Cellular respiration — glycolysis, Krebs cycle, ETC (KCSE Biology Form 3)

TL;DR

Cellular respiration is how your cells break down glucose to make energy (ATP). It involves three main stages: glycolysis, the Krebs cycle, and the electron transport chain. Each stage happens in a specific part of the cell and produces different amounts of energy.

1. The Mental Model

Think of cellular respiration as your body's power plant. It takes fuel (glucose) and, through a series of steps, converts it into usable energy (ATP) that powers all your activities.

2. The Core Material

Cellular respiration is the process by which living cells break down organic substances, mainly glucose, to release energy in the form of ATP (adenosine triphosphate). This energy is then used for various life processes like muscle contraction, active transport, and synthesis of new molecules.

There are two main types of cellular respiration:
1. Aerobic Respiration: Requires oxygen. It's much more efficient at producing ATP.
2. Anaerobic Respiration: Does not require oxygen. It produces much less ATP.

For KCSE Biology, we focus mainly on aerobic respiration, which has three main stages: Glycolysis, Krebs Cycle, and the Electron Transport Chain (ETC).

Glycolysis

This is the first stage of both aerobic and anaerobic respiration.
* Location: Occurs in the cytoplasm of the cell.
* Input: One molecule of glucose (a 6-carbon sugar).
* Process: Glucose is broken down into two molecules of pyruvate (a 3-carbon compound). This involves several steps, but you don't need to memorize all of them.
* Output:
* 2 molecules of Pyruvate
* 2 molecules of ATP (net gain, as 2 ATP are used initially)
* 2 molecules of NADH (an energy-carrying molecule)
* Oxygen Requirement: Does not require oxygen.

Krebs Cycle (Citric Acid Cycle)

This stage only occurs if oxygen is present.
* Location: Occurs in the matrix of the mitochondria.
* Input: The two molecules of pyruvate from glycolysis are first converted into acetyl-CoA before entering the cycle. This conversion also produces CO2 and NADH.
* Process: Acetyl-CoA combines with a 4-carbon molecule to form a 6-carbon molecule (citrate). This molecule then goes through a series of reactions, releasing carbon dioxide and generating energy carriers.
* Output (per glucose molecule, meaning two turns of the cycle):
* 6 molecules of CO2 (4 from the cycle, 2 from pyruvate-to-acetyl-CoA conversion)
* 2 molecules of ATP (or GTP, which is equivalent)
* 8 molecules of NADH (6 from the cycle, 2 from pyruvate-to-acetyl-CoA conversion)
* 2 molecules of FADH2 (another energy-carrying molecule)
* Oxygen Requirement: Does not directly use oxygen, but it can only proceed if oxygen is available for the next stage (ETC).

Electron Transport Chain (ETC)

This is the final and most productive stage of aerobic respiration.
* Location: Occurs on the inner mitochondrial membrane.
* Input: The NADH and FADH2 molecules produced in glycolysis and the Krebs cycle.
* Process: NADH and FADH2 donate their high-energy electrons to a series of protein complexes embedded in the inner mitochondrial membrane. As electrons move down the chain, energy is released, which is used to pump protons (H+) from the mitochondrial matrix into the intermembrane space. This creates a proton gradient. Oxygen acts as the final electron acceptor, combining with electrons and protons to form water. The protons then flow back into the matrix through an enzyme called ATP synthase, which uses this flow to generate a large amount of ATP.
* Output:
* Approximately 32-34 molecules of ATP (this number can vary slightly)
* Water (H2O)
* Oxygen Requirement: Directly requires oxygen as the final electron acceptor. Without oxygen, the ETC stops, and the entire aerobic respiration process backs up.

Here's a simplified flow of the process:

graph TD
    A[Glucose] -->|Glycolysis (Cytoplasm)| B[2 Pyruvate]
    B -->|Conversion (Mitochondrial Matrix)| C[2 Acetyl-CoA]
    C -->|Krebs Cycle (Mitochondrial Matrix)| D[CO2, NADH, FADH2, ATP]
    B -- No Oxygen --> E[Lactic Acid/Ethanol (Anaerobic Respiration)]
    D -->|Electron Transport Chain (Inner Mitochondrial Membrane)| F[Large ATP, H2O]
    F -- Requires Oxygen --> G[Energy for Cell]

Overall Equation for Aerobic Respiration:

C6H12O6 (Glucose) + 6O2 (Oxygen) → 6CO2 (Carbon Dioxide) + 6H2O (Water) + Energy (ATP)

3. Worked Example

Let's trace what happens to one molecule of glucose through aerobic respiration, focusing on the main products at each stage.

Problem: Describe the fate of one glucose molecule during aerobic respiration, detailing the key products formed at each major stage.

Solution:

1. Glycolysis (in the cytoplasm):

   • One molecule of glucose (6 carbons) is broken down.
   • It produces 2 molecules of pyruvate (3 carbons each).
   • Net gain of 2 ATP.
   • 2 NADH molecules are also formed.

2. Pyruvate Oxidation (conversion to Acetyl-CoA, in the mitochondrial matrix):

   • The two pyruvate molecules are transported into the mitochondria.
   • Each pyruvate is converted into 1 molecule of acetyl-CoA (2 carbons).
   • This process releases 2 molecules of CO2 (one from each pyruvate).
   • 2 NADH molecules are also formed (one from each pyruvate).

3. Krebs Cycle (in the mitochondrial matrix):

   • The two acetyl-CoA molecules enter the cycle.
   • For each acetyl-CoA (so, two turns for one glucose):
     • 2 molecules of CO2 are released.
     • 3 NADH molecules are formed.
     • 1 FADH2 molecule is formed.
     • 1 ATP (or GTP) is formed.
   • Therefore, for one glucose molecule (two turns): 4 CO2, 6 NADH, 2 FADH2, and 2 ATP are produced.

4. Electron Transport Chain (on the inner mitochondrial membrane):

   • All the NADH (2 from glycolysis + 2 from pyruvate oxidation + 6 from Krebs = 10 NADH total) and FADH2 (2 from Krebs) molecules donate their electrons.
   • These electrons power the pumping of protons, leading to the synthesis of a large amount of ATP.
   • Approximately 32-34 ATP molecules are generated.
   • Water (H2O) is formed as oxygen accepts the electrons and protons.

Summary for one glucose molecule:
* Total ATP: ~36-38 ATP (2 from glycolysis + 2 from Krebs + ~32-34 from ETC)
* Total CO2: 6 CO2 (2 from pyruvate oxidation + 4 from Krebs)
* Total H2O: 6 H2O (produced in ETC)

4. Key Takeaways

• Cellular respiration breaks down glucose to release energy in the form of ATP.
• Glycolysis occurs in the cytoplasm and produces pyruvate, 2 ATP, and 2 NADH.
• The Krebs cycle occurs in the mitochondrial matrix, producing CO2, ATP, NADH, and FADH2.
• The Electron Transport Chain on the inner mitochondrial membrane produces the most ATP, using NADH and FADH2, and requires oxygen.
• Oxygen is essential for the high ATP yield of aerobic respiration, acting as the final electron acceptor in the ETC.
• Anaerobic respiration occurs without oxygen and produces much less ATP, along with lactic acid or ethanol.
• Mitochondria are the powerhouses of the cell because they host the Krebs cycle and ETC.

Common mistakes you should avoid:
- Confusing the location of each stage (e.g., saying Krebs cycle is in the cytoplasm).
- Forgetting that oxygen is crucial for the ETC and overall high ATP production.
- Mixing up the products of each stage, especially the energy carriers (NADH, FADH2).
- Not understanding that pyruvate needs to be converted to acetyl-CoA before entering the Krebs cycle.

5. Now Try It

Draw a simple diagram showing the three main stages of aerobic respiration, including where each stage occurs within the cell and the key molecules that enter and leave each stage. Label all parts clearly. Your diagram should visually represent the flow from glucose to ATP. Success looks like a clear, accurate, and well-labelled diagram that correctly places each stage and its inputs/outputs.