Transport in flowering plants

Transpiration

http://croptechnology.unl.edu/animationOut.cgi?anim_name=transpiration.swf

Lesson Objectives

By the end of this lesson you should be able to: • 1) Define the term transpiration and explain that

transpiration is a consequence of gaseous exchange in plants.

• 2) Describe the factors that affect transpiration rate

Get this right

• Water potential= a measure of the tendency of water to move from one place to another.

• A dilute solution has a ............... water potential than a concentrated one.

• Water moves from a solution of higher water potential to a solution of lower water potential

higher

Capillary action- cohesion

• Each molecule of water consists of 1 oxygen and 2 hydrogen atoms

• Oxygen carries ............. charge• Hydrogen carries .............

charge• Charges results in forces of

attraction between water molecules: force of cohesion

negative

positive

Capillary action- adhesion

• Water also sticks to the inner surface of xylem vessels

• The wall of the xylem just above the water surface attracts water molecules upwards: force of adhesion

Transpiration- How?• Definition: The loss of water vapour from the

aerial parts of a plant• Stomata on the surface of leaves open for

gaseous exchange • Air surrounding the leaves are “drier”• Water molecules exit the leaves through the

stomata

Lower water potential

Higher water potential

Transpiration- the process•Stomata open for gaseous exchange

•More water vapour molecules in-between the spongy mesophyll cells than the outside air

•Water molecules diffuses out of the leaves through the stomata

•Loss of water creates water potential difference between xylem and mesophyll cells

•Water moves out of xylem vessels through osmosis

•Chain of water molecules pulled upwards with the assistance of the forces of cohesion and adhesion

Factors affecting the rate of transpiration

Factors Rate of transpiration will

be fast if

Rate of transpiration will

be slow if

http://passel.unl.edu/pages/animation.php?a=transpiration.swf

Factors affecting the rate of transpiration

Factors Rate of transpiration will

be fast if

Rate of transpiration will

be slow if

Humidity dry damp/ humid

Temperature warm cold/ cool

Light bright dim

Wind windy still

Quick check

Are you able to:

• 1) Define the term transpiration and explain that transpiration is a consequence of gaseous exchange in plants.

• 2) Describe the factors that affect transpiration rate

1. Windier conditions increases transpiration rate because the boundary layer around the leaf is smaller. (T/F)

2. Transpiration rate is lower at night due to lower temperature. (T/F)

3. The water vapour concentration gradient is due to the difference in water vapour concentration between the atmosphere and the leaf. (T/F)