How Xylem and Phloem Transport Sap in Plants
Consider that you want to go on a vacation to someplace far away from you. How would you travel to that place? You would obviously use a mode of transport. A mode of transport is very necessary, the same stands true even with respect to our body. In the human body, blood is responsible for carrying out all the transport. The transport of energy, the transport of oxygen, hormones, nutrients, carbon dioxide, and humanly waste. Similarly, even the plants we see around us need a system of transport. The Sap Plant Physiology is the transport system in plants. It is responsible for carrying around all the nutrients and water in the plant. Today, let us take a deeper look and understand more about the transport system of plants: Sap Plant Physiology.
The Sap System
As iterated earlier, the sap system or often simply referred to as sap is a fluid transport system in the plant. It consists of xylem tubes of phloem cells. The xylem and the phloem together form something called the vascular bundle. This vascular bundle plays a major role in contributing to the formation of the transport system in plants. The vascular bundle runs right from the top of the plant or tree to the bottom. The xylem tubes are responsible for the transportation of water and inorganic nutrients in a plant. On the other hand, the phloem cell's duty is to transport sugary fluids and other biological molecules. Remember that the latex and resins aren't a part of the sap system of the plant. They are produced separately in the plant body and have distinguished functions.
What is the Phloem Sap Made Out of?
The plant sap consists of two components, the phloem sap, and the xylem sap. The phloem sap primarily consists of water, the plant manufactured sugar, several other biological molecules, and essential minerals. The sap system mainly transports energy through the plant. It traverses through every part of the plant and provides every part with the amount of energy required. On the other hand, xylem sap mainly consists of water, plant hormones, minerals, and nutrients which exist in a diluted form in water.
The Phloem
The phloem is a major constituent of the plant sap system. The phloem of a plant is responsible for the carriage of energy to each part of the plant's body. Therefore, the phloem must run from the energy house of the plants to the other parts. We know that the leaves are the energy house of the plants. Therefore, the phloem of the plant needs to run from the leaves to all the other parts of the plants. This is because like our human body even each cell in the plant body requires energy to function normally. Each part of the plant cannot produce its own energy and hence the energy is transported to each part from the leaves. Hence, we can conclude that the phloem runs from the leaves to the other parts of the body!
The Xylem
The Xylem is a watery tissue that is responsible for the transportation of minerals and essential nutrients. The plant has roots in the soil. The soil is filled with nutrients. These nutrients are generally minerals such as potassium, nitrogen, phosphorus, and several other essential nutrients and vitamins. These minerals are essential for the production of energy in the body of the plant. As we've seen earlier, roots are responsible for absorbing minerals from the soil. Therefore, the xylem runs from the roots of the leaves of the plants. The xylem cells are actually dead cells; these dead cells are called vessels. They make a continuous, capillary passage for the xylem constituents. The xylem transports these nutrients to the leaves to help the leaves to produce the required amount of energy in the plant!
Differences Between Xylem and Phloem
FAQs on Understanding Sap in Plant Physiology
1. What is sap in the context of plant physiology?
In plant physiology, sap is the vital fluid that circulates through a plant's vascular tissues, namely the xylem and phloem. It acts as the primary transport system, distributing water, minerals, and nutrients throughout the plant. There are two types of sap: xylem sap, which is a dilute solution of water and minerals, and phloem sap, which is a concentrated solution of sugars and other organic molecules.
2. What are the main components that differentiate xylem sap from phloem sap?
Xylem and phloem saps are distinguished by their composition and function, directly related to the tissues they flow through:
- Xylem Sap: Primarily consists of water, inorganic mineral ions absorbed from the soil (like nitrates, phosphates, potassium), and some plant hormones. Its main function is to transport these materials from the roots to the rest of the plant.
- Phloem Sap: Primarily consists of water and a high concentration of sucrose (sugar). It also contains amino acids, organic acids, and other hormones. Its main function is to transport energy in the form of sugars from the leaves (source) to other parts of the plant like roots, fruits, and flowers (sinks).
3. How does the ascent of sap occur in tall trees, overcoming gravity?
The ascent of sap in tall trees is primarily explained by the cohesion-tension theory. This mechanism does not rely on a pump but on the physical properties of water. The main driving force is the transpiration pull created when water evaporates from the leaves. This pull creates tension in the xylem. Due to the strong cohesive forces between water molecules and adhesive forces between water and xylem walls, the entire column of water is pulled upwards from the roots to the leaves in a continuous stream.
4. What is the mechanism of phloem transport, as explained by the Pressure-Flow Hypothesis?
The Pressure-Flow Hypothesis, or Mass-Flow Hypothesis, explains how sap moves through the phloem. It occurs in the following steps:
- Loading: Sugar produced during photosynthesis is actively loaded from the source (e.g., leaves) into the phloem's sieve tubes, making the sap highly concentrated.
- Osmosis: Water moves from the adjacent xylem into the sieve tubes via osmosis, creating high hydrostatic (turgor) pressure at the source.
- Mass Flow: This high pressure pushes the sap along the sieve tubes towards areas of lower pressure.
- Unloading: At the sink (e.g., roots or fruits), sugar is actively unloaded for use or storage. As sugar leaves the phloem, water also moves out, reducing the pressure at the sink and maintaining the pressure gradient for continuous flow.
5. What are the key differences between xylem and phloem transport?
Xylem and phloem transport differ significantly in their direction, composition, and mechanism:
- Direction of Flow: Transport in xylem is typically unidirectional, moving water and minerals upwards from the roots. In contrast, transport in phloem is bidirectional, moving sugars from a source to a sink, with the direction changing based on the plant's needs.
- Transported Substances: Xylem transports water and inorganic minerals. Phloem transports organic food, mainly sucrose.
- Tissues Involved: Xylem vessels and tracheids are dead cells, forming hollow tubes. Phloem sieve tubes are living cells that require support from companion cells.
- Driving Force: Xylem transport is a passive process driven by the physical force of transpiration pull. Phloem transport is an active process requiring metabolic energy to create a pressure gradient.
6. Why is transport in phloem considered bidirectional, while transport in xylem is always unidirectional?
The direction of transport is determined by the location of the source and sink. For xylem, the source is always the roots (where water is absorbed), and the rest of the plant parts are sinks. Therefore, the flow is always upwards and is unidirectional. For phloem, the source (where sugar is produced or stored) and sink (where sugar is needed) can change. For instance, mature leaves are the source and roots are the sink during summer. However, in early spring, stored sugar in the roots becomes the source for growing buds, reversing the flow. This variable relationship makes phloem transport bidirectional.
7. What would happen to a plant if its phloem tissue was completely blocked near the base of the stem?
If the phloem tissue were blocked, the transport of sugars from the leaves to the roots and other non-photosynthetic parts would cease. The roots would be starved of energy and unable to perform their functions, including the absorption of water and minerals. Although the leaves could continue to photosynthesise for a while, the eventual death of the root system would lead to the death of the entire plant, demonstrating the critical importance of phloem in distributing energy.
8. What is the importance of root pressure in the ascent of sap?
Root pressure is a positive pressure that develops in the root xylem when mineral ions are actively transported into the vascular tissue, causing water to follow by osmosis. This pressure can push water upwards in the xylem to a small height. While it is not strong enough to account for the ascent of sap in tall trees, it is significant in smaller plants and can contribute to water movement, especially at night when transpiration is low. It is also responsible for the phenomenon of guttation, where water droplets are exuded from the tips of leaves.
