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Population and ecosystem

Populations and ecosystems

·       A population is all the organisms of one species in a habitat at any given time. 

·       Populations of different species form a community. 

·       Within a habitat a species occupies a niche governed by adaptation to both biotic (living) and abiotic (non-living) conditions.

·       Niche – where an organism is found and what it does there ie  abiotic factors in it’s habitat and biotic factors affecting it.  No two species can occupy exactly the same niche in a community.

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Investigating populations

Investigating populations


·       Large sample size to increase reliability and reduce effects of chance

·       Random sampling with quadrats  - prevent bias

Grid and random numbers from a table to determine co-ordinates

·       Counting along transects ( a line through the study area)  for systematic data.

·       The use of the following as measures of abundance:

percentage cover - proportion of ground covered by a species using a frame quadrat frequency – number of sampling units (quadrats) species found in

population density – number of individuals per square metre

·       mark–release–recapture for more mobile species.

Population =   no of individuals in 1st sample x no of individuals in 2nd sample

                                        Number of marked individuals recaptured

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mark and release method

Assumptions made

o   no immigration into or emigration

o   few deaths and ‘births’

o   The marked animals mix fully with the unmarked animals

o   The marked animals are no more or less likely to die than the unmarked ones

o   The mark or label is not lost or washed off during the investigation

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Variation in popn size

Variation in population size

Population size may vary as a result of

·       the effect of abiotic factors

·       interactions between organisms:

o   interspecific and intraspecific competition

o   predation – predator prey cycles

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3.4.3   In photosynthesis, energy is transferred to ATP in the light-dependent reaction and the ATP is utilised in the light-independent reaction.


Light-dependent reaction of photosynthesis

·       light energy excites electrons in chlorophyll

·       energy from these excited electrons generates ATP and reduced NADP

·       excited electrons leave the chlorophyll and pass along electron carriers in the electron transfer chain in the chloroplast membranes

·       Energy released as electrons pass along electron transfer chain is used to produce ATP from ADP and phosphate

·       Energy from excited electrons used in the photolysis of water, which produces protons, electrons and oxygen.

·       The protons and electrons reduce NADP to reduced NADP

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Light dependent reaction

Light-independent reaction

·       carbon dioxide is accepted by ribulose bisphosphate (RuBP) to form two molecules of glycerate 3-phosphate (GP)

·       ATP and reduced NADP are required for the reduction of GP to triose phosphate

·       RuBP is regenerated in the Calvin cycle

·       1/6 of the triose phosphate formed is converted to useful organic substances eg glucose, pyruvate, starch

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Limiting factors

Limiting factors

·       A factor that prevents photosynthesis taking place at a faster rate is a limiting factor.

·       If the limiting factor is increased then photosynthesis will take place at a faster rate until the factor becomes non-limiting; then the rate will remain constant as some other factor is now limiting the rate.

·       Increasing a non-limiting factor has no effect on rate of photosynthesis

·       Factors limiting the rate of photosynthesis can be temperature, carbon dioxide concentration and light intensity.

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Limiting factors

·       Knowledge of limiting factors can enable growers to increase the yield of crops grown in in commercial glasshouses.  The cost of maintaining optimum temperature, light intensity and carbon dioxide concentration need to be outweighed by the greater income from the crop. 

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Aerobic respiration

3.4.4  In respiration, glycolysis takes place in the cytoplasm and the remaining steps in the mitochondria. ATP synthesis is associated with the electron transfer chain in the

membranes of mitochondria.


Aerobic respiration

·       Glycolysis takes place in the cytoplasm and involves the oxidation of glucose to pyruvate with a net gain of ATP and reduced NAD

·       Pyruvate combines with coenzyme A in the link reaction to produce acetylcoenzyme A.

·       Acetylcoenzyme A is effectively a two carbon molecule that combines with a four carbon molecule to produce a six carbon molecule which enters the Krebs cycle

·       In a series of oxidation-reduction reactions the Krebs cycle generates reduced coenzymes and ATP by substrate-level phosphorylation, and carbon dioxide is lost

·       Synthesis of ATP by oxidative phosphorylation is associated with the transfer of electrons down the electron transport chain and passage of protons across mitochondrial membranes.

·       Oxygen is the final acceptor of electrons and protons to produce water

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Anerobic respiration


Anaerobic respiration

Glycolysis followed by the production of ethanol or lactate and the regeneration of NAD in anaerobic respiration.

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Energy transfere

Energy transfer 

·       Photosynthesis is the main route by which energy enters an ecosystem.

·       Less than 10% (generally about 2%) of available solar energy is incorporated into carbon compounds by photosynthesis.

o   A large proportion of solar energy does not reach the producers.  It is reflected by clouds, dust; heats the earths surface; evaporates water

o   Of the solar energy that does reach the producers only a small proportion is converted into plant tissue.  The rest is reflected from the producers surface; the wrong wavelength; converted to heat / evaporation

·       Energy is transferred through the trophic levels in food chains and food webs and is dissipated.

·       About 10% of the energy consumed by the primary consumers is used to produce new organic material  (production) The rest of the energy consumed is lost from the primary consumer through:

o   Respiration

o   Energy in faeces

o   Energy in urine

·       Pyramids of numbers, biomass and energy represent the quantity of each of these at each trophic level – see notes for details

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Energy and food production

Energy and food production

·       Primary productivity is the amount of energy per unit area per unit time that is transferred from sunlight to chemical energy in plants.

·       Only about 10% of the available energy is transferred

·       Gross primary productivity is the total amount of energy fixed into organic molecules by photosynthesis

·       Net productivity = Gross productivity – Respiratory loss

·       The ways in which productivity is affected by farming practices that increase the efficiency of energy conversion include

o   the use of natural and artificial fertilizers:

§  fertilsers used to ensure that productivity is not limited by lack of inorganic ions such as nitrate.  Natural fertilizers such as farmyard manure improve soil structure but their ion content is not known.

o   the use of chemical pesticides, biological agents and integrated systems in controlling pests on agricultural crops

§  chemical pesticides may kill useful insects as well as pests; build up in food chains

§  Biological agents including predators are more specific and once introduced does not need further reintroductions but may become a pest themselves

§  Integrated control uses a carefully worked out combination of biological control and pesticides to control the pest.

o   intensive rearing of domestic livestock can involve restricting movement so less energy is used in muscle contraction, environment kept warm to reduce heat loss so more energy can go to growth and feeding controlled so animals receive the optimum amounts.

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Nirtogen cycle


Nutrient cycles 

Microorganisms have important roles in the carbon and nitrogen cycles

Nitrogen cycle:

  • Nitrogen fixing - Nitrogen fixing bacteria convert nitrogen gas to ammonium ions
  • Nitrification – nitrifying bacteria oxidise ammonium ions to nitrite then nitrate ions
  • Plants absorb nitrate ions through their roots and produce amino acids, then proteins.
  • Animals obtain their nitrogen (amino acids) by feeding on plants
  • Ammonification - Saprobionts (decomposers) break down dead plants and animals and animal urine and faeces by extracellular digestion.  They release ammonia into the soil.
  • Denitrification - In water logged soil with little oxygen denitrifying bacteria use nitrate ions in respiration, which produces nitrogen gas.
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Carbon cycle and carbon

Carbon Cycle

  •  Producers take in carbon dioxide in photosynthesis

·       Carbon dioxide is released as a result of respiration by bacteria, by producers and by primary and secondary consumers.


§  Respiration, photosynthesis and human activity (burning fossil fuels and deforestation) is important in giving rise to short-term fluctuation and long-term change in global carbon dioxide concentration.

§  Carbon dioxide and methane are greenhouse gases, which help prevent long wave radiation from leaving earth.   The concentration of these is increasing which has led to an increased greenhouse effect, bringing about global warming.

§  Increased carbon dioxide concentrations may result in increased rates of photosynthesis therefore productivity and yield of crop plants.  However, in some parts of the world increased temperature may reduce yield or prevent certain crops being grown.

§  Warmer winters and longer summers may affect the life cycles of insects and increase populations.  Therefore insect pests may become a greater threat to health of humans and other animals and decrease crop yield

§  The distribution and numbers of wild animals and plants may change as temperatures increase and some may become extinct.

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The environmental issues arising from the use of fertilizers include leaching and eutrophication:

§  Fertilisers drains into rivers and lakes increasing the level of nitrates, phosphates and other nutrients

§  There is a bloom of algae, blocking out light and killing native aquatic plants

§  The nutrients run out and the algae diesBacteria decompose the algae. 

§  The respiration of these aerobic bacteria removes most of the available oxygen from the water

§  Native aquatic animals die from lack of oxygen

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3.4.7  Ecosystems are dynamic systems, usually moving from colonisation to climax

communities in the process of succession.


§  Succession is from pioneer species to climax community.

§  At each stage in succession, certain species may be recognised which change the environment so that it becomes more suitable for other species.

§  The changes in the abiotic environment result in a less hostile environment and changing diversity.

§  Conservation of habitats frequently involves management of succession.

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