B. Biochemistry · BiochemistryFour Reactions That Run the Cell: Condensation, Hydrolysis, Neutralization and Redox
B3.5 B2.1 A1.12About 70 minutes
Learning goals- I can identify and describe condensation (dehydration synthesis), hydrolysis, neutralization and oxidation-reduction reactions in cells.
- I can recognize which type a biochemical equation shows and explain my reasoning.
- I can explain how buffers use neutralization to keep body fluids at a steady pH.
- I can describe biological oxidation and reduction in terms of electrons and hydrogen atoms, using NAD⁺ as an example.
Words to know
condensation reaction (dehydration synthesis)
A reaction that joins two molecules with a covalent bond and releases a molecule of water.
hydrolysis
A reaction that breaks a covalent bond by adding water: –H goes to one product and –OH to the other.
neutralization
A reaction between an acid and a base that produces water and a salt (or, in cells, that removes excess H⁺ or OH⁻).
buffer
A pair of substances, a weak acid and its base, that takes up or releases H⁺ to keep the pH nearly constant.
oxidation
Loss of electrons. In biology it often appears as a loss of hydrogen atoms (or a gain of oxygen).
reduction
Gain of electrons. In biology it often appears as a gain of hydrogen atoms.
redox reaction
A reaction in which one substance is oxidized and another is reduced at the same time.
NAD⁺ / NADH
An electron carrier (a coenzyme). NAD⁺ is the oxidized form; it accepts two electrons and one H⁺ to become NADH, the reduced form.
Thousands of reactions, four main types
A single cell carries out thousands of different chemical reactions every second. That sounds impossible to learn, but most of them belong to just four types. Once you can recognize them, you can read almost any pathway in this course, from digestion to cellular respiration to DNA copying.
- Condensation (building up by removing water)
- Hydrolysis (breaking down by adding water)
- Neutralization (acid meets base)
- Oxidation-reduction, or redox (electrons move from one molecule to another)
Remember from Grade 11
In SBI3U Animals: Structure and Function you met dehydration synthesis and hydrolysis as the reactions that build and digest food molecules. In SCH3U you balanced neutralization reactions (acid + base → salt + water). Here you will see all four types as tools the cell uses every second.
1. Condensation: building polymers
Most large biological molecules are polymers: chains of similar units called monomers. Cells build them by condensation reactions (also called dehydration synthesis). One monomer loses an –OH, the other loses an –H, and these leave together as H₂O. A new covalent bond forms between the monomers.
- Sugars join by glycosidic bonds (carbohydrates).
- Glycerol and fatty acids join by ester bonds (fats).
- Amino acids join by peptide bonds (proteins).
- Nucleotides join by phosphodiester bonds (DNA and RNA).
Building a polymer needs energy (usually supplied by ATP) and an enzyme. A chain of n monomers is held by n − 1 bonds, so making it releases n − 1 water molecules.
2. Hydrolysis: breaking polymers
Hydrolysis (hydro = water, lysis = splitting) is the reverse. A water molecule is split across a bond: –H attaches to one piece and –OH to the other. Digestion is hydrolysis. So is the breakdown of ATP: ATP + H₂O → ADP + Pᵢ (inorganic phosphate), a reaction that releases energy the cell uses to do work.
3. Neutralization: controlling pH
An acid releases H⁺; a base accepts H⁺ (or releases OH⁻). When they react, they neutralize each other: acid + base → salt + water. For example, the antacid milk of magnesia neutralizes stomach acid:
Mg(OH)₂ + 2 HCl → MgCl₂ + 2 H₂O
Inside the body, neutralization happens all the time through buffers. Enzymes and other proteins only keep their shapes within a narrow pH range, so pH must stay steady. Human blood is kept between about pH 7.35 and 7.45 mainly by the carbonic acid–bicarbonate buffer:
- If H⁺ is added (for example, lactic acid from hard exercise), bicarbonate neutralizes it: H⁺ + HCO₃⁻ → H₂CO₃
- If a base is added, carbonic acid neutralizes it: H₂CO₃ + OH⁻ → HCO₃⁻ + H₂O
In digestion, the pancreas releases bicarbonate into the small intestine to neutralize acidic food arriving from the stomach. Amino acids act as buffers too, because their carboxyl group can give H⁺ and their amino group can take H⁺.
4. Redox: moving electrons
In oxidation, a substance loses electrons; in reduction, a substance gains electrons. They always happen together (one molecule's loss is another's gain), so the pair is called a redox reaction. Memory aid: OIL RIG: Oxidation Is Loss, Reduction Is Gain.
In cells, electrons usually travel with protons, as hydrogen atoms (H = H⁺ + e⁻). So in biology:
- Oxidation often looks like losing H atoms (or gaining O).
- Reduction often looks like gaining H atoms (or losing O).
Cells use special electron carriers to move electrons between reactions. The most important is NAD⁺. An enzyme (a dehydrogenase) removes two H atoms from a fuel molecule. NAD⁺ takes two electrons and one proton and becomes NADH; the other proton is released as H⁺:
NAD⁺ + 2 H → NADH + H⁺
The fuel molecule was oxidized; NAD⁺ was reduced. A similar carrier, FAD, becomes FADH₂. In cellular respiration, glucose is gradually oxidized to CO₂, and oxygen is finally reduced to H₂O:
C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O
You will follow every step of this in Metabolic Processes.
| Reaction type | What happens | Clue in the equation | Cell example |
|---|
| Condensation | Two molecules join | H₂O is a product | Amino acids → protein |
| Hydrolysis | A bond breaks | H₂O is a reactant | Starch → maltose; ATP → ADP + Pᵢ |
| Neutralization | Acid + base → salt + water | H⁺ meets OH⁻ or a base such as HCO₃⁻ | Bicarbonate buffer in blood |
| Redox | Electrons (often as H atoms) move | One molecule loses H (or gains O); another gains H | NAD⁺ → NADH; glucose → CO₂ |
Worked exampleExample: Classify the reaction
Classify each reaction and justify your answer.
(a) glycerol + 3 fatty acids → triglyceride + 3 H₂O (b) ATP + H₂O → ADP + Pᵢ (c) lactate + NAD⁺ → pyruvate + NADH + H⁺ (d) H⁺ + HCO₃⁻ → H₂CO₃ (e) glucose + fructose → sucrose + H₂O. Also find the formula of sucrose.
- (a) Water is a product and the molecules join: condensation (three ester bonds form, so three waters are released).
- (b) Water is a reactant and a bond breaks: hydrolysis.
- (c) NAD⁺ gains H and becomes NADH, so NAD⁺ is reduced; lactate loses H, so it is oxidized: redox.
- (d) A base (HCO₃⁻) accepts H⁺ from an acid: neutralization (buffering).
- (e) Water is a product: condensation. Formula: C₆H₁₂O₆ + C₆H₁₂O₆ = C₁₂H₂₄O₁₂; subtract H₂O to get C₁₂H₂₂O₁₁.
(a) condensation, (b) hydrolysis, (c) redox, (d) neutralization, (e) condensation; sucrose is C₁₂H₂₂O₁₁.
Just for funBiochemists' favourite memory aid is OIL RIG. Chemists who prefer lions use LEO says GER: Lose Electrons Oxidation, Gain Electrons Reduction. Either way, no actual oil rigs or lions are involved.
Hands-on activityKitchen reactions: neutralization and redox
Grown-up needed for the boiling water and the knife.
You need: a quarter of a red cabbage, a pot or kettle, a strainer, 4 clear glasses, vinegar, baking soda, water, a teaspoon, an apple, a lemon (or bottled lemon juice), and a plate.
Part A: neutralization with a natural indicator
- With a grown-up, chop the cabbage and cover it with about 500 mL of boiling water. Let it stand 10 minutes, then strain. The purple liquid is an acid-base indicator: pink-red in acid, purple near neutral, blue to green in base.
- Put 100 mL of indicator in a glass and add 1 teaspoon of vinegar (acetic acid). Note the colour.
- Dissolve 1 teaspoon of baking soda (sodium bicarbonate, a base) in 100 mL of water. Add it to the vinegar glass a little at a time, stirring, and watch the colour pass through purple. You are neutralizing the acid. (The fizz is CO₂: H⁺ + HCO₃⁻ → H₂CO₃ → H₂O + CO₂, the same buffer reaction as in your blood.)
Part B: redox on an apple
- Cut 3 apple slices. Brush one with lemon juice, dip one in water, and leave one untreated. Wait 30 minutes.
- The untreated slice browns: an enzyme in the apple uses oxygen to oxidize compounds called polyphenols, and the products form brown pigments. Lemon juice contains ascorbic acid (vitamin C), which is easily oxidized itself, so it reduces the oxidized compounds back before they turn brown; its acidity also slows the enzyme.
What to look for: a colour change from pink to purple to blue as base is added in Part A; in Part B, the lemon slice stays palest and the untreated slice is darkest. Write one sentence naming the reaction type in each part.
Your turn
1What is released in every condensation reaction that joins two monomers?
A molecule of water (H₂O).
2How many water molecules are needed to hydrolyze a chain of 10 glucose units completely into glucose?
A chain of 10 has 9 glycosidic bonds, so 9 water molecules are needed.
3In the reaction FAD + 2 H → FADH₂, which substance is reduced? Explain.
FAD is reduced: it gains hydrogen atoms (and so gains electrons).
4Write the neutralization of stomach acid (HCl) by sodium bicarbonate (NaHCO₃), and say what gas is produced.
HCl + NaHCO₃ → NaCl + H₂O + CO₂. The gas is carbon dioxide. (The carbonic acid formed breaks down into water and CO₂.)
5Name the bond formed when (a) two amino acids join, (b) two sugars join, (c) glycerol joins a fatty acid, (d) two nucleotides join.
(a) peptide bond, (b) glycosidic bond, (c) ester bond, (d) phosphodiester bond. All are formed by condensation.
6Why must blood pH be kept within a narrow range?
Enzymes and other proteins hold their shapes (and so work) only in a narrow pH range. A large change in H⁺ concentration changes the charges on their groups, which can change their shape and stop them working.
More practice: warm-up
1What does OIL RIG stand for?
Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).
2Is digestion of starch a condensation or a hydrolysis reaction?
3What are the products of a neutralization reaction?
4Which form of the carrier holds more energy-rich electrons: NAD⁺ or NADH?
5What is the normal pH range of human blood?
More practice: on your own
1Maltose (C₁₂H₂₂O₁₁) is hydrolyzed into two glucose molecules. Write the balanced equation.
C₁₂H₂₂O₁₁ + H₂O → 2 C₆H₁₂O₆. Check: C 12 = 12; H 22 + 2 = 24 = 2 × 12; O 11 + 1 = 12 = 2 × 6.
2During hard exercise, muscles release lactic acid. Explain, with an equation, how the blood resists a drop in pH.
Bicarbonate ions neutralize the extra H⁺: H⁺ + HCO₃⁻ → H₂CO₃. The carbonic acid can break down to H₂O + CO₂, and the extra CO₂ is breathed out. So free H⁺ concentration (and pH) changes only a little.
3In photosynthesis, CO₂ becomes glucose (C₆H₁₂O₆). Is carbon oxidized or reduced? Explain using hydrogen and oxygen.
Reduced. In CO₂ each carbon is bonded only to oxygen; in glucose the carbons have gained hydrogen atoms and have fewer oxygens per carbon. Gaining H (losing O) means gaining electrons: reduction.
4A polymer made of 250 amino acids has how many peptide bonds, and how many water molecules were released when it was made?
249 peptide bonds, and 249 water molecules released (one per bond).
5Classify: ethanol + NAD⁺ → acetaldehyde + NADH + H⁺. Which molecule is the reducing agent?
Redox. Ethanol loses H atoms, so it is oxidized; it gives electrons to NAD⁺, so ethanol is the reducing agent. (This is the first step in breaking down alcohol in the liver.)
Challenge
1Condensation and hydrolysis are reverse reactions, yet the cell can run both at once in different places. Using what you learned about organelles, explain how.
Compartmentalization and different enzymes. Building reactions (for example, protein synthesis at ribosomes, lipid synthesis in the smooth ER) and breakdown reactions (hydrolysis in lysosomes or the gut) happen in different compartments, each with its own specific enzymes and conditions such as pH. Building also uses ATP, so it is driven forward even though the hydrolysis direction releases energy.
2Gram for gram, fats release about twice as much energy as carbohydrates when oxidized in respiration. Use the idea of oxidation and reduction to explain why. (Compare a fatty acid such as palmitic acid, C₁₆H₃₂O₂, with glucose, C₆H₁₂O₆.)
Fats are more reduced. Palmitic acid has 32 H and only 2 O for 16 C (mostly C–H bonds), while glucose already has one O per C, so it is partly oxidized already. A more reduced molecule has more electrons (in C–H bonds) that can be passed to oxygen during respiration, so more energy is released per gram as it is oxidized to CO₂ and H₂O.
3A patient has a condition that makes the blood too acidic. Breathing faster removes CO₂. Use the equilibrium H⁺ + HCO₃⁻ ⇌ H₂CO₃ ⇌ H₂O + CO₂ to explain why faster breathing raises the pH.
Removing CO₂ pulls the equilibrium to the right (Le Châtelier's principle): more H₂CO₃ breaks down, so more H⁺ combines with HCO₃⁻ to replace it. Free H⁺ concentration falls, so the pH rises toward normal.
Quiz
1. Which reaction type has water as a reactant and breaks a polymer into monomers?
2. A chain of 6 glucose molecules is built by condensation. How many water molecules are released?
3. In the reaction NAD⁺ + 2 H → NADH + H⁺, NAD⁺ is:
4. Which equation shows a neutralization reaction?
5. The bonds that join nucleotides in DNA are formed by:
6. During cellular respiration, glucose is:
Big idea: This lesson covers B3.5: the four main types of biochemical reactions (redox, hydrolysis, condensation and neutralization).
How to help: Give your teen equations from this lesson and ask, "Where is the water?" (product = condensation, reactant = hydrolysis), "Is there an acid and a base?" (neutralization), and "Who gained or lost H or electrons?" (redox). Those three questions classify almost every reaction in the course.
Common difficulties:
- Thinking "oxidation" must involve oxygen. In biology it often means losing hydrogen atoms (electrons).
- Forgetting that oxidation and reduction always happen together.
- Counting water molecules: n monomers need n − 1 bonds.
- The cabbage indicator activity needs boiling water and a knife: please supervise.