Transport in plants

• xylem tissue transports water and mineral ions in solution. These substances move up the plant from the roots to the leaves
• Phloem tissue transports organic substances like sugars (also in solution) both up and down the plant
• Xylem vessels are the part of the xylem tissue that actually transports the water and ions. Xylem vessels are very long, tube-like structures formed from dead cells (vessel elements) joined end to end. There are no end walls on these cells making an uninterrupted tube that allows water to pass up through the middle easily
• water moves up a plant against the force of gravity, from roots to leaves - can be explained by the combined action of cohesion and tension
• • water evaporates from the leaves at the 'top' of the xylem - transpiration
  • this creates tension (suction) which pulls more water into the leaf
  • water molecules are cohesive (they stick together) so when some are pulled into the leaf others follow.
  • this means the whole column of water in the xylem from the leaves down to the roots moves upwards
  • water then enters the stem through the roots
• water movement up a plant increases as the transpiration rate increases
• Transpiration is the evaporation of water from a plant's surface, especially the leaves. Water evaporates from the moist cell walls and accumulates in the spaces between cells in the leaf. When the stomata open, it moves out of the leaf down the water potential gradient (bc there is more water inside the leaf than in the air outside)
• factors affecting transpiration rate:

  • light intensity
  • temperature
  • humidity
  • wind
• light intensity:

  • the lighter it is the faster the transpiration rate
  • positive correlation between light int and transp rate
  • bc stomata open when it gets light to let in CO2 for photosynth
  • when dark - stomata usually closed - little transp
• Temperature:

  • higher the temp faster the rate
  • warmer water molecules have more energy so they evaporate from the cells inside the leaf faster
  • increases water potential gradient between inside and outside leaf
  • making water diffuse out of lead faster
• Humidity:

  • lower humidity faster the rate
  • negative correlation between humidity and transp rate
  • if the air around a plant is dry the water potential gradient between leaf and air is increased
  • increases transp rate
• wind:

  • windier it is faster transp rate
  • lots of air movement blows away water molecules from around the stomata
  • increases water potential gradient
  • increases rate of transp
• transpiration is really a side effect of photosynth - plant needs to open stomata to let in CO2 so that it can produce glucose - also lets water out
• transp rate not exactly same as water uptake by plants - some water used in reactions e.g. in photosynth and to support the plant and some water produced during resp
• structure and function of the phloem:

  • solutes are dissolved substances
  • phloem tissue transports organic solutes (mainly sugars like sucrose) round plants
  • like xylem, phloem is formed from cells arranged in tubes
• sieve tube elements and companion cells are important cell types in phloem tissue
• sieve tube elements are living cells that form the tube for transporting solutes. They have no nucleus and few organelles
• There is a companion cell for each sieve tube element. They carry out living functions for sieve cells e.g. providing the energy needed for the active transport of solutes
• translocation is the movement of solutes (e.g. amino acids and sugars like sucrose) to where they are needed in the plant. Solutes are sometimes called assimilates.
• Translocation is an energy requiring process that happens in the phloem
• translocation moves solutes from 'sources' to 'sinks'.
• the source is where assimilates are produced (so they are at high concentration here).
• the sink is where assimilates are used up (so they are at lower conc. there)
• assimilates are substances that become incorporated into the plant tissue
• the source for sucrose is usually the leaves (where it is made) and the sinks are the other parts of the plant, especially the food storage organs and the meristems (areas of growth) in the roots, stems and leaves
• enzymes maintain a conc. gradient from source to sink by changing the solutes at the sink (e.g. be breaking them down or making them into something else). This makes sure there is always a lower conc. at the sink than at the source
• in potatoes, sucrose is converted into starch in the sink areas, so there is always a lower conc. of sucrose at the sink than inside the phloem. This makes sure a constant supply of new sucrose reaches the sink from the phloem