The following pathways describe the movement of carbon within this ecosystem:

• Pathway 1: Atmospheric carbon dioxide (${CO}_2$) is fixed into sugars and starches in green plants.
• Pathway 2: Carbon in organic compounds within plants is released back into the atmosphere as carbon dioxide during energy release.
• Pathway 3: When fossilized organic remains are burned, carbon dioxide is released into the atmosphere.
• Pathway 4: Carbon compounds in plants are transferred to animals.

Match each Pathway 1–4 to its correct process from the list below:

• Combustion
• Feeding (Eating)
• Photosynthesis
• Respiration

The student first sets up the terrarium using sterilized soil that contains minerals but no living microorganisms.

Over time, plant growth ceases because the minerals in the soil are depleted. Explain why the minerals are not replenished in this setup.

The student then introduces beneficial microbes to the soil. Although the terrarium remains completely sealed, both the plants and the soil microbes are able to survive because they supply each other with the gases they need. Explain how this gas exchange occurs.