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Transport in Plants - Xylem and phloem

Grade 11IGCSEBiology

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

🔑Concepts

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Xylem vessels are responsible for the transport of water (H2OH_2O) and dissolved mineral ions (such as Mg2+Mg^{2+} and NO3−NO_3^-) from the roots to the leaves.

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The structure of Xylem includes dead cells joined end-to-end with no cross-walls, forming a continuous tube, and walls thickened with lignin for support.

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Phloem tissue transports sucrose (C12H22O11C_{12}H_{22}O_{11}) and amino acids from 'sources' (leaves/storage organs) to 'sinks' (roots/flowers/fruits) via a process called translocation.

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Root hair cells increase the surface area for the absorption of H2OH_2O through osmosis and mineral ions via active transport.

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Transpiration is the loss of water vapor from plant leaves by evaporation of H2OH_2O at the surfaces of the mesophyll cells followed by diffusion of water vapor through the stomata.

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The 'Transpiration Stream' is maintained by cohesive forces between H2OH_2O molecules and adhesive forces between H2OH_2O and the xylem walls.

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Factors affecting the rate of transpiration include light intensity, temperature, humidity, and wind speed.

📐Formulae

Rate of Transpiration=Distance moved by air bubble (mm)Time (min)\text{Rate of Transpiration} = \frac{\text{Distance moved by air bubble (mm)}}{\text{Time (min)}}

Ψcell=Ψs+Ψp\Psi_{cell} = \Psi_s + \Psi_p

C12H22O11+H2O→2C6H12O6C_{12}H_{22}O_{11} + H_2O \rightarrow 2C_6H_{12}O_6

💡Examples

Problem 1:

In a potometer experiment, the air bubble moves 30 mm30\text{ mm} in 5 minutes5\text{ minutes}. Calculate the rate of transpiration.

Solution:

Rate=30 mm5 min=6 mm/min\text{Rate} = \frac{30\text{ mm}}{5\text{ min}} = 6\text{ mm/min}

Explanation:

The rate of transpiration is calculated by dividing the distance the water (represented by the air bubble) travels by the time taken for that movement.

Problem 2:

Explain the movement of H2OH_2O from the soil into the root hair cell using water potential terminology.

Solution:

The soil has a higher water potential (Ψ\Psi) than the cytoplasm of the root hair cell. Therefore, H2OH_2O moves down a water potential gradient via osmosis through a partially permeable membrane.

Explanation:

Osmosis is the net movement of water from a region of higher water potential to a region of lower water potential.

Problem 3:

Identify the 'Source' and 'Sink' during the spring when a potato tuber starts to grow new shoots.

Solution:

Source: Potato tuber (stored starch converted to C12H22O11C_{12}H_{22}O_{11}); Sink: Developing shoots/leaves.

Explanation:

A source is an organ that exports sugars, while a sink is an organ that imports sugars for growth or storage.

Xylem and phloem - Revision Notes & Key Diagrams | IGCSE Grade 11 Biology