Metabolism: Anaerobic Cellular Respiration
Overview
Anaerobic = refers to without oxygen
Cellular Respiration = metabolic reactions that convert energy from nutrients into ATP
Anaerobic Cellular Respiration occurs in organisms when oxygen is not available to continue the process of aerobic cellular respiration.If there is not enough oxygen present, glycolysis stops because NADH cannot be oxidized back into NAD+. Organisms have evolved two processes to allow glycolysis to continue in the absence of oxygen. These processes are known as Lactate (Lactic Acid) Fermentation and Ethanol Fermentation.
Cellular Respiration = metabolic reactions that convert energy from nutrients into ATP
Anaerobic Cellular Respiration occurs in organisms when oxygen is not available to continue the process of aerobic cellular respiration.If there is not enough oxygen present, glycolysis stops because NADH cannot be oxidized back into NAD+. Organisms have evolved two processes to allow glycolysis to continue in the absence of oxygen. These processes are known as Lactate (Lactic Acid) Fermentation and Ethanol Fermentation.
Anaerobic vs. aerobic
curriculum expectations
Overall Expectations
Specific Expectations
- C2. investigate the products of metabolic processes such as cellular respiration and photosynthesis;
- C3. demonstrate an understanding of the chemical changes and energy conversions that occur in metabolic processes
Specific Expectations
- C2.2 conduct a laboratory investigation into the process of cellular respiration to identify the products of the process, interpret the qualitative observations, and display them in an appropriate format
- C3.1 explain the chemical changes and energy conversions associated with the processes of aerobic and anaerobic cellular respiration
- C3.4 describe, compare, and illustrate (e.g., using flow charts) the matter and energy transformations that occur during the processes of cellular respiration (aerobic and anaerobic) and photosynthesis, including the roles of oxygen and organelles such as mitochondria and chloroplasts
anaerobic pathways
Glycolysis allows organisms to obtain energy from nutrients in the absence of oxygen. However, step 6 (See diagram below) of the glycolytic pathway reduces NAD+ to NADH. There is a limited supply of NAD+ in cells. If glycolysis continues without oxidizing NADH back into NAD+, step 6 will be blocked and glycolysis will stop. In aerobic cellular respiration, the Electron Transport Chain (ETC) oxidizes NADH to NAD+, allowing glycolysis to continue. So without oxygen how cells oxidize NADH back into NAD+?
Organisms have evolved a couple of ways of recycling NAD+ and allowing glycolysis to continue when oxygen is not available. One method involves transferring the hydrogen atoms of NADH to certain organic molecules instead of the ETC. This process is called fermentation. Bacteria have evolved multiple forms of fermentation, however, eukaryotes use ethanol fermentation and lactate (lactic acid) fermentation.
Organisms have evolved a couple of ways of recycling NAD+ and allowing glycolysis to continue when oxygen is not available. One method involves transferring the hydrogen atoms of NADH to certain organic molecules instead of the ETC. This process is called fermentation. Bacteria have evolved multiple forms of fermentation, however, eukaryotes use ethanol fermentation and lactate (lactic acid) fermentation.
Lactate (Lactic Acid) Fermentation
In normal circumstances, animal cells use aerobic respiration. However, during strenuous exercise when there is a shortage of oxygen muscle cells respire glucose faster than oxygen can be supplied.
The limited number of NADH molecules present can't give up their H atoms to the ETC. This causes all of the remaining NAD+ to remain in NADH form causing glycolysis to stop.
This problem has been solved evolutionarily by converting the pyruvate molecules to Lactate which reduces the NADH to NAD+. The NAD+ can then go back into glycolysis to produce more pyruvate molecules. (See diagram below)
The limited number of NADH molecules present can't give up their H atoms to the ETC. This causes all of the remaining NAD+ to remain in NADH form causing glycolysis to stop.
This problem has been solved evolutionarily by converting the pyruvate molecules to Lactate which reduces the NADH to NAD+. The NAD+ can then go back into glycolysis to produce more pyruvate molecules. (See diagram below)
Related Activity
1. Share this video with the class to demonstrate the effects of lactic acid build up in the muscles due to a lack of oxygen.
Oxygen Debt
The accumulation of lactate molecules 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. The extra oxygen required to catabolize lactate to CO2 and H2O (through the aerobic pathway) is referred to as oxygen debt. Panting after bouts of strenuous exercise is the body’ s way of “paying” the oxygen debt.
When vigorous exercise ceases, lactate is oxidized back to pyruvate, which then goes through the Krebs cycle and oxidative phosphorylation. The extra oxygen required to catabolize lactate to CO2 and H2O (through the aerobic pathway) is referred to as oxygen debt. Panting after bouts of strenuous exercise is the body’ s way of “paying” the oxygen debt.
Related Activity
1. Get students to hold their breath and see who can hold their breath the longest! After, discuss with the students what they felt once they could breathe again. The should have felt out of breath and as a result were "panting".
2. Show this video to help illustrate what happens when you run out of oxygen:
2. Show this video to help illustrate what happens when you run out of oxygen:
Ethanol Fermentation
Ethanol Fermentation is a biological process that converts sugars into energy that can be used by the organism.The waste products from this process are ethanol and carbon dioxide.
Similar to lactate fermentation, in step 6 of glycolysis, 2 NADH are produced. During the process of glycolysis total of 4 ATP are produced. 2 ATP were used in steps 1 and 3 with a net production of 2 ATP and 2 NADH. The remaining energy is trapped in 2 pyruvate molecules.
When oxygen levels are low, ethanol fermentation begins. CO2 is removed from the pyruvate by the enzyme pyruvate decarboxylase and 2 acetaldehyde are formed. The 2 NADH oxidizes to NAD+ by reducing 2 acetaldehyde to ethanol.The NAD+ can then be used in glycolysis again.The by-products CO2 & Ethanol are then removed as waste.
Similar to lactate fermentation, in step 6 of glycolysis, 2 NADH are produced. During the process of glycolysis total of 4 ATP are produced. 2 ATP were used in steps 1 and 3 with a net production of 2 ATP and 2 NADH. The remaining energy is trapped in 2 pyruvate molecules.
When oxygen levels are low, ethanol fermentation begins. CO2 is removed from the pyruvate by the enzyme pyruvate decarboxylase and 2 acetaldehyde are formed. The 2 NADH oxidizes to NAD+ by reducing 2 acetaldehyde to ethanol.The NAD+ can then be used in glycolysis again.The by-products CO2 & Ethanol are then removed as waste.
The following pictures are some other ways humans have learned to use the waste products from ethanol fermentation.
Related activity
1. The following is a exciting "visual" experiment to demonstrate ethanol fermentation to the class! Materials: water, balloons, water bottles, yeast, and sugar. Directions are explained in the video
handout (fill-in-the-blank)
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Family feud game
This is a fun way to end of the lesson:
- Split the class into two groups
- Have one student from each group come up to the front of the class
- Read one of the provided questions
- The students must race to answer the question
- Award one point to the student who gets the question correct
- Some sample questions can be found in the document below
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Glossary
Electron Transport Chain - is a series of compounds that transfer electrons from electron donors to electron acceptors via redox reactions, and couples this electron transfer with transfer of protons (H+ ions) across a membrane
Ethanol Fermentation - is a biological process that converts sugars from glucose into cellular energy, producing ethanol and carbon dioxide as byproducts.
Glycolysis - is a metabolic pathway that converts glucose into pyruvate
Lactate (Lactic Acid) Fermentation - is a biological process that converts sugars from glucose into cellular energy and metabolic lactate
Ethanol Fermentation - is a biological process that converts sugars from glucose into cellular energy, producing ethanol and carbon dioxide as byproducts.
Glycolysis - is a metabolic pathway that converts glucose into pyruvate
Lactate (Lactic Acid) Fermentation - is a biological process that converts sugars from glucose into cellular energy and metabolic lactate