Aqa - a level biology paper 2

• Photosynthesis is a two-stage reaction with light dependent and light independent reactions
• Chloroplast


  • Thylakoid stacks and membrane where light dependent reactions occur
  • Stroma where light independent reactions occur
  • Inner and outer membranes
• Light dependent reactions


  1. Photolysis

  2. Photo ionization of chlorophyll

  3. Chemiosmosis

  4. Production of ATP and reduced NADP
• Photolysis


  Light energy splits water into oxygen, electrons and protons
• Photo ionization of chlorophyll

  Light energy excites chlorophyll electrons, causing them to be released
• Chemiosmosis


  1. Electrons move along electron transport chain, releasing energy to pump protons into thylakoid lumen

  2. Protons flow back through ATP synthase, generating ATP

  3. NADP picks up electrons and protons to form NADPH
• Light independent reactions (Calvin cycle)

  1. Carbon dioxide reacts with RuBP

  2. GP is reduced using ATP and NADPH

  3. GP is used to regenerate RuBP or form hexose sugars
• Limiting factors


  Anything that reduces the rate of photosynthesis, e.g. light intensity, CO2 concentration, temperature
• Aerobic respiration


  1. Glycolysis in cytoplasm

  2. Link reaction in mitochondrial matrix

  3. Krebs cycle in mitochondrial matrix

  4. Oxidative phosphorylation in mitochondrial membrane
• Glycolysis


  Glucose is phosphorylated, converted to pyruvate, producing some ATP and NADH
• Link reaction


  Pyruvate is oxidized to acetyl-CoA, producing CO2 and more NADH
• Krebs cycle


  Acetyl-CoA combines with 4-carbon molecule, undergoing redox reactions to produce ATP, NADH and FADH2
• Oxidative phosphorylation


  NADH and FADH2 donate electrons to electron transport chain, generating proton gradient used by ATP synthase to produce ATP
• Anaerobic respiration


  Glycolysis occurs, pyruvate is reduced to lactate or ethanol to regenerate NAD and allow glycolysis to continue
• Food web


  Begins with producers (green plants) that photosynthesize, energy is lost between trophic levels due to respiration and excretion, remaining energy forms biomass
• NPP and GPP


  Measures of ecosystem productivity, dependent on abiotic and biotic factors
• ATP


  Molecule produced for metabolic reactions to continue
• The next bit in this topic is looking at how energy is transferred in an ecosystem
• Food webs begin with producers which are green plants because they can photosynthesize to use the light energy to make their own organic compounds
• Trophic level


  Each stage in a food web
• Between each trophic level the majority of energy is lost due to respiration and excretion
• Biomass


  The mass of carbon within an organism or the dry mass
• GPP (Gross Primary Production)


  The chemical energy stored in the plant biomass in a given area or volume, the total energy resulting from photosynthesis
• NPP (Net Primary Production)

  The GPP minus the energy lost by respiration, the energy left to go towards making the biomass
• Calculating net production in consumers

  1. I = Chemical energy stored in ingested food

  2. F = Chemical energy lost to the environment in feces and urine

  3. R = Respiratory losses

  4. N = I - F + R (Net production of biomass)
• Productivity units


  Kilojoules per hectare per year
• The units of productivity include per unit area to standardize results for comparison, and per year to account for seasonal impacts
• Nitrogen is essential as it is within proteins, ATP, and nucleic acids
• Nitrogen cycle


  1. Nitrogen fixation

  2. Nitrification

  3. Denitrification
• Saprobiotic nutrition


  Microbes feeding on dead plant matter or waste
• Denitrification is the only anaerobic stage in the nitrogen cycle
• Phosphorus is essential as it is in phosphate groups of DNA, RNA, ATP, and phospholipids
• Phosphorus cycle


  1. Absorption of phosphate ions by plants

  2. Consumption by animals

  3. Excretion

  4. Decomposition

  5. Sedimentation

  6. Erosion of rocks
• Fertilizers are added to replace minerals and nutrients removed when crops are harvested
• Natural fertilizers


  Animal manure, cheaper but less controlled composition
• Artificial fertilizers


  Synthetic chemicals with exact mineral proportions, but more soluble and prone to leaching
• Leaching of fertilizers into waterways can cause eutrophication
• Stimulus


  A detectable change in the environment
• Receptor


  Cells that can detect changes in the environment
• Tropism


  A plant's response to its surroundings via growth