photosynthesis

• photosynthesis: carbon dioxide + water -> glucose + oxygen

  • light energy is absorbed by chlorophyll - a green substance found in chloroplasts in the palisade cells in the leaf
  • absorbed light energy is used to convert carbon dioxide (from the air) and water (from the soil) into a sugar called glucose
  • oxygen is released as a by-product
• • plant life depend on it as a source of food
  • all animals rely on plants for food, directly or indirectly
  • produces oxygen which is necessary for respiration
  • early plant life added oxygen to atmosphere
• • glucose is used as:
  • transported to the growing parts of the plant for use in respiration
  • transformed into cellulose, proteins and oils
  • turned into starch for storage
• needed for photosynthesis

  • carbon dioxide
  • oxygen - in the day plants make more oxygen than they need for respiration, at night photosynthesis stops and they need to obtain oxygen from the air
  • water - needed for photosynthesis and other living processes
  • minerals - needed for a range of processes, nitrates are needed to make proteins from glucose made by photosynthesis
  • light energy from the sun
• testing for starch:

  • leaf is boiled to kill the cells and melt waxy covering so liquids can soak into the leaf
  • leaf is placed in boiling ethanol for discoloration so that the color change can been seen
  • leaf is washed in warm water to soften in and remove ethanol
  • iodine solution is added a positive test turns blue/black
• The leaf on the right shows a positive test for starch in the areas which contained chlorophyll, and a negative test for starch in the areas which lack chlorophyll.

  This is evidence that chlorophyll is required for photosynthesis.

  variegated leaves

  
• factors limiting photosynthesis: one factor is more important than the other in setting the rate of photosynthesis

  • light intensity - increasing this increases the rate of photosynthesis providing plenty of carbon dioxide and water are available, maximum rate is determined by other factors (at night)
  • temperature - rate increases as temperature rises, enzymes denatured if temp is too high, has an optimum temperature (cold winter days)
  • carbon dioxide - increasing concentration increases rate provided plant is warm enough and has light and water (limiting whenever light and temperature is not)
• • sodium hydroxide removes carbon dioxide from the air so acts as a control
  • sodium bicarbonate is used as it releases carbon dioxide
  • a de-starched plant can be established by leaving it in darkness for 48 hours with water, important to know starch was formed during experiment not before