Q.Aspirin is an –
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Enzyme Definition
What is an Enzyme? — From Intuition to Precision
Imagine you're trying to dissolve a sugar cube in a glass of cold water. It takes a while, doesn't it? Now imagine you had a tiny pair of molecular tweezers that could grab the sugar cube, break it into smaller pieces, and release them into the water instantly. That's what an enzyme does — but inside your body, with the molecules that make life possible.
Every chemical reaction in your body — digesting food, contracting muscles, thinking thoughts — needs to happen fast enough to keep you alive. Without help, most of these reactions would take hours or days at body temperature. Enzymes are the biological catalysts that make them happen in milliseconds.
A catalyst is any substance that speeds up a chemical reaction without being consumed or permanently changed by it. Enzymes are nature's catalysts — they get the job done and walk away ready to work again.
The Precise Definition
An enzyme is a protein (or, in rare cases, an RNA molecule called a ribozyme) that acts as a biological catalyst — it increases the rate of a specific chemical reaction by lowering the activation energy required for that reaction, without being consumed or altered in the process.
Let's unpack each part of that definition:
1. It's a protein (mostly). Enzymes are large, folded chains of amino acids. Their three-dimensional shape creates a special pocket called the active site — the "tweezer" that grabs the reacting molecule (called the substrate).
2. It lowers activation energy. Every reaction needs a little "push" to get started — that's activation energy. Think of it like pushing a boulder over a hill. The enzyme doesn't change where the boulder ends up, but it carves a tunnel through the hill instead of forcing you to go over the top. The reaction happens much faster.
3. It's specific. Each enzyme typically works on only one substrate or a very small group of similar substrates. This is often called the lock-and-key model (the substrate fits the active site like a key fits a lock) or the induced-fit model (the active site changes shape slightly to wrap around the substrate).
4. It's not consumed. After the reaction, the enzyme releases the product and is ready to catalyze another reaction. One enzyme molecule can process thousands of substrate molecules per second.
E+S⇌ES→E+P
Where E = enzyme, S = substrate, ES = enzyme-substrate complex, P = product
A Concrete Example
The enzyme lactase breaks down lactose (the sugar in milk) into glucose and galactose. People who are lactose intolerant don't produce enough lactase, so lactose stays undigested and causes discomfort. The reaction:
lactoselactaseglucose+galactose
Without lactase, this reaction happens so slowly at body temperature that it's essentially useless. With lactase, it happens in the blink of an eye.
Key Points to Remember for Exams …
Why this formula?
Enzyme Definition: Why the Key Concepts Hold
Enzymes are biological catalysts — proteins (or sometimes RNA) that speed up chemical reactions without being consumed. The "why" behind their definition comes from two core ideas: activation energy and specificity.
1. Why Enzymes Lower Activation Energy (Ea)
Every chemical reaction has an energy barrier — the minimum energy needed for reactants to collide and form products. This is the activation energy (Ea).
- Without an enzyme, reactants must overcome a high Ea — like pushing a boulder up a steep hill.
- An enzyme provides an alternative reaction pathway with a lower Ea.
The Arrhenius Equation Connection
The rate constant k of a reaction depends on Ea via the Arrhenius equation:
k=Ae−Ea/RT
Where:
- A = frequency factor (collision rate)
- R = gas constant
- T = temperature (Kelvin)
Why this matters:
If Ea decreases, the exponential term e−Ea/RT becomes larger — so k increases dramatically. Even a small drop in Ea (e.g., from 75 kJ/mol to 50 kJ/mol) can speed up a reaction millions of times.
Key insight: Enzymes don't change the equilibrium constant (Keq) — they only speed up the rate at which equilibrium is reached.
2. Why Enzymes Are Specific — The "Lock and Key" & "Induced Fit"
Enzymes have an active site — a 3D pocket that binds the substrate (reactant molecule).
Lock and Key Model (Emil Fischer, 1894)
- The active site has a pre-formed shape that exactly fits the substrate.
- Why it works: Only the correct substrate fits perfectly — like a key in a lock. Wrong shapes don't bind, so no reaction occurs.
Induced Fit Model (Daniel Koshland, 1958)
- The active site is flexible — it changes shape slightly when the substrate binds.
- Why it works: The binding itself induces a conformational change that strains the substrate or aligns catalytic groups — this lowers Ea further.
Exam tip: The induced fit model is more accurate — it explains how enzymes can stabilize the transition state (the highest-energy intermediate).
3. Why Enzymes Are Not Consumed — The Catalyst Cycle
An enzyme participates in the reaction but is regenerated at the end:
E+S⇌ES→E+P
- E = enzyme, S = substrate, ES = enzyme-substrate complex, P = product.
- The enzyme is released unchanged — it can bind another substrate molecule immediately.
Why this holds:
The enzyme lowers Ea for both the forward and reverse reactions equally — so it doesn't get "used up" in the net reaction.
4. The Michaelis-Menten Equation — Why It Describes Enzyme Kinetics
The rate of an enzyme-catalyzed reaction (v) depends on substrate concentration [S]:
v=Km+[S]Vmax[S]
Where:
- Vmax = maximum rate (when all enzyme is bound to substrate) …
Aspirin (acetylsalicylic acid) is a well-known drug used to reduce fever and relieve pain. …
The correct option is (i) Antipyretic.
Concept. Drugs are classified by their action:
- Antipyretics reduce body temperature (fever) — e.g. aspirin, paracetamol.
- Antibiotics kill/inhibit microorganisms — e.g. penicillin.
- Antiseptics are applied to living tissue to prevent infection — e.g. Dettol. …
Showing the 12 most recent of 14 on this concept.
- CBSE 2024Set A11 markMCQQ.The enzyme which can catalyse the conversion of glucose to ethanol is :(a) Invertase(b) Maltase(c) Zymase(d) Sucrase
›Reveal solutionSolution
The enzyme converting glucose to ethanol is zymase — option (c).
The enzyme zymase, present in yeast, catalyses the fermentation of glucose to ethanol:
C6H12O6zymase2C2H5OH+2CO2 …
- CBSE 2024Set B1 markMCQQ.The following enzyme decompose maltose into glucose.(a) Invertase(b) Zymase(c) Maltase(d) Urease
›Reveal solutionSolution
Maltase is the specific enzyme that breaks maltose down into two glucose units.
Each enzyme listed acts on a specific substrate:
- Invertase (sucrase): hydrolyses sucrose into glucose + fructose.
- Zymase: converts glucose into ethanol and CO2 during fermentation.
- Maltase: hydrolyses the disaccharide maltose (two glucose units joined by a glycosidic bond) into two molecules of glucose — Maltose + H2O -> 2 Glucose. …
- CBSE 2023Set ANNUAL1 markMCQQ.Glucose is converted into Ethyl Alcohol by -(a) Invertase(b) Zymase(c) Maltase(d) Amylase
›Reveal solutionSolution
Zymase converts glucose into ethyl alcohol during fermentation.
Zymase is an enzyme complex (present in yeast) that catalyses the fermentation of glucose into ethanol and carbon dioxide: C6H12O6Zymase2C2H5OH+2CO2. Invertase converts sucrose into glucose and fructose, maltase converts mal …
- CBSE 2023Set F1 markMCQQ.Enzymes are(a) Carbohydrates(b) Lipids(c) Proteins(d) None of these
›Reveal solutionSolution
Almost all enzymes are proteins (a few are catalytic RNAs), acting as highly specific biological catalysts.
Enzymes are biocatalysts that speed up biochemical reactions in living organisms. Chemically they are globular proteins with a specific three-dimensional shape and active site. Their catalytic activity depends on this native protein structure, whi …
- CBSE 2023Set ANNUAL1 markQ.Name the enzyme which is present in saliva. What is its important function? (½+½=1)
›Reveal solutionSolution
The enzyme in saliva is salivary amylase (ptyalin); its function is to hydrolyse dietary starch into the disaccharide maltose, starting carbohydrate digestion as soon as food is chewed.
Enzyme: Salivary amylase, commonly called ptyalin, is secreted by the salivary glands and mixed with food during chewing.
Function (21 mark): It catalyses the hydrolysis of the glycosidic (α-1,4) linkages in starch (a polysaccharide), breaking it down into the smaller disaccharide maltose (and some dextrins):
…
- CBSE 2022Set E1 markMCQQ.Enzymes are(a) Carbohydrates(b) Lipids(c) Proteins(d) None of these
›Reveal solutionSolution
Enzymes are proteins that act as highly specific biological catalysts.
Enzymes speed up biochemical reactions in living organisms. Chemically they are globular proteins built from amino acids linked by peptide bonds. Each enzyme has an active site whose specific shape lets it act on a particular substrate (lock-and-key / induced-fit).
…
- CBSE 2022Set E1 markMCQQ.Which of the following enzymes is used in the hydrolysis of starch ?(a) Maltase(b) Zymase(c) Invertase(d) Diastase
›Reveal solutionSolution
Diastase (an amylase) hydrolyses starch → maltose; the specific enzyme for each substrate is different.
Enzymes are substrate-specific biological catalysts. For the given options:
- Diastase (amylase): starch → maltose. This is the enzyme that hydrolyses starch.
- Maltase: maltose → glucose.
- Invertase: sucrose → glucose + fructose. …
- CBSE 2021Set A1 markMCQQ.What is an enzyme?(a) Carbohydrate(b) Lipid(c) Protein(d) None of these
›Reveal solutionSolution
Enzymes are biocatalysts, and chemically almost all of them are proteins.
Enzymes are highly specific biological catalysts that speed up biochemical reactions in living systems. Chemically, enzymes are proteins (specifically globular proteins) with a definite three-dimensional structure and an active site that binds the substrate. For example, the enzyme that hydrolyses maltose is maltase.
…
- CBSE 2019Set ANNUAL1 markMCQQ.Which enzyme helps in converting sucrose into glucose and fructose?(a) Lactase(b) Invertase(c) Urease(d) none of these.
›Reveal solutionSolution
Invertase (sucrase) is the enzyme that hydrolyses the glycosidic bond in sucrose to release its two monosaccharide units, glucose and fructose.
Sucrose is a disaccharide made of one glucose unit and one fructose unit joined by a glycosidic linkage. The enzyme invertase catalyses hydrolysis of this bond:
Sucrose + H2O --invertase--> Glucose + Fructose …
- CBSE 2019Set ANNUAL1 markMCQQ.Aspirin is an –(a)(i) Antipyretic(b)(ii) Antibiotics(c)(iii) Antiseptic(d)(iv) None of these
›Reveal solutionSolution
The correct option is (i) Antipyretic.
Concept. Drugs are classified by their action:
- Antipyretics reduce body temperature (fever) — e.g. aspirin, paracetamol.
- Antibiotics kill/inhibit microorganisms — e.g. penicillin.
- Antiseptics are applied to living tissue to prevent infection — e.g. Dettol. …
- CBSE 2018Set ANNUAL1 markMCQQ.Enzymes are(a) Carbohydrates(b) Proteins(c) Vitamins(d) None of these
›Reveal solutionSolution
Enzymes are the catalysts of living cells, and structurally almost all of them are proteins whose folded shape creates an active site specific to one reaction.
Enzymes catalyse the biochemical reactions that keep an organism alive - digestion, respiration, DNA replication, and so on - typically speeding up a reaction by many orders of magnitude while remaining highly specific to their substrate. Chemically, enzymes are (with a small number of exceptions, such as catalytic RNA) globular proteins: long polypeptide chains folded into a precise three-dimensional structure that creates an 'active site' shaped to bind one parti …
- CBSE 2018Set ANNUAL1 markQ.What are antiseptics ? Give an example.
›Reveal solutionSolution
Antiseptics kill/stop microbes and are applied to living tissue; e.g. Dettol, tincture of iodine.
Concept. Antiseptics and disinfectants both destroy or inhibit microorganisms, but antiseptics are applied to living tissues (skin, wounds, mouth) whereas disinfectants are used on non-living surfaces (floors, instruments). Many chemicals act as either depending on concentration.
…
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