Structure and Function of Xylem and Phloem
The xylem and phloem are two of the four types of plant tissue, both central to the process of photosynthesis. The other two types are called “parenchyma” or “collenchyma.”
Xylem is a type of vascular plant tissue that transports water and minerals from the roots to parts of the plant, like leaves and flowers. Phloem is a type of vascular plant tissue that carries sugar (carbohydrates) synthesized during photosynthesis. These are both found in the “phloem” part of a plant.
Structure and Function of Xylem
The xylem is used for support in many plants and transporting water and nutrients to all areas of the plant. Xylem is made of mainly cellulose, and it is composed of three different types of plant cells.
- The first type is called tracheophytes, which are found in the xylem and are long and tubular.
- The second type is called fern gametophytes, which are found in the xylem and are shorter than the tracheophytes.
- The third type is called lycophytes, which are found in the xylem and are the smallest of the three.
Xylem carries water and minerals to all parts of the plant, and it does this by using water pressure. During transpiration, water moves from the roots through the xylem upwards. This causes air pressure to build up above the xylem in a process called root pressure.
When the water reaches the top of the plant, pressure in the xylem builds up and causes water to flow out of small openings on the xylem ends.
Xylem provides structural support to plants. The tracheophytes are responsible for most of this support because they are long and have a hollow tube shape that makes them strong. Because they also contain lignin (a polymeric phenolic substance), they can withstand tension and compression.
Xylem is important in the process of photosynthesis. During photosynthesis, plants use sunlight to convert carbon dioxide into glucose. Xylem plays a vital role in the transportation of this product from the leaves to other parts of the plant.
The xylem and phloem are collectively known as “conducting tissue.” Conducting tissue helps to transport all nutrients throughout the plant for various uses.
The xylem also aids in a plant’s adaptation to its environment. When a plant is experiencing acute water stress (as in a drought), the wood cells shrink, pulling turgor pressure inside the xylem cell’s walls and making it easier for them to close their openings.
This process closes all but one opening per pipe cell, thereby reducing water flow from the roots to all shoot and leaf areas of the plant.
Structure of Xylem
The xylem is constructed of tracheids, vessel elements, and parenchyma cells. Tracheids are rigid cells that have a long central lumen or cavity lined with thin walls. Vessel elements are similar to tracheids, but they are larger, have flanges on the cell wall’s inner side, and lack a lumen.
Parenchyma cells are thin-walled living tissue that functions in storage and support.
Xylem cells are contained within a thin layer of living tissues called the endodermis. The endodermis is a tissue that forms the barrier around water, nutrients, and hormones that flow through the xylem.
Xylem cells use turgor pressure to close off their lumen, thus closing off flow to the outside of the plant and preventing gravity from moving water out of the cell.
Xylem can also close in response to internal signals such as low temperatures. Acute temperature shocks such as a sudden temperature drop can cause the xylem to close to reduce water loss.
All of the components of xylem tissue are important for the plant’s structural support, and they provide mechanical strength and stiffness in the woody tissues.
However, tracheids contain significant amounts of lignin. This lignin adds additional strength and stiffness to the tracheid’s wall because it resists tension and compression. The presence of lignin also makes the cell resistant to enzymatic breakdown when it is in the tree.
The color of the cortex and pith, which show through the xylem, depends on the species’ age, growing conditions, and exposure to sunlight. Xylem exposed to bright sunlight usually has a lighter colored cortex than under dark conditions.
Phloem Structure and Function
Phloem is a complex tissue in vascular plants that serves as a transport mechanism for soluble organic compounds.
The phloem is made of living tissue that uses turgor pressure and energy in the form of ATP to effectively transport sugars to plant organs such as fruits, flowers, buds, and roots; the xylem, the other material that makes up the vascular plant transport system, is made of non-living material that moves water and minerals from the root.
The phloem transports photoassimilates, primarily in the form of sucrose sugars and proteins, from the leaves where they are formed by photosynthesis to the rest of the plant through the translocation method.
Active transport transports sugars from the source, normally the leaves, to the phloem. The pressure-flow hypothesis explains the next step, photoassimilate translocation.
An osmotic gradient is formed when there is a high concentration of organic material (in this case, sugar) within the cells. Water is passively drawn over the adjacent xylem gradient, resulting in a sugar solution and a high turgor pressure within the phloem.
The high turgor pressure allows water and sugars to pass through the phloem tubes and into the sink tissues (e.g., the roots, growing tips of stems and leaves, flowers, and fruits).
When the sugar solution reaches the drain, the sugars are used for growth and other processes. If the concentration of sugars in the solution decreases, so does the volume of water influx from the xylem, resulting in low pressure in the phloem at the sink.
Photoassimilates and water are continuously transferred around the plant in both directions where there are areas of high and low pressure.
The phloem structure is made up of many components. Each of the components cooperates to promote sugars and amino acids’ transport from a source to sink tissues where they are ingested or processed.
Phloem is a type of tissue in plants, and it’s highly specialized. It transports food substances, typically sucrose, from the leaves to other places where the plant needs them. The cells are long and tube-like and have companion cells.
Phloem cells are found primarily in the inner bark of a tree, and they are aligned so that the cell bodies lie end to end. They contain a watery fluid known as phloem sap. The phloem sap transports organic molecules (carbohydrates) produced in the leaves throughout the plant’s entire system.
Structure and Function of Xylem and Phloem
The xylem and phloem are both part of the vascular system that moves water, minerals, nutrients and more throughout plants. They basically work together to transport resources from where they’re made (leaves) to where they’re needed by the plant (roots).
Xylem is a hard thick stem-like tissue found inside stems or roots in many land plants with these structures. It’s actually composed of several tissues
- Tracheids which carry water up through the plant
- Vessel elements which form tubes for transporting sap downward
- Parenchyma cells that provide structural support; sometimes even lignin as well.
These tissues collectively make up what looks like woody material but it does not have any special properties related to decomposition.
Phloem is a soft tissue found inside stems or the main stem of roots in many land plants with these structures. It’s made up of sieve-tube members that act as kind of water pipes, and companion cells which provide structural support for those tubes to stretch out from one cell wall to another.
Together they form what looks like jelly but it doesn’t have any special properties related to decomposition either!
Xylem transport functions are most often studied because this part can be easily isolated by cutting open vascular bundles (cylinders composed primarily of xylem) and treating them independently.
Phloem transport functions are less frequently studied because there isn’t an easy way to isolate the pure function without compromising its location.
It’s made up of sieve-tube members that act as kind of water pipes, and companion cells which provide structural support for those tubes to stretch out from one cell wall to another. Together they form what looks like jelly but it doesn’t have any special properties related to decomposition either!
Xylem tissue is involved in water and nutrient transport, while phloem transports sugars manufactured during photosynthesis to other plant tissues. They have a circulatory system that can be thought of as paired channels
- Xylem tubes for transporting solute from the roots to the leaves
- Phloem tubes for delivering sugar-rich sap produced by mesophyll cells in leaf tissue back down to the rest of the plant.