3.1.3 Transport in Plants

• What do plants need to transport?

  Water:

  • required for photosynthesis
  • transported from roots to all tissues

  Mineral ions:

  • required for protein synthesis
  • transported from roots to all tissues

  Sugars:

  • required for respiration, anabolism
  • transported from leaves (stems) to other tissues

  Oxygen and Carbon Dioxide:

  • oxygen required for respiration and carbon dioxide for photosynthesis
  • plants do not transport gases
• What shape adaptations do many plants have to increase their surface area?

  • branching shape
  • thin, flat leaves
  • long, thin root hairs
• What does the vascular bundle consist of and What do they transport?

  • xylem vessels: moves water and minerals from root upwards. process is called transpiration
  • phloem tissue: moves products of photosynthesis (assimilates) and amino acids. process is called translocation
• What is the function of root hairs in a dicotyledonous root?


  absorption of water through large surface area.
• What is the function of the epidermis/exodermis in a dicotyledonous root?


  protection against desiccation / protection against invasion by pathogens.
• What is the function of the cortex in a dicotyledonous root?


  may store carbohydrates (starch), can form nodules.
• What is the function of the endodermis in a dicotyledonous root?


  complete band around vascular bundle: involved in water transport into xylem.
• What is the function of phloem in dicotyledonous root and stem?


  transport of organic solutes (sugars, amino acids).
• What is the function of cambium in a dicotyledonous root and stem?


  meristem cells that actively divide.
• What is the function of xylem in a dicotyledonous root and stem?


  water and mineral transport.
• What is the function of the epidermis in a dicotyledonous stem?


  protection against desiccation / protection against invasion by pathogens.
• What is the function of the cortex in dicotyledonous stem?


  long support cells, may store carbohydrates (starch).
• What is the function of the pith/parenchyma in a dicotyledonous stem?


  are unspecialised storage cells.
• What is the function of the cuticle in a dicotyledonous leaf?


  water-repellent to protect against desiccation.
• What is the function of the upper/lower epidermis in a dicotyledonous leaf?


  protection against desiccation / protection against invasion by pathogens.
• what is the function of the palisade layer in a dicotyledonous leaf?


  efficient absorption of light.
• What is the function of spongy mesophyll in a dicotyledonous leaf?


  allows exchange of gases within leaf for photosynthesis.
• What is the function of a guard cell in a dicotyledonous leaf?


  control gas exchange through the stoma.
• Why is lignin important in the xylem?

  • walls of xylem are strengthened with lignin to resist forces generated by water moving up the stem
  • lignin also makes xylem vessels waterproof
  • using different types of lignification enables the xylem to stretch as the plant grows and enables the stem to bend
• Describe the structure and function of the xylem.

  Xylem vessels:

  • lignified cell walls: strengthens/stiffens to prevent collapse
  • no end walls: aids water flow
  • no cytoplasm: no resistance to water flow
  • bordered pits: allows water to move laterally between xylem vessels

  Xylem parenchyma:

  • thick walls: to avoid collapse
  • tannin: tastes bitter to carnivores

  Xylem fibres:

  • lignified cell walls: stiffens/strengthens to prevent collapse
• What is the structure of sieve tube elements?

  • sieve tube cells are connected to each other through sieve plates with many holes to allow sugar transport
  • sieve tube cells have a modified cytoplasm, no vacuole and few organelles making a long, hollow structure
  • energy for transport comes from the companion cells
• What is the role of companion cells in the phloem?

  • companion cells provide materials to keep sieve tube elements alive
  • they maintain their nucleus and organelles
  • companion cells are connected to the sieve tube elements by plasmodesmata, which permits transport of materials
• Describe the structure and function of the phloem.

  Sieve tube elements:

  • ends connected by sieve plates: aid transport of solutes, including carbohydrates
  • modified cytoplasm: aids flow of dissolved solutes, including carbohydrates
  • few organelles: long hollow structure minimises resistance to flow

  Companion cells:

  • plasmodesmata: transport to sieve tube elements
  • nucleus and organelles: maintained to undertake cell processes (i.e. respiration)

  Fibres and Sclereids:

  • thick cell walls: strengthens to prevent collapse
• Compare the xylem and phloem.

  Xylem:

  • transpiration
  • carries water and minerals to the leaves (upwards)
  • dead cells
  • thicker cell wall
  • has lignification in cell wall
  • has interconnected cells to form tubes

  Phloem:

  • translocation
  • carries biomolecules to growing parts and storage (up and down)
  • living cells
  • thinner cell wall
  • no lignification
  • has interconnected cells to form tubes
• Explain why a plant needs water.

  • water is a raw material for photosynthesis
  • mineral ions and products of photosynthesis are transported in aqueous solution
  • loss of water by evaporation helps keep plants cool
  • turgor drives cell expansion. turgor is the force that enables roots to force their way through tarmac and concrete
  • turgor pressure provides a hydrostatic skeleton to support the stems and leaves and maintain turgidity
• Define the term adhesion.

  attraction of water molecules to surfaces, like the walls of cells, or xylem vessels.
• Define the term cohesion.

  attraction of water molecules to each other by hydrogen bonding.
• What is transpiration?

  Transpiration is the process of evaporation of water from leaves. It causes movement of water upwards through a plant.
• Outline the route water takes from the roots upwards through the plant.

  • root hair cell
  • cortex
  • endodermis around vascular tissue
  • xylem in root
  • xylem in stem
  • xylem in leaf
  • air spaces between spongy mesophyll
  • stomata
• Explain the process of the intake of water into a plant.

  • water enters plant through root hair cells of the epidermis
  • soil water has a HIGH water potential, as it contains a low concentration of dissolved minerals
  • cytoplasm and vascular sap of the root hair cell contain different solvents, so the water potential is LOWER inside the cell
  • as a result, water moves into the root hair cells via osmosis through the permeable cell walls and partially permeable plasma membrane
• What adaptations do root hair cells have for water uptake?

  • microscopic size: allows them to penetrate between soil particles, increasing the likelihood of encountering water
  • lots of long, thin root hairs: increases the total root surface area for water absorption
  • thin cell walls: give a shorter diffusion distance increasing the rate of water uptake
  • solutes in cytoplasm: maintain a water potential gradient with the soil
• Name the two pathways by which water travels across the root to the xylem.

  As water moves through the cortex, it can take two different paths:

  • apoplast pathway: water travels along cellulose cell walls
  • symplast pathway: water travels along the cell cytoplasm and plasmodesmata
• Describe the apoplast pathway of water movement.

  • water diffuses down a water potential gradient through a space between cellulose fibres in plant cells
  • cohesive forces between water molecules creates tension which pulls water through cortex cells to the endodermis
  • this is the fastest way that water moves as it doesn't need to cross membranes
  • doesn't pass through a plasma membrane, so the plant has no way of preventing any of the dissolved substances from the soil travelling via this pathway as well as water
• Describe the symplast pathway of water movement.

  • water crosses plasma membrane and enters cytoplasm
  • each cell has a lower water potential than the previous, so the water moves down the maintained water potential gradient across the cortex
  • then travels from cell to cell through the plasmodesmata until it reaches the xylem
  • pathway involves crossing membranes, so it is selective
  • dissolved substances in soil water cannot enter root hair cell unless cells have channel or carrier proteins to allow them to cross the plasma membrane
• What is the role of the endodermis in the symplast pathway?

  • at the endodermis, the apoplast pathway is blocked and all water and mineral ions have to enter the symplast pathway to continue on to the xylem
  • this ensures everything entering the xylem has been through the selective symplast pathway
  • endodermis blocks the apoplast pathway by having a ring of hydrophobic molecule called suberin embedded within its cell wall
  • this ring is called the Casparian strip
• What is the Casparian strip and How is it involved in water movement through a plant?

  The Casparian strip is an impermeable layer of suberin, which is a waxy substance in the cell wall.

  The Casparian strip of suberin prevents water moving through the apoplast pathway and forces water into the symplast pathway. This means all water and dissolved solutes must pass through a cell membrane giving cells control over what enters the xylem.
• Describe the movement of water into the xylem.

  • endodermis moves ions into the xylem by active transport, lowering the water potential in the xylem
  • water moves down the water potential gradient from the endodermis into the xylem via both the symplast and apoplast pathways
  • active movement of ions promotes movement of water into the xylem and creates root pressure
  • the upward force generated by water entering the xylem vessels due to the active transport of solutes is called root pressure
  • water moves through the xylem in roots, stems and leaves
  • xylem contributes to capillary pressure
• Explain the movement of water in the leaves?

  • water moves from the leaf xylem into leaf cells by osmosis. following both the symplast and apoplast pathways
  • much water evaporates from cellulose cell walls of mesophyll cells into air spaces and diffuses out of stomata down a water potential gradient
  • stomata need to open to allow exchange of gases, so some water loss is unavoidable
• What are the benefits of water loss from leaves?

  • evaporation of water cools leaves
  • it aids transport of water and minerals
• Explain how guard cells can open and close stomata.

  Flaccid guard cells close stoma:

  • water is scarce in a leaf, so water moves out of the guard cell vacuoles by osmosis
  • cells become flaccid
  • thin outer wall of guard cells becomes less bent and thick inner wall of guard cells straightens
  • stoma closes

  Turgid guard cells open stoma:

  • water is plentiful in a leaf, so water moves into the guard cell vacuoles by osmosis
  • cells become turgid
  • thin outer wall of guard cells bends and thick inner wall is forced to bend too
  • stoma opens