A-level Biology - 3.3.5 Nutrient Cycles

food webs

saprobionts

Feed on remains of dead plants & animals & on waste products = break them down

Allows chemical elements to be recycled

Saprobionts secrete enzymes & digest their food externally, then absorb soluble molecules (nutrients) they need

Known as extracellular digestion

Organic molecules are broken down into inorganic ions

Obtaining nutrients from dead organic matter using extracellular digestion

symbiotic

Relationships between fungi and the roots of plants are known as mycorrhizae

Fungi made up of long, thin strands called hyphae which connect to plant’s roots

Hyphae increase SA of plant’s root system = helps plant to absorb ions from soil that usually are scare (e.g. phosphorus)

Also increase uptake of water

organic compounds

To make proteins and nucleic acids (DNA & RNA)

It’s inert

Shows how nitrogen is converted into usable form & then passed between different living + non-living organisms

Nitrogen Fixation

Ammonification

Nitrification

Denitrification

Nitrogen gas → ammonia

N2 + 6H → 2NH3

Bacteria e.g. Rhizobium turns nitrogen into ammonia

Inside root nodules (growths on roots) of leguminous plants (e.g. peas, beans)

They provide the plant with nitrogen compounds & plant provides them with carbohydrates

Lightining

Fixes atmospheric nitrogen

Artificial fertilisers

Produced from atmospheric nitrogen on industrial scale in Haber process

Nitrogen compounds from dead organisms + animal waste are turned into ammonia by saprobionts via decay, which then forms ammonium ions

Ammonia can also dissolve in water to produce ammonium ions

Ammonium ions in soil are changed into nitrogen compounds which can be used by plants (nitrates)

ammonium ions → nitrites

nitrites → nitrates

Nitrifying bacteria (Nitrosomonas) change ammonium ions → nitrites

Other nitrifying bacteria (Nitrobacter) change nitrites → nitrates

NH4+ → NO2-

NO2- → NO3-

Called chemoautotrophs ∵ use chemical energy released from these reactions (nitrification) to live

When nitrates in soil → nitrogen gas by denitrifying bacteria

To carry out respiration (use NO as source of O) & produce nitrogen gas

Happens under anaerobic conditions e.g. in waterlogged soils

(Bacteria use NO as source of O ∵ of anaerobic conditions)

To replace lost minerals = more energy from ecosystem can used for growth

= increases efficiency of energy transfer

Inorganic

Contain pure chemicals (e.g ammonium nitrate) as powders or pellets

Organic matter

Include manure, compost vegetables, crop residues & sewage sludge

Crops take in minerals from soil as they grow

When crops are harvested, they're removed from field & ∴ don't decompose there

∴ minerals ions they contain (e.g. phosphates and nitrates) = not returned to soil by decomposers in nitrogen or phosphorous cycles

Animal eat plants = take in their nutrients

∴ when removed = nutrients aren't replaced though their remains or waste products

Leads to fertilisers leaching into water ways & thus eutrophication

When water-soluble compounds in soil are washed way (e.g. by rain or irrigation systems) into nearby ponds/rivers

Inorganic ions in chemical fertiliser = relatively soluble

Excess minerals = not used immediately are more likely to leach into waterways

VS natural fertilisers = nitrogen and phosphorus are contained in organic molecules that need to be decomposed by microorganisms before they can be absorbed by plants

∴ their release into soil = more controlled & leaching is less likely

∵ phosphates less soluble in water

having too much of one nutrient

Mineral ions leached from fertilised fields stimulate rapid growth of algae in ponds + rivers

Large amounts of algae block light from reaching plants below

Eventually plants die ∵ unable to photosynthesise enough

Bacteria feed on dead plant matter

Increased no. of bacteria reduce oxygen concentration in water by carrying out aerobic respiration

Fish & other aquatic organisms die ∵ not enough dissolved oxygen

Variations in rates of respiration and photosynthesis

e.g. concentration of CO2 at night is greater than during the day ∵ no photosynthesis occurs, but respiration occurs

ocean

Shells and bones sink to bottom of ocean and form rock such as chalk and limestone

Carbon returns to atmosphere as rocks weather

Sun's radiation reaches Earth

Some of it is reflected back & some radiated back to Earth by clouds + greenhouse gases that form part of the atmosphere

Greenhouse gases absorb heat

Gases trap this heat close to Earth's surface keeping it warm

so much longer than

When micro-organisms break down organic molecules which organisms are made of:

Decomposers break down dead remains of organisms

Micro-organism in intestines of primary consumers (e.g. cattle) digest food that's been eaten

1. It'll affect niches that are available in a community

2. ∴ distribution of species will change

3. Species may be able to migrate & compete for niches

4. Leads to loss of native species that occupy those niches

Life cycles and populations of insect pests would change to adapt to changed conditions ∴ tropical diseases could spread towards poles (as they carry pathogens)

Carbon dioxide taken in as a result of photosynthesis

Carbon is incorporated into compounds in the trees

Plants already have enough nitrate / nitrate no longer limiting

Another named factor is limiting growth e.g. light intensity

Increased temperature could decrease yield ∵ not optimum for enzyme action

Increase rate of transpiration

Increased prediation by insect pests

Unpredicated effects on rainfall

Ammonia formed by decay

On nitrogenous waste / nitrogenous compounds