Sap - Plant Physiology

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Sap: The Transport System of 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

Xylem

Phloem 

1. Facilitates the only unidirectional flow.

2. The xylem sap is made out of dead cells. 

3. Xylem fibres are tiny.

4. Transport minerals and water.

5. Strengthens the stem and provides the stem mechanical support.

6. A plant has more xylem tissues than Phloem tissues.

1.  Facilitates the bi-directional flow

2. The phloem sap consists of live cells.

3. Phloem fibres are larger

4. Transport energy from the leaves to other parts.

5. Has no role in strengthening the stem.

6. The number of phloem tissues is less. 

FAQ (Frequently Asked Questions)

1. What is the Mechanism of Phloem Transport? 

A: The phloem consists of several sieve tubes. These sieve tubes form long columns with holes in the end walls. Through these holes, the cytoplasmic strands pass continuously forming a continuous chain. We can compare it to a pipeline which we see around us. The cytoplasmic strands are like the pipelines that facilitate the transport of energy from the leaves to the other parts. The phloem transport follows a method called the pressure-flow technique or mass flow hypothesis. 

Following are the steps for phloem transportation. 

  • The glucose from the leaves is converted to soluble sugars 

  • The sugar is then moved to the companion cell

  • Water from the xylem vessels moves to the phloem by endosmosis

  • The osmotic pressure rises and hence the phloem sap moves from higher pressure to lower pressure

  • The osmotic pressure at the sink (near the roots is low) hence the phloem sap moves from the top to the bottom. 

2. What is the Mechanism of Xylem Sap Transport? 

A: Xylem sap is responsible for the upward pull of nutrients and minerals from the roots. The root pressure in the xylem sap is the pressure caused by the continuous influx of water and nutrients from the soil. The xylem vessels are very narrow, this causes the easier upward pull of the xylem sap constituents. They also have a capillary action, this generates a suction pull and creates a vacuum. Due to this vacuum, there is minimal loss of water due to evaporation and transpiration. The xylem transport is more difficult since it is against gravity, going against gravity is tougher since the force of gravity needs to be overcome. Hence, the capillary flow in the xylem is more favourable!