How Is Water Absorbed in Plants?

Overview

Plants absorb water for nutrition from both soil and atmosphere, converting sunlight to an energy source in a photochemical process called photolysis. Although water can be absorbed through the stems and leaves of the plant, the majority is absorbed by the roots.

Roots

The roots themselves contain millions of root hairs which are extensions of the root epidemal cells. These provide a large surface area within the soil which the plant uses for absorption in a process called osmosis. The roots of the plant grow outward through the soil to bring needed minerals into the plant roots by this absorption process. The soil in which the plants grow comprises both organic and inorganic material which get mixed with air and which get dissolved in water that falls as rain. Ideally the soil is fertile and contains the nutrients that plants require for growth in a readily available form.

Absorption

Water and dissolved mineral nutrients are absorbed by the root and transported to the plant. This passage of water is referred to as the transpiration stream and it occurs via two routes know as active absorption and passive absorption. In active absorption the water enters the cells despite a concentration gradient which precludes the process of osmosis, but if the soil has a higher concentration of nutrients and minerals, then the water will move into the plant. The plant maintains a low concentration gradient by absorbing large amounts of nutrients as the water enters and is moved up the stem to the rest of the plant. Passive absorption occurs by osmosis when the concentration gradient is such that the fluid concentration within the cell is higher than that of the water contained in the soil. Water is absorbed when the plant's transpiration rate is high causing a water deficit, or when the soil becomes drier. During periods of extreme transpiration the loss of water from the plant reduces the internal pressure in the roots cells creating an upward force that pulls water from surrounding root cells up through the plant to make up water deficit.

Regulation

When a plant requires water, as when the concentration of liquid in the roots drops below a certain point, the plant is capable of drawing moisture from the higher concentration of water found in the soil. The same mechanism occurs when there is a difference in the concentration between cells within the plant's structure. The root cells permit osmosis to occur which introduces additional water to the internal structure and thus helps to equalize the concentration within the cells throughout the entire organism. Plants are uniquely capable of producing self-regulating macromolecules by manipulations of the sugars formed in the process of photosynthesis.