Root Uptake

• Plants take up water and mineral ions from the soil via their roots and thus need a maximal surface area to optimise this uptake
• Some plants have a fibrous, highly branching root system which increases the surface area available for absorption
• Other plants have a main tap root with lateral branches, which can penetrate the soil to access deeper reservoirs of water
• The epidermis of roots may have cellular extensions called root hairs, which further increases the surface area for absorption
• Materials absorbed by the root epidermis diffuse across the cortex towards a central stele, where the xylem is located
• The stele is surrounded by an endodermis layer that is impermeable to the passive flow of water and ions called the Casparian strip
• Water and minerals are pumped across the Casparian strip by specialised cells, allowing the rate of uptake to be controlled
• Fertile soil typically contains negatively charged clay particles to which positively charged mineral ions (cations) may attach
• Minerals that need to be taken up from the soil include Mg2+ (for chlorophyll), nitrates (for amino acids), Na+, K+ and PO43–
• Mineral ions may passively diffuse into the roots, but will more commonly be actively uploaded by indirect active transport
• Root cells contain proton pumps that actively expel H+ ions (stored in the vacuole of root cells) into the surrounding soil
• The H+ ions displace the positively charged mineral ions from the clay, allowing them to diffuse into the root along a gradient
• Negatively charged mineral ions (anions) may bind to the H+ ions and be reabsorbed along with the proton
• Water will follow the mineral ions into the root via osmosis – moving towards the region with a higher solute concentration
• The rate of water uptake will be regulated by specialised water channels called aquaporins on the root cell membrane
• What is it called when water moves towards the xylem via the cytoplasm?

  Symplastic
• What is it called when water moves towards the xylem via the cell wall?

  apoplastic
• In the symplastic pathway, water moves continuously through the cytoplasm of cells
• In the apoplastic pathway, water cannot cross the Casparian strip and is transferred to the cytoplasm of the endodermis
• What are the cytoplasm of cells connected by?

  plasmodesmata