Grade 10-11 (age 15-17)

high school Metabolism and Energy

Cells do not use the energy they get from breaking down nutrients such as glucose right away; they first store it in ATP and then draw on it wherever energy is needed, as in muscle contraction or building molecules. Energy is released when ATP's last phosphate bond breaks to give ADP and phosphate (Pi), and energy from cellular respiration joins ADP and phosphate back into ATP. These metabolic reactions run quickly even at body temperature because enzymes lower their activation energy. In the diagram you can change how active the cell is and see the rate at which ATP turns into ADP and the rate at which it is remade change together.

ATP, the Energy Currency

💰 ATP Is the Cell's Currency

①Just as you earn money and spend it when needed, a cell stores the energy from nutrients in ATP and uses it when needed

②The energy in glucose ends up in ATP through cellular respiration, and the energy in ATP powers muscle contraction, building molecules, active transport and keeping the body warm

③Because it serves as the common means of passing energy around, ATP is called the energy currency

The ATP–ADP Cycle

Cell activity5

🔑 ATP's Structure and Cycle

①ATP is adenosine (adenine + ribose) with three phosphate groups attached

②When the last phosphate bond breaks, ATP becomes ADP and phosphate and releases energy (about 7.3 kcal/mol under standard conditions)

③A cell holds only a small amount of ATP, so it remakes ATP as fast as it uses it, with energy from cellular respiration

④So when activity rises, breakdown and synthesis both speed up, while the amount of ATP in the cell stays nearly the same

Key Reactions

ATP Breakdown (Energy Released)

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

the energy released when the last phosphate bond breaks powers life activities such as muscle contraction and building molecules

ATP Synthesis (Energy Stored)

ADP + Pi + energy → ATP

energy from breaking down glucose in cellular respiration is stored in ATP

Energy Metabolism Comparison

Anabolism and Catabolism

Item	Anabolism	Catabolism
Change in matter	simple → complex molecules	complex → simple molecules
Energy	absorbed (runs on ATP)	released (part of it stored in ATP)
Examples	photosynthesis, protein synthesis	cellular respiration, digestion

Enzymes and Metabolism

⚡ Enzymes Are Biological Catalysts

①Without enzymes, metabolic reactions barely happen at body temperature

②Enzymes speed up reactions by lowering the activation energy, but they do not change the energy difference between the start and end of the reaction

③An enzyme acts only on the particular substrate that fits its active site (substrate specificity)

④An enzyme is left unchanged after the reaction, so it can help the same reaction again and again

Enzyme Reaction

E + S → ES → E + P

the enzyme (E) binds the substrate (S) to form an enzyme–substrate complex (ES); when the reaction is done the product (P) leaves and the enzyme remains unchanged

Worked Examples

Example 1

Write the ATP reaction that takes place when a muscle contracts and uses energy, and name the substances it produces.

The last phosphate bond of ATP breaks and energy is released.

ATP → ADP + Pi (inorganic phosphate) + energy, and this energy powers the muscle contraction.

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

The ADP and Pi are not thrown away; energy from cellular respiration turns them back into ATP. That is why, the harder you exercise, the thicker both arrows in the diagram become together.

Example 2

Using activation energy, explain how an enzyme speeds up a chemical reaction.

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

With a lower activation energy, more molecules clear that barrier at the same temperature, so the reaction goes faster. The energy difference between the start and end of the reaction stays the same.

▸ it lowers the activation energy, which raises the reaction rate (the energy difference of the reaction is unchanged)

An enzyme leaves the heights of the starting and finishing points alone and only lowers the hill between them. So the amount of energy released by the reaction does not change; only how fast it happens does.

Summary

The Core of the ATP Cycle

cellular respiration (catabolism) → ADP + Pi + energy → ATP → life activities (muscle contraction, building molecules) → ADP + Pi

ATP links the reactions that gain energy with the reactions that use it

CSAT-style problem, adapted

Which statement about ATP and metabolism is INCORRECT?

①ATP consists of adenosine bound to three phosphate groups.

②Energy is released when ATP breaks down to ADP.

③Anabolism is a building-up reaction that absorbs energy.

④Enzymes speed up reactions by raising the activation energy.

⑤Energy obtained from cellular respiration is stored in ATP.

▸ ④ Enzymes speed up reactions by raising the activation energy.

Enzymes are biological catalysts that speed up reactions by lowering the activation energy.

④ says the opposite, raising it, so it is incorrect. All the other statements are correct.

🎯 Exam Points

①ATP is adenosine bound to three phosphate groups and serves as the energy currency

②The energy released by ATP → ADP + Pi powers life activities

③Energy from cellular respiration drives ADP + Pi → ATP

④Enzymes lower the activation energy and show substrate specificity, without changing the energy difference of the reaction

⑤Anabolism is building up (energy absorbed); catabolism is breaking down (energy released)

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Stimulus and Response — Nervous System

Last updated 2026-10-17

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