Phloem – Definitions, Structure, Types, Functions

In the intricate network of plant biology, the phloem stands out as a critical component of the plant’s long-distance transport system. Together with the xylem, the phloem forms a duo of tissues that are pivotal in sustaining plant life, facilitating the movement of vital nutrients and water throughout the plant’s structure. This article delves into the fascinating world of phloem, exploring its origin, structure, function, and the increasing recognition of its role in plant signaling.

What is Phloem?

Location

• In Stems and Leaves: Phloem is part of the vascular bundles found throughout these organs, usually located closer to the surface than xylem.
• In Roots: Phloem is found within the vascular cylinder and is crucial for transporting nutrients to and from the root system.

Phloem Structure in Plants

Sieve Tube Elements

• Structure: Sieve tube elements are elongated, tube-like cells that align end-to-end, forming continuous channels known as sieve tubes. The end walls between these elements have pores and are referred to as sieve plates. The pores allow for the free flow of phloem sap, which contains water, sugars, hormones, and other nutrients.
• Features: Unlike most cells, mature sieve tube elements lack a nucleus, ribosomes, and other typical cell organelles. This reduction in cellular components maximizes the internal space available for the transport of phloem sap.

Companion Cells

• Association: Companion cells are closely associated with sieve tube elements. Each sieve tube element is typically connected to one or more companion cells through plasmodesmata, which are small channels in the cell walls that allow for the exchange of materials between cells.
• Role: Companion cells are crucial for the functioning of sieve tube elements. They help in loading and unloading sugars and other substances into and out of the sieve tube elements. Companion cells retain their organelles and carry out metabolic activities necessary to support the sieve tubes, compensating for the lack of organelles in the sieve tube elements.

Phloem Parenchyma

• Function: Phloem parenchyma cells are involved in the storage and lateral transport of nutrients within the phloem. They can store starch and other organic substances.
• Structure: These cells have a more traditional plant cell structure with a nucleus, cytoplasm, and various organelles. They are less specialized than sieve tube elements and companion cells and are found interspersed among them.

Phloem Fibers

• Support: Phloem fibers are elongated, thick-walled cells that provide mechanical support to the phloem tissue. They help in protecting the delicate sieve tubes and companion cells from physical damage.
• Composition: The cell walls of phloem fibers are often lignified, which makes them rigid and strong. Lignification helps in maintaining the structural integrity of the phloem, especially in woody plants where the phloem is subjected to more mechanical stress.

Additional Structural Features

• Sieve Plates: The presence of sieve plates is a defining feature of sieve tube elements. These plates are essentially the porous end walls that facilitate the flow of sap between consecutive sieve tube elements. The size and distribution of the pores can vary, affecting the flow rate of phloem sap.
• Plasmodesmata: These microscopic channels connect the cytoplasm of companion cells with that of sieve tube elements, allowing for the direct exchange of molecular signals and nutrients. This connection is vital for the coordinated functioning of the phloem tissue.
• Callose: A polysaccharide that can deposit around the sieve pores, particularly in response to injury. Callose deposition can help to seal off damaged parts of the sieve tube, preventing loss of sap and entry of pathogens.

Phloem Cell Types

Sieve Elements

• Sieve elements are the key conducting cells within the phloem. They are elongated cells that align end to end to form sieve tubes. These cells are unique because they lack a nucleus at maturity, which allows for a more efficient flow of nutrients. The end walls of these cells are perforated, forming structures known as sieve plates, which facilitate the movement of sap between cells. There are two main types of sieve elements:
• Sieve-Tube Members: Predominant in angiosperms, these cells are connected by sieve plates and are associated with companion cells, which help in the maintenance and function of the sieve-tube members.
• Sieve Cells: Common in gymnosperms and some non-flowering plants, these are less specialized than sieve-tube members and lack companion cells.

Companion Cells

Companion cells are only found in angiosperms and are closely associated with sieve-tube members. They originate from the same parent cell and have a dense cytoplasm and a prominent nucleus. Companion cells play a crucial role in loading and unloading sugars into the sieve-tube elements, essentially regulating the material that gets transported through the phloem.

Phloem Fibres

Phloem fibres, also known as bast fibres, are specialized sclerenchyma cells that provide structural support to the phloem. They are characterized by their thick cell walls, which are lignified, making them strong and rigid. Phloem fibres are not involved in nutrient transport but are crucial for the plant’s mechanical strength.

Phloem Parenchyma

Phloem parenchyma cells are the most versatile cells within the phloem. These cells are involved in the storage and lateral transport of nutrients. They are living cells that can store starch and other organic substances. Phloem parenchyma also aids in the repair and regeneration of sieve elements and plays a role in defense against pathogens.

Phloem Function in Plants

• Transport of Sugars and Nutrients: The primary function of the phloem is to transport sugars, primarily sucrose, synthesized in the leaves during photosynthesis to various parts of the plant. This transport process, known as translocation, ensures that all parts of the plant, including roots, stems, and developing organs like fruits and flowers, receive the necessary nutrients for growth and development.
• Bidirectional Flow: Unlike the xylem, which only transports water and minerals upwards from the roots, the phloem is capable of bidirectional movement. This means it can transport nutrients both from the leaves to the roots and from the roots to the leaves, depending on the plant’s needs.
• Storage and Redistribution: The phloem is not just a transportation highway; it also plays a role in storing and redistributing nutrients. During times of high photosynthetic activity, excess sugars can be stored in the form of starch in various parts of the plant. These stored nutrients can later be mobilized and transported via the phloem to areas of the plant requiring more energy, such as budding flowers or growing fruits.
• Signaling: The phloem is also involved in transporting signaling molecules that help coordinate various physiological processes within the plant. These signals can include hormones, proteins, and RNA molecules that regulate growth, development, and responses to environmental stimuli.

FAQ’S

What is the Main Function of Phloem?

Phloem transports nutrients, especially sugars from leaves to all plant parts.

What is the Xylem and Phloem?

Xylem carries water and minerals upward; phloem distributes sugars throughout the plant.

What is the Definition of a Phloem?

Phloem is plant tissue that translocates sugars and other organic compounds.

Is Phloem a Food or Water?

Phloem is responsible for transporting food, mainly sugars, not water.

Is Phloem Alive or Dead?

Phloem is composed of living cells, unlike some parts of xylem.