Water and Food Transport

Xylem and Phloem Xylem transport water and minerals from the soil to all parts of the plant Phloem transport sugars made in the leaves to all parts of the plant How these tasks are accomplished is not fully understood

Active Transport vs. Osmosis Active transport is the movement of ions, molecules or minerals from an area of low concentration (hypotonic) to an area of high concentration (hypertonic) This requires energy Osmosis is the movement of water from an area of low concentration of ions, molecules or minerals (hypotonic) to an area of high concentration (hypertonic)

Moving stuff around link

Active transport

Water Transport in the Root Root hairs absorb essential minerals from the soil by active transport The energy required comes from respiration Once the minerals are inside the epidermis they are moved towards the center of the root where the wax coated endodermis prevents them from diffusing back out

Water Transport in the Root The concentration of minerals in the xylem sap is hypertonic relative to the soil water (has more minerals dissolved in it than the water in the soil) This causes the hypotonic soil water to enter the root by osmosis As long as the root hairs absorb minerals by active transport, water will follow by osmosis

Minerals in the xylem sap Minerals in the soil water Flow of Water by Osmosis

Root Pressure The accumulation of water and minerals in the root creates pressure which pushes the xylem sap up This is called root pressure which can be measured Root pressure can also cause guttation

Root Pressure Root pressure can only push water up to a maximum of a few metres in some plants There are trees that are over 100 m tall In many plants there is no measurable root pressure Root pressure cannot entirely account for xylem sap movement

Water Transport in the Stem Water clings readily to a variety of non oily surfaces – adhesion Adhesion accounts for the movement of water even against gravity Water adheres to the inner walls of the xylem, creating a pulling effect on the column of sap

Water Transport in the Stem Water clings readily to itself -- cohesion The hydrogen bonds between adjacent molecules create very strong forces The cohesion of the water molecules in the very thin xylem tubes results in the column holding together continuously from the ground up to the highest leaves

Water Transport in the Stem The “pull” at the leaves draws water all the way up from the roots Leaf pull or transpiration pull provides the pull

Water Transport in the Leaves Most of the water entering the leaves is lost through transpiration As each water molecule evaporates it “pulls” on the next molecule in an unbroken cohesive chain all the way from the root hairs

Water Transport in the Leaves Transpiration rate depends on the temperature In high temperatures, transpiration rate is high

Food Transport in Vascular Plants The movement of sugars through the phloem is called the pressure-flow theory Leaves are called the source The storage cells in the roots are called the sink

Food Transport in Leaves In the leaves, the sugars are pumped into the phloem by active transport thus creating a hypertonic solution inside water moves by osmosis into the phloem This causes pressure that pushes the sugars away from the leaves into the stem towards the roots

Food Transport in Roots In the roots, the sugars move out of the phloem into storage parenchyma cells (the sink) This happens either passively or actively depending on the relative concentration of sugars in the roots Water molecules follow the sugars out of the phloem by osmosis

Food Transport in Roots As the leaves add sugars and the roots remove sugars the phloem sap experiences two different pressures High pressure in the leaves pushes the sap and the low pressure in the roots pulls the sap

Support for the Pressure Flow Theory Aphid colonies are usually found on thin stems just below leaves. An aphid’s mouth-parts form an extremely fine tube or proboscis, which it inserts into individual phloem sieve cells to withdraw phloem sap for food. While an aphid is actively feeding, researchers are able to “snip” the aphid away from its embedded proboscis. The proboscis continues to ooze phloem sap, clearly indicating that the phloem is under pressure.

Homework Answer questions on Page 404 # 1-3 Page 407 # 1, 2,