topic 4

• Photosynthesis


  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

  
• Limiting factor


  Restrict rate of photosynthesis (stop from increasing) if in low levels

  
• How temperature affects rate of photosynthesis

  1. As temperature increases, rate increases (so temperature =limiting factor)

  2. Above an optimum temperature, rate decreases

  
• How light intensity affects rate of photosynthesis

  1. As light intensity increases, rate increases (so light intensity = limiting factor)

  2. Above a certain light intensity, rate stops increasing

  
• How CO2 concentration affects rate of photosynthesis

  1. As CO2 conc. increases, rate increases (so CO2 conc. = limiting factor)

  2. Above a certain CO2 conc., rate stops increasing

  
• 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

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

  
• Water is not considered a limiting factor 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

  
• Respiration (releases CO2) occurs

  No photosynthesis as no light

  
• Photosynthesis occurs as there is light


  Photosynthesis rate greater than respiration rate

  
• Optimum temperature for maximum rate of photosynthesis is 21.5 / 22 oC

  
• Investigating the effect of light intensity on rate of photosynthesis

  1. Change distance of white light source from pondweed

  2. Control temperature, CO2 supply, pondweed

  3. Measure volume of O2 released / number of O2 bubbles produced per minute

  
• 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, pondweed
• Sodium hydrogen carbonate solution is used to provide carbon dioxide so that it is not a limiting factor
• Counting bubbles is not an accurate way of measuring volume of oxygen produced
• LED light source does not emit a lot of infrared / thermal radiation, so temperature is controlled
• Modifying the experiment to investigate other factors

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

  2. Temperature - different temperature water baths

  3. Wavelength - different coloured filters over bulb

  
• Uses of glucose produced in photosynthesis

  • Used for respiration
  • 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

  
• Leaf left in a cupboard with no light for 2 days

  Starch has been converted to glucose, so glucose can be used for respiration / to release energy

  
• TMV causes plants to produce less chlorophyll


  Lack of chlorophyll reduces photosynthesis, so less glucose made, less energy released for growth, less amino acids / proteins / cellulose produced
• Cellulose


  Strengthens cell wall
• Used to produce amino acids

  For protein synthesis
• Combined with nitrate ions from soil


  To form amino acids
• Glucose is used for growth, but this is too vague. Use the acronym 'SCARF' to remember the specific uses of glucose - starch, cellulose, amino acids, respiration, fats (and oils)

  
• Cellular respiration


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

  
• Energy released in respiration

  Used for 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


  Occurs in cytoplasm

  
• Economic importance of anaerobic respiration in yeast

  Fermentation used in production of bread (CO2 makes dough rise)<|>Fermentation used in production of alcoholic drinks (ethanol is a type of alcohol)
• Respiration releases, or transfers energy. Energy can't be created.

  
• Respiration is a series of chemical reactions in cells to release energy, not just breathing.

  
• Respiration happens in every cell, including plant cells.