Plants need a transport system to provide:
- A way to carrying water and ions from the root to the leaves.
- A way of carrying sugars and aminoacids from the leaves to the other parts of the plant when it is photosynthesis.
- A way of carrying sugar and amino acids from the storage organs to the leaves when it is not photosynthesis
Plants have two different networks of tubes inside them;
- Phloem: transports sucrose and amino acids up and down the stem.
- Xylem: transports water and minerals up the stem.
Phloem vessels are involved in translocation. This is the movement of food substances from the stems to growing tissues and storage tissues.
It is tissue composed of tubes called xylem vessels and long pointed fiber cells.
Comparison of xylem and phloem
|Cell wall material||Lignin
|Takes place in||Roots, stems and leaves||Roots, stems and leaves|
|Nature||Non living tissue at maturity||Living tissue|
|Made up of||Tracheids, vessel elements, xylemparenchyma, xylem sclerenchyma.||Sieve tubes, companion cells, phloem parenchyma, bast fibers, intermediary cells.|
|Physical structure||Tubular with hard walled cells||Tubular with soft walled cells|
|Movement||Unidirectional – Moves up the plant’s stem||Bi-directional – Moves up or down the plant’s stem|
How does water and ions moves in the plant
The different in water potential in a plant and surrounding dictate the movement the movement of water into and within the plant. Water moves from an area of higher water potential to an area of lower water potential.
Water and ions moves from the roots to the leaves of plants due to:
- Root pressure
- Capillary action
- The cells and root hair cells that surround the xylem in root actively pump ions into the xylem vessels
- The water potential in the xylem vessels decreases creating a water potential gradient.
- Water moves by osmosis from these cells into the xylem vessels. As water continues to enter the xylem, the increase in pressure pushes the water column upwards. However, root pressure is unable to bring water all the way to the top of the tall trees.
- Capillary action is the spontaneous movement of water along narrow tubes. It contributes to the upward movement of water in small plants.
- Capillary action is due to the force of attraction between the water molecules and the force of attraction between the water molecules and the inner surface of capillary tubes.
- However, the maximum height to which water can rise to capillary action is 3 meter.
- Transpiration pull is the main force that pulls water up the plant.
- Transpiration lowers the water potential of mesophyll cells, which causes water move from the xylem vessel into the mesophyll cells
- As water leaves the xylem vessels, tension is created. Water is able to move up the xylem vessel.
- The continuous column of water moving up xylem due to the transpiration pull is called the transpiration stream.
The main force which draws water from the soil and through the plant is cued by a process called transpiration.
Water is lost from three main area of plant:
- Stomata: Plants ‘breathe’ too, but they do it through tiny openings in leaves
- Cuticle: the outer cellular layer of a hair
- Lenticels:Oneof the small, corky, oval or elongated areas on the surface of a plant stem, trunk, or fruit thatallow the interchange of gases between the interior tissue and the surrounding air
Water on the surface of spongy and palisade cell evaporates and then diffuses out of the leaf. This is called transpiration. More water is drawn out of the xylem cells inside the leaf to replace what’s lost. As the xylem cells make a continuous tube from the leaf, down the stem to the roots, these acts like a drinking straw, producing a flow of water and dissolved minerals from roots to leaves.
Factors affecting the rate of transpiration:
|Light||In bright light transpiration increases||The stomata (openings in the leaf) open wider to allow more carbon dioxide into the leaf for photosynthesis|
|Temperature||Transpiration is faster in higher temperatures||Evaporation and diffusion are faster at higher temperatures|
|Wind||Transpiration is faster in windy conditions||Water vapour is removed quickly by air movement, speeding up diffusion of more water vapour out of the leaf|
|Humidity||Transpiration is slower in humid conditions||Diffusion of water vapour out of the leaf slows down if the leaf is already surrounded by moist air|
Measuring rate of transpiration
The best experiment is a potometer, which measures how quickly a little bubble of air moves up a glass tube attached to the bottom of the stem. Adding a fan, changing the humidity, increasing the temperature etc will all change the speed the bubble moves up the tube.
How to calculate rate of transpiration;
- Volume of water column from B to A=vcm3.
- Time taken for water meniscus to move from B to A=t min.
- Rate of transpiration=v/t