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Plant Biology (AHL) - Transport in the xylem of plants (HL)

Grade 11IBBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Transpiration is the inevitable consequence of gas exchange in the leaf; CO2CO_2 enters for photosynthesis through stomata, while H2OH_2O vapor escapes down a concentration gradient.

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The cohesive property of water and the structure of xylem vessels allow transport under tension. Water molecules are polar and form hydrogen bonds, a property known as cohesion.

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Adhesion between H2OH_2O molecules and the hydrophilic cellulose in the xylem cell walls allows water to be pulled up the xylem in a continuous column.

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Xylem vessels are formed from dead cells that are arranged end-to-end; their walls are thickened with lignin to withstand very low pressures (suction) without collapsing.

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Active uptake of mineral ions (such as K+K^+, Na+Na^+, and PO43−PO_4^{3-}) in the roots causes absorption of water by osmosis. This creates root pressure, though the primary mover is transpiration pull.

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Stomata regulate transpiration rates. Guard cells control the aperture of the stomatal pore. Abscisic acid (ABAABA) is a plant hormone produced during drought stress that causes stomatal closure.

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Xerophytes are plants adapted to arid climates. Adaptations include thick waxy cuticles, reduced number of stomata, rolled leaves, and CAMCAM physiology to minimize H2OH_2O loss.

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Halophytes are plants adapted to saline soils. They maintain a lower water potential than the soil by sequestering inorganic ions (e.g., Na+Na^+, Cl−Cl^-) in vacuoles to facilitate H2OH_2O uptake via osmosis.

📐Formulae

Rate of Transpiration=Volume of H2O lostTime (t)\text{Rate of Transpiration} = \frac{\text{Volume of } H_2O \text{ lost}}{\text{Time (t)}}

V=πr2d (Volume of water in a potometer capillary tube, where d is distance bubble moved)V = \pi r^2 d \text{ (Volume of water in a potometer capillary tube, where } d \text{ is distance bubble moved)}

Ψ=Ψs+Ψp (Water potential equation where Ψs is solute potential and Ψp is pressure potential)\Psi = \Psi_s + \Psi_p \text{ (Water potential equation where } \Psi_s \text{ is solute potential and } \Psi_p \text{ is pressure potential)}

💡Examples

Problem 1:

A student uses a potometer to measure the transpiration rate of a temperate plant. The internal radius of the capillary tube is 0.8 mm0.8\text{ mm}. If the air bubble moves 25 mm25\text{ mm} in 5 minutes5\text{ minutes}, calculate the volume of water lost per minute.

Solution:

V=π×(0.8 mm)2×25 mm≈50.27 mm3V = \pi \times (0.8\text{ mm})^2 \times 25\text{ mm} \approx 50.27\text{ mm}^3 Rate=50.27 mm35 min=10.05 mm3 min−1\text{Rate} = \frac{50.27\text{ mm}^3}{5\text{ min}} = 10.05\text{ mm}^3\text{ min}^{-1}

Explanation:

The volume of water transpirated is approximated by the volume of the cylinder of water moved in the capillary tube (V=πr2hV = \pi r^2 h). Dividing this total volume by the time gives the rate of transpiration.

Problem 2:

Describe how the cohesive and adhesive properties of water facilitate its movement in the xylem.

Solution:

Cohesion: H2OH_2O molecules stick to each other via hydrogen bonds. Adhesion: H2OH_2O molecules stick to the lignin/cellulose walls of the xylem.

Explanation:

Because of cohesion, when H2OH_2O evaporates from the mesophyll into the leaf air spaces, it pulls on the adjacent water molecules, creating a continuous 'string' of water. Adhesion prevents the water column from dropping back down due to gravity and helps maintain the tension required for the transpiration pull.