3 - Tissues: Plant and Animal Tissues

A tissue is a collection of similar cells that work together to perform a particular function. In multicellular organisms, such as plants and animals, tissues are fundamental building blocks, which means they are a primary way in which these organisms are organized. In plants, the main functions of tissues include providing structural support and facilitating the transportation of nutrients and water. In contrast, animal tissues are generally more complex and diverse, designed to perform a wider variety of specialized functions such as movement, protection, and regulation.

Plant tissues can be categorized into two main types: meristematic tissues and permanent tissues. Meristematic tissues consist of actively dividing cells that enable plants to grow, particularly in new areas such as the tips of roots and shoots. In contrast, permanent tissues develop from meristematic tissues once they differentiate, meaning they lose the ability to divide. These permanent tissues can further be classified as simple or complex, highlighting their varied functions in plants.

Meristematic tissues are specialized tissues composed of cells that are constantly dividing, which contributes to the growth of the plant. These cells are smaller than mature plant cells and contain dense cytoplasm and large nuclei. There are three types of meristematic tissues: Apical meristem is located at the tips of roots and shoots and is responsible for the primary growth, which increases the length of the plant. Lateral meristem is found along the sides of stems and roots to increase girth, known as secondary growth. Lastly, intercalary meristem is found at the base of leaves or internodes, allowing growth in length, particularly in mature tissues.

Permanent tissues arise from meristematic tissues after they differentiate and are no longer capable of division. These tissues are categorized into simple and complex permanent tissues. Simple permanent tissues consist of one type of cell, including parenchyma, collenchyma, and sclerenchyma. Parenchyma is primarily involved in storage and photosynthesis, collenchyma provides support and flexibility, and sclerenchyma gives strength to the plant. On the other hand, complex permanent tissues consist of multiple cell types and are mainly involved in the transport of materials. The xylem is responsible for transporting water and minerals from the roots to other parts of the plant, while phloem transports food produced in the leaves.

Animal tissues are classified into four main types: epithelial, connective, muscular, and nervous tissues. Epithelial tissue covers and protects surfaces of the body and lines cavities, while connective tissue supports and binds different tissues together. Muscular tissue is responsible for movement through contraction, and nervous tissue facilitates communication within the body by transmitting signals.

Epithelial tissue serves as a protective layer on body surfaces and lines internal cavities. It comes in various shapes and arrangements: simple squamous is made of flat cells ideal for diffusion, cuboidal consists of cube-shaped cells often found in organs like kidneys, columnar features tall cells that line the intestines, ciliated tissue has hair-like structures to move particles, and stratified tissue, which has multiple layers, offers additional protection like that seen in skin.

Connective tissue's main role is to connect and support other types of tissues. Areolar tissue holds organs in place and fills spaces, adipose tissue stores fat, tendons connect muscles to bones, ligaments connect bones to other bones, cartilage provides flexible support like in your ears and nose, bone offers a hard structure, and blood acts as a fluid connective tissue that transports nutrients and waste throughout the body.

Muscular tissue is specialized for movement, allowing the body to perform various actions. It comes in three types: striated (skeletal) muscle, which is under voluntary control and helps with movements like lifting weights; unstriated (smooth) muscle, which is involuntary and found in organs like the intestines; and cardiac muscle, which is also involuntary and keeps the heart beating. On the other hand, nervous tissue includes neurons that are responsible for transmitting signals throughout the body, with parts like the cell body, dendrites (which receive signals), and axon (which transmits impulses).

There are key differences between plant and animal tissues. In plants, growth occurs in localized areas, primarily at meristems, while in animals, growth is more uniformly distributed throughout the body. The types of tissues also differ, with plants having simple and complex tissues, while animals feature four main types (epithelial, connective, muscular, nervous). Additionally, in plants, support often comes from dead tissues (like sclerenchyma), whereas in animals, support derives from living tissues. The functions also vary significantly; plant tissues mainly provide support and facilitate transport, while animal tissues aid in movement, coordination, and protection.