentation occurs continuously as you exercise. However, as exercise intensity increases, lactate production increases. The increase in Lactate also causes an interference between areas of muscle fibres which inhibit muscular contraction. The lactate threshold (LT) is the value of exercise intensity at which blood lactate concentration begins to increase sharply.

In this graph the amount of Lactate that remains with in the blood will sharply increase as the intensity of an activity increases.  This point is called the “Onset of Blood Lactate” which is the area on the line where there is a dramatic increase. This graph shows that levels of blood Lactate change as an individual trains more often. This means the person can sustain a high work output for longer without the onset of Lactate interfering with their performance.

One measure of performance is the percentage of VO2 max (maximum oxygen consumption) at which the LT is reached.  This is the maximum volume of oxygen (in mL) that the cells of the body can remove from the bloodstream in one minute per kg of body mass.