Parenchyma

Parenchyma tissue is characterized by its simple structure, thin cell walls, large central vacuoles, and functional versatility. Parenchyma cells may either be closely packed or have small intercellular spaces. The cells of parenchyma are generally isodiametric. They may be spherical, oval , elongated or polygonal in shape. It is found in cortex, pericycle, medullary rays, pith, leaf mesophyll etc.It forms the major component within organs. It plays essential roles in photosynthesis, storage, gas exchange, and tissue repair, making it a crucial component of plant health and development.

1.0What is Tissue?

Tissue refers to a group of cells having a common origin and usually performing a common function within an organism. These cells usually share similarities in structure and function and are organized into layers or specialized structures. A plant is made up of different types of tissues.The tissues are classified into two main groups, namely, meristematic and permanent tissue based on whether the cells are dividing or not.

2.0Types of Tissues

Plant tissues can be broadly classified based on the ability of the cells to divide into meristematic tissues and permanent tissues. Meristematic tissues -consist of a group of cells that have the ability to divide. Cells of the meristematic tissue are found in meristems that have the ability to divide continuously and are generally undifferentiated. In contrast, permanent tissue consists of plant cells that are no longer actively dividing.

3.0Simple Permanent Tissue

Simple permanent tissue is a type of plant tissue that is composed of similar cells to perform a common function. It is categorized into three main types: parenchyma, collenchyma, and sclerenchyma.

1. Parenchyma: These tissues are found in the soft parts of a plant such as the roots, stems, leaves and flowers. The cells of this tissue are loosely packed and contain large intercellular spaces between them. Each cell has a vacuole at the centre. The functions of parenchyma tissues are storage, photosynthesis and helping the plant float on water. 
2. Collenchyma: Are similar to parenchyma cells with thicker cell walls. They are meant to provide mechanical support to the plant structure in parts such as the petiole of the leaf.  
3. Sclerenchyma: The cells of this tissue are dead. They are rigid, contain thick and lignified secondary walls. Their main function is to provide strength and support to parts of the plant.

4.0Parenchyma Tissue

• It is a type of simple permanent tissue that makes a major part of ground tissues in plant parts, where other tissues like vascular tissues are embedded. It is composed of simple, living and undifferentiated cells, that are modified to perform various functions.
• Cells of parenchyma tissue are usually isodiametric, thin-walled and equally expanded cells. These cells are oval, rounded or polygonal in shape with well developed intercellular spaces. The cells have active protoplast.
• Pith, mesophyll of leaves, endosperm of seeds, cortex of stems and roots and other organs of plants consist mainly of parenchyma. The parenchyma cells also occur in xylem and phloem that are involved in the radial conduction of water and storage of food materials respectively. The parenchyma cells that perform the function of mechanical support are known as prosenchyma.

5.0Parenchyma Cell

Parenchyma cells are the most common type of plant cell. Their adaptability makes them essential for the plant's growth and survival. These cells are thin-walled, living, and unspecialized in structure. They are found in many parts of a plant and perform a variety of functions, including storage, transport, photosynthesis, growth, repair and support.

6.0Characteristics of Parenchyma Cells

The cells of parenchyma are isodiametric or they may be polygonal, oval, round or elongated.

• These are living cells that have a prominent nucleus and protoplast.
• These cells are closely packed or may have small intercellular spaces especially in aerenchyma cells, facilitating gas exchange (e.g., oxygen and carbon dioxide).
• Cell wall is thin and is made up of cellulose and hemicellulose.
• These cells contain a large central vacuole that maintains turgor pressure and stores nutrients.
• These cells may have Chloroplasts to perform photosynthesis.
• The parenchyma cells that perform secretory function, have dense protoplasm, rich in ribosomes, Golgi bodies and a highly developed endoplasmic reticulum.
• Present throughout the plant in structures like the cortex and  pith of roots and stems, and the mesophyll of leaves.

7.0Structure of Parenchyma Cells

• Parenchyma in plant cells is either polyhedral or isodiametric in shape. They can also be oval, polygonal, elongated, or round. Such cells are packed tightly or may have limited intercellular space. They are built up of a thin cell wall that is mainly composed of cellulose, hemicellulose. Thin primary cell walls with no secondary walls, making them flexible and less rigid compared to other plant cells like collenchyma or sclerenchyma
• Size  of parenchyma varies with function. Storage parenchyma cells are often larger due to extensive vacuoles, while photosynthetic parenchyma cells are generally smaller and more compact.

8.0Types of Parenchyma Cell

Parenchyma cells play various roles in the plant body like  growth, metabolism, and storage. These are the most common type of plant cell and are characterized by their thin cell walls and large central vacuoles. There are several types of parenchyma cells, each with specific functions:

1. Chlorenchyma Cells: Chlorenchyma Cells are specialized for photosynthesis. These cells Contain chloroplasts, typically found in the mesophyll of leaves. Example: Cells in the palisade layer of leaves.

2. Aerenchyma Cells: Aerenchyma cells facilitate gas exchange and buoyancy in aquatic plants. These cells have large air spaces (intercellular spaces) that allow for the movement of gasses like oxygen and carbon dioxide. Example: Cells in the stems of water lilies and other aquatic plants.

3. Storage Parenchyma: Storage parenchyma cells store various substances such as starch, oils, and water. Often found in roots, tubers, and seeds. These cells have large vacuoles to accommodate storage. Example: Cells in potato tubers (for starch storage) and in fruits like apples (for sugar storage).

4. Stellate parenchyma: Stellate parenchyma cells are present in the petioles and midribs of Canna and banana leaves. It is a form of aeration tissue in plants, which helps with internal air circulation in plants.

5. Secretory Parenchyma: Secretory Parenchyma cells are involved in the secretion of substances such as resins, essential oils, or nectar. Can have specialized structures like secretory cavities or canals. Example: Cells in citrus fruits that produce essential oils.
6. Prosenchyma: Prosenchyma cells are typically long and narrow, with tapered or pointed ends. They have thin primary cell walls, similar to parenchyma cells, but are usually more elongated.Generally, prosenchyma cells have fewer intercellular spaces compared to other types of parenchyma cells. Provides mechanical support to plant tissues, contributing to the overall structural integrity of the plant.
7. Xylem parenchyma: Xylem parenchyma are only living cells in xylem. xylem parenchyma stores food, starch, latex, and fats. It also helps with the radial conduction of water, and with healing and regeneration during water stress. Xylem parenchyma has thin cellulose walls. 
8. Phloem parenchyma: Phloem parenchyma are made up  of elongated, tapering cylindrical cells which have dense cytoplasm and nucleus and store food material and other substances like resins, latex, and mucilage.

9.0Functions of Parenchyma Cells

Parenchyma cells are a type of plant cell that play several crucial roles in plants. Main functions of parenchyma cells are-

1. Photosynthesis: In green plant tissues, particularly in leaves, parenchyma cells contain chloroplasts that facilitate photosynthesis, the process of converting light energy into chemical energy stored in glucose.
2. Storage: Parenchyma cells store various substances, including starch, oils, and proteins in roots and tubers.
3. Wound Healing: Parenchyma cells have the ability to divide and differentiate into other types of cells, which helps in healing wounds and repairing damaged tissues.
4. Support: Although not as rigid as sclerenchyma or collenchyma cells, parenchyma cells provide some support to plant tissues. They often form the bulk of soft plant organs like fruits and vegetables.
5. Transport: Parenchyma cells play a role in the movement of nutrients and water within the plant. In some cases, they help in the lateral movement of substances between vascular tissues and other parts of the plant.
6. Gas Exchange: In some plants, parenchyma cells in the leaf's spongy mesophyll help with gas exchange by providing air spaces that facilitate the movement of oxygen and carbon dioxide.