ATP and Cellular Respiration Study Notes

ATP (Adenosine Triphosphate) is a phosphorylated nucleotide structurally related to RNA nucleotides, containing adenine, ribose sugar, and three phosphate groups.
Composition and phosphate counts:
- Adenosine: Adenine + Ribose ( $0$ phosphate groups)
- AMP (adenosine monophosphate): Adenosine + $1$ \, $\text{P}_i$ ( $1$ phosphate group)
- ADP (adenosine diphosphate): Adenosine + $2$ \, $\text{P}_i$ ( $2$ phosphate groups)
- ATP (adenosine triphosphate): Adenosine + $3$ \, $\text{P}_i$ ( $3$ phosphate groups)
Serves as the universal energy currency linking energy-yielding (exergonic) reactions like respiration with energy-requiring (endergonic) reactions like active transport, muscle contraction, and biosynthesis.
Key properties as energy currency:
- Intermediate energy yield to prevent waste.
- Controlled by a single enzyme (ATPase) for rapid energy release.
- Stable at cellular pH until hydrolysed.
- Small, water-soluble, and diffuses easily.
- Continuously recyclable via the ATP–ADP cycle.

ATP Hydrolysis (Dephosphorylation):
- Catalysed by ATP hydrolase (ATPase).
- Cleaves the terminal ( $\gamma$ ) phosphate bond by adding water: $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{Energy} \quad (\approx 30.5\,\text{kJ\,mol}^{-1})$
- Inorganic phosphate ( $\text{P}_i$ ) can be transferred to activate other molecules (phosphorylation).
ATP Synthesis:
- Reverse condensation reaction: $\text{ADP} + \text{P}_i + \text{Energy} \rightarrow \text{ATP} + \text{H}_2\text{O}$
- The body stores $\approx 200\,\text{g}$ of ATP but uses over $50\,\text{kg}$ per day, requiring continuous regeneration.
Phosphorylation Mechanisms:
- Substrate-Level Phosphorylation: Direct transfer of a phosphate group from a substrate to ADP. Occurs in the cytoplasm (glycolysis) and mitochondrial matrix (Krebs cycle). Does not require $\text{O}_2$ . Produces a small yield ( $\approx 4\,\text{ATP}$ per glucose).
- Oxidative Phosphorylation (Chemiosmosis): Proton-motive force across a membrane drives ATP synthase. Occurs in the inner mitochondrial membrane. Requires $\text{O}_2$ as the final electron acceptor. Produces a large yield ( $\approx 32\text{–}34\,\text{ATP}$ per glucose).

Step 1: Glucose is phosphorylated by hexokinase (or glucokinase in liver) using $1\,\text{ATP}$ to form glucose-6-phosphate, trapping it inside the cell.
Step 2: Glucose-6-phosphate is isomerised to fructose-6-phosphate by phosphoglucose isomerase.
Step 3: Fructose-6-phosphate is phosphorylated by phosphofructokinase-1 (PFK-1) using a second ATP to form fructose-1,6-bisphosphate (the main rate-limiting, committed step).
Step 4: Fructose-1,6-bisphosphate is cleaved by aldolase into dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P / GLAP).
Step 5: DHAP is converted to a second G3P molecule by triose phosphate isomerase, doubling all subsequent reaction yields ( $\times 2$ ).

Step 6: G3P is oxidised and phosphorylated by G3P dehydrogenase (GLAP Hydrogenase), reducing $\text{NAD}^+$ to $\text{NADH}$ and producing 1,3-bisphosphoglycerate ( $\times 2$ ).
Step 7: 1,3-bisphosphoglycerate transfers a phosphate to ADP via phosphoglycerate kinase (substrate-level phosphorylation), forming 3-phosphoglycerate and generating $2\,\text{ATP}$ total ( $\times 2$ ).
Step 8: 3-phosphoglycerate is rearranged to 2-phosphoglycerate by phosphoglycerate mutase.
Step 9: 2-phosphoglycerate is dehydrated by enolase to form phosphoenolpyruvate (PEP).
Step 10: PEP transfers its phosphate to ADP via pyruvate kinase (substrate-level phosphorylation), forming pyruvate and generating $2\,\text{ATP}$ total ( $\times 2$ ).

Net Yield per Glucose Molecule:
- ATP: $4$ produced (Steps 7 & 10), $2$ consumed (Steps 1 & 3) = Net $2\,\text{ATP}$
- NADH: $2$ produced (Step 6) = Net $2\,\text{NADH}$
- Pyruvate: $2$ produced
- Overall Net Reaction: $\text{Glucose} + 2\,\text{NAD}^+ + 2\,\text{ADP} + 2\,\text{P}_i \rightarrow 2\,\text{Pyruvate} + 2\,\text{NADH} + 2\,\text{H}^+ + 2\,\text{ATP} + 2\,\text{H}_2\text{O}$
Regulation:
- Hexokinase (Step 1): Inhibited by glucose-6-phosphate via feedback inhibition.
- Phosphofructokinase-1 / PFK-1 (Step 3): Main rate-limiting enzyme; inhibited by high ATP and citrate; activated by AMP and ADP.
- Pyruvate kinase (Step 10): Inhibited by high ATP and acetyl-CoA; activated by fructose-1,6-bisphosphate via feed-forward activation.

Question: The net ATP yield from the glycolytic breakdown of one glucose molecule to two pyruvate molecules is:
- (A) 4 ATP
- (B) 2 ATP
- (C) 6 ATP
- (D) 8 ATP
Question: Which of the following enzymes catalyses the main rate-limiting step of glycolysis?
- (A) Hexokinase
- (B) Aldolase
- (C) Phosphofructokinase-1
- (D) Pyruvate kinase
Question: During glycolysis, the reduction of NAD⁺ to NADH occurs during which step?
- (A) Conversion of glucose to glucose-6-phosphate
- (B) Cleavage of fructose-1,6-bisphosphate
- (C) Oxidation of glyceraldehyde-3-phosphate
- (D) Conversion of phosphoenolpyruvate to pyruvate