Tissues in ActionClass 9 Science Notes

Introduction to Tissues

In multicellular organisms, life begins as a single cell that divides many times. These cells then specialize to form different parts of the body, such as skin, muscles, and bones. This organization allows for a division of labour, where different groups of cells perform specific tasks, making the organism more efficient.

A tissue is a group of cells that are similar in structure and work together to perform a specific function.

The hierarchy of organization in multicellular organisms is:

1. Cells: The basic building blocks of life.
2. Tissues: Groups of similar cells performing a common function.
3. Organs: Structures made of more than one type of tissue working together (e.g., the heart, stomach).
4. Organ Systems: Groups of organs that work together for a major function (e.g., the digestive system).
5. Organism: A complete living being.

Why are Plant and Animal Tissues Different?

Plant and animal tissues are fundamentally different because their lifestyles and needs are different.

• Mobility and Structure:

  • Plants are stationary (fixed in one place). They need strong, rigid tissues for support. Their cells have a cell wall that provides this rigidity.
  • Animals are generally mobile. Their cells lack a rigid cell wall, which allows for flexibility and helps make their bodies suitable for movement.

• Nutrition:

  • Plants are autotrophs; they make their own food through photosynthesis. They have tissues designed to capture solar energy.
  • Animals are heterotrophs; they get food from external sources. They have tissues specialized for digestion.

• Growth:

  • Growth in plants is often localized to specific regions (meristems), and some tissues can grow throughout the plant's life.
  • Growth in animals is more uniform and generally stops after reaching maturity, although cell repair and replacement continue.

Tissues for Growth in Plants

Plant growth, such as an increase in height, thickness, or regrowth after being cut, is carried out by tissues made of actively dividing cells. This type of tissue is called meristematic tissue.

Apical Meristem - Growth in Length

The apical meristem is a type of meristematic tissue located at the very tips of roots and shoots.

• Function: It is responsible for the increase in the length of the plant. The cells in the apical meristem divide continuously, adding new cells that cause the stem to grow taller and the roots to grow deeper.
• Location: Tips of roots and shoots.

Lateral Meristem - Growth in Girth

The lateral meristem is responsible for the increase in the diameter, or girth, of a plant's stem and roots.

• Function: The cells of the lateral meristem are arranged in a ring within the stem. They divide to produce new cells towards the inside and outside, causing the stem to become thicker over time.
• Annual Rings: This growth creates visible annual growth rings in the trunk of a tree. The width of these rings can indicate the climatic conditions during a particular year, and counting them can help estimate the tree's age.

Intercalary Meristem - Regrowth After Cutting

The intercalary meristem is found at the base of internodes or just above the nodes (the point on a stem where leaves or branches arise).

• Function: This tissue is responsible for the regrowth of plants like grasses after they are grazed by animals or mowed. It also allows for the growth of new branches from the nodes if the tip of a stem is cut.
• Location: At the nodes or base of internodes, especially common in grasses.

• Cells are small and tightly packed with little intercellular space.
• Cell walls are thin.
• They have a large, prominent nucleus and dense cytoplasm.
• Vacuoles are generally absent, which allows for more space for the nucleus and cytoplasm needed for rapid cell division.

Differentiation

As meristematic tissues divide, some of the new cells lose their ability to divide and begin to specialize for a particular function. This process, where a cell takes on a specific role, structure, and function, is called differentiation. Through differentiation, meristematic tissue transforms into permanent tissue.

Permanent Tissues

Permanent tissues are composed of cells that have lost the ability to divide and have become specialized to perform specific functions like protection, support, or transport. They can be simple or complex.

Protective Tissue - Epidermis

The epidermis is the outermost layer of the plant body, usually consisting of a single layer of tightly packed cells.

• Function: It protects the plant from mechanical injury, water loss, and invasion by harmful microorganisms.
• Cuticle: The outer surface of the epidermis is often covered by a waxy, waterproof layer called the cuticle, made of cutin. This layer is thicker in plants living in dry habitats to prevent water loss.
• Root Hairs: In roots, epidermal cells form long projections called root hairs, which increase the surface area for absorbing water and minerals.
• Stomata: In leaves, the epidermis contains tiny pores called stomata. Stomata are crucial for gas exchange (carbon dioxide in, oxygen out) and for transpiration (the evaporation of water), which helps pull water up from the roots.

Supporting Tissue - Simple Permanent Tissues

These tissues are called "simple" because they are made up of only one type of cell. They primarily provide support to the plant.

• Parenchyma:

  • Structure: Consists of living cells with thin walls and intercellular spaces between them.
  • Function: Its main role is to store food. In green parts of the plant, it also performs photosynthesis. In aquatic plants, specialized parenchyma contains large air spaces to provide buoyancy, helping the plant float.

• Collenchyma:

  • Structure: Consists of living cells with corners that are unevenly thickened with a substance called pectin.
  • Function: It provides flexible support, allowing parts like stems and tendrils to bend without breaking.

• Sclerenchyma:

  • Structure: Consists of dead cells with very thick, hard walls due to the deposition of lignin.
  • Function: It provides mechanical strength and rigidity to the plant. It is found in stems, leaf veins, and the hard coverings of seeds and nuts (e.g., coconut husk).

Conducting Tissues - Complex Permanent Tissues

These tissues are "complex" because they are made of more than one type of cell working together as a unit. They are responsible for transport within the plant.

• Xylem:

  • Function: Transports water and minerals from the roots to all other parts of the plant. It also provides structural support.
  • Components: Xylem is made of four types of cells: tracheids, vessels, xylem parenchyma, and xylem fibres.
  • Structure: Most xylem cells (tracheids, vessels, fibres) are dead and have thick, lignified walls, forming a continuous tube for water flow. Xylem parenchyma is the only living component.

• Phloem:

  • Function: Transports food (sugars produced during photosynthesis) from the leaves to other parts of the plant for use or storage.
  • Components: Phloem is made of sieve tubes, companion cells, phloem parenchyma, and phloem fibres.
  • Structure: Unlike xylem, phloem is mostly composed of living cells. Sieve tubes are long, tubular cells that transport the food. They are supported by companion cells, which regulate their functions.

Plant Tissue Systems

Plant tissues are organized into three major systems:

1. Dermal Tissue System: The outer protective covering (epidermis).
2. Ground Tissue System: The main body of the plant, located between the dermal and vascular tissues (includes parenchyma, collenchyma, and sclerenchyma).
3. Vascular Tissue System: The conducting tissues (xylem and phloem).

Animal Tissues

Animal tissues are groups of cells specialized for various functions like movement, communication, and connection. There are four main types of animal tissues.

Epithelial Tissues

Epithelial tissue forms the covering and lining of the body.

• Location: It forms the outer layer of the skin and lines internal organs like the mouth, lungs, and intestines.
• Structure: The cells are very closely packed with little space between them.
• Functions:
  • Protection: The skin protects underlying tissues from injury, friction, and microbes.
  • Exchange: Thin, single-layered epithelium in the lungs and blood vessels allows for rapid diffusion of gases and liquids.
  • Secretion: Specialized epithelial cells in glands produce and release substances like mucus, enzymes, and hormones.
  • Absorption: Tall, pillar-like cells in the small intestine are adapted for efficient uptake of nutrients.
  • Sensory Functions: Specialized receptor cells in the nose, taste buds, and inner ear detect stimuli.

Connective Tissues

Connective tissue connects, supports, and binds other tissues in the body. A key feature is that its cells are embedded in a non-living material called the matrix. The nature of the matrix determines the tissue's function.

• Blood:

  • Matrix: A fluid matrix called plasma.
  • Function: Transports nutrients, gases, hormones, and waste products throughout the body.
  • Components:
    • Red Blood Cells (RBCs): Contain haemoglobin and carry oxygen.
    • White Blood Cells (WBCs): Fight infection.
    • Platelets: Help in blood clotting.

• Bone:

  • Matrix: A hard, rigid matrix made of calcium and phosphorus compounds.
  • Function: Provides a strong framework for the body, gives support, and protects internal organs.

• Cartilage:

  • Matrix: A soft, flexible, jelly-like matrix.
  • Function: Provides flexibility and cushions the ends of bones at joints to absorb shock. Found in the ear, nose, and between vertebrae.

• Tendon:

  • Structure: Tough, fibrous connective tissue.
  • Function: Connects muscle to bone, transmitting the force of muscle contraction to produce movement.

• Ligament:

  • Structure: Strong, flexible connective tissue.
  • Function: Connects bone to bone at joints, providing stability and preventing excessive movement or dislocation.

Muscular Tissues

Muscular tissue is responsible for all types of movement in the body. It is composed of cells called muscle fibres that can contract and relax.

• Skeletal Muscle:

  • Control: Voluntary (under conscious control).
  • Function: Responsible for body movements like running, writing, and lifting.
  • Structure: Attached to the skeleton. Cells are long, cylindrical, unbranched, have multiple nuclei (multinucleate), and show alternating light and dark bands (striated).

• Smooth Muscle:

  • Control: Involuntary (not under conscious control).
  • Function: Found in the walls of internal organs like the stomach and intestines. Responsible for slow, continuous movements like digestion.
  • Structure: Cells are spindle-shaped, have a single nucleus, and lack striations (non-striated).

• Cardiac Muscle:

  • Control: Involuntary.
  • Function: Found only in the heart. It contracts rhythmically and tirelessly throughout life to pump blood.
  • Structure: Cells are cylindrical, branched, have a single nucleus, and show faint striations.

Nervous Tissue

Nervous tissue forms the body's control and communication network, including the brain, spinal cord, and nerves.

• Function: It receives, processes, and transmits messages, coordinating all body activities.
• Neuron: The basic cell of nervous tissue is the neuron, or nerve cell.
• Structure of a Neuron:
  • Cell Body: Contains the nucleus and controls the cell's activities.
  • Dendrites: Branch-like extensions that receive signals from other neurons.
  • Axon: A long fibre that carries messages away from the cell body to other cells.

The Musculoskeletal System

The musculoskeletal system is an integrated system made of bones, muscles, joints, cartilage, tendons, and ligaments.

• Function: It gives the body its shape, allows movement, helps maintain posture, and protects delicate organs.
• How it Works: The nervous system sends signals to muscles. Muscles contract and pull on bones via tendons, causing movement to occur at joints.

Skeletal System

The skeletal system is the framework of bones that provides strength, support, and protection.

• Vertebral Column (Spine): A flexible column of small bones called vertebrae that extends from the base of the skull. It supports the body and allows us to stand upright. Between each vertebra is a cartilage disc that acts as a cushion and allows for bending and twisting.

• Rib Cage: A protective cage formed by 12 pairs of ribs. It protects vital organs like the heart and lungs. The ribs are attached to the spine at the back and the breast bone (sternum) in the front, with flexible cartilage allowing the cage to expand and contract during breathing.

From one cell to an organism: Totipotency

Totipotency is the remarkable ability of certain mature plant cells to dedifferentiate (regain the ability to divide), then divide and redifferentiate to regenerate an entire new plant. This concept was demonstrated by F. C. Steward in 1958.

In his experiment, he took cells from the phloem of a carrot and grew them in a special nutrient medium. He observed that these specialized cells began to divide, forming a mass of unspecialized cells. This mass then differentiated to form roots, a shoot, and eventually a complete carrot plant. This shows that, under the right conditions, a single specialized plant cell can give rise to a whole organism, much like a zygote does.