6. Nutrient Cycles

Question 1

Compare the flow of nutrients vs the flow of energy in an ecosystem.

Answer 1

Flow of energy is linear.

Flow of nutrients is cyclic (nutrients are recycled)

Question 2

What is the simple sequence that all nutrient cycles have at heart?

Answer 2

• Nutrients taken up by producer
• Producer assimilates the nutrients into their structure by turning them into complex organic molecules
• Producer is eaten and the nutrients pass to the consumer
• Passes along food chain where the consumer is eaten by more consumers
• When the producers and consumers die, they are decomposed by saprobionts (decomposers) that release the nutrients in its original simple form
• Cycle is complete

Question 3

Why are there short term fluctuations in the proportions of oxygen and carbon dioxide in the atmosphere?

Answer 3

Variations in the rate of photosynthesis and respiration

Question 4

Describe how the concentration of carbon dioxide (eg in a woodland area) varies with the time of day.

Answer 4

-Conc of co2 is greater at night than during the day as no light means no photosynthesis. Respiration is still occuring (however the rate may be slightly lower as the temperatures tend to drop at night) so no photosynth means less co2 taken up in photosynthesis

Question 5

What are the two main reasons (human activities) that the global co2 concentration has risen over the past few hundreds of years?

Answer 5

• Deforrestation- fewer plants to absorb co2 through photosynthesis and act as a carbon sink
• Increased burning of fossil fuels which release co2 into the atmosphere

Question 6

When is methane, a gas that contributes to the enhanced greenhouse effect, produced?

Answer 6

• When decomposers decompose remains of dead organisms

- When the microorganisms in the intestines of primary consumers (eg cattle) digest the food that has been eaten

Question 7

What are the consequences of global warming?

Answer 7

-Changes in temperature
-Changes in precipitation
-Increase in storms, floods and droughts
-Melting of polar icecaps
-Rise in sea level due to thermal expansion of water
>Flooding
>Salty sea water may extend further up rivers, making cultivation of crop plants difficult
-Failure of crops present in some areas due to increased temperature and less rainfall
-

Question 8

Why do organisms need nitrogen?

Answer 8

-Manufacture nucleic acids and other nitrogen containing compounds such as amino acids and therefore proteins

Question 9

State the 4 main stages in the nitrogen cycle

Answer 9

• Ammonification
• Nitrofication
• Nitrogen fixation
• Denitrification

Question 10

Describe the process of ammonification.

Answer 10

• Production of ammonia from organic ammonium containing compounds such as urea and proteins, nucleic acids and vitamins
• Saprobiotic organisms (bacteria and fungi) feed on these materials, releasing ammonia
• This ammonia then forms ammonium ions in the soil

Question 11

Describe the process of nitrification.

Answer 11

• Some bacteria obtain their energy from chemical reactions involving inorganic ions
• Conversion of ammonium ions to nitrate ions (this is an oxidation reaction and so releases energy)
• Carried out by free living soil microorganisms called nitrifying bacteria

Conversion occurs in two stages:

• Conversion of ammonium ions to nitrite ions (NO2-)
• Conversion of nitrite ions to nitrate ions (NO3-)

Question 12

What conditions do nitrifying bacteria need in order to convert ammonium ions to nitrite ions and nitrite ions to nitrate ions? How may this knowledge be used to increase productivity in farming?

Answer 12

• They need aerobic conditions- soil with lots of air spaces in it
• Therefore farmers can plough their fields to keep them aerated and good drainage prevents the soil from becoming waterlogged (which would force air out of the soil)

Question 13

Describe the process of nitrogen fixation.

Answer 13

-Nitrogen gas converted into nitrogen containing compounds
-Occurs naturally when lightning passes through the atmosphere
-Carried out by microorganisms
>FREE LIVING NITROGEN FIXING BACTERIA - reduce nitrogen gas to ammonia which they then use to manufacture amino acids. Nitrogen rich compounds are released when they die
>MUTUALISTIC NITROGEN FIXING BACTERIA- live on the root nodules of legumes (they obtain carbohydrates from the plant and the plant acquires amino acids from the bacteria)

Question 14

What do free living and mutualistic nitrogen fixing bacteria do?

Answer 14

Free living- reduce nitrogen gas to ammonia and nitrogen rich compounds are released when they die and decompose

Mutualistic- obtain carbohydrates from the plant and the plant acquires amino acids from the bacteria

Question 15

Describe the process of denitrification.

Answer 15

• When soil becomes waterlogged/ there are anaerobic conditions, there are denitrifying bacteria rather than nitrifying bacteria
• They convert soil nitrates into gaseous nitrogen
• This reduces the nitrogen containing compound availability for plants so in order for productivity to be high, aerobic conditions must be maintained

Question 16

Why are fertilisers necessary in an agricultural system?

Answer 16

Nutrients are recycled when the plants/ organisms die. In agricultural systems, plants are harvested so therefore the nutrients aren’t recycled so fertilizers are needed to maintain the concentration of nitrogen containing compounds in the soil so crops can continue to grow

Question 17

Describe the two types of fertilisers.

Answer 17

-Organic/natural fertilisers
>Dead and decaying remains of plants and animals (also animal waste such as manure and bone meal)
-Artificial/ inorganic fertilisers- mined from rocks and deposits and mixed to contain the right blend of nitrogen, phosphorous and potassium.

Question 18

How do fertilisers increase productivity?

Answer 18

Provide a source of nitrogen for nucleic acid and amino acid/ protein production so plants can grow taller/ increased area of leaves so there will likely be more photosynthesis so therefore increases the crop’s productivity

Question 19

What are the detrimental effects of nitrogen fertilisers? (Briefly describe)

Answer 19

• Reduced species diversity- nitrogen rich soils encourages the growth of some species more than others (eg grasses and other rapidly growing species are favored)
• Leaching- may lead to the pollution of watercourses
• Eutrophication- caused by leaching fertilisers into watercourses

Question 20

Describe how leaching of fertilisers can occur and suggest the harmful effects on humans and the environment.

Answer 20

• Rain washes the fertilisers deep into the soil, eventually beyond the reach of the plants roots. It can then be washed/ make its way into watercourses such as lakes and rivers
• If they get into human drinking water the nitrates, in high concentrations, can prevent efficient oxygen transport in babies
• Also a higher risk of stomach cancer has been suggested and linked to high concentrations of nitrates in drinking water
• Harmful to the environment as they can cause eutrophication

Question 21

Describe the steps of eutrophication.

Answer 21

• Increased nitrate concentration causes an increase in growth of plants and algae (as it is no longer a limiting factor)
• Algal bloom occurs where there is lots of algae growth at the surface of the water
• This prevents light reaching the lower depths of the water
• Light becomes a limiting factor for plants and algae at lower depths so they die
• Increased dead plants and algae is no longer a limiting factor for saprobionts so the number of saprobionts increases exponentially
• Oxygen in the water is reduced as it is used up by the saprobionts and decomposers
• Less oxygen for aerobic organisms such as fish so they die
• Less competition for anaerobic organisms from the aerobic organisms so their numbers increase exponentially
• Anaerobic organisms further decompose dead materials releasing more NITRATES and TOXIC WASTE such as HYDROGEN SULPHIDE which makes the water putrid

Question 22

(3 marks)
Describe the role of microorganisms in producing nitrates from the remains of dead
organisms.

Answer 22

1. Saprobiotic (microorganisms/bacteria) break
   down remains/dead material/protein/DNA into
   ammonia/ammonium;
2. Ammonia/ammonium ions into nitrite and then
   into nitrate;
3. (By) Nitrifying bacteria / nitrification;

Question 23

(2 marks)
Leguminous crop plants have nitrogen-fixing bacteria in nodules on their roots.
On soils with a low concentration of nitrate ions, leguminous crops often grow better
than other types of crop. Explain why.

Answer 23

1. (Nitrogen) to
   ammonia/NH3/ammonium;
2. Produce protein/amino acids/
   named protein/DNA/RNA;

Question 24

(5 marks)
Much of Indonesia is covered with forest. Large areas of forest have been cleared and
planted with oil-palm trees to be used in the production of fuel.

In these forests, nitrogen in dead leaves is made available to growing plants by the
action of bacteria. Describe the role of bacteria in making the nitrogen in dead leaves
available to growing plants.

Answer 24

. Saprobionts/saprophytes;
2. Digest/break down proteins/DNA/nitrogen-containing
substances;
3. Extracellular digestion/release of enzymes;
4. Ammonia/ammonium produced;
5. Ammonia converted to nitrite to nitrate/ammonia to nitrate;
6. Nitrifying (bacteria)/ nitrification;
7. Oxidation;

Question 25

(2 marks)
Reeds have hollow, air-filled tissue in their stems which supplies oxygen to their roots.
Explain how this enables the roots to take up nitrogen-containing substances.

Answer 25

1. Uptake (by roots) involves
   active transport;
2. Requires ATP/ aerobic
   respiration;