What is transpiration?
Transpiration is the water loss in the form of water vapour from plants by evaporation.
Where does transpiration occurs?
Stomata, cuticle and lenticels
Process of tranpiration
- Water diffuses out of the mesophyll cells to form a water film.
- Water of the water film evaporates to form water vapour & moves into the air space.
- The air space will be nearly saturated with water vapour.
The water vapour concentration in the air space is highter than that of the atmosphere. Water vapour then diffuses to the atmosphere through the stomata.
Creation of transpiration
- Mesophyll cells near the air space lose water due to transpiration. Their water potential lowers.
- As a result, water moves from neighbouring cells with relatively higher water potental into these cells by osmosis & is repeated across the layer of mesophyll cells.
- Eventually, water moves out of the xylem vessels by osmosis to replace the waer loss of the mesophyll cells.
- Overall, a water potential gradient is created along a chain of cells across the leaf. Transpiration pull is created to pull water up the xylem vessels from roots.
Significance of transpiration
- Transpiration creates transpiration pull
1. Enable transport of water & minerals from roots to other parts of the plant
2. Facilitates water absorption by roots - Evaporation of water (from the mesophyll cells) removes heat from the leaves
→ produces a cooling effect to prevent the plant from being overheated under hot condition.
Adaptation of leaves to prevent excessive water loss
- Epidermis is covered with a waxy cuticle → impermeable to water → reduce water loss
- Fewer stomata in the upper epidermis
1. Layers of dicotyledonous plant are usually oriented horizontally, their upper epidermis is directly illuminated under sunlight & has a higher temperature than the lower epidermis → prevent excessive water loss. - Guard cells control the opening and closing of stomata
1. In the daytime, stomata open to allow gas exchange.
2. At night, most stomata close because the need of gas exchange decreases in the absence of photosynthesis → prevent excessive water loss.
How different factors affect the rate of transpiration?
- Rate of evaporation of water
- Rate of gas diffusion
- Degree of opening of stomata
How does light intensity affect the rate of transpiration?
- The rate of light intensity is low in darkness
Light intensity low → stomata closed → only a small amount of water vapour can diffuse out → low transpiration rate. - The rate of transpiration increases when the light intensity increases
Light intensity increases → stomata open wider → water vapour in the air space diffuses out more rapidly → higher transpiration rate. - The rate of transpiration decreases when the light intensity is too high
Light intensity is too high → stomata closed → only a small amount of water vapour can diffuse out → lower transpiration rate.
Degree of opening of stomata
How does temperature affect the rate of transpiration?
The rate of transpiration increases when the temperature increases.
Temperature increases → rate of evaporation from mesophyll cells increases (for cooling down the plants) → concentration gradient of water vapour between the air space and the atmosphere increases → water vapour in the air space diffuses out more rapidly → higher transpiration rate.
Rate of evaporation of water
How does air movement affect the rate of transpiration?
- The rate of transpiration is low in still air.
In still air → water vapour accumulates around the stomata → concentration gradient of water vapour between the air space and the atmosphere decreases → water vapour in the air space diffuses out less rapidly → low transpiration rate. - The rate of transpiration increases when the wind speed increases.
Wind speed increases → wind blows away water vapour around the stomata → maintain a steep concentration gradient of water vapour → water vapour in air space diffuses out more rapidly → higher transpiration rate. - The rate of transpiration decreases in strong wind.
Strong wind → most stomata closed → water vapour in air space diffuses out less rapidly → lower transpiration rate.
How does relative humidity affect the rate of transpiration?
The rate of transpiration decreases when the relative humidity increases.
Relative humidity increases → concentration gradient of water vapour between the air space and atmoshere decreases → water vapour in air space diffuses out less rapidly → lower transpiration rate.
Other factors affecting the rate of transpiration
- Surface area of the leaves
- Thickness of the cuticle
- Abundance of stomata
- Use vaseline to block the stomata
- Cut all the leaves
- Waterlogged / Too much water / Cyanide
- Too much fertilizer (water is drawn out of the roots & there will be not enough water for the plant)
Process of water absorption by roots
- Water is lost due to transpiration and transpiration pull is created.
- Water is drawn up the xylem vessels from the roots to the leaves by transpiration pull.
- Water in the cortex cells enters the xylem vessels and thus their water potential decrease. A water potential gradient is set up across the whole cortex.
- Water travels from cells to cells:
a. Water moves (along a water potential gradient) through the cytoplasm by osmosis
b. Water moves through the vacuoles by osmosis
c. Water moves through the cell wall freely - +++? Water in the soil enters the root hair by osmosis.
Process of minerals absorption by roots
- Minerals are absorbed by active transport against concentration gradient using energy from respiration (the reason why many mitochondria are found in root hair cells).
- Absortion of minerals helps absorption of water into root hair cells by osmosis as it lowers the water potential of the cells.
1. Concentration of dissolved minerals in the soil is lower than that of the root hair cells.
2. MInerals are absorbed into the root hair cells against a concentration gradient by active transport.
3. Water potential of root hair cells decreases, facilitating water absorption.
Adaptative features of roots for absorption of water and minerals
- The epidermis is not covered by cuticle → allows water and minerals to easily pass through the epidermis into the root.
- The root is highly branched & there are numerous root hairs on the root → provides a larger water surface for the absorption of water and minerals.
- Root hairs are long and fine → allows root hairs to grow between soil particles to absorb water and minerals more easily.
- Root hair cells contain many mitochondria → ensure enough energy is supplied to absorb minerals from the soil by active transport.
Xylem’s function and adaptive features
- Transport water and minerals.
- Provides support for plants.
- Mainly consists of xylem vessels. Xylem vessels are hollow tubes made up of dead cells.
1. Hollow tube (with no cell contents & walls between cells) → allow the passage of water with little resistance.
2. Cell walls are thick and lignified → prevent vessels from collapsing under the great tension of transpiration pull.
Phloem’s function and adaptive features
- Transport organic nutrients (mainly sugar(sucrose))
- Mainly consists of sieve tubes (: elongated living cells joined end to end, which ended in sieve plates) & companion cells.
1. Sieve tubes contain little cytoplasm & no nucleus → allows orgainc nutrients to move along.
2. Sieve tubes have pores
→ allows organic nutrients to pass through.
3. Companion cell has dense cytoplasm & many organelles (including nucleus)
→ supports metabolism of the sieve tube.
Process of transportation of water and minerals
- Water is lost due to transpiration. Transpiration pull is created.
- Water with dissolved minerals is drawn up the xylem vessels by transpiration pull.
- Water in the soil is absorbed into the roots by osmosis. Dissolved minerals are absorbed by active transport.
Translocation of organic nutrients in plants
Supporting in plants enable?
Stand upright & stretch branches
* Allow leaves to recieve maximum amount of sunlight for photosynthesis.
* Facilitates pollination by lifting up flowers.
* Facilitates disperal of fruits and seeds by lifting them up.
Parts with many thin-walled cells?
- Leaves (e.g. mesophyll cells)
- Herbaceous stems (e.g. cells in cortex and pith in stems)
- Other non-woody parts of plants
TWS are closely packed and their turgidity provides support to the plant
What happen to the plant when the water supply is adequate?
Thin-walled cells in the leaves and stems gain water by osmosis. They become turgid and press against each other. The turgidity of the cells makes the whole stem strong enough to stand upright.
What happen to the plant when the water supply is inadequate?
Thin-walled cells lose water (as the rate of transpiration is higher than the rate of water uptake). The cells become flaccid and can no longer press against each other and provide support to the plant. The plant wilts. If the plant can take up enough water shortly, the cells will become turgid and the plant will stand upright again.
Parts with thick-walled cells?
- Xylem vessels
- Tissue outside each vascular bundle in young dicot stems
Cell walls of TkWS contain lignin, which makes rhe cells hard and rigid
