Question

Photosynthesis and Cellular Respiration in a Closed Ecosystem

Photosynthesis is the process in which plants use the Sun’s energy to create their own food. During photosynthetic reactions, carbon dioxide and water are converted into glucose and oxygen. The chemical equation for photosynthesis is shown.

6CO₂ + 6H₂O + Light Energy → C₆H₁₂O₆ + 6O₂

Cellular respiration is a process in which plants and animals gain the energy required for cellular activity. In cellular respiration, plants use the glucose and oxygen that is produced during photosynthesis and release carbon dioxide and water as waste products. While animals also perform cellular respiration, they must take in glucose from food sources and oxygen from the air they breathe. Similar to plants, animals release carbon dioxide and water as waste products of cellular respiration. The chemical equation for cellular respiration is shown.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP

When plants undergo photosynthesis at a greater rate than cellular respiration, they produce glucose and oxygen in excess of the amount needed for cellular respiration. This is known as net positive photosynthesis. When net positive photosynthesis occurs, plants store the glucose produced during photosynthesis and release the oxygen gas as a waste product.

To model the process of photosynthesis and cellular respiration, a small ecosystem is created. The ecosystem is closed within a bottle and contains water, snails, elodea, and air. In this ecosystem, the snails eat the elodea for food. The ecosystem is located close to a light source to ensure that the elodea experiences net positive photosynthesis.

A second ecosystem is created in a bottle that contains water, elodea, and air. Which ecosystem allows the elodea to continue the process of photosynthesis over a longer period of time?

A.
The first ecosystem, because the snails take in carbon dioxide and release oxygen.

B.
The first ecosystem, because the snails take in oxygen and release carbon dioxide.

C.
The second ecosystem, because the elodea will no longer need to compete with the snails for oxygen.

D.
The second ecosystem, because the elodea will no longer need to compete with the snails for carbon dioxide.

Answer 1

B.

Snails respire (C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP), so they consume O2 and release CO2. In a closed bottle the snails’ CO2 replenishes the carbon dioxide the elodea needs for photosynthesis, letting photosynthesis continue longer.