Biology 12

SKILL FOCUS

Predicting

Performing and recording

Analyzing and interpreting

Communicating results

Investigation 1•A

22 MHR • Unit 1 Metabolic Processes

Pre-lab Questions

What factors might influence the pH of cells?

Why might cells need to maintain a constant pH

environment?

How do cells regulate pH?

Problem

How do acids and bases affect living cells?

Prediction

Make a prediction about how cells maintain a

constant pH.

CAUTION: Acids and bases are corrosive and

caustic substances. Avoid any contact with skin,

eyes, or clothes. If contact does occur, rinse

thoroughly with water and inform your teacher.

Clean up any spill immediately. Dispose of any

materials as instructed by your teacher. Wash

your hands before leaving the laboratory.

Materials

universal indicator paper or pH meter

forceps

pH scale

0.1 mol/L HCl

0.1 mol/L NaOH

commercial buffer solution, pH 7

10% homogenized potato solution

50 mL beaker

distilled water in squirt bottle

medicine dropper

tap water

graphing paper

Procedure

1.Work in a small group. Read steps 2 to 7, then

design a data table to record your results.

2.Add 20 mL of tap water to a clean beaker, and

measure the pH of the water. If you are using

universal indicator paper, use only a small piece.

Immerse the paper into the water using the forceps.

Compare the colour of the paper against the pH

chart and determine the pH.

3.To the 20 mL of tap water, add one drop of the

HCl solution. Gently swirl the contents, and then

measure the pH. Continue adding drops of the HCl

solution to the beaker, recording the pH after each

drop, until a total of five drops has been added.

Gently swirl the contents of the beaker after each

drop is added.

Acids, Bases, and Cellular pH

The pH value indicates whether a substance is acidic or basic. Acids have a pH

value less than 7 and bases have a pH value greater than 7. Most cells function

at around pH 7, which is considered to be a neutral environment. To maintain

this neutral environment, cells must control, or buffer, the pH level so it does not

become too acidic or too basic. In this investigation you will determine the effects

of adding an acid and a base to several solutions. You will use a commercially

prepared buffer solution and a solution made from living cells.

An acid is any substance that donates H+ions

when it dissolves or dissociates in water. Therefore,

acids increase the concentration of H+ions in water

solutions. Bases, on the other hand, decrease the

concentration of H+ions in solution. Usually this

occurs because bases attract H+ions, thus reducing

their concentration. As a result, the concentration

of OH−ions increases when bases dissolve or

dissociate in water. The pH scale, shown in

Figure 1.15, is a means for ranking substances

according to the relative concentrations of their

hydrogen and hydroxide ions. Water, with equal

concentrations of these ions, is considered neutral

and has a pH of 7. Substances with a pH that is

lower than 7 have higher concentrations of H+ions

(and lower concentrations of OH−ions), so they are

acids. Substances with a pH that is higher than 7

have lower concentrations of H+ions (and higher

concentrations of OH−ions), so they are bases.

When acids and bases react, they produce two

products: water and a salt (an ionic compound). This