transport in plants Flashcards
structural adaptations of xylem
-continuous lumen without any cross walls and protoplasm, hence reduces resistance to the flow of water and ions
-cellulose cell walls of xylem are further lignified to provide mechanical support for the plant to stand upright
structural adaptations of phloem
-sieve tube of phloem only has a thin layer of cytoplasm without vacuole, nucleus and mitochondria, hence reduces resistance to the flow of sucrose and amino acids
-sieve tube of phloem has sieve plates with pores, hence further reduces resistance to the flow of sucrose and amino acids
-companion cells of phloem have numerous mitochondria to release energy for loading of sucrose into the sieve tube of phloem
location of vascular tissues in root and stem
in the root,
-the xylem and phloem are not bundled together
-they are positioned at the centre of the root with the xylem and phloem alternating with each other
in the stem,
-the xylem and phloem are grouped together to form vascular bundles
-vascular bundles are positioned in a ring arrangement around the pith
translocation
-the transport of sucrose and amino acids using the phloem, from a source to a sink
entry of water and ions
-root hair cells absorb water molecules by osmosis from soil into cell sap
-root hair cells absorb ions by facilitated diffusion and active transport from soil into cell sap
structural adaptations of root hair cell
-root hair cell have elongated structure to increase the surface area to volume ratio. hence, higher rate of water and ions absorption.
-root hair cells have numerous mitochondria. hence, a lot of respiration can occur to release a lot of energy for active transport of ions from the soil into cell sap.
-root hair cells have much larger vacuole. hence, there is maximal storage of ions and water.
movement of water in the root
-the uptake of water molecules by osmosis dilutes the root hair cell’s cell sap. the cell sap of the root hair cell now has a less negative water potential than that of adjacent cell.
-hence, water molecules move by osmosis from the root hair cell into the adjacent cell.
-similarly, water molecules move by osmosis from the adjacent cell into the next adjacent cell.
-osmosis continues until the water molecules enter into the xylem of the roots
processes by which water flows from the roots to aerial parts of a plant
-root pressure
-capillary action
-transpiration pull
root pressure
-root pressure is present in the xylem of the roots due to the continual absorption of water by the root hair cells
-root hair cells absorb ions by active transport into its cell sap, hence lowering the water potential of cell sap
-henceforth, water molecules move by osmosis into the root hair cells and subsequently into inner root cells and the xylem of the roots.
capillary action
-due to cohesion forces between water molecules due to hydrogen bonds and adhesion forces between water molecules and cellulose cell wall of xylem
-hence, there can be continual movement of water up xylem vessels
transpiration pull
-transpiration is the loss of water vapour by diffusion through the stomata in leaves
-is a consequence of gaseous exchange in plants due to the opening of stomata
-transpiration pull is the main suction force to draw water and ions up the xylem vessels from roots to all other parts of the plant
-is a result of transpiration, enhanced with capillary action
transpiration
-water molecules moves by osmosis continuously out of the mesophyll cells to form a thin film of water over cell wall surfaces
-water evaporates from film of water to form water vapour in the intercellular air spaces. this results in a high concentration of water vapour.
-water vapour diffuses through the stomata of the leaf into the atmospheric air with lower concentration of water vapour
-as water moves out from mesophyll cells, the water potential of the cell sap becomes more negative. water molecules move by osmosis from xylem into mesophyll cells.
-transpiration pull will be created to draw the whole column of water and ions up the xylem from the stem into the xylem of the leaf
factors affecting rate of transpiration
-air temperature
-air humidity
-wind/ air movement
-light intensity
air temperature
-a rise in temperature increases the rate of evaporation of water from the mesophyll cell surfaces
-this increases the water vapour concentration gradient between the intercellular airspaces in leaf and the atmospheric air
-increase in air temperature also increases the kinetic energy of water vapour molecules. this increases the rate of diffusion of water vapour through the stomata.
-therefore, rate of transpiration increases
air humidity
-a rise in air humidity increases the concentration of water vapour in the atmospheric air
-this decreases the water vapour concentration gradient between the intercellular airspaces in leaf and the atmospheric air. this decreases the rate of diffusion of water vapour through the stomata.
-therefore, rate of transpiration decreases
