How does plant nutrient metabolism work?

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Contributed by: aallonharja
Submitted: June 18, 2003

How does plant nutrient metabolism work?

In other words, how do plants eat?

In order to live, plants need these 16 essential elements, called macronutrients and micronutrients.

Macro Nutrients
(primary nutes)
  • carbon (C)
  • hydrogen (H)
  • oxygen (O2)
  • nitrogen (N)
  • phosphorus (P)
  • potassium (K)
  • calcium (Ca)
  • magnesium (Mg)
  • sulfur (S)

    Micro Nutrients
    (Secondary nutes)
  • Boron (B)
  • Chlorine (Cl)
  • copper (Cu)
  • iron (Fe)
  • Manganese (Mn)
  • Zinc (Zn)
  • Molybdenum (Mo)

    Macronutrients and micronutrients
    Most of the plant is formed from Hydrogen, Carbon and Oxygen (~95% of the dry mass). Carbon comes from carbon dioxide (CO2) in the air. Hydrogen and Oxygen come from water. Note that this Oxygen must be available 'mixed in the water', as Dissolved Oxygen.

    The remaining macronutrients, Nitrogen, Phosphorus, Potassium, Calcium, Magnesium and Sulfur must be available to the plants root-hairs from the soil or from fertilizers, as part of the solution the plants roots are in contact with. Same applies to the micronutrients, Iron, Manganese, Boron, Copper, Zinc, Molybdenum and Chlorine.

    These essential elements are mostly used by the plants in ionic form, as inorganic salts that have dissolved into the nutrient solution.

    Next we will follow the course of an water drop with some fertilizers in it through the plant, to learn how plants metabolism works.

    The solution in the root-zone
    Whether plants are grown in soil, rockwool or water, solution with dissolved nutrients must come into contact with the plants roots. This nutrient solution should be of the suitable temperature, concentration, acidity and chemical composition to be healthy and to contribute positively to the plants growth and well-being.

    For 'our-favorite-plant' temperatures should be between 16-26 C degrees, or 60-80 F degrees. Low temperatures slow down the metabolism of the plant and its growth. On the other hand, in high temperatures there will be less of Dissolved Oxygen in the solution, causing the roots to be more vulnerable to diseases and

    Acidity in the root zone effects the intake of nutrient ions. Generally for hydroponic applications the recommended pH range for our favorite is between pH 5.2 and pH 6.0. If the the nutrient solution should become more acidic or alkaline then the availability of certain nutrients would decrease, making nutrients less available or even completely unavailable to the plant. Also problems like nutrient ions precipitating out of the solution could arise.

    The concentration of the nutrient solution should not be too strong, ie. over 1300-1500 ppm, nor should it be too weak. A strong solution would cause negative osmotic pressure on the plant. Because of high salinity, ie. the amount of dissolved solids outside the plants root cells, water flow would reverse to flow out of the plants, causing plants to lose their turgor, to wilt. Too weak solution wouldn't contain enough nutrients and might cause osmotic flow of nutrients to reverse, causing nutrient ions to flow out of the cells, leaving the plant hungry for more.

    "If a cell is in contact with a solution of lower water concentration than its own contents, then water leaves the cell by osmosis, through the cell membrane. Water is lost first from the cytoplasm, then the vacuole through the tonoplast. The living contents of the cell contracts and eventually pulls away from the cell wall and shrinks, this is known as Plasmolysis."
    Quote from CourseworkHelp: AT1- Osmosis In potatoes.

    Chemical composition of the nutrient solution is likewise important. Without certain nutrients plants cannot live, or cannot complete their life cycle. Toxic substances in the solution could cause the plant to die, or perhaps cause the grower, enjoying the fruits of his/her labors, to fall sick or die. Sufficient Dissolved oxygen levels should be present in the solution, root-cells need this to breathe, like fish, underwater. Also the essential elements should be in a such a form as to be available to the plant, as inorganic ions. With the plethora of nutrient products currently available to most growers, the nutrient composition is rarely a problem.

    Rosa Root hair meets Wally Waterdrop
    To simplify, plants roots are basically composed of surface cells that absorb the water and the elements, and of inner structures of veins that translocate the water & elements, called nutrient solution from here on, upwards to the stem.

    The cells on the root surfaces, called root hairs because of their 'fuzzy' nature, can passively diffuse the nutrient solution, or expend energy and actively transport water and nutrient ions across their cell membrane.

    The Cell
    Every organism on our planet, according to the science is composed of one or more cells. An average human might have billions of cells. On the other hand, bacteria are single celled organisms. Plants are multicelled, of course. Cells always have an cell wall, surface membrane, and internal organs.

    Cell wall
    The cell wall, often called the primary cell wall serves to protect the cell from the surrounding environment and to support the cell. The primary cell walls of plants are made of tiny cellulose fibers intertwining on the surface of the cell, pumped out by tiny cellulose rosettes moving across the surface of the cells plasma membrane right 'beneath' the cell wall.

    "If you put a plant cell in water, water enters by Osmosis, then swells up. However, the cell will not burst. This is due to the fact that the cell walls are made from cellulose, which is extremely strong. Eventually, the cell stops swelling, and when this point is reached, we say the cell is turgid. This is important, because it makes plant stems strong and upright."
    Quote from CourseworkHelp: AT1- Osmosis In Potatoes.

    Plasma membrane
    The surface membrane is also called plasma-membrane or double lipid layer membrane, and the internal organs the cytoplasm. The cell membrane inside the outer primary cell wall is an complex, living tissue of biochemical wonders and little molecular machines that can move molecules back and forth across the membrane and build the cell wall. There are also little conduits between the adjacent individual cells, to make transport of water and ions even easier. These pores are called plasmodesmata.

    Functions of the membrane
    This plasma membrane has many functions, each function covered by particular tiny organs, made of proteins:
  • Keeping the solution balance suitable in- and outside the cell. There are proteins on the membrane that can pump water and ions in and out of the cell wall. It's also referred to with an really advanced term 'Maintaining ionic homeostasis'. Be sure to dazzle your friends with this term.
  • Signaling and sensing the environment. Such as receiving hormonal messages.
  • Building the primary cell wall. Small organs moving on the membrane spewing out long strands of cellulose that form the external cell wall matrix.
  • Regulating the turgidity. Adjusting the osmotic pressure.
  • Communicating with the adjacent cells, through the plasmodesmata mentioned earlier.

    So once an root-hair-cell starts to feel a little thirsty, or perhaps gets an message from its neighbor to move in more nitrogen, it can utilize several strategies to 'transport' the required molecyles from the nutrient solution, into the cell and onwards. If no energy is required upon the cells part, this is called passive transport, and, logically, if energy is expended, active transport is in progress.

    Passive Transport
    Because of the physical and chemical nature of the nutrients ions, the substances dissolved in the nutrient solution, all the substances and even the solution itself are subject to osmosis, diffusion through the selectively permeable plasma membrane. This is because each molecule has an electric charge, and differing concentrations of the molecules create electric potential between the differing concentration areas, called gradients (concentration gradient, potential gradient, trans-membrane electrochemical gradient...).

    What is diffusion?
    In diffusion solutes (molecyles) seek to move from the stronger concentration towards the more diluted thus equalizing any possible differences in the concentration.

    In other words, diffusion is the effect of molecyles dissolved in solution, diffusing from the area of higher concentration towards the area of lower concentration of dilutes.

    Suppose two solutions are mixed in an container: water and pH down. Right after mixing, concentrations of pH down in the water are uneven. After a while, after diffusion, pH down will be equally concentrated across the volume of the water.

    Diffusion occurs in solutions consisting of particles. The energy to diffusion is created from the random thermal motion of molecyles, also called the brownian motion.

    Diffusion happens through cell walls also, except where blocked by the selectively permeable cell wall.

    Diffusion through an cell wall
    Anything will permeate the double lipid layer of the cell wall given enough time. However, there are large differences in the time period required.
    High permeability (through cell walls)
    Cl+ (Chlorine ion)
    K- (Potassium anion)
    NA+ (Natrium ion)
    Low permeability

    Table 1. Permeability for some substances
    Higher the permeability, the faster is the movement through the cell walls into the cells.

    What is Osmosis?
    Well, Osmosis is actually diffusion of water with an permeable layer of some kind that's permeable by the solution. In cell biology terms words, Osmosis is what diffusion of water through the cell wall is actually called.

    To give an more practical example, Osmosis is the diffusion of water from a hypotonic solution, solution that is low in dissolved solids, into a hypertonic solution which contains higher amount of dissolved solids across and selectively permeable membrane.

    Osmotic diffusion through cell walls is passive transport mechanism, because it requires no energy from the cell's part.

    As you can see above, cell walls can permeate water and some molecyles easily. However, some of the molecyles require active effort from the cells to transport into the cells. This is called active transport.

    What is Reverse Osmosis?
    Reverse Osmosis-term is most often used of water purification systems that use water-permeable layer to purify water. Reverse Osmosis water contains only water molecyles (H2O) or molecyles smaller than that. Reverse osmosis -layers are capable of rejecting bacteria, salts, sugars, proteins, particles and dyes among other things (molecule size smaller than ~200 daltons).

    In plants the condition of Reverse Osmosis suggests that the concentration of solutes in solution outside the (root) cells is higher than inside the (root) cells and thus the direction of the water movement is out of the (root) cells, and not inwards. Simply put, the salty solution draws water from the plants, often causing plants to wilt.

    Active Transport
    The root hair cells can utilize the transport proteins and ion pumps, located on the plasma membrane, to actively move solutes across the membrane. This way plants can control the intake of water and nutrients from the solution that is in contact with the root hairs.

    There is much more to the whole transport-business. To learn more about the issue, type some of these keywords into your favorite search engine: "cell wall transport active facilitated diffusion cytosis ATPase".

  • drawing by: ReSoNiC420

    Normal 'hypertonic' situation
    Normally all plants cells are filled with water, and the whole plant is 'rigid' with the water. This is caused by the high positive internal osmotic pressure, also called turgor. This state of high internal pressure in cells is called hypotonic. Should a plant lose its turgor, it would wilt and its leaves would be completely limp. This opposite state would be hypertonic, ie. when a cell would have an negative internal osmotic pressure, causing water to flow out and the cell to shrink (or in case of rigid-walled cells, the interal cell membrane (plasma membrane) to shrink).

    Most energy for keeping the cells hypertonic results from the transpiration, the evaporative pull resulting from water evaporated through the stomata, small openings on the lower leaf surfaces, and from the cohesion & capillary action of the water in the plants veins (xylem).

    Roots control the environment and intake and exhaust of solutes Some of the pressure is created actively by the root-hair cells - cells pump water inside the plant, using their cellular energy (ATP). In similar fashion plant can actively transport nutrients, like mentioned above.

    Note that the above is simply one theory to explain the phenomena that happens in plants and cells. There are different theories on how cell-walls, diffusion etc work. For more info on this theory, do an web search with 'Donnan equilibria'.

    Root structure
    Roots are responsible for extracting water and the nutrient minerals from the growing medium. The root tip, also called apical meristem grows into the medium, pushing through it covered by the root cap, an protective shield of cells.

    On the roots surface layer, the epidermis, root hairs have developed on top of the cortex, which in turn is formed around the internal layer of the roots, also called the endodermis. Root hairs have large surface area which effectively absorbs nutrients from the medium. Symbiotic, mycorrhizal fungi can also increase the surface area, greatly enhancing the intake of nutrients.

    Root hairs
    Root hairs cover the mature root surface. They are tiny hair-like structures that grow right into the medium and increase the surface area of the root to asphyxiating numbers. There can be more than 20000 root hairs on an area equal to fingernail. On the average length of 5 mm, the surface area of these root hairs would exceed 1/3 square meter, over 3 square feet!!!(h=0,005m, r=0,0005m) So thanks to this huge surface area, roots can supply water and nutrients to an very large plant.
    Root hairs are often visible by the naked eye. Root hairs are quite short lived and often mature roots have no visible root hairs.

    Nutrient movement across membranes
    The nutrients, minerals dissolved into water-solution, are transported as ions. Ions are soluble in water but cannot cross membranes without the presence of transport proteins, little organs on the surface of the membrane. The transport of the negatively charged ions requires the transport of an positively charged particle in the opposite direction. These positively charged particles are protons, H+ - hydrogen without an electron. This way the electric potential and chemical potential stays in equilibria, with equal electric potentials on both sides of the membrane. These protons are pumped actively across the membrane using ATP, adenosine triphosphate, as an energy source.

    There are basically three mechanisms that transport the nutrient ions: primary and secondary ion pumps and ion channels. These are proteins that sit in the plasma membrane, each type specific to the nutrients they carry.

    Some of the ion pumps move the H+ protons out of the cell, an some into the cell. These are known as primary ion pumps. This movement of H+ changes the potential/gradient, and facilitates the movement of the other ions. There are also the secondary ion pumps that move the other ions in and out of the cell.

    Finally there are the ion channels, little channels with opening and closing 'gates' that permit the nutrient ions to move across the membranes, driven by the potential & electrochemical gradient.

    Movement of nutrient solution inside the plant
    Once the water and the nutrient ions have been absorbed by the root hair-cells, these are transported across the cell plasma membranes, directly in the cells, symplastically, or between the cells, in intracellular spaces, apoplastically.

    Once the solution has traveled through the root-hairs and the cortex, into the inner parts of the roots (endodermis), it can only travel into the vascular system inside the cells, symplastically, transported through the membranes, in the cells. In the vascular system, the bundles of veins, there are two types of veins - the xylem, and the phloem. A these vascular bundles are basically vertical veins running from the roots to the growing tip.

    The bulk of the flow is created by the transpiration pull, drawing the solution upwards, towards the leaves. Diffusion and active transport also help in the movement of the solution. The physical properties of water, cohesion ie. the attraction of water molecules to one another and the resulting capillary action also helps in creating the strong vertical upward movement of water. This is an very efficient system - plants can move large volumes of nutrient solution from the roots up to the foliage often very high above the root level.

    Leaving leaves
    The vascular bundles run throughout the plants, in the stems and the leaves. You can actually see the bundles in the leaves - the veins of the leaves. Once aboard the 'plants internal transport system', the solution is moved around the plant, and the nutrients used for building blocks, to create energy in the photosynthetic process, and to regulate the metabolism and the turgidity of the plant.

    Most of the water is transported into leaves, where it is evaporated through small openings on the lower surfaces of leaves. These openings are called stomata (singular stoma). Plants can open and close these to control the amount of evaporation. As water evaporates, it contributes to the total transpiration pull. In nature the evaporated water floats in the air, condenses into clouds, rains down on the plants and the cycle is completed.

    How does all this apply to Cannabis -plants?
    So how do the plants roots, or the roothairs in them, control the nutrient intake?!? Wouldn't any and all nutrient ions diffuse themselves all around the plant and the nutrient solution (as opposed to Nitrogen going to leaves and Kalium to the stems)?!?
    With active transport-mechanisms root cells can 'select' the ions (and other substances) that are transported into the cell. This way they can adjust (to) the environment, and actually even work against the osmotic imbalance. Looking at the larger context, plants use the energy from photosynthesis to keep the juices flowing in the right places.

    "Excessive flow of water into a cell by osmosis can burst the cell. Cells protect against this using processes of osmoregulation. If external pressure is applied to the stronger solution, osmosis is arrested. By this mechanism plant cells can osmoregulate, since the cell wall of a fully turgid cell exerts pressure on the solution within the cell." Quote from CourseworkHelp: AT1- Osmosis In Potatoes.

    Nutrient solution and soil management
    So once an grower understands these principles (s)he can apply these to practice. Its easy to understand that strong changes in the amount of dissolved substances in the root-zone would stress the roots by changing the direction of the osmotic flow. A plant could suddenly experience strong stress, and possibly even direct physical damage to the roots.

    For each plant there exists an optimal environment. By measuring the pH, TDS or EC one can understand the conditions in the root-zone and act accordingly. The suitable range was discussed in the second paragraph of this text, The solution in the root-zone.

    DISCLAIMER: Information in this text may not be completely correct. This text is meant as an starting point for further study.
      Last modified: 04:00 - Jul 08, 2003  

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