5.4 nutrient cycles

What is the role of saprobionts in recycling chemical elements

Decompose/ break down organic compounds (protein, urea, DNA, organic waste)

How do saprobionts break down organic compounds

• by secreting enzymes for extracellular digestions (saprobiotic nutrition)

• Absorb soluble nutrients that are needed and release mineral ions (e.g phosphate ions)

What is mycorrhizae

A Symbiotic association between fungi and plant roots

What’s the role of mycorrhizae

• fungi (hyphae) act as an extension of plant roots to increase surface area of root system

• To increase the rate of uptake/ absorption of water and inorganic ions

• In return, fungi receives organic compounds (e.g carbohydrates)

Give examples of biological molecules that contain nitrogen

• amino acids

• Proteins or enzymes

• Urea

• DNA or RNA

• Chlorophyll

• ATP or ADP

• NAD or NADP

What are the key stages of the nitrogen cycle

1. Nitrogen fixation

2. Ammonification

3. Nitrification

4. Dentrification

Draw the stages of the nitrogen cycle

.

Describe the role of bacteria in nitrogen fixation

• nitrogen gas (N2) is converted into ammonia (NH3) which forms ammonium ions (NH4+) in soil

• This is done by nitrogen fixing bacteria, this may be found in root nodules

Describe the role of bacteria in ammonification

• nitrogen-containing compounds (e,g proteins/ urea/ waste) are broken down/ decomposed

• Converted to ammonia which forms ammonium ions in soil

• This is done by saprobionts, which secrete enzymes for extracellular digestion

Describe the role of bacteria in Nitrification

• ammonium ions in soil are converted into nitrites then nitrates via a two step oxidation reaction

• Allows for uptake by plant roots hair cells by active transport

• Done by nitrifying bacteria in aerobic conditions (oxygen)

Describe the role of bacteria in dentrification

• nitrates in soil converted into nitrogen gas (reduction)

• Done by denitrifying bacteria in anaerobic conditions (no oxygen, e.g waterlogged soil)

Why does ploughing/ aerating soil increase its fertility

• more ammonium converted into nitrite and nitrate, more Nitrification, more active nitrifying bacteria

• Less nitrate is converted to nitrogen gas, less denitrification, fewer active denitrifying bacteria

Give examples of biological molecules that contain phosphorus

• Phospholipids

• DNA or RNA

• ATP or ADP

• NADP

• TP or GP

• RuBP

Describe the phosphorus cycle

1. Phosphate ions in rocks are released by erosion/ weathering

2. These ions are taken up by producers/ plants/ algae and incorporated into their biomass (rate of absorption is increased by mycorrhizae)

3. Phosphate ions are transferred through food chain

4. Some are lost in water products/ excretion

5. Saprobionts decompose organic compounds releasing phosphate ions

Why are fertilisers used

• to replace nitrates/ phosphates lost when plants are harvested and livestock are removed

• As those removed from soil on incorporated into biomass can’t be released back into the soil through decomposition of saprobionts

• So it improves efficiency of energy transfer, increasing productivity/ yield

What is a natural fertiliser

• organic, e.g manure, compost, sewage

• Ions released during decomposition of saprobionts

What are artificial fertilisers

• contain inorganic compounds of nitrogen, phosphorus and potassium

What are the environmental issues from using fertilisers (1)

• phosphates/ nitrates dissolve in water, leading to leaching of nutrients into lakes, rivers and oceans

• This leads to eutrophication

What is eutrophication (2)

• rapid growth of algae in ponds/ rivers (called algal bloom), so light is blocked

• So submerged plants die as they cannot photosynthesise

• So saprobionts decompose dead plant matter, using oxygen in aerobic respiration

• So there’s less oxygen for fish to aerobically respire, so they die

What are the advantages of natural fertilisers

• less soluble in water, so less leaching. Making eutrophication less likely

• Organic molecules require breaking down by saprobionts, so there’s a slower release of nitrates/ phosphates