Mass Transport in Plants

• Transport of water in the xylem:

  • water is absorbed by the roots through extensions called root hairs by osmosis
  • in flowering plants, the vast majority of water is transported through hollow, tick-walled tubes called xylem vessels
  • the main force that pulls water through the xylem vessels in the stem of a plant is the evaporation of water from the leaves - a process called transpiration
  • the energy for this is supplied by the sun and the process is passive
• Xylem vessels:

  • xylem vessels transport water and dissolved mineral ions
  • they are long, tube-like structures formed from dead cells
• Structure and function of xylem vessels:

  • the cells walls contain lignin - lignin strengthens the xylem walls against the tension within them and makes them waterproof
  • the lignified vessel walls cause the cell contents to die - this leaves a hollow lumen with no cytoplasm that offers little resistance to the mass flow of water (so water flows as a column) and minerals
• Structure and function of xylem vessels:

  • the walls of xylem vessels contain tiny holes called pits - if a vessel becomes blocked or damaged, the water can be diverted laterally (into the neighbouring xylem vessels), so the upwards movement of water can continue in an adjacent vessel
  • the vessels also lose their end walls - again, this reduces resistance to flow. They form a continuous vessel for a column of water to move from root to leaves
• Transpiration:

  • transpiration is the process of water movement through a plant and its evaporation from leaves
  • the continuous columns of water that move from the soil through the roots, stems and leaves to the air are known as the transpiration stream
• The cohesion-tension theory:

  • water vapour molecules diffuse out of the air spaces into the surrounding air down a water potential gradient. This is known as transpiration. By changing the size of the stomatal pores, plants can control their rate
  • to replace this, water evaporates from the mesophyll cells into the air spaces forming water vapour, which builds up in the air spaces
  • the water potential in the mesophyll cells is now lower than in the xylem cells, so water from the xylem moves into the mesophyll cells by osmosis
• The cohesion-tension theory:

  • this creates low hydrostatic pressure at the top of the xylem in the stem (as water has moved out, lowering the volume in the top of the xylem)
  • water in the xylem is under tension and is pulled towards the leaves
  • water molecules form hydrogen bonds between one another and hence tend to stick together. This is known as cohesion. Continuous columns of water are maintained due to this cohesion, and is pulled up the xylem as a result of transpiration
  • water can then move by osmosis from the soil into the roots
• The cohesion-tension theory:

  • there is also adhesion of water molecules to the walls of the xylem vessels. This creates an inward pull on the vessel walls as the water is pulled up, creating a negative pressure within the xylem, causing the xylem vessels to decrease in diameter
  • this process is entirely passive as no ATP from metabolic processes is required for this to occur; the energy needed to drive the process comes from heat energy from the Sun. This cohesion-tension theory is the best current explanation of how water moves up the stems of plants and can account for the movement of water even to the top of very tall trees
• Evidence for the cohesion-tension theory:

  • change in the diameter of tree trunks according to the rate of transpiration. During the day, when transpiration is at its greatest, there is more tension (more negative pressure) in the xylem due to adhesion of the water molecules. This pulls the walls of the xylem vessels inwards and causes the trunk to shrink in diameter. At night when transpiration is at its lowest, there is less tension in the xylem and so the diameter of the trunk increases
  • this can be measured using a dendrometer
• Evidence for the cohesion-tension theory:

  • if a xylem vessel is broken and air enters it, the tree can no longer draw up water. This is because air bubbles form in the xylem, meaning the continuous column of water is broken as the cohesion of water molecules is prevented
  • when a xylem vessel is broken, water does not leak out, as it would be the case if it were under pressure. Instead, air is drawn in, which is consistent with it being under tension
  • respiratory inhibitors, such as cyanide or a lack of oxygen, do not inhibit this process
• Factors affecting transpiration rates:

  • light
  • temperature
  • wind speed
  • humidity
• Light and transpiration rates:

  • higher light intensity leads to a faster rate of transpiration (positive correlation)
  • stomata open when it is light to let $CO_2$ in for photosynthesis
  • when its dark the stomata are usually closed so there is little transpiration
• Temperature and transpiration rates:

  • higher temperature leads to a faster rate of transpiration (positive correlation)
  • warmer water molecules have more energy so evaporate from cells inside the leaf faster
  • this increases the water potential gradient between inside and outside the leaf, so water diffuses out faster
  • this will plateau if the temperature is too high as proteins and enzymes will denature
• Wind speed and transpiration rates:

  • faster wind speed leads to a faster rate of transpiration (positive correlation)
  • lots of air movement blows away water molecules from around the stomata
  • this increases the water potential gradient between inside and outside the leaf, so water diffuses out faster
  • however, this will plateau when the stomata become the limiting factor
• Humidity and transpiration rates:

  • lower humidity leads to faster rates of transpiration (negative correlation)
  • if air around the plant is dry, the water potential gradient between the leaf and the air outside is increased, so water diffuses out faster
• Measuring transpiration rates

  • it is almost impossible to measure transpiration because it is difficult to condense and collect all the water vapour that leaves all the parts of a plant
  • water can be used within the plant and can be used in photosynthesis, hydrolysis reactions or for turgidity, and can be produced in respiration
  • we are able to measure the amount of water that is taken up in a given time by part of the plant such as a leafy shoot
  • about 99% of the water taken up by the plant is lost during transpiration, which means that the rate of uptake is almost the same as the rate of transpiration occurring (assume that all water taken up is the same as the transpiration rate)
  • the rate of water loss in a plant can be measured using a potometer
• Potometer experiment:

  • a leafy shoot is cut underwater (at a slant to increase surface area). Care is taken to not get water on the leaves
  • the potometer is filled completely with water making sure there are no air bubbles
  • using a rubber tube, the leafy shoot is fitted to the potometer underwater
  • the potometer is removed from under the water and all joints are sealed (waterproof jelly)
  • an air bubble is introduced to the capillary tube
  • the distance moved by the air bubble in a given time is measured a number of times and the mean is calculated
• Potometer experiment:

  • rate of movement can then be calculated; you need to know the mean distance the bubble has moved, how long it has taken to move, and the diameter of the tube
  • the bubble can be returned to the starting position by opening the tap on the reservoir
  • the experiment can then be repeated to compare the rates of water uptake under different conditions, for example at different temperatures humidity, light intensity or the differences in water uptake between different species under the same condition
• Transport in the phloem:

  • plants have another transport tissue called the phloem
  • the process by which soluble organic molecules (sucrose and amino acids) and some mineral ions (potassium, chloride, phosphate) are transported around the plant as sap is known as translocation
• Structure of phloem vessels:

  • phloem tissue is made up of sieve tube elements, long, thin, living cells arranged end to end. Unlike in the xylem, end walls of the sieve tube elements do not breakdown; instead, they have small pores called perforations and are referred to as sieve plates
  • sieve tube elements contain little cytoplasm, no nucleus and very few organelles, allowing more space for transporting materials
  • sieve tube elements are associated with companion cells, which carry out many of the living functions of sieve tube elements. The companion cells are biochemically highly active, containing a large nucleus, dense cytoplasm, and many mitochondria. They are connected to the sieve tube elements by cytoplasmic connections called plasmodesmata
• Translocation:

  • experiments have demonstrated that translocation can be bidirectional not just unidirectional like transpiration, and that rates of translocation are around 1ms$^{-1}$
  • this is too rapid to simply be explained by diffusion
  • translocation move organic solutes from 'sources' to 'sinks'
• The mass flow theory:

  the mass flow theory is the currently accepted explanation of translocation
• Transfer of sucrose into sieve elements from photosynthesising tissue:

  • sucrose is synthesised from the products of photosynthesis in cells with chloroplasts
  • the sucrose diffuses down a concentration gradient by facilitated diffusion from the photosynthesising cells to the companion cells
  • hydrogen ions and actively transported from companion cells into the spaces within cell walls using ATP
  • these hydrogen ions then diffuse down a concentration gradient through carrier proteins into the sieve tube elements
  • sucrose molecules are transported along with the hydrogen ions in a process known as co-transport. The protein carriers are therefore also known as co-transport proteins
• Mass flow of sucrose through sieve tube elements:

  • mass flow is the bulk movement of a substance through a given channel or area in a specified time
• Mass flow of sucrose through sieve tube elements:

  • the sucrose produced by photosynthesising cells (source) is actively transported into the sieve tubes
  • this causes the sieve tubes to have a lower (more negative) water potential
  • as the xylem has a much higher water potential, water moves from the xylem into the sieve tubes by osmosis, creating a high hydrostatic pressure within them
• Mass flow of sucrose through sieve tube elements:

  • at the respiring cells (sink), sucrose is actively transported from sieve tube elements through companion cells into sink cells. Sucrose is then either used up in respiration or converted to starch for storage
  • this reduces the water potential of this part of the sieve tube element and so water leaves the sieve tubes by osmosis, re-entering the xylem
  • the hydrostatic pressure of the sieve tubes in this region is therefore lowered
  • as a result of water entering the sieve tube elements at the source and leaving at the sink, there is a high hydrostatic pressure at the source and a low one at the sink
  • there is therefore a mass flow of sucrose solution down this hydrostatic pressure gradient in the sieve tubes
• Mass flow:

  • while mass flow is a passive process, it occurs as a result of the active transport of sugars
  • therefore, the process as a whole is active which is why is it affected by, for example, temperature and metabolic inhibitors
• Evidence supporting the mass flow theory:

  • there is hydrostatic pressure in the phloem, as shown by the release of sap when it is cut
  • the concentration of sucrose is higher in the leaves (source) than it is in the roots (sink)
  • flow in the phloem occurs in daylight, but ceases when leaves are shaded, or at night
  • increases in sucrose levels in the leaf are followed by increases in the phloem a little later
  • metabolic inhibitors and/or a lack of oxygen inhibit translocation of sucrose in the phloem
  • companion cells have many mitochondria and readily produce ATP
• Evidence against the mass flow theory:

  • the theory leaves the function of sieve plates unclear as they would hinder mass flow of sucrose. It has been suggested that they may have a structural function in preventing bursting of sieve tubes under pressure
  • not all solutes move at the same speed - they should if moved by mass flow
  • sucrose is delivered at more or less the same rate to all sinks, rather than going faster to those with the lowest sucrose concentration as the mass flow theory would suggest
• Investigating transport in plants:

  • ringing experiments
  • tracer experiments
  • puncture experiments
• Ringing experiments:

  • if a complete ring of bark containing the phloem is removed while leaving the wood (xylem) intact, the downward movement of sugars is prevented
  • in time, the region of the stem immediately above the missing ring of tissue swells with liquid that is rich in sugars and other dissolved organic substances
  • some non-photosynthetic tissues in the region below the ring (towards the roots) wither and die, but the region above the ring continues to grow
• Conclusion from ringing experiments:

  • the conclusion drawn from this type of ringing experiment is that the phloem, rather than the xylem, is the tissue responsible for translocating sugars downwards in plants
  • as the ring of tissue removed has not extended into the xylem, it continuity had not been broken
  • if it were the tissue responsible for translocating sugars you would not have expected sugars to accumulate above the ring nor tissues below it to die
• Tracer experiments:

  • the isotope 14C can be used to make radioactively labelled carbon dioxide (14$CO_2$)
  • if a plant is then grown in an atmosphere containing 14$CO_2$ the 14C isotope woll be incorporated into the sugars produced during photosynthesis
  • these radioactive sugars can then be traced as they move within the plant using autoradiography which involves taking thin cross sections of the plant stem and placing them on a piece of X-ray film
  • the film becomes blackened where it has been exposed to the radiation produced by the 14C in the sugars. The blackened regions are found to correspond to where the phloem tissue is in the stem. As the other tissues do not blacken the film, it follows that they do not carry sugars and that the phloem alone is responsible for their translocation
• Tracer experiments:

  • in the photomicrograph, carbohydrates including sucrose from the leaves appear in the sieve tube elements of the phloem in the stem
  • the location of the label is revealed in the tissue cross sections by the presence of dark grains on the film
  • the label is confined almost entirely to the sieve elements and companion cells of the phloem, demonstrating the organic solutes such as carbohydrates travel in the phloem
• Puncture experiments:

  • if the phloem is punctured with a hollow tube then the sap oozes out, showing that there is a high pressure (compression) inside the phloem (this is how maple syrup is tapped)
  • if the xylem is punctured then air is sucked in, showing that there is a low pressure (tension) inside the xylem
• Puncture experiments:

  • aphids, such as greenfly, have specialised mouthparts called stylets, which they use to penetrate the phloem tubes and suck up the sugary sap therein
  • if the aphids are anaesthetised with carbon dioxide and cut off, the stylet remains in the phloem so pure phloem sap can be collected through the stylet for analysis
  • this technique is more accurate than a human with a syringe and the aphid's enzymes ensure that the stylet doesn't get blocked