15- Nutrient Cycles

What are some facts about nutrient cycles?

• Elements in organisms are constantly being recycled between biotic and abiotic pahes

• Nutrients cycles between biotic and abiotic components of ecosystems within which they exist as organic and inorganic compounds 

What are organic compounds?

tend to be found inside organisms (living or dead) and are large, complex molecules that all contain the element carbon

What are inorganic compound?

consist of simple, small molecules and tend to be found in the abiotic phase

What are the features of nutrient cycles?

• Nutrients in a simple, inorganic form are taken up by producers

• Producers use nutrients to synthesise more complex organic molecules

• Then passed along food chains as producers and animals are eaten by consumers

• When these die, decomposed by saprobionts and nutrients are returned to original inorganic form to complete the cycle

Which biological molecules contain nitrogen?

• nucleic acids DNA and RNA 

• proteins and their monomers (amino acids)

• ATP/ ADP 

Tell me about nitrogen

• Makes up 80% of earth’s atmosphere

• Scarce resource for living organisms

• Nitrogen gas is inert/ unreactive, so not available to most living organisms when in gaseous state

Why will lack of nitrogen lead to stunted growth?

Organisms is unable to make proteins which are needed for growth

What is the order of the nitrogen cycle?

1. Nitrogen fixation 

2. Nitrification 

3. Decomposition and Ammonification 

4. Denitrification 

Describe nitrogen fixation

• carried out by nitrogen fixing bacteria

• convert nitrogen gas into ammonia/ ammonium ions

• some bacteria live in swelling on roots (nodules) of plants called legumes

• bacterium receives water and sugars from plant and in return supplies plant with ammonia

• ammonia can be used to make proteins and other nitrogen containing compounds

• some ammonia ends up in soil

• free living nitrogen fixing bacteria in soil- ammonia they produce dissolves in water in soil to form ammonium ions

• plants take up nitrogen from soil to form nitrates by active transport- this is how most nitrogen returns to biotic phase

Describe nitrogen flow through food webs 

• inorganic nitrogen taken up by plants is largely in the form of nitrates

• used to synthesise molecules like proteins or DNA 

• moves into other organisms when eat plants, and digest and absorb the organic compounds 

What type of organisms will consume nitrogen in living plant material?

• herbivores

• primary consumers

• omnivores

What type of organisms will consume nitrogen in dead plant and animal material?

• saprobionts

• decomposers

What type of organisms will consume nitrogen in living animals?

• carnivores

• secondary consumers

• tertiary consumers

• omnivores

Describe Nitrification

• carried out by nitrifying bacteria which convert ammonium ions into nitrates

• 2 stages each carried out by different nitrifying bacteria

1. oxidation of ammonium ions to nitrite ions by nitrifying bacteria

2. oxidation of nitrites to nitrates by nitrifying bacteria

• plants take up nitrates through roots by active transport, returning nitrogen to biotic phase

Describe Decomposition and ammonification (saprobiotic nutrition)

• when organisms die, excrete urea or egest faeces, saprobiotic microogranisms convert organic nitrogen in these organic compounds into ammonium ions

• returns nitrogen to abiotic phase

• saprobionts secrete enzymes in detritus so nitrogen-containing compounds are digested externally

• this is extracellular digestion

• products of digestion are absorbed by saprobionts but some ions/ molecules are released into the environment

Describe Denitrification

• loss of nitrates from soil under anaerobic conditions

• nitrates converted to nitrogen gas

• aerating soil will reduce this process 

What does nitrogen fixing bacteria do?

reduce nitrogen gas in atmosphere to ammonia which dissolves to form ammonium ions

What does saprobiotic bacteria do?

decompose dead/ waste material and convert nitrogen-containing organic matter into ammonia/ ammonium ions

What does nitrifying bacteria do?

oxidise ammonium ions to nitrites, and nitrites to nitrates

What does denitrifying bacteria do?

convert nitrate ions in soil to nitrogen gas in the atmosphere 

Explain the nitrogen cycle (paragraph)

• Nitrogen in atmosphere can be converted into ammonium ions in soil by nitrogen fixing bacteria.

• These bacteria can either be free living in soil or live in root nodules of plants called legumes.

• Ammonium is converted firstly to nitrite and then nitrate by other soil bacteria called nitrifying bacteria.

• This process uses oxygen so is oxidation.

• The nitrate can be taken up by plant roots by active transport which goes against a conc gradient so uses energy in the form of ATP.

• Respiration is used to release this energy/ make ATP.

• The plants use nitrates to build nitrogen containing compounds like DNA, proteins, amino acids, RNA etc, so increasing their yield.

• These nitrogen-containing organic compounds are passed to animals when plants are eaten by herbivores/ omnivores/ consumers.

• When plants and animals die, bacteria called saprobionts break down nitrogen containing organic compounds.

• These are hydrolysis reactions, using extracellular enzymes and the small molecules produced are absorbed.

• The waste product of this process is ammonia/ ammonium which can be converted into nitrate by nitrification and taken up by plants. 

What are the features of nitrogen fixing bacteria?

• converts nitrogen gas to ammonia

• can be found in soil

• can be found in root nodules

What are the features of nitrifying bacteria?

• converts ammonia to nitrates

• can be found in soil

What are the features of denitrifying bacteria?

• converts nitrates to nitrogen gas

• works in anaerobic conditions

• can be found in soil

What are the features of saprobiotic bacteria?

• breaks down proteins in dead material 

• can be found in soil 

Give 2 sources of Ammonia in the soil (2 marks)

• from nitrogen-fixing bacteria converting nitrogen gas to ammonia 

• breakdown of proteins in dead material by saprobiotic bacteria leads to the release of ammonia 

What biological molecules is phosphorus found in?

• phospholipids

• nucleic acids (DNA/ RNA)

• ATP/ ADP

• NADPH

Describe the phosphorus cycle

• Saprobionts decompose dead animals/ plants/ algae/ waste materials using extracellular digestion.

• Saprobionts absorb products of digestion and release phosphate ions into soils, lakes etc. where they dissolve (stored mainly in sedimentary rock).

• Some phosphate ions are absorbed by plants and are used to form organic biological molecules used in growth (phospholipids, ATP, nucleic acids etc).

• Phosphate ions passed to animals that feed on plants. Phosphate-containing organic compounds are digested then absorbed and assimilated.

• Some dissolved phosphates in oceans can form rocks by sedimentation.

• Phosphate in sedimentary rocks only return to cycle if geological upheavel and erosion occurs

What is mycorrhizae?

Fungi that grow in association with plant roots and improve plant’s uptake of water and inorganic ions

• example of a mutualistic relationship between 2 species in which they both benefit nutritionally

Explain how mycorrhizae works?

• Fungus colonises host plant’s roots.

• Grow in and around roots, producing large system of threads or hyphae.

• Provides fungus with carbohydrates from plant (e.g. glucose and sucrose).

• In return plant gains benefits of fungal hyphae’s greater ability to absorb water and minerals due to large SA of thread like hyphae.

Explain the difference in growth of crops A and B (2 marks)

• Increased growth in B due to presence of fungus which, due to their large SA, allow more efficient absorption of water and the minerals that are essential for growth (and are often limiting factors in plant growth)

Why do we need fertilisers?

• Nitrates and phosphates are 2 very important mineral ions (often limiting factors in plant growth)

• Intensive food production results in nitrates and phosphates continually being lost from soil

• Feeding livestock depletes nitrates and phosphates

• Plants not left to decompose so minerals not recycled- necessary farmers use fertilisers to replace lost mineral ions and maintain crop productivity

What ions are needed to produce proteins?

• nitrates 

What ions are needed to produce nucleic acids?

• nitrates

• phosphates

What ions are needed to produce phospholipids?

• phosphates

What ions are needed to produce ATP?

• phosphates

• nitrates

What are natural fertilisers?

consist of organic plant or animal matter, containing complex organic compounds e.g. urea which releases mineral ions as it decays 

What are artificial fertilisers?

manufactured and contain concentrated mineral ions in an inorganic, soluble and ready to use form

What widespread environmental impacts can using fertilisers have?

• reduced species diversity

• leaching

• eutrophication

Describe leaching

• excess ions can be washed out of soil by rain

• leached ions then find way into streams

• causes eutrophication

Describe eutrophication

• in most lakes/ rivers, there is naturally very little nitrate= nitrate is a limiting factor for plant growth

• once nitrate leaches into water course, ceases to be a limiting factor and plants/ algae grow exponentially resulting in:

  • an algal bloom occurs (rapid algal growth)

  • dense layer of algae forms on surface, absorbing light and preventing it penetrating lower depths so plants below cannot photosynthesise and die

  • increased competition between algae results in algal death

  • when algae die, decomposed by bacteria and other saprobiotic organisms, whose population suddenly increase

  • increase in aerobic respiration by bacteria uses up oxygen

  • dissolved oxygen in water become depleted, causing many invertebrates and fish to die

Explain how pollution of a river with sewage can also cause eutrophication (3 marks)

The organic material in sewage is fed upon by bacteria, which will respire aerobically, using up the oxygen in the river. If detergents are present, these contain phosphates which will encourage algal growth.