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Transport in Plants: CSEC Biology

Plants move around materials using two sets of tissues: the xylem and the phloem. The xylem transport water and dissolved mineral ions in one direction, i.e. from the roots to the leaves (for use in photosynthesis and other processes). The phloem on the other hand, transport sucrose bi-directionally, that is, from the leaves either up or down the plant in a process known as translocation.

Xylem consist of dead cells. The cells of the xylem lose their end walls so the xylem forms a continuous hollow tube through which the water and minerals pass. The xylem cells are also adapted to their function in that they become strengthened by a substance called lignin in order to support and give structure to the plant.

Transpiration plays a significant role in how water makes its way up the plant. Transpiration is the loss of water by plants through their aerial parts (i.e. stems and leaves). This usually occurs through the diffusion of water vapour from the spongy mesophyll layer out of the leaf through the stomata. This loss of water creates a negative pressure (suction force) in the leaf called transpiration pull. The lessened concentration of water in the mesophyll cells causes an upward pull.

Transpiration pull draws water from the roots to the leaves because of the effects of capillary action. The primary forces that create the capillary action are adhesion and cohesion. Adhesion is the attraction that occurs between substances and other surfaces, in this case it is between water and the surface of the xylem. Cohesion is the intermolecular attraction between like water molecules. Capillary action occurs when the adhesion of water molecules to the walls of the vessel is stronger than the cohesive forces between the water molecules. The thinner the vessel, the farther up the liquid will travel.

You can watch this video for more information on transpiration.

You may be wondering how plants are able to lose so much water and still survive, especially in adverse conditions of heat and low water availability.

Desert plants (xerophytes) and plants that grow in high salinity (halophytes) possess various adaptations for water conservation

  • Xerophytes will have high rates of transpiration due to the high temperatures and low humidity of desert environments

  • Halophytes will lose water as the high intake of salt from the surrounding soils will draw water from plant tissue via osmosis

These plants have various adaptations based on their environment to conserve water. In these drier/arid climates, plants have developed certain mechanisms to better survive in these conditions by conserving water. These include:

  • Reduced leaves- Typically, leaves will have 3 basic layers- the epidermis (upper and lower) and the mesophyll layers in the center. The epidermis has opening known as stomata through which transpiration and water loss occurs. By having a reduced leaf size, they reduce water loss through the epidermis as there are fewer stomata. Some plants also only have stomata on the lower epidermis, and some even have multiple layers of epidermal cells.

  • Water storage- The plants best known for water storage are succulents, which have multiple structural components for water storage. When water is available, they absorb it through their roots and bind it in place in interior water storage cells. The plant will keep the water there until it is needed. A good example of this is the Aloe vera. These plants have large fleshy leaves capable of storing large volumes of water for later usage. The leaf cuticle is also thick and covered in a thick layer of wax, which brings us to our next point...

image: Live Science

  • Coated Leaves- Plants in drier conditions tend to have coverings on their leaves that include wax or hairs to prevent water loss. The hairs on the leaves of certain plants also help to reduce air movement over the leaves, therefore reducing transpiration. As discussed in the video, higher wind speed results in higher rates of transpiration. So, by slowing the movement of air using the hairs on their leaves, these plants can reduce water loss. The hairs also trap moisture. The Brittlebush, Encelia farinosa, has small leaves with white hairs that are also reflective and help to reflect sunlight and reduce the temperature of the plant.

  • Physiological Mechanisms- Most succulents and xerophytes perform a specialized form of photosynthesis known as Crassulacean Acid Metabolism (CAM) (which involves the formation of malic acid due to an adaptation to the photosynthetic pathway). Their stomata can be opened only at night (to reduce water loss via evaporation) and then they store the carbon dioxide they absorb. The plants can then use the carbon dioxide stored for photosynthesis during daylight hours. During extended droughts, these plants can decrease their metabolism rate, keeping their stomata closed day and night, and maintaining in moist internal tissues a low level of activity sufficient to sustain life. The pineapple is a notable example of a plant that uses CAM photosynthesis.

  • Rolled Leaves- Several xerophytes have rolled leaves to reduce the exposure of the stomata to the air, and hence reduce water loss via evaporation. This is seen in Marram grass (Ammophila), where the stomata are on the inside of the rolled leaf.

  • Stomata Sunken in Pits- By having the stomata in small individual pits, moist air is trapped, reducing the rate of diffusion and, therefore, transpiration.

  • Very Long Roots- Plants in drier conditions must have longer roots that reach deeper into the soil to find water, since the high temperatures would evaporate most water near the surface. Different plants have different root systems to deal with this, as seen in the image. The saguaro cactus has wider reaching roots to absorb water as soon as it reaches the surface before leaching downwards. The other xerophytes there have deeper roots to reach the water table directly.

(You can find more here at ck12)

We have already discussed the xylem, however there is another vascular tissue that is essential to the plant's survival: the phloem. The phloem transports sugars (in the form of sucrose) from source tissues (leaves/photosynthetic leaf cells and storage tissues) to sink tissues (roots, flowers, other non-photosynthetic cells). This is known as translocation. During translocation, a pressure gradient is created causing the materials to move upwards and downwards in the phloem. Phloem is composed of several cell types including sclerenchyma, parenchyma, sieve elements and companion cells (you only need to know about sieve elements and companion cells).

Sieve Element Cells

These are narrow cells interconnected to form a phloem sieve tube. Sieve elements are connected by porous sieve plates that allow the flow of materials between the cells. These element cells have no nuclei and reduced organelles so that there is maximal space for the translocation of materials. Their walls are also very rigid and thick to withstand the hydrostatic pressure.

Since these cells have no nuclei and few organelles they need to be supported by the companion cells.

Companion Cells

These cells are responsible for supporting the sieve element cells metabolically. They also facilitate the loading and unloading of materials at source and sink. They have many mitochondria to facilitate the active transport of materials between the sieve tube and source or sink. These specialized cells also have appropriate transport proteins within the plasma membrane to move materials into or out of the sieve tube.

phloem diagram

The best-supported theory to explain the movement of food through the phloem (translocation) is called the pressure-flow hypothesis. This theory proposes that the material in the phloem flows under pressure created by the difference in water potential between the solution in the phloem and the relatively pure water in the xylem ducts.

As sugars accumulate in the phloem, water enters from the xylem through osmosis (flow of water from an area of high water potential to an area of lower water potential).

Turgor pressure, that is, the force causing turgidity, builds up in the sieve elements. As the fluid is pushed through the phloem, sugars are removed by sinks and consumed or converted into starch. Starch is insoluble and causes no difference in water potential. So, the water potential of the contents of the phloem increases. Finally, relatively pure water is left in the phloem, and this leaves by osmosis or is drawn back into nearby xylem vessels by the suction of transpiration-pull.

That may have been a bit confusing worded like that, but a diagram may be useful in understanding:

Additional Reading:

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