Topic 5 - Biology Paper 2

• Photosynthesis

  Reaction in which light energy is used to produce glucose in plants
• Photosynthesis

  1. Requires water and carbon dioxide

  2. Products are glucose and oxygen

  3. Two stages: light dependent and light independent

  4. Rate determined by carbon dioxide concentration, light intensity, and temperature
• Chloroplasts

  • Site of photosynthesis
  • Contains stacks of thylakoid membranes called grana which provides a large surface area for the attachment of chlorophyll, electrons and enzymes
  • Network of proteins in the grana hold the chlorophyll in a specific manner to absorb the maximum amount of light
  • Granal membrane has ATP synthase channels embedded allowing ATP to be synthesised as well as being selectively permeable allowing the establishment of a proton gradient
  • Contain DNA and ribosomes allowing them to synthesise proteins needed in the light dependent reaction
• Light Dependent Reaction

  1. Photons of light hit chlorophyll molecules in PSII causing the electrons to become excited

  2. Photolysis: splitting of water with light, producing oxygen, protons and electrons

  3. Excited electron moves down a series of protein complexes, pumping protons from the stroma to the thylakoid space

  4. Electron moves to PSI, absorbing more photons, then combines with a proton to reduce NADP

  5. Proton gradient drives photophosphorylation, producing ATP
• Light Independent Reaction

  1. Carbon dioxide fixation: carbon dioxide is fixed with ribulose bisphosphate (RuBP) by Rubisco, forming 2 molecules of glycerate-3-phosphate

  2. Reduction phase: glycerate-3-phosphate is reduced using the reducing power of reduced NADP and energy from ATP, forming 2 molecules of triose phosphate

  3. Regeneration of RuBP: 5 triose phosphate molecules are used to regenerate 3 RuBP molecules

  4. Organic molecule production: 2 triose phosphate molecules can combine to form fructose 1,6 bisphosphate, then glucose
• 6 turns of the Calvin Cycle are required to produce 1 molecule of glucose per molecule of CO2
• Aerobic respiration

  Splitting of a respiratory substrate to release carbon dioxide as a waste product, with hydrogen reunited with atmospheric oxygen and a large amount of energy released
• Anaerobic respiration

  Respiration that occurs in the absence of oxygen
• Respiration

  1. Glycolysis: glucose is phosphorylated to produce 2 pyruvate, 2 ATP, and 2 NADH

  2. Link reaction: pyruvate is transported into mitochondria, decarboxylated and combined with coenzyme A to form acetyl coenzyme A

  3. Krebs cycle: acetyl coenzyme A enters the Krebs cycle, producing 2 ATP, 6 NADH, 2 FADH, and 4 CO2

  4. Oxidative phosphorylation: NADH and FADH from Krebs cycle pass electrons down electron transport chain, pumping protons to create proton gradient that drives ATP synthase

  
• Ecosystem

  Includes all the organisms living in a particular area (community) and all the non-living elements of that environment
• Ecosystem

  • Distribution and abundance of organisms controlled by biotic (living) and abiotic (non-living) factors
  • Each species has a particular role (niche) consisting of its biotic and abiotic interactions with the environment
• Autotrophs

  Organisms that can synthesize their own food, using energy from the sun
• Heterotrophs

  Organisms that cannot synthesize their own food
• Only around 10% of chemical food energy is passed on between organisms in the food chain, the other 90% is lost to the surroundings
• Percentage efficiency of energy transfer

  Calculated as: (Energy content of consumers / Energy content of producers) x 100
• Biomass

  Measured in terms of mass of carbon or dry mass of tissue per given area per given time
• Calorimetry

  Measuring the energy content of a sample by burning it in a bomb calorimeter and measuring the change in water temperature
• Net primary productivity (NPP)

  Rate at which energy is transferred into the organic molecules that make up new plant biomass, after respiratory losses
• Gross primary productivity (GPP)


  Rate at which energy is incorporated into organic molecules in plants through photosynthesis

  
• NPP = GPP - R
• Net production of consumers (N)

  Calculated as: N = I - (F+R), where I is chemical energy in ingested food, F is chemical energy lost in faeces and urine, and R is respiratory losses
• Nitrogen Cycle

  1. Ammonification: microbes break down organic matter to ammonia

  2. Nitrification: nitrifying bacteria convert ammonia to nitrite, then nitrate

  3. Denitrification: denitrifying bacteria convert nitrate to nitrogen gas

  4. Nitrogen fixation: nitrogen-fixing bacteria convert nitrogen gas to ammonia
• Phosphorus Cycle

  1. Phosphate released from weathering of rocks and decay of bones, shells, and excreta

  2. Plants take up phosphate ions from soil

  3. Mycorrhizae facilitate uptake of water and inorganic ions by plants
• Nitrogen fertilisers increase crop yields but have negative environmental effects like reducing biodiversity, leaching, and eutrophication
• Leaching

  Process by which mineral ions like nitrate dissolve in rainwater and are carried from soil to rivers and lakes
• Eutrophication

  Rapid growth of algae in waterways due to excess nutrients, blocking light and causing decay and oxygen depletion