Water & Chemical Elements (College Board AP® Biology): Exam Questions

Scientists investigated how different substances affect the cohesion of water molecules. They designed an experiment where a droplet of water was placed on a glass slide, and the contact angle (θ) of the droplet with the surface was measured. The higher the contact angle, the higher the cohesion of water molecules. Figure 1. shows two water droplets and their contact angles.

The scientists repeated the experiment after adding different substances to water. The data is recorded in Table 1.

Table 1: Contact angle of water droplets containing different additional substances.

Describe the effect of adding a substance on the contact angle of water droplets.

Calculate the percentage change in contact angle when oil is added to pure water.

(i) Identify the solute that disrupts water cohesion the most.

(ii) Explain how the differences in contact angle observed in Table 1 may relate to the chemical properties of the added substance.

A student proposes an additional experiment to test the effect of solute concentration on cohesion in water droplets.

(i) State a suitable hypothesis for this investigation.

(ii) Identify the independent and dependent variables.

(iii) State the control condition that would need to be used in this experiment.

(iv) Justify the inclusion of a control condition in this experiment.

Hydrogen bonding in water affects surface tension. Surface tension can affect biological processes, including water transport in plants and the ability of aquatic organisms to move across the surface of ponds and lakes.

Explain how the polarity of water leads to hydrogen bonding.

Scientists investigated the effect of temperature on surface tension in water, They measured the surface tension of water at different temperatures, as shown in Table 1.

Table 1: Surface tension of water at different temperatures

Construct an appropriately labeled graph that represents the data in Table 1.

Phosphorus is an essential element that allows living organisms to build other biological molecules required within cells.

(i) Predict how a decrease in phosphorus availability would affect ATP production.

(ii) Identify one cellular process that would be affected by the change described in (i).

Phosphorus is a structural component in lipids.

(i) Describe the role of phosphorus in lipid function.

(ii) State one location of the lipids described in (i) within a eukaryotic cell.

Water, and elements such as carbon, hydrogen, oxygen, nitrogen and phosphorus are essential for biological processes. In photosynthesis, water provides electrons, while phosphorus is required for ATP synthesis. Scientists studied how limited water and phosphorus availability affect photosynthesis by measuring oxygen production in aquatic plants. Their results are shown in Figure 1.

Describe how limiting water availability affects oxygen production.

Identify the independent and dependent variables in the study described in part a).

Predict how a long-term decrease in both water and phosphorus availability would affect an aquatic ecosystem.

Phosphorus is needed for production of ATP, which powers many metabolic processes.

State two other macromolecules that require phosphorus atoms.