Nutrient Cycles

Question 1

Why is essential that nutrients such as nitrogen and carbon are recycled?

Answer 1

Only a certain quantity of them on Earth.

Question 2

What is the general sequence of nutrient cycles?

Answer 2

• Nutrient is taken up by producers (plants) as simple, inorganic molecules.
• The producer incorporates the nutrient into complex organic molecules.
• When the producer is eaten, the nutrient passes into consumers.
• It then passes along the food chain when these animals are eaten by other consumers.
• When the producers and consumers die, their complex molecules are broken down by saprobiotic microorganisms (decomposers) that release the nutrient in its original form.

Question 3

What is carbon?

Answer 3

A component of all the major macromolecules in living organisms. Basic building block of life.

Question 4

What is the main source of carbon for terrestrial organisms?

Answer 4

CO2 in atmosphere.

Question 5

What do photosynthetic organisms turn CO2 into?

Answer 5

macromolecules- carbohydrates, fats and proteins.

Question 6

How is CO2 returned to air?

Answer 6

Through respiration.

Question 7

What causes short term variations in the proportions of O2 and CO2 in the atmosphere?

Answer 7

Variations in the rates of resp. and photosynth.
(CO2 conc. greater at night than during the day. no light so no photosynthesis)
Also varies seasonally.

Question 8

What are the two main reasons for the increase in CO2 in the atmosphere?

Answer 8

• Combustion of fossil fuels, such as coal, oil and peat, has released CO2 that was previously locked up in fuels.
• Deforestation, especially of the rainforests, has removed enormous amounts of photosynthesising biomass and so less CO2 is being removed from the atmosphere.

Question 9

What helps to keep the CO2 level relatively constant?

Answer 9

The ocean- contains massive reserve of CO2. This store is sometimes 50X greater than that in the atmosphere.
Some of excess carbon in atmosphere dissolves in ocean.
When CO2 levels are low the reverse occurs.

Question 10

What do aquatic photosynthetic organisms do?

Answer 10

(phytoplankton) use dissolved CO2 to form macromolecules that make up their bodies.

Question 11

What are saprobiotic organisms?

Answer 11

Organisms which secrete enzymes on to the dead organisms. These enzymes break down complex molecules into smaller, soluble molecules that the saprobiotic microorganisms absorb by diffusion. The carbon in the dead organism is then released as CO2 during respiration by the decomposer.

Question 12

How to organisms become fossilised into coal, oil or peat?

Answer 12

If decay is prevented.

Question 13

What happens to the shells and bones of aquatic organisms?

Answer 13

DO not decompose.
Sink to bottom of ocean and, over millions of years, form carbon-containing sedimentary rocks such as chalk and limestone This carbon eventually returns to the atmosphere as these rocks are weathered.

Question 14

Describe the carbon cycle.

Answer 14

• There are carbon containing compounds in producers.
• Consumers feed on producers gaining their carbon.
• Consumers and producers respire, releasing CO2 into atmosphere.
• Producers also photosynthesis, taking in CO2 from atmosphere.
• Saprobiotic microorganisms cause producers and consumers to decompose, releasing CO2 by respiration.
• If decay is prevented, fossil fuels burned and CO2 released to atmosphere
• Some CO2 dissolved in oceans.

Question 15

What is the greenhouse effect?

Answer 15

A natural process that occurs all the time and keeps average global temp. at around 17C. Without it average temp. of surface of the earth would be about minus 18C.

Result of heat and light of the SUn (solar radiation) that reaches our planet. Some solar radiation is reflected back into space, some is absorbed by the atmosphere and, fortunately, some reaches Earth’s surface. Some radiation reaching Earth’s surface is reflected back as heat and is lost into space. However some is radiated back to Earth by clouds and greenhouse gases.

Question 16

What do greenhouse gases do?

Answer 16

Trap heat close to Earth’s surface, keeping it warm.

Question 17

Why is CO2 the most important greenhouse gas?

Answer 17

Partly because there is so much of it and partly because it remains in the atmosphere for much longer than the other greenhouse gases (100yrs compared to 10yrs for methane).
It is estimated that 50-70% of global warming is caused by CO2 in the atmosphere.

Question 18

How is methane produced?

Answer 18

When microorganisms break down the organic molecules of which organisms are made.
Occurs mostly in two situations:
-when decomposers break down dead remains of organisms.
-When microorganisms in the intestines of primary consumers, such as cattle, digest the food that has been eaten.

Question 19

What is global warming?

Answer 19

The mean global temp. has increased by 0.6C since 1900, a change known as global warming.

Question 20

What are the consequences of global warming?

Answer 20

Global warming is said to bring about changes in temperature and precipitation, the timing of seasons and the frequency of extreme events such as storms, floods and drought.
Climate change will affect the niches available in a community. As each organism is adapted to a particular niche the distribution of species will alter. If the rate of climate change is slow, species may have time to gradually migrate to new areas, where they will compete for available niches. Lead to loss of native species.

• Polar ice caps melt
• Rise in sea levels due to thermal expansion of oceans- flood. Salt water further up rivers.
• Higher temp. and less rainfall could lead to failure of certain crops in some areas.
• Greater rainfall and intense storms.
• Life cycles and populations of insect pests would alter as they adapt.

Question 21

What do organisms manufacture from nitrogen?

Answer 21

Proteins, nucleic acids and other nitrogen containing compounds.

Question 22

How do plants take up nitrogen?

Answer 22

As NO3- from soil. These ions are absorbed, using active transport, by the root hairs.
This is where nitrogen enters the living component of the ecosystem.

Question 23

How do animals obtain nitrogen?

Answer 23

By eating and digesting plants.

Question 24

What is the issue with nitrate ions and how are they restored in natural ecosystems?

Answer 24

Nitrate ions are very soluble and easily leach through the soil, beyond the reach of plant roots.
In natural ecosystems, the nitrate levels are restored through the recycling of nitrogen-containing compounds.

Question 25

How are nitrate levels in the soil increased in agricultural ecosystems?

Answer 25

Addition of fertilisers.

Question 26

How are nitrates returned to soil?

Answer 26

Decomposition of dead plants and animals.

Question 27

What are the four stages of the nitrogen cycle?

Answer 27

• Ammonification
• Nitrification
• Nitrogen fixing
• Denitrification

Question 28

Describe the nitrogen cycle?

Answer 28

• Ammonium ions are turned into nitrite ions by nitrification.
• Nitrite ions are turned into nitrate ions by nitrification.
• Nitrate ions are then absorbed by producers and turned into ammonium-containing molecules.
• These are then passed on to consumers by feeding and digestion.
• Death and excretion allow decomposers (saprobiotic microorganisms) to break down the compounds.
• Through ammonification ammonium containing molecules are turned back into ammonium ions.
• Not all nitrogen ions are absorbed by producers, some nitrogen is released into atmosphere by denitrification.
• Nitrogen in atmosphere can be absorbed by producers by nitrogen fixation by mutualistic bacteria, also turned back into ammonia through nitrogen fixation by free-living bacteria.

Question 29

Describe ammonification.

Answer 29

The production of ammonia from organic ammonium containing compounds. In nature, these compounds include urea (from the breakdown of excess amino acids) and proteins, nucleic acids and vitamins (found in faeces and dead organisms).
Saprobiotic microorganisms, mainly fungi and bacteria, feed on these materials, releasing ammonia, which then forms ammonium ions in the soil.
THIS IS WHERE NITROGEN RETURNS TO THE NON-LIVING COMPONENT OF THE ECOSYSTEM

Question 30

Describe nitrification.

Answer 30

Nitrification is when ammonium compounds in the soil are changed into nitrogen compounds that can then be used by plants.

First nitrifying bacteria (e.g. nitrosomonas) change ammonium compounds into nitrites.
Then other nitrifying bacteria called nitrobacter change nitrites in to nitrates.

Question 31

What is required for nitrification?

Answer 31

Nitrifying bacteria require oxygen to carry out these conversions and so they need a soil that has many air spaces. To raise productivity, it is important for farmers to keep soil structure light and well aerated by ploughing. Good drainage also prevents the air spaces from being filled with water and so prevents air being forced out of soil.

Question 32

Describe nitrogen fixation.

Answer 32

Nitrogen fixation is when nitrogen gas in the atmosphere is turned into ammonia by bacteria called Rhizobium. The ammonia can then be used by plants.

Rhizobium are found inside root nodules of leguminous plants (e.g. peas, beans and clover).

They form a mutualistic relationship with the plants- they provide the plant with nitrogen compounds and the plant provides them with carbohydrates.

Question 33

What do free-living nitrogen-fixing bacteria do?

Answer 33

These bacteria reduce gaseous nitrogen to ammonia, which they then use to manufacture amino acids. Nitrogen-rich compounds are released from them when they die and decay.

Question 34

What do mutualistic free-living bacteria do?

Answer 34

These bacteria live in nodules on the roots of plants such as peas and beans. They obtain carbohydrates from the plant and the plant acquires amino acids from the bacteria.

Question 35

What is denitrification?

Answer 35

When soils become waterlogged, and therefore short of oxygen, the type of microorganism present changes. Fewer aerobic nitrifying and nitrogen-fixing bacteria are found, and there is an increase in anaerobic denitrifying bacteria.

These convert soil nitrates into gaseous nitrogen. This reduces the availability of nitrogen-containing compounds for plants.

For land to be productive, the soils on which crops grow must therefore be kept well aerated to prevent the build up of denitrifying bacteria.

Question 36

Why are fertilisers needed?

Answer 36

Intensive farming- mineral ions removed from soil need to be replaced.

In natural ecosystems minerals are removed from soil by plants and replaced when plant is broken down by microorganisms when dead.
In agricultural ecosystems the crop is harvested and then transported for consumption.
Urine, faeces and dead remains of the consumer are rarely returned to the same area of land.

Question 37

What are the two types of fertiliser?

Answer 37

• Natural (organic) fertilisers- which consist of the dead and decaying remains of plants and animals as well as animal wastes such as manure and bone meal.
• Artificial (inorganic) fertilisers- mined from rocks and deposits and then converted into different forms and blended together to give the appropriate balance of minerals for a particular crop. Compounds containing the three elements, nitrogen, phosphorus and potassium, are almost always present.

Question 38

How do fertilisers increase productivity?

Answer 38

Plants require minerals for growth. If more minerals, plant grows faster (develop earlier), grow taller and have a greater leaf area. This increase rate of photosynthesis and improves crop productivity.

Question 39

What are the negative effects of nitrogen fertilisers?

Answer 39

REDUCED SPECIES DIVERSITY: nitrogen rich soils favour the growth of grasses, nettles and other rapidly growing species. These out-compete many other species.
LEACHING: which may lead to pollution of watercourses
EUTROPHICATION: caused by leaching of fertilisers into watercourses.

Question 40

What is leaching?

Answer 40

The process by which nutrients are removed from the soil. Rain water will dissolve any soluble nutrients, such as nitrates, and carry them deep into the soil, eventually beyond the reach of roots.

The leached nitrates find their way into watercourses, such as streams and rivers, which may in turn drain into freshwater lakes.

Question 41

Why can leaching be harmful?

Answer 41

If lake water or river water is a source of drinking water.
Very high nitrate levels in drinking water can prevent efficient oxygen transportat in babies and link to stomach cancer in humans.
Also cause eutrophication.

Question 42

What is eutrophication?

Answer 42

The process by which nutrients build up in bodies of water.

Question 43

How does eutrophication work?

Answer 43

1. In most freshwater lakes and rivers there is naturally very little nitrate and so nitrate is a limiting factor for plant and algal growth.
2. As the nitrate concentration increases as a result of leaching, it ceases to be a limiting factor for the growth of plants and algae and both grow exponentially.
3. As algae mostly grows at the surface, the upper layer of water becomes densely populated with algae. This is called algal bloom.
4. This dense surface layer of algae absorbs light and prevents it from penetrating to lower depths.
5. Light then becomes the limiting factor for the growth of plants and algae at lower depths and so they eventually die.
6. The lack of dead plants and algae is no longer a limiting factor for the growth of saprobiotic algae and so these too grow exponentially, using the dead organisms as food.
7. The saprobiotic bacteria require oxygen for their respiration, creating an increased demand for O2.
8. The concentration of O2 in the water id reduced and nitrates are released from the decaying organisms.
9. Oxygen then becomes a limiting factor for the population of aerobic organisms, such as fish. These organisms ultimately die as the O2 is used up altogether.
10. Without aerobic organisms, there is less competition for the anaerobic organisms, whose populations now rise exponentially.
11. The anaerobic organisms further decompose dead material, releasing more nitrates and some toxic wastes, such as hydrogen sulphide, which makes water putrid.

Question 44

What bacteria cause nitrogen gas from the atmosphere to be turned into ammonia? (NITROGEN FIXATION)

Answer 44

Rhizobium

Question 45

What bacteria change ammonium compounds into nitrites? (NITRIFICATION)

Answer 45

NITROSOMONAS

Question 46

What bacteria change nitrites into nitrates? (NITRIFICATION)

Answer 46

Nitrobacter

Question 47

What causes a daily change in CO2 conc?

Answer 47

• Respiration is carried out constantly through the day and night.
• Photosynthesis only takes place during daylight hours.
• CO2 conc. falls during day because it’s being used by plants as they carry out photosynthesis.
• CO2 conc. increases at night because it’s no longer removed, but all organisms are still respiring and adding CO2 to the atmosphere.

Question 48

What causes a yearly change in CO2 conc.?

Answer 48

• Most plant life exists in the northern hemisphere because that’s where most land is.
• Most plant growth occurs in summer (june-aug in the northern hemisphere) because that’s when light intensity is greatest- more photosynthesis can take place, so more energy to grow.
• CO2 conc. falls during summer because more is being removed from the atmosphere as more plants are photosynthesising.
• CO2 conc. increases throughout autumn and winter because less is being removed from the atmosphere, as fewer plants are photosynthesising.

Question 49

How does global warming affect crop yield?

Answer 49

The increasing CO2 conc. that’s causing global warming could also be causing an increase in crop yields (the amount of crops produced from and area). CO2 concentration is a limiting factor for photosynthesis, so increasing global CO2 conc. could mean crops grow faster, increasing crop yields.

Question 50

How does global warming affect insect pests?

Answer 50

1) Climate change may affect the life cycle of some insect species. For example it’s thought that increasing global temperature means some insects their larval stage quicker and emerge as adults earlier. E.g. some butterflies may spend 10 fewer days as larvae for every 1C temp. rise.

2) Climate change may affect the numbers of some insect species:
- Some species are becoming more abundant (e.g. warmer, wetter summers in some places have led to an increase in the number of mosquitoes.)
- Other species may become less abundant, e.g. some tropical insect species can only thrive in specific temperature ranges, so if it gets too hot fewer insects may be able to reproduce successfully.

Question 51

How does global warming affect wild animals and plants?

Answer 51

1) Climate change could affect the distribution of many wild animal and plant species:
- Some species may become more widely distributed, e.g. species that need warmer temperatures may spread further as the conditions they thrive in exist over a wider area.
- Other species may become less widely distributed, e.g. species that need cooler temperatures may have smaller ranges as the conditions they thrive in exist over a smaller area.

2) Climate change could also affect the number of wild animals and plants:
- Some species are becoming more abundant, e.g. boarfish are increasing in number in parts of the Atlantic Ocean where sea temp. is rising.
- Other species are becoming less abundant, e.g. polar bears need frozen sea ice to hunt and global warming is causing more sea ice to melt.