B4 – Bioenergetics

🔋 What We’re Covering

Topic	Key Learning Points
Energy in Biology	Organic → chemical → stored in ATP; exergonic vs endergonic.
ATP (adenosine triphosphate)	Structure, power‑station role, synthesis routes.
Enzymes	Catalysts, specificity, temperature/ pH influence, inhibitors.
Aerobic & Anaerobic Respiration	Glycolysis, pyruvate oxidation, Krebs cycle, electron‑transport chain.
Fermentation	Lactic acid & alcohol fermentation, ATP yield.
Photosynthesis	Light‑dependent & independent reactions, location in chloroplasts.
Energy Balance	Supply vs demand; exercise & metabolism.
Analysing Bioenergetic Processes	Equations, net ATP gains, real‑world examples.

---

⚡ Energy Transfer in Living Systems

Concept	What it Means	Example	Key Question Types
Exergonic Reaction	Releases energy; ΔG < 0.	Oxidation of glucose → CO₂ + H₂O.	“Explain why respiration releases energy.”
Endergonic Reaction	Requires energy input; ΔG > 0.	Conversion of ADP → ATP.	“Why does photosynthesis need light energy?”
Unit of Energy	Joule (J) & calorie/food‑kcal.	1 kcal ≈ 4.184 kJ.	“Convert 200 kcal to J.”
Law of Conservation	Energy can’t be created/destroyed.	Reactants → products + heat.	“Show energy is balanced in a given reaction.”

---

🏗️ ATP – The Cell’s Power‑Station

Feature	Detail
Structure	Adenine + ribose + 3 phosphate chains.
Why it’s a good energy carrier	3 high energy phosphate bonds release ~10 kJ mol⁻¹ when hydrolysed to ADP.
ATP Synthesis	Anabolic pathway: substrate‑level phosphorylation (glycolysis, Krebs). <br>Oxidative phosphorylation (Electron‑Transport Chain + ATP synthase).
ATP Turnover	1 s¹–10 s¹ in cells; quick regeneration essential.
Equation	A+H3PO4→ADP+ATPA+H3​PO4​→ADP+ATP + water + heat.

🔬 “Power‑Analogy” Qs

• “Why is ATP often called the ‘molecular unit of currency’ in cells?”
• “Contrast substrate‑level phosphorylation with oxidative phosphorylation.”

---

🔬 Enzymes – Biological Accelerators

Attribute	Definition	Example
Catalysis	Lowers activation energy.	Lactase converting lactose → glucose + galactose.
Active Site	Specific region; shaped like lock‑and‑key (or induced fit).	Hexokinase binds glucose + ATP.
Optimal Conditions	Temperature & pH.	Human enzymes: ~37 °C, pH 7.4.
Inhibitors	Competitive / non‑competitive.	Trypsin inhibitor – reduces digestion.

📚 Typical Exam Questions

• Diagram the active‑site interaction between an enzyme and substrate.
• Explain the effect of a temperature rise from 36 °C to 42 °C on enzyme activity.

---

---

🏃‍♂️ Cellular Respiration – From Glucose to ATP

Stage	Process	Output	Net ATP (per glucose)
Glycolysis	Cytosol	2 ATP (substrate), 2 NADH	2 ATP
Anaerobic		2 Lactic acid	2 ATP
Aerobic		Pyruvate → Acetyl‑CoA	2 ATP
Pyruvate Oxidation	Mitochondrial matrix	1 NADH per pyruvate	2 NADH
Krebs Cycle	Matrix	3 NADH, 1 FADH₂, 1 GTP	2 ATP (via GTP)
Electron Transport Chain (ETC)	Inner mitochondrial membrane	6 ATP per NADH, 4 ATP per FADH₂	28–30 ATP (≈30 total)

Note: NET ATP ≈ 32, but 2 lost to produce 2 × O₂ + H₂O (≈ 3 ATP).

💡 Gates & Regulatory Points

Gate	Substrate	Enzyme (rate‑limiting)
Embden–Meyerhof (glycolysis)	Glucose	Hexokinase (first)
Pyruvate dehydrogenase complex	Pyruvate	Pyruvate → Acetyl‑CoA
Citrate synthase (Krebs)	Acetyl‑CoA	Citrate synthase

📏 Energy Balance Qs

• “Show the stoichiometry for production of 30 ATP from one glucose.”
• “Why does anaerobic respiration produce only 2 ATP?”

---

🍺 Fermentation – When Oxygen’s Out of the Picture

Type	Substrate → Product	Key enzyme	Net ATP
Lactic Acid	Glucose → 2 lactate	Lactate dehydrogenase	2
Alcohol	Glucose → 2 ethanol + 2 CO₂	Alcohol dehydrogenase	2

People use lactic acid fermentation during intense exercise to maintain glycolysis when O₂ is limited.

---

☀️ Photosynthesis – Turning Light Into Chemical Energy

Stage	Light‑Dependent	Light‑Independent
Inputs	Light, H₂O, ADP + Pi	CO₂ + NADPH + ATP
Outputs	O₂ + NADPH + ATP	Glucose + H₂O
Location	Thylakoid membranes	Stroma
Chromophores	Chlorophyll a/b, Carotenoids	—
Key Reactions	Photon energy → Ephoton→Ephoton​→ electron transfer (Photosystem II/ I) → ATP‑synthase & NADPH‑reductase	Calvin cycle; enzymes: Rubisco → RuBP → glucose

📘 Important Numbers

• Stomatal regulation – balances CO₂ uptake vs water loss.
• Photosystem II absorbs 680 nm; PSI 700 nm.
• Quantum yield – 1 photon ∼ 2 e⁻ transferred.

---

🏋️‍♀️ Energy Balance in The Human Body

Moment	Energy Intake	Energy Expenditure	Discussion
Basal Metabolic Rate (BMR)	2000 kcal day⁻¹	25% energy	Heat, resting functions
Exercise	0	5–60% BMR (running, swimming)	Glycolysis & aerobic pathways
Thermoregulation	0	Variable	Thermogenesis (brown adipose)
Growth / Repair	0	10–15% BMR	Protein synthesis, cell division

Example Question: “If a student runs 5 km in 30 min at a heart rate of 170 bpm, estimate the energy expended and compare to their BMR.”

---

🔧 How to Analyse Bio‐Energetic Processes

1️⃣ Write All Steps

• Identify the metabolic pathway (glycolysis, photosynthesis, etc.).
• Draw a simple schematic (substrate → intermediate → product).

2️⃣ Count Energy Compounds

• Reactants: ATP, NAD⁺, O₂, etc.
• Products: ADP, NADH, H₂O, CO₂, O₂, glucose.

3️⃣ Calculate Net ATP

• Subtract ATP used in biosynthetic steps from ATP produced.

4️⃣ Explain Directionality

• Reference the Gibbs‑free‑energy change (ΔG).

5️⃣ Draw Real‑World Correlation

• “In heavy exercise, lactic acid builds up because aerobic pathway is limited; the cell shifts to fermentation to keep glycolysis running.”

---

🎯 Exam Focus Areas

Section	Typical Question Format	Sample
Enzyme Details	Diagram, fill‑in	Label the active site.
Respiration	Calculate net ATP, causal relationships	“How many ATP are produced from one glucose when the electron transport chain is fully functional?”
Photosynthesis	Mechanism & stoichiometry	Write the equation for photosynthesis.
Energy Balance	Case studies	“Explain why an athlete’s body weight remains stable during a diet plan.”
Process Comparison	Endergonic vs exergonic	“Contrast the energy changes in glucose oxidation and photosynthesis.”

---

✅ Quick‑Check Checklist

•  I can explain how ATP is produced in the three main stages of respiration.
•  I know the roles of hexokinase, pyruvate dehydrogenase, and citrate synthase.
•  I can write the balanced equations for fermentation (both lactic and alcoholic).
•  I understand how light‑dependent and independent reactions in photosynthesis relate.
•  I can calculate a net ATP gain from one molecule of glucose under aerobic conditions.
•  I know how enzymes are affected by temperature, pH, and inhibitors.