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Agriculture LibreTexts

Section D

Active transport (using ATP energy)When a molecule moves from lower concentration to higher concentration , requires input of energy (uses ATP), and is known as active transport. For uncharged substances active transport moves molecules against their concentration gradients. For charged substances (ions), active transport moves molecules against concentration + electrical gradients.

Active transport is always mediated by carrier proteins , similar to those of facilitated diffusion, but these carrier proteins act as pumps, using the energy from splitting ATP to pump specific molecules against the concentration gradient.Two types of active transport :-

  1. ATP driven active transport systems :- Hydrolysis of ATP takes place by
    • Na+ ,K+ transport (sodium pump),Na+ is maintained at low concentrations inside the cell and K+ is at higher concentrations.
    • Ca++ transport
    • H+,K+ transport iv ABC transporter superfamily, v osteocast proton pumps.
  2. Ion -gradient driven active transport systems orCo-transport ;-The gradients of ions (cations or anions) established by ATPase or by light driven transport may lead to secondary active transport of substance like amino acid and sugar. These molecules move in same direction-Symport or in opposite directions Antiport .

In plants, active transport enables roots to absorb mineral ions from the soil, which are therefore more concentrated inside plant cells than in the soil. This requires ATP energy from aerobic respiration, and therefore roots need oxygen to allow mineral uptake and a waterlogged (thus anaerobic) soil will kill most roots.

Bulk transport Large molecules, such as polysaccharides and proteins,are too large to cross the cell membrane and cross the membrane via vesicles, occurs by exocytosis (substances leaving the cell.) or endocytosis.

Mineral nutrients uptakeVascular plants have the vascular system ; trumpet hyphae in bryophytes. Water and dissolved mineral nutrients enter the plant in two ways:-

  1. Apoplastic(or outer space):- It is a non-living continuous system made up of water-filled cellulosic cell wall and intercellular spaces from epidermis to xylem.The ions which enter the cell walll of epidermis, cell walls of cortex, cytoplasm of endodermis, cell walls of pericycle and finally accumulate in xylem vessels.The passive movement of ions into the apoplast usually occurs through ion-channels, the trans-membrane proteins that function as selective pores. .
  2. Symplastic( or inner space ) It is a living continuous system formed by cytoplasm and plasmodesmata from epidermis to xylem parenchyma. An ion enter the cell wall of the root hair and pass between the wall and plasma membrane and moves across the cytoplasm, cortex, endodermis, pericycle through plasmodesmata endodermis, a layer of cells that they must pass through to enter the xylem.The entry or exit of ions to and from the symplast requires the expenditure of metabolic energy, which is an process.

The mineral ions stored in the root hairs pass into xylem from where these are translocated through xylem along with the ascending stream of water, which is pulled up through the plant by transpiration stream. Mineral elements brought to the leaves are subsequently assimilated into organic molecules and are redistributed to other parts of the plant through the phloem.