Ecosystems — Answer Key
Part A: Fill in the Blank
Write the missing word or number on each line.
1. During photosynthesis, plants take in carbon dioxide from the air and convert it into glucose.
Photosynthesis is the process where plants use sunlight, water, and carbon dioxide to produce glucose (sugar) for energy and release oxygen as a byproduct.
2. When animals breathe, they release carbon dioxide back into the atmosphere.
During respiration, animals break down glucose for energy and release carbon dioxide gas, which returns to the atmosphere and gets used by plants again.
3. Burning fossil fuels adds extra carbon to the atmosphere, which contributes to climate change.
Fossil fuels contain carbon from ancient organisms, and burning them releases that stored carbon into the atmosphere as CO2, trapping heat and warming the planet.
4. Nitrogen makes up about 78 percent of Earth's atmosphere.
Nitrogen is by far the most abundant gas in our atmosphere at 78 percent, compared to oxygen at about 21 percent and all other gases combined at just 1 percent.
5. Special bacteria in the soil convert nitrogen gas into forms that plants can absorb through their roots.
Plants cannot use nitrogen gas directly from the air, so they rely on special soil bacteria to convert it into usable compounds like nitrates that roots can absorb.
6. Plants need nitrogen to build proteins, which are essential for growth and repair.
Nitrogen is a key building block of proteins, which plants need to grow new cells, repair tissue, and carry out essential life functions.
7. When organisms die, decomposers break down their bodies and return nitrogen to the soil.
Decomposers break down the proteins in dead organisms, releasing nitrogen compounds back into the soil where new plants can absorb them and start the cycle again.
8. The process of breaking down glucose for energy and releasing CO2 is called respiration.
Respiration is essentially the reverse of photosynthesis: cells break apart glucose molecules to release stored energy, producing carbon dioxide and water as waste.
9. Unlike energy, matter is recycled and reused over and over in an ecosystem.
Energy flows in one direction and is eventually lost as heat, but matter like carbon, nitrogen, and water cycles through ecosystems and gets reused endlessly.
Part B: Matching
Match each item on the left to the correct answer on the right.
1. Match each item to its correct answer.
Photosynthesis
→ Plants absorb carbon dioxide and produce oxygen and glucose
Bacteria convert nitrogen gas into usable forms for plants
Respiration
→ Organisms break down glucose and release carbon dioxide
Organisms break down glucose and release carbon dioxide
Decomposition
→ Fungi and bacteria break down dead matter and return nutrients to soil
Fungi and bacteria break down dead matter and return nutrients to soil
Nitrogen fixation
→ Bacteria convert nitrogen gas into usable forms for plants
Plants absorb carbon dioxide and produce oxygen and glucose
Correct matches: Photosynthesis → Plants absorb carbon dioxide and produce oxygen and glucose; Respiration → Organisms break down glucose and release carbon dioxide; Decomposition → Fungi and bacteria break down dead matter and return nutrients to soil; Nitrogen fixation → Bacteria convert nitrogen gas into usable forms for plants.