Photosynthesis

• green plants make their own food using light energy from the sun, they use this energy to make glucose
• contain chloroplasts that contain a green pigment called chlorophyll that absorbs sunlight
• raw materials of photosynthesis:

  • carbon dioxide, diffuses into leaves from air
  • water, absorbed by the roots from the soil
• photosynthesis is an endothermic reaction, absorbs heat
• carbon dioxide + water -> glucose + oxygen
• 6CO2 + 6H2O -> C6H12O6 + 6O2
• Plants store glucose as starch
• to de-starch a plant leave in the dark for 48 hours
• test for starch:

  1. dip leaf in boiling water (kill it + stop reactions)
  2. turn off bunsen burner (ethanol is flammable)
  3. put leaf in test tube of ethanol and stand it in a beaker of boiling water for 10 minutes (kill chlorophyll)
  4. wash leaf in hot water (soften it)
  5. spread leaf on a white tile and cover with iodine solution (to stain any starch present)
• if starch is present iodine goes yellow-brown to blue-black
• a variegated leaf has both white and green parts, the white parts don't contain chlorophyll
• sodium hydroxide absorbs CO2
• limiting factors of photosynthesis are light intensity and wavelength, temperature and CO2 concentration
• more light means more photosynthesis, artificial red or blue lights
• CO2 pumped into greenhouses/ burners, cost of pumping low compared to increased profit, burners can also provide heat
• temperature increase of 10 degrees can cause doubled photosynthesis rates but over 40 degrees can denature a plant and it dies, costs money to heat
• fertilisers cost money, ensure not raining when applied
• to maximum crop yield, limiting effects minimised , consider if cost is more or less than expected value of crop for maximum profit
• spongy mesophyll layer, more rounded cells with lots of air spaces between them
• upper epidermis, single layer of cells with no chloroplasts and light goes straight through
• lower epidermis, no thick cuticle, has lots of tiny holes called stomata that allow gases to diffuse in and out
• palisade mesophyll layer, contains lots of chloroplasts, where most photosynthesis happens
• cuticle, waterproof layer that also cuts down water lost by evaporation
• in order to photosynthesise efficiently the leaf has to adapt for light absorption and gas exchange
• leaf adapted for light absorption:

  • leaves are thin so there is a short distance from top to bottom to allow all cells to receive light
  • waxy cuticle is transparent and thin to allow light through
  • upper epidermis is transparent, no chloroplasts
  • have lots of chloroplasts in palisade layer that contain chlorophyll to absorb sunlight
• leaf adaption for gas exchange:

  • air spaces, in spongy mesophyll which allows CO2 to enter and oxygen to leave the photosynthesising cells
  • stomata (pores), allow CO2 to enter and oxygen to leave the leaf, each stomata is surrounded by two guard cells that regulate the opening and closing of the stomata
• epidermis is thin and transparent to allow more light to reach the palisade cells
• thin cuticle made of wax is waterproof to reduce water loss and transparent to allow light through
• palisade cell layer at top of leaf is tightly packed with many chloroplasts to absorb more light
• spongy mesophyll layer has few chloroplasts and a large surface area for gas exchange
• palisade cells contain many chloroplasts to absorb all available light
• intercellular spaces are air spaces that allow CO2 to diffuse through the lead and increase the surface area
• guard cells and stomata allow gases to diffuse in and out of the leaf
• glucose made in photosynthesis is converted into:

  • starch for energy storage
  • cellulose for cell walls
  • amino acids to make proteins for growth
  • lipids used as an energy store
  • respiration to release energy
• for respiration plants need to take in oxygen and give out CO2, respiration takes place all the time
• for photosynthesis plants need to take in CO2 and give out oxygen, photosynthesis only takes place in the light
• bicarbonate indicator solution gives an indication of changes in concentration of CO2 present in a solution
• more CO2 bicarbonate indicator goes yellow

  normal CO2 bicarbonate indicator goes red

  less CO2 bicarbonate indicator goes purple
• if there is an increase in CO2 concentration the plant must be respiring more than photosynthesising (yellow/night)
• if there is no change in CO2 concentration the respiration and photosynthesis is equal (red/dawn-dusk)