The process by which liquids are absorbed by colloidal dry or semi-dry materials is called imbibition. Materials that swell by absorbing water are called hydrophilic materials. For example, glue, cellulose, gelatin, protein, starch etc.

The process by which water enters the cell from a source outside the cell is called water absorption. Higher plants absorb water by roots, lower plants by rhizoids and aquatic plants by whole body. Rhizomes are the main organ of plant water absorption. Rhizomes extend into the spaces between soil particles. On the other hand, there is a large amount of capillary water in the spaces between soil particles. As the concentration of capillary water in the interstices of soil particles is less than that of the root cell sap, the pore pressure deficit occurs. ie DPD decreases. Water enters the rhizome in the process of osmosis to equalize this diffusion pressure deficit. It reduces the density of mullein and root cells. But cortex cells have higher cell density. Due to this, there is a diffusion pressure deficit between the cortex and cortex cells. To equalize this diffusion pressure deficit, water moves from the root into the first cells of the cortex. Then the water passes through the second, third, fourth, fifth etc. cells of the cortex and finally reaches the intestine. This results in a decrease in endodermal cell density, but a higher pericycle cell density. As a result a diffusion pressure deficiency occurs. To equalize this pressure deficit, water enters the xylem vessel cavity from the endoderm through the pericycle by the process of osmosis. Later, the water reaches the necessary parts of the plant through the xylem.

1. Ion concentration: If the concentration of ions in the soil solution is high, the rate of absorption of mineral salts increases. Increasing the ion concentration up to a certain limit increases the rate of mineral salt absorption.
2. Temperature: Increase in temperature up to a narrow range increases the rate of absorption of mineral salts. Absorption rate decreases when the temperature is below or above a certain limit.
3. Light: Light indirectly increases the rate of absorption of mineral salts. Light regulates stomatal opening and closing and the rate of respiration. Therefore, light regulates the absorption of mineral salts.
4. Perfusion: If the perfusion rate increases, the absorption rate also increases. Increased transpiration rate increases water transport to plant roots, stems and leaves. As a result, the absorption rate of mineral salts also increases.
5. Oxygen: When oxygen is low, respiration is low and absorption rate is also low.
6. Respiratory matter: If respiratory matter is low, respiration rate decreases and mineral absorption also decreases.
7. Ion interactions: The presence of Ca+ + and Mg+ + ions inhibits the absorption of K+ ions.
8. Plant Growth: Physical growth of plants increases mineral salt absorption rate. Therefore, large plants absorb mineral salts to a greater extent and smaller plants to a lesser extent.
9. pH: A change in pH beyond a certain range causes cell damage. Hence mineral salt absorption is disturbed.
10. Growth Zone: Absorption of mineral salts occurs mostly in the growth zone and cell division zone of the plant.

The process by which protons and anions enter the cell from outside the cell through co-localization is called the proton-anion cotransport theory. This theory is based on the chemi-osmotic model of Peter Mitchell (1968).
According to this theory a specific ion is transported by a specific carrier. Cell membranes contain ATP. ATP-ase enzymes break down ATP to generate energy. Under the influence of this force, protons (H+) come from the inner surface of the cell to the outer surface of the cell. This phenomenon is called proton pump. This increases the pH on the inside of the cell and decreases the pH on the outside. This phenomenon is called pH gradient. For the same reason the ‘+’ charge on the inside of the cell decreases and the ‘+’ charge on the outside increases. This phenomenon is called Protential gradient. The pH gradient and proton gradient together are called Proton Motive Force (PMF). When the Proton Motive Force is generated, the inactive proteins on the outer surface of the cell are converted into active proteins. Active proteins act as carriers and transport cations (K+) from the cell surface to the cell surface. This increases the amount of cations in the cell’s interior. Because of this, protons (H+) from the outer layer of the cell want to enter the inner layer of the cell. Then protons and anions co-exist from the outer surface of the cell to the inner surface of the cell. Anions and cations then combine to meet the cell’s mineral salt needs.

In 1957, scientist Bennett Clark introduced the protein lecithin theory. According to this theory, a type of phospholipid called lecithin acts as a carrier. On the outer layer of cells, lecithin combines with anions and cations to form ‘ion lecithin complexes’. The ‘ion lecithin complex’ moves from the outer surface of the cell to the inner surface. Under the influence of lecithinase enzyme on the inside of the cell, the compound is broken down to produce choline and phosphotidic acid and cation and anion are released. With the help of ATP, choline and phosphotidic acid move to the outer layer of the cell and combine to form lecithin. Thus anions and cations are absorbed through the cyclic synthesis and breakdown of lecithin.

In 1937, scientist Vanden Honert introduced the ion carrier theory. According to this theory, there is outer space outside the cell membrane and inner space inside the cell membrane. Ions can move freely between the extracellular and intracellular compartments, but not within the cell membrane. Ions from the outside environment come directly to the outside. On the outside, ions and carriers join to form ‘ion complexes’. The ‘ion complex’ compound passes through the cell membrane and reaches the endoplasmic reticulum. On reaching the bottom, the ‘ion complex’ breaks down and the ions and carriers are released. The free ion accumulates inside the cell. The carrier returns to its previous location and participates in another ion transport. In this case the metabolic energy to create, move and break down compounds comes from respiration.

Although Lundigaard’s theory has been proven to be true, it has some flaws. These are-
1. Although anion absorption is said to be active, energy (ATP) is not shown to be used.
2. Cation transport through cytochromes is not clear.
3. Anions are transported through the cytochrome, but cations are not. This is not acceptable to many.
4. In this theory, respiration rate increases during anion absorption, but not during cation absorption. Scientists Epstein (1955) and Handey (1955) proved by their experiments that respiration rate increases during cation absorption.

In 1933, scientist Lundigaard and his colleague Bersatrum introduced the theory about the absorption of mineral salts by plants called Lundigaard’s theory. According to this theory, cytochromes play an important role in the absorption of iron minerals, so it is also called the cytochrome pump theory. They observed that placing the plant in a solution of salt from water increased the respiration rate of the plant. They termed this increased respiration as anion-respiration or salt-respiration.
According to Lundigaard’s cytochrome pump theory, every cell has two compartments. Outer floor and inner floor. Protons (H+) and electrons (e+) are produced in the dehydrogenase reaction inside the cell. Protons (H+) move directly to the outer layer of the cell and combine with oxygen to form water (H2O). On the other hand, electrons (e+) enter the cytochrome chain. Inside the cell, cytochromes are oxidized by accepting electrons. After entering the cytochrome chain, electrons (e+) cycle to the outside of the cell. On the outer surface of the cell, cytochromes are oxidized by releasing electrons. Oxidized cytochrome combines with the anion (A-) located in the cell membrane to form a ‘cytochrome-anion’ compound. The anion (A-) cycles through the cytochrome chain to the intracellular surface and accumulates. An excess of anions (A-) on the inside of the cell causes an electrical-potential difference. To equalize this electric-potential difference, cations (K+) from the outer surface of the cell move directly into the inner surface of the cell. Later, cations and anions combine to meet the cell’s mineral salt needs.
Fe++ (2A-) – e- + A- → Fe+++ (3A-)

1. Active absorption requires direct metabolic energy.
2. Active absorption requires carriers.
3. Respiration rate increases during ion absorption.
4. Enzymes play an important role in ion absorption.
5. Cations and anions can be adsorbed simultaneously.
6. Ions can be adsorbed against the gradient of concentration.

The process of absorption that requires metabolic energy is called active absorption. Most mineral salts are actively absorbed by the plant body. Professor D.R. Hogland observed that the concentration of inorganic material in cells of Nitella algae is higher than the concentration of inorganic material present in water.