Plant Nutrition: Absorption and Transport of Water and Nutrients
**Thallophyte Organization**
Thallophyte organization refers to a type of organization where there are no tissues or organs, such as in algae and lichens.
**Cormophyte Organization**
Cormophyte organization refers to an organization in which cells are grouped into tissues and organs, such as in Pteridophytes and Spermatophytes.
Cormophytes have organs for nutrition:
- Roots: for absorption of water (H2O) and mineral salts.
- Leaves: for capturing light and atmospheric carbon dioxide (CO2).
- Conducting vessels: transport both substances, such as in phloem sap.
**Osmosis**
Osmosis is the movement of water across a membrane. The membranes of plant cells are permeable to water and relatively impermeable to solutes, so water moves through them from where the concentration of solutes is lower to where it is higher.
**Colloid**
A colloid is a type of mixture in which a substance is contained within another without actually dissolving.
**Water Absorption**
The roots maintain a continual supply of water that plants need for nourishment and compensate for its loss by transpiration. The root zone where water is absorbed is the piliferous area. It comprises epithelial cells with absorbent hairs, whose walls are thin and lack a cuticle. The cell membrane is a semipermeable barrier that separates the outside and inside of the cell, determining the selective absorption of nutrients. Water passes through the membrane and enters the hairs by osmosis.
**Factors Affecting Water Absorption**
- Temperature: Promotes cell metabolism and increases absorption.
- Aeration: Soil aeration causes an increase in absorptive surface, forming highly branched roots and long, numerous root hairs.
- Quantity of Water: An increase in soil water promotes its entry into the roots, where the concentration of salts in the soil is less than inside the root hairs.
- Holding Capacity: This is important because, in many cases, the water in the soil is retained in the form of colloids.
**Absorption of Mineral Salts**
Mineral salts are absorbed in the form of ions. The mechanism of entry is by active transport and takes place against a concentration gradient, so it requires an expenditure of energy. It requires the participation of plasma membrane transport enzymes, introducing foreign ions within the epidermis and the absorbent hair cells. Once the water and mineral salts have penetrated, they form the xylem sap. This circulates inside the root to the central cylinder where the xylem is located. Transport to get to the xylem vessels can be:
- Via A or Symplastic: Water and solutes are transported by osmosis and active transport between cells through plasmodesmata (cell-wall pores that communicate with neighboring cells).
- Via B or Apoplastic: The movement is by simple diffusion from the outer cell membrane and comprises the cell walls and intercellular spaces. It is interrupted in the endodermis of the root by the Casparian strip, which regulates the passage of substances.
**The Transport of Phloem Sap**
Organic molecules produced in photosynthesis form the phloem sap. The transport of these substances from the tissues that produce them to all parts of the plant takes place through the phloem vessels, sieve tubes, and companion cells. Phloem vessels are living cells whose separation walls (sieve plates) are perforated by pores, allowing the flow of sap. When organic compounds accumulate in the roots, transport is carried out in that direction. The organic components produced in the chlorophyll parenchyma of the leaf undergo active transport to the phloem companion cells and, through the plasmodesmata, enter the sieve tubes.
**Mechanisms of Transport**
- Passive Mechanism (Mass Flow Hypothesis): The movement is performed by a pressure gradient between photosynthetic producer organs and consumer organs. The increase in carbohydrates in the sieve tubes tends to raise the concentration of solutes, causing water to enter by osmosis. This water comes from the xylem cells and produces a thrust of the phloem sap toward the consumer organs, where they pass by active transport into cells that require it. The resulting decrease in the concentration of solutes causes water to exit the sieve tubes by osmosis to the surrounding tissues.
- Active Mechanism (Cytoplasmic Streaming Hypothesis): This view suggests that much of the organic compounds could be transported through the cytoplasm of sieve tubes with energy consumption. The motion of the phloem sap is slow, as the lumen of the phloem vessels is interrupted by sieve plates, whose holes are plugged with callose during the cold season, disrupting transport, which is resumed in spring.