Chapter 3: Tissues in Action

Life begins from a single cell. This single cell divides repeatedly and forms many cells. In multicellular organisms, these cells do not work randomly. Cells become specialised and form tissues, tissues form organs, organs form organ systems, and organ systems together form the complete organism.

A tissue is a group of similar cells that work together to perform a particular function.

For example:

The formation of tissues leads to division of labour, which means different groups of cells perform different functions. This makes the body more efficient.

Plant and animal tissues are different because plants and animals have different body structures, needs, and ways of living.

Main differences between plant and animal tissues

Plant cells have cell walls, so plant tissues are often more rigid. Animal cells do not have cell walls, so they can change shape more easily. This flexibility helps animals in movement.

Plants grow in different ways:

• They grow in length.
• They grow in girth or thickness.
• They regrow after cutting or grazing.

This growth happens due to meristematic tissue.

Meristematic Tissue

Meristematic tissue is made up of cells that divide continuously.

Features of meristematic cells

Meristematic cells:

• are small in size,
• have thin cell walls,
• have dense cytoplasm,
• have a large and prominent nucleus,
• have many organelles,
• usually do not have vacuoles,
• are tightly packed with little or no intercellular space.

These features help them divide rapidly and continuously.

Apical meristem is found at the tips of roots and shoots.

Function

It helps plants grow in length.

Example

In the onion root experiment, roots continue to grow if the root tips are present. But when the root tips are cut, root growth stops. This shows that the tip contains actively dividing cells.

Location

Lateral meristem is found along the sides or circumference of stems.

Function

It increases the girth or thickness of the stem.

In woody plants, lateral meristem forms new cells in concentric layers. This increases the diameter of the stem.

Annual rings

In the cut trunk of a tree, ring-like structures are seen. These are called annual growth rings. They help scientists estimate:

• the age of the tree,
• the growth conditions of different years,
• climatic conditions during growth.

Wide rings show favourable conditions, while narrow rings show unfavourable conditions.

Intercalary meristem is located at the base of internodes or just above the nodes.

Important terms

Function

Intercalary meristem helps in regrowth after cutting or grazing.

Example

Grass grows again after being cut or eaten by animals because it has intercalary meristem near the nodes.

Types of Meristematic Tissues

Differentiation

When some meristematic cells lose the ability to divide, they become specialised to perform specific functions. This process is called differentiation.

Through differentiation, meristematic tissue changes into permanent tissue.

Permanent tissues are formed from meristematic tissues. Their cells have lost the ability to divide and are specialised for particular functions.

Permanent tissues are of two main types:

• Simple permanent tissues
• Complex permanent tissues

(i) Protective Tissue — Epidermis

The epidermis is the outermost layer of the plant body.

Structure

• It is usually a single layer of tightly packed cells.
• Cells are flat and rectangular.
• It may be covered by a waxy layer called cuticle.

Functions of epidermis

The epidermis:

• protects the plant from mechanical injury,
• prevents entry of harmful microorganisms,
• reduces water loss,
• protects against parasites,
• helps in absorption through root hairs,
• helps in gaseous exchange through stomata.

Cuticle

The cuticle is a waxy layer made of cutin. It reduces water loss. It is thick in desert plants because they need to conserve water.

Root hairs

Root hairs are hair-like projections from epidermal cells of roots. They increase the surface area for absorption of water and minerals.

Stomata

Stomata are small pores present in the epidermis of leaves. They help in:

• gaseous exchange,
• transpiration,
• elimination of some wastes.

Transpiration is the loss of water vapour through stomata. It helps create a transpiration pull, which helps water move upward through xylem.

(ii) Supporting Tissue — Simple Permanent Tissues

Supporting tissues help plants stay upright, flexible, strong, and protected.

There are three types of simple permanent tissues:

• Parenchyma
• Collenchyma
• Sclerenchyma

a. Parenchyma

Parenchyma is made up of living cells with thin cell walls.

Structure

• Cells are living.
• Cell walls are thin.
• Cells are loosely packed.
• Intercellular spaces are present.

Functions

Parenchyma:

• stores food,
• performs photosynthesis in green parts,
• helps aquatic plants float when it forms air spaces.

In aquatic plants, parenchyma with air spaces is called aerenchyma.

b. Collenchyma

Collenchyma is made up of living cells with unevenly thickened corners.

Structure

• Cells are living.
• Corners are thickened due to pectin deposition.
• Cells provide flexibility.

Functions

Collenchyma:

• provides support,
• gives flexibility,
• allows plant parts to bend without breaking.

Example

Young stems and tendrils bend without breaking because of collenchyma.

c. Sclerenchyma

Sclerenchyma cells have thick, hard walls due to lignin deposition.

Structure

• Cells have thick lignified walls.
• Most cells are dead.
• Cells are hard and strong.

Functions

Sclerenchyma:

• provides strength,
• forms woody parts,
• gives hardness to seed coats and nut shells.

Examples

Sclerenchyma is found in:

• stems,
• leaf veins,
• coconut husk,
• walnut shell,
• hard seed coats.

Difference between Parenchyma, Collenchyma and Sclerenchyma

(iii) Conducting Tissues — Complex Permanent Tissues

Plants need tissues to transport water, minerals, and food. These tissues are called conducting tissues.

There are two main conducting tissues:

• Xylem
• Phloem

Together, xylem and phloem form the vascular tissue system.

Xylem

Xylem transports water and minerals from roots to other parts of the plant.

Components of xylem

Xylem consists of:

• tracheids,
• vessels,
• xylem parenchyma,
• xylem fibres.

Nature of xylem cells

Most xylem cells are dead, except xylem parenchyma, which is living.

Functions of xylem

Xylem:

• transports water,
• transports minerals,
• provides mechanical strength.

Phloem

Phloem transports food prepared in leaves to other parts of the plant.

Components of phloem

Phloem consists of:

• sieve tubes,
• companion cells,
• phloem parenchyma,
• phloem fibres.

Nature of phloem cells

Phloem is mostly made up of living cells.

Functions of phloem

Phloem:

• transports food,
• stores food materials, resins, tannins, and latex,
• provides support through phloem fibres.

Companion cells

Companion cells help sieve tubes in loading and unloading sugars.

Difference between Xylem and Phloem

Tissue Systems in Plants

Plant tissues are organised into three tissue systems:

Animal tissues are groups of specialised cells that perform different functions in the animal body.

The four main types of animal tissues are:

• Epithelial tissue
• Connective tissue
• Muscular tissue
• Nervous tissue

Epithelial tissue forms the outer covering of the body and lines internal organs.

Locations

Epithelial tissue is found in:

• skin,
• mouth,
• lungs,
• blood vessels,
• intestine,
• stomach lining,
• glands.

Structure

• Cells are closely packed.
• Very little intercellular space is present.
• It forms protective coverings and linings.

Functions

Epithelial tissue helps in:

• protection,
• absorption,
• secretion,
• exchange of gases and liquids,
• sensory functions,
• movement of substances.

Types of Epithelial Tissue Based on Function

The tissue that connects and supports other tissues is called connective tissue.

Examples of connective tissue:

• blood,
• bone,
• cartilage,
• tendon,
• ligament.

Connective tissues differ because of differences in their matrix.

The matrix may be:

• fluid,
• jelly-like,
• hard,
• rigid.

Blood as Connective Tissue

Blood is a fluid connective tissue. It connects different parts of the body by transporting materials.

Components of blood

Blood contains:

• plasma,
• red blood cells,
• white blood cells,
• platelets.

Functions of blood components

The red colour of blood is due to haemoglobin, an iron-rich protein present in RBCs.

Bone

Bone is a hard connective tissue.

Function

Bones:

• provide support,
• give shape to the body,
• protect internal organs,
• help in movement with muscles.

The matrix of bone contains calcium and phosphorus compounds, which make it hard and strong.

Cartilage

Cartilage is a flexible connective tissue.

Function

Cartilage:

• provides flexibility,
• cushions the ends of bones,
• absorbs shock.

Examples

Cartilage is found in:

• ear,
• nose,
• joints,
• between vertebrae.

Tendon

Tendon connects muscle to bone.

Function

It transmits the force of muscle contraction to bones and helps in movement.

Ligament

Ligament connects bone to bone.

Function

It:

• provides stability to joints,
• limits excessive movement,
• prevents dislocation.

Difference between Tendon and Ligament

Movement in our body happens due to muscular tissue.

There are two types of movements:

• Voluntary movements
• Involuntary movements

Voluntary movements

Movements that are under our conscious control are called voluntary movements.

Examples:

• running,
• writing,
• lifting objects,
• walking.

These movements are performed by skeletal muscles.

Involuntary movements

Movements that occur automatically without conscious control are called involuntary movements.

Examples:

• movement of food in the intestine,
• heartbeat,
• movement of internal organs.

These movements are performed by smooth muscles and cardiac muscles.

Types of Muscles

1. Skeletal Muscle

Skeletal muscles are attached to bones.

Features

• Cells are long and cylindrical.
• Cells are unbranched.
• Cells are multinucleate.
• They have light and dark bands, so they are striated.
• They are voluntary.

Function

They help in body movement.

2. Smooth Muscle

Smooth muscles are found in internal organs like the stomach and intestine.

Features

• Cells are spindle-shaped.
• Each cell has one nucleus.
• No striations are present.
• They are involuntary.

Function

They help in slow and continuous movements, such as movement of food in the digestive tract.

3. Cardiac Muscle

Cardiac muscles are found only in the heart.

Features

• Cells are cylindrical and branched.
• Each cell has one nucleus.
• They have faint striations.
• They are involuntary.
• They work rhythmically without fatigue.

Function

They help the heart beat throughout life.

Difference between Skeletal, Smooth and Cardiac Muscles

The body senses, communicates, and responds through nervous tissue.

Nervous tissue forms the control and coordination network of the body.

Main functions of nervous tissue

Nervous tissue:

• receives messages,
• processes information,
• sends signals to different body parts,
• controls muscles,
• helps in memory,
• coordinates responses.

The brain acts as the control centre of the body.

Neuron

The cells of nervous tissue are called neurons or nerve cells.

Parts of a neuron

A neuron has three main parts:

• Cell body
• Dendrites
• Axon

The musculoskeletal system is made up of:

• bones,
• muscles,
• joints,
• cartilage,
• tendons,
• ligaments.

Functions of musculoskeletal system

It helps us to:

• stand upright,
• move,
• maintain posture,
• protect delicate organs.

The musculoskeletal system works under the control of the nervous system.

How movement occurs

• The brain sends signals through nervous tissue.
• Muscles receive the signal.
• Muscles contract.
• Tendons transmit the force to bones.
• Bones move at joints.

Muscles pull bones to produce movement.

Different body parts move in different ways because they have different types of joints.

A joint is a place where two or more bones meet.

Joints allow movement, but they do not move bones by themselves. Movement happens when muscles pull bones.

Examples:

There are different types of joints in the human body.

In a ball and socket joint, the rounded end of one bone fits into a hollow part of another bone.

Movement allowed

It allows movement in many directions:

• forward,
• backward,
• sideways,
• circular movement.

Example

Shoulder joint and hip joint.

A hinge joint allows movement in one direction, like a door hinge.

Movement allowed

It allows bending and straightening.

Examples

• Elbow joint
• Knee joint
• Ankle joint

A pivot joint allows rotation.

Example

The joint between skull and backbone allows us to move the head side to side, like saying “no”.

Fixed joints do not allow movement.

Example

The bones of the skull are joined by fixed joints. This protects the brain, eyes, and ears.

Types of Joints at a Glance

The skeletal system is the framework of bones in the body.

Functions of skeletal system

The skeletal system:

• gives shape to the body,
• provides support,
• protects delicate organs,
• helps in movement,
• supports muscles.

Main Parts of the Skeletal System

1. Skull

The skull protects the brain, eyes, and ears. The bones of the skull are joined by fixed joints.

2. Vertebral Column

The vertebral column is also called the backbone or spine.

It is made up of many small bones called vertebrae.

Functions

The vertebral column:

• supports the body,
• helps us stand upright,
• protects the spinal cord,
• allows bending and twisting.

Between vertebrae, cartilage discs act as cushions and provide flexibility.

3. Rib Cage

Humans have 12 pairs of ribs. Together, they form the rib cage.

Functions

The rib cage:

• protects the heart,
• protects the lungs,
• helps in breathing.

The ribs are attached to the spine at the back and to the breastbone or sternum in the front. Flexible cartilage allows the rib cage to expand and contract during breathing.

Health of Bones and Muscles

To keep the musculoskeletal system healthy, we should maintain:

• correct posture,
• proper nutrition,
• regular exercise,
• yoga,
• physical activity.

Yoga improves flexibility, posture, breathing, and reduces stress.

In 1958, F. C. Steward showed that single cells from the phloem of carrot could develop into a complete carrot plant under suitable laboratory conditions.

What happened in the experiment?

• Phloem cells from carrot were taken.
• These cells were grown in a nutrient medium containing sugars and hormones.
• The cells divided to form a mass of cells.
• These cells differentiated into roots and shoots.
• A complete plant developed.

Important terms

Dedifferentiation

When specialised cells regain the ability to divide, it is called dedifferentiation.

Redifferentiation

When these dividing cells again become specialised to form different tissues and organs, it is called redifferentiation.

Totipotency

The ability of a single plant cell to divide and develop into a complete plant is called totipotency.

Such cells are called totipotent cells.

Importance of Totipotency

Totipotency is useful in:

• plant tissue culture,
• production of disease-free plants,
• crop improvement,
• genetic engineering,
• production of valuable plant chemicals.

Crown Gall Disease and Science

Crown gall disease is caused by a bacterium called Agrobacterium tumefaciens.

In this disease, tumour-like swellings develop on plant stems due to uncontrolled cell division.

Scientists studied how this bacterium transfers genetic material into plant cells. This knowledge is now used in genetic engineering to introduce useful genes into plants.

Important Keywords

Quick Revision Tables

Plant Tissues

Animal Tissues

In this chapter, we learn that tissues are groups of similar cells that work together to perform specific functions. In multicellular organisms, tissues help in division of labour and increase efficiency.

Plant and animal tissues are different because plants are mostly fixed, have cell walls, prepare their own food, and grow throughout life in specific regions. Animals usually move, do not have cell walls, and have tissues specialised for digestion, movement, support, and coordination.

Plant growth occurs due to meristematic tissues. Apical meristem increases length, lateral meristem increases girth, and intercalary meristem helps in regrowth after cutting or grazing. When meristematic cells become specialised, they form permanent tissues. Permanent tissues may be simple or complex. Simple permanent tissues include parenchyma, collenchyma, and sclerenchyma. Complex permanent tissues include xylem and phloem. Xylem transports water and minerals, while phloem transports food.

Animal tissues are mainly of four types: epithelial, connective, muscular, and nervous. Epithelial tissue protects body surfaces and lines internal organs. Connective tissue connects and supports body parts. Blood, bone, cartilage, tendons, and ligaments are examples of connective tissues. Muscular tissue helps in movement. Skeletal muscles control voluntary movements, while smooth and cardiac muscles control involuntary movements. Nervous tissue contains neurons that receive and transmit messages.

The musculoskeletal system consists of bones, muscles, joints, cartilage, tendons, and ligaments. It helps in movement, posture, and protection of organs. Different types of joints allow different movements. Ball and socket joints allow movement in many directions, hinge joints allow bending and straightening, pivot joints allow rotation, and fixed joints do not allow movement.

The chapter also explains totipotency, the ability of a single plant cell to develop into a complete plant under suitable conditions. This idea is important in plant tissue culture, crop improvement, and genetic engineering.

Important Keywords and Definitions

Plant Tissues

Protective and Supporting Plant Tissues

Conducting Plant Tissues

Animal Tissues

Epithelial Tissue

Connective Tissue

Muscular Tissue

Nervous Tissue

Musculoskeletal System and Joints

Advanced Keywords