Tissues — Study Notes

1. Introduction

In the previous chapter we learned that the cell is the fundamental unit of life. But in a multicellular organism, millions of cells do not work in isolation. Cells that are similar in structure and perform the same function group together to form tissues. A group of cells that are similar in structure and work together to perform a particular function is called a tissue.

In unicellular organisms, a single cell performs all the life processes. But in multicellular organisms, there is a division of labour - different groups of cells specialise in different functions. For example, some cells carry oxygen, some help in movement and some help in conduction. This specialisation of cells into tissues allows complex organisms to function efficiently.

In this chapter we study the different types of tissues in plants and animals, their structure, location and functions. Plant tissues are mainly classified as meristematic and permanent tissues, while animal tissues are classified into epithelial, connective, muscular and nervous tissues.

2. Plant Tissues

Plants are stationary organisms, so they do not need many of the tissues that animals require, such as those for locomotion. Plants also have a different body plan, with growth occurring in specific regions throughout life. Plant tissues are broadly of two types: meristematic tissues and permanent tissues.

Meristematic tissue: Meristematic tissue consists of cells which have the power of continuous division. These cells are young, have thin cell walls, dense cytoplasm and prominent nuclei, and do not contain vacuoles. The meristematic tissue is found in those regions of the plant where growth takes place. Based on the region of occurrence, meristematic tissues are of three types:

1. Apical meristem: Present at the growing tips of stems and roots; it increases the length of the plant (primary growth).
2. Intercalary meristem: Present at the base of leaves and internodes; it helps in the growth of these regions.
3. Lateral meristem: Present on the sides of stems and roots; it increases the girth of the plant (secondary growth).

When the cells of the meristem divide, some of the cells remain meristematic while others lose the ability to divide and differentiate into permanent tissues.

Permanent tissue: Permanent tissues are formed from meristematic tissue when its cells lose the ability to divide and attain a permanent shape, size and function. Permanent tissues are of two types - simple and complex.

Simple permanent tissue: A simple permanent tissue is made up of only one type of cells. It is of three types:

1. Parenchyma: Parenchyma cells have thin cell walls and large intercellular spaces. They store food and provide support to the plant. In leaves and green stems, some parenchyma cells contain chloroplasts and are called chlorenchyma, which carry out photosynthesis. In aquatic plants, the parenchyma cells have large air spaces and are called aerenchyma, which provide buoyancy.

2. Collenchyma: Collenchyma cells have cell walls thickened at the corners and little intercellular space. They provide mechanical support and flexibility to the plant. Collenchyma is found in the leaf stalks and below the epidermis of stems. It also allows bending without breaking, as in the stem of a plant bending in the wind.

3. Sclerenchyma: Sclerenchyma cells have thickened, lignified cell walls and no intercellular spaces. These cells are dead at maturity. They provide strength and rigidity to the plant. Sclerenchyma is found in the husk of a coconut, in the stems of plants and around the vascular bundles. Fibres of jute and the gritty cells in the pear fruit are sclerenchyma cells.

Complex permanent tissue: A complex permanent tissue is made up of more than one type of cells which work together as a unit. Xylem and phloem are complex tissues.

Xylem: Xylem conducts water and dissolved minerals from the roots to all parts of the plant. It is made up of tracheids, vessels, xylem parenchyma and xylem fibres. Except for xylem parenchyma, all xylem cells are dead. Conduction in xylem is unidirectional (upward only).

Phloem: Phloem transports food materials (produced in leaves by photosynthesis) to all parts of the plant. It is made up of sieve tubes, companion cells, phloem parenchyma and phloem fibres. Sieve tubes and companion cells are living cells. Conduction in phloem is bidirectional.

3. Animal Tissues

Animal tissues are of four main types: epithelial tissue, connective tissue, muscular tissue and nervous tissue.

Epithelial tissue: Epithelial tissue forms the covering or lining of body surfaces and organs, both inside and outside. It protects the body from injury and infection. The cells of epithelial tissue are packed closely with little intercellular material and rest on a basement membrane. Epithelial tissue has no blood supply of its own. It is classified on the basis of cell shape and the number of layers:

1. Squamous epithelium: Thin, flat cells arranged like tiles; it forms the lining of the mouth, oesophagus and the air sacs of lungs.
2. Cuboidal epithelium: Cube-shaped cells; it forms the lining of kidney tubules and the ducts of glands.
3. Columnar epithelium: Tall, pillar-like cells; it lines the intestine and helps in absorption and secretion.
4. Ciliated epithelium: Columnar cells with hair-like cilia on their free surface; it is found in the respiratory tract and helps to move mucus and dust particles.

Connective tissue: Connective tissue connects, supports and binds various parts of the body. It is the most abundant tissue in animals. Its cells are loosely spaced and embedded in an intercellular matrix. Types of connective tissue include:

1. Areolar tissue: It is the most widely distributed connective tissue and fills the spaces between organs. It supports internal organs and helps in the repair of tissues.
2. Adipose tissue: It stores fat, insulates the body and acts as a shock absorber. It is found below the skin and between internal organs.
3. Bone: It is the hardest connective tissue and forms the framework of the body. The matrix of bone has calcium and phosphorus, making it rigid and strong.
4. Cartilage: It is a flexible connective tissue found in the ear, nose and at the joints. It provides support and flexibility.
5. Blood: Blood is a fluid connective tissue. Its matrix, called plasma, contains blood cells. Blood transports oxygen, nutrients, hormones and waste products throughout the body.

Muscular tissue: Muscular tissue is made up of elongated muscle cells called muscle fibres which can contract and relax, producing movement. There are three types of muscles:

1. Skeletal (striated) muscles: These muscles have alternate light and dark bands (striations) and are attached to bones. They are voluntary, that is, under our control. They help in the movement of limbs and body.
2. Smooth (unstriated) muscles: These muscles have no striations and are found in the walls of internal organs like the stomach, intestine and blood vessels. They are involuntary and contract slowly.
3. Cardiac muscles: These muscles are found only in the heart. They have faint striations and are involuntary. Cardiac muscles never get tired and beat throughout life.

Nervous tissue: Nervous tissue is specialised to receive stimuli and conduct impulses. It is made up of neurons (nerve cells). A neuron has a cell body with a nucleus, dendrites (short branches that receive impulses) and an axon (a long fibre that carries impulses away). Nervous tissue is found in the brain, spinal cord and nerves. It controls and coordinates all the activities of the body.

Quick Revision Tables

Mind Map

Important Diagrams (SVG)

Diagram 1: Types of Plant Tissues

Diagram 2: Types of Animal Tissues

Common Mistakes

1. Thinking that all plant tissues are permanent; meristematic tissue continuously divides and produces permanent tissues.
2. Confusing collenchyma with sclerenchyma; collenchyma has thickened corners and is living, while sclerenchyma has thickened, lignified walls and is dead.
3. Believing that xylem carries food; xylem conducts water and minerals, while phloem conducts food.
4. Thinking that skeletal muscles are involuntary; skeletal (striated) muscles are voluntary, while smooth and cardiac muscles are involuntary.
5. Forgetting that blood is a connective tissue; it is a fluid connective tissue with plasma as its matrix.
6. Saying that cardiac muscles get tired; cardiac muscles contract and relax throughout life without getting fatigued.
7. Confusing ciliated epithelium with columnar epithelium; ciliated epithelium has hair-like cilia that move mucus and particles.

Exam Tips

1. Classify plant tissues into meristematic and permanent, and remember the location of apical, intercalary and lateral meristems.
2. Learn the three simple permanent tissues - parenchyma, collenchyma, sclerenchyma - with their functions and one example each.
3. Remember that xylem conducts water and minerals (unidirectional) and phloem conducts food (bidirectional).
4. For animal tissues, know the four main types and one function of each sub-type, including blood as a connective tissue.
5. Be able to distinguish skeletal, smooth and cardiac muscles on the basis of striations, location and voluntary/involuntary control.
6. Understand the structure of a neuron: dendrites receive impulses and the axon carries them away.
7. Know the function of epithelial tissue as protection, absorption and secretion with examples like the lining of the intestine and lung air sacs.

Conclusion

In this chapter we learned that a tissue is a group of cells similar in structure and function, and that tissues are essential for division of labour in multicellular organisms. Plant tissues are classified into meristematic tissues, which divide continuously to promote growth, and permanent tissues, which include simple tissues (parenchyma, collenchyma, sclerenchyma) and complex tissues (xylem and phloem). Animal tissues are of four types - epithelial, connective, muscular and nervous - each with specialised cells and functions. We compared the different muscle types, understood the structure of the neuron, and saw how blood acts as a fluid connective tissue. The study of tissues connects cell biology with the functioning of whole organisms and prepares us for the study of organs and organ systems.