Transport in Plants Ch. 36

Transport of Xylem Fluid rises in the xylem without requiring energy Fluid rises AGAINST gravity Two ways this happens: Pushed up by root pressure Results from water flowing into stele from the ground because of high mineral content in root cells Can only push xylem sap up a little bit Guttation: escape of water droplets on leaves that can be seen as morning dew

Pulled up by transpirational pull Much stronger force Transpiration: evaporation of water from leaves Causes tension (negative pressure) in the xylem Due to the cohesion of water you can pull a large amount of water up through the xylem Cohesion: binding together of similar molecules, usually by hydrogen bonds For each molecule of water that is lost by transpiration, another is gained at the roots Factors that can change rate of transpiration: Humidity Closing stomates Light Intensity

Controlling Transpiration Guard cells control opening and closing of stomates Cell walls of guard cells are not uniform in thickness When guard cells become turgid they curve, causing the stomates to open Due to radially oriented cellulose microfibrils When guard cells become flaccid the stomata close

Things that cause stomates to open: Low CO2 levels in the leaf: happens when photosynthesis begins Increase in potassium ions in guard cells: causes water to diffuse into them, making them turgid Stimulating the Blue light receptor: causes proton pumps in the guard cell’s membrane to turn on, which causes uptake of potassium ions (see above) Things that cause the stomates to close: Lack of water: causes guard cells to lose their turgor Hot Temperatures: starts cellular respiration, which increases CO2 levels in the leaves (see above) Abscisic Acid: made in mesophyll cells due to dehydration, signals guard cells to close stomates

Translocation of Phloem Sap Translocation: transport of food (sugar) in a plant Phloem sap travels from sugar source to sugar sink Sugar source: plant organ in which sugar is being produced Ex: Mature leaves Sugar sink: plant organ that is a net consumer or storer of sugar Ex: Growing roots and fruit

Water and Mineral Absorption Lateral movement: movement of water and nutrients across a plant Symplast: system of cytoplasm in cells connected by plasmodesmata When water reaches the endodermis it can continue to the xylem through the symplast Apoplast: system of cell walls and intercellular spaces that allows extracellular movement of water in a plant When water reaches the endodermis it must pass through the endodermis by diffusion

Helping Out the Roots Mycorrhizae: Symbiotic structures constructed from plants roots and filaments (hyphae) of a fungus These structures increase the amount of nutrients a plant can absorb This helps older regions of roots that no longer have root hairs, thus poor absorption Rhizobium: Symbiotic bacteria nodules that live on roots and fix nitrogen gas from air into a nitrogenous form that the plant requires.

Mycorrhizae

Rhizobium