Transport in Plants and Transpiration

• Transpiration


  The process of water loss by evaporation in plants
• As with animal cells, water and other aerials can move on to call in a plent by a number of processes. As in multicellular inks, plans have specialised odis that ere adapted for the mass dow of and other suevances.
• Plant vascular tissues


  Specialised for the transport of water and dissolved molecules such as sugars and amino acids (phloem)
• Structure of plant roots


  • The outer layer is the epidermis and the vascular tissue is in a central stele (vascular cylinder). The single layer of cells bordering the stele is the endodermis.
  • The xylem is differentiated into a number of types but the main type involved in water and ion transport is the xylem vessel
  • Xylem vessels have no end walls, no cell contents and are dead when fully formed. They have specially thickened cell walls with lignin to provide strength and prevent collapsing
  • Protoxylem has annular or spiral thickening, allowing elongation. Metaxylem has more complete lignin covering in a reticulate or pitted pattern
• Phloem tissue


  • The cells primarily involved in transport are the sieve tube elements, which are aligned end to end to form a continuous row called the sieve tube
  • Sieve tube elements are living cells but have no nuclei and reduced cytoplasm, with the cytoplasm displaced to the side walls
  • Each sieve tube element is closely associated with one or more companion cells, which have a dense cytoplasm and high metabolic rate to support the sieve tube elements
• Structure of plant stems


  • The vascular tissue is arranged as vascular bundles around the outside of the stem, with the protoxylem closer to the centre and the metaxylem closer to the outer edge
  • Vascular bundles continue into the leaf as the midrib, which branches to form smaller veins distributed throughout the leaf
• The uptake and transport of water and ions in plants

  1. The transport of water (and ions) into and across the root

  2. The transport of water up the root and stem in the xylem

  3. The transport of water through the leaf and the evaporation of water from the leaf
• Transpiration


  The evaporation of water from the mesophyll surface and the subsequent diffusion of water vapour through the stomata and into the atmosphere
• Functions of transpiration


  • Transport of water to the leaves to provide turgor and for photosynthesis
  • Transport of minerals up the plant
  • Cooling effect as water evaporates out of the leaf, preventing leaves overheating
• Transport of water and ions into and across the root


  1. Water enters by osmosis through root hair cells, moving by the apoplast pathway through cell walls and the symplast pathway from cell to cell

  2. Ions enter by facilitated diffusion or active transport, often being actively pumped into the xylem by the endodermis
• Role of the endodermis


  • The Casparian strip blocks the apoplast pathway, forcing water to move by the symplast pathway under metabolic control
  • The endodermis pumps ions into the xylem, creating a water potential gradient that draws water into the xylem
• Transport of water up the root and stem in the xylem

  1. Evaporation of water from the leaves creates a negative pressure (transpiration pull) that pulls the water column up through the xylem

  2. The cohesive properties of water allow it to form a continuous column that is 'sucked up' the xylem (cohesion-tension theory)
• Evidence for the cohesion-tension theory includes the fact that if the water column is broken, water below the gap cannot be pulled up, and cut flowers will wilt if the water column is disrupted before being placed in water
• The xylem vessel cannot be pulled up (in reality each column of xylem vessels can be treated as a single column of water, so a water column can be disrupted in one xylem column but continue in others)
• The same principle applies when keeping cut flowers in containers of water in shops- if the time between buying the flowers and placing them in a vase of water at home is too long, air will enter the bottom of the cut stems as the water column moves up due to loss of water by evaporation
• Placing them in water after this has happened will make no difference (as the column is broken) and the flowers will rapidly wilt as they are starved of water even though the stems may be immersed in it
• Xylem vessel


  Part of the vascular cylinder
• Stele


  Vascular cylinder
• Endodermis


  Role in the transport of water into the xylem
• During the day, when transpiration is normally at its greatest, there is much more tension, or negative pressure, in the xylem
• This negative pressure tends to pull the walls of the xylem vessels in and can reduce the diameter of a tree trunk
• Changing diameters in tree trunks is more obvious then in herbaceous plants due to the fact that a tree trunk is almost all xylem and therefore much of it is involved in water uptake
• The lignin in the xylem vessels gives it two important properties: it is waterproof, avoiding the loss of water and it is very strong, allowing the vessels to withstand the pressures exerted on them by the transpiration pull
• The missing end walls and absence of cell contents in xylem vessels make them highly adapted for water transport by the cohesion-tension method
• In most plants the cohesion-tension theory is probably the most important process involved in moving water up through the xylem in roots and stems
• Root pressure also contributes somewhat (albeit a minor role) in most species
• The adhesive property of water is also important. If a drinking straw or a capillary tube is placed in water, the water will move up the tube to some extent. This is due to the adhesive forces between the substances in the straw, or capillary tube, and the water being stronger than the cohesive forces between the water molecules themselves
• Adhesion is therefore the attraction of unlike material. This phenomenon, known as capillarity, has a role in water transport in the xylem. It may not have a significant role in moving the water up the xylem but the adhesive forces between the water column and the xylem walls may reduce the forces necessary for the transpiration pull
• Water enters the leaf in the midrib (vascular bundle). In most leaves this midrib splits into a number of veins that distribute water across the leaf
• Water passes from the vein to the surrounding cells, where some is used in photosynthesis or in providing turgor but most is lost in transpiration
• Transpiration losses are due to water evaporating from the cell surface membranes of the spongy mesophyll cells into the air spaces of the spongy mesophyll, with the water vapour diffusing down the concentration gradient out of the stomata (transpiration)
• This process sets up the water potential gradient that is ultimately responsible for the transpirational pull
• The water travels across the leaf (from the xylem to the spongy mesophyll cells) by the same processes involved in water transport across the root, ie the apoplast and symplast pathways
• Stomatal density refers to the number of stomata per unit area of leaf. Normally the more stomata per unit area in a leaf, the more evaporation (and transpiration)
• Normally the greater the leaf surface area, the more evaporation of water (and transpiration). This is because there is usually a positive correlation between leaf area and stomatal number
• Thicker cuticles tend to lose less water by evaporation than thinner cuticles
• In most leaves, the cuticle on the upper surface of the leaf is thicker than the cuticle on the lower surface
• Light has little effect directly on evaporation rate. However, the rate of evaporation and transpiration will be greater during the day (in light) compared to during the night (in darkness), as the stomata of most plants are often closed when it is dark
• Increasing wind speed increases the rate of evaporation as the wind removes diffusion shells by blowing humid air away from the leaf. This maintains a steep water potential gradient between the inside and outside of the leaf, allowing water to evaporate capilly from the spongy mesophyll cells into the air spaces of the spongy mesophyll, water vapour which then diffuses out of the stomata
• The higher the temperature, the faster the rate of evaporation of water from the spongy mesophyll cells (and the faster diffusion of water vapour through the stomata)