Study sheet: Bioenergetics and ATP

📋 Course Outline

1. Bioenergetics Exam Framework
2. ATP Structure and Recycling
3. Energy Coupling by ATP
4. Redox and Electron Carriers
5. Glycolysis and Pyruvate Fate
6. Aerobic Respiration Pathways
7. Fermentation Pathways
8. Photosynthesis and Chemiosmosis
9. IMAT Pathway Comparisons

📖 1. Bioenergetics Exam Framework

📝 Essential Points

📌 Bioenergetics questions commonly test pathway location, inputs and outputs, and ATP or electron-carrier counts together.

📌 The figures 36 or 38 ATP per glucose are theoretical maximum values, whereas the realistic yield is approximately 30–32 ATP because efficiency and NADH transport costs vary.

• Glycolysis occurs in the cytoplasm, the Krebs cycle occurs in the mitochondrial matrix, and the respiratory electron transport chain occurs across the inner mitochondrial membrane.

💡 Memory Hook

Location → inputs and outputs → ATP or carrier count

📖 2. ATP Structure and Recycling

🔑 Key Concepts & Definitions

• ATP : an immediately usable cellular energy currency made of adenine, ribose, and three phosphate groups

★ Must-know

• The bonds linking ATP’s second and third phosphate groups are phosphoanhydride bonds, whose hydrolysis is favourable because ADP and inorganic phosphate are more stable than ATP.

• ATP is hydrolysed to ADP and inorganic phosphate to drive cellular work, and ADP is then re-phosphorylated using energy harvested from food breakdown.

Further detail

• A typical ATP molecule in an active human cell is consumed and regenerated within roughly a minute, while the body recycles an amount of ATP per day approximately equal to body weight.

💡 Memory Hook

Glucose is a bank deposit; ATP is small-denomination cash.

📖 3. Energy Coupling by ATP

🔑 Key Concepts & Definitions

• Exergonic reaction : a reaction that releases free energy because its products have lower free energy than its reactants
• Endergonic reaction : a reaction that requires free-energy input because its products have higher free energy than its reactants

📝 Essential Points

• ATP hydrolysis is coupled to an endergonic cellular reaction, usually by transferring ATP’s terminal phosphate to a substrate and temporarily increasing the substrate’s free energy.

💡 Memory Hook

ATP hydrolysis → phosphorylation → endergonic reaction proceeds

📖 4. Redox and Electron Carriers

★ Must-know

📌 Oxidation is the loss of electrons, whereas reduction is the gain of electrons, and the two processes always occur together in a redox reaction.

• NAD⁺ accepts two electrons and two protons overall to become NADH plus one proton released into solution.

Further detail

• FAD accepts two hydrogen atoms to become FADH₂ and later delivers their electrons to the electron transport chain.

💡 Memory Hook

OIL RIG: Oxidation Is Loss, Reduction Is Gain.

📖 5. Glycolysis and Pyruvate Fate

★ Must-know

• Glycolysis occurs in the cytosol without requiring oxygen and splits one six-carbon glucose into two three-carbon pyruvate molecules through ten enzyme-catalysed steps.

📐 Formula — For each glucose, glycolysis consumes two ATP, produces four ATP by substrate-level phosphorylation, and yields a net of $4-2=2$ ATP plus two NADH.

• When oxygen is available, pyruvate enters the mitochondrial matrix and becomes acetyl-CoA through the link reaction; when oxygen is unavailable, fermentation regenerates NAD⁺ so glycolysis can continue.

Further detail

📌 Phosphofructokinase is inhibited by high ATP levels and activated by high AMP levels, slowing glycolysis when energy is abundant and accelerating it when energy is scarce.

💡 Memory Hook

Invest ATP → split glucose → pay off ATP and NADH → choose respiration or fermentation

📖 6. Aerobic Respiration Pathways

🔑 Key Concepts & Definitions

• Chemiosmosis : ATP production driven by proton flow through ATP synthase down an electrochemical gradient

★ Must-know

• In the link reaction, each pyruvate loses carbon dioxide, its two-carbon remainder joins coenzyme A to form acetyl-CoA, and NAD⁺ is reduced to NADH in the mitochondrial matrix.

• Because one glucose produces two pyruvate, the link reaction occurs twice per glucose and yields two acetyl-CoA, two carbon dioxide, and two NADH.

• The Krebs cycle occurs in the mitochondrial matrix and turns twice per glucose, producing four carbon dioxide, six NADH, two FADH₂, and two ATP.

• The respiratory electron transport chain passes electrons from NADH and FADH₂ through inner-membrane protein complexes, pumping protons into the intercristal space and creating a proton gradient.

📌 Oxygen is the final electron acceptor of aerobic respiration and combines with electrons and protons to form water, allowing the electron transport chain to continue.

Further detail

📌 Cyanide blocks the final electron transport complex, whereas dinitrophenol dissipates the proton gradient by making the inner membrane leaky and generates heat instead of ATP.

💡 Memory Hook

Link reaction → Krebs cycle → electron transport chain → chemiosmosis

📖 7. Fermentation Pathways

📝 Essential Points

📌 Fermentation regenerates NAD⁺ when oxidative phosphorylation cannot reoxidise NADH, allowing glycolysis to continue producing its net two ATP per glucose.

📌 Lactic acid fermentation reduces pyruvate to lactate in animal cells and some bacteria without releasing carbon dioxide, whereas alcoholic fermentation produces ethanol and carbon dioxide in yeast and some bacteria.

• Both lactic acid and alcoholic fermentation regenerate NAD⁺ and preserve the two-ATP net yield of glycolysis without producing extra ATP.

💡 Memory Hook

Lactate in animal cells versus ethanol and CO₂ in yeast.

📖 8. Photosynthesis and Chemiosmosis

📝 Essential Points

• A chloroplast contains thylakoid membranes stacked into grana and suspended in the stroma, whereas a mitochondrion uses its inner membrane and matrix for respiration.

• The light-dependent reactions occur across the thylakoid membrane, where light excites electrons, photosystem II splits water and releases oxygen, electron transport pumps protons, and ATP synthase produces ATP by photophosphorylation.

• NADP⁺ is the final electron acceptor of the light-dependent reactions and is reduced to NADPH.

• The Calvin cycle occurs in the stroma, where RuBisCO fixes carbon dioxide onto RuBP and ATP and NADPH convert the resulting three-carbon compounds into G3P while most G3P regenerates RuBP.

💡 Memory Hook

Mitochondria use NADH and oxygen; chloroplasts use water and NADP⁺.

📖 9. IMAT Pathway Comparisons

★ Must-know

📌 The respiratory chain uses NADH and FADH₂ as electron sources and oxygen as final acceptor, whereas the thylakoid chain uses water as electron source and NADP⁺ as final acceptor.

📌 Oxidative phosphorylation produces most ATP from glucose through the respiratory electron transport chain, whereas substrate-level phosphorylation produces ATP directly in glycolysis and the Krebs cycle.

• The realistic ATP yield from one glucose is approximately 30–32 ATP, and well over twenty of these molecules come from oxidative phosphorylation rather than direct substrate-level phosphorylation.

Further detail

• In respiration, protons are pumped from the mitochondrial matrix into the intercristal space and flow back into the matrix; in photosynthesis, protons accumulate in the thylakoid space and flow back into the stroma.

📊 Synthesis Tables

Respiration pathway comparison

Pathway	Location	Main outputs	ATP yield
Glycolysis	Cytoplasm	2 pyruvate, 2 NADH	Net 2 ATP
Link reaction	Mitochondrial matrix	2 acetyl-CoA, 2 CO₂, 2 NADH	None directly
Krebs cycle	Mitochondrial matrix	4 CO₂, 6 NADH, 2 FADH₂	2 ATP
Electron transport and chemiosmosis	Inner mitochondrial membrane	Water and proton gradient	Majority of ATP

Photosynthesis and respiration comparison

Feature	Respiration	Photosynthesis
Final electron acceptor	Oxygen, forming water	NADP⁺, forming NADPH
Electron source	NADH and FADH₂	Water split by photosystem II
Proton accumulation	Intercristal space	Thylakoid space
Main ATP-producing process	Oxidative phosphorylation	Photophosphorylation