CELLULAR RESPIRATION

The process of aerobic respiration occurs in several stages, primarily in eukaryotic cells, and requires oxygen to fully break down glucose into energy, carbon dioxide (CO₂), and water (H₂O). Below is a detailed explanation of each stage:

1. Glycolysis (Cytoplasm)

• What Happens:

  • One molecule of glucose (6-carbon) is broken down into two molecules of pyruvate (3-carbon).

  • ATP and NADH (electron carriers) are produced.

• Products:

  • 2 ATP (net gain).

  • 2 NADH (carrying electrons to later stages).

  • 2 pyruvate molecules.

STEP 1: The first step in glycolysis is catalyzed by hexokinase, an enzyme with broad specificity that catalyzes the phosphorylation of six-carbon sugars. The product is GLUCOSE-6-PHOSPHATE, a more reactive form of glucose. 

STEP 2: In the second step of glycolysis, an isomerase converts glucose-6-phosphate into one of its isomers, FRUCTOSE-6-PHOSPHATE.

STEP 3: The third step is the phosphorylation of fructose-6-phosphate, catalyzed by the enzyme phosphofructokinase. A second ATP molecule donates a high-energy phosphate to fructose-6-phosphate, producing FRUCTOSE-1,6-BISPHOSPHATE.

STEP 4: The fourth step in glycolysis employs an enzyme, aldolase (fructose biphosphate aldolase), to cleave 1,6-bisphosphate into two three-carbon isomers: DIHYDROXYACETONE-PHOSPHATE and GLYCERALDEHYDE-3-PHOSPHATE.

STEP 5: In the fifth step, an isomerase (triose phosphate isomerase) transforms the dihydroxyacetone-phosphate into its isomer, GLYCERALDEHYDE-3-PHOSPHATE.

STEP 6: The sixth step in glycolysis oxidizes the sugar (glyceraldehyde-3-phosphate) by glyceraldehyde-3-phosphate dehydrogenase, extracting high-energy electrons, which are picked up by the electron carrier NAD+, producing NADH. The sugar is then phosphorylated by the addition of a second phosphate group, producing 1,3-BISPHOSPHOGLYCERATE.​​

​STEP 7: In the seventh step, catalyzed by phosphoglycerate kinase, 1,3-bisphosphoglycerate donates a high-energy phosphate to ADP, forming one molecule of ATP. A carbonyl group on the 1,3-bisphosphoglycerate is oxidized to a carboxyl group, and 3-PHOSPHOGLYCERATE is formed.​

STEP 8: In the eighth step, the remaining phosphate group in 3-phosphoglycerate moves from the third carbon to the second carbon, producing 2-PHOSPHOGLYCERATE. The enzyme catalyzing this step is a mutase (phosphoglycerate mutase).​

​STEP 9: Enolase catalyzes the ninth step. This enzyme causes 2-phosphoglycerate to lose water from its structure resulting in the formation of a double bond that increases the potential energy in the remaining phosphate bond and produces PHOSPHOENOLPYRUVATE (PEP).​

STEP 10: The last step in glycolysis is catalyzed by the enzyme ph substrate-level phosphorylation and the compound PYRUVIC ACID (or its salt form, PYRUVATE).

After the process of glycolysis, its products: pyruvic acid/pyruvate, NADH, and ATP will be utilized in the next parts of aerobic cellular respiration. The next stage is the Krebs Cycle. However, the molecule needed for this stage is not pyruvic acid itself, so it needs to be oxidized into acetyl coenzymeA (acetyl coA) as shown in the figure below.

2. Pyruvate Oxidation (Mitochondrial Matrix)

• What Happens:

  • Each pyruvate molecule is converted into acetyl-CoA (2-carbon molecule).

  • CO₂ is released as a byproduct.

  • NAD⁺ is reduced to NADH.

• Products per Pyruvate:

  • 1 NADH.

  • 1 CO₂.

  • 1 Acetyl-CoA.

3. Citric Acid Cycle (Krebs Cycle) (Mitochondrial Matrix)

• What Happens:

  • Acetyl-CoA enters the cycle and combines with a 4-carbon molecule (oxaloacetate) to form citrate (6-carbon molecule).

  • The cycle releases CO₂ and transfers high-energy electrons to NAD⁺ and FAD to form NADH and FADH₂.

  • ATP is produced through substrate-level phosphorylation.

Step 1: Acetyl CoA (two-carbon molecule) joins with oxaloacetate (4 carbon molecules) to form citrate (6 carbon molecules).

Step 2: Citrate is converted to isocitrate (an isomer of citrate)

Step 3: Isocitrate is oxidized to alpha-ketoglutarate (a five-carbon molecule) which results in the release of carbon dioxide. One NADH molecule is formed. The enzyme responsible for catalyzing this step is isocitrate dehydrogenase. This is a rate-limiting step, as isocitrate dehydrogenase is an allosterically controlled enzyme

.

Step 4: Alpha-ketoglutarate is oxidized to form a 4 carbon molecule. This binds to coenzyme A, forming succinyl CoA. A second molecule of NADH is produced, alongside a second molecule of carbon dioxide.

Step 5: Succinyl CoA is then converted to succinate (4 carbon molecules) and one GTP molecule is produced.

Step 6: Succinate is converted into fumarate (4 carbon molecule) and a molecule of FADH₂ is produced.

Step 7: Fumarate is converted to malate (another 4 carbon molecule).

Step 8: Malate is then converted into oxaloacetate. The third molecule of NADH is also produced.

• Products per Acetyl-CoA:

  • 3 NADH.

  • 1 FADH₂.

  • 1 ATP.

  • 2 CO₂.

4. Electron Transport Chain (ETC) and Oxidative Phosphorylation (Inner Mitochondrial Membrane)

• What Happens:

  • NADH and FADH₂ donate electrons to the ETC, a series of protein complexes.

  • As electrons move through the ETC, energy is used to pump protons (H⁺) across the inner mitochondrial membrane, creating a proton gradient.

  • Oxygen acts as the final electron acceptor, forming water when it combines with electrons and protons.

  • Protons flow back into the mitochondrial matrix through ATP synthase, driving the synthesis of ATP.

• The ETC is a series of proteins located in the mitochondrial membrane.

• It uses high-energy electrons from the NADH and FADH2 provided by the Krebs Cycle to move H+(protons) across the concentration gradient.

• These protons pass back down the concentration gradient through ATP synthase to form ATP.

• Products:

  • Approximately 32-34 ATP.

  • H₂O.

Summary of Aerobic Respiration Products (Per Glucose Molecule):

• ATP: 36-38 (2 from glycolysis, 2 from Krebs cycle, 32-34 from ETC).

• Byproducts: CO₂ and H₂O.

This highly efficient process allows organisms to maximize energy extraction from glucose, enabling complex life processes.

The process of anaerobic respiration occurs in the absence of oxygen and involves the partial breakdown of glucose to generate energy. It is less efficient than aerobic respiration and results in different byproducts depending on the organism. Below is a detailed explanation of the anaerobic process:

Anaerobic respiration is the type of respiration through which cells can break down sugars to generate energy in the absence of oxygen.

SIMILARITIES:

• These are methods of harvesting energy from a food source, such as fats or sugars. Both processes begin with the splitting of a six-carbon sugar molecule into 2 three-carbon pyruvate molecules in a process called glycolysis. This process consumes two ATP molecules and creates four ATP, for a net gain of two ATP per sugar molecule that is split.

• Likewise, the two pyruvate molecules are subject to another series of reactions that use electron transport chains to generate more ATP. It is these reactions that require an electron acceptor – be it oxygen, sulfate, nitrate, etc. –  in order to drive them.

DIFFERENCES:

• After glycolysis, both the aerobic and anaerobic cells send the two pyruvate molecules through a series of chemical reactions to generate more ATP and extract electrons for use in their electron transport chain. However, what these reactions are, and where they happen, varies between aerobic and anaerobic respiration

• During aerobic respiration, the electron transport chain, and most of the chemical reactions of respiration, occur in the mitochondria. The mitochondria’s system of membranes makes the process much more efficient by concentrating the chemical reactants of respiration together in one small space.

• In contrast, anaerobic respiration typically takes place in the cytoplasm. This is because most cells that exclusively carry out anaerobic respiration do not have specialized organelles. The series of reactions is typically shorter in anaerobic respiration and uses a final electron acceptor such as sulfate, nitrate, sulfur, or fumarate instead of oxygen.

• Anaerobic respiration also produces less ATP for each sugar molecule digested than aerobic respiration, making it a less efficient method of generating cellular energy. In addition, it produces different waste products – including, in some cases, alcohol.

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1. Glycolysis (Cytoplasm)

• What Happens:

  • Glucose (6-carbon molecule) is broken down into two pyruvate molecules (3-carbon each).

  • ATP and NADH are produced as intermediate energy carriers.

• Products per Glucose Molecule:

  • 2 ATP (net gain).

  • 2 NADH (used in fermentation).

  • 2 Pyruvate molecules.

2. Fermentation (Cytoplasm)

After glycolysis, pyruvate is converted into other products to regenerate NAD⁺, which is necessary for glycolysis to continue. This occurs through different types of fermentation depending on the organism:

a. Lactic Acid Fermentation (in Animals and Certain Bacteria):

• What Happens:

  • Pyruvate is reduced to lactic acid (lactate) by lactate dehydrogenase using NADH, regenerating NAD⁺.

  • Lactic acid accumulates in cells, such as muscle cells during intense exercise.

• Products per Glucose Molecule:

  • 2 Lactate molecules.

  • 2 ATP (from glycolysis).

b. Alcoholic Fermentation (in Yeast and Some Microorganisms):

• What Happens:

  • Pyruvate is converted into ethanol and CO₂ in two steps:

    1. Pyruvate decarboxylase removes a carbon from pyruvate, forming acetaldehyde and releasing CO₂.

    2. Alcohol dehydrogenase reduces acetaldehyde into ethanol, regenerating NAD⁺.

• Products per Glucose Molecule:

  • 2 Ethanol molecules.

  • 2 CO₂ molecules.

  • 2 ATP (from glycolysis).

Key Characteristics of Anaerobic Respiration:

1. Energy Yield: Only 2 ATP per glucose molecule (much less efficient than aerobic respiration).

2. Byproducts: Lactic acid or ethanol and CO₂, depending on the type of fermentation.

3. Location: Entire process occurs in the cytoplasm, as it does not involve mitochondria.

Anaerobic Respiration Products:

• ATP: 2 (from glycolysis).

• Byproducts:

  • Lactic acid (in animals and some bacteria).

  • Ethanol and CO₂ (in yeast and some microorganisms).

Anaerobic respiration is essential for energy production in low-oxygen conditions, allowing organisms to survive and function when oxygen is unavailable.

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Aerobic Respiration Questions and Answers

1. You are running a marathon at a steady pace. Explain how your body primarily generates energy during this activity and why aerobic respiration is efficient for prolonged exercise.

   • During a marathon, your body primarily relies on aerobic respiration, as oxygen is available for sustained energy production. This process efficiently breaks down glucose into CO₂ and H₂O, producing up to 38 ATP per glucose molecule, which provides a steady supply of energy needed for endurance activities.

2. A scientist observes that mitochondria in a sample of muscle cells are damaged. Predict how this would affect aerobic respiration and overall energy production.

   • Damaged mitochondria would impair aerobic respiration because the citric acid cycle and electron transport chain occur in the mitochondria. This would drastically reduce ATP production, forcing the cells to rely on less efficient anaerobic respiration, leading to fatigue and accumulation of byproducts like lactic acid.

3. Plants undergo aerobic respiration at night when photosynthesis is not occurring. How does this process contribute to a plant's survival and growth?

   • At night, aerobic respiration provides the energy (ATP) required for cellular processes such as cell division, nutrient transport, and repair. This energy is vital for maintaining plant functions even in the absence of sunlight.

4. Imagine a scenario where oxygen levels drop significantly in the atmosphere. How would this affect organisms that rely on aerobic respiration?

   • Organisms relying on aerobic respiration would experience reduced ATP production due to insufficient oxygen. This could lead to energy shortages, reliance on anaerobic processes, and potential survival challenges for organisms with high energy demands.

5. How does aerobic respiration contribute to the balance of CO₂ in the environment, especially in humans and other animals?

   • Aerobic respiration releases CO₂ as a byproduct, which is absorbed by plants during photosynthesis. This exchange maintains a balance of CO₂ in the atmosphere, supporting ecosystems and regulating the Earth's climate.

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Anaerobic Respiration Questions and Answers

1. During an intense sprint, your leg muscles feel a burning sensation. Explain why this happens and how anaerobic respiration is involved.

   • During intense sprints, oxygen supply to muscles is insufficient for aerobic respiration. As a result, muscles switch to anaerobic respiration, producing ATP quickly but generating lactic acid as a byproduct. The accumulation of lactic acid causes the burning sensation.

2. Bread rises due to alcoholic fermentation by yeast. Explain the role of anaerobic respiration in this process and its byproducts.

   • Yeast undergoes anaerobic respiration (alcoholic fermentation) in the dough, breaking down sugars into ethanol and CO₂. The CO₂ forms bubbles in the dough, causing it to rise, while ethanol evaporates during baking.

3. In a sealed pond with low oxygen levels, how do certain bacteria survive, and what byproducts might they produce through anaerobic respiration?

   • In low-oxygen conditions, anaerobic bacteria metabolize organic material using alternative electron acceptors. Depending on the species, they may produce byproducts such as methane (CH₄), hydrogen sulfide (H₂S), or lactic acid, allowing them to survive in oxygen-deprived environments.

4. After a strenuous workout, your body needs time to recover. Why does your breathing rate remain elevated, and how does this relate to anaerobic respiration?

   • After a workout, your body undergoes "oxygen debt" recovery. Elevated breathing supplies oxygen to convert accumulated lactic acid back into pyruvate or glucose in the liver, restoring normal energy levels and clearing the byproducts of anaerobic respiration.

5. In industries like brewing and biofuel production, anaerobic respiration is used. How is the ethanol produced in fermentation beneficial in these contexts?

   • In brewing, ethanol produced during anaerobic fermentation gives alcoholic beverages their characteristic properties. In biofuel production, ethanol is used as a renewable energy source, reducing reliance on fossil fuels and contributing to sustainable energy practices.

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General Questions and Answers about Aerobic and Anaerobic Respiration

1. What is the main difference between aerobic and anaerobic respiration?

   • The main difference is that aerobic respiration requires oxygen to fully break down glucose into energy, producing CO₂ and H₂O, while anaerobic respiration occurs without oxygen and results in less energy production with byproducts like lactic acid or ethanol and CO₂.

2. What are the byproducts of aerobic respiration?

   • The byproducts of aerobic respiration are carbon dioxide (CO₂) and water (H₂O).

3. Why is anaerobic respiration less efficient than aerobic respiration?

   • Anaerobic respiration is less efficient because glucose is only partially broken down, yielding just 2 ATP per molecule, compared to the 36-38 ATP produced in aerobic respiration where glucose is fully oxidized.

4. What role does oxygen play in aerobic respiration?

   • Oxygen acts as the final electron acceptor in the electron transport chain, combining with electrons and protons to form water, which allows the chain to continue and maximize ATP production.

5. Which organisms use anaerobic respiration as their primary energy source?

   • Certain bacteria (e.g., those in oxygen-deprived environments), yeast, and some parasites rely on anaerobic respiration as their primary energy source.

6. How does lactic acid affect muscles during anaerobic respiration?

   • Lactic acid lowers the pH in muscles, leading to fatigue and the burning sensation during intense activity. It must be cleared after exercise to prevent prolonged discomfort.

7. What is the role of NAD⁺ in both aerobic and anaerobic respiration?

   • NAD⁺ acts as an electron carrier, accepting electrons during glycolysis. In aerobic respiration, NADH transfers these electrons to the electron transport chain, while in anaerobic respiration, it is regenerated during fermentation to sustain glycolysis.

8. What is the significance of glycolysis in both types of respiration?

   • Glycolysis is the first step in both aerobic and anaerobic respiration. It breaks glucose into pyruvate, producing a small amount of ATP and NADH, which are essential for subsequent processes.

9. In what part of the cell does aerobic respiration occur?

   • Aerobic respiration occurs in the mitochondria (specifically the matrix and inner mitochondrial membrane), except for glycolysis, which occurs in the cytoplasm.

10. What are the industrial applications of anaerobic respiration?

    • Anaerobic respiration is used in brewing (ethanol production in alcoholic beverages), baking (CO₂ production for leavening bread), biofuel production (ethanol as a renewable fuel), and biogas production (methane from anaerobic digestion).

11. How is energy stored and used in cells after respiration?

    • Energy produced during respiration is stored as ATP. Cells use ATP to power various processes such as muscle contraction, active transport, and biosynthesis.

12. What happens to pyruvate in the absence of oxygen?

    • In the absence of oxygen, pyruvate undergoes fermentation. In animals, it is converted to lactic acid, while in yeast and some microorganisms, it is converted to ethanol and CO₂.

13. Why is aerobic respiration considered more sustainable for multicellular organisms?

    • Aerobic respiration produces much more ATP per glucose molecule, meeting the high energy demands of multicellular organisms more efficiently compared to anaerobic respiration.

14. What is the role of the electron transport chain in aerobic respiration?

    • The electron transport chain transfers electrons from NADH and FADH₂ to oxygen, using the energy released to pump protons across the inner mitochondrial membrane. This creates a proton gradient that drives ATP synthesis.

15. How does anaerobic respiration support short bursts of intense activity?

    • Anaerobic respiration provides a rapid supply of ATP when oxygen delivery is insufficient, enabling muscles to perform short bursts of high-intensity activity. However, it is not sustainable for long periods due to limited ATP yield and lactic acid buildup.