Unit 9: Cycles

What does it mean when materials "cycle" through an ecosystem?

Elements are continually taken from the abiotic environment by living organisms, passed along food chains, and returned to the environment via excretion and decomposition.

What is the only biological process that removes carbon dioxide from the atmosphere?

Photosynthesis by green plants and algae.

What two major biological processes return carbon dioxide to the atmosphere?

Respiration by living organisms and decomposition by microorganisms.

What is the role of microorganisms as decomposers in the carbon cycle?

Bacteria and fungi break down dead organic matter and waste materials, releasing carbon back into the atmosphere through their own respiration.

What non-biological process releases large volumes of carbon into the atmosphere from fossil fuels or wood?

Combustion burning.

What are the four core physical processes involved in the water cycle?

Evaporation, condensation, precipitation, and transpiration water loss from plants.

What is potable water?

Water that is safe for humans to drink.

What is desalination and when is it used?

The chemical process of removing salt from seawater to produce potable water, used in areas of severe drought where fresh water is scarce.

What is a common thermal method used in desalination?

Distillation, where seawater is boiled so the water evaporates and condenses into pure fresh water, leaving the salt behind.

What is reverse osmosis in the context of desalination?

A method where seawater is forced under high pressure through a semi-permeable membrane that allows water molecules through but traps the salt ions.

Why cannot plants absorb nitrogen directly from the air despite it making up 78% of the atmosphere?

Nitrogen gas is unreactive and contains a triple covalent bond that plants cannot break down without the help of specific bacteria.

What is the role of nitrogen-fixing bacteria?

They convert atmospheric nitrogen gas into nitrogen-containing compounds like ammonia in the soil.

Where are nitrogen-fixing bacteria often found?

Free-living in the soil, or inside root nodules of leguminous plants like peas, beans, and clover.

What do nitrifying bacteria do in the soil?

They convert ammonia into nitrites, and then convert those nitrites into nitrates, which plants can easily absorb through their roots.

What is the role of denitrifying bacteria?

They convert soil nitrates back into atmospheric nitrogen gas, typically thriving in anaerobic conditions like waterlogged, un-aerated soils.

How does crop rotation help make nitrates available for plant uptake?

Farmers grow a leguminous crop like clover or peas every few years to naturally increase soil nitrate levels via root nodule bacteria, reducing the need for chemical fertilizers.

Why do farmers use artificial fertilizers on crops?

To directly add ready-to-absorb nitrates to the soil, replacing nutrients lost during harvest and maximizing plant growth.