Plant TransportMoving water, minerals, and sugars

Vascular Tissue

Xylem• Xylem tissue

transports water from roots to leaves.

• Xylem vessels are dead at maturity. Why? Think of both structural and functional reasons.

Phloem• Phloem tissue

transports sap (water and sugar) from “source” to “sink.”

• Phloem vessels are live at maturity, but need companion cells. Why?

Source and Sink• Source: where

the sugar starts its journey (either where it is produced or stored).

• Sink: where sugar ends up (either where it is needed or will be stored).

• Animated tutorial: Nutrient uptake

Transpiration

Water transport in 3 parts

• Transpiration (or evapo-transpiration) is the transport of water and minerals from roots to leaves. It involves three basic steps:

• Absorption at the roots.

• Capillary action in the xylem vessels.

• Evaporation at the leaf.

Part 1: Roots

• Roots absorb water and minerals in a 4-step process:

• Active transport of minerals into root hairs.

• Diffusion to the pericycle.

• Active transport into the vascular cylinder.

• Diffusion into the xylem.

Mineral and water uptake

Casparian Strip• The Casparian

strip controls water movement into the vascular cylinder of the root.

• Water cannot move between cells. It must move through the cells by osmosis. Why is this important?

Thinking question:

• What would happen in a root that had no Casparian strip? Why would this be a problem?

Microbial helpers

• Mycorrhizal fungi help plants absorb minerals by extending the surface area of roots.

Microbial helpers• Nitrogen-

fixing bacteria in root nodules help plants acquire nitrogen.

• N-fixing bacteria are associated mostly with legumes and alder trees.

Thinking Question

• Suppose a gardener notices that several of her favorite flowering plants are infected with a fungal disease. She decides to spray not only the plants, but the soil all over her garden to prevent the disease. What negative effect could this have on her plants?

Step 2: Capillary action

• Cohesion: polar water molecules tend to stick together with hydrogen bonds.

• Adhesion: water molecules tend to stick to polar surfaces.

Capillary action

• Cohesion and adhesion cause water to “crawl” up narrow tubes. The narrower the tube the higher the same mass of water can climb.

• Maximum height: 32 feet.

Cohesion-tension theory• Cohesion between water molecules creates a

“water chain” effect.

• As molecules are removed from the column by evaporation in the leaf, more are drawn up.

Thinking question

• If the forces of cohesion-tension theory move water up a stem, what happens to water pressure at the roots? Will that affect water moving into the roots?

Back to the roots...• Pressure differences created by transpiration

draws water out of the roots and up the stems.

• This creates lower water pressure in the roots, which draws in more water.

Part 3: Evaporation• Evaporation at the surface of the leaf keeps the

water column moving.

• This is the strongest force involved in transpiration.

• Transpiration:http://www.kscience.co.uk/animations/transpiration.htm

Stomata control• Guard cells

around the stomata are sensitive to light, CO2, and water loss.

• Cells expand in response to light and low CO2 levels, and collapse in response to water loss.

• Guard Cell Function: http://www.phschool.com/science/biology_place/labbench/lab9/stomamov.html

Stomata

• When stomata are open, evaporation draws water out of the leaf. Gas exchange can also occur to keep photosynthesis and respiration running.

• When stomata are closed, evaporation cannot occur, nor can gas exchange. What happens to photosynthesis and transpiration?

Thinking question

• Suppose you were testing transpiration in trees on a warm, humid summer day, and again on a windy day when the air is dry. How would transpiration on these two days differ?

Sugar Transport

The trouble with phloem

• Phloem tissue is living tissue, unlike xylem. When scientists studying how it works cut into it, the plants responded by plugging up the phloem.

Aphid helpers

• But aphids can pierce phloem tissue and suck out sap without any problem.

• Scientists used aphids to study the flow of sap in phloem.

Sap

• Sap consists of sugar dissolved in water at high concentrations: usually between 10% and 25%.

• Since this is highly concentrated, plants have to use active transport to work against a diffusion gradient as part of the sap-moving process.

Pressure-flow theory

• The pressure-flow theory explains how sap moves in a plant from source to sink:

• Sugars begin at a source and are pumped into phloem tube cells.

• Osmosis moves water into the cells and raises pressure.

• Pressure moves the sap.

Pressure flow 1• The leaf is a source

of sugar, since it makes sugar by photosynthesis. Glucose and fructose made by photosynthesis are linked to make sucrose, which does not move easily through the cell membranes. Why is this important?

Pressure-flow 2• Active transport is

used to load sucrose into phloem tubes against a diffusion gradient. As sugar is loaded into the cell, what else moves in on its own? What will happen to the pressure in the cell?

Pressure-flow 3• The high

concentration of sucrose in the sieve tube cells of the phloem causes water to move in by osmosis, which raises pressure in the cell. What happens to the sap?

Pressure-flow 4• A developing fruit

is one example of a sink. Sucrose may be actively transported out of phloem into the fruit cells. In a root, sucrose is converted into starch, which keeps sugar moving in by diffusion.

Pressure-flow 5

• As the sugar concentration drops in the sieve tube cells, osmosis moves water out of the tube.

Pressure-flow 6

• As water moves out by osmosis, the pressure in the sieve tube cells drops. The pressure difference along the column of sieve tube cells keeps the sap flowing.

Pressure-flow: Review

Thinking question

• Sugars are often stored in roots, sometimes as starch. Why can a root be both a “source” and a “sink” when it comes to sugar transport?

• Pressure-Flow animation:http://www.tvdsb.on.ca/westmin/science/sbioac/plants/sucrose.htm

• Plant transport animated tutorial