Absorption by roots

Absorption by roots | Diffusion | Active transport | Osmosis


  • It is the branch of biology which deals with the study of external structures of plants and animals.
  • A plant consists of a root system (underground part) and a shoot system (above the ground parts).
  • Roots are the parts of the root system; and stem, leaves, flowers, and fruits are parts of the shoot system.
  • Also Read Transpiration


  • Roots help in anchoring plant and absorption of water and minerals take place by Roots.
  • Developed from the radicle part of a cotyledon
  • It consists of a region of meristematic activity covered by a root cap, a region of elongation, and a region of maturation having root hairs.

Types of roots system

  1. Tap root system
    • It consists of a primary root that grows deep inside the soil.
    • It also bears lateral roots referred to as secondary and tertiary roots.
    • Example- Dicotyledons (mustard)
  2. Fibrous root system
    • Primary root is short-lived and is replaced by a large number of secondary roots.
    • Example- Monocotyledons (wheat)
  3. Adventitious roots
    • Roots arise from parts other than the radicle.
    • Example- Banyan tree

Root modifications

  • Prop roots – Example: banyan tree
  • Stilt roots – Example: maize and sugarcane
  • Pneumatophores (that helps in respiration) – Example: Rhizophora

Characteristics of Root for Absorbing Water

  • Enormous surface area
  • Root hairs containing cell sap at higher concentration
  • Thin walled root hairs


  • It is a biological process that involves the transport of dissolved material within a plant.
  • It mainly occurs with the help of xylem and phloem.
  • The transport of food from leaves to other parts of plant occurs by phloem. Movement of food in phloem is bidirectional.
  • The conduction of water and minerals from soil to the rest of the plant occurs by xylem. Movement of water in xylem is unidirectional.
  • Also read Photosynthesis

Need of Water and Minerals for Plants

Need of Water

  • For photosynthesis
  • For transpiration
  • For transportation
  • For mechanical stiffness

Need of Minerals

  • Needed as nutrients for the plants
  • For the synthesis of a variety of compounds and enzymes

Means of Transport


  • It is the spontaneous movement of molecules from a region of high concentration to a region of low concentration.
  • It is a slow process and does not require any energy expenditure.
  • It does not require a semi-permeable membrane and can take place through any membrane along concentration gradient.
  • Rate of diffusion is affected by
    • concentration gradient
    • membrane permeability
    • temperature
    • pressure

Facilitated diffusion

  • It involves the movement of molecule from the region of higher concentration to lower concentration, mediated by a carrier (mainly protein) molecule.
  • Movement of molecules across membrane occurs without expenditure of energy.
  • Porins – They are large protein molecules that form pores in membranes of plastids, mitochondria, and some bacteria
  • Porins allow the movement of small-sized proteins across membrane. Aquaporins are proteins, which form a water-permeable channel.
  • Some protein molecules allow diffusion only if two molecules are present. Based upon the direction which is followed by both molecules, the path can be of three types.
  • Symport – when both molecules cross the membrane in same direction
  • Antiport – when both the molecules move in the opposite directions
  • Uniport – when single molecule moves across a membrane independent of other molecule

Active transport

  • It involves the transport of molecules from a region of low concentration to a region of high concentration with an expenditure of energy.
  • It is carried out by membrane proteins.


  • Osmosis is a special type of diffusion which involves the movement of water molecules from the region of high concentration to the region of low concentration through a semi-permeable membrane.
  • Semi-permeable membrane = Selectively permeable membrane.
  • Types of Osmosis:
    • Endosmois
    • Exosmosis
  • Rate of osmosis is affected by
    • pressure gradient
    • concentration gradient
  • Osmotic pressure is the hydrostatic pressure produced by a difference in concentration between solutions on the two sides of a semi-permeable membrane.
  • Tonicity : Relative concentration of solution and its surroundings.
  • Isotonic solution: Solution that has the same salt concentration as the normal cells
  • Hypotonic solution: Solution that has lower salt concentration than the normal cells
  • Hypertonic solution: Solution that has higher salt concentration than the normal cells


  • It is the contraction of cells within plants due to the loss of water through osmosis.
  • When cells are placed in hypertonic solution, a cell tends to lose water to the surrounding solution due to exosmosis. The plasma membrane shrinks and the cell is said to be plasmolysed.
  • When cells are placed in hypotonic solution, cells get deplasmolysed (turgid) due to movement of
    water into the cell from surrounding as a result of endosmosis.


  • The opposite of plasmolysis.
  • If not dead, the protoplasm absorbs water
  • The cell swells up

Difference between Diffusion and Osmosis

Movement of substances from higher concentration to lower
Movement of selective substances thro
It occurs in any mediumIt occurs in liquid medium.
It helps in equalising the concentration in the available space.It does not equalise the concentration
It does not depend on solute potentialIt depends on the solute potential.

Water movement

Root pressure

  • It is the positive pressure that develops in the roots of plants by active absorption of nutrients from soil.
  • It pushes the water up to small heights.
  • Root pressure is linked to the phenomenon of guttation.
  • Guttation: It involves the loss of water in the form of liquid droplets through the vein endings of the leaves.
  • Guttation occurs early in the morning and late in the evening when evaporation is low and root pressure is high.

Transpiration Pull

  • Water transport in tall trees occurs by transpiration pull.
  • Transpiration pull is generated by transpiration. It is also called cohesion – tension – transpiration pull model of water transport.
  • The ascent of xylem sap is dependent on three physical properties of water:
    • Cohesion
    • Surface tension
    • Adhesion

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