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Transport in Plants - Water uptake

Grade 12IGCSEBiology

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

🔑Concepts

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Root Hair Cells: Specialized cells with long extensions that increase the surface area for the absorption of H2OH_2O and mineral ions from the soil.

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Water Potential (psi\\psi): Water moves down a water potential gradient from the soil (higher psi\\psi) to the root hair cells (lower psi\\psi) via osmosis through a partially permeable membrane.

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Active Transport of Ions: Root hair cells use ATPATP to pump mineral ions from the soil into the cytoplasm against a concentration gradient. This lowers the psi\\psi inside the cell, facilitating further H2OH_2O uptake.

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Apoplast Pathway: The movement of water and dissolved minerals through the cell walls and intercellular spaces. This movement is fast but is eventually blocked by the Casparian strip.

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Symplast Pathway: The movement of water through the cytoplasm and plasmodesmata (cytoplasmic channels between cells). This movement is slower as it is regulated by the selectively permeable plasma membranes.

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The Endodermis and Casparian Strip: The Casparian strip is a band of waterproof suberin in the endodermal cell walls. It forces water from the apoplast pathway into the symplast pathway, allowing the plant to regulate which minerals enter the XylemXylem.

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Xylem Loading: Water and minerals are moved into the XylemXylem vessels to be transported upwards to the leaves via the transpiration stream.

📐Formulae

ψ=ψs+ψp\psi = \psi_s + \psi_p

Rate of Water Uptake=Distance moved by bubble in potometer (mm)Time (min)\text{Rate of Water Uptake} = \frac{\text{Distance moved by bubble in potometer (mm)}}{\text{Time (min)}}

Surface Area to Volume Ratio=Surface AreaVolume\text{Surface Area to Volume Ratio} = \frac{\text{Surface Area}}{\text{Volume}}

💡Examples

Problem 1:

Calculate the water potential (ψ\psi) of a root cortical cell if its solute potential (ψs\psi_s) is −600 kPa-600\text{ kPa} and its pressure potential (ψp\psi_p) is 150 kPa150\text{ kPa}. If the soil water potential is −200 kPa-200\text{ kPa}, determine the direction of water movement.

Solution:

ψcell=−600 kPa+150 kPa=−450 kPa\psi_{cell} = -600\text{ kPa} + 150\text{ kPa} = -450\text{ kPa}. Since the soil has a ψ\psi of −200 kPa-200\text{ kPa} and the cell has a ψ\psi of −450 kPa-450\text{ kPa}, water will move from the soil into the cell.

Explanation:

Water always moves from a region of higher (less negative) water potential to a region of lower (more negative) water potential. −200 kPa>−450 kPa-200\text{ kPa} > -450\text{ kPa}.

Problem 2:

A student uses a potometer to measure water uptake. The air bubble moves 45 mm45\text{ mm} in 15 minutes15\text{ minutes}. Calculate the rate of water uptake in mm/min\text{mm/min}.

Solution:

Rate=45 mm15 min=3.0 mm/min\text{Rate} = \frac{45\text{ mm}}{15\text{ min}} = 3.0\text{ mm/min}.

Explanation:

The rate is determined by dividing the total distance the bubble traveled by the time interval recorded during the experiment.

Water uptake - Revision Notes & Key Diagrams | IGCSE Grade 12 Biology