photosynthesis and plants

• The balanced chemical equation for photosynthesis is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
• Photosynthesis is an endothermic reaction as it requires (light) energy to work
• The word equation for photosynthesis is: carbon dioxide + water → glucose + oxygen
• Glucose produced during photosynthesis is usually converted into starch for storage
• Starch test steps:

  • Put the leaf in boiling water
  • Boil the leaf in ethanol (alcohol) in a water bath
  • Dip the leaf in boiling water again
  • Spread the leaf on a white tile and add iodine
• Before investigating photosynthesis, it's essential to destarch the plant by leaving it in darkness for 48 hours to ensure any starch produced is only during the investigation
• If starch is present, the iodine will turn from yellow-brown to blue-black in the starch test
• Hydrogencarbonate (bicarbonate) indicator is red in normal atmospheric (0.04%) carbon dioxide levels, turns yellow in increased levels, and purple in decreased levels
• To find the amount of light required to reach the compensation point in pondweed using hydrogencarbonate indicator:

  1. Add a section of pondweed to hydrogencarbonate indicator in a boiling tube.

  2. Add a bung.

  3. Leave the pondweed for 20-30 minutes in a particular light intensity and check the color of the hydrogencarbonate indicator.

  4. Increase or decrease the light intensity and repeat step 3.

  5. Repeat step 4 until the indicator remains red
• The compensation point is the point at which the rates of photosynthesis and respiration are equal
• Leaves are highly adapted for light absorption and gas exchange:

  • Adaptations for light absorption:
  • Large surface area
  • Thin, transparent cuticle
  • Many tightly packed palisade mesophyll cells, end-on to the upper surface, with chloroplasts rich in chlorophyll
  • Adaptations for gas exchange:
  • Spongy mesophyll cells have a large surface area for gas exchange
  • Intercellular spaces in the spongy mesophyll allow carbon dioxide to enter and oxygen to leave the photosynthesising cells, mainly concentrated in the palisade layer
  • Stomata allow carbon dioxide and oxygen to enter the leaf; guard cells can open and close the stomatal pore - in many plants, stomata are open during the day and closed at night