What are Cells and Tissues?

The smallest, basic, and structural component of the body is a cell. It is the basic functional unit of life. No organism can exist on this Earth without a cell. Thus, it is the fundamental unit. These cells perform the major functions of the body. The term cell was given by Robert Hooke, who stated that all organisms’ bodies, whether unicellular or multicellular, are made of cells. In single-celled organisms such as amoeba and protozoa, all the body functions are performed by a single cell. The multicellular organisms have collective cells which perform a specific function in the body.

A tissue is a group of cells that work together to perform a particular function in the body. The tissues are similar to cells. The word tissue was derived from the Latin word weave. Plants and animals have different kinds of tissues. They are made of specialized cells like nerve cell that transmits electrical impulses. So, the cluster of cells performs a certain function to maintain the metabolism of the body. The organs usually perform well only if the worn-out cells are replaced quickly.

Cells

The study of cells is called cell biology or cytology. There are primarily two types of cells: prokaryotic cells and eukaryotic cells. 

Prokaryotic cell

Prokaryotes comprise bacteria and archaea and are considered primitive organisms on Earth. The prokaryotes are simpler than eukaryotes. They lack a nucleus and are membrane-less. The DNA (deoxyribonucleic acid) of prokaryotes is circular and is in direct contact with the cytoplasm. A prokaryotic cell has three regions: cell envelope, cytoplasmic region, and flagella. A cell envelope consists of a plasma membrane covered by a cell wall. It gives rigidity to the cell and demarcates the interior of the cell to the exterior environment. The cell of prokaryotes has peptidoglycan, which serves as a barrier and prevents the cell from bursting or expanding. The cytoplasmic region has genomic DNA, inclusions, and ribosomes. The genetic material floats freely in the cytoplasm. The prokaryotic cells also have extrachromosomal DNA called plasmids. The prokaryotic DNA is condensed to form a nucleoid. Some prokaryotes have flagella and pili that project outwards from the membrane to facilitate the movement of the cell.

Eukaryotic cell

Eukaryotes include fungi, slime molds, animals, and algae. The size of the eukaryotic cell is 15 times larger than a prokaryotic cell. Compartmentalization is the main distinguishing feature in both cells; the eukaryotes have much compartmentalized spaces for membranous organelles. The eukaryotic cell nucleus is the most important. The DNA is linearly associated with the histone proteins and is folded to fit inside the nucleus. The membrane-bound organelle mitochondrion has its DNA. Some eukaryotic cells possess flagella that work as sensory receptors antennae. Motile cilia and flagella are present to give movement; these structures are more complex than prokaryotes.

Tissues

Tissues make the cellular organization between cells and organs. The grouping of cells makes the tissue, and the grouping of tissues makes an organ. The study of tissue is called histology. The tissues can be stained and observed under a microscope to enhance their visualization.

Animals have four types of tissues: connective, epithelial, nervous, and muscle. 

1. Connective tissue

The connective tissue is made of an extracellular matrix, just like blood has plasma. Its function is to maintain the position of the organs. A certain example of connective tissue is ligament, tendon, bone, adipose, and areolar. Ligaments do the bone-to-bone attachment. The rigidity of these tissues helps maintain the body structure. The bone stores minerals and need calcium for maintaining its structure. Further classification of the connective is fibrous, skeletal, and fluid.

2. Epithelial tissue

The epithelial tissue covers the skin surface of various organs. They have semi-permeable tight junctions that provide a barrier between the organ and the external environment. The epithelial tissues are specialized to perform excretion, absorption, and secretion. Certain epithelial tissues are simple squamous, simple cuboidal, ciliated, stratified keratinized, stratified non-keratinized. 

3. Nervous tissue

The nervous tissue is a part of the central nervous system (CNS) and peripheral nervous system (PNS). The brain and the spinal cord form the CNS. The cranial and spinal nerves form the peripheral nervous system and transmit information to and from the brain.

4. Muscle

The muscle forms the contractile tissue of the body and is categorized into skeletal, cardiac, and smooth tissues. The skeletal tissue is attached to the bone(providing structure, stability, and rigidity) and generates movement. The cardiac muscle is found in the heart. The contraction of this muscle helps pump blood throughout the body. The smooth muscle is found in the inner linings of visceral organs.

Tissues in Plants

Plants have three types of tissues: epidermis, vascular, and ground.

1. The epidermis is the outer layer of tissue present on leaves and young plants, making their structures.
2. Vascular tissue makes the xylem and phloem that transport fluids and food throughout the plant.
3. The ground tissue makes nutrients through photosynthesis and stores them in the leaves. This tissue is less differentiated.

The plants also have other types of tissues: meristematic and permanent.

• Meristematic tissues are composed of actively dividing cells that increase the thickness and length of the plant. These tissues are present at the extremities of stems and roots. Certain meristematic tissues are apical meristem, lateral meristem, and intercalary meristem.
• Permanent tissues have a specialized function and cannot divide further. Parenchyma, collenchyma, and sclerenchyma are the different types of permanent tissues. The parenchyma is an unspecialized living cell present in between the spaces of cells and tissues. The collenchyma is a living tissue that provides strength, structure, and support to the plant. The sclerenchyma consists of thick and dead cells that provide mechanical support to the plant.

The Connection between Cells and Tissues

The cells coordinate with the tissues through messenger proteins. These proteins include hormones that transmit certain signals for coordination. They provide movement, structure, and support to the cells. These proteins also form the backbone of the cells and are required to replenish, grow, and repair cells and muscles. If excess protein is present compared to carbohydrates in the body, these proteins will start providing dietary energy. Thus, a protein-rich diet is important for overall development.

Common Mistakes

• The organization of cells and tissues is different.
• Cells are undifferentiated, but tissues perform the specific task as they are composed of differentiated cells.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• Bachelor of Science in Biology
• Master of Science in Anatomy and Physiology
• Master of Science in Zoology

Related Concepts

• Digestive system
• Excretory system
• Histopathology