Metabolism and Energy

Cells convert food energy into ATP for muscle contraction and synthesis, so ATP is called the energy currency. When ATP is hydrolyzed to ADP and phosphate, energy is released, and cellular respiration rebuilds ATP. Enzymes speed metabolism by lowering activation energy and act with substrate specificity. Slide the ATP production rate and watch the ATP–ADP cycle change.

ATP — the Energy Currency

💰 ATP = Cellular Money

①Just as we use cash to buy things, cells use ATP for energy

②Food (glucose) → exchanged into ATP → used for muscle contraction, synthesis, etc.

③ATP is called the "energy currency"

ATP ↔ ADP Cycle

ATP production rate5

🔑 Secret of ATP Structure

①ATP = adenosine + 3 phosphate groups (triphosphate)

②Energy is released when high-energy phosphate bond is broken

③ATP → ADP + Pi + energy (~7.3 kcal/mol)

Key Formulas

ATP Hydrolysis (Releases Energy)

ATP → ADP + Pi + energy (7.3 kcal/mol)

Energy is released as high-energy phosphate bond breaks

ATP Synthesis (Stores Energy)

ADP + Pi + energy → ATP

Energy from cellular respiration (catabolism) is stored in ATP

Energy Metabolism Comparison

ChartAnabolism vs Catabolism

Item	Anabolism	Catabolism
Definition	simple → complex molecules	complex → simple molecules
Energy	consumes ATP (endothermic)	produces ATP (exothermic)
Examples	photosynthesis, protein synthesis	cellular respiration, digestion

Enzymes and Metabolism

⚡ Enzymes = Biological Catalysts

①Metabolism is difficult to proceed without enzymes

②Enzymes lower activation energy to speed up reactions

③Enzyme specificity: one enzyme acts on one substrate (lock-and-key model)

Enzyme Reaction

E + S → ES → E + P

Enzyme (E) + Substrate (S) → Enzyme-Substrate complex (ES) → Enzyme (E) + Product (P)

Worked Examples

Example 1

Write the ATP reaction that occurs when a muscle contracts and uses energy, and the products released.

The high-energy phosphate bond of ATP breaks, releasing energy (hydrolysis).

ATP → ADP + Pi (inorganic phosphate) + energy. This energy powers muscle contraction.

▸ ATP → ADP + Pi + energy (products: ADP, Pi)

ATP hydrolysis releases energy (catabolic in nature). Respiration energy then re-synthesizes ATP from ADP+Pi.

Example 2

Explain how an enzyme speeds up a chemical reaction, in terms of activation energy.

An enzyme is a biological catalyst that lowers the activation energy needed for the reaction.

With lower activation energy, more molecules cross the reaction threshold, so the rate increases. However, the overall energy change (ΔE) of the reaction is unchanged.

▸ It lowers activation energy to raise the rate (ΔE unchanged)

An enzyme only lowers the activation energy; the start-to-end energy difference stays the same. Substrate specificity (lock-and-key) is also key.

Summary

ATP Cycle Core

Cellular respiration (catabolism) → ATP synthesis → Life activity (anabolism) → ADP + Pi

ATP is the currency linking energy storage and usage

CSAT-style

Which statement about ATP and metabolism is INCORRECT?

①ATP is adenosine bound to three phosphate groups

②Energy is released when ATP breaks down to ADP

③Anabolism is a synthesis reaction that absorbs energy

④Enzymes promote reactions by raising the activation energy

⑤Energy obtained from cellular respiration is stored in ATP

▸ ④ Enzymes promote reactions by raising the activation energy

Enzymes are catalysts that "lower" activation energy to speed up reactions.

It should be "lower," not "raise," so ④ is incorrect. The others are all correct.

🎯 Exam Points

①ATP = adenosine + 3 phosphates → energy currency

②ATP → ADP + Pi: releases energy (catabolism)

③ADP + Pi → ATP: stores energy (cellular respiration)

④Enzyme: lowers activation energy, substrate specificity (lock-and-key)

⑤Anabolism = synthesis (absorbs energy); Catabolism = breakdown (releases energy)

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Cells and Cell Division

Stimulus and Response — Nervous System

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