How does plant nutrient metabolism work?

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
pathogens.

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.

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"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.

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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.

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"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.

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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)
Water
Urea
Glycerol
Tryptofan
Glucose
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".