TISSUE LEVEL OF ORGANIZATION: CHARACTERISTICS AND EXAMPLES - BIOLOGY - 2025

The tissue level of organization refers to one of the levels of hierarchical organization observed in living beings that has to do with the ordering of cells with different functions for the formation of tissues in multicellular organisms.

Just as the level of chemical organization is composed of atoms and molecules, and at the cellular level different molecules are configured to form cells, the tissue level consists of the ordered arrangement of multiple cells with similar properties and in close connection with each other.

Cholelenchymal tissue in plants (Source: Snowman frosty at English Wikipedia via Wikimedia Commons).

Like any level of organization, the tissue level has emergent properties that characterize it, that are inherent to it and that are not found in any of the individual parts that make it up.

Plants and animals are made up of tissues, these tissues function in the formation of organs and these, in turn, constitute functional systems in organisms, whose associations can be further identified in the different ecological systems that have been described (populations, communities, among others).

characteristics

All known tissues are made up of a complex combination of cells with common specific functions that retain their identity despite sharing a common environment with other cells.

Each tissue is made up of a large number of cells with a specific size, arrangement and shape. The type of cell it is made of gives tissue its function, which can be to transport materials and substances, regulate processes, provide rigidity, stability, and even movement and protection.

In all tissues one of the fundamental characteristics is the close association and communication that their cells have, which are generally in physical contact with each other, sending and receiving signals from each other and from cells belonging to other tissues.

Characteristics of animal tissues

The types of tissues most commonly found in animals are epithelial tissue, connective or connective tissue, muscle tissue, and nervous tissue.

Epithelial tissues cover the body and internal cavities, connective tissues are responsible for generating a continuum between other tissues and supporting them, muscle tissue is responsible for contraction, and nervous tissue participates in multiple tasks, including the conduction of electrical impulses. in response to external and internal signals or stimuli.

Dermal tissue in animals (Source: Normal_Epidermis_and_Dermis_with_Intradermal_Nevus_10x.JPG: KilbadCropped and labeled by Fama Clamosa (talk) and Mikael Häggström, respectively via Wikimedia Commons)

In the adult tissues of animals and plants, but especially in animals, cells die and are permanently renewed and during this process tissue integrity must be maintained, a fact that is possible thanks to three elements: cell communication, intercellular adhesion and memory.

- Cellular communication

Each cell present in a tissue controls its environment and is in a continuous search for extracellular signals sent by the cells that are around it, this ensures both the survival and the formation of new cells when they are necessary.

- Specific intercellular adhesion

Since animal cells do not have but a plasma membrane that surrounds them, outside they have specific proteins that mediate adhesion processes with their neighboring cells. This process appears to be highly specific between cells in a given tissue.

- Cell memory

When a type of cell belonging to a tissue divides it gives rise to a cell of the same class and this is genetically determined thanks to particular patterns of gene expression in each specialized cell.

There are animal tissues that have cells that are so specialized and differentiated that they are not capable of dividing to form a new identical cell, in these cases, special cells known as "stem cells" are in charge of continually replenishing them.

Characteristics of plant tissues

Also multicellular plants are organized into tissues and these are responsible for the formation of organs such as leaves, stems and roots, flowers, fruits, among others.

In plant tissues, the cell walls form a continuum known as apoplast, through which an important part of the rapid transport of molecules occurs around the cytoplasms, without temporarily coming into contact with the filtering plasma membranes.

One difference with animals is that two types of tissues are recognized in plants: simple tissues (made up of a single type of cell) and complex tissues (made up of two or more types of cells).

Vascular plants organize both types of tissues into what have been called tissue systems, which extend throughout the plant body and are the dermal tissue system, the vascular tissue system, and the fundamental tissue system.

- Dermal tissue system

This system, analogous to the dermal system of some animals, is responsible for the formation of the outer covering of the entire plant and therefore is one of the first contact systems between the environment and its body structure.

- Vascular tissue system

It is made up of two complex tissues: the xylem and the phloem. This system is essential for the transport of water and nutrients throughout the entire plant.

Cells in the xylem do not divide, since they are dead, and are responsible for transporting water. Phloem cells, on the contrary, are responsible for the transport of sugar and organic nutrients produced by photosynthesis.

- Fundamental tissue system

It represents all tissue that is neither dermal nor vascular. It is made up of parenchyma, collenchyma and sclerenchyma, three simple tissues each characterized by the composition of its cell walls. These fabrics specialize in structural support, where each one provides specific properties.

Examples

There are several and numerous examples that can be cited about the level of tissue organization, both in plants and animals.

In animals, blood is a connective tissue specialized in the distribution and exchange of substances with the environment. The nervous and endocrine tissues contribute to the coordination and regulation of the different organic functions.

In plants, the parenchymal tissue (included in the fundamental tissue system) mainly contains the cells that are responsible for photosynthetic processes and the assimilation of nutrients, which makes it essential for the growth and development of the other surrounding cells.

References

1. Alberts, B., Johnson, A., Lewis, J., Morgan, D., Raff, M., Roberts, K., & Walter, P. (2015). Molecular Biology of the Cell (6th ed.). New York: Garland Science.
2. Dudek, RW (1950). High-Yield Histology (2nd ed.). Philadelphia, Pennsylvania: Lippincott Williams & Wilkins.
3. Johnson, K. (1991). Histology and Cell Biology (2nd ed.). Baltimore, Maryland: The National medical series for independent study.
4. Nabors, M. (2004). Introduction to Botany (1st ed.). Pearson Education.
5. Solomon, E., Berg, L., & Martin, D. (1999). Biology (5th ed.). Philadelphia, Pennsylvania: Saunders College Publishing.