Topic 9.1: Transport in the Xylem of Plants

Essential Idea:  Structure and function are correlated in the xylem in plants.

At SHS, Topic 9.1 is taught in the following class unit(s):

• Xylem Structure and Function (unit 35)

Statements & Objectives:

9.1.U1  Transpiration is the inevitable consequence of gas exchange in the leaf.​

• Define transpiration.
• Outline gas exchange that occurs through leaf stomata.

9.1.U2  Plants transport water from the roots to the leaves to replace losses from transpiration.

• Outline structures and mechanisms involved in the flow of water from roots to leaves. 

9.1.U3  The cohesive property of water and the structure of the xylem vessels allow transport under tension.

• Describe structure of xylem.
• Outline how xylem is able to maintain rigidity even under low pressure or mechanical disturbance.
• Outline polarity of water molecule.
• Define cohesion.​

9.1.U4  The adhesive property of water and evaporation generate tension forces in leaf cell walls.

• Explain the decrease in pressure and transpiration-pull that results from evaporation of water from the leaf.
• State the transpiration is a passive processes.

9.1.U5  Active uptake of mineral ions in the roots causes absorption of water by osmosis.

• Explain why roots are hypertonic relative to the soil.
• Outline the role of active transport in maintaining root tonicity.
• Describe how water enters roots from the soil.
• Compare the symplastic and apoplastic pathways of water transport through the root.

9.1.A1  Adaptations of plants in deserts and in saline soils for water conservation.

• Define xerophyte and halophytic.
• Outline strategies used by xerophytes  and halophytes to reduce water loss. ​

9.1.A2  Models of water transport in xylem using simple apparatus including blotting or filter paper, porous pots and capillary tubing.

• Explain use of models in science.
• Describe simple models of water transport, inclusive of evaporation, adhesion and cohesion. ​

9.1.S1  Drawing the structure of primary xylem vessels in sections of stems based on microscope images.

• Draw a xylem vessel tube, labeling cellulose wall and helical lignin thickening.

9.1.S2  Measurement of transpiration rates using photometers. (Practical 7)

• Describe the use of a potometer to measure transpiration rates.

9.1.S3  Design of an experiment to test hypothesis about the effects of temperatures or humidity on transpiration rates.

• ​Identify the manipulated, responding and controlled variables in an experiment to test the effect of an abiotic factor on the rate of transpiration.

9.1.NOS  Use models as representations of the real world-mechanisms involved in water transport in the xylem can be investigated using apparatus and material that show similarities in structure to plant tissues.

• State a similarity and a difference between transpiration models and transpiration in plant tissues. ​

In the News:

• Sensors applied to plant leaves warn of water shortage (2017-11-08)
• Trees in the Amazon make their own rain (2017-08-04)
• Drought-affected trees die from hydraulic failure and carbon starvation (2017-08-07)
• Changing of the guard: Research sheds light on how plants breathe (2017-09-21)
• To Clean Drinking Water, All You Need Is A Stick (2014-03-05)
• Breakthrough discovery reveals how thirsty trees pull water to their canopies (2016-01-20)
• How Is A 1,600-Year-Old Tree Weathering California's Drought (2016-10-27)
• As rains ease in the west, cactus shines brighter than ever (2017-04-24)
• ​During drought, dry air can stress plants more than dry soil (2016-09-05)