Biology

• The two essential reactions for life on Earth: photosynthesis and respiration

  
• Photosynthetic reaction


  1. Light energy is absorbed

  2. By chlorophyll (pigment) in chloroplasts in plant / algal cells

  3. This energy is used to convert carbon dioxide and water into glucose (sugar) and oxygen (by-product)

  
• Photosynthesis


  Endothermic reaction - energy transferred from environment to chloroplast by light

  
• Photosynthesis absorbs light energy and uses it to make glucose

  
• Oxygen is made in photosynthesis, but it is only a by-product. The aim of photosynthesis is to make glucose

  
• Rate of photosynthesis


  1. Limiting factor restricts rate of photosynthesis (stop from increasing) if in low levels

  2. As temperature increases, rate increases (as enzymes controlling photosynthesis gain kinetic energy)

  3. Above an optimum temperature, rate decreases (as enzymes denature)

  4. As light intensity increases, rate increases (as light provides energy needed for photosynthesis)

  5. Above a certain light intensity, rate stops increasing (another factor is limiting)

  6. As CO2 conc. increases, rate increases (as CO2 is a reactant / raw material needed for photosynthesis)

  7. Above a certain CO2 conc., rate stops increasing (another factor is limiting)

  
• Chlorophyll


  Affects how much light energy can be absorbed
• Inverse square law describes the relationship between distance of a light source from a plant and light intensity

  
• Interpreting graphs of photosynthesis rate showing two or three factors, to identify which is the limiting factor


  If increasing a particular factor increases rate of photosynthesis, that factor is the limiting factor

  
• Removing limiting factors


  Gain maximum rate of photosynthesis, increase plant yield
• Water is not considered a limiting factor in photosynthesis as the volume needed is very small and water shortages affect other processes in the plant before affecting photosynthesis

  
• Factors that can limit rate of photosynthesis


  • Carbon dioxide concentration
  • Temperature
  • Light intensity

  
• Concentration of CO2 in greenhouse air


  • Increases between X & Y (respiration releases CO2, no photosynthesis)
  • Decreases between Y & Z (photosynthesis rate greater than respiration rate)

  
• Optimum temperature for greenhouse lettuce growth


  21.5 / 22 oC - maximum rate of photosynthesis, most economical / cheapest heating
• Required practical 6: Investigate the effect of light intensity on the rate of photosynthesis using an aquatic organism


  1. Vary light intensity by changing distance of white light source from pondweed

  2. Control temperature, CO2 concentration, pondweed type/length

  3. Measure photosynthesis rate by counting oxygen bubbles or collecting gas in syringe

  
• Independent variable


  Distance of pondweed from light source (light intensity)

  
• Dependent variable


  Volume of O2 released / number of O2 bubbles produced per minute (photosynthesis rate)
• Control variables


  Temperature, CO2 supply / concentration, pondweed type/length, colour of light
• Sodium hydrogen carbonate solution provides carbon dioxide so it is not a limiting factor
• Counting bubbles is not an accurate way of measuring volume of oxygen produced as bubbles are different sizes and can be easy to miss
• LED light source does not emit a lot of infrared / thermal radiation, so temperature is controlled
• Modifying the experiment to investigate the effect of CO2 / temperature / wavelength of light on the rate of photosynthesis


  1. Carbon dioxide - use different concentrations of sodium hydrogencarbonate solution

  2. Temperature - different temperature water baths

  3. Wavelength - different coloured filters over bulb

  4. Control distance from light source, type / length of pondweed

  
• Uses of glucose produced in photosynthesis


  • Used for respiration to release energy
  • Converted into insoluble starch for storage
  • Used to produce fat / oil (lipids) for energy storage
  • Used to produce cellulose to strengthen cell wall
  • Used to produce amino acids for protein synthesis

  
• Glucose is not used directly for growth, but for the specific uses of starch, cellulose, amino acids, respiration, and fats/oils
• Leaf left in cupboard with no light for 2 days


  Contained glucose but did not contain starch (starch has been converted to glucose for respiration)
• Plants with less chlorophyll


  Produce less glucose by photosynthesis, so less energy available for growth
• Uses of glucose produced in photosynthesis


  • Used for respiration to release energy
  • Converted into insoluble starch for storage
  • Used to produce fat/oil (lipids) for energy storage
  • Used to produce cellulose to strengthen cell wall
  • Used to produce amino acids for protein synthesis

  
• Glucose is combined with nitrate ions from soil to form amino acids
• Misconception: "Glucose is used for growth"
• Cellular respiration


  • A series of chemical reactions that release/transfer energy from glucose
  • Occurs continuously in all living cells
  • Exothermic reaction

  
• How energy released in respiration is used


  • Chemical reactions to build larger molecules (eg. metabolism)
  • Keeping warm (birds and mammals only)
  • Movement eg. muscle contraction in animals
• Aerobic respiration


  Occurs in mitochondria

  
• Anaerobic respiration in animals, plants & yeast


  Occurs in cytoplasm

  
• Economic importance of anaerobic respiration in yeast


  Fermentation is used in production of bread (CO2 makes dough rise) and alcoholic drinks (ethanol is a type of alcohol)
• Misconception: "Respiration makes/creates energy"
• Misconception: "Respiration is breathing"
• Misconception: "Respiration only occurs in animal cells"
• How the body reacts to increased energy demand during exercise


  1. Increased heart rate -> increased blood flow to muscles/lungs

  2. Increased breathing rate and volume -> more air/oxygen enters lungs

  3. To supply more blood containing more oxygen (and glucose) to muscles

  4. For more aerobic respiration to release energy/to reduce anaerobic respiration & lactic acid build up
• What happens when muscles are supplied with insufficient oxygen


  • Anaerobic respiration -> oxidation of glucose is incomplete
  • Causing a buildup of lactic acid which causes oxygen debt and muscles become fatigued and stop contracting efficiently (if activity long/vigorous)
• Oxygen debt


  Amount of extra oxygen the body needs after exercise to react with lactic acid and remove it from cells