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

Grade 12IBBiology

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.

Transport in the xylem of plants - Revision Notes & Key Diagrams | IB Grade 12 Biology