19 April 2015

# 67 Summary of Ecology

1 A habitat is a place where an organism lives. The niche of an organism is the role that it plays in the community.
2 A population is a group of organisms of the same species, living in the same place at the same time, that can interbreed with one another. A community is all the organisms, of all the diff erent species, living in the same place at the same time.

3 An ecosystem is an interacting system of organisms and their environment, more or less self-contained.

 4 Energy flows from one organism to another in the form of chemical energy in organic molecules in food. The pathways of energy flow can be shown in a food chain or food web, in which the arrows show the direction of energy flow.

 5 The first organism in a food chain or food web is a producer. In most food chains, plants are the
producers. They transfer energy from sunlight into chemical energy in organic molecules in the process of photosynthesis. All other organisms in a food chain are consumers.

 6 Each step in a food chain is a trophic level. Energy is lost as it passes from one trophic level to the next. The percentage of energy in one trophic level that passes to the next trophic level is generally around 10%. Th is value is the effi ciency of energy transfer. It is generally relatively low for transfers from producers to primary consumers, because of the high content of cellulose in plants, which is not easily digested by most animals but contains a lot of energy.

 7 Nitrogen atoms are an essential part of many organic molecules, especially proteins and nucleic acids. Although a high percentage of the atmosphere is nitrogen gas, nitrogen molecules are very unreactive and cannot be used by most living organisms. The nitrogen must be fi xed – converted into a more reactive form such as ammonium ions or nitrate ions – before plants can make use of it. Lightning and nitrogen-fixing bacteria are the two most important natural methods of nitrogen fixation.

 8 Plants use ammonium ions or nitrate ions to make amino acids and then proteins. Consumers obtain amino acids from plants.

 9 Decomposers break down nitrogen-containing molecules in dead plants and animals, or in their
waste products, such as urea. Ammonia is produced, which is converted to nitrite ions and then nitrate ions by nitrifying bacteria. Denitrifying bacteria convert nitrate ions back to nitrogen gas.

1. Multiple-choice test

1 Which ecological term is defined as ‘the particular location and type 
of environment occupied by an organism’?

A  ecosystem
B   habitat
C   niche
D   population

2 What is meant by an ecosystem?
  A  all the biotic and abiotic components of a given environment
  B  all the populations of organisms in a given environment
  C  all the chemical and physical factors acting in a given environment
  D  the community of organisms in a given environment

 Which step in a marine food chain involves transfer of energy from trophic level 2 to trophic level 3?
   A  phytoplankton carry out photosynthesis
   B  zooplankton feed on phytoplankton
   C  small fish feed on zooplankton
   D  large fish feed on smaller fish

4 Which step in a marine food chain involves the smallest transfer of energy?

  A  phytoplankton carry out photosynthesis
  B  zooplankton feed on phytoplankton
  C  small fish feed on zooplankton
  D  large fish feed on smaller fish

5 Which statement explains why herbivores obtain and store in their bodies less than 10% of the net primary productivity of the plants available for them to eat?

  A  Not all parts of a plant are available to be eaten.
  B  Not all parts of a plant can be digested.
  C  Some plant material is tough or distasteful and is not eaten.
  D  Some plant material is eaten by other herbivores.

6 Which bacterial reaction in the nitrogen cycle decreases soil fertility?

  A oxidation of NH4+ to NO2 by Nitrosomonas
  B oxidation of NO2 to NO3 by Nitrobacter
  C conversion of N2 to NH4+ by bacteria using nitrogenase

  D conversion of NO3 to N2 by bacteria in water-logged soil

7  Rhizobium bacteria form colonies in nodules on the roots of some plants.
   What is the role of these bacteria in the nitrogen cycle?

  A  to produce ammonium ions from nitrogen gas
  B  to produce ammonium ions from urea
  C  to produce nitrate ions from nitrite ions

  D  to produce nitrogen gas from nitrate ions

8 The percentage of energy consumed by an organism that is actually absorbed by the organism varies for different foods.

The table shows the percentage absorption of different foods by small birds.

What explains the differences in percentage absorption?
  A  A bird in trophic level 3 uses more energy to digest its food than any of the birds in trophic level 2.
  B  More energy is used to digest caterpillars than to digest seeds.
  C  Nectar requires little energy to digest.

  D  Seeds contain more cellulose than do plant leaves.

9  The energy trapped by grassland plants is 10 000 kJm−2 year−1 .
    2000 kJm−2 year−1  is used by the plants in respiration.
   The energy passed along a food chain based on this grassland is shown, in kJm−2 year−1  . 

What percentage of the energy available to the herbivore passes to carnivore 2?
  A 0.32%
  B 0.40%
  C 3.2%
  D 4.0%

10 Plots of white clover plants were inoculated with three different strains of Rhizobium bacteria, P, Q and R, and the mass of harvested clover compared with that from plots that were not inoculated and plots that were treated with nitrate fertiliser. The results are shown in the table.

What may be concluded from these results?

  1 Adding nitrate fertiliser increases the growth of white clover.
  2 Inoculating unfertilised plots of white clover with Rhizobium increases growth of the clover.
  3 Different strains of Rhizobium differ in their ability to fix nitrogen.
  A 1, 2 and 3
  B 1 and 2 only
  C 2 and 3 only

  D 3 only

Answers to Multiple choice test
1. B
2. A
3. C
4. D
5. D
6. D 
7. A
8. C
9. B
10. D

2. End-of-chapter questions
1  Nitrogenase   is an enzyme   found   in  nitrogen-fixing bacteria. What   reaction   is catalysed   by nitrogenase?
   A   the  conversion   of nitrate   ions  to  nitrite   ions
   B   the conversion    of nitrite   ions  to  nitrate   ions
   C   the conversion    of nitrogen   gas to ammonium     ions

   D  the  conversion    of nitrogen   gas to nitrate   ions

2  Which  is not a way in which energy is lost between a producer  and  a primary consumer?

  A   as chemical   energy  in  the  faeces  of the  primary   consumer
  B  as chemical   energy  in  roots  that  are not  eaten  by the  primary   consumer
  C  as heat  from  respiration    in the  cells of the  primary   consumer
  D  as heat  from  respiration    in  the  cells of the  producer

3  Draw  a flow diagram   to show  how  an  atom  of nitrogen    in  a nitrogen    molecule   in  the  air could   become   part  of a protein  molecule   in a muscle  in  a person's   arm.

Your diagram   should:

•    name  the  molecules   or  ions  in which   the  nitrogen    atom   is present,   at each  stage
•    name  the  processes   involved   in  the  conversion    of one  substance    to another,   or  in  the  passing   of a substance    from one  organism   to  another

You will need  to  use knowledge    of the  nitrogen    cycle  in your  answer.  You may  also  use knowledge    of digestion, absorption,   transport    in  mammals    and  protein   synthesis   in  cells.

4    a     Explain   what   is meant   by  the  term  community.
       b   The  figure  shows  the  How of energy through a woodland.  All figures  are in  kJ m−2 per  week.  [2]

   i   Calculate  the  energy  trapped  by the  producers and  converted to biomass, as a percentage of the  light energy   absorbed. Express your answer to the nearest 0.1 %. Show your working.  [2]                                                
 ii  Suggest, in terms of energy How, why there are no tertiary consumers in the woodland.    [2]                                    
iii  Leaf  litter  is composed  of dead leaves and twigs.
     The total energy in the leaf litter was 15 899 kJm−2 but only  153 kJm−2 per  week  was transferred to decomposers.  When   animal   wastes  rich  in nitrogen   were  mixed with   the  leaf  litter the  energy flow to decomposers    increased   significantly.

Suggest  why  the  addition    of animal   wastes  rich  in  nitrogen    increased   the  energy  How to  decomposers.       [3]

[Total:  9]

[Cambridge  International  AS  and A Level Biology 9700  Paper 22,  Question  6, November 2009]

5   Many  species  of legume   grow  in  nitrate-deficient  Soils  in  the  tropics.  Some  of these  are large  trees  such  as the flamboyant   tree,  Delonix   regia.

Bacteria  of the  genus  Rhizobium  live inside  swellings   along  the  roots  of legumes.   These  swellings   are known   as root nodules.

A student   followed   the  cycling  of nitrogen    in  an area  with  many  flamboyant    trees.

The  figure  summarises    the  flow  of nitrogen    in the  area.

 a    Name   the  processes   that  occur  at H, J and  K.  [3]
 b  Suggest  the  advantages   gained   by legumes   of having  Rhizobium living  in  their  roots. [2]

[Total:   5]

[Cambridge  International AS and A   Level Biology 9700  Paper 21,  Question  6, June  2010 ]

3. End-of-chapter answers

Cambridge International Examinations bears no responsibility for the example answers to questions taken from its past question papers which are contained in this publication. 

1 C

2 C

3 The answer could include the following points, shown in the form of a flow diagram:
 • nitrogen fixation – conversion of N2 to a named compound or compound ion by a named process     (e.g. to NH4 +  by Rhizobium in root nodules)
 • uptake (e.g. of nitrate, NO3 - ) or assimilation of NH4 + by a plant to produce amino acids
 • production of proteins in the plant on ribosomes
 • eating of plant by a person (or by another animal that will eventually provide food for a person)
 • digestion of protein in the alimentary canal by protease enzymes, followed by absorption through the walls of the small intestine as amino acids
 • transport of amino acids in solution in blood plasma
 • uptake of amino acids from blood by muscle cells (reference to diff usion through capillary wall, tissue fluid)
 • protein synthesis on ribosomes in muscle cell, involving mRNA and tRNA

Exam-style questions

4 a all the organisms of all species; living in the same place at the same time; [2]
   b i 2946 ÷ 65 000 × 100; = 4.5%;                                                                    [2]
      ii not enough energy reaching them; only 2 kJ m−2 per week;                     [2]
      iii (leaf litter is) difficult to break down; reference to cellulose/lignin;
          (leaf litter contains) little protein/nitrogen;
           lack of nitrogen is a limiting factor for decomposer activity;
          animal wastes contain urea/amino acids/ organic nitrogen sources;
          provide materials for decomposers/increase rate of decomposer activity; [max. 3]

[Total: 9]

5 a H = nitrogen fi xation;
      J = nitrifi cation/oxidation;

      K = denitrifi cation/reduction; [3]

   b provide ammonium ions;
     for use in making amino acids/proteins;
     allow plant to grow in soil that is low in nitrate ions; [max. 2]

  [Total: 5]

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