- IB DP Biology 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Biology 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Biology 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
- IB DP Biology 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
C1.1 Enzymes and metabolism
Metabolism
Anabolism + Catabolism = Metabolism
Anabolism : The making up of big molecules from small molecules
= Condensation = Polymerization
Monomers to Polymers
Catabolism: The breaking up of big molecules into small molecules
= Hydrolysis = Depolymerization
Polymers to Monomers
Anabolic and catabolic reactions
Anabolism : The making up of big molecules from small molecules
= Condensation = Polymerization
Monomers to Polymers
Catabolism: The breaking up of big molecules into small molecules
= Hydrolysis = Depolymerization
Polymers to Monomers
Role of enzymes in metabolsim
Aerobic respiration : enzymatic reactions that use oxygen to break down nutrients molecules to release energy.
Word Equation
Glucose + Oxygen→ Carbon dioxide + water (+Energy)
Chemical Equation
\(C_6H_{12}+O_2 → CO_2 + H_2O\) (+Energy)
Balanced chemical equation
\(C_6H_{12}+6O_2 → 6CO_2 + 6H_2O\) (+Energy)
In the actual human cells:
Metabolic Pathways: One part of cell respiration
Metabolic Pathways
In the actual plant cells :
Enzymes have three types of functions
1. Breaking down large molecules into smaller ones
e.g. in digestion in nutrition proteins are broken down into individual amino acids that can be absorbed
e.g. in digestion in nutrition starch is broken down into individual glucose that can be absorbed
2. Building up large molecules from smaller ones
e.g. in assimilation in nutrition when individual amino acids are linked together into human proteins
e.g. in plants when they make starch out of individual molecules of glucose for storage
e.g. in plants when they make cellulose out of individual molecules of glucose for cell wall
3. Converting one small molecule into another
e.g. in plants when converting some of the glucose into fructose
e.g. in many organisms when one amino acid is converted into another amino acid
Names of enzymes very often end in –ase
Substrate Enzyme
Protein……………………Protease
Lipid………………………Lipase
Maltose……………………Maltase
Starch = Amylum…………Amylase
Enzymes as biological catalysts
Enzymes are proteins
Living organisms produce proteins
Enzymes are catalysts
⇒ Enzymes are biological catalysts
1. An enzyme speeds up a chemical reaction
2. An enzyme reduces the energy needed to start a reaction
3. An enzyme is unchanged by the reaction
1. An enzyme speeds up a chemical reaction
Without enzyme: normal speed
With enzyme: higher speed
Reaction is sped up by a factor of \(10^8\) to \(10^{26}\)
100.000.000 to 100.000.000.000.000.000.000.000.000 FASTER
Chemical reactions cannot start by themselves
All reactions begin with breaking up of chemical bonds in substrate(s)
– They need an initial input of energy = ACTIVATION ENERGY
2. An enzyme reduces the energy needed to start a reaction
3. An enzyme is unchanged by the reaction
The enzyme can be used again and again
Millions of times
Active site of enzymes
Active Site
A restricted region of an enzyme molecule which binds to the substrate
Specificity enzyme-substrate and Induced-fit binding
Enzymes are globular proteins
A few amino acids make up the active site
Its shape fits the shape of the substrate(s)
– Specificity of the enzyme for its substrate
Both substrate and enzyme change shape
when binding occurs
Substrate can bind the enzyme
“Induced-fit binding”
Denaturation of enzymes
From active to inactive enzyme
Change in the 3-D shape of the active site
Substrate(s) do not fit
Collisions between substrate and active site
Enzyme and substrate need to interact = to “collide”
To form the temporary enzyme-substrate complex
– Movement is needed
Both enzyme and substrate
Or enzyme only
Or substrate only
Reaction occurs: Enzyme uses the substrate(s) to produce the product(s)
Products are released as they no longer fit in the active site
Enzyme is available for the next molecule of substrate
Effects of temperature, pH and substrate concentration
- Effects of temperature
To observe influence of temperature on enzyme action
1. Measure rate of reaction rate at different temperatures
2. Report the results on a graph
Let’s look at the reaction rate…
1. Zero activity at 0 degree C
In water (frozen)
Remember the increase factor of rate by enzymes
Without enzyme, the reaction “NOT” happening
2. Increases until a maximum (120 AU at 40 degree C)
Optimum temperature (Temperature at which reaction rate is maximum)
3. Decreases if temperature above 40 degree C
Enzyme gets denatured
4. Reaches zero at 65 degree C
Shape active site depends on bonds inside enzyme
If temperature higher than optimum temperature
These bonds start to break
– Whole enzyme loses its shape
– Active site loses its shape
– Enzyme is denatured
Substrate cannot fit inside active site anymore
– Rate of reaction decreases
Until zero
Optimum temperature depends on the organism’s own temperature
- Effect of pH
Bonds holding the 3-D shape of
proteins can be broken by changes in pH
As for temperature, enzymes have their own optimum pH
X: pH 3 Y: pH 8
If the pH is below (more acidic) than
the optimum pH, enzymes are denatured
If the pH is above (more alkaline) than the optimum pH, enzymes are denatured
Optimum pH
depends on the part of the body the enzyme works in
depends on the part of the cell the enzyme works in
cytoplasm pH 7 lysosome pH 5
- Effect of substrate concentration
Reason:
Point of saturation = Substrate concentration that saturates the enzyme
Measurement of enzyme activity rate
Why is it better to measure the initial rate of reaction ?
Before any factors, e.g. substrate /product concentration and conditions, have had time to change
Catalase detoxifies hydrogen peroxyde
\(2H_2O_2 → 2H_2O +O_2\)
Catalase uses up hydrogen peroxide
Concentration of hydrogen peroxyde changes
Concentration of substrate influences rate of reaction
– Rate of reaction changes fast
Measure it as soon as possible
Other reactions change the pH or temperature of the solution
– Rate of reaction changes fast
Measure it as soon as possible
About the graph;
pH of one solution was NOT increased from 4 to 10
1. Reaction performed at pH4, pH5, pH6, pH7, pH8, pH9, and pH10
2. Initial rate of reaction measured at pH4, pH5, pH6, pH7, pH8, pH9, and pH10
3. IRR plotted on the graph
4. Curve drawn by joining the points
About the graph;
Temperature of one solution was NOT increased from 10 to 70 degree C
1. Reaction performed at temperatures 10, 20, 30… 70 degree C
2. Initial rate of reaction measured at at temperatures 10, 20, 30… 70 degree C
3. IRRs plotted on the graph
4. Curve drawn by joining the points
Intra- and extra-cellular enzyme reactions (HL only)
Intra : Inside the cells
Glycolysis and Krebs cycle
Extra: Outside the cells
Chemical digestion in the gut
Cyclical and linear pathways in metabolism (HL only)
Cyclic
- Calvin cycle
- Krebs cycle
- Glycolysis
Linear
- Glycolysis
Generation of heat (HL only)
Metabolic reactions are not 100% in energy transfer
Each time a reaction occurs, heat is generated… and lost to surroundings
| Ectotherms | Endotherms |
| They are cold blooded animals | They are warm blooded animals |
| Their temperature varies with the surrounding temperature | They can maintain constant body temperature |
| eg. fish, amphibians, and reptiles | eg. birds and mammals |
Enzyme inhibition (HL only)
Competitive and non-competitive inhibition
Inhibitors of enzymes are competitive or non- competitive
- Competitive inhibition
Competitive inhibition as a consequence of an inhibitor binding reversibly to an active site
The shape of the CI resembles the substrate’s: fits into active site
Competitive Inhibitor and the substrate compete to access the active site
Is it possible to reach maximum rate?
YES, A lot of substrate outcompetes the competitive inhibitor
Cholesterol from food builds up in arteries
Creates plaque
Decreases blood flow
Serious heart disease complications
- Non- Competitive inhibition
The shape of the NCI does not resemble the substrate’s: does not fit into the active site
Non-Competitive Inhibitor and the substrate do not compete to access the active site
1. NCI binds the enzyme at the allosteric site
2. NCI hides(left) the active site OR changes its shape(right)
Is it possible to reach maximum rate?
NO , A lot of substrate does not dislodge the NCI from the allosteric site
Competitive or non-competitive inhibition?
End-product inhibition: a form of negative feedback to control metabolic pathways
Final product in metabolic pathway inhibits an enzyme from an earlier step in the sequence
This product is a a non-competitive inhibitor
The whole pathway is stopped
This ensures levels of an essential product are tightly regulated
If product levels build up, further product formation is prevented
If product levels drop, tmore product is produced
Transpeptidase = enzyme used by bacteria to make their cell wall
Antibiotics fight bacterial infections
Example = penicillin
Penicillin = competitive inhibitor of transpeptidase
Binding is irreversible
Some mutations in bacteria make them
resistant to penicillin
Shape change of transpeptidase’s active site
Production of penicillinase