Cellular Respiration
Understand how cells break down glucose to release energy in the form of ATP through glycolysis, the Krebs cycle, and the electron transport chain.
Overview of Cellular Respiration
Cellular respiration is the process by which cells break down glucose and other organic molecules to release energy stored in their chemical bonds. This energy is captured in the form of ATP (adenosine triphosphate), the universal energy currency of cells. The process occurs in three main stages: glycolysis, the Krebs cycle, and the electron transport chain.
Overall Equation (Aerobic Respiration)
C6H12O6 + 6O2 → 6CO2 + 6H2O + ~36-38 ATP
Glucose + Oxygen → Carbon dioxide + Water + Energy (ATP)
Photosynthesis vs Cellular Respiration
Photosynthesis
Builds glucose from CO2 + H2O
Stores energy
Occurs in chloroplasts
Releases O2
Cellular Respiration
Breaks down glucose using O2
Releases energy (ATP)
Occurs in mitochondria
Releases CO2
Key point: Cellular respiration is essentially the reverse of photosynthesis. Both processes are connected in the global carbon and energy cycle.
The Three Stages of Aerobic Respiration
Aerobic respiration consists of three linked stages. Glycolysis occurs in the cytoplasm, while the Krebs cycle and electron transport chain occur in the mitochondria.
The Pathway of Aerobic Respiration
1. Glycolysis
Location: Cytoplasm
Glucose (6C) → 2 Pyruvate (3C)
Net yield: 2 ATP + 2 NADH
2. Krebs Cycle
Location: Mitochondrial matrix
Acetyl CoA (2C) enters the cycle, CO2 released
Yield per glucose: 2 ATP + 6 NADH + 2 FADH2
3. Electron Transport Chain
Location: Inner mitochondrial membrane
NADH and FADH2 donate electrons; O2 is final electron acceptor
Yield: ~32-34 ATP
ATP tally: Glycolysis produces 2 ATP, the Krebs cycle produces 2 ATP, and the ETC produces ~32-34 ATP. Total: approximately 36-38 ATP per glucose molecule. The ETC produces the vast majority of ATP.
Anaerobic Respiration (Fermentation)
When oxygen is unavailable, cells can still produce ATP through anaerobic respiration (fermentation). However, this process is far less efficient than aerobic respiration, producing only 2 ATP per glucose (from glycolysis alone).
Lactic Acid Fermentation
Occurs in animal muscle cells during intense exercise
Glucose → 2 Lactic Acid + 2 ATP
Causes muscle fatigue and soreness
Alcoholic Fermentation
Occurs in yeast and some bacteria
Glucose → 2 Ethanol + 2CO2 + 2 ATP
Used in bread-making and brewing
Why Aerobic Respiration is More Efficient
Aerobic respiration produces ~36-38 ATP per glucose molecule, while anaerobic respiration produces only 2 ATP. This is because anaerobic respiration cannot fully oxidise glucose -- the energy remains locked in the products (lactic acid or ethanol). The Krebs cycle and ETC require oxygen to extract this remaining energy.
Key Vocabulary
ATP
Adenosine triphosphate -- the primary energy currency of cells. Energy is released when the terminal phosphate bond is broken.
Glycolysis
The first stage of cellular respiration, occurring in the cytoplasm, where glucose (6C) is split into two pyruvate molecules (3C).
Krebs Cycle
A cyclic series of reactions in the mitochondrial matrix that oxidises acetyl CoA, producing CO2, NADH, FADH2, and ATP.
Electron Transport Chain
A series of protein complexes in the inner mitochondrial membrane that transfer electrons from NADH/FADH2 to oxygen, generating most of the ATP.
Worked Examples
Calculate the total ATP yield from one glucose molecule undergoing aerobic respiration.
Step 1: Glycolysis: 2 ATP (net) + 2 NADH
Step 2: Link reaction: 2 NADH (one per pyruvate)
Step 3: Krebs cycle (x2): 2 ATP + 6 NADH + 2 FADH2
Step 4: ETC: 10 NADH x ~2.5 ATP = 25 ATP; 2 FADH2 x ~1.5 ATP = 3 ATP
Total: 2 + 2 + 25 + 3 = approximately 32 ATP (can vary to 36-38 depending on shuttle systems).
Why do your muscles produce lactic acid during a sprint?
Step 1: During intense exercise, muscles demand more ATP than aerobic respiration can supply.
Step 2: Oxygen delivery cannot keep up with demand, so cells switch to anaerobic respiration (lactic acid fermentation).
Answer: Pyruvate from glycolysis is converted to lactic acid instead of entering the Krebs cycle. This regenerates NAD+ needed for glycolysis to continue, providing 2 ATP rapidly, but lactic acid accumulation causes muscle fatigue.
What is the role of oxygen in aerobic respiration?
Step 1: Oxygen acts as the final electron acceptor in the electron transport chain.
Step 2: After electrons pass through the chain, they combine with O2 and H+ ions to form water.
Answer: Without oxygen, the ETC would stop, NADH and FADH2 could not be recycled, and the Krebs cycle would halt. Only glycolysis could continue (anaerobically).
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
Where does glycolysis occur in the cell?
Question 2
Which stage of cellular respiration produces the most ATP?
Question 3
What is the role of oxygen in aerobic respiration?
Question 4
During lactic acid fermentation, what happens to pyruvate?
Question 5
How many ATP molecules are produced (net) by glycolysis alone from one glucose molecule?
Key Concepts Summary
- ●Cellular respiration converts glucose + O2 into CO2 + H2O + ATP.
- ●Glycolysis (cytoplasm) splits glucose into 2 pyruvate, yielding 2 ATP net.
- ●The Krebs cycle (mitochondrial matrix) fully oxidises carbon, producing CO2, NADH, FADH2, and 2 ATP.
- ●The electron transport chain (inner mitochondrial membrane) produces ~32-34 ATP using O2 as final electron acceptor.
- ●Anaerobic respiration (fermentation) yields only 2 ATP and produces lactic acid (animals) or ethanol + CO2 (yeast).