Topic 6: Plant Structures

• Photosynthetic organisms (green plants/algae) use light energy to make glucose which can be used to form larger complex molecules for growth
• Photosynthetic organisms are main producers as the energy stored in their biomass typically supplied the entire food chain

  carbon dioxide + water --light-> glucose + oxygen
• Photosynthesis is an endothermic reaction (absorbs energy) that happens inside chloroplasts - chlorophyll absorbs light energy
• The rate of photosynthesis is affected by light intensity, temperature and CO2 concentration meaning they can be limiting factors
• Light Intensity - transfers energy for photosynthesis

  • begins directly proportional and increases steadily
  • graph flattens out as light intensity is limited by temperature/CO2 concentration
• Carbon Dioxide Concentration - raw material for photosynthesis

  • increasing CO2 concentration increases rate steadily
  • graph flattens out because of other limiting factors
• Temperature - affect speed of enzymes

  • enzymes denature when too hot (around 45'C)
  • limiting factor when too low as enzymes work slowly
• Light Intensity Practical:

  1. Place algal balls inside a conical flask connected to a gas syringe
  2. Add sodium hydrocarbonate which releases CO2 in solution
  3. Place a light source away from the flask and measure the distance with a ruler
  4. Leave the plants to photosynthesis for a set amount of time
  5. O2 releases will collect in the gas syringe, accurately showing volume
  6. Repeat the experiment at different distances
  7. All variables should be controlled such as temperature (water bath) and CO2 concentration (add set amount of sodium hydrocarbonate to set volume of water)
• The distance from lamp and light intensity in the practical are inversely proportional (inverse law) meaning halving distance quadruples light intensity and vice versa
• Root hair cells grow on plant roots and stick out into the soil in the millions:

  • large surface area for absorbing water/mineral ions
  • mineral ion concentration is higher in the root hair cells so they can be absorbed by active transport
  • water is absorbed by osmosis
• Xylem Tissue - transports up water:

  • made of dead cells joined without wall in between which creates a hole at the centre
  • strengthened by lignin
  • carry water/mineral ions from the roots to the stem and leaves (transpiration)
• Phloem Tissue - transports food:

  • made of elongated live cells with small pores in end walls
  • transport food substances (sucrose) from leaves for immediate use or storage
  • transport in both directions (translocation)
• Transpiration - the loss of water:

  • caused by evaporation and diffusion (mostly in leaves)
  • water shortage causes xylem vessels to replace it (includes dissolved mineral ions)
  • more water from roots required meaning constant transpiration stream
• Transpiration rate is affected by environmental factors:

  • Light Intensity - the brighter light, the greater transpiration rate
  • Temperature - the higher temperature, the faster transpiration happens
  • Air Flow - the better air flow around a leaf the greater transpiration rate
• A potometer can estimate transpiration rate by measuring water uptake - assuming its directly related to water loss
• Stomata - tiny pores mostly found on the lower surface of leaves that allow gases (oxygen/carbon dioxide) to diffuse directly in and out of the leaf
• The stomata allow water vapour to escape during transpiration:

  • more water inside plant than outside air (diffusion gradient)
  • surrounded by guard cells that changes stomata shape

  Turgid - guard cells are swollen with water so stomata is open

  Flaccid - guard cells are low on water and limp so stomata are closed
• Environmental Factors on Stomata:

  • stomata close when light intensity decreases as photosynthesis needs light (little water loss)
  • high temperatures mean water particles have more energy to diffuse and evaporate out the stomata (great water loss)
  • poor air flow creates a high water vapour concentration so diffusion is slow (small diffusion gradient means little water loss)
• Structure of a Leaf: (top to bottom)

  • large surface area to absorb light for photosynthesis
  • upper epidermis is transparent so light can pass through
  • epidermal tissues covered in a waxy cuticle so reduce water loss
  • palisade layer has lots of chloroplasts
  • xylem and phloem support structure and transport water/glucose
  • spongy mesophyll contain air spaces to increase gas diffusion
  • lower epidermis contain stomata for gas diffusion
• Plants in Deserts (conserve water):

  • spines and small leaves reduces surface area and acts as a defence
  • hairy/curled leaves and spines reduce air flow
  • thick waxy cuticles reduces water evaporation
  • fewer stomata or they only open at night
  • sunken stomata reduces air flow
• Auxins - plant hormones which control growth at the tips of shoots and roots by moving around the plant in solution
• Shoots are positively phototropic - when exposed to light, the auxins move to the shaded area to stimulate cell elongation so the shoot grows in the direction of the light

  • absorb more light for photosynthesis
  • plants grown without light will be tall and spindly as auxins cause it to elongate quickly on all sides to find light
• Roots are positively gravitropic - extra auxins inhibit growth so cells on top elongate faster which causes the root to grow downwards

  • absorb more nutrients
  • stabilise plant
• Plant hormones have many commercial uses:

  • Selective Weedkiller - auxins cause uncontrollable growth in weeds with wide leaves meaning grass is unharmed
  • Cloning - rooting powder encourages cuttings to grow (branch)
  • Flower and Fruit Formations - gibberellins stimulate seed germination, stem growth and flowering
  • Seedless Fruit - gibberellins applied to unpollinated flowers prevents seed growth in fruits
  • Ripening Fruit - ethene ripens fruit only for supermarkets so they are undamaged during transportation
  • Seed Germination - gibberellins can cause seeds to grow out of season at once