BIOL1020 Chap.4 Cellular Energy and Respiration
Cellular Energy and Respiration
Cellular energy questions combine thermodynamics with pathway logic. This chapter distinguishes free-energy change from activation energy, explains how enzymes affect rate without changing reaction favourability, and shows how ATP hydrolysis can drive an endergonic step through coupling. It then follows carbon and electrons through glycolysis, pyruvate oxidation, the citric-acid cycle and oxidative phosphorylation.
Perturbation problems are solved locally: identify the blocked component, determine which intermediate changes, and trace the effect on electron carriers, proton gradients and ATP production.
What this chapter covers
- 01
Exergonic and endergonic reactions
- 02
Activation barriers and enzymes
- 03
ATP hydrolysis and reaction coupling
- 04
Glycolysis and pyruvate oxidation
- 05
Citric-acid cycle
- 06
Electron transport and chemiosmosis
Worked example · free
Tracing a proton leak across the inner membrane
- +1The leak dissipates the proton electrochemical gradient across the inner membrane.
- +1ATP synthase receives less proton-motive force and makes much less ATP.
- +1Electron transport may continue and consume oxygen because the leak relieves the gradient opposing further pumping.
- +1Glycolysis can still provide direct ATP, so cellular ATP production is reduced rather than automatically zero.
Key terms
- Free Energy
- Energy available to perform work, used to compare the states of reactants and products.
- Activation Energy
- The barrier a reaction must cross before products can form.
- Reaction Coupling
- The linking of an endergonic process to a sufficiently exergonic process so their combined free-energy change is favourable.
- Chemiosmosis
- ATP production driven by ions moving down an electrochemical gradient through ATP synthase.
- Terminal Acceptor
- The final recipient of electrons in a transport chain, which is oxygen during aerobic respiration.
Cellular Energy and Respiration FAQ
Do enzymes make reactions exergonic?
No. Enzymes lower the activation barrier and increase the rate at which equilibrium is approached, but they do not change the free energies of reactants and products. An endergonic reaction still needs coupling or another energy input to proceed net forward.
Why does ATP hydrolysis release free energy?
The complete products, ADP and inorganic phosphate, are collectively more stable than ATP and water because of reduced charge repulsion, resonance and improved hydration. Breaking the phosphate bond itself requires energy; the net reaction is favourable because product stabilisation is greater.
What happens when oxygen is unavailable?
Oxygen can no longer accept electrons at the end of the aerobic transport chain, so electron flow and proton pumping stop. Oxidised carriers become scarce, constraining pyruvate oxidation and the citric-acid cycle. Glycolysis can continue only if cells regenerate the carriers it requires.
Why do enzymes not change reaction free energy?
An enzyme lowers the activation barrier by providing another reaction pathway, which changes reaction rate. It does not alter the free-energy difference between reactants and products, so it does not make an energetically unfavourable reaction favourable. Keep kinetics separate from the thermodynamic direction of change.
How can ATP hydrolysis drive another reaction?
Coupling links an energy-releasing process to an energy-requiring process through shared intermediates or transferred phosphate. The combined pathway can proceed when its total free-energy change is negative. Simply placing reactions beside each other is insufficient; the mechanism must connect the favourable step to the unfavourable transformation.
What establishes the mitochondrial proton gradient?
Electrons move through inner-membrane transport complexes toward oxygen. Energy released along that route powers proton movement from the matrix to the intermembrane space, creating electrochemical potential. Protons returning through ATP synthase drive phosphorylation, so the membrane must preserve the separation for efficient ATP production.
Why does blocked pyruvate transport reduce ATP yield?
Glycolysis can still make pyruvate and a small amount of ATP in the cytosol, but pyruvate cannot reach mitochondrial enzymes for oxidation. The citric-acid cycle receives less carbon input, fewer reduced electron carriers reach the transport chain, and oxidative phosphorylation consequently produces less ATP.
Why does transport-chain failure affect earlier pathways?
When electrons cannot move efficiently to the terminal acceptor, reduced carriers are not regenerated to their oxidised forms. Pyruvate oxidation and the citric-acid cycle then lose carriers needed to accept more electrons. The effect propagates backward through carrier availability even though the blocked complexes sit later in the overall respiratory sequence.
Exam move
Separate three questions on every reaction: Is it thermodynamically favourable, how large is the activation barrier, and how fast does it proceed with an enzyme? Then rebuild respiration as a location map, marking cytosol, mitochondrial matrix and inner membrane. For perturbation drills, begin at a transporter, enzyme, membrane or terminal acceptor and follow the effect through substrates, products, carriers, gradient and ATP.
Write oxidation and reduction beside the molecules that lose and gain electrons. Do not memorise ATP yield as an isolated number when the source emphasis is mechanism. Practise explaining why glycolysis can persist when later mitochondrial stages fail. Audit each answer for the carrier whose oxidised or reduced form connects one stage to the next.
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