A-level Biology Paper 2

• Photosynthesis


  The process in plants from which energy from sunlight is used to convert simple molecules into complex molecules
• Light-dependent reaction


  1. Chlorophyll molecules absorb light energy in photoionisation

  2. Electrons pass through electron transfer chain, providing energy to pump hydrogen ions into inner membrane space

  3. Photolysis of water requires light energy to break bonds between oxygen and hydrogen atoms

  4. Photoionised chlorophyll electrons replaced by electrons from photolysis of water

  5. Hydrogen ions move down electrochemical gradient through ATP synthase channel, producing ATP

  6. Hydrogen ions and electrons combine with NADP, producing reducing NADP

  7. Cyclic photophosphorylation recycles electrons back to chlorophyll, producing only a small amount of ATP
• Light-independent reaction


  1. Calvin cycle depends on products from light-dependent stage

  2. Carbon dioxide fixation catalysed by RuBisCo

  3. 5 out of 6 TP molecules used to regenerate RuBP instead of producing hexose sugars

  4. TP used to produce hexose sugars, 2 TP molecules needed

  
• Factors limiting rate of photosynthesis

  • Temperature
  • Availability of carbon dioxide
  • Availability of water
  • Availability of light energy
• Law of limiting factors


  The rate of a physiological process is limited by the factor that is at its least favourable value
• Chromatography can be used to separate out photosynthetic pigments, identifying them by their Rf value

  
• Anaerobic respiration


  The process that releases energy stored in organic molecules such as glucose, occurring in living cells

  
• Glycolysis


  The first stage of respiration, occurring in the cytoplasm of cells, with a net yield of 2 pyruvate, 2 reduced NAD and 2 ATP molecules

  
• Anaerobic respiration


  1. Glycolysis continues if reduced NAD is reoxidised so NAD is available to accept a hydrogen atom again

  2. In mammals, the lactate fermentation pathway is used

  3. In plants and fungi, the ethanol fermentation pathway is used

  
• Aerobic respiration


  1. Pyruvate enters the mitochondrial matrix by active transport

  2. Link reaction occurs

  3. Krebs cycle occurs

  4. Oxidative phosphorylation occurs, with reduced NAD and FAD donating electrons to the electron transfer chain, creating a proton gradient used to synthesise ATP

  5. Oxygen combines with protons and electrons as the final electron acceptor

  
• Aerobic respiration


  Produces 32 ATP, 30 more than anaerobic respiration
• Respiratory substrates


  • Sugars
  • Lipids
  • Proteins
• Lipids as respiratory substrate

  Hydrolysed to fatty acids and glycerol, glycerol phosphorylated and converted to triose phosphate, fatty acids broken down into 2-carbon fragments and converted to acetyl CoA
• Proteins as respiratory substrate

  Hydrolysed to amino acids, amino group removed in liver and converted to urea, remaining amino acid converted to an intermediate
• Biomass


  The total mass of living material in a specific area at a given time
• Dry biomass


  Shows the chemical energy store in an organism, measured by calorimetry
• Gross primary production (GPP)


  The total quantity of chemical energy stored in plant biomass, in a given area or volume
• Net primary production (NPP)

  The chemical energy store in plant biomass after respiratory losses to the environment have been taken into account
• Net production of consumers (N)

  The total chemical energy consumers store after energy losses to faeces, urine and respiration have been taken away from the chemical energy store of the ingested plant food
• Primary and secondary productivity

  The rate of primary or secondary production, respectively, measured as biomass in a given area in a given time
• Farming practices to increase energy transfer efficiency

  • Reducing respiratory losses in human food chain
  • Simplifying food chains to reduce energy loss to non-human food chains
• Microorganisms in nutrient cycles

  • Mycorrhizae
  • Free-living nitrogen-fixing bacteria
  • Mutualistic nitrogen-fixing bacteria
  • Saprobiontic organisms
  • Nitrifying bacteria
  • Anaerobic denitrifying bacteria
• Fertilisers


  Can be used to provide plants with minerals, particularly nitrates, to support their growth
• Effects of using fertilisers

  • Reduced species diversity
  • Leaching (pollutes waterways)
  • Eutrophication
• Eutrophication


  Nitrate levels increase in rivers and lakes due to leaching, leading to increased algal growth, blocking light and reducing oxygen levels, killing other aerobic organisms
• Tropisms


  Directional growth responses in plants, where the direction of the response is determined by the direction of the external stimulus
• Indoleacetic acid (IAA)


  Growth factor that causes elongation of shoot cells and inhibits root cell elongation, causing positive geotropism and phototropism
• Taxis


  Movement of an animal towards or away from a stimulus
• Kinesis


  Change in rate of movement (turning or speed) in animals to move towards favourable conditions
• Reflexes


  Rapid responses that don't require conscious thought, using simple mechanisms and localized to the part of the body where they occur
• Sensory receptors


  Specialised cells in the nervous system that detect physical stimuli and convert them into electrical signals
• Pacinian corpuscles


  Detect changes in pressure in the skin, with pressure causing deformation of lamellae and opening of stretch-mediated sodium channels, generating a generator potential
• Photoreceptors in the retina

  • Rod cells
  • Cone cells
• Sinoatrial node (SAN)


  Sends out regular waves of electrical activity to the atrial wall, causing contraction, which is then passed to the atrioventricular node (AVN) and ventricles
• Stimuli affecting heart rate

  • High blood pressure
  • Low blood pressure
  • High blood O2, pH or low CO2
  • Low blood O2, pH or high CO2
• Action potential


  All-or-nothing response triggered when depolarisation reaches threshold, involving opening of voltage-gated sodium channels, repolarisation as potassium channels open, and hyperpolarisation as potassium ions diffuse out
• Medulla sends impulses


  1. Along parasympathetic neurones, using acetylcholine to reduce the heart rate

  2. Along sympathetic neurones, using noradrenaline to increase the heart rate
• Action Potentials


  1. Neurone receives impulse from sensory receptors, sodium channels on dendrites open, leading to movement of Na+ ions into cell causing depolarisation

  2. If depolarisation reaches threshold potential, it activates voltage-gated sodium channels causing an action potential

  3. Voltage-gated sodium ion channels close, and voltage-gated potassium channels open, causing repolarisation as K+ ions leave the cell

  4. Outward diffusion of K+ ions causes hyperpolarisation and the voltage-gated potassium channels close

  5. Sodium-potassium pump returns the cell to the resting membrane potential
• Action potentials


  • An all or nothing response because once the threshold is reached each action potential always depolarises the axon to the same voltage by voltage-gated sodium channels
  • The refractory period is the period where the axon can't be depolarised to initiate a new action potential, limiting the frequency of action potentials and ensuring they are discrete & only travel in one direction
• Resting potential


  The difference in electrical charge across the membrane while the neurone is at rest