Bio Chap 12 - Nutrition and Transport in Flowering Plants

• Xylem:

  • Xylem cells are dead at maturity
  • Xylem vessels are long and hollow tubes made of xylem cells
  • Transport water and mineral salts from roots to leaves
  • Absence of protoplasm and cross-walls which could impede water flow through the lumen
  • Inner walls of vessels are deposited with lignin in different shapes
  • Provides mechanical support for the plant
• Phloem:

  
  

  • Made up of sieve tube cell and companion cells

  
  

  • Sieve tube cell only have degenerate protoplasm and a thin layer of cytoplasm, no organelles

  
  

  • Porous walls between sieve tube cell called sieve plates

  
  

  • Companion cells closely associated with sieve tube elements and contain organelles

  
  

  • Companion cells have many mitochondria

  
  

  • Companion cells located beside sieve tube cells providing energy for transport of food (sucrose and amino acids) from the leaves/storage organs to the rest of the plant, known as translocation
• Translocation:

  • Transport of sugars from leaves to other parts of the plant via phloem tissues
  • Source of sugars is leaves, while other parts of the plant requiring sugar are known as the sink
  • Energy is required for active transport of sugars into sieve tube elements
  • In ringing experiment, removal of phloem ring leads to bulge formation due to sugar accumulation
  • Aphids can be used to collect sap by feeding on sucrose in phloem sieve tubes
• Root Hair:

  • Function: Water and ion uptake by root hair
  • Long and narrow structure increases surface area to volume ratio for rapid absorption
  • Cell sap contains sugars, amino acids, and mineral salts, lowering water potential compared to soil solution
  • Water uptake occurs by osmosis from soil into root hair cells
• Root Pressure:

  • Mechanism for water movement from roots to leaves against gravity
  • Root hair cells pump mineral salts into xylem vessels using active transport
  • Creates pressure forcing water to move upwards
  • Not the main mechanism for water movement in most plants
• Capillary Action:

  • Tendency of water to move up narrow xylem tubes due to cohesion and adhesion forces
  • Usually observed in young plants with narrow veins and not significant in larger plants
• Transpiration:

  • Loss of water vapor from stomata of leaves through diffusion
  • Transpiration pull is the main force causing water to travel upwards in plants
  • Effects of air movement, temperature, humidity, and light intensity on transpiration rate
  • Water lost via transpiration must be replaced by absorption from roots to leaves by xylem
  • Rate of water absorption vs. transpiration affects plant turgidity and wilting
• Leaf Structure:

  • Upper epidermis: single layer of closely packed cells that allows light to pass through
  • Palisade Mesophyll: long and cylindrical shape, main site of photosynthesis, contains the largest amount of chloroplasts among the tissue in leaves to increase absorption of sunlight for photosynthesis
  • Spongy Mesophyll: irregular shape with large intercellular air spaces among the cells to increase surface area for gaseous exchange, covered with a thin film of moisture for carbon dioxide to dissolve in it, contains vascular bundle and less chloroplasts than palisade mesophyll
• • Lower Epidermis: similar to upper epidermis, single layer of closely-packed cells covered by cuticle, contains guard cells which control the opening and closing of the stoma for gaseous exchange
• Gaseous Exchange:

  • Plants open their stomata during the day for carbon dioxide intake and close them during the night to minimise water loss through transpiration
  • During the day, guard cells photosynthesise and pump potassium ions into the guard cells, leading to the opening of the stoma
  • During the night, potassium ions move out of the guard cells, causing the stoma to close
• Photosynthesis:

  • Word equation: Carbon Dioxide + Water + Light Energy (chlorophyll) → Glucose + Oxygen + Water
  • Chlorophyll: pigment in chloroplast that traps light energy for the formation of carbohydrates
  • Intake of Carbon Dioxide: carbon dioxide diffuses into the leaf through stomatal openings and dissolves in the thin film of moisture on mesophyll cell surfaces
  • Intake of Water: xylem transports water and mineral salts from roots to leaves, water enters cells via osmosis
• Limiting Factors:

  • Light intensity, carbon dioxide concentration, and temperature are limiting factors on the rate of photosynthesis
  • Effect of Light Intensity: rate of photosynthesis increases with increasing light intensity until it reaches a plateau, light saturation point is reached
  • Effect of Carbon Dioxide Concentration: increasing carbon dioxide concentration increases the rate of photosynthesis until it reaches a plateau
  • Effect of Temperature: increasing temperature increases the rate of photosynthesis until enzymes are denatured
• All life on earth ultimately depends on plants for food and energy
• Plants have an efficient transport system which allows them to transport food and water
• Structure and Function of Parts of a Plant
• External Structure of Leaf:

  • Veins: Large network of veins across the leaf (xylem and phloem) to carry water and mineral salts to leaf cells, and to carry food from the leaf cells to other parts of the plant
  • Blade: Leaf blade has a large surface area to obtain as much light as possible for photosynthesis and is thin to allow carbon dioxide to rapidly reach inner cells of the leaf
• Petiole (Leaf stalk): Stalk attaching the leaf to the stem holds the leaf away from the stem to allow the leaf to obtain as much light and carbon dioxide as possible
• • Guard cells: A pair of cells containing chloroplasts, surrounding each stoma, to regulate the opening and closing of the stomata
• Internal Structure of Leaf:

  • Palisade mesophyll: Closely packed, cylindrical cells containing many chloroplasts for photosynthesis
  • Spongy mesophyll: Loosely packed, irregularly shaped cells with some chloroplasts, many large intercellular air spaces between cells, and covered with a thin film of moisture for photosynthesis and gaseous exchange
  • Cuticle: Waxy layer covering the upper and lower epidermis to prevent water loss from leaf cells
  • Stomata: Small openings found mostly on the lower epidermis for gaseous exchange with the surrounding air
• Xylem and Phloem
• Identifying locations of xylem and phloem in transverse section of dicot stem and leaf:

  • Xylem transports water and mineral salts
  • Phloem transports food
  • Vascular bundle refers to the xylem and phloem of a plant
• Guard Cells and Stomata
• What are stomata:

  • Guard cells surround each stoma, contain chloroplasts, and are able to photosynthesize
  • Opening of stomata: In the presence of light, guard cells photosynthesize, glucose is produced, water enters guard cells by osmosis, guard cells become turgid and more curved, and stomata open
  • Closing of stomata: At night or in extreme heat, guard cells become flaccid and straighter, and stomata close
• • Entry of carbon dioxide into the plant: Carbon dioxide diffuses from the surrounding air through the stomata into the intercellular air spaces, dissolves into the thin film of moisture covering the spongy mesophyll cells, and then diffuses into the spongy mesophyll cells
• Photosynthesis
• Equation of Photosynthesis: Chlorophyll absorbs energy from light to make glucose from carbon dioxide and water

  • Importance of Photosynthesis: Conversion of light energy to chemical energy, production of oxygen, removal of carbon dioxide, and storage of glucose as starch in plants
• Fate of Glucose after Photosynthesis:

  • Glucose is stored as starch and converted to sucrose and amino acids for transport around the plant
• Factors Affecting the Rate of Photosynthesis:

  • Light intensity: Required for photosynthesis, increase leads to an increase in the rate until a constant rate is reached
  • Carbon dioxide: Required for photosynthesis, increase in concentration leads to an increase in the rate until a constant rate is reached
• Concentration leads to an increase in the rate of photosynthesis until a constant rate is reached
• Further increase in carbon dioxide concentration does not lead to a further increase in the rate of photosynthesis
• Low rate of photosynthesis at low temperatures as enzymes are inactive
• Increase in temperature to the optimum temperature leads to an increase in the rate of photosynthesis
• Further increase in temperature past the optimum temperature causes enzymes to get denatured, leading to a decrease in the rate of photosynthesis
• Root hair cells have narrow extensions to increase surface area-to-volume ratio to maximize the absorption of water and dissolved mineral salts
• Water potential in the soil is higher than that of the root hair cells, so water enters the root hair cells by osmosis
• Absorption of dissolved mineral salts:

  • By diffusion when the concentration of dissolved mineral salts in the soil solution is higher than in the root hair cell
  • By active transport when the concentration of dissolved mineral salts in the soil solution is lower than in the root hair cell
  • Root hair cells contain many mitochondria to release energy for active transport
• Transpiration is the loss of water vapor from aerial parts of a plant, mainly through the stomata
• Transpiration pull is the main force that causes the movement of water up the xylem of a plant
• Factors affecting the rate of transpiration:

  • Stronger wind leads to a higher rate of transpiration
  • Higher temperature leads to a higher rate of transpiration
  • Higher light intensity leads to a higher rate of transpiration
  • Higher humidity leads to a lower rate of transpiration
• Wilting occurs when plants have insufficient water (rate of transpiration > rate of water absorption)