Transpiration + Translocation

• Mineral ions


  Required for plant growth, e.g. nitrates are required to produce proteins
• Root hairs


  • Adapted for the uptake of water and minerals
• Water transport into root hairs


  1. Lower concentration of water in root hair cells than in the soil

  2. Water diffuses down its concentration gradient into root hair cells by osmosis
• Mineral transport into root hairs


  1. Lower concentration of mineral ions in the soil than in the root

  2. Root hair cells take up mineral ions by active transport
• Plant roots


  • Composed of millions of root hair cells with long hairs that extend from the cell body, increasing the surface area for absorption
  • Many mitochondria which produce ATP for active transport of mineral ions
• Xylem


  One of the two plant transport tissues
• Phloem


  One of the two plant transport tissues
• Function of xylem


  Transports water and minerals up the plant, from the roots to the leaves via the transpiration stream
• Xylem


  • Composed of dead cells laid end-to-end to form a long, hollow, continuous column
  • No end walls which provides little resistance to the passage of water
  • Thick cell wall strengthened with lignin to provide support
• Function of phloem


  Transports sugars up and down the stem from photosynthetic tissues (e.g. mature green leaves) to non-photosynthetic tissues (e.g. developing seeds) via translocation
• Sieve tube elements


  One of the two cell types that make up the phloem
• Companion cells


  One of the two cell types that make up the phloem
• Phloem


  • Sieve tube elements are long, thin cells, laid end-to-end with perforated end plates to enable the flow of sugars. They contain no nucleus and little cytoplasm to allow sugars to flow easily.
  • Companion cells (adjacent to sieve tube elements) contain a dense cytoplasm, nucleus and mitochondria. They provide energy for processes in both cell types.
• Transpiration


  The loss of water vapour from the parts of a plant exposed to the air due to evaporation and diffusion
• The majority of transpiration takes place in the leaves
• Transpiration process


  1. Water evaporates from the mesophyll cell surfaces and diffuses out of the stomata

  2. Water molecules (which have cohesive properties) are drawn up the xylem vessels to replace the water that has been lost

  3. This causes more water molecules to be absorbed from the soil into root hair cells
• Transpiration stream


  Transports mineral ions dissolved in the water
• Stomata


  Pores found in the lower epidermis of a leaf which allow gas exchange
• Guard cells


  Specialised cells surrounding the stoma that change shape to control the size of the pore
• How guard cells control stomata size


  1. To open the stomata: Water enters guard cells. They swell and become turgid. They bend and draw away from each other, opening the stomata.

  2. To close the stomata: Water leaves guard cells. They become flaccid, closing the stomata.
• Factors affecting rate of transpiration


  • Light intensity
  • Temperature
  • Air movement
• High light intensity


  Increases the rate of transpiration
• Guard cells


  • Specialised cells surrounding the stoma
  • Change shape to control the size of the pore
• How do guard cells control the size of stomata?


  1. To open: Water enters, they swell and become turgid, they bend and draw away from each other, opening the stomata

  2. To close: Water leaves, they become flaccid, closing the stomata
• Factors affecting the rate of transpiration


  • Light intensity
  • Temperature
  • Air movement
• High light intensity


  • Greater number of stomata open to allow gas exchange for photosynthesis
  • Rate of photosynthesis increases so more water is taken up from the soil, pushing water up the xylem
  • More water vapour diffuses out of the stomata, rate of transpiration increases
• Low light intensity


  Fewer stomata are open so the rate of transpiration decreases
• Temperature increases


  • Water molecules have more kinetic energy so rate of diffusion increases
  • Photosynthesis also increases so more water is taken up from the soil, pushing water up the xylem
  • More water vapour diffuses out of the stomata, rate of transpiration increases
• Air movement increases


  • High water concentration gradient maintained between the air spaces in the leaf and atmosphere
  • Increased rate of diffusion of water molecules out of the stomata
  • Rate of transpiration increases
• Potometer


  Apparatus used to measure the rate of transpiration

  
• When measuring the rate of transpiration using a potometer, it is assumed that the rate of water uptake ≈ rate of transpiration

  
• Translocation


  The movement of sugars (sucrose, amino acids etc.) up and down a plant, from the source to the sink, via the phloem. Requires ATP
• Structures of the leaf


  • Waxy cuticle
  • Vascular bundle
  • Upper epidermis
  • Palisade mesophyll tissue
  • Spongy mesophyll tissue
  • Lower epidermis
  • Air-filled space
  • Stoma
  • Guard cell
• Leaf adaptations for photosynthesis and gas exchange

  • Broad - large surface area for light absorption
  • Thin - short diffusion distance for gases, allows light to reach all cells
  • Vascular bundles (xylem and phloem) form a network to deliver water and remove glucose. Also provide support.
  • Photosynthetic pigments (e.g. chlorophyll) absorb light
• Tissue adaptations of leaves for photosynthesis and gas exchange


  • Palisade mesophyll layer - receives most light so contains greatest concentration of chloroplasts
  • Upper epidermis - transparent, allows light to reach palisade layer
  • Spongy mesophyll layer - air spaces increase the rate of diffusion
  • Lower epidermis - contains many stomata for gas exchange
• Plant adaptations to live in hot, dry conditions


  • Small leaves/spines - reduce surface area for water loss
  • Thick waxy cuticle - reduces evaporation, conserving water
  • Thick stem - provides a storage of water
  • Shallow but widespread roots - large surface area to absorb water
  • Stomata sunken in pits and leaves curled - reduces air flow, lowering diffusion gradient and reducing water loss by evaporation
  • Stomata close to reduce water loss