Define metabolism, energy, and ATP — Biology notes

1. Metabolism

WHAT — It splits into two opposite directions:

Branch	Direction	Energy	Example
Catabolism	breaks big → small	releases energy (exergonic)	glucose → CO₂ + H₂O
Anabolism	builds small → big	needs energy (endergonic)	amino acids → protein

WHY split it this way? Because life must do two jobs at once: demolish fuel to get energy, and construct structures (DNA, membranes, muscle) that energy then pays for. Catabolism is the income, anabolism is the spending.

HOW are they linked? Through a shared energy currency — and that currency is ATP. Catabolism charges the battery; anabolism drains it.

2. Energy

WHY care about energy types? Because cells convert energy from one form to another:

Chemical energy — stored in bonds (e.g. glucose, ATP).
Mechanical — muscle contraction.
Electrical — nerve impulses, ion gradients.
Thermal/heat — body warmth (often a "waste" output).

The key idea — free energy ( $G$ ):

Formula

Deriving the sign rule from first principles. Total energy change splits into useful + dissipated: $\Delta H = \Delta G + T\Delta S$ where $\Delta H$ = total enthalpy (heat) change, $T$ = temperature (K), $\Delta S$ = entropy (disorder) change. Rearranging for the useful part: $\boxed{\Delta G = \Delta H - T\Delta S}$ Reading the sign:

$\Delta G < 0$ → reaction releases free energy → spontaneous / exergonic (catabolism).
$\Delta G > 0$ → reaction needs an input → non-spontaneous / endergonic (anabolism).
$\Delta G = 0$ → equilibrium, no net change.

Why this step? We subtract $T\Delta S$ because energy lost to increasing disorder cannot be captured to do work — only what's left ( $G$ ) is available.

Figure — Define metabolism, energy, and ATP

3. ATP — the Energy Currency

WHAT is it made of? Three parts:

Adenine (a nitrogenous base)
Ribose (a 5-carbon sugar)
Three phosphate groups linked in a chain: P–P–P

WHY do the bonds store so much? The last two P–P bonds are phosphoanhydride bonds. The phosphate groups each carry negative charges crammed close together — like-charges repel. Breaking them relieves that repulsion and the products are more stable (resonance + better hydration), so a lot of free energy is released.

Formula

The energy-releasing reaction (hydrolysis): $\text{ATP} + \text{H}_2\text{O} \longrightarrow \text{ADP} + \text{P}_i + \text{energy}$ Standard free energy change: $\Delta G^{\circ\prime} \approx -30.5\ \text{kJ/mol}\ \ (\approx -7.3\ \text{kcal/mol})$ Why negative? Because hydrolysis is exergonic — it releases usable energy that anabolic reactions then borrow. Recharging (the reverse): $\text{ADP} + \text{P}_i + \text{energy} \longrightarrow \text{ATP} + \text{H}_2\text{O}\quad (\Delta G^{\circ\prime} \approx +30.5\ \text{kJ/mol})$ This costs energy, supplied by catabolism (respiration).

Common Mistakes

Recall Feynman: explain to a 12-year-old

Your body is like a busy LEGO city. Metabolism is everything happening in that city — tearing old buildings down and putting new ones up. To do any building or moving you need energy, which is just "the power to make something happen." But you can't carry a whole power plant around, so the city uses little rechargeable ATP batteries. When you eat food, you charge the batteries. When you move, think, or grow, you snap a battery from "full" (ATP) to "low" (ADP) and use that pop of energy. Then food charges it back up — over and over, all day long.

Flashcards

Metabolism is defined as

the sum total of all chemical reactions occurring in a living organism.

Catabolism vs anabolism (direction & energy)

Catabolism breaks big→small, releases energy (exergonic); Anabolism builds small→big, requires energy (endergonic).

Energy is defined as

the capacity to do work (cause change).

The equation linking free energy, enthalpy, entropy

\Delta G = \Delta H - T\Delta S

Sign of ΔG for a spontaneous (exergonic) reaction

\Delta G < 0

Full form of ATP

Adenosine Triphosphate.

Three components of ATP

Adenine (base) + ribose (sugar) + three phosphate groups.

The reaction releasing energy from ATP

ATP + H₂O → ADP + Pᵢ + energy (hydrolysis).

Approximate ΔG°′ of ATP hydrolysis

about −30.5 kJ/mol (−7.3 kcal/mol).

Why ATP phosphate bonds release much energy

phosphate groups' negative charges repel; products (ADP + Pᵢ) are more stable, so breaking the phosphoanhydride bond releases free energy.

What is energy coupling

linking an endergonic reaction to ATP hydrolysis so the combined ΔG is negative and the reaction proceeds.

Which branch of metabolism charges ATP

catabolism (breakdown/respiration).

Connections

Enzymes – Lowering Activation Energy
Cellular Respiration – Glycolysis
Gibbs Free Energy & Spontaneity
Oxidative Phosphorylation & ATP Synthase
Endergonic vs Exergonic Reactions
Entropy & the Second Law of Thermodynamics

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, teen simple words hain jo poori cell biology ka base hain: metabolism, energy, aur ATP. Metabolism matlab cell ke andar ho rahi saari chemical reactions ka total — kuch reactions cheezon ko tod-ti hain (yeh catabolism, jaise glucose ko todna, isme energy release hoti hai), aur kuch cheezon ko banati hain (yeh anabolism, jaise protein banana, isme energy kharch hoti hai). Income aur expense wali baat hai: catabolism paisa kamata hai, anabolism paisa kharchta hai.

Energy ka matlab simple — "kaam karne ki capacity", yaani change laane ki power. Par yaad rakho, molecule ki saari energy useful nahi hoti. Jo part actually use ho sakta hai use free energy (G) kehte hain, aur formula hai $\Delta G = \Delta H - T\Delta S$ . Agar $\Delta G$ negative hai to reaction apne aap chalegi (exergonic), agar positive hai to bahar se push chahiye (endergonic).

Ab ATP — yeh cell ki rechargeable battery hai. Iske teen phosphate group hote hain, aur jab last phosphate todte ho (ATP → ADP + Pᵢ) to lagbhag −30.5 kJ/mol energy nikalti hai. Yeh energy un reactions ko power deti hai jo akele nahi ho sakti — ise coupling kehte hain: ek positive ΔG wali reaction ko ATP ke negative ΔG ke saath jod do, total negative ho jata hai, aur reaction chal padti hai. Food khate ho to respiration battery ko wapas charge kar deta hai (ADP → ATP). Yeh charge-discharge cycle din bhar chalta rehta hai — isliye humein continuously khana aur oxygen chahiye.

Test yourself — Enzymes & Bioenergetics Basics

Connections