3 Tissues in Action | Class 9 Science | PDF and Web notes

3	TISSUES IN ACTION

• Life begins when a single cell divides to form many cells. These cells form skin, muscles, bones, nerves etc. This process is a great engineering marvel of nature.
• To understand, replicate and modify this process for human welfare, it requires understanding biological processes controlling growth and development.
• Multicellular organisms show hierarchy of organisation. Similar cells form tissues, tissues form organs, organs form organ system and organ systems form an organism.
• In unicellular organisms like amoeba, a single cell performs all functions. In multicellular organisms, different groups of cells perform different functions.
• A tissue is a group of similar cells working together to perform a specific function.
• Different tissues create division of labour, increasing efficiency and enabling complex life processes.

WHY ARE PLANT AND ANIMAL TISSUES DIFFERENT?

• Most plants are fixed in one place and need support to stay upright. They have cell wall that provides rigidity and strength.
• Animals lack cell wall. So, they can change shape easily (cellular flexibility) helping locomotion.
• Muscle tissue enables movement and nervous tissue carries messages to different body parts.
• Plants and animals have distinct tissues for transporting food and water. In plants, xylem transports water and minerals, and phloem transports food.
• Mode of nutrition: Animals have tissues that help digest food. Plants have tissues that help utilise solar energy to synthesise food through photosynthesis.
• Growth patterns differ in plants and animals due to different growth tissues.

TISSUES FOR GROWTH IN PLANTS

• Plant growth includes growth in length & girth and branching. It requires actively dividing cells that form a tissue called meristematic tissue (meristem).
• It is 3 types:
  • Apical meristem: Located at the root and shoot tips, increases its length.
  • Lateral meristem: Located along the circumference of stems increases girth (thickness of stem).
  • Intercalary meristem: Located at the base of plants such as grasses, helps them regenerate after cutting.

Meet a Scientist

• B. G. L. Swamy (Indian botanist - plant morphology and anatomy) wrote a Kannada book Hasuru Honnu.
• It describes botanical excursion in the Western Ghats, including plant descriptions, utilisation, conservation practices, botanical folklores and myths.
• It won the Kendra Sahitya Akademi Award in 1978.

Apical meristem - How do plants grow in length?

Activity:

• Fill two jars with water.
• Place one onion bulb in each jar.
• Observe root growth for a few days.
• Measure root length on days 1, 2, and 3.
• On day 3, cut about 1 cm of root tips in Jar B.
• Observe and measure root growth for four more days.

Length of onion root (cm) from the base of the bulb

Jars	Day 1	Day 2	Day 3	Day 4	Day 5	Day 6	Day 7
A	0.25	0.75	1.8	2.9	3.8	4.9	5.8
B	0.25	0.70	1.6	1.6	1.6	1.6	1.6

Observation:

• Roots in Jar A continue to grow in length.
• Roots in Jar B stop growing after the tips are cut.

Inference:

• Roots grow only from their tips.
• This confirms that root tips contain actively dividing cells.
• Shoot tips also contain actively dividing cells that help the shoots to grow in length.
• The growth zones at the tips of roots and shoots are called apical meristems; they help the plant grow in length.

Lateral meristem - How do plants grow in girth?

• In dicot stems, an increase in diameter or girth occurs due to actively dividing cells arranged in a ring in the stem. This tissue is called the lateral meristem.
• It produces new cells inside and outside in a concentric manner, increasing the stem diameter.
• Cut tree trunks show annual growth rings.
• Wide or narrow rings indicate favorable or unfavorable growth conditions for the lateral meristem in a given year.
• Thus, counting annual rings helps estimate tree age and past climatic conditions.

Intercalary Meristem - How do plants grow after being cut?

• If the tip of a young stem is cut, lengthwise growth stops and new branches arise from the nodes. This is due to intercalary meristem.
• A node is a point on the stem where branches or leaves arise. The region between two nodes is called an internode.
• Intercalary meristem is located at the base of an internode or just above a node.
• When a hedge is trimmed, more branches appear, giving it a bushy appearance. Grass also continues to grow after being mowed or grazed by animals. These growth patterns are due to intercalary meristem.

Features of Meristematic Cells Favouring Cell Division

• Cells are small with thin walls, dense cytoplasm, and a large nucleus.
• Tightly packed with little or no intercellular space.
• Cells usually lack vacuoles because they undergo continuous and rapid cell division and do not need to store food or waste.

Permanent Tissues

• Some newly formed meristematic cells remain meristematic, while others lose the ability to divide to form permanent tissues.
• These cells become specialised for specific functions like support, transport, or storage.
• The process by which meristematic tissue becomes specialised to perform specific functions is called differentiation.
• Permanent tissues may be simple (only one type of cell) or complex (more than one type of cell).
• Transverse Section (T.S) of the root/stem, or the Vertical Section (V.S) of the leaf of an herbaceous plant (e.g., sunflower) under a microscope shows various permanent tissues.

Internal structure of a sunflower stem

(i) Protective Tissue - Epidermis

• Epidermis is the outermost layer of the plant body.
• It consists of a tightly packed, single layer of flat and rectangular cells.
• It protects the plant from mechanical injury, water loss, harmful microbes, and extreme environmental conditions.
• Cells are covered with a cuticle (a waxy layer of cutin).
• In plants living in very dry habitats, the epidermis may have a thick cuticle to reduce water loss by transpiration through stomata.
• The cuticle also provides protection against mechanical injury and invasion by parasites.
• In many plants, hair-like projections arise from epidermal cells. In roots, these are called root hairs, which increase surface area for the absorption of water and minerals.
• In leaves, the epidermis contains pores called stomata, which help in gaseous exchange and transpiration.
• Transpiration creates the pull necessary for water transport through the xylem and helps in the elimination of wastes.

(ii) Supporting Tissue (Simple Permanent Tissues)

• Supports the plant and keeps it upright.
• Comprises 3 types: parenchyma, collenchyma, and sclerenchyma.

a. Parenchyma

• Consists of loosely packed living cells with thin walls and intercellular spaces.
• Stores food and performs photosynthesis.
• In aquatic plants, specialised parenchyma forms air spaces (aerenchyma), which help them float.

b. Collenchyma

• Consists of living cells with unevenly thickened corners due to pectin (a chemical that provides flexibility) deposition.
• Provides support and flexibility, allowing stems, twigs, and tendrils to bend without breaking.

c. Sclerenchyma

• Has cells with thick walls due to the deposition of lignin, making them hard and strong (forming the plant's woody structure).
• Most of these cells are dead.
• Found in stems, leaf veins, and the hard coverings of seeds and nuts, such as coconut husks and walnut shells.
• It makes seed coats hard and causes dry twigs to break.

(iii) Conducting Tissues - Complex Permanent Tissues

• Made up of different types of cells working together.
• Two main types: Xylem and Phloem.

Xylem:

• Transports water and minerals from the roots to other parts of the plant.
• Provides structural strength to the plant.
• Consists of tracheids, vessels, xylem parenchyma, and xylem fibres.
• Tracheids and vessels are tubular and thick-walled; xylem parenchyma is the only living component.

Phloem:

• Mostly made up of living cells.
• Consists of sieve tubes, companion cells, phloem parenchyma, and phloem fibres.
• Sieve tubes: Long, tubular cells joined end-to-end to transport food from leaves to the rest of the plant.
• Companion cells: Specialised parenchyma that regulate sieve tube functions and monitor sugar loading/unloading.
• Phloem parenchyma: Stores food, resin, tannins, and latex.
• Phloem fibres: Primarily sclerenchymatous; provide strength and support.

Vascular tissues: (a) xylem (b) phloem

In a plant body, the tissues are organised together into larger groups called tissue systems.

Plant tissues are organised into three distinct systems:

• Dermal tissue system: Forms the outer covering; protects the plant and reduces water loss.
• Ground tissue system: Forms the main body between dermal and vascular tissues (includes parenchyma, collenchyma, and sclerenchyma).
• Vascular tissue system: The conducting tissues (xylem and phloem).

Ready to Go Beyond

• In young plants, the outer layer is the epidermis.
• As plants age, stem cells below the epidermis become the cork cambium, which produces cork cells.
• Cork cells are dead, compact, and impermeable, forming the bark of the tree.

Meet the Scientists

Sipra Guha Mukherjee and S. C. Maheshwari revolutionized modern agriculture by developing complete plants through anther culture using artificial nutrient media.

Pause and Ponder

1. You may have noticed that fibres of coconut husk are hard and brittle, whereas the leaf stalks of coriander are soft and flexible. Find out the reason.
   Answer: Coconut husk is hard and brittle because it contains sclerenchyma tissue with thick, lignified walls. Coriander leaf stalks are soft and flexible because they contain collenchyma tissue, which provides support without rigidity.
2. Why do you think that a thick cuticle on the outer wall of epidermis is advantageous for a plant living in the desert but disadvantageous for a plant living underwater?
   Answer: In the desert, a thick cuticle prevents water loss (transpiration) due to heat. Underwater, it is disadvantageous because it blocks the direct absorption of water and gases from the surrounding environment through the epidermis.
3. Once water is absorbed by plant roots, it has to travel against gravity through xylem. How do the 'dead' cells of the xylem work together with the living cells of leaves at the top to keep the water moving?
   Answer: Water moves against gravity through a suction pull (transpiration pull) created by the living cells in the leaves. As water evaporates through stomata, it creates a continuous chain of water molecules that is pulled upward through the hollow, pipe-like dead xylem vessels.
4. What do you think will happen if there were no stomata in the epidermis of the stem or leaves?
   Answer: If there were no stomata, the plant would be unable to exchange gases CO₂ and O₂ for photosynthesis and respiration. Also, the transpiration pull would stop, preventing the transport of water and minerals from the roots to the leaves.

ANIMAL TISSUES

There are 4 main types: Epithelial, Connective, Muscular, and Neural.

Epithelial Tissues - Structure and Functions

• Function: Forms the outer covering (skin) and lines internal organs (mouth, lungs, blood vessels, intestine, etc.).
• Structure: Closely packed cells with very little intercellular space. This prevents germ entry, reduces water loss, and assists in absorption, secretion, and movement of substances.

Characteristics of Epithelial Tissues

Function	Structure	Location
Exchange: Rapid diffusion of liquids and gases.	Single layer of thin, flat cells (a)	Blood vessels and lungs.
Protection: Guards against mechanical injury, friction, and microbes.	Many layers; outer cells are flat and tightly packed (b)	Skin, mouth, and oesophagus.
Secretion: Produces mucus, enzymes, hormones, sweat, saliva.	Specialised cells (cuboidal or columnar) (c)	Salivary/sweat glands, stomach lining.
Sensory: Smell, taste, sound, and balance.	Receptor cells with hair-like cilia (d)	Nostrils, taste buds, inner ear.
Absorption: Efficient uptake of nutrients and water.	Single layer of tall, pillar-like cells (often with cilia) (e)	Lining of small intestine.

How are various parts connected in our body?

• Connective tissue: A tissue that connects and supports other tissues (e.g., Blood, Bone, Cartilage, Tendon, Ligament).
• Blood: A fluid tissue (watery/jelly-like matrix) that transports nutrients, gases, and hormones.
  • Plasma (55%): The fluid component.
  • Formed elements (45%):
    • RBCs: Contain iron-rich haemoglobin for oxygen/CO2 transport (live ~4 months).
    • WBCs: Provide immunity.
    • Platelets: Assist in blood clotting.
• Bones: Hard/rigid matrix containing calcium and phosphorus; provides structural support.
• Cartilage: Soft, jelly-like matrix; provides flexibility and cushioning.
• Tendons: Connect muscles to bones.
• Ligaments: Connect bones to bones; prevent excessive movement.

Components of blood

Types of bones

Arrangement of tissues at a joint

Activity

Experiences	Observations	Questions	Answers
A small cut on skin	Red blood oozes out; a clot forms after some time.	What causes blood to clot?	Platelets help in blood clotting at the site of injury.
Getting a skin infection	Area turns red, swollen, and may have a fever.	Why does the area swell during infection?	WBCs collect at the site, causing inflammation, redness, and pus formation.
Exercise or run	Breathe faster; face may turn red.	Why do we breathe faster and turn red during exercise?	Muscles need more oxygen; breathing speeds up and blood flow to the skin increases.

Connective Tissues

Action	Experience	Function	Connective Tissue
Touch elbow gently	Hard and rigid structure	Gives strength, support, and protection	Bone
Press/fold ear or nose	Soft, flexible structure that retains shape	Provides flexibility and cushions bone ends	Cartilage
Touch forearm muscles and wiggle fingers	Feel movement in the forearm	Connects muscle to bone to bring about movement	Tendon
Move leg upwards at knee	Joint stops at a limit	Connects bone to bone; provides stability	Ligament

Can we control movement in our body?

The tissues responsible for body movements are called muscle tissues. There are three types:

1. Skeletal Muscles

• Function: Attached to the skeleton; carry out voluntary movements (under conscious control) like running, writing, or lifting objects.
• Structure: Made of bundles of long, cylindrical cells called muscle fibres.
• Characteristics: Striated (have light and dark bands), unbranched, and multinucleate.

2. Smooth Muscles

• Found in the stomach, intestines, etc.
• Responsible for involuntary movements (automatic movements like food movement in the intestine).
• Cells are spindle-shaped, have a single nucleus, lack striations, and help in slow, continuous movements.

3. Cardiac Muscles

• Found only in the heart.
• Fibres are cylindrical and branched, with a single nucleus and faint striations.
• Enable rhythmic heartbeat throughout life without fatigue.

How does the body sense, communicate, and respond?

• Nervous tissue controls quick responses (like pulling a hand away from a hot object), memory, and coordination; forms the body's control and coordination network.
• Brain is the control centre that coordinates activities, memory, and responses.
• Muscles require instructions from the nervous tissue to function (e.g., the brain signaling the heart to beat faster during exercise).

The Neuron (Nerve Cell)

Structure of a neuron

• The cells of nervous tissue are called neurons or nerve cells, which are specialised to receive, process and transmit messages. A neuron has 3 parts:
• A neuron has 3 parts:
  • Cell body: Contains the nucleus; controls cell activities.
  • Dendrites: Receive signals from other neurons.
  • Axon: A long fibre that carries messages away from the cell to axon terminals for transmission.

THE MUSCULOSKELETAL SYSTEM

• Composed of bones, muscles, joints, cartilage, tendons, and ligaments.
• Functions to maintain posture, enable movement, and protect delicate organs under the control of the nervous system.

Human Musculoskeletal system

• Muscles: Attached to bones by tendons (flexible bands). Contraction of muscles pulls on tendons, exerting force on bones to create movement at a joint.
• Joints: Junctions between two or more bones. They allow movement but require muscles to initiate it.

Different types of movements our body can make

Body Part	Complete Rotation	Partial Rotation	Bending	Other Movement
Elbow	No	No	Yes	Up-down movement
Shoulder	Yes	Yes	Yes	Side-raising, all directions
Knee	No	No	Yes	Up-down movement
Neck	No	Yes	Yes	Turning (left/right), up-down
Fingers	No	No	Yes	Gripping, minor side movement
Toes	No	No	Yes	Minor up-down
Wrist	No	Yes	Yes	Side-to-side, up-down

Ready to Go Beyond: Stem Cells

• Stem cells in the bone marrow can divide and make new cells.
• In bone marrow transplant, stem cells from a healthy person are given to patients with blood cancers like Leukemia or disorders such as Thalassemia.

TYPES OF JOINTS

Ball and Socket Joint

• This allows forward, backwards, sideways and circular movements.
• E.g., shoulder joint allows movement of the arm. In this, rounded top of the upper arm bone fits into a shallow hollow of the shoulder bone.
• Together with the collarbone, the shoulder forms the shoulder girdle. It connects the arm to the skeleton.

Hinge Joint

• Allows movement in only one direction, similar to a door hinge.
• Examples: Elbow joint and knee joint.
• The knee joint is protected by the kneecap.

Types of joints

Pivot Joint

• Allows rotational movement, such as shaking the head "no" (like a doorknob turning).
• The skull connects to the backbone through a pivot joint.

Fixed Joints

• Joints that do not allow movement.
• Example: Bones of the skull, which form a hard protective case for the brain, eyes, and ears.

Pause and Ponder

Look at the picture given carefully and observe the various poses of classical and folk dances of India. Can you identify which joints are involved? Also, what type of movement each joint allows

Answer

• Shoulder & Hip (Ball & Socket): Enables wide-angle arm/leg movements and complete circles.
• Elbow & Knee (Hinge): Allows rhythmic folding of arms and bending of legs.
• Neck (Pivot): Facilitates head rotation and tilting to complement hand gestures.
• Wrist & Ankle (Gliding): Provides the flexibility required for intricate gestures and precise footwork.

SKELETAL SYSTEM

• It 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 extends the flexible vertebral column (backbone), made of vertebrae (series of small bones). It supports the body and helps stand upright.
• Between each vertebra is a cartilage disc. It acts as a cushion and gives flexibility to bend and twist without injuring the spinal cord.
• The bones under the chest are ribs. 12 pairs of ribs form rib cage. It protects vital organs like heart and lungs.
• Ribs are attached to the spine at the back and to the breast bone (sternum) in the front. They are joined by flexible cartilage so that the rib cage can expand and contract during breathing. This changes space in the chest, allowing air to move in and out of the lungs.
• Injury to ribs makes breathing painful and difficult.

Bridging Science and Society: Yoga

• Yoga integrates physical postures, breathing, and meditation.
• Benefits: Improves flexibility, posture, and breathing; reduces stress; helps prevent lifestyle-related diseases.
• International Yoga Day: Celebrated on June 21st.
• Regular practice maintains strong bones, fit muscles, and flexible joints, protecting the body from stiffness.

Think as a Scientist

From one cell to an organism: Totipotency

• Ability of some mature plant cells to form a whole plant is called totipotency. Such cells are totipotent cells. This is similar to the ability of a zygote.
• In 1958, F. C. Steward showed that single carrot phloem cells can regenerate into whole plants.
• He grew carrot phloem cells in a nutrient medium with sugars and hormones. The cells divided and later differentiated into complete plants.
• It shows that phloem cells first dedifferentiated (regain the ability to divide) to form an undifferentiated mass of cells. Under suitable conditions, they divided and redifferentiated into roots, shoots, and a whole plant.

• In the experiment, he obtained the following results.

Effect of light, air and nutrient medium on growth of the cultured plant cells

Conditions		Nutrient Medium	Weight Increase (mg)
Light	Air	Composition	Result
✓	X	Solid + nutrients	Reduced
✓	✓	Liquid + nutrients	20% increase
X	✓	Liquid + nutrients	Reduced

1. What do you conclude about the characteristics of phloem cells of carrot?
   Answer: They are totipotent, i.e., they can form a complete new plant.
2. In which of the three combinations would you obtain the highest and lowest biomass? What could be the possible reason(s) for this observation?
   Answer:

   • Highest: Light + Air + Liquid medium + nutrients.
     Reason: Oxygen is available for respiration. Liquid medium allows for better nutrient absorption.
   • Lowest: Combinations lacking Air or Light.
     Reason: Without oxygen (Air), cells cannot produce energy for growth. Without light, growth and metabolic pathways are hindered.
3. Will you get the same results if you culture animal cells instead of carrot cells?
   Answer: No. Most mature animal cells cannot naturally revert to an undifferentiated state.
4. Think and mention any two commercial applications of the study above.
   Answer:

   • Micropropagation: Mass production of identical, disease-free plants quickly.
   • Conservation: Multiplying endangered plant species from a few healthy cells.

Bridging Science and Society

• Agrobacterium tumefaciens is a bacterium that causes crown gall disease in plants.
• In this, tumour-like swellings develop on the stems due to rapid and uncontrolled cell division.
• Scientists studied how this bacterium transfers genes into plant cells. This led to development in plant tissue culture and genetic engineering.
• Today, Agrobacterium is used to introduce useful genes into plants to produce phytochemicals, improved crops and disease-resistant varieties.