Enzymes & Metabolism

Apréndelo jugando

Responde estas preguntas para ganar energía, luego pesca y explora. Sin cuenta.

Para educadores: diapositivas de la lección, apuntes de repaso listos para usar sobre Enzymes & Metabolism (Biology, HL) — úsalos en tu lección, o presenta el tema como una actividad interactiva de clase que tus aprendices juegan como un juego en vivo.

Apuntes de la lección

Metabolism and Enzymes

Metabolism is all the chemical reactions that take place within cells and organisms; the molecules involved are metabolites.
Metabolic reactions are classified as anabolic (building large molecules from smaller ones) or catabolic (breaking down large molecules into smaller ones).
Anabolic reactions are endergonic (require energy input), often involve condensation reactions, and store energy; examples include photosynthesis, protein synthesis, and glycogen formation.
Catabolic reactions are exergonic (release energy), often involve hydrolysis reactions, and provide energy for cellular processes; examples include respiration, deamination, and digestion.
Enzymes are globular proteins that act as biological catalysts, speeding up reactions by lowering activation energy without being used up or permanently changed.
Enzymes are specific to their substrates due to the complementary shape and chemical properties of the active site and substrate.

Enzyme Action and Specificity

The active site is the region of an enzyme where the substrate binds; it is made of only a few amino acids but its shape is determined by the enzyme's complex 3D structure.
For a reaction to occur, substrate molecules must collide with the active site at the correct orientation and speed, forming a temporary enzyme-substrate complex.
The lock-and-key model proposed that the active site is precisely complementary to the substrate, like a key fitting a lock.
The induced-fit hypothesis is the current model: the enzyme and active site (and sometimes the substrate) change shape slightly (conformational changes) as the substrate binds, achieving an ideal binding arrangement.
Enzymes lower the activation energy needed for a reaction, so the reaction proceeds at a faster rate; the overall energy released is unchanged.
Denaturation occurs when high temperatures or extreme pH break bonds (e.g., hydrogen bonds) holding the enzyme's 3D shape, permanently changing the active site so the substrate can no longer bind.

The lock and key model

Factors Affecting Enzyme Activity

Temperature: lower temperatures slow reactions because molecules have less kinetic energy and fewer successful collisions; higher temperatures increase collisions until the optimum temperature, above which the enzyme denatures and the rate drops sharply.
pH: each enzyme has an optimum pH; extremes of pH alter hydrogen bonding and cause irreversible denaturation. For example, pepsin works best at pH 2, while some bacterial enzymes work at pH 9–10.
Substrate concentration: as substrate concentration increases, the rate increases until all active sites are saturated; beyond this point, adding more substrate has no effect unless enzyme concentration is increased.
Enzyme concentration: with substrate concentration constant, increasing enzyme concentration increases the rate until substrate becomes limiting.
The rate of reaction can be measured as the rate of product formation or substrate disappearance over time.
The rate can also be calculated as 1 ÷ time taken (s⁻¹) when quantitative data on amount is not available.

Effect of temperature on enzyme activity

Investigating Enzyme Activity: Skills

Catalase breaks down toxic hydrogen peroxide into water and oxygen; the volume of oxygen produced over time measures the rate of reaction.
When investigating temperature, the conical flask is held in a water bath with the water level higher than the H₂O₂ level to ensure even heating; swirling maintains an even temperature.
Amylase hydrolyses starch into maltose and glucose; its activity can be followed by testing samples with iodine in potassium iodide solution (blue-black with starch, yellow-brown without).
Colorimetry measures light absorbance or transmission; as starch is broken down, absorbance decreases (or transmission increases), allowing the rate to be measured.
A calibration graph of starch concentration against absorbance or transmission is plotted using serial dilutions of a stock starch solution.
When drawing graphs, the initial rate is found by drawing a tangent at the origin and calculating its gradient (change in volume ÷ change in time).

Investigating enzyme activity

Metabolic Pathways

A metabolic pathway is a series of interlinked reactions, each stage catalysed by a separate enzyme.
Linear (chain) pathways have a distinct beginning and end; glycolysis is an example.
Cyclical pathways involve the end product starting the next cycle; examples include the Calvin cycle and the Krebs cycle.
Enzymes can be intracellular (function inside the cell, e.g., glycolysis and Krebs cycle enzymes) or extracellular (secreted to function outside the cell, e.g., digestive enzymes).
End-product inhibition occurs when the end product of a pathway acts as a non-competitive inhibitor of an earlier enzyme, binding to an allosteric site and changing the active site shape.
End-product inhibition prevents build-up of intermediates and regulates the pathway; it is a reversible feedback loop.
Example: isoleucine inhibits threonine deaminase, the enzyme catalysing the first step of its own synthesis from threonine.

Enzyme Inhibition

Competitive inhibitors have a similar shape to the substrate and bind to the active site, blocking the substrate; their effect can be overcome by increasing substrate concentration.
Non-competitive inhibitors bind to an allosteric site, causing conformational changes that alter the active site; increasing substrate concentration cannot overcome this inhibition.
Statins are competitive inhibitors that block the enzyme synthesising cholesterol, lowering LDL levels and reducing risk of coronary heart disease.
Mechanism-based inhibition is irreversible: a substrate analogue forms covalent bonds with the active site, producing a stable inhibitor-enzyme complex.
Penicillin is an example: it inhibits DD-transpeptidase, preventing cross-link formation in bacterial cell walls; the weakened wall bursts by osmosis (death by lysis).
Penicillin is only effective against growing bacteria because autolysins (which create holes for wall expansion) are only active during growth.
Bacteria can develop resistance to penicillin through DNA mutations altering the active site of DD-transpeptidase; these mutations can be shared via plasmids during conjugation.

Diapositivas

Step through every slide for this topic — plus flashcards and revision notes — with a free account.

REGÍSTRATE — GRATIS INICIAR SESIÓN

Preguntas de práctica

Vista previa gratis — 8 de 63 preguntas. Regístrate para verlas todas.

1.Which of the options below refers to the features of catabolism?
Easy
- AExergonic, condensation reaction, an example is polypeptide synthesis
- BEndergonic, hydrolysis reaction, an example is respiration
- CEndergonic, condensation reaction, an example is photosynthesis
- DExergonic, hydrolysis reaction, an example is deamination
2.Which of the following statements apply to enzymes? (select all that apply)
Easy
- AThey speed up the rate of chemical reactions in the body
- BThey are fibrous proteins
- CThe active site is where the substrate binds
- DThe shape of the active site is complementary to the shape of the substrate molecule
3.The following diagram shows the three stages involved in enzyme catalysis. Which of the following provides the most accurate description of the events occurring at each stage?
Easy
- AThe substrate collides with the enzyme; New bonds are forming within the substrate molecule; Chemical substances are released from the enzyme
- BThe substrate collides with the active site of the enzyme; The substrate is bound to the active site of the enzyme; The products are released from the active site
- CThe substrate binds to the enzyme; New chemical substances are formed while attached to the enzyme; These substances are released from the enzyme
- DThe substrate collides with the active site of the enzyme; The substrate changes into different chemical substances; The products are released from the active site
4.An enzyme catalysed reaction is heated to a temperature of 65 °C. Which of the following best explains the effect this temperature increase would have on the enzymes?
Medium
- AThe enzyme and substrate molecules will gain kinetic energy and collide more frequently
- BThe bonds in the enzyme will vibrate more and break, which will cause a permanent change in the shape of the active site
- CThe bonds in the enzyme will vibrate more and break, causing a temporary change in the shape of the active site
- DThe active site of the enzyme will permanently change shape due to the increased speed of collisions between the substrate molecule and the active site
5.Lactase is an enzyme that is often immobilised and used in the food industry to produce lactose-free milk. Which of the following would not be an advantage of using lactase?
Medium
- AIncreases the sweetness of many dairy products, such as yoghurt and milk shakes
- BIt may increase the rate of crystallisation of frozen dairy products, such as ice cream
- CIt may increase the fermentation rate of products such as yoghurt and cheese
- DLactase is able to function closer to its optimum conditions in a controlled factory environment
6.Students investigated the effect of pH on catalase activity. Each experiment was repeated at a different pH value (pH = 2, 4, 7, 9, 11) and was set up as follows: Five potato cubes of similar dimensions were used as a source of catalase; this was added to 50 cm³ of hydrogen peroxide; the volume of oxygen released from this reaction was collected in a measuring cylinder; this was used to calculate the initial rate of the reaction in dm³ min⁻¹. Which of the rows in the following table correctly identifies the variables in this experiment?
Medium
- AIndependent variable: pH; Dependent variable: Initial reaction rate; Control variable: Volume of oxygen released
- BIndependent variable: Initial reaction rate; Dependent variable: pH; Control variable: Volume of hydrogen peroxide
- CIndependent variable: pH; Dependent variable: Initial reaction rate; Control variable: Volume of hydrogen peroxide
- DIndependent variable: Volume of oxygen released; Dependent variable: pH; Control variable: Initial reaction rate
7.Which of the following statements is not true about enzymes in metabolic pathways?
Easy
- AAll metabolic reactions are catalysed by an enzyme.
- BAll proteins are enzymes.
- CAll enzymes are proteins.
- DAll enzymes reduce activation energy of metabolic reactions in living organisms.
8.The following statements are about enzymes: 1. The speed that they function at can be reduced by competitive inhibitors. 2. Their primary structure is translated from mRNA. 3. They can be embedded in the plasma membrane of a cell. 4. They all have quaternary structures. Which of these statements are correct for all enzymes? (select all that apply)
Medium
- A1 and 3 only
- B1, 2 and 3 only
- C2, 3 and 4 only
- DAll of them are correct

Más preguntas

9.Which of the following sketch graphs best shows the effect of an enzyme on a biochemical reaction profile?
Medium
- AA graph showing a lower activation energy curve compared to the uncatalysed reaction
- BA graph showing a higher activation energy curve compared to the uncatalysed reaction
- CA graph showing no change in activation energy
- DA graph showing the enzyme increasing the energy of the products
10.Which one of the following phenomena is not a consequence of end-product inhibition in biochemical pathways?
Medium
- ARegulation of the pathway.
- BPrevention of a build-up of biochemical intermediate compounds.
- CShutdown of the pathway once product levels reach zero.
- DAllosteric changes to an enzyme involved in the pathway.
11.Enzymes alter the activation energy required for a biochemical reaction to take place. Which of the following gives the best definition of activation energy?
Medium
- AThe energy input required to break bonds in order for a reaction to take place.
- BThe energy is released as bonds breaks in a reaction.
- CThe energy required to allow substrates and enzymes to be closely aligned at the active site.
- DThe energy given off when an enzyme-substrate complex forms.
12.The four statements below are examples of the two types of metabolism, anabolism and catabolism. Which one of the statements represents a type of metabolism which is different to the other three options?
Medium
- ADeamination of polypeptides to form urea.
- BFormation of glycosidic bonds between glucose and fructose.
- CDepletion of fat stores during a period of starvation.
- DAnaerobic respiration.
13.The following statements apply to enzyme-catalysed reactions. Which row of the table best describes the sequence of events in an anabolic reaction?
Medium
- ASubstrate binds to active site; substrate is broken down; products released
- BSubstrates bind to active site; bonds form between substrates; product released
- CEnzyme is used up; substrate is converted to product; enzyme is recycled
- DSubstrate binds to allosteric site; active site changes shape; product released
14.Which statement below best describes the concept of enzyme-substrate specificity?
Medium
- AEnzymes and substrates fit together because they are the same type of biological molecule.
- BThe enzyme's precise 3-D shape and chemical properties are compatible with the substrate allowing formation of enzyme-substrate complexes.
- CAn enzyme can adapt its active site to fit a range of substrates to give maximum catalytic ability.
- DProjections on the enzyme's surface match projections on the substrate's surface.
15.When investigating the rate of reaction of the enzyme lipase on the hydrolysis of triglycerides, the pH must be maintained to prevent the lipase denaturing. Why might fluctuations in pH occur?
Medium
- AThe products of hydrolysis will decrease the pH.
- BLipase has a very low optimum pH.
- CThe products of hydrolysis will increase the pH.
- DThe water molecules produced as a byproduct of hydrolysis decreases pH.
16.The breakdown of hydrogen peroxide to water and oxygen is catalysed by the enzyme catalase. In an investigation into the effect of temperature on the rate of reaction, which of the following is the most appropriate control variable?
Medium
- AThe temperature of the reaction mixture
- BThe volume of oxygen produced
- CThe concentration of hydrogen peroxide
- DThe rate of reaction
17.Which of the following are true about the induced-fit hypothesis of enzyme action? (select all that apply)
Medium
- AThe active site is precisely complementary to the substrate before binding.
- BThe enzyme and active site can change shape slightly as the substrate binds.
- CConformational changes occur to achieve an ideal binding arrangement.
- DThe substrate fits exactly into the active site like a key in a lock.
18.Enzymes are globular proteins.
Easy
True or false?
19.The active site of an enzyme and its substrate have exactly the same shape.
Easy
True or false?
20.Match each term with its correct description.
Medium
- Anabolism
- Catabolism
- Metabolism
- The building of large molecules from smaller ones
- The breakdown of large molecules into smaller ones
- All the chemical reactions that take place within cells and organisms
21.Place the stages of an enzyme-catalysed reaction in the correct order.
Medium
- Substrate collides with the active site of the enzyme
- Enzyme-substrate complex forms
- Substrate is converted to product
- Products are released from the active site
22.Which of the following is the dependent variable in this investigation?
Medium
- AThe pH of the buffer solution
- BThe volume of oxygen produced
- CThe number of potato cubes
- DThe concentration of hydrogen peroxide
23.Which of the following is a control variable in this investigation?
Medium
- AThe pH value
- BThe initial rate of reaction
- CThe volume of hydrogen peroxide
- DThe volume of oxygen produced
24.Match each term relating to enzyme inhibition with its correct description.
Medium
- Competitive inhibitor
- Non-competitive inhibitor
- Allosteric site
- End-product inhibition
- Binds to the active site because it chemically resembles the substrate
- Binds at a site away from the active site and changes the shape of the active site
- A site on an enzyme, located away from the active site, to which effectors bind
- The end product of a pathway acts as an inhibitor of an earlier enzyme in the pathway
25.Place the following events of an enzyme-catalysed reaction in the correct sequence, starting with the first event.
Medium
- The substrate collides with the active site of the enzyme
- An enzyme-substrate complex forms temporarily
- The substrate is converted into product(s) while bound to the enzyme
- The products are released from the active site and the enzyme is reused
26.Which of the following statements about anabolism are correct? (select all that apply)
Medium
- AIt builds large molecules from smaller ones
- BIt is endergonic, requiring an input of energy
- CIt often involves condensation reactions
- DIt is exergonic, releasing free energy
- EIt typically involves hydrolysis reactions
27.Which of the following statements about non-competitive inhibition are correct? (select all that apply)
Medium
- AThe inhibitor binds to an allosteric site
- BThe inhibitor chemically resembles the substrate
- CBinding of the inhibitor changes the shape of the active site
- DIncreasing substrate concentration can fully overcome the inhibition
- EBinding of the inhibitor to the allosteric site is reversible
28.Which of the following are advantages of using immobilised enzymes in industry? (select all that apply)
Medium
- AThe enzyme can be reused multiple times
- BThe product is uncontaminated by enzyme, so no separation step is needed
- CImmobilised enzymes often have greater tolerance of temperature and pH changes
- DThe enzyme is permanently destroyed after a single use
- EThe substrate can be exposed to higher enzyme concentrations, increasing throughput
29.Which of the following statements about mechanism-based inhibition by penicillin are correct? (select all that apply)
Medium
- APenicillin has a similar structure to parts of the growing peptide chain of the bacterial cell wall
- BPenicillin forms a stable, irreversible enzyme-inhibitor complex with DD-transpeptidase
- CPenicillin inhibits the formation of cross-links between peptidoglycan molecules
- DPenicillin works by competitively inhibiting autolysins
- EPenicillin is effective against bacteria that are not growing
30.Enzymes are fibrous proteins.
Easy
True or false?
31.In the induced-fit hypothesis, the active site of the enzyme can change shape slightly as the substrate binds.
Easy
True or false?
32.Increasing the substrate concentration can fully overcome inhibition by a non-competitive inhibitor.
Easy
True or false?
33.Denaturation of an enzyme breaks the peptide bonds between its amino acids.
Easy
True or false?
34.A student measures the rate of an enzyme-catalysed reaction at a range of substrate concentrations while keeping enzyme concentration constant. The rate rises steeply at first and then levels off. Which of the following best explains why the rate stops increasing at high substrate concentration?
Medium
- AAll active sites of the enzyme molecules are occupied at any one moment
- BThe substrate molecules begin to denature the enzyme at high concentration
- CThe substrate has been used up so no further product can form
- DThe pH of the reaction mixture falls as more substrate is added
35.An enzyme-catalysed reaction produces 24 cm³ of gas in the first 8 seconds. A second identical experiment uses twice the enzyme concentration but the same substrate concentration. Which of the following is the most likely volume of gas produced in the first 8 seconds?
Hard
- A48 cm³
- B24 cm³
- C12 cm³
- D6 cm³
36.A colorimeter is used to follow the amylase-catalysed breakdown of starch. The starch-iodine mixture is blue-black at the start and becomes yellow-brown as starch is digested. Which of the following describes the expected colorimeter readings?
Hard
- APercentage transmission increases as the reaction proceeds
- BPercentage transmission decreases as the reaction proceeds
- CPercentage absorbance increases as the reaction proceeds
- DPercentage transmission stays constant throughout
37.A student investigates the effect of temperature on catalase activity using potato cubes and hydrogen peroxide, collecting the oxygen produced. Which of the following are valid control variables for this investigation? (select all that apply)
Hard
- AVolume of hydrogen peroxide used
- BpH of the hydrogen peroxide solution
- CTemperature of the water bath
- DVolume of oxygen collected
- ETime for which the reaction is measured
38.An enzyme is assayed with a competitive inhibitor present. Which of the following statements about this inhibited reaction are correct? (select all that apply)
Hard
- AThe inhibitor binds to the active site
- BThe inhibitor has a shape similar to the substrate
- CIncreasing substrate concentration can restore the maximum rate
- DThe maximum rate of reaction is permanently lowered
- EThe inhibitor binds to an allosteric site
39.In an enzyme-controlled reaction, increasing the substrate concentration can fully restore the reaction rate when a non-competitive inhibitor is present.
Medium
True or false?
40.A student investigating the effect of pH on catalase activity uses five potato cubes of similar dimensions added to 50 cm³ of hydrogen peroxide and measures the volume of oxygen released. Which row correctly identifies the variables in this experiment?
Medium
- AIndependent variable: pH; Dependent variable: initial reaction rate; Control variable: volume of hydrogen peroxide
- BIndependent variable: initial reaction rate; Dependent variable: pH; Control variable: volume of hydrogen peroxide
- CIndependent variable: pH; Dependent variable: volume of oxygen released; Control variable: initial reaction rate
- DIndependent variable: volume of oxygen released; Dependent variable: pH; Control variable: initial reaction rate
41.Complete the sentence about enzyme action.
Easy
The temporary complex formed when a substrate binds to an enzyme is called the enzyme-____ complex.
42.What is the name of the model that describes enzyme action where the active site changes shape slightly to fit the substrate?
Medium
43.Calculate the initial rate of reaction in cm³ s⁻¹ if 30 cm³ of oxygen is produced in 10 seconds.
Medium
44.If 45 cm³ of oxygen is produced in 15 seconds, what is the initial rate of reaction in cm³ s⁻¹?
Medium
45.Give the name of the model of enzyme action in which the active site is precisely complementary to the substrate and the substrate fits like a key in a lock.
Easy
46.Name the site on an enzyme, away from the active site, to which a non-competitive inhibitor binds.
Medium
47.In an investigation, 30 cm³ of oxygen is produced in 10 seconds. Calculate the initial rate of reaction in cm³ s⁻¹.
Medium
48.A reaction produces 45 cm³ of oxygen in 15 seconds. Calculate the rate of reaction in cm³ s⁻¹.
Medium
49.Complete the sentence about the effect of temperature on enzyme activity.
Easy
As temperature increases, enzyme and substrate molecules gain more kinetic energy, so they collide more ____, leading to more enzyme-substrate complexes forming.
50.Complete the sentence about the effect of substrate concentration on enzyme activity.
Medium
As substrate concentration increases, the rate of reaction increases until the active sites become ____, after which further increases in substrate concentration have no effect.
51.In a catalase investigation, 36 cm³ of oxygen is collected in 12 seconds. Calculate the initial rate of reaction in cm³ s⁻¹.
Medium
52.An amylase-starch reaction reaches its endpoint (no starch remaining) in 50 seconds. Using the equation rate = 1 ÷ time, calculate the rate of reaction in s⁻¹. Give your answer to 2 decimal places.
Hard
53.Explain how enzymes lower the activation energy of a reaction.
Hard
54.Describe how end-product inhibition regulates a metabolic pathway.
Hard
55.A sample of an enzyme was heated to 70 °C and then cooled back to its optimum temperature of 37 °C. Predict and explain the effect on the rate of the reaction it catalyses.
Hard
56.Explain why penicillin is only effective against bacteria that are still growing.
Hard
57.Describe the induced-fit hypothesis of enzyme action and explain how it differs from the lock-and-key model.
Hard
58.Calculate the initial rate of reaction in cm³ s⁻¹ if 30 cm³ of oxygen is produced in 10 seconds.
Medium
- A2 cm³ s⁻¹
- B5 cm³ s⁻¹
- C3 cm³ s⁻¹
- D4 cm³ s⁻¹
59.If 45 cm³ of oxygen is produced in 15 seconds, what is the initial rate of reaction in cm³ s⁻¹?
Medium
- A4 cm³ s⁻¹
- B2 cm³ s⁻¹
- C5 cm³ s⁻¹
- D3 cm³ s⁻¹
60.In an investigation, 30 cm³ of oxygen is produced in 10 seconds. Calculate the initial rate of reaction in cm³ s⁻¹.
Medium
- A3 cm³ s⁻¹
- B2 cm³ s⁻¹
- C4 cm³ s⁻¹
- D5 cm³ s⁻¹
61.A reaction produces 45 cm³ of oxygen in 15 seconds. Calculate the rate of reaction in cm³ s⁻¹.
Medium
- A5 cm³ s⁻¹
- B4 cm³ s⁻¹
- C3 cm³ s⁻¹
- D2 cm³ s⁻¹
62.In a catalase investigation, 36 cm³ of oxygen is collected in 12 seconds. Calculate the initial rate of reaction in cm³ s⁻¹.
Medium
- A4 cm³ s⁻¹
- B2 cm³ s⁻¹
- C3 cm³ s⁻¹
- D5 cm³ s⁻¹
63.An amylase-starch reaction reaches its endpoint (no starch remaining) in 50 seconds. Using the equation rate = 1 ÷ time, calculate the rate of reaction in s⁻¹. Give your answer to 2 decimal places.
Hard
- A0.02 s⁻¹
- B-0.98 s⁻¹
- C1.02 s⁻¹
- D2.02 s⁻¹

Unlock all 63 questions & more

Crea una cuenta gratis para ver todas las preguntas, las diapositivas, las tarjetas y los apuntes de repaso de este tema.

REGÍSTRATE — GRATIS INICIAR SESIÓN

Exámenes anteriores

La práctica con exámenes anteriores de este tema llegará pronto.

Próximamente