Plant Nutrition

• Photosynthesis


  The process during which green plants absorb sunlight energy and use it to combine CO2 and H2O to produce glucose and O2 as by-product in the presence of chlorophyll
• Photosynthesis


  • Takes place inside chloroplast
  • 6 H2O + 6 CO2 → C6H12O6 + 6O2 (water + carbon dioxide → glucose + oxygen)
• Mechanism of photosynthesis


  1. Light reaction

  2. Dark reaction
• Light reaction of photosynthesis

  1. Chlorophyll absorbs light energy

  2. Light energy converted into chemical energy

  3. H2O molecules split into H & O2

  4. O2 diffuses out

  5. Hydrogen used in dark reaction
• Dark reaction of photosynthesis

  1. Carbon dioxide fixation occurs

  2. Hydrogen from light reaction combines with CO2 to form sugar & water

  3. Controlled by enzyme

  4. 24H + 6CO2 → C6H12O6 + 6H2O
• The wavelengths that are absorbed most effectively (blue & red light) by chlorophyll are also the wavelengths that give the highest rates of photosynthesis
• Limiting factor


  A factor that directly affects a process if its quantity is changed
• As light intensity increases (initially)


  Rate of photosynthesis increases from 0 to A
• At point A

  Light becomes the limiting factor
• Beyond point A


  Light is no longer the limiting factor, some other factor like temperature or CO2 becomes the limiting factor
• Increasing temperature from 20°C to 30°C


  Increases the rate of photosynthesis as more heat energy is available to speed up enzymes activity
• Increasing CO2 concentration up to the optimum point around 0.1%


  Increases the rate of photosynthesis to max
• Beyond 0.1% CO2


  CO2 is no more the limiting factor
• Adaptations of leaf structure for photosynthesis

  • Lamina: large & thin flat surface to absorb max sunlight

  Petiole: holds lamina away from stem to obtain sufficient sunlight & air

  Network of veins: provide mechanical support, bring water & minerals, distribute food
• Transverse section of a leaf

  • Cuticle: waxy layer that protects & prevents water loss

  Upper epidermis: single layer of closely packed cells

  Palisade mesophyll: 1-2 layers of long cylindrical cells with many chloroplasts

  Spongy mesophyll: irregular cells with intercellular air spaces for gas diffusion

  Stomata: important for gaseous exchange

  Guard cells: regulate opening & closing of stomata
• How guard cells control stomata size

  In sunlight: K+ concentration increases, chloroplasts photosynthesize, K+ pumped in, water enters by osmosis, guard cells swell & open stomata

  At night: K+ diffuses out, water leaves by osmosis, guard cells deflate & close stomata
• Mineral nutrition in plants

  Needed in small amounts for healthy growth

  Absorbed as ions by roots from soil, mainly by active transport

  Ions formed from weathering of rocks & decay of plants/animals

  Chemical fertilizers & compost can increase soil mineral content
• Water-culture experiment


  Prepare 3 jars: A with complete culture solution, B lacking nitrate, C lacking magnesium

  Insert seedling in each jar & allow to stand for 1 month

  1. seedling grows healthily with green leaves
  2. poor growth with smaller & pale yellow leaves (stunted growth)
  3. poor growth with yellow leaves (chlorosis)
• Transport


  Movement of substances in and out of cells
• Transport in plants


  1. Uptake

  2. Movement

  3. Transpiration

  4. Photosynthesis

  
• Cell types


  • Parenchyma
  • Collenchyma
  • Sclerenchyma
• Collenchyma cells


  • Provide structural support
  • Flexible
• Sclerenchyma cells


  • Thick-walled
  • Provide rigid support
• Stomata allow gas exchange
• Xylem transports water and minerals
• Phloem transports organic nutrients
• Phloem


  • Living cells
  • Transport organic nutrients
• Translocation


  Movement of organic nutrients in phloem
• Translocation is bidirectional
• Translocation is pressure-flow mechanism
• Roots absorb water and minerals

  
• Roots anchor the plant in the soil
• Roots increase the surface area for absorption</b>
• Root pressure pushes water up the xylem

  
• Transpiration pull draws water up the xylem

  
• Photosynthesis occurs in chloroplasts
• Chloroplasts contain chlorophyll
• Chlorophyll absorbs light energy
• Respiration occurs in mitochondria
• Respiration releases energy from glucose