Topic 5

• Ecosystem


  All the organisms living in a particular area, known as the community, as well as all the nonliving elements of that particular environment (e.g. climate, nutrients being cycled etc.)
• Community


  All of the populations of all the organisms living in a particular habitat at a particular time
• Population


  All of the organisms of a particular species living in a particular habitat at a particular time
• Habitat


  The place where an organism lives
• Factors controlling distribution and abundance of organisms in a habitat

  • Biotic (living) factors (e.g. predators, food availability, parasitism or disease)
  • Abiotic (non-living) factors (such as light, oxygen or moisture levels and temperature)
• Ecological niche


  The particular role of a species in its habitat, consisting of its biotic and abiotic interactions with the environment
• The niche concept states that only one organism can occupy each niche in a given habitat at a given time - if two or more species have a niche that overlaps, the best adapted will out-compete the others in surviving to reproduce
• Succession


  The change in species inhabiting an area over time, brought about by changes to the environment made by the organisms colonising it themselves
• Primary succession


  Occurs when an area previously devoid of life is first colonised by communities of organisms
• Secondary succession


  Occurs with existing soil that is clear of vegetation, such as after a forest fire
• Pioneer species


  • Adapted to survive in harsh conditions where other species would not survive, e.g. lichens that can penetrate rock surface and break it down, or hold together loose shifting sands
• Humus


  The organic component of soil formed from the decomposition of organisms
• Climax community


  The most productive, self-sustaining and stable community of organisms that the environment can support, usually with only one or two species
• Light-dependent reaction of photosynthesis


  1. Electrons are excited to a higher energy level using the energy trapped by chlorophyll molecules in the thylakoid membranes

  2. Electrons are passed down the electron transport chain from one electron carrier to the next, generating ATP from ADP and inorganic phosphate in a process called photophosphorylation

  3. Photolysis: light energy breaks apart the strong bonds in water molecules, forming hydrogen and hydroxide ions, with the hydroxide ions reacting to form water and oxygen
• Cyclic phosphorylation


  Photon hits chlorophyll in photosystem I, electrons are excited and taken up by an electron acceptor, passed along an electron transport chain, energy is released and ATP is synthesised, electrons return to chlorophyll in photosystem I
• Non-cyclic phosphorylation


  Photon hits chlorophyll in photosystem II, electrons are excited and taken up by an electron acceptor, passed along an electron transport chain to photosystem I, energy is released and ATP is synthesised, electrons replace lost electrons in chlorophyll of photosystem II, photon hits chlorophyll in photosystem I, electrons are excited and taken up by an electron acceptor, passed along an electron transport chain to NADP, NADP takes up an H+ ion and forms reduced NADP
• Light-independent reaction (Calvin cycle)


  1. RuBP (5 carbon compound) is combined with carbon dioxide in a reaction called carbon fixation, catalysed by RUBISCO

  2. The unstable 6 carbon intermediate formed immediately splits into two molecules of glycerate 3-phosphate (GP)

  3. Reduced NADP and ATP are used to reduce two GP molecules to GALP

  4. One in six GALP molecules are used to make glucose, which is then converted to essential organic compounds

  5. The remaining five in six TP molecules are used to reform RuBP with the help of ATP
• Net primary productivity (NPP)

  The rate at which energy from the sun is converted into the organic compounds by photosynthesis
• Chloroplasts


  The site of photosynthesis
• Chloroplasts


  • Contain stacks of thylakoid membranes, called grana, which contain the photosynthetic pigments, such as chlorophyll, arranged as photosystems. This is the site of the light-dependent stage of photosynthesis.
  • Contain stroma, which is the fluid surrounding the grana. Stroma contains all the enzymes required for the light-independent stage of photosynthesis.
  • Have a chloroplast envelope that controls movement of substances into & out of the organelle. Their double membrane supports the endosymbiotic theory.
  • Contain starch granules that store the products of photosynthesis.
• Net primary productivity (NPP)

  The rate at which energy from the sun is converted into the organic molecules that make up new plant biomass
• Gross primary productivity (GPP)


  The energy transferred to primary consumers
• NPP


  GPP - R
• Energy losses at each trophic level


  • Undigested matter
  • Respiration (exothermic, transfers thermal energy to the surroundings)
  • Metabolic waste products like urea
• Global warming


  A gradual increase in the average temperature of the Earth's atmosphere and surface
• Evidence for climate change

  • Records of carbon dioxide levels
  • Temperature records
  • Pollen in peat bogs
  • Dendrochronology
• Extrapolation


  Making predictions based on data
• Models


  Used to predict future climate change, but have limitations as they do not include factors like human efforts to reduce greenhouse gas emissions
• Greenhouse effect


  Short-wavelength UV radiation passes through Earth's atmosphere and is reflected from Earth's surface. The reflected rays are of a longer wavelength (infrared radiation) and are trapped by gases such as carbon dioxide and methane, leading to an increase in the temperature of the Earth's surface and atmosphere.
• Effects of climate change

  • Changes in distribution of species
  • Changes to development (e.g. sex ratio)
  • Disrupted life cycles
• As temperature increases


  The rate of enzyme-catalysed reactions initially increases, but then decreases above the optimum temperature as enzymes become denatured
• Carbon cycle


  Knowledge of the carbon cycle can help humans make decisions to reduce the levels of greenhouse gases in the atmosphere
• Ways to reduce global warming

  • Growing plants to use as biofuels, which are carbon neutral
  • Reforestation, to increase the rate at which carbon dioxide is removed from the atmosphere by plants
• Scientific consensus


  Ideas are validated and conclusions are drawn via scientific conferences and publication in peer-reviewed scientific journals
• Evolution


  Change in the heritable traits of biological populations over successive generations, occurring as a result of change in allele frequency which is affected by changing selection pressures
• Evolution via natural selection

  1. A variety of phenotypes exist within a population due to random mutation

  2. An environmental change occurs and the selection pressure changes

  3. Some individuals possess advantageous alleles which give them a selective advantage and allow them to survive and reproduce

  4. The advantageous alleles are passed on to their offspring

  5. Over time, the frequency of alleles in a population changes
• Reproductive isolation


  When gene flow between two populations is reduced or stopped, accumulation of genetic differences can lead to new species being formed
• Speciation


  When two populations can no longer interbreed to produce fertile offspring, they are said to be a separate species
• Types of speciation


  • Allopatric (where the isolation is geographic)
  • Sympatric (isolation caused by non-geographic factors e.g. temporal, behavioural, gametic etc.)