Theme C: C1 Molecules - C1.1 Enzymes

Question 1

enzymes as catalysts

Answer 1

most reaction within a cell proceed too slowly on their own to sustain the life processes. However, in the presence of catalysts these reaction occur much faster, so that essential life functions can be maintained. in organisms, organic catalysts are known as enzymes.

they do not work in isolation, however; they interact with substrates and other molecules, and such interactions can control the rate of reactions.

Question 2

metabolism

Answer 2

includes all the chemical reactions that occur in an organism. these chemical reactions may be independent of one another, or they may interact with other reactions.

each chemical reaction is controlled by a specific enzyme. because of this specificity, there are many, many enzymes in each organism.

Question 3

what do all chemical reactions involve?

Answer 3

all chemical reactions involve reactants and products. reactants are substances that participate in a reaction, while products are the subtsances that are formed.

Question 4

anabolic reactions (process called anabolism)

Answer 4

reactions that use energy to build complex organic molecules from simpler organic molecules are said to be anabolic.

e.g. photosynthesis, protein synthesis, and glycogen formation

Question 5

catabolic reactions (process is known as catabolism)

Answer 5

reactions that break down complez organic molecules with the realease of energy.

e.g. digestion and the oxidation of substrates in cell respiration.

Question 6

compare anabolic and catabolic reactions

Answer 6

anabolic reactions build macromolecules (and realease water) from monomers by condensation reactions, while catabolic reactions break down macromolecules into monomers by hydrolysis (the splitting of molecules by adding water)

anabolic reaction require energy input to occur, while catabolic reactions release energy as they occur.

Question 7

what two general metabolic reactions (from b1.1) should you keep in mind for this chapter?

Answer 7

condensation and hydrolysis reactions

Question 8

in general, what is energy?

Answer 8

all organisms maintain their strcuture and function through chemical energy. in general, energy is the capacity to cause change, to do work.

Question 9

forms of energy that are particularly important to organisms - and briefly what they are

Answer 9

1. kinetic energy: energy of motion, including movement of molecules within objects
2. potential energy: stored energy or energy in a form that is not being used at a point of time
3. chemical energy: a form of potential energy that is available for release when a chemical reaction occurs
4. thermal energy: a form of kinetic energy stored within objects. capable of being transferred from one object to another as heat.

Question 10

how to measure energy and the unit of heat

Answer 10

various different forms of energy can require different methods of measurement, but one that is often used is heat. the unit to measure heat in biology is the kilocalorie (kcal).

a kilocalorie is 1000 calories (cal). a calorie is the amount of heat necesarry to raise the temperature of 1 gram of water by 1 degree celcius (C).

joule is also a unit of energy, one joule = 0.239 cal.

Question 11

what is the energy currency of the cell

Answer 11

Adenosine triphosphate (ATP)

Question 12

functions of ATP in an organism

Answer 12

• supplying the energy needed to sunthesise large molecules called macromolecules
• supplying the neergy necessary for mechanical work, such as muscle action, chromosome movement and cilia or flagellum motion
• providing energy to move substances across the cell membrane, such as sodium-potassium pump

Question 13

chemical structure of ATP

Answer 13

adenine and ribose combine to form the molecule adenosine. ATP has three phosphate groups attacthed to adenosine, while ADP only has two. note the locations of high-energy bonds represented by wavy lines. the high-energy bonds, espeically the one located between the second and third phosphate in ATP, are the source of energy for chemical reactions within a cell.

Question 14

what are enzymes?

Answer 14

almost all enzymes are proteins

Question 15

protein enzymes and their general structure

Answer 15

protein enzymes are long chains of amino acids that have taken on a very specific three-dimensional shape. think of a flexible metal wire that can be bent many times into what is called a globular shape. this hsape is complex and a first glance appears to be random, but in enzymes (an other globular proteins) the complex shape is not random: it is very specific.

Question 16

active site

Answer 16

somewhere in the 3D enzyme shape is an area that matches the shape of that enzyme’s substrate. this area is called the active site. the shape of the active site closely matches the shape of one particular substrate. it is important to note that the active site is composed of only a few amino acids. it is the interaction between the amino acids in the overall 3D enzyme chape that provides the active site with the properties necessary to carry out catalysis.

Question 17

lock-and-key model

Answer 17

in the 1890s Emil Fischer proposed the lock-and-key model for enzyme action. in this model, the lock represents the enzyme’s active site, and the key represents the substrate. because the 3D shape of the internal portion of the lock is complex and specific, only one key will fit it. at the time this model provided a good explanation of the specificity of enzyme action. however, as knowledge about enzyme action has increased, Fischer’s model has been modified into what is now as the induced-fit-model of enzyme atcion.

Question 18

induced fit

what happens to the enzyme when subtrates combine with their active site?

Answer 18

many enzymes undergo significant changes in their conformation (shape) when substrates combine with their active site. a good way to visualise this model of enzyme action is to think of a hand and glove, the hand being the substrate and the glove being the enzymes, the glove looks a bit like the hand. however when the hand is placed in the glove, there is an interaction that results in chape chnages of both the hand and the glove, the providing and induced fit.

the changes in the shape of the subtrate casues stresses pon its chemical bonds. the bonds become destabilised, which favours reactions and increase reaction rates.

Question 19

activation energy

Answer 19

it is not enough for an enzyme’s substrate(s) to enter an active site. the substrate(s) must enter with a minimum rate of motion, kinetic energy, that will provide the energy necessary for the reaction to occur. enzymes do not provide this energy; they simply lower the energy minimum that is required. the energy being referred to is called the activiation energy of the reaction. thus, enzymes lower the activiation energy of reactions. enzymes are not considered to be reactants and are not used up in the reaction. an enzyme can function as a catalyst many, many times.

activation energy is the energy necessary to destabilise the existing bonds in a substrate so that a reaction can proceed. reactions that require large amounts of activiation energy tend to proceed more slowly than those requiring smaller amounts. this is because there are fewer molecules collsiding with sufficient energy to overcome the initial energy requirement.

Question 20

what are the 2 ways of overcoming the energy barrier and increasing the rate of chemical reactions?

Answer 20

1. increasing the energy of the reacting molecules and thus increasing the rate of collisions, usuall by the addition of heat
2. lowering activation energy that is required to stress particular chemical bonds in the reactants so that the bonds can be broken more easily

because living systems are vulnerable to higher temperatures, most chemical reaction raes in organisms are increased by the action of enzymes.

in addition, an enzyme cannot force a reaction to occur that would not otherwise happen without the enzyme. however, the reaction will be much more likely to occur with an enzyme present because the input of energy (activation energy) required will be lower.

Question 21

graph

how do enzymes effect exothermic reactions?

Answer 21

enzymes accelerate exothermic reactions by lowering the activation energy required. the activiation energy is needed to destabilise the chemical bonds in the reactant. the upper curve shows the activation energy when no enzyme is involved. the lwoer curve shows the activation energy requried when an enzyme is prsent to catalyse the reaction.

Question 22

until when do reactions proceed?

Answer 22

reactions proceed until there is equilibrium between the relative amounts of reactants and products. it is important to note that, even though enzymes lower the activation energy of a particular reaction, they do not later the proportion of reactants to products at equilibrium. some reversible chemical reactions require a different enzyme to lower the activation enegry in the reverse direction.

Question 23

what do catalysts do

Answer 23

Catalysts lower the activation energy needed for a reaction to proceed.

Chemical reactions are reversible, which means they can occur in both directions. By reducing activation energy, catalysts increase the rate of a chemical reaction in both the forward and reverse directions.

You can think of a catalyst as lowering the energy barrier that is preventing or hindering a reaction from occurring.
In living systems enzymes act as catalysts.

Question 24

energy of exergonic and exthermic reactions

Answer 24

Exergonic and exothermic reactions release energy when they occur.

The products of an exergonic reaction have less energy than the reactants had because of this released energy.

Endergonic and endothermic reactions result in products that have a higher energy level than the reactants

Question 25

what type of reactions are exergonic and endergonic reactions?

Answer 25

Exergonic and endergonic reactions are **bioenergetic reactions**. Molecules are rearranged and energy can be used to do work. Exothermic and endothermic reactions are primarily **thermodynamic reactions**. The energy is given out or taken in in the form of heat. In chemistry it is more usual to have exothermic and endothermic reactions. In biology. exergonic and endergonic are more common.

Question 26

role of molecular motion in substrate-active site collisions in enzyme catalysis

Answer 26

Active sites and movement are both important to enzyme action and control of chemical reactions. The active site must join with the substrate based on shape. However, movement is also important. In order for substrates to react, they need to collide and they need to find the enzyme's active site. Molecules therefore need enough energy to move and collide. Often, the substrate or enzyme is anchored or immobilized in a membrane, allowing a more efficient joining of substrate and active site. The processes of cellular respiration and photosynthesis utilize enzymes embedded in membranes to carry out essential reactions efficiently. The evolution of life has progressed largely as the result of the development of more efficient chemical reactions.

Question 27

mechanism of enzyme action

Answer 27

* The surface of the substrate makes contact with the active site of the enzyme. * The enzyme and substrate change shape to provide a fit. * A temporary complex called the enzyme-substrate complex forms. * The activation energy is lowered, and the substrate is altered by the rearrangement of the existing atoms. * The transformed substrate, the product, is released from the active site. * The unchanged enzyme is then free to combine with other substrate molecules.