ecosystems

• How do niches affect the abundance and distribution of organisms in a habitat?


  No two species can occupy the same niche; interspecific competition excludes one species or the niche is divided according to adaptations
• How are quadrats used for estimating population size?


  They can be places on randomly-generated grid coordinates or at intervals along a belt transect. Results are reported as either percentage cover or frequency. For slow-moving or non-motile organisms.
• What is meant by primary succession?


  The initial colonization and establishment of plant and animal life in an area that was previously devoid of life.
• Summary of primary succession:

  Pioneer species can survive harsh conditions and colonise the area, they change abiotic factors of their environment (e.g. decomposition adds nutrients to ground), over time more complex organisms survive and outcompete the initial species.
• What is a climax community?


  The final stage of succession - mature community.

  Ecosystem is stable in dynamic equilibrium over a long period of time - usually dominated by large trees or shrubs in rich soil
• How might a species alter the environment that develops during succession?


  May improve environment to make it more suitable for other species, may worsen the environment by making it less suitable for other species
• Overview of photosynthesis:

  In the light-dependent stage, light energy breaks strong covalent bonds in water and oxygen is released into the atmosphere.

  In light-independent stage (the Calvin cycle), fixation with carbon dioxide enables hydrogen to be stored as glucose.
• Where does the light-dependent reactions occur in plants?


  In the thylakoids of chloroplasts
• Where does the light-independent reactions occur in plants?


  stroma of chloroplasts
• What are the cyclic processes of the light-dependent reaction?

  photoionisation, electron transfer chain, chemiosmosis
• What are the non-cyclic processes of the light-dependent reaction?


  reduction of NADP, photolysis of water
• What is the role of light in photoionisation?


  Chlorophyll molecules absorb energy from photons of light. This 'excites' 2 electrons (raising them to a higher energy level), causing them to be released from the chlorophyll.
• What happens in the electron transfer chain (ETC)?


  Electrons released from chlorophyll move down a series of carrier proteins embedded in the thylakoid membrane and undergo a series of redox reactions, which releases energy.
• How is a proton concentration gradient established during chemiosmosis?


  some energy released from the ETC is coupled to the active transport of the Hydrogen ions (protons) from the stroma into the thylakoid space.
• How does chemisomosis produce ATP in the light-dependent stage?


  Hydrogen ions (protons) move down their concentration gradient from the thylakoid space into the stroma via the transmembrane channel protein ATP synthase.
• What does ATP synthase catalyse?


  ADP + Pi -> ATP (requires energy)
• Describe non-cyclic photophosphorylation: Uses photosystems I and II. Excited electrons enter ETC to produce ATP. NADP acts as the final electron acceptor and is reduced. Water is photolysed to release electrons to replace those lost from PS II.