Transport in Vascular PlantsChapter 36

Review: Cell Transport

• Passive transport:– Diffusion across membrane

with concentration gradient, no energy required

• Active transport:– Pumping of materials against

concentration gradient, ATP required• May involve transport proteins

(ex: proton pump)• Cotransport• Chemiosmosis

Recall: Water Potential

• The combined effects of solute concentration and physical pressure

• Determines the direction of movement of water

• Water moves from areas of high water potential to areas of low water potential

• Bozeman biology

Transport

• Transport in plants occurs on three levels:– Cellular

• Absorption of water and minerals from soil by root cells

– Short distance• Cell-to-cell at tissue level• Ex: loading sugar into

phloem

– Long distance• Sap in xylem and phloem

throughout plant

Transport of Water• Moving water up the xylem to the leaves is known as transport• There are 3 major parts to the transport of water:

– Absorption at the roots– Capillary action up the xylem– Transpiration (evaporation) at the leaves

• The water also brings nutrients and mineral vital for plant growth with it

Step 1: Absorption at the roots• Active transport of minerals into root hairs• Diffusion to the pericycle• Active transport into the vascular cylinder• Diffusion into the xylem• Mycorrhizal fungi help plants absorb minerals and water• Nitrogen fixing bacteria live in nodules of roots to assist in nitrogen 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?

Step 2: Capillary Action

• Due to hydrogen bonds, water molecules stick to one another and other polar surfaces

• Cohesion and adhesion cause water to climb up xylem as if it were a chain being pulled

Cohesion-Tension Theory

• Cohesion causes molecules to stick to one another

• As water evaporates, it “pulls” the chain upwards from the roots to the leaves

• As more water is drawn up the stem, the water pressure in the roots is lowered, therefore, it takes in more water

Step 3: Transpiration

• Evaporation occurs in the leaves• Why is this important?• This keeps water flowing

upward from the roots• This is the strongest force

involved in water transport• Stomata help regulate the rate

of transpiration by opening/closing to help balance conservation of water with gas exchange for photosynthesis

Sugar Transport

• Sugar is made in the leaves and plants store it as a mixture of sugar and water called sap

• Due to the sugar content, plants must move sap against a concentration gradient

• The leaves (where sugar is made) are called a source

• Where sugar ends up are called sinks

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 sap

Step 1: Getting Glucose Out of Parenchyma

• The leaf is a source of sugar

• Glucose and fructose made by photosynthesis are linked to make sucrose

• Sucrose will need assistance moving across the membranes of cells– Companion cells

Companion cell

Parenchyma cell

Step 2: Using Water to Help Sucrose Move

• Active transport moves sucrose into the xylem cells

• As sugar moves in, water will move in also to attempt to reach a state of equilibrium

• This raises the pressure in the tubes

Step 3: Reaching the Sink

• Active transport will move sugar out of xylem into a sink (like a fruit)

• As sugar concentration drops, water moves out of the cell, lowering the pressure

• The lower pressure the more the column of sap will move

Pressure Flow Hypothesis