Nutrient cycles (A-level only) Flashcards – AQA A-Level Biology - (7402)

What are saprobionts?

Organisms that obtain nutrients from dead organisms and waste by extracellular digestion.

How do saprobionts digest and absorb organic material?

They secrete enzymes onto it, breaking large, insoluble molecules into small, soluble products that can be absorbed.

What role do saprobiont bacteria and fungi play in nutrient recycling?

They decompose organic matter, returning inorganic nutrients to the environment where plants and other organisms can use them.

What is a mycorrhiza?

A mutually beneficial association between a fungus and plant roots.

What does a plant gain from its mycorrhizal association?

Improved uptake of water and inorganic ions, including phosphate.

How do fungal hyphae increase a plant’s effective surface area for absorption?

They extend into the soil.

What does the fungus receive from the plant in a mycorrhizal association?

Organic compounds, such as sugars produced by the plant.

Between which three stores does the nitrogen cycle recycle nitrogen?

The atmosphere, organisms and soil.

What do nitrogen-fixing bacteria convert atmospheric nitrogen into?

Ammonia or ammonium compounds.

Where do nitrogen-fixing bacteria live?

Some live freely in soil; others live in association with plant roots.

Which nitrogen ions can plants use to make organic nitrogen compounds?

Ammonium ions or nitrate ions.

How do organic nitrogen compounds pass from plants to animals?

Through feeding.

What do saprobionts break down in the nitrogen cycle?

Dead organisms and waste.

What happens to organic nitrogen compounds during ammonification?

Microorganisms convert them into ammonia, which forms ammonium ions in the soil.

What transformations occur during nitrification, and under what conditions?

Nitrifying bacteria oxidise ammonium to nitrite, then nitrite to nitrate, under aerobic conditions.

What happens during denitrification?

Denitrifying bacteria convert nitrate into nitrogen gas, returning it to the atmosphere, particularly in anaerobic soil.

Between which four parts of the environment does the phosphorus cycle recycle phosphorus?

Rocks, soil, water and organisms.

How is phosphate released from phosphate-containing rocks?

Weathering releases phosphate ions.

How do plants take up and use phosphorus?

They absorb inorganic phosphate from soil or water and incorporate it into organic compounds.

How does phosphorus pass through food chains?

It passes between organisms when they feed.

How does organic phosphorus return to the environment from organisms?

Through waste and dead organisms.

How do decomposers make phosphorus available for uptake again?

They release inorganic phosphate from organic phosphorus.

How can phosphate in bodies of water become part of rock?

It becomes part of sediments and, over long periods, phosphate-containing rocks.

How can phosphate-containing rocks become available for weathering again?

Geological processes can expose them.

How do harvesting plants and removing livestock reduce soil nutrient return?

They remove nitrogen- and phosphorus-containing material from the ecosystem, leaving less to return to the soil.

What is the role of fertilisers in agricultural nutrient replacement?

They replenish nutrients needed for plant growth, including sources of nitrate and phosphate.

What is a natural fertiliser?

A fertiliser containing organic material from living organisms, such as manure.

How do natural fertilisers make nutrients available to plants?

Their organic material decomposes, releasing inorganic nutrients that plants can absorb.

What is an artificial fertiliser?

A manufactured product that supplies mineral nutrients in inorganic forms.

How can fertiliser nutrients be leached from soil into water bodies?

Water carries soluble nutrients out of the soil and into water bodies.

What effect can elevated nitrate and phosphate concentrations have on algae in a water body?

They can cause rapid algal growth, producing an algal bloom.

Why can an algal bloom cause submerged plants and algae to die?

It reduces light penetration, so they cannot photosynthesise sufficiently.

How does the decomposition of dead material deplete dissolved oxygen in water?

Saprobionts decompose the material and respire aerobically, consuming dissolved oxygen.

What can happen to aquatic organisms when dissolved oxygen is depleted?

Fish and other aquatic organisms can die.

What is eutrophication?

The enrichment of water with nutrients and its resulting consequences.