The passage of nutrients across small distances is carried out through active transport or passive transport using the plasmodesmata.
Passive transport does not require any energy, and by this process, the solutes or molecules move from the regions of higher concentration to those of lower concentration. This movement could be from one cell to its neighboring cells or via the spaces in between the cells. It is the major means of gaseous flux in plants.
The substances could be lipid-soluble or water-soluble, hydrophobic or hydrophilic, and may need to cross lipid layers or through some other membranes. Facilitated diffusion utilizes special proteins that help in transportation. It is a selective method of uptake by nutrients. Example- Aquaporin transports only water molecules.
The transportation carried out by transport proteins could be uniport, symport, or antiport.
Active transportation as the name indicates requires energy to move a substance uphill, which is from lower concentration to higher concentration. Membrane-associated proteins like “pump†carry out active transportation in plants. The saturation point could be achieved when maximum transportation takes place. Example- Loading and unloading sugar molecules via the phloem.
Transport of water over long distances:
The diffusion of substances over long distances is not a feasible action since it is a very slow process. Hence, the plant uses the mass flow system of transportation.
The mass flow of substances is achieved by either a positive or negative hydrostatic pressure gradient by which water is transported.
Organized tissues like the xylem and phloem are linked with the translocation of substances across the plant. Xylem functions like a bus to its passengers which are the water molecules, minerals, and some hormones mainly from roots to other plant parts, whereas the passengers of phloem include nutrients, solutes, from the leaves to other body parts.
In particular, the tracheids and vessels of the xylem are dead cells that transport water and the sieve elements of the phloem (live cells) transport other nutrients. The movement of water in plants cover only a few centimeters, but in the case of trees, the distance covered could be around 100 meters or more.
The absorption of water from the soil is mainly performed by the roots. The thin walls of the root have a larger surface area and facilitate the absorption of water and solutes via its tiny root hairs. This water passes through the root through two different pathways.
1. Apoplast: It is the pathway tunneling through the living cells, spaces between the cells, and their rigid cell wall. It is a continuous pathway and need not traverse through the endodermis’s Casparian strip. It is a gradient dependent movement and has no barriers.
2. Symplast: This pathway is through the plasmodesmata and the cytoplasm of the cells. It is comparatively slow and is gradient dependent. Example- The water movement across the endodermis region of the roots can only be made possible via the symplastic pathway.
Root pressure: The positive pressure exerted by the roots to take up water molecules through the vessels of the xylem is called root pressure. It acts as a lift to water molecules but only up to a certain height in the stem. The root pressure can be visualized in the veins when the rate of evaporation is low. This way the water molecules are lost and are referred to as guttation.
The water is taken up by the plant using several mechanisms. The cohesion-tension-transpirational pull is also one of the reasons. It helps in the absorption and transportation of water.
Translocation of nutrients:
Minerals are charged particles and are taken up from the soil by roots through active or passive transport mechanisms.
Some special proteins present in the membranes of the root actively transport the minerals from the soil into the cells.
The ions have now entered inside the plant and are now ready to be transported through the xylem and phloem to reach the target sites. The young and developing parts of the plant like the stem, flowers, fruits, and meristematic tissues are the destination of these nutrients.
The nutrients present in the almost dead parts and the old parts are drawn back from these sources and sent to the budding young regions of the plant. The much-needed macronutrients from the old parts like nitrogen, phosphorus, sulfur are repacked and sent. The structural components of the plants like the calcium are an exception to this.
The mass flow hypothesis:
The source is the region of the plant where the food is cooked and the sink is the members that require the food. The source and sink are variable and change depending on the needs, and the season.
Usually, the xylem functions in the unidirectional motion while the phloem in bidirectional motion.
The mass flow hypothesis is the mechanism followed by the phloem sap which are live cells to translocate sugars, hormones, water, and in some cases amino acids. Xylem and phloem are not strict in transporting the substances, they sometimes exchange the substances they carry.
The photosynthates in the form of glucose are produced by the mesophyll cells during photosynthesis. It is transformed into a non-reducing form of sugar (sucrose).
As shown in the diagram, the sucrose transports via the plasmodesmata, into the neighboring cells, the phloem parenchyma, companion cells, sieve elements by either the symplast pathway or the apoplast pathways. The mode of active transport is mediated.
When the sucrose is loaded into the cells (loading process), a hypertonic environment develops in the phloem (since more nutrients are accumulated). Since a hypertonic environment has been created, the water molecules from the nearby xylem elements visit the phloem, this is known as osmosis.
The osmotic pressure is high in the source but is lower in the sink. This environment moves solutes from the source to the sink, again by the mode of active transport. The nutrients are now accessible to the plant parts that need them.
After this movement, the osmotic pressure drops down, and the water molecules pave their path outside the phloem. This way by building up pressure, the movement of nutrients is carried out successively by the xylem and phloem.
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