Chapters
In this article, we will discuss how water and mineral ions are transported in plants. Before discussing the mechanism of movement of water in plants, first, let us discuss why water is critical for plants.
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Importance of Water in Plants
Water is important for plants because of the following reasons:
- Water is essential for photosynthesis which is a process by which plants make their food.
- Products of photosynthesis and mineral ions are moved around the plant in the form of a solution
- When water evaporates from the plant’s leaf, it decreases its temperature, i.e. it cools it down
- Water is needed to create a turgor pressure which makes cells rigid. This pressure provides support to the plant in the form of a hydrostatic skeleton
- Turgor pressure also gives the root cells sufficient strength to force their way through hard surfaces like concrete
In the next section of the article, we will discuss how water moves into the root.
How Does Water Move into the Root?
Water enters the plant through root hair. It then moves across the root cells into the xylem which moves it up and around the plant.
What are root hairs?
Root hairs refer to the thin, long extensions of root hair cells. Root hair cells have the following attributes which allow them to absorb sufficient water from the soil:
- Their small size allows them to grow between soil particles
- They have a large surface area to volume ratio which gives them more space for diffusion
- Their surface area is one cell thick that allowing the osmosis to occur quickly
- The water potential gradient is maintained by the solutes in the root hair cells so that the water from the soil can osmose in continuously
It is already discussed above that xylem transports water and mineral ions up and around the plant. In the next section of the article, we will discuss the xylem and its structure in detail.
Xylem
There are three main components of xylem tissue:
- Xylem vessels: The vessel elements include tracheid
- Fibres: They refer to the elongated cells having lignified walls that provide support to the plant
- Parenchyma cells: They are normal plant cells, however, they do not have chloroplasts
Pathways and Mechanism of Water and Mineral Ions Transport in Plants
- Inside a plant, mineral ions and organic compounds such as sucrose are transported after being dissolved in water. In other words, we can say that these substances are transferred in the form of a solution. The dissolved mineral ions are moved in the xylem tissue, whereas the dissolved organic compounds are moved in the phloem tissue
- The roots of the plant have the responsibility to uptake water and mineral ions. They contain root hairs which increase their surface area for more efficient absorption of substances from the soil.
- The water uptake is a passive process that takes place through osmosis. Osmosis refers to the diffusion of water from a higher water potential to a region with lower water potential.
- The uptake of minerals can be either active or passive. It takes place through active transport or diffusion respectively.
- Transpiration is the process by which the water vapours are lost by the plant to its environment through diffusion and the transpiration stream means the movement of water from the roots to the leaves.
- Since water is lost continuously from the leaves of the plant by a process known as transpiration, therefore plants must get a constant supply of water and mineral ions from the roots to compensate for this water loss.
- The movement of water through the xylem of the plants takes place mainly because of the evaporation of water vapours from the leaves and the cohesive and adhesive attributes of the water molecules.
- Plants constantly take water in their roots and lose water through stomata (tiny pores) in the leaves.
- Approximately, 99% of water absorbed is lost via evaporation from the stem of the plant and its leaves through the process of transpiration.
- The continuous supply of water is also essential in plants for carrying out photosynthesis.
Water and dissolved solutes can take the following two pathways to move across the cortex:
- Apoplast pathway
- Symplast pathway.
In the next section of the article, we will discuss the apoplast pathway in detail.
Apoplast Pathway
- The majority of the water moves through the apoplast pathway (when there are high transpiration rates. This pathway refers to the series of spaces that run through the cell walls composed of cellulose, dead cells, and hollow xylem tubes.
- The process by which the water moves is known as diffusion (water does not cross the partially permeable membrane.
- The water either moves from the cell wall to the cell wall or through the intercellular spaces.
- Water moves through the apoplast pathway more rapidly than the symplast pathway
- When the water arrives at the endodermis, a thick, waterproof, waxy band of suberin within the cell wall blocks the apoplast pathway
- This band is referred to as the Casparian strip and creates an impassable barrier for the water
- The water and dissolved minerals must take the symplastic pathway after they reach the Casparian strip. Why this strip is present there is not yet comprehended fully, however, it is believed that it may assist the plant to control which mineral ions reach the xylem and produce root pressure
- The Casparian strip thickens when the plant ages because of the more deposition of suberin except in the cells known as the passage cells. This enables more control of the mineral ions.
In the next section, we will discuss the symplast pathway in detail.
Symplast Pathway
- A tiny amount of water moves through the symplastic pathway which includes cytoplasm and plasmodesmata or vacuoles of the cells
- The movement of water occurs through osmosis into the cell (across the cell surface membrane which is partially permeable), probably into the vacuole through the tonoplast by osmosis and between the cells by the plasmodesmata.
- The movement of water through the symplastic pathway is slower as compared to the apoplastic pathway.
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