Transport in Plants

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  • Transport in Plants
    • Plants have developed a mas transport system made of columns of plant cells that form 2 distinct types of transport tissue;
      • Mass transport is 'the bulk movement of water and organic solutes'
      • Xylem: Transports water and mineral ions from the roots to the leaves
      • Phloem: Transports sugars (sucrose) and amino acids made by photosynthesis from the leaves to the rest of the plant
    • Xylem is made up of 4 types of cell;
      • Vessels
        • Unspecialised plant cells (parenchyma) form long columns of cells which lose their end walls to create vessels
          • The vessel walls are strengthened by impermeable lignin. This prevents the tubes from collapsing inwards during transpiration due to the negative pressure inside the vessels
            • The lignin is impermeable, so water and solutes cannot pass into the xylem cells, so the protoplasm dies. This results in the formation of hollow vessels which offer no resistance to the passage of water
              • The lignin is often laid down in rings, giving a characteristic appearance under microscopes
      • Tracheids
        • Elongated cells with tapering ends that conduct water, but are less well adapted than vessels
          • Cell walls also strengthened by impermeable lignin
            • Tracheids do not have open ends, so water passes from cell to cell via pits
              • Tracheids are usually found in the finest branches of xylem in roots and leaves
      • Fibres
        • Long, narrow cells with tapered ends
          • They are very strong because of their thick, lignified walls and interlocking ends
            • They are dead cells and their function is to provide support
      • Parenchyma
        • Living cells that store food and allow lateral transport of materials through the vascular tissue. They also provide some support for plant tissue
    • Phloem is made up of 4 cell
      • Parenchyma
      • Sieve tube cells
        • The sieve tube cells are formed from cells called sieve elements placed end to end
          • The end walls are perforated by pores, these areas are called sieve plates
            • The sieve tubes do not posess a nuleus and during their development, most of the other organelles disintegrate
              • The cytoplasm forms a thin layer around the periphery of the cell and cytoplasmic filaments extend from one cell to the next through pores in the sieve plate
                • Function is to transport organic solutes
      • Companion Cells
        • Each sieve tube cell is closely associated with a companion cell which has a dense cytoplasm, large centrally placed nuclei, many mitochondria and they are connected to the sieve tube element by the plasmodesmata
          • The mitochondria are needed to produce ATP for active transport of sucrose into the phloem
            • Function is to provide the energy needed to move the sucrose into the sieve tube cell
      • Fibres
    • Uptake and movement of water and mineral ions in the roots
      • Most absorption of water is through the root hair by osmosis
      • Mineral ions are absorbed by diffusion and active transport
      • Adaptations of root hairs;
        • Large surface area
        • Short diffusion pathway
        • Many mitochondria to produce ATP for active transport
        • Lower water potential than soil to generate water potential gradient
    • Uptake of water into the root
      • Water moves from the soil into root hairs and across the cortex towards the xylem by osmosis
        • The water in the soil contains only a weak solution of mineral ions so it has a high water potential
          • Inside the root hair, there is a strong solution of dissolved glucose and other substances so there is a low water potential
            • Water passes into the root, down the water potential gradient by osmosis
              • Once inside, the water potential in the xylem is lower than that of the root hair, so water passes across the cortex and enters the xylem in the centre of the root by osmosis
    • How water gets from the root hair to the xylem
      • There are two main pathways for water to travel across the cells of the cortex of the root to the xylem;
        • Symplast Pathway
          • Water moves through the root cortex form cell cytoplasm to cell cytoplasm, down a water potential gradient until it reaches the xylem in the centre of the root
            • It passes through small pores (plasmodesmata) in adjacent cell walls
        • Apoplast Pathway
          • Through the cell walls and spaces between the cells
            • Water and mineral ions, carried in solution pass through the cell walls of the cortex until they reach the endodermis which surrounds the centralcore of the vascular bundle
              • Here, the apoplast pathway is blocked by a waxy material called suberin
                • The suberin is waterproof and forms impermeable bands around the endodermal cells. These are known as the casparian strip
                  • To bypass the casparian strips, the water has to leave the apoplast pathway and enter the symplast pathway
                    • To enable this to happen, mineral ions are actively transported into the cytoplasm of the cells to lower water potential, causing water to move into the symplast pathway by osmosis
                      • The endodermal cells actively pump mineral ions into the xylem to help generate a water potential gradient right across the root, drawing water in from the soil
                        • The endodermis allows the plant to selectively uptake ions from the soil
        • Small amounts of water can also move through the vacuolar pathway from vacuole to vacuole in adjacent cells by osmosis down a water potential gradient
    • Movement of water through a stem;
      • Root pressure theory
        • Endodermal cells actively transport mineral ions into the xylem vessels. This lowers the water potential, so water enters the xylem by osmosis
          • This pulling of water into the xylem produces a positive hydrostatic pressure (root pressure)
            • This root pressure forces water up the stem (but is not sufficient to push water to the leaves at the top of tall plants)
              • Root pressure is thought to be a major force in the movement of water up the stems of mature plants, but a major force in developing plants
      • Cohesion-tension theory
        • As water is drawn out of the xylem by osmosis, it creates a tension on the column of water molecules in the xylem
          • This is because the water molecules show cohesion - they are bonded together by weak hydrogen bonds
            • Because of this, transpiration from leaves pulls water up the stem in a continuous column called the transpiration stream
      • Capillary/adhesion theory
        • The tendency of water to rise in narrow tubes
          • Xylem vessels are narrow, and have a hydrophilic lining. Water molecules are attracted and adhere to the hydrophilic walls
            • This causes water to move up the vessels
    • Transpiration
      • Transpiration is the evaporation of water from plant leaves
        • Water evaporates from the surface of cells of the spongy mesophyll into air spaces, and then diffuses out of the stomata down a water potential gradient
          • This sets up a water potential gradient across the leaf from a higher potential in the xylem to a lower potential in the air spaces

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