Transpiration
This
process of 'sucking up' of minerals
and water by the xylem is called transpiration. It
is just like
sucking up a straw. Water
and dissolved minerals are transported, through the Xylem, from the root to the leaves. The transpiration ('sucking up') is caused
by the evaporation of water inside the leaves to the surrounding
air. The water vapour passes out of stomata by diffusion. This
leads to a lack of water in the spongy
mesophyll cells of the leaves. The lost water is then replaced by more water from the xylem which in turn
draws up more from the roots. Transpiration
benefits the plants in two ways: Transpiration Stream
Remember:
Transpiration
- this an important process for a
plant!
Amount of light - more light will increase transpiration as rate of photosynthesis
increase.
Temperature - increased temperature increases both evaporation and photosynthesis,
thus increasing transpiration.
Extent of air
movement - increased air movement increases evaporation, thus
increasing transpiration.
Humidity of the surrounding air - decreased humidity increases evaporation, thus
increasing transpiration.
The condition of stomata - Turgid stomata allows more water to escape by diffusion, hence
increasing transpiration.
From
the above information, it is quite apparent that the highest rate of transpiration
occurs during sunny, hot, dry and windy conditions. Perfect
conditions for putting your washing out! When plants have ample supply
of water, all their cells 'suck up' water
into themselves by osmosis and turn turgid.
The 'watery' content of
vacuoles in the cells start to press against the cell wall, just like tire
tube against the tire, and so gives support to the plant tissue.
Turgor
pressure also help the leaves to stay
'stiff'. You have probably seen this
with potted plant, if you don't water it for some weeks, the plant starts
to 'wilt' and the leaves 'droop'.
This is due to the fact that cells start to lose
water and thereby lose their turgor
pressure.
Factors
affecting the speed of transpiration.
Remember: The TURGIDITY (fluid
pressure) of
the cells help a plant to stay upright and keep leaves firm.
Test your understanding of the transpiration stream and how plants transport water. 1. Which specific vessels are responsible for transporting water from the roots up to the leaves? 2. What happens to the rate of transpiration on a very windy day? 3. Through which structures on the leaf does water vapour primarily escape into the atmosphere? 4. Why does transpiration occur slower in humid (damp) air? 5. Which cells are responsible for opening and closing the pores on a leaf to control water loss?🔬 Knowledge Check: Transpiration
Click to Reveal Answers
2. It increases (Wind moves water vapour away, encouraging more evaporation).
3. Stomata (Tiny pores usually found on the underside of the leaf).
4. Smaller difference in water concentration (Humid air is already full of water, slowing evaporation).
5. Guard cells (Two cells that flank each stoma to control its size).
Tags:Photosynthesis, Turgidity, Osmosis, Turgor, Stomata, Transpiration