Worksheet Solutions: Our Home: Earth, a Unique Life Sustaining Planet | Worksheets with Solutions for Class 8 PDF Download

Multiple Choice Questions (MCQs)

Instruction: Select the correct option for each question.

Q1. If Earth had never known life, which change in its atmosphere would most likely be true?
a) High oxygen from photosynthesis
b) Thick ozone layer formed naturally without oxygen
c) Very low oxygen and no biologically produced ozone
d) Same composition as today
Ans: c) Very low oxygen and no biologically produced ozone
Without life (photosynthesis), atmospheric O₂ and the ozone layer would not build up.

Q2. Which surface feature would most likely be less abundant without life-driven soil formation?
a) Thick, organic-rich soils
b) Basaltic rocks
c) Impact craters
d) Volcanoes
Ans: a) Thick, organic-rich soils
Soils on Earth are greatly enriched and structured by organisms; without life, organic-rich soils would be scarce.

Q3. In a lifeless Earth, which gas would most likely be higher due to unbalanced volcanic emissions?
a) Oxygen (O₂)
b) Carbon dioxide (CO₂)
c) Ozone (O₃)
d) Methane (CH₄) from microbes
Ans: b) Carbon dioxide (CO₂)
Without biological carbon sinks, volcanic CO₂ would accumulate more.

Q4. Without plants and microbes, the long-term carbon cycle would be mainly controlled by:
a) Photosynthesis and respiration
b) Weathering, volcanism, and ocean chemistry
c) Pollination and seed dispersal
d) Decomposition by fungi
Ans: b) Weathering, volcanism, and ocean chemistry
Abiotic processes would dominate cycles in a lifeless world.

Q5. Which color would Earth most likely appear from space without life?
a) Deep blue with green land patches
b) Mostly brown/grey rocks with blue oceans and white clouds
c) Entirely green
d) Mostly white
Ans: b) Mostly brown/grey rocks with blue oceans and white clouds
No vegetation means no green “biosignature” on land.

Fill in the Blanks

Instruction: Fill in the blanks with the correct word based on the chapter.

Q1. Land surfaces would show more visible impact __________ without rapid biological weathering and soil cover.
Ans: craters
Less erosion by roots/biota means more preserved craters.

Q2. Without forests and phytoplankton, Earth’s surface would lack the green __________ of life.
Ans: biosignature
Vegetation color is a life indicator.

Q3. Climate would be set mainly by orbital position, volcanism, oceans, and the greenhouse effect, not by __________ feedbacks.
Ans: biological
No vegetation or microbial feedback loops.

Q4. The atmosphere, hydrosphere, geosphere, and magnetosphere would still interact, but the __________ would be missing.
Ans: biosphere
Life is the missing sphere.

Q5. Satellite images would show bare rock, deserts, ice, clouds, and oceans, but no __________ patterns from crops or forests.
Ans: vegetation
No green canopies or agricultural mosaics.

Very Short Answer Questions

Instruction: Answer the following questions in one line.

Q1. What key atmospheric layer would be weaker without life?
Ans: The ozone layer, because it depends on oxygen from photosynthesis.

Q2. How would continents look from space without life?
Ans: Mostly brown/grey bare rock and sand, lacking green vegetation.

Q3. Which major biogeochemical cycles would be missing their main drivers?
Ans: The carbon and nitrogen cycles would lack biological processes.

Q4. Would Earth still have plate tectonics without life?
Ans: Yes; tectonics is driven by internal heat, not biology.

Q5. What would replace forests in shaping land surfaces?
Ans: Physical processes like wind, water erosion, volcanism, and impacts.

Short Answer Questions

Instruction: Answer the following questions in 2–3 lines.

Q1. Explain why oxygen would be scarce on a lifeless Earth.
Ans: Free O₂ on Earth is mainly produced by photosynthesis. Without life, oxygen would react with rocks and gases and not accumulate, leaving a thin or negligible O₂ atmosphere.

Q2. How would oceans differ without marine life?
Ans: No plankton blooms, coral reefs, or biological pump; carbon cycling would be set by chemistry and physics alone, and dissolved oxygen patterns would be purely abiotic.

Q3. What visual signatures used by satellites to track ecosystems would disappear?
Ans: Vegetation greenness (NDVI), crop patterns, algal blooms, and deforestation scars would be absent; images would be dominated by rock, sand, ice, water, and clouds.

Q4. Would weather and climate still vary in a lifeless world?
Ans: Yes. Seasons, winds, clouds, and rainfall would still occur due to solar heating, rotation, and ocean–atmosphere physics, but without biological feedbacks.

Q5. How would the carbon cycle be balanced without photosynthesis and respiration?
Ans: By slow abiotic processes: volcanic outgassing adding CO₂, silicate weathering and ocean chemistry removing CO₂, and carbonate rock formation over geologic timescales.

Match the Following

Instruction: Match Column A with the correct option in Column B.
Column A:
1) Ozone layer
2) Biological pump
3) Nitrogen fixation
4) Humus-rich soil
5) Green vegetation signal

Column B:
a) Satellite NDVI/green cover from plants
b) Deep-ocean carbon transport by plankton
c) Thicker, organic topsoil maintained by decomposers
d) UV shield formed from atmospheric O₂
e) Conversion of N₂ to usable forms by microbes

Ans:

1. Ozone layer — d) UV shield formed from atmospheric O₂
   O₃ derives from O₂; both depend on life today.

2. Biological pump — b) Deep-ocean carbon transport by plankton
   Marine life moves carbon into the deep sea.

3. Nitrogen fixation — e) Conversion of N₂ to usable forms by microbes
   Key biological pathway for bioavailable nitrogen.

4. Humus-rich soil — c) Thicker, organic topsoil maintained by decomposers
   Decomposition builds and sustains fertile soils.

5. Green vegetation signal — a) Satellite NDVI/green cover from plants
   The “green” biosignature indicates living plant cover.