ap09_biology_form_b

AP

®

BIOLOGY

2009 SCORING GUIDELINES (Form B)

Visit the College Board on the Web: www.collegeboard.com.

Question 3

Water is essential to all living things.

(a) Discuss THREE properties of water.

(b) Explain each of the following in terms of the properties of water. You are not limited to the three

properties discussed in part (a):

• the role of water as a medium for the metabolic processes of cells

• the ability of water to moderate temperature within living organisms and in organisms’

environments

• the movement of water from the roots to the leaves of plants

(a) Discuss THREE properties of water (6 points maximum):

Name of property and correct description (2 points). Points MUST provide both property and

description.

Property and Description (2 points jointly)

Polarity of water Polar covalent bonds created by unequal sharing of electrons

between O and H within the molecule

Specific heat/high heat

capacity

Heat absorption without temperature change

High heat of vaporization Water molecules absorb energy as it changes state/breaking of

bonds by absorbing energy

Adhesion Attraction to other molecules that are polar or have charge

Cohesion Attraction to other water molecules due to polar nature of water/

surface tension

Three states of matter Ice–liquid–gas (vapor)

Kinetic energy differences

Expands at 4°C to become less dense

Repels hydrophobic material Moves aside nonpolar substances

(b) Explain each of the following in terms of water properties (6 points maximum; 2 points for each

part). To earn 10 points, students must get at least 1 application point for each area.

Water’s role as a medium for the metabolic processes of cells (2 points maximum):

• Diffusion—allows for movement of materials through an aqueous solution down the concentration

gradient

• Osmosis—movement of water across membranes due to water potential differences (down the

gradient)

• Solvent—dissociation/ionization of materials

• Buffer—explanation of role water plays in formation of bicarbonate ion

AP

®

BIOLOGY

2009 SCORING GUIDELINES (Form B)

Visit the College Board on the Web: www.collegeboard.com.

Question 3 (continued)

Water’s ability to moderate temperature within living organisms/environments (2 points

maximum):

• Specific heat—moderates climates, maintains stable temperature in cells, constant internal

environment

• High heat of vaporization—perspiration cooling, evaporative cooling

• Ice forming and acting as insulator for lakes, keeping water in liquid state

Water from the roots to the leaves of plants (2 points maximum):

• Transpiration—moving water away from leaves due to water potential differences/evaporation

through stomata

• Capillary action of water due to adhesion and cohesion

• Root pressure—driven by osmosis/movement of water into roots

• Negative pressure potential—caused by surface tension of water as it is pulled up xylem

Visit the College Board on the Web: www.collegeboard.com.

AP

®

BIOLOGY

2009 SCORING COMMENTARY (Form B)

Visit the College Board on the Web: www.collegeboard.com.

Question 3

Sample: 3A

Score: 10

For the most part this response follows the structure of the question. In part (a) the response demonstrates

how discussion of a property of water must include a description of that property to earn 2 points. The first

property the response describes is the polar nature of water, earning 2 points. The response also earned

1 point for describing a use of water as a medium when it includes a solvent, which comes from part (b) but

was awarded the point here. This happened often in students’ responses, as students would mix in

statements from part (b) during a description of a property in part (a). The response describes the properties

of specific heat (2 points) and cohesion (2 points). The property of adhesion was also described, but the

response had already earned the maximum 6 points in part (a) so this property was not counted in the score.

The response addresses part (b) by explaining each of the three bullets. One point was earned for describing

the ability of the high heat of vaporizations to regulate temperature. Two points were earned for addressing

the role of the properties of water in the roots and leaves of plants. One point was earned for explaining how

root pressure is acquired from diffusion in the root, and 1 point was earned for explaining that capillary action

assists water as it rises in the plant by adhesion and cohesion.

Sample: 3B

Score: 8

The response earned all 6 available points for the discussion of three properties of water and 2 points for the

explanation of how those properties play out in the biological world.

The response earned its first 2 property points by identifying and describing the polarity of water. The next

2 property points were earned for the identification and description of cohesion of water. Ice is given as the

means to regulate temperature in an environment, which relates to part (b) of the question and earned 1 point

for that part. The final 2 property points were earned for the description and discussion of specific heat.

The response earned a final point for the discussion of the ability of water to moderate temperature: water

creates an environment for stabilizing temperature in relationship to its specific heat.

Sample: 3C

Score: 3

The response is clearly based on the question. However, although it contains the term “polarity” and some

description of it, the response is vague in terms of where the attraction takes place (between or within the

water molecule). The diagrams provided to left of the description appear accurate, but without the use of

labels they do not clarify the description.

The response earned 1 point for providing a role of water in temperature regulation when it describes how

water is used to cool the body. One point was earned for explaining that water stabilizes temperature

through its ability to hold energy or heat. The response earned a final point for the description of root

pressure as water moving from higher to lower in the root.