Biology 12

SKILL FOCUS

Initiating and planning

Predicting

Analyzing and interpreting

Communicating results

Investigation

DESIGN YOUR OWN

2 • B

46 MHR • Unit 1 Metabolic Processes

Problem

How effectively does the enzyme peroxidase work at

different temperatures and pH values?

Hypotheses

Make hypotheses about how you think temperature and

pH will affect the rate at which the enzyme peroxidase

breaks down hydrogen peroxide. Consider both low and

high temperatures and pH values.

CAUTION: Hydrochloric acid and sodium

hydroxide are corrosive chemicals. Avoid any

contact with skin, eyes, or clothes. Flush spills

on your skin immediately with copious amounts

of cool water and inform your teacher. Exercise

care when heating liquids and using a hot plate.

Take care when using sharp instruments. Do not

taste any substances in the laboratory. Wash

your hands before leaving the laboratory.

Materials

test tubes clock or timer

test tube markers ice

test tube brush thermometer or probe

test tube rack pH indicator paper or probe

beaker for hot water bath 0.1 mol/L hydrochloric acid

hot plate (HCl)

beaker tongs 0.1 mol/L sodium hydroxide

medicine droppers (NaOH)

distilled water 3% hydrogen peroxide

scalpel or sharp knife (H 2 O 2 )

ruler raw peeled potato

forceps

Experimental Plan

1.Examine the materials provided by your teacher. As

a group, list ways you might test your hypotheses.

2.Agree on a method(s) your group will use to test

your hypotheses.

Affecting Enzyme Activity

The compound hydrogen peroxide, H 2 O 2 , is a by-product of metabolic reactions

in most living organisms. However, hydrogen peroxide is damaging to delicate

molecules inside cells. As a result, nearly all organisms produce the enzyme

peroxidase, which breaks down H 2 O 2 as it is formed. Potatoes are one source of

peroxidase. Peroxidase speeds up the breakdown of hydrogen peroxide into water

and gaseous oxygen. This reaction can be detected by observing the oxygen

bubbles generated.

Each enzyme also works optimally (best) at a

specific pH. Figure 2.8 shows the activity ranges

for the enzymes pepsin and trypsin at different

pH levels. At pH values where the enzymes

work optimally, the enzymes have their normal

configurations. The bonds that hold peptides in

position in the enzyme are sensitive to hydrogen

ion concentrations. A change in pH can alter the

ionization of these peptides and disrupt normal

interactions. Under extreme conditions of pH, the

enzyme will eventually denature. Most enzymes

function best in the pH range of 6 to 8. Pepsin,

which digests proteins in the human stomach,

works best under very acidic conditions (pH of 2).

In the next investigation, you will design an

experiment to study the effects of temperature and

pH on enzyme activity.

Figure 2.8Rate of an enzymatic reaction as a function of

pH. The maximum rate of reaction for the enzyme pepsin,

which is found in the stomach, occurs at a pH of about 2.

Trypsin, an enzyme found in the small intestine, performs

best at a pH of about 8.

0 2 4 6 8 10 12

Rate of Reaction

pH

pepsin trypsin