Transport in Plants Flashcards
What is the main function of xylem in which direction does xylem sap move?
Xylem carries mainly water and inorganic ions (mineral salts) from the roots to the parts of the plant above ground and xylem sap moves in only one direction, from the roots to the rest of the plant.
What does phloem transport and in which direction does phloem sap move?
Phloem transports substances made by photosynthesis from the leaves to other parts of the plant. It can also carry substances from storage organs to other parts of the plant. Phloem sap can move in different directions in different tubes, either up or down the plant.
Describe the structure of xylem.
• Xylem vessels are made from cells joined end to end to form tubes and xylem vessel elements are elongated.
• The cells are dead. There is an empty space or lumen inside.
• The walls of the cells are thickened with lignin, a hard, strong, waterproof material.
• Pits in the walls allow water in and out of the tubes.
Xylem also has the function of support.
Explain the movement of water through xylem from root to leaf.
Mass flow through xylem is helped by the fact that water molecules are attracted to each other by hydrogen bonding; this attraction is called cohesion. The water molecules are also attracted to the cellulose and lignin in the walls of the xylem vessels which are hydrophilic. This attraction is called adhesion. Cohesion and adhesion keep the water moving as a continuous column. The cells are dead and empty so there is no protoplasm to get in the way of transport. The small diameter of xylem vessels helps to prevent air breaks from occurring. The pits in the vessel walls allow water to move out from one vessel to a neighbouring vessel and so bypass an air lock. Pits are also important because they allow water to move in and out of xylem vessels from and to surrounding living cells.
Explain the movement of water across the root from root hairs to xylem.
Water is absorbed by root hairs, which increase surface area for water and mineral uptake. Water moves across the root cortex toward the xylem due to a water potential gradient. It follows two pathways:
1. Apoplast Pathway – Water moves through cell walls without entering cells.
2. Symplast Pathway – Water enters cells by osmosis and moves through plasmodesmata.
When water reaches the endodermis, the Casparian strip (thick, waterproof, waxy band of suberin in the cell walls of endodermis cells) blocks the apoplast pathway, forcing water into the symplast route through passage cells. This ensures selective mineral uptake. Water then continues to move down the water potential gradient towards the xylem through either the symplast or apoplast pathways. Water moves into the xylem through pits in the vessel walls, and moves upward toward the leaves.
Describe the structure of a phloem sieve tube element.
Sieve tubes are made of living cells. Sieve tube elements are elongated in shape. The cells are joined end to end vertically to form a continuous tube. A sieve tube element has a cell wall containing cellulose, a cell surface membrane and little amount of cytoplasm which only forms a thin layer lining the inside of the cell wall, containing endoplasmic reticulum and mitochondria. There is no nucleus, no tonoplast and no ribosomes. Where the end walls of two sieve tube elements meet, a sieve plate is formed. There are pores in between which are called sieve pores.
Describe the structure of a companion cell.
Companion cells have a cell wall containing cellulose, a cell surface membrane, cytoplasm, a small vacuole and a nucleus. However, the number of mitochondria and ribosomes is greater than normal, and the cells are metabolically very active. Numerous plasmodesmata pass through their cell walls, making direct contact between the cytoplasm of the companion cell and that of the sieve tube element.
Explain transport in sieve tubes.
• decreased water potential
• osmosis
• hydrostatic pressure
• mass flow through sieve tubes
• sucrose removal and removal of water
How is sucrose loaded into phloem.
• loading of sucrose by active transport
• pumping of hydrogen ions
• excess of hydrogen ions in the apoplast pathway outside the companion cell
• co-transportation of sucrose and hydrogen ions
• sucrose molecules diffuse into sieve tube
• move by symplast pathway
What is suberin, and where is it found?
Suberin is a waterproof substance found in the cell walls of the root endodermis, forming a barrier to water movement and aiding in the control of water and mineral ion uptake.
