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.

Hydrogen bonding and the unique properties of water are critical for biological processes, especially in extreme environments. Scientists investigated microbial life in Lake Vostok, a subglacial lake buried under 4 km of Antarctic ice. The microbes in this ecosystem endure high pressure, low temperatures (~−3°C), and limited access to essential elements like carbon, nitrogen, and phosphorus.

The researchers examined the freezing point of water under different environmental conditions. Table 1 shows the freezing point of water in the presence of various solutes found in Lake Vostok.

Table 1: Freezing point of water with different solutes

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

(ii) Calculate the mean freezing point for each solute.

Based on the data in Table 1,

(i) Identify the solute where most hydrogen bonding was disrupted.

(ii) Describe the effect polar and non-polar solutes have on freezing point of water.

Based on Table 1, identify a dependent variable in the scientists’ experiment. NEEDS MORE TO IT

c

Hydrogen water (HW) is water with added molecular hydrogen gas (H₂). Scientists investigated the effects of HW consumption on oxidative stress and immune functions in healthy adults. Healthy adults (20–59 years) consumed either 1.5 L/d of HW or plain water (PW) for 4 weeks. Changes from baseline to the fourth week in biological antioxidant potential (BAP) were measured. BAP is a measure of the total antioxidant properties in the blood and is used to study neurodegenerative diseases, such as Alzheimer's disease and dementia. The scientists' results are shown in figure 1.

Based on the data in Figure 1, describe the effect of hydrogen water on blood antioxidant potential in adults.

Differences between baseline and week 4 within each group were determined with the use of a statistical test. P values were obtained and P < 0.05 was considered statistically significant. In adults under 30 years, P = 0.534; whilst in adults 30 years or over P = 0.028.

(i) Based on the data, determine which age group hydrogen water had a beneficial effect on.

(ii) Provide a reasoning to justify your answer.

The scientists hypothesizes that hydrogen-rich water reduces inflammatory responses in healthy adults. Use all the data to evaluate their hypothesis.

In a follow up study the scientists investigated the effect of hydrogen water (HW) on programmed cell death (apoptosis) of blood cells. This was compared with apoptosis of blood cells in adults who drank only plain water (PW). Both groups drank the waters for four weeks and apoptosis rates where measured weekly. The results are shown in Table 1.

Table 1. Apoptosis in blood cells in adults consuming HW and PW

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

(ii) Based on the data, determine whether apoptosis differs in adults who have consumed HW and PW.