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Biology: Biochemistry and Plant Physiology - Plant Transport (Transpiration and Translocation)

Grade 8IB

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

🔑Concepts

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Transpiration is the loss of water vapor from the leaves and stems of plants. It occurs mainly through the stomata. This process creates a negative pressure (tension) that pulls water upwards from the roots through the xylem, known as the transpiration pull.

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The Xylem tissue is a system of dead, hollow tubes reinforced with a waterproof substance called lignin. It transports water and dissolved mineral ions unidirectionally from the roots to the leaves.

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Cohesion and Adhesion: Cohesion is the attraction between water molecules due to hydrogen bonding, while adhesion is the attraction between water molecules and the xylem walls. These forces maintain a continuous column of water: H2O (liquid)→H2O (vapor)H_{2}O \text{ (liquid)} \rightarrow H_{2}O \text{ (vapor)}.

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Factors affecting Transpiration: 1. Light intensity (increases stomatal opening), 2. Temperature (increases kinetic energy of water molecules), 3. Humidity (high humidity decreases the concentration gradient, slowing transpiration), 4. Wind speed (removes water vapor from the leaf surface, maintaining the gradient).

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Translocation is the movement of organic solutes (mainly sucrose and amino acids) through the Phloem. Unlike xylem transport, translocation is bidirectional, moving substances from sources (where they are made, like leaves) to sinks (where they are used or stored, like roots, fruits, or growing tips).

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The Phloem consists of living cells: sieve tube elements (which lack nuclei for easier flow) and companion cells (which provide metabolic support and energy for loading sucrose).

📐Formulae

Rate of Transpiration=Total volume of water lost (mL)Time (minutes)\text{Rate of Transpiration} = \frac{\text{Total volume of water lost (mL)}}{\text{Time (minutes)}}

Ψ=Ψs+Ψp\Psi = \Psi_{s} + \Psi_{p}

Stomatal Density=Number of stomata observedArea of the field of view (mm2)\text{Stomatal Density} = \frac{\text{Number of stomata observed}}{\text{Area of the field of view (mm}^2\text{)}}

💡Examples

Problem 1:

A student uses a potometer to measure the rate of transpiration in a plant shoot. The capillary tube has a radius of 0.5 mm0.5\text{ mm}. Over a period of 10 minutes10\text{ minutes}, the air bubble moves a distance of 40 mm40\text{ mm}. Calculate the rate of water uptake in mm3/min\text{mm}^3/\text{min}. (Use π=3.14\pi = 3.14)

Solution:

  1. Calculate the volume of water uptake (Volume of a cylinder): V=πr2hV = \pi r^2 h V=3.14×(0.5)2×40V = 3.14 \times (0.5)^2 \times 40 V=3.14×0.25×40=31.4 mm3V = 3.14 \times 0.25 \times 40 = 31.4\text{ mm}^3

  2. Calculate the rate: Rate=VolumeTime\text{Rate} = \frac{\text{Volume}}{\text{Time}} Rate=31.4 mm310 min=3.14 mm3/min\text{Rate} = \frac{31.4\text{ mm}^3}{10\text{ min}} = 3.14\text{ mm}^3/\text{min}

Explanation:

The movement of the bubble in the potometer represents the volume of water taken up by the plant to replace what was lost via transpiration. We calculate the volume using the geometry of the capillary tube and divide by time.

Problem 2:

Explain the difference in the concentration of sucrose between the 'source' and the 'sink' during the summer and the mechanism of movement.

Solution:

In summer, the leaves are the source where photosynthesis produces glucose, converted to sucrose (C12H22O11C_{12}H_{22}O_{11}). The concentration of sucrose is higher at the source than at the sink (roots). Water enters the phloem via osmosis due to high solute concentration, creating a high hydrostatic pressure (PP). This pushes the sap toward the sink.

Explanation:

Translocation follows the pressure-flow hypothesis. The movement occurs from high pressure (source) to low pressure (sink) regions.