CONCEPTUAL LIFE SCIENCE
Cellular Respiration
Introduction
Cellular respiration is the oxidative, chemical attack on energy-rich molecules to provide useful energy for the cell. Enzymes catalyze the oxidation reactions. These reactions are known as catabolic reactions because they break molecules down to release energy.
Anaerobic respiration
The first part of respiratory pathways in the cell is anaerobic. This term means that oxygen is not involved. Even cells of organisms that utilize oxygen, such as humans, have an anaerobic component. The anaerobic component is known as glycolysis and the Embden-Meyerhof pathway carries it out. This pathway produces ATP and NADH as energy-storage molecules. NADH is similar to the NADPH produced in photosynthesis. In anaerobic respiration, this NADH needs to be recycled. The total anaerobic pathway including the recycling process is called fermentation. We will consider two kinds of fermentation reactions. The first will be the lactic acid fermentation as it occurs in muscles. The second will be the alcohol fermentation as it occurs in yeast.
Glycolysis
Glycolysis is a pathway that the cell uses to oxidize glucose. Some of the steps in this pathway are the reverse of steps found in the dark reactions of photosynthesis. Glycolysis is part of the fermentation process found in muscle and yeast cells.
It is important to avoid confusion between photosynthesis and cellular respiration. Photosynthesis is reductive and anabolic. It produces glucose and releases oxygen as a waste product. In contrast, cellular respiration is oxidative and catabolic. It uses glucose and oxidizes it to produce CO2 as the waste product.
Fermentation
Fermentation is used by muscle cells to provide quick energy. Yeast cells use it to produce alcohol and CO2. This is why yeast is used for brewing and baking. Lactic acid bacteria produce lactic acid, which is used to make fermented foods like pickles, sauerkraut and yogurt.
Fermentation is required under anaerobic conditions because of the need to recycle NADH. The Embden-Meyerhof pathway produces two molecules of NADH. These cannot be used to make ATP under anaerobic conditions so they are recycled and oxidized using the fermentation process. This recycling permits the Embden-Meyerhof pathway to continue functioning.
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Glycolysis is the conversion of glucose to pyruvate. Energy is made available. It is stored as ATP. This process occurs in all cells, all the time.
Figure 7-1. Glycolysis. Energy is made available all the time. Two ATP are used to start and the pathway makes 4 ATP, so there is a net yield of two ATP. The PGAL to PGA step is the reverse of a similar step in the Calvin-Benson cycle. The resulting two NADH molecules must be reoxidized (recycled) to permit the pathway to continue working.
Fermentation is the reduction of pyruvate (using the NADH) to produce a reduced product such as lactic acid or alcohol.
Net reactions of anaerobic respiration.
A. Lactic acid fermentation in muscle
Glucose + 2 Pi + 2 ADP à 2 lactate + 2 ATP
B. Alcohol fermentation in yeast
Glucose + 2 Pi + 2 ADP à 2 ethanol + 2 CO2 + 2 ATP
Each of the reactions begins with glucose and produces a net yield of two ATP. This low number of ATP results from the lack of aerobic (oxidative) conditions. Muscles use fermentation under conditions when oxygen is low or when energy is needed quickly.
Yeast cells are used in the processes of brewing and baking. In brewing, the product, such as wine, contains alcohol. In the case of beer, the product contains both alcohol and CO2. In baking, the product of interest is the CO2. The carbon dioxide helps the baked products rise. That is why yeast is used in making bread, cakes and other baked products.
Recycling of NADH
The NADH produced in the glycolysis reactions is in its reduced form. It must become oxidized again in order for the pathway to function. The final fermentation reactions are used to regenerate or recycle NAD+ for use in the Embden-Meyerhof pathway.
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Figure 7-2. Production of NAD+ by recycling.
The central reaction of the Embden-Meyerhof pathway requires the oxidized form of NAD+. The pathway converts it to the reduced form of NADH. In order for the pathway to continue, the NADH must be recycled.
In oxidative conditions, the NADH is not recycled but is sent to the Electron Transport/Chemiosmosis pathway instead. Under oxidative conditions, fermentation does not take place. The pyruvate resulting from the Embden-Meyerhof pathway is sent to the Krebs Citric Acid Cycle, instead.
Aerobic respiration
Aerobic respiration consists of three parts.
· Glycolysis
· Krebs Citric Acid Cycle
· Electron Transport/Chemiosmosis
Overall reaction of aerobic respiration
C6H12O6 + 6 O2 à 6 CO2 + 6 H2O + energy
Aerobic respiration is the reverse of photosynthesis. The glucose made by the plant is oxidized in the cell to produce the waste products, CO2 and H2O, and energy. Cellular respiration is the opposite of photosynthesis. The energy stored by the plant is released in our cells for our use.
Glycolysis
Glycolysis produces two pyruvates, two ATP and two NADH. Unlike the situation in fermentation, the two NADH molecules are not recycled. The process occurs in the cytoplasm of the cell.
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Krebs Citric Acid Cycle
The Krebs Citric Acid Cycle is a series of eight reactions that go around in a circle. It is found in mitochondria. Each pyruvate molecule resulting from glycolysis enters the cycle and is oxidized to produce three CO2, and the following energy molecules:
4 NADH, 1 FADH2 and 1 ATP.
Electron Transport/Chemiosmosis
This is the final pathway of aerobic respiration. It occurs in the mitochondria. The pathway removes electrons and protons from the NADH and FADH2 molecules. FAD is Flavin Adenine Dinucleotide. It is an electron carrier like NADH. FADH2 is the reduced form that carries the electrons and protons.
Electron Transport and Chemiosmosis occur in the mitochondria. The electrons are passed through the electron transport chain, a series of enzymes that transport electrons. The electrons eventually end up on oxygen, which is the terminal electron acceptor of the chain. The protons are pumped out of the mitochondria through the mitochondrial membrane. The pumping mechanism is called chemiosmosis. Energy from chemiosmosis produces 3 ATP for each NADH and 2 ATP for each FADH2. So, the result of all of these pathways is to make useful energy for the cell in the form of ATP.
Final elimination of electrons is done by adding electrons and protons to a molecule of oxygen. The result is the production of hydrogen peroxide (H2O2). Hydrogen peroxide is a toxic substance. The cell has an enzyme called catalase that destroys hydrogen peroxide producing oxygen and water, thereby rendering in harmless by removing it.
Figure 7-3. The catalase reaction.
Virtually all enzymes are proteins. Their job is to catalyze biological reactions and allow these reactions to occur at room temperature. Catalase is the name of a specific enzyme that performs the reaction responsible for the destruction of hydrogen peroxide. Each enzyme is a catalyst, which is specific for its own substrate. The substrate is the specific chemical substance reacted on by the enzyme. The enzyme changes the substrate into products. In the case of the destruction of hydrogen peroxide
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by catalase, the substrate was hydrogen peroxide (the starting material) and the products were water and oxygen (the resulting materials).
Total energy yield for one glucose by aerobic respiration
a. Yield from glycolysis: 2 ATP direct
6 ATP from 2 NADH
b. Krebs Cycle (per pyruvate): 1 ATP direct
2 ATP from FADH2
12 ATP from NADH
c. Total Yield: 8 ATP from glycolysis
30 ATP from 2 pyruvates in Krebs Cycle
38 ATP total yield
Note: Normal totals in eukaryotic cells are 36 molecules of ATP. The figure of 38 ATP comes from prokaryotic cells, which do not have mitochondria.
Study guide for cellular respiration
A. General considerations
1. Respiration is used to obtain energy.
2. The energy is in the form of ATP.
3. The by-products of respiration are H2O and CO2.
4. The process of respiration is conducted by all living cells, all of the time.
B. Anaerobic respiration (fermentation)
1. Anaerobic respiration does not require air or O2.
2. The process is used to obtain energy.
3. Few ATP molecules (2 ATP) are produced.
4. The end-products are:
a. muscle: lactic acid
b. yeast: alcohol and CO2
5. The process occurs in the cytoplasm of the cell.
C. Aerobic respiration
1. Aerobic respiration uses anaerobic respiration (glycolysis) as the first set
of reactions.
2. Conversion of pyruvate to CO2 and H2O gives much more energy than
anaerobic respiration.
3. Aerobic respiration occurs in the mitochondria.
4. ATP is produced from NADH and FADH2.