NEET'22 Crash Course

Introduction

The groups of cells that are found to have a similar structure and act together to perform a specific function are called tissues. The word tissue is derived from a verb meaning “to weave” which is a form of an old French language. In animals, there are four different types of tissues such as connective tissue, muscular tissue, nervous tissue, and epithelial tissue

 

What is Phloem Tissue?

The complex tissue that acts as a transport system found in the vascular plants for the transport of soluble organic compounds, the above mentioned is the phloem definition. 

 

The food conducting tissue in plants is made up of living tissues that use turgor pressure and energy in the form of ATP to transport sugars to the plant organs such as the fruits, flowers, buds, and roots. The other material that makes up the vascular plant transport system, the xylem, moves water and minerals from the root to various parts of the plant.

 

Components of Phloem Tissue

The phloem tissue is made up of several various components. Each of these components performs functions by working together, these functions include facilitating the conduction of sugars and the amino acids, from source tissues to the sink tissues where they are consumed or stored. The elements of phloem are as follows;

 

The Sieve Elements

The sieve elements are elongated and narrow cells that are connected together to form the phloem’s sieve tube structure. They are considered the highly specialized types of cells that are found in plants. These elements lack the nucleus at maturity and are also lacking in organelles such as ribosomes, cytosol, and Golgi apparatus, to maximize the available space for the translocation of materials.

 

There are two main types of sieve elements: both are derived from a common mother cell form.

  1. Sieve Member: It is found in angiosperms.

  2. Sieve Cells: These are associated with gymnosperms. 

 

Sieve Plates

Sieve plates are located in between the connections of sieve member cells, which are modified plasmodesmata. They are large and thin in structure, these are the areas of pores that help to facilitate the exchange of materials between the element cells.

 

When the phloem is cut or damaged then in the prevention of loss of sap the sieve plates also act as a barrier, often by an insect or herbivorous animal. After the injury, a unique protein called “Phloem-protein or P-protein”, which is formed within the sieve element is released from its anchor site and accumulates to form a ‘clot’. These clots are present on the pores of the sieve plate that helps in preventing the loss of sap at the damage site.

 

In gymnosperms, the sieve elements have more primitive features compared to the angiosperms. They have numerous pores at the tapered end of the cell walls instead of sieve plates for material to pass through directly.

 

The Companion Cells

Each of the sieve element cells is closely associated with a ‘companion cell’ in angiosperms and ‘Strasburger cell’ or an albuminous cell in gymnosperms. Companion cells consist of a nucleus, that is filled with dense cytoplasm. The cytoplasm is made up of numerous ribosomes and mitochondria. Due to this reason the companion cells are responsible for performing several metabolic reactions and other cellular functions. The sieve element is a lack in appropriate organelles due to this these elements cannot involve in the process of metabolic reactions as it lacks the appropriate organelles. For function and survival, the sieve elements are dependent upon or in need of the companion cells.

 

The sieve tube and companion cells are connected via plasmodesmata, a microscopic channel connecting the cytoplasm of the cells, which allows the transfer of the sucrose, proteins, and other molecules to the sieve elements. The transport of materials around the plant and to the sink tissues is done by companion cells, also helps in the loading of sieve tubes with the products of photosynthesis, and the loaded products get unloaded at the sink tissues. 

 

Phloem Parenchyma

The parenchyma is a collection of cells, which make up the ‘filler’ of plant tissues. They have thin and flexible walls that are made of cellulose. The parenchyma’s main function that is present in the phloem is the storage of starch, fats, and proteins and in the case of some plants, they help in the storage of tannins and resins also.

 

Phloem Sclerenchyma

The sclerenchyma is the main tissue of the phloem that provides support, stiffness, and strength to the plant. Sclerenchyma comes in two forms: fibers and sclereids; both are usually dead upon reaching maturity and are characterized by a thick secondary cell wall.

 

The bast fibers provide support to the tensile strength while allowing flexibility of the phloem. They are narrow, elongated cells where the walls consist of thick cellulose, hemicellulose, and lignin, and a narrow lumen.

 

Sclereids are slightly shorter, irregularly shaped cells that help to add compression strength to the phloem but restrict the flexibility. Sclereids act as protective measures for herbivory by generating a gritty texture when chewed.


Phloem, also known as bast, are plant tissues that transport nourishment from the leaves to the rest of the plant. Phloem tubes, companion cells, phloem fibers, and phloem parenchyma cells are all types of special cells found in the phloem.


The apical meristems (zones of new cell production) of root and shoot tips create primary phloem, which can be either protophloem or metaphloem, depending on whether the cells mature before or after elongation (growth) of the area in which it is found. Protophloem sieve tubes are unable to stretch with the elongating tissues, and as the plant ages, they are torn and destroyed. The phloem's various cell types may be transformed into fibers. In plants with a cambium, the later mature metaphloem is not destroyed and may operate for the rest of the plant's life, but it is replaced by secondary phloem.


Food material flows through a sieve tube, which is a row of sieve tube cells with a sieve-like section with holes in the sidewalls or ends walls.


Metastatic and marginal parenchymal cells, also called phloem parenchyma, are found at the thinnest branches and ends of the vein sieving tubes, where they also play a role in the food supply.


Phloem fibers are long, flexible cells that make up the soft fibers used in commerce (such as flax and hemp).


Phloem is the biological tissue of vascular plants that transports photosynthesis, a soluble organic compound produced during photosynthesis, to various regions of the plant. Translocation is the name of this type of transport. Since phloem is the innermost layer of bark, the name comes from the ancient Greek word o (phloiós), meaning "bark". Carl Nägeli first coined the term in 1858.

FAQs (Frequently Asked Questions)

1. Define phloem. What are the constituents of phloem?

Phloem is the complex tissue that acts as a transport system, commonly found in vascular plants for the transport of soluble organic compounds.

The elements of phloem tissue are:

  • Sieve elements

  • Sieve plates

  • Companion cells

  • Phloem parenchyma

  • Phloem sclerenchyma

2. What is phloem?

The vascular plant tissue phloem is in charge of transporting and distributing the sugars produced by photosynthesis. Phloem can be found in the exterior section of root cylinders, the stem vascular bundles, and the abaxial region of the venations of every single leaf because the plant is a continuity. While the most typical arrangement is for the phloem to be external to the xylem in roots and stems and abaxial in leaves, there are notable variations that are taxon-specific. Internal or intraxylary phloem is the phloem present on the interior.

3. What's the difference between xylem and phloem, and how do you tell them apart?

Xylem and Phloem are two types of vascular tissues that play a key role in the transportation process. These tissues form a circulatory bundle that functions as a whole. Xylem movement is unidirectional, but phloem movement is bidirectional.


Xylem:

  • The tubular-shaped structure of xylem tissues is devoid of cross walls. This tissue has the appearance of a star.

  • It is found in the vascular bundle's center.

  • Xylem fibers are smaller than ectoderm fibers.

  • They can be found in the roots, stems, and leaves of plants.

  • These tissues travel in a straight line. They live in hollow dead cells (only in one direction – upward way).

  • There is more xylem tissue in total.

  • Tracheids, vessel components, xylem parenchyma, xylem sclerenchyma, and xylem fibers make up the xylem.

  • It transports water molecules and soluble minerals from the roots to the above-ground parts of the plant.

  • With the help of phloem, forms vascular bundles.

  • It gives the plant mechanical strength and aids in the strengthening of the stem.

  • It is in charge of refilling all of the water molecules lost during transpiration and photosynthesis.

Phloem:

  • The phloem tissue is a tubular elongated structure with a thin sieving tube on the wall.

  • It is found on the vascular bundle's outer edge.

  • The fibers in the phloem are bigger.

  • They are found in stems and leaves, which then go to roots, fruits, and seeds, where they grow.

  • These tissues have a bidirectional movement. They dwell in the cytoplasm without a nucleus (both ways – up and down).

  • There is less phloem tissue overall.

  • Companion cells, sieve tubes, bast fibers, phloem fibers, intermediary cells, and the phloem parenchyma are the four components.

  • Food and other nutrients, such as sugar and amino acids, are transported from the leaves to the plant's storage organs and growth portions.

  • With the help of the xylem, forms vascular bundles.

  • Photosynthetic parts of plants generate sugars, which are then transported to storage organs such as roots, bulbs, and tubers.

  • It's in charge of transporting proteins and mRNAs all across the plant.

4. What are the components of phloem?

Sieve tubes, companion cells, phloem fibers, and the phloem parenchyma are the four types of constituents that make up phloem.


(i) Sieve tubes: These are perforated-walled tubular cells. Food and nutrient transfer is the responsibility of these cells.


(ii) Companion cells: These cells are found near sieve tubes and are connected to them by pits. The walls of these cells are long, narrow, and thin. These are living cells.


(iii) Phloem fibers: These fibers have thick cell walls, and support the plant mechanically.

(iv) Phloem parenchyma: These are live cells that are frequently cylindrical.


Translocation refers to the movement of food from leaves to other regions of the plant.

5. What are the functions of phloem?

The phloem transports photoassimilates, primarily in the form of sucrose sugars and proteins, from the leaves, where they are created by photosynthesis, to the remainder of the plant.


Active transport transports sugars from the source, usually the leaves, to the phloem. The pressure-flow concept explains the following stage, the photoassimilates' translocation.


An osmotic gradient is formed when there is a high concentration of an organic substance (in this example sugar) within the cells. Water is passively pulled across the gradient from the nearby xylem, resulting in a sugar solution and a high turgor pressure within the phloem. Water and carbohydrates migrate through the tubes of the phloem and into the sink tissues' due to the high turgor pressure (e.g. Roots, stems and leaf growing tips, flowers and fruits). Sugars are needed for growth and other functions once the sugar solution reaches the sink. The amount of water input from the xylem decreases as the concentration of sugars in the solution decreases, resulting in low pressure in the phloem at the sink. Photoassimilates and water are regularly circulated across the plant in both directions where there are zones of high and low pressure.

6. What are xylem tissue and phloem tissue?

Xylem is one of the most critical issues in a plant's vascular system. This tissue's major function is to transmit minerals and water from the roots to other sections of the plant. It consists of two separate chambers, tracheids, and vessels for the transportation of minerals and water. The word xylem comes from the Greek word o (xylon), which means wood.


Phloem is also vital for the vascular system of plants as xylem tissues. This tissue's primary function is to carry nutrients and food from leaves to other plant growth components. Phloem is derived from the Greek word "photos," which means "bark."

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