Chapter 6:transport systems Part 2: plant transport Flashcards
What are transport systems needed to do? (metabolic demands)
-meet metabolic demands (only areas with photosynthetic pigments can produce glucose, all cells need a supply of oxygen and glucose, all cells need to remove waste products of metabolism, growth plant regulators need to be transported from their site of synthesis to site of use, mineral ions need to be taken up and transported to all cells for proteins and nucleic acid production)
What are transport systems needed to do? (cope with size requirements)
-large multicellular plants have relatively high metabolic rate
-need effective transport system to move substances from root tip to growing tip.
What are transport system needed to do? (surface area to volume ratio restrictions)
-whole plant has a small surface area to volume ratio which is too small to enable gas exchange demand to be met by simple diffusion (move substances to all cells at a quick rate and hence meet demand)
What is the role of water within a plant?
Water is important for:
-turgor pressure (hydrostatic pressure) which provides hydrostatic skeleton to support stems and leaves and causes cell expansion and this force enables plant roots to force their way through soil.
-transpiration stream acts as a cooling mechanism for plant
-main component of cytosol
-main component of cell sap
-enables mineral ions and products of photosynthesis (assimilates) to be transported from where they are obtained to where they are needed.
-water is a key reactant in photosynthesis
Description of phloem
-made of several different types of cells: sieve tube elements, phloem fibres, phloem parenchyma, companion cells
-transports assimilates around plant
-transports dissolved organic solutes e.g assimilates such as sucrose
-transports substances up and down the plant
Description of Xylem
-made of several different types of cell: hollow, dead xylem vessels supported by living parenchyma cells
-transport of water and mineral ions (nitrates, phosphates, sulphates and magnesium from root to leaves (up the plant only)
Definition of transpiration
Transpiration=the loss of water vapour (by diffusion) from the underside of the leaf via the open stomata
Definition of transpiration stream
Transpiration stream= the movement of water from the roots to the leaves via mass flow and transpiration. (cohesion tension theory and transpiration pull)
Mechanisms involved in the transpiration stream
-cohesion: water molecules are attracted to each other via hydrogen bonds (as water molecules are polar and have dipoles) hence they are cohesive
-tension: the evaporation of water molecules from the walls of the PMC to the mesophyll air spaces generate a suction (tension) that draws more water into the leaf.
-adhesion: water molecules are attracted to the cellulose cell walls of the xylem vessels and the hydrophilic parts of lignin which aids the mass flow of water up the cell
Transpiration at the leaves
-Water evaporates from the cellulose cell walls of the mesophyll cells into the mesophyll air spaces
-Water vapour then diffuses through the mesophyll air spaces to the sub-stomatal air spaces.
-Water vapour them diffuses down the concentration gradient through the open stomata to the surrounding external air.
-loss of water vapour is called transpiration
Movement of water up the xylem
-Xylem vessels transport water up the stem of the plant from roots to the leaves due to a difference in water potential at the top of the stem compared to the bottom.
-Water is drawn up the xylem by capillary action due to hydrogen bonds between the adjacent water molecules which are polar.
-Water moves up the xylem in a continuous column due to cohesion forces.
-At the leaves, the water moves out of the xylem into the cells by apoplast pathway (transpiration pull)
Water moves from the root hair cells into the xylem tissue
-different pathways exist for water to move from root hair cell to the xylem vessels
Symplast pathway
Water travels through the protoplasm of the cell via osmosis through the plasmodesmata straight through endodermis to xylem from the root hair cell through the cells of the root cortex.
Apoplast pathway
Water travels from cell to cell through the cellulose cell walls which are very absorbent so water diffuses along the cellulose fibres by osmosis through spaces between fibres (as cell walls are freely permeable this route offers little resistance) until it reaches the endodermis where it is blocked by the casparian strip (present in cell wall, made from suberin) so water cannot continue and joins the symplastic pathway.
Water uptake at root hair cell
-Water is taken up from the soil by root hair cell via osmosis down the water potential gradient.
