Noesis
2.5.3Enzymes & Bioenergetics Basics

Distinguish exergonic and endergonic reactions

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🇮🇳 Yeh note Hinglish mein padho →

WHAT are we distinguishing?

WHY is ΔG\Delta G the deciding number? (derive it)

We want a quantity that predicts spontaneity. The Second Law says a process is spontaneous if total entropy increases: ΔSuniverse=ΔSsystem+ΔSsurroundings>0\Delta S_{\text{universe}} = \Delta S_{\text{system}} + \Delta S_{\text{surroundings}} > 0

The surroundings receive heat from the system. At constant temperature/pressure, the heat the system releases is its enthalpy change −ΔHsys-\Delta H_{\text{sys}}, so: ΔSsurroundings=−ΔHsysT\Delta S_{\text{surroundings}} = \frac{-\Delta H_{\text{sys}}}{T}

Why this step? Pouring heat −ΔH-\Delta H into surroundings at temperature TT raises their entropy by heat/TT (definition of entropy change).

Substitute: ΔSuniverse=ΔSsys−ΔHsysT>0\Delta S_{\text{universe}} = \Delta S_{\text{sys}} - \frac{\Delta H_{\text{sys}}}{T} > 0

Multiply by −T-T (flips the inequality): ΔHsys−TΔSsys<0\Delta H_{\text{sys}} - T\Delta S_{\text{sys}} < 0

Define this combination as Gibbs free energy change:

Figure — Distinguish exergonic and endergonic reactions

HOW to read it from an energy diagram

  • Exergonic: products sit lower on the free-energy axis than reactants → energy ramps downhill → energy released = ∣ΔG∣|\Delta G|.
  • Endergonic: products sit higher → energy must be pushed uphill → energy absorbed.
  • In both cases there is a hump (activation energy EaE_a) the reaction must climb first — that controls speed, not direction.

Worked examples

Recall Feynman: explain to a 12-year-old

Imagine a ball on a hill. If the ball is at the top, it can roll down by itself and even spin a little windmill on the way — that's an exergonic reaction: it gives energy out, no help needed. If the ball is at the bottom, it won't climb up on its own — you have to push it. That pushing is the energy you must spend, and that's an endergonic reaction. The "height" is called free energy. Going down = energy released (negative ΔG\Delta G). Going up = energy needed (positive ΔG\Delta G). And a ball rolling down can drag an uphill ball up if you tie them together with a rope — that rope is ATP inside your cells!

Recall checkpoint

Flashcards

Exergonic reaction sign of ΔG
ΔG<0\Delta G < 0 (negative); energy released, spontaneous.
Endergonic reaction sign of ΔG
ΔG>0\Delta G > 0 (positive); energy absorbed, non-spontaneous.
Gibbs free energy equation
ΔG=ΔH−TΔS\Delta G = \Delta H - T\Delta S
What does ΔG\Delta G secretly measure?
The entropy change of the whole universe, rewritten using only system quantities (ΔG<0⇔ΔSuniv>0\Delta G<0 \Leftrightarrow \Delta S_{univ}>0).
Does negative ΔG mean the reaction is fast?
No — ΔG sets direction/spontaneity; speed depends on activation energy EaE_a (lowered by enzymes).
How can a cell run an endergonic reaction?
By coupling it to an exergonic one (e.g. ATP hydrolysis) so the summed ΔG is negative.
Crossover temperature where ΔG = 0
T=ΔH/ΔST = \Delta H/\Delta S.
Approximate ΔG of ATP hydrolysis
about −30.5-30.5 kJ/mol (exergonic).
Why does high temperature favor entropy-driven reactions?
The −TΔS-T\Delta S term grows with TT, so positive ΔS\Delta S pulls ΔG\Delta G more negative.

Connections

Concept Map

change gives

equals

from

from

negative

positive

is

needs

derives

not same as

depends on

lowered by

Free energy G

Delta G

Delta G = H - T S

Enthalpy H

Entropy tax T S

Exergonic releases energy

Endergonic absorbs energy

Spontaneous

Energy input

Second Law: S universe up

Reaction speed

Activation energy Ea

Enzymes

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, har chemical reaction ek "energy ka sauda" hai. Jo number batata hai ki sauda fayde ka hai ya nuksaan ka, woh hai Gibbs free energy change, yaani ΔG\Delta G. Agar ΔG\Delta G negative hai (matlab products ki energy reactants se kam), to reaction energy release karti hai — isse exergonic kehte hain, aur yeh apne aap chal sakti hai (spontaneous). Agar ΔG\Delta G positive hai, to reaction ko bahar se energy chahiye — yeh endergonic hai, jo akele nahi chalegi.

Formula yaad rakho: ΔG=ΔH−TΔS\Delta G = \Delta H - T\Delta S. Yahan ΔH\Delta H heat content ka change hai aur ΔS\Delta S disorder (entropy) ka. Mast baat yeh hai ki TΔST\Delta S ek "disorder tax" ki tarah kaam karta hai — temperature jitna zyada, utna yeh term bada. Isiliye ice melting cold mein endergonic hai par garmi mein exergonic ho jaati hai. Crossover point: T=ΔH/ΔST = \Delta H/\Delta S.

Ek common galti: log sochte hain "spontaneous matlab fast". Nahi! ΔG\Delta G sirf direction batata hai, speed nahi. Speed activation energy (EaE_a) pe depend karti hai, jo enzymes kam karte hain. Tabhi cheeni jar mein saalon tak safe rehti hai jabki uska jalna bahut exergonic hai.

Body ke andar magic kaise hota hai? Coupling se. Cell endergonic reaction (jaise protein banana) ko ek strongly exergonic reaction — ATP hydrolysis (ΔG≈−30.5\Delta G \approx -30.5 kJ/mol) — ke saath jod deta hai. Dono ke ΔG\Delta G add ho jaate hain; agar total negative aaya, kaam ho gaya! Yahi poora metabolism ka funda hai.

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Connections

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