Transport Of Water In The Xylem Flashcards Preview

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How can plants control their rate of transpirtation ?

By changing size of the Stomatal pores


What is transpiration

The evaporation of water from a plants surface, especially the leaves


How is water lost during transpiration

Mesophyll cells lose water to the air spaces by evaporation due to heat (sun)

These cells now have low water potential and so water enters by osmosis from neighbouring cells

Loss of water from these neighbouring cells lowers water potential

They in turn take in water from their neighbours by osmosis

A water potential gradient is established that pulls water from the xylem across the leaf mesophyll, and finally out into the atmosphere


Describe the cohesion tension theory ( movement of water up the stem)

Water evaporates from mesophyll cells due to heat from the sun leading to transpiration

This creates tension (suction) which pulls more water into the leaves

Water molecules form H bonds between one another and hence stick together (cohesion)

This means the whole column of water in the xylem from the leaves down to the roots, moves upwards

As water evaporates from the mesophyll cells in the leaf into the air spaces beneath the stomata, more molecules of water are drawn up behind it as a result of this cohesion

A column of water is therefore pulled up the xylem as a result of transpiration ( transpiration pull)


What evidence supports the cohesion tension theory

Change in diameter of tree trunks according to rate of transpiration - more tension through the day when transpiration is at its greatest, this pulls the walls of the xylem inwards and causes the trunk to shrink in diameter

If a xylem vessel is broke and air enters it- can no longer draw up water, as water molecules cannot stick together

When xylem vessel broken - water not leaked out, as would be the case if under pressure. Instead air is drawn in, which is consistent with it being under tension


Why is transpiration passive

Doesn’t require metabolic energy to take place

However energy is needed to drive the process of transpiration as energy from sun to evaporate


How can the rate of water loss in a plant be measured using a potometer

A leafy shoot is cut under water. Care is taken not to get water on the leaves

The potometer is filled completely with water, making sure there are no air bubbles

Using a rubber tube the leafy shoot is fitted to the potometer under water

Potometer is remover from under the water and all joints are sealed with waterproof jelly

An air bubble is introduced in the capillary tube

The distance moved by the air bubble in a given time is measured a number of times and the mean is calculated


What’s the 4 main factors that increase transpiration rate

Lighter it is - stomata open when it gets lite to let in co2 for photosynthesis

Higher temp - molecules have more energy so they evaporate quicker

Lowe humidity- if air around plants is dry, conc gradient between leaf and air is increased

Windier it is - blows away water molecules from around stomata, increasing concentration gradient