M3:S3 Transport in plants

• Multicellular plants need transport systems
• Plants need substances like water, minerals and sugars to live. They also need to get rid of waste substances. Plants also need carbon dioxide, but this enters at the leaves (where it's needed)
• Like animals, plants are multicellular - so they have a small surface area : volume ratio (SA:V, see page 70). They're also relatively big with a relatively high metabolic rate
• Exchanging substances by direct diffusion from the outer surface to the cells would be too slow to meet their metabolic needs
• Plants need transport systems to move substances to and from individual cells quickly
• Xylem tissue

  Transports water and mineral ions in solution. These substances move up the plant from the roots to the leaves
• Phloem tissue

  Mainly transports sugars (also in solution) both up and down the plant
• Xylem and phloem make up a plant's vascular system. They are found throughout a plant and transport materials to all parts
• The position of the xylem and phloem in the root, leaf and stem are shown in these transverse cross-sections. Transverse means the sections cut through each structure at a right angle to its length
• You can also get longitudinal cross-sections. These are taken along the length of a structure
• Xylem vessels

  • They are very long, tube-like structures formed from 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
  • The cells are dead, so they contain no cytoplasm
  • Their walls are thickened with a woody substance called lignin, which helps to support the xylem vessels and stops them collapsing inwards
  • The amount of lignin increases as the cell gets older
  • Water and ions move into and out of the vessels through small pits in the walls where there's no lignin
• Phloem tissue

  Transports solutes (dissolved substances), mainly sugars like sucrose, round plants
• Phloem tissue

  • It contains phloem fibres, phloem parenchyma, sieve tube elements and companion cells
  • Sieve tube elements and companion cells are the most important cell types in phloem for transport
• Sieve tube elements

  • They are living cells that form the tube for transporting solutes through the plant
  • They are joined end to end to form sieve tubes
  • The 'sieve' parts are the end walls, which have lots of holes in them to allow solutes to pass through
  • Unusually for living cells, sieve tube elements have no nucleus, a very thin layer of cytoplasm and few organelles
  • The cytoplasm of adjacent cells is connected through the holes in the sieve plates
• Companion cells

  The lack of a nucleus and other organelles in sieve tube elements means that they can't survive on their own. So there's a companion cell for every sieve tube element. Companion cells carry out the living functions for both themselves and their sieve tube elements
• Dissecting plant stems

  1. Use a scalpel (or razor blade) to cut a cross-section of the stem (transverse or longitudinal)

  2. Use tweezers to gently place the cut sections in water until you come to use them

  3. Transfer each section to a dish containing a stain, e.g. toluidine blue O (TBO), and leave for one minute

  4. Rinse off the sections in water and mount each one onto a slide
• Toluidine blue O (TBO) stains the lignin in the walls of the xylem vessels blue-green. This will let you see the position of the xylem vessels and examine their structure
• Water enters a plant through its root hair cells
• Osmosis

  Water always moves from areas of higher water potential to areas of lower water potential - it goes down a water potential gradient
• Water moves through the roots into the xylem

  1. Symplast pathway - goes through the living parts of cells - the cytoplasm

  2. Apoplast pathway - goes through the non-living parts of the cells - the cell walls
• When water in the apoplast pathway gets to the endodermis cells in the root, its path is blocked by a waxy strip in the cell walls, called the Casparian strip. Now the water has to take the symplast pathway
• Both pathways are used, but the main one is the apoplast pathway because it provides the least resistance
• Water moves up the xylem and out at the leaves

  1. Water evaporates from the cell walls into the spaces between cells in the leaf

  2. When the stomata (tiny pores in the surface of the leaf) open, the water diffuses out of the leaf (down the water potential gradient) into the surrounding air
• Transpiration stream

  The movement of water from roots to leaves
• Mechanisms that move water up a plant

  • Cohesion and tension help water move up plants, from roots to leaves, against the force of gravity
  • Adhesion is also partly responsible for the movement of water
• Transpiration is the evaporation of water from a plant's surface, especially the leaves
• Transpiration happens as a result of gas exchange. Plants need to open their stomata to let in carbon dioxide so that they can produce glucose (by photosynthesis). But this also lets water out
• Four main factors that affect transpiration rate

  • Temperature
  • Humidity
  • Wind
  • Light
• Light - the lighter it is the faster the transpiration rate. This is because the stomata open when it gets light, so CO2 can diffuse into the leaf for photosynthesis. When it's dark the stomata are usually closed, so there's little transpiration
• Transpiration

  The evaporation of water from a plant's surface, especially the leaves
• Transpiration happens as a result of gas exchange
• Gas exchange in plants

  1. Plant opens stomata to let in carbon dioxide for photosynthesis

  2. Water moves out of the leaf down the water potential gradient when the stomata open
• Transpiration

  A side effect of the gas exchange needed for photosynthesis
• Four Main Factors Affecting Transpiration Rate

  • Temperature
  • Humidity
  • Wind
  • Light
• Light

  The lighter it is the faster the transpiration rate. This is because the stomata open when it gets light, so CO2 can diffuse into the leaf for photosynthesis. When it's dark the stomata are usually closed, so there's little transpiration.
• Temperature

  The higher the temperature the faster the transpiration rate. Warmer water molecules have more energy so they evaporate from the cells inside the leaf faster. This increases the water potential gradient between the inside and outside of the leaf, making water diffuse out of the leaf faster.
• Humidity

  The lower the humidity, the faster the transpiration rate. If the air around the plant is dry, the water potential gradient between the leaf and the air is increased, which increases transpiration.
• Wind

  The windier it is, the faster the transpiration rate. Lots of air movement blows away water molecules from around the stomata. This increases the water potential gradient, which increases the rate of transpiration.
• Potometer

  A special piece of apparatus used to estimate transpiration rates. It actually measures water uptake by a plant, but it's assumed that water uptake by the plant is directly related to water loss by the leaves.
• Using a potometer

  1. Cut a shoot underwater to prevent air from entering the xylem

  2. Assemble the potometer in water and insert the shoot underwater

  3. Remove the apparatus from the water but keep the end of the capillary tube submerged in a beaker of water

  4. Dry the leaves, allow time for the shoot to acclimatise, and then shut the tap

  5. Remove the end of the capillary tube from the beaker of water until one air bubble has formed, then put the end of the tube back into the water

  6. Record the starting position of the air bubble

  7. Start a stopwatch and record the distance moved by the bubble per unit time, e.g. per hour