SBI4U Test 2

Question 1

Distinguish between anabolism and catabolism.

Answer 1

Anabolism is when simple substances are built up into complex substances (storing energy). Catabolism means that complex substances are being broken down into simpler substances (releasing energy).

Question 2

State the 2 Laws of Thermodynamics.

Answer 2

1. The Law of Conservation of Energy - enerygy can neither be created nor destroyed, but can be transformed from one form to another.
2. Second Law of Thermodynamics - Energy cannot be transformed from one form to another without a loss of useful energy. *Entropy increases, never decreases*

Question 3

What is entropy?

Answer 3

A measure of usless, unorganized energy in the universe.

Question 4

How do some chemical reactions appear to defy the Laws of Thermodynamics? Why is this not true?

Answer 4

In some reactions, two low energy reactants produce a high energy product, e.g. in photosynthesis, carbon dioxide and water produce glucose. This is actually not contrary to the Laws of Thermodynamics because energy is needed to drive this reaction (sunlight energy).

Question 5

How do enzymes reduce the activation energy required?

Answer 5

1. R-groups in the active sites stress bonds of substrate, weakening them
2. Reactive R-groups found on the bond sites help to transfer hydrogen ions to and from the substrate.
3. A temporary attraction between the enzyme and the substrate is formed, making the substrate unstable so that it reacts easier. (attraction forms when electrons are transferred)
4. Oxidative enzymes catalyze the transfer of electrons from substrates to oxygen molecules.

Question 6

Explain how an enzyme works. Your explanation should include, but not be limited to, the following terms: induced fit, catalytic cycle, activation energy, substrate, active site.

Answer 6

An enzyme works in a catalytic cycle where a substrate first attaches to an enzyme on one of its active sites. The active site will change in shape to accomodate the substrate, creating an induced fit. Some bonds form between the substrate and the enzyme are bonded together with hydrogen forces or weak ionic bonds, forming an enzyme-substrate complex. The enzyme will then work to reduce the activation energy needed for the substrate to react (the method by which this is done can vary, ex. R-groups in the active sites of the enzyme put stress on the bonds in the substrate molecule, weakening the bonds) and speed up the reaction. After the reaction is complete, the enzyme releases the products from the enzyme-substrate complex.

Question 7

Define induced fit.

Answer 7

An induced fit is the change in the shape of an active sight on an enzyme to accomodate a certain substrate.

Question 8

Define catalytic cycle.

Answer 8

The process in which substrates form an enzyme-substrate complex and are then released from the complex as a product or products, freeing the enzyme for further reactions. (while the substrate is on the enzyme, the enzyme speeds up the reaction by reducing the activation energy required.)

Question 9

Define activation energy

Answer 9

Input of energy required to start a chemical reaction.

Question 10

Define substrate

Answer 10

A reactant in any given enzymatic reaction

Question 11

Define active site

Answer 11

Depressions on the surface of an enzyme where substrates fit and where catalysis occurs.

Question 12

Define catalysis

Answer 12

The acceleration of a chemical reaction by some substance which itself undergoes no chemical change.

Question 13

Define oxidative enzymes

Answer 13

The enzymes which catalyze the transfer of electrons form substrates to oxygen molecules in a oxidation reduction reaction.

Question 14

Define enzyme-substrate complex.

Answer 14

The combination of the substrate induced-fitted into the active site of an enzyme and the enzyme itself.

Question 15

Explain how temperature affects enzyme activity.

Answer 15

As temperature increases, so do kinetic energy levels, making molecules speed up. This increases chance of collision and the amount of energy in the collisions.

When the temperature is too low for enzyme activity, enzymes are less flexible and therefore less able to provide an induced fit for substrates.

When the temperature is too high, bonds in the enzyme are weakened and cannot hold the peptide chains together in the proper orientation. Enzyme loses its shape and is denatured because bonds are permanently altered.

Question 16

How does pH affect enzyme activity?

Answer 16

When an enzyme is in its optimal pH environment, it is in its normal configuration.

pH can disrupt the electrical forces of attraction between oppositely charged amino acids in the enzyme. The bonds that hold the peptides of the enzyme together in their correct positions are sensitive to hydrogen ion concentration.

Enzymes can denature under extreme pH conditions.

Question 17

Explain competitive inhibition and give examples of how it works in different cases.

Answer 17

In competitive inhibition, the inhibitor mimics the normal substrate and binds to the active site. The inhibitor competes with the normal substrate for access to the active site.

Ex. Penicillin is a competitive inhibitor. It binds to the active site of transpeptidase, the enzyme involved in bacterial cell wall construction.

Ex. DDT - inhibits enzymes of the nervous system.

Competitive inhibition can be permanent if it forms strong covalent bonds with the enzyme, e.g. hydrogen cyanide binds permanently to cytochrome c oxidase (respiratory enzyme.

Question 18

Explain non-competitive inhibtion.

Answer 18

The inhibitor bonds to the allosteric site on the enzyme and alters the enzyme’s shape. This prevents the substrate from binding to the active site.

Question 19

Explain how feedback inhibition works.

Answer 19

Feedback inhibition is where a substance attaches to an enzyme. Then after the reaction has occured in the pathway, the end product of the enzyme pathway will bind to an allosteric site on one of the enzymes (probably the first enzyme?) in the pathway. The pathway can now no longer catalyse reactions because it is switched off.

Question 20

Distinguish between cofactors and coenzymes.

Answer 20

Cofactors are inorganic substances that contain mineral ions like F2+ or K+. They are located on the active sites of enzymes, and because they are cations they attract electrons from the substrate.

Coenzymes are organic substances containing vitamins like riboflavin (a B vitamin). Deficiency in any of these vitamins can effect enzymatic reactions.

Question 21

Define energy

Answer 21

The ability to do work.

Question 22

Define kinetic energy.

Answer 22

Energy associated with movement

Question 23

Define potential energy

Answer 23

energy stored by matter as a result of its position or arrangement in space

Question 24

Define chemical energy

Answer 24

potential energy stored in the bonds of a molecule

Question 25

Define thermodynamics

Answer 25

The study of how energy continually flows through living and non-living systems

Question 26

What 3 things hold the shape of an enzyme?

Answer 26

1. hydrogen forces
2. disulphide bridges
3. hydrophobic/hydrophilic interactions

Question 27

What 3 factors affect enzyme activity?

Answer 27

Temperature, pH and Chemicals (like inhibitors and activators)

Question 28

What are activators?

Answer 28

Activators speed up a chemical reaction. They are non-protein molecules which help enzymes catalyze a reaction and help to draw off electrons from a substrate. They may attach to the enzyme or the substrate.