The movement of molecules, specifically water and any solutes, is vital to understand in light of plant processes. This will be more or less a quick review of several guiding principles of water motion in reference to plants.
- Diffusion—Diffusion is the net movement of molecules or ions from an area of higher concentration to an area of lower concentration. Think of it as a rebalancing. The molecules or ions are said to be moving along a diffusion gradient. If molecules or ions moving in the opposite direction are said to be moving against a diffusion gradient. Diffusion will continue until a state of equilibrium is reached. Rates of diffusion are affected by temperature and the density of the involved molecules among other things. In the leaves, water diffuses out via the stomata into the atmosphere.
- Osmosis—Osmosis in plant cells is basically the diffusion of molecules through a semipermeable, or differentially permeable, membrane from a region of higher solute concentration to a region of lower solute concentration. The application of pressure can prevent osmosis from occurring. Plant physiologists like to describe osmosis more precisely in terms of potentials. Osmotic potential is the minimum pressure required to prevent fluid from moving as a result of osmosis. Fluid will enter the cell via osmosis until the osmotic potential is balanced by the cell wall resistance to expansion. Any water gained by osmosis may help keep a plant cell rigid or turgid. The turgor pressure that develops against the cell walls as a result of water entering the cell’s vacuole. This pressure is also referred to as the pressure potential. The crunch when you bite into a celery stick is as a result of the violation of the cell’s turgor pressure. The osmotic potential and pressure potential combined make up the water potential of a plant cell. If there are two cells next to each other of different water potentials, water will move from the cell with the higher water potential to the cell with the lower water potential. Water enters plant cells from the environment via osmosis. Water moves because the overall water potential in the soil is higher than the water potential in the roots and plant parts. If the soil is desiccated then there will be no net movement into the plant cells and the plant will die.
- Plasmolysis—Plasmolysis is the loss of water via osmosis and accompanying shrinkage of the protoplasm away from the cell wall. When this occurs, the cell is said to be plasmolyzed. This process can be reversed if the cell is placed in fresh water and the cell is allowed to regain its turgor pressure. However, as with anything living, there is a point of no return and permanent or fatal damage to the cell can occur.
- Imbibition—Imbibition is the swelling of tissues, alive or dead, to several times their original volume. This is a result of the electrical charges on materials in suspension (colloidal) such as minerals, cellulose and starches attracting highly polar water molecules which then move into the cell. This swelling process is the initial step in the germination of seeds.
- Active Transport—Active transport is the energy assisted movement of substances against a diffusion or electrical gradient. This process requires enzymes and a ‘proton-pump’ embedded in the plasma membrane. The pumps are energized by ATP molecules—a cellular energy storage molecule.
Water and its Movement Through the Plant
Roughly 90% of the water that enters a plant is lost via transpiration. Transpiration is the loss of water vapor through the leaves, just to refresh you. In addition, less than 5% of the water entering the plant is lost through the cuticle. Water is vital to plant life, not just for turgor pressure reasons, but much of the cellular activities occur in the presence of water molecules and the internal temperature of the plant is regulated by water. Recall that the xylem pathways go from the smallest part of the youngest roots all the way up the plant and out to the tip of the smallest and newest leaf. This internal plumbing system, paired with phloem and its nutrient transportation system, maintains the water needs and resources in the plant. The issue of the processes by which water is raised through columns—of considerable height at times—has been studied and debated for years in botany circles. The end result is the cohesion-tension theory.
The Cohesion-Tension Theory
Polar water molecules adhere to the walls of xylem tracheids and vessels and cohere to each other which allows an overall tension and form ‘columns’ of water in the plant. The columns of water move from root to shoot and the water content of the soil supplies the ‘columns’ with water that enters the roots via osmosis. The difference between the water potentials of the soil and the air around the stomata are capable of producing enough force to transport water through the plant—from bottom to top and thus goes the cycle.
Transport of Food Substances (Organic Solutes) in Solution
Phloem is responsible for transporting food substance throughout the plant. As with water movement in plants, the movement of organic solutes in plants has been studied and debated for years. The currently accepted hypothesis is the pressure-flow hypothesis for the translocation of solutes.
The Pressure-Flow Hypothesis
This is essentially a source and sink hypothesis. Food substances that are in solution flow from a source, which is generally where water is taken up by osmosis (roots; food storage tissues, such as root cortex or rhizomes; and food producing tissues such as mesophyll in leaves), and the food substances are then given up at a destination or a sink where the food resources will be utilized in growth. The idea is that the organic solutes are moved along concentration gradients existing between sources and sinks.
At the source, phloem-loading occurs and sugars are moved by active transport into the sieve tubes of the smallest veins. The overall water potential in the sieve tube drops and then water enters the phloem cells via osmosis. The resulting turgor pressure from the movement of the water is enough to drive the solution through the sieve network to the sink . The sugar is unloaded at the sink via active transport and water then exits the ends of the sieve tubes. The pressure drops as the water exits, which causes a mass flow from the higher pressure at the source to the now lowered pressure at the sink. Much of the water that exits the sieve tubes will diffuse back into the xylem where it can be recirculated, transpired once it reaches the source. In a nutshell the mass flow is caused by drops in turgor pressure at the sink as the sugar molecules are removed. This generates the next push of materials toward the sink.
Regulation of Transpiration
It is the responsibility of the stomata to regulate transpiration and gas exchange via the actions of the guard cells. The pores of the stomata are closed when turgor pressure in the guard cells is low, and they are open when turgor pressure is high. Changes occur when light intensity, carbon dioxide concentration or water concentration change. The guard cells of the stomata use energy to take up potassium ions from adjacent epidermal cells.The uptake opens the stomata because water potential in the stomata drops and water moves into the guard cells and increases turgor pressure. When the potassium ions are released, the water then leaves the cells as the water potential shifts again. There is evidence that stomata will close with water stresses, but there also seems to be some indication that hormones are involved cause a loss of potassium ions from the guard cells and thus a pore closure.
Most plants keep their stomata open during the day and close them at night. However, there are plants that do the opposite and open their stomata during the night when overall water stress is lower. These plants have a specialized form of photosynthesis called CAM photosynthesis since the standard source of carbon dioxide is shut off as the stomata are closed during daylight hours. There are desert plants that are able to store carbon dioxide in their vacuoles in the form of organic acids that are converted back into carbon dioxide during the daytime for standard photosynthetic processes. As mentioned earlier, there are also adaptations such as sunken stomata which reduce the loss of water. Submerged or partially submerged plants generally do not have stomata on the underwater portions of their leaves.
High humidity will reduce transpiration rates while low humidity accelerates the process. There is a direct correlation between temperature and water movement out of the leaf. At high temperatures the rate of transpiration increases, while the opposite occurs at lower temperatures.
Mineral Requirements for Growth
Many external factors will affect growth rates and quality. The minerals available in the local soil is one such source of external input. Essential plant elements include: carbon, hydrogen, oxygen, phosphorus, potassium, nitrogen, sulphur, calcium, iron, magnesium sodium, chlorine, copper, manganese, cobalt, zinc, molybdenum, and boron to name the most common. Other minerals are required but they vary greatly from plant to plant. For example some algae need large amounts of iodine and silicon, while some loco weed species need selenium—which is poisonous to cattle on its own.
Macronutrients and Micronutrients
When any of these elements are lacking in the soil and the deficiencies are not compensated for by adding fertilizer compounds of compost the plant will demonstrate characteristic symptoms of mineral deficiencies. Most commercial fertilizers are some ratio of nitrogen, phosphorus and potassium and thus are able to compensate for a wide variety of issues. As an example of uses for the essential element in plants we will look at a few elements and how they are utilized:
Nitrogen—used in the building of proteins, nucleic acids and chlorophyll
Potassium—responsible in the process of enzyme activation, usually found in the Meristems
Calcium—vital part of the middle lamella and has a direct role in the movement of substances through cell membranes
Phosphorus—vital role in respiration and cellular division also used in the synthesis of energy compounds—ATP and ADP
Magnesium—central component of the chlorophyll molecule and involved in enzyme activation
Sulphur—structural component of many amino acids
Iron—integral in chlorophyll production and plays role in respiration
Boron—role in calcium ion use, not clearly understood
As you can see by scanning through the list, all of these elements are involved to one degree or another in vital life sustaining processes!