Tissues in Action

Class 09 Science

Life begins when a single cell divides itself several times to give rise to a large number of cells. These cells gradually form the skin (protection), muscles (movement), bones (support), nerves (control and coordination), and all other organs.

The cell is the basic unit of life. Many cells come together to form a multicellular organism. In all multicellular organisms, there is a hierarchy of organisation. Cells of similar type performing similar function group together to form a tissue, more than one type of tissues form organs, different organs form organ systems and organ systems form an organism.

In unicellular organisms, such as amoeba, a single cell performs all functions of life. In multicellular organisms like plants and animals, different groups of cells perform different functions.

A tissue is a group of cells (similar in structure) that work together to perform a specific function. The formation of different types of tissues leads to the division of labour, which increases the efficiency of the body and enables it to carry out complex life processes.

Plant and Animal Tissues

Most plants are fixed in one place and do not move from place to place like animals. They need support to stay firm and upright. Plant cells have a cell wall that provides rigidity and strength. In general, animals can move (although some, such as sponges, are immobile). Without a rigid cell wall, animal cells can change shape easily. This cellular flexibility eventually helps make their bodies suitable for locomotion.

Another major difference between plants and animals is their mode of nutrition. Animals have tissues that help them digest food obtained from different food sources, while plants have tissues that help them utilise solar energy for synthesising the food components through photosynthesis.

Plants and animals have distinct tissues for transporting food and water to different parts of the body. The growth patterns in plants and animals also vary because the tissues responsible for growth differ in structure and function.

Tissues for Growth in Plants (Meristematic Tissues)

Plants grow in different ways:

• increase in length (height of stem and depth of roots)
• increase in girth (thickness of stem)
• regrowth after cutting the branches or grazing by animals

This growth require actively dividing cells that together form a tissue called a meristematic tissue. Plants have three types of meristematic tissues:

1. Apical
2. Lateral
3. Intercalary

The apical meristem, located at the root and shoot tips, increases its length. The lateral meristem located along the circumference of stems increases girth. The intercalary meristem located at the base of certain plants, such as grasses, helps them regenerate after cutting. Together, these meristems account for growth in length, girth and branching in plants.

Apical meristem - How do plants grow in length?

Root tips contain actively dividing cells. Similarly, shoot tips also contain actively dividing cells that help the shoots to grow in length. Thus, plants have growth zones at the tips of their roots and shoots, called the apical meristems, which help the plant grow in length.

Lateral meristem - How do plants grow in girth?

The increase in girth occurs due to the presence of actively dividing cells arranged in a ring in the stem. These cells divide and produce new cells inside and outside in a concentric manner, leading to an increase in diameter of stem. This meristematic tissue is called the lateral meristem.

Intercalary meristem - How do plants grow after branch being cut?

The intercalary meristem is located at the base of internode or just above the node. The node is point on plant stem where branches or leaves arise. The part of stem between the two nodes is called internode.

When the hedge around a garden is cut, after sometime more branches appear again, giving the hedge a bushy appearance. Grass also appear after sometime being mowed or grazed by animals. This happens because of the presence of intercalary meristem at the nodes of its stem. These meristematic tissues are called intercalary meristems.

The cells of the meristematic tissues are small, have thin cell walls, a large and prominent nucleus, and dense cytoplasm with many organelles. Vacuoles are generally absent and the cells are tightly packed with little or no intercellular space. These characteristics of meristematic tissue allow them continuous and rapid cell division.

Due to continuous cell division, meristematic tissue adds new cells to the plant body. Some of the newly formed cells remain meristematic while others lose the ability to divide. The cells that loose the ability to divide undergo changes in structure and function, and become permanent tissues. These cells become specialised to perform specific functions, such as support, transport or storage. This process, by which meristematic tissue becomes specialised to perform specific functions, is called differentiation. Meristemetic tissue becomes permanent by the process of differentiation.

Permanent Tissues

Permanent tissues can be simple (composed of only one type of cell) or complex (composed of more than one types of cells).

(i) Protective tissue - Epidermis

The epidermis forms the outermost layer of the plant body. It consists of a tightly packed, single layer of flat and rectangular cells. It protects all parts of the plants. These cells are covered with a waxy layer of cutin called cuticle.

In some plants, living in very dry habitat, the epidemis may be covered by a thick layer of cuticle to reduce the water loss in the process of transpiration by stomata. The cuticle also provides protection against mechenical injury and invasion by prasites. In many plants, hair-like projections arise from epidermal cells. In roots, these projections are called root hair, which increase the surface area for absorption of water and minerals from the soil.

In leaves, the epidermis contains pores called stomata, which apart from gaseous exchange helps in transpiration, i.e., evaporation of water vapours through stomata. Thus, transpiration helps in water transportation by creating a transpiration pull in xylem. Transpiration also helps in elimination of wastes from the plant body.

(ii) Supporting tissue - Simple permanent tissues

There are three types of supporting tissues or simple permanent tissues:

1. Parenchyma
2. Collenchyma
3. Sclerenchyma

Each differs in structure and performs supporting functions.

Parenchyma

Parenchyma consists of living cells with thin walls. These cells are loosely packed with intercellular spaces. Parenchyma mainly stores food but also performs photosynthesis in the green parts of the plants. In aquatic plants, specialised parenchyma forms air spaces, which help them float.

Collenchyma

Collenchyma consists of living cells with unevenly thickened corners due to pectin (a chemical that gives flexibility like rubber) deposition. This tissue provides support and flexibility, allowing parts of the plant like stems and tendrils to bend without breaking.

Sclerenchyma

Sclerenchyma cells have thick walls due to deposition of lignin, making them hard and strong (forms the woody structure). Most of these cells are dead. This tissue is found in stems, leaf veins, and hard coverings of seeds and nuts, such as coconut husk and walnut shell.

(iii) Conducting tissues - Complex permanent tissues

Plants have specialised conducting tissues called xylem and phloem, together known as complex permanent tissues, because they are made up of different types of cells working together.

Xylem is the tissue that transports water and minerals from the roots to other parts of the plant. It also provides strength to the plant. Xylem consists of tracheids, vessels, xylem parenchyma and xylem fibres. Tracheids and vessels are tubular and thick-walled. Xylem parenchyma are the only living component of xylem while trachieds, vessels, and xylem fibres are primarily sclerenchymatous.

Unlike the xylem, phloem is mostly made up of living cells. It consists of sieve tubes, companion cells, phloem parenchyma and phloem fibres. Some cells are long and tubular, joined end to end by perforated walls. These cells form sieve tubes. Sieve tubes transport food from leaves to other parts of the plant. The cellular functions of the sieve tube cells are regulated by companion cells. Companion cells are specialised parenchyma cells. Main function of companion cells is to monitor loading and unloading of sugars in sieve tubes.

Phloem parenchyma store food materials, and resin, tannins and latex. The sieve tubes are also supported by phloem fibres which are primarily schlerenchymatous and provide strength.

Plant Tissue Systems

Different types of plant tissues based on their functions are protection, support and conduction. In a plant body, these tissues do not work alone. They are organised together into larger groups called tissue systems. Plant tissues are organised into three tissue systems:

1. Dermal tissue system: This forms the outer covering of the plant. It protects the inner parts and reduces water loss.

2. Ground tissue system: This forms the main body of a plant between the dermal and conducting tissues. It includes parenchyma, collenchyma and sclerenchyma.

3. Vascular tissue system: This consists of conducting tissues - xylem and phloem.

Animal Tissues

Like plants, animal cells also group together and specialised in performing different functions. These groups of similar cells form animal tissues.

(i) Epithelial Tissues - Structure and Functions

Epithelial tissue forms the outer covering of the body (skin) and also lines the internal organs, such as the mouth, lungs, blood vessels and intestine. It is composed of closely packed cells with very little space between them. This structure prevents the entry of the germs, reduces water loss, and also helps in the absorption, secretion and movement of substances. Different types of epithelial tissues are structurally adapted to perform different functions.

(ii) Connective Tissue

Blood connects different parts of the body by transporting nutrients, gases, hormones, etc. In the same way, bones connect and support the body from head to toe. A tissue that connects and supports other tissues is called a connective tissue.

Both blood and bones are connective tissues. Though both are connective tissues, they differ in composition and consistency. Blood is fluid, while bone is hard. This difference is due to the matrix, which is watery, soft and jelly‐like in blood but hard, solid, and rigid in bones.

Blood

1. The red colour of blood is due to haemoglobin, an iron-rich protein in the Red Blood Cells (RBCs). RBCs live for about 4 months and are replaced regularly.
2. Platelets help in blood clotting at the site of the injury.
3. During exercise or running, muscles need more oxygen, so breathing becomes faster and blood flow increases (face appears red).
4. White Blood Cells (WBCs) collect at infected areas, causing pus formation and inflammation (causing redness and swelling, and possible pus formation at the infected area).

Bones

Bones have a rigid matrix containing calcium and phosphorus compounds, giving them strength and rigidity. In contrast, cartilage has a soft, jelly-like matrix, and provides flexibility and cushioning.

Other connective tissues include tendons and ligaments. Tendons connect muscles to bones, while ligaments connect bones to bones and prevent excessive movement.

(iii) Muscular Tissue

Some movements are under our conscious control, such as running, writing or lifting objects. These are called voluntary movements and are carried out by skeletal muscles, which are attached to the skeleton. They are made up of bundles of long, cylindrical cells called muscle fibres, which are unbranched, multinucleate (having many nuclei) and striated (showing light and dark bands).

Many body movements occur automatically without conscious control. For example the movement of food in the intestine and the beating of the heart. These are called involuntary movements. The muscles responsible for these movements include smooth muscles, which are found in organs like the stomach and intestines. Their cells are spindle-shaped, have a single nucleus and lack striations. They help in slow, continuous movements like digestion.

The cardiac muscles are found only in the heart. Their fibres are cylindrical and branched with a single nucleus, and have faint striations. Cardiac muscles work tirelessly and rhythmically, enabling the heart to beat throughout life without fatigue.

(iv) Nervous Tissue

Nervous tissue forms the body’s control and coordination network. The brain acts as the control centre - coordinating activities, memory and responses across the body. Muscles, both voluntary and involuntary, cannot function independently. They receive instructions from the nervous tissue. For example, during exercise, the brain signals the heart to beat faster to meet the body’s increased oxygen demand.

The cells of nervous tissue are called neurons or nerve cells, which are specialised to receive, process and transmit messages. Each neuron has three main parts - the cell body, which contains the nucleus and controls cell activities; dendrites, which receive signals from other neurons; and an axon, a long fibre that carries messages from the cell and ends at axon terminals. The axon terminals transmit the messages to other cells.

The Musculoskeletal System

The musculoskeletal system is made up of bones, muscles, joints, cartilage, tendons and ligaments. This system helps us stand upright, move, maintain posture and protect delicate organs. The musculoskeletal system functions under the control of the nervous system.

Muscles pull on bones to produce movement. They are attached to bones by strong, flexible bands called tendons. When a muscle contracts, the tendon transmits this force to the bone, resulting in movement at a joint.

Types of Joints

A joint is a junction between two or more bones. Joints allow movement but they cannot move the bones on their own.

1. Ball and socket joint

The shoulder joint allows free movement of the arm. This is because the rounded top of the upper arm bone fits into a shallow hollow of the shoulder bone, forming the ball and socket joint. Together with the collarbone, the shoulder forms the shoulder girdle, which connects the arm to the skeleton. This joint allows forward, backwards, sideways and circular movements.

2. Hinge joint

Unlike the shoulder, the elbow bends and straightens in one direction only like a door hinge. This type of joint is called a hinge joint. A similar hinge joint is present in the knee, where a small bone called the kneecap protects the joint.

3. Pivot joint

The skull is connected to the backbone through a pivot joint, which allows the head to move side to side like a doorknob turning in its socket.

4. Fixed joints

The skull is a hard case of flat bones joined together to protect the brain, eyes and ears. The bones of the skull are connected by fixed joints, which means the bones of the skull cannot move. This keeps the brain safe even when the body moves.

Skeletal System

The skeletal system consists of a framework of bones that provides strength and protects delicate internal organs. It includes the skull, vertebral column and rib cage. From the base of the skull, it extends a flexible column called the backbone or vertebral column (spine), made up of a series of small bones called vertebrae. It supports the body and helps us stand upright. Between each vertebra is a cartilage disc, which acts as a cushion and allows flexibility, so we can bend and twist without injuring the internal spinal cord.

There are 12 pairs of ribs and together they form the rib cage. The rib cage acts like a protective cage to protect vital organs, such as the heart and lungs. The ribs are attached to the spine at the back and to the breast bone (sternum) in the front. They are joined by flexible cartilage. This flexibility allows the rib cage to expand and contract during breathing. This movement increases and decreases space in the chest, allowing air to move in and out of the lungs. Injury to the ribs can make breathing painful and difficult.