Transport in Plants 2006-2007

Transport in plants H2O & minerals Sugars Gas exchange transport in xylem transpiration Water potential, adhesion & cohesion Sugars transport in phloem bulk flow Photosynthesis in leaves loads sucrose into phloem Gas exchange photosynthesis CO2 in; O2 out stomata respiration O2 in; CO2 out roots exchange gases within air spaces in soil

Overview Sugars travel from leaves to roots through phloem Water and dissolved minerals travel from root to shoot through xylem It defies gravity! http://www.youtube.com/watch?v=w6f2BiFiXiM

1. Transport of Water and Minerals Amount of water needed daily by plants is small compared to the amount that is lost through transpiration Transpiration: evaporation of water from plant surface If water is not replaced, the plant will wilt and may die.

Hydrostatic pressure causes water to travel up tube Water Potential Water movement is governed by differences in water potential The potential energy of water molecules Solute concentration and pressure Water moves from an area of higher water potential to lower water potential High solute concentration = low water potential Low solute concentration – high water potential PE High Water Potential Low Water Potential Hydrostatic pressure causes water to travel up tube PE

The Process - Roots Minerals from the soil Actively transported into the root hairs and start to accumulate Increase solute concentration in root cells, decrease water potential Water moves in through osmosis to xylem cells High Water Potential WATER Low Water Potential root hair H2O

Hydrostatic pressure causes water to travel up xylem As water enters the xylem, it forces fluid up the xylem due to hydrostatic root pressure positive pressure This pressure can only move fluid a short distance. The most significant force moving the water and dissolved minerals in the xylem The “pull” of water from transpiration cohesion & adhesion Hydrostatic pressure causes water to travel up xylem Pull = Negative Pressure

Adhesion and Cohesion Water is a polar molecule unequal sharing of electrons in the covalent bonds oxygen atom has a stronger attraction for electrons then hydrogen O becomes slightly negatively charged H becomes slightly positively charged d– O H H d+ d+ H2O

Cohesion and Adhesion Water molecules are attracted to one another and other materials Cohesion Due to: Hydrogen Bonds Force of attraction between slightly “–” oxygen and slightly “+” hydrogen of adjacent water molecules Adhesion Attraction between water molecules and the side of xylem cells

Transpirational Pull Starts in Leaves Evaporation of water through stomata Lowers WP in the surrounding air spaces Water moves from spongy cells (higher WP) to air spaces (lower WP) Water in spongy cells exerts a pull on column of water molecules in the xylem all the way from the leaves to the roots (adhesion, cohesion)

LOW water potential HIGH water potential

Mycorrhizae increase absorption Symbiotic relationship between fungi & plant symbiotic fungi greatly increases surface area for absorption of water & minerals increases volume of soil reached by plant increases transport of minerals to host plant

Mycorrhizae The hyphae of mycorrhizal fungi extend into soil, where their large surface area and efficient absorption enable them to obtain mineral nutrients, even if these are in short supply or are relatively immobile. Mycorrhizal fungi seem to be particularly important for absorption of phosphorus, a poorly mobile element, and a proportion of the phosphate that they absorb has been shown to be passed to the plant.

2. Transport of Sugars Photosynthesis: CO2 + H2O  C6H12O6 + O2 Storage form of sugar: Starch Cannot be transported, must be broken down into smaller components Transport form of sugar: Sucrose Very sweet sap Usable form of sugar: Glucose

Push and Pull Water and minerals are mainly transported via transpiration negative pressure or “pull” Sucrose is mainly transported via: positive pressure (hydrostatic pressure) “push” force (+) pressure due to accumulation of water “pull” force (-) pressure due to adhesion & cohesion

Companion cells ATP Cells that surround phloem Contain a lot of mitochondria Why? A lot of active transport! ATP

Transport of Sugars ATP Mass flow hypothesis Phloem loading in leaf “source to sink” flow Source = leaf, Sink = root Phloem loading in leaf active transport of sucrose into phloem increased sucrose concentration decreases water potential Water flows in from xylem cells increase in pressure due to increase in water causes flow Hydrostatic pressure ATP can flow 1m/hr In contrast to the unidirectional transport of xylem sap from roots to leaves, the direction that phloem sap travels is variable. However, sieve tubes always carry sugars from a sugar source to a sugar sink. A sugar source is a plant organ that is a net producer of sugar, by photosynthesis or by breakdown of starch. Mature leaves are the primary sugar sources. A sugar sink is an organ that is a net consumer or storer of sugar. Growing roots, buds, stems, and fruits are sugar sinks. A storage organ, such as a tuber or a bulb, may be a source or a sink, depending on the season. When stockpiling carbohydrates in the summer, it is a sugar sink. After breaking dormancy in the spring, it is a source as its starch is broken down to sugar, which is carried to the growing tips of the plant. A sugar sink usually receives sugar from the nearest sources. Upper leaves on a branch may send sugar to the growing shoot tip, whereas lower leaves export sugar to roots. A growing fruit may monopolize sugar sources around it. For each sieve tube, the direction of transport depends on the locations of the source and sink connected by that tube. Therefore, neighboring tubes may carry sap in opposite directions. Direction of flow may also vary by season or developmental stage of the plant.

ATP Phloem unloading into root cells active transport of sucrose into root cells Decreases pressure in bottom of plant Sucrose will travel from high pressure near leaves to low pressure near roots ATP

3. Gas Exchange What environmental conditions might impact transpiration of water?

Gas Exchange Regulation Epidermal cell Guard cell Chloroplasts Nucleus In dry conditions water leaves guard cells by osmosis guard cells become flaccid stomata close to prevent water loss In humid conditions water enters guard cells by osmosis guard cells become turgid stomata open to facilitate water flow H2O H2O H2O H2O H2O H2O H2O H2O Thickened inner cell wall (rigid) H2O H2O H2O H2O Stoma open Stoma closed water moves into guard cells water moves out of guard cells

Control of transpiration Balancing stomate function always a compromise between photosynthesis & transpiration leaf may transpire more than its weight in water in a day…this loss must be balanced with plant’s need for CO2 for photosynthesis

Don’t get mad… Get answers!! Ask Questions! 2006-2007

Homework Section 9.5 – pg. 326 #1-9 Read Transpiration Lab