ATP as a battery and its synthesis Flashcards
Why is ATP often referred to as a ‘battery’ in cells
ATP stores energy from catabolism or photosynthesis and releases it to power biosynthetic reactions. It acts like a rechargeable battery—charged by energy-yielding processes and discharged to drive cellular work.
Why is ATP a useful energy carrier in cells
ATP → ADP + Pi has a high equilibrium constant (Keq), proceeds spontaneously, and releases significant free energy. This makes it ideal for coupling to thermodynamically unfavorable reactions.
What is the typical [ATP]:[ADP] ratio in cells and why is it important
About 3:1. This keeps ATP far from equilibrium, ensuring it remains a high-energy molecule ready to drive cellular processes.
Why is ATP described as thermodynamically unstable but kinetically stable
While ATP hydrolysis is energetically favourable, it does not occur spontaneously without enzymes - allowing the cell to tightly regulate when and where energy is released.
What is substrate-level phosphorylation
A process where ATP is synthesized by directly transferring a phosphate group from a high-energy intermediate to ADP, often during glycolysis or fermentation.
How does glycolysis contribute to ATP production
Glycolysis breaks glucose into pyruvate, forming a doubly phosphorylated sugar that splits and donates phosphate groups to ADP via substrate-level phosphorylation, yielding 2 ATP per glucose.
What role does fermentation play in ATP production
In the absence of oxygen, fermentation regenerates NAD⁺, allowing glycolysis to continue producing ATP through substrate-level phosphorylation.
What is oxidative phosphorylation
A process where ATP is synthesized using a proton gradient generated by the electron transport chain (ETC), powered by electrons from NADH and FADH₂ produced in the TCA cycle.
How does the TCA cycle support ATP production
It oxidizes pyruvate to produce NADH and FADH₂, which donate electrons to the ETC, leading to proton pumping and ATP synthesis via ATP synthase.
What is the role of oxygen in oxidative phosphorylation
Oxygen is the terminal electron acceptor in the ETC. It accepts electrons and is reduced to water - preventing toxic accumulation of electrons.
What happens as electrons pass through the ETC
Each complex is reduced and oxidized, passing electrons along the chain and using the released energy to pump protons from the cytoplasm to the periplasmic space.
How does Complex I contribute to the proton gradient
It transfers electrons from NADH to ubiquinone, releasing free energy that drives conformational changes. This opens proton channels that pump H⁺ across the membrane.
What determines whether a compound is an electron donor or acceptor in the ETC
Reduction potential: more negative compounds like NADH donate electrons, while more positive ones like O₂ accept them.
How does the proton gradient drive ATP synthesis
Proton flow rotates the ATP synthase complex, causing conformational changes (open, loose, tight states) that bind ADP + Pi and synthesize ATP.
What are the open, loose, and tight states in ATP synthase
Open: ADP + Pi bind. Loose: H⁺ flow causes partial closing. Tight: rotation causes spontaneous ATP formation. These states rotate with the rotor’s movement.
Why is oxidative phosphorylation preferred over substrate-level phosphorylation (Reason 1)
Flexibility - ETC can extract energy from various sources using universal electron carriers. SLP is limited to specific reactions.
Why is oxidative phosphorylation preferred over substrate-level phosphorylation (Reason 2)
Efficiency - ETC enables maximal energy extraction from carbon sources and efficient NAD⁺ regeneration, boosting ATP yield.
What is the likely evolutionary origin of oxidative phosphorylation
Carboxylic acids, such as those in the TCA cycle, were likely among the first metabolic molecules to arise spontaneously from CO₂ and H₂, laying the groundwork for electron transport chains.
What is the main function of the electron transport chain (ETC) in ATP synthesis
The ETC transfers electrons through a series of protein complexes, releasing energy that pumps protons across the membrane, creating an electrochemical gradient used by ATP synthase.
What are the four components of the electron transport chain
The ETC consists of Complexes I, II, III, and IV, along with electron carriers like ubiquinone and cytochrome c, embedded in the cell membrane.
What is the role of ubiquinone (CoQ) in the ETC
Ubiquinone shuttles electrons between Complex I (or II) and Complex III and participates in proton pumping via redox-driven conformational changes.
How does Complex I use redox energy to move protons
It transfers electrons from NADH to ubiquinone, releasing energy that drives piston-like movements and conformational shifts that pump protons across the membrane.
How are protons moved through Complex I during ATP generation
Conformational changes open proton channels from the cell interior to a central water-filled ‘river’ in the membrane arm, then eject protons to the outside.
What is the relationship between redox potential and electron transfer
Compounds with low (negative) redox potential like NADH donate electrons to those with high (positive) redox potential like oxygen, releasing free energy.
What is the proton-motive force (PMF)
PMF is the electrochemical gradient of protons across the membrane, composed of a pH difference and charge separation, which drives ATP synthesis.
What protein complex synthesises ATP from the proton gradient
ATP synthase, a rotary motor complex, uses the flow of protons through its base to drive conformational changes in its catalytic head, forming ATP from ADP and Pi.
What is the rotational catalysis (binding change) mechanism in ATP synthase
As protons enter, the base rotates, causing three active sites in the catalytic head to cycle through Open, Loose, and Tight states, enabling ATP formation.
What happens during the ‘Open’ state of ATP synthase
ADP and inorganic phosphate (Pi) bind to the enzyme’s active site, preparing for ATP formation.
What happens during the ‘Loose’ state of ATP synthase
ADP and Pi are held in place within the enzyme, positioned for condensation but not yet reacted.
What happens during the ‘Tight’ state of ATP synthase
The conformational change forces ADP and Pi to combine, forming ATP, which is then released in the next Open state.
How does the ETC ensure the continued flow of electrons
Electrons flow downhill in energy from NADH/FADH₂ to oxygen, and each redox step releases energy that powers proton pumping.
Why is oxygen an effective terminal electron acceptor
Oxygen has a high positive redox potential, readily accepts electrons, and is reduced to non-toxic water.
Why is water the final product of oxidative phosphorylation
Because oxygen accepts electrons at the end of the ETC and combines with protons to form H₂O, preventing harmful electron buildup.
Why is oxidative phosphorylation more efficient than substrate-level phosphorylation (SLP)
It extracts more energy from carbon substrates and regenerates NAD⁺ more efficiently, producing many more ATP molecules than SLP.
What limits the flexibility of substrate-level phosphorylation
SLP is tied to specific metabolic reactions and intermediates, limiting its energy-generating capacity and adaptability.
What role did carboxylic acids play in the origin of oxidative phosphorylation
Carboxylic acids were likely the first metabolites to arise spontaneously from CO₂ and H₂, forming the basis of early metabolic pathways like the TCA cycle.
What connects the TCA cycle and ETC in ATP production
The TCA cycle generates NADH and FADH₂, which supply high-energy electrons to the ETC for generating a proton gradient and driving ATP synthesis.
What is the energetic benefit of displacing ATP synthesis from equilibrium in the cell
Maintaining high ATP relative to ADP ensures energy-releasing hydrolysis reactions remain favorable and capable of driving essential cellular processes.
