Tissues in Action — Class 9 Science

Tissues in Action

• 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 and function group together to form a tissue → more than one type of tissue forms an organ → different organs form organ systems → 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 division of labour, which increases efficiency and enables complex life processes.
• Example: in animals, muscle tissue enables movement and nervous tissue carries messages; in plants, xylem transports water and minerals, while phloem transports food.

3.1 Why are Plant and Animal Tissues Different?

• 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 helps make their bodies suitable for locomotion.
• Mode of nutrition also differs: animals have tissues to digest food from different sources, while plants have tissues that utilise solar energy to synthesise food through photosynthesis.
• Plants and animals have distinct tissues for transporting food and water to different parts of the body.
• Growth patterns in plants and animals vary because the tissues responsible for growth differ in structure and function.

3.2 Tissues for Growth in Plants

• A small seedling grows into a tall tree, roots grow deep into the soil, stems become thicker with time, and grass grows again after being eaten by grazing animals.
• 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 requires actively dividing cells that together form a tissue called meristematic tissue.

3.2.1 Apical meristem — How do plants grow in length?

Fig 3.3: Location of apical meristem in a sapling

• This observation confirms that root tips contain actively dividing cells; shoot tips also contain actively dividing cells that help shoots grow in length.
• Plants have growth zones at the tips of their roots and shoots, called apical meristems, which help plants grow in length.

3.2.2 Lateral meristem — How do plants grow in girth?

• The stems of dicot plants not only grow in length but also increase in diameter or girth over time.
• The cut trunk of a tree shows several ring-like patterns called annual growth rings.
• By counting these annual rings, scientists can estimate the age of a tree and understand the climatic conditions under which it grew.

Fig 3.4: T.S. of a tree trunk showing annual growth rings

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

3.2.3 Intercalary meristem — How do plants grow after being cut?

• If the tip of a young stem is cut, the stem stops growing in length, but new branches arise from the nodes of the stem.
• The intercalary meristem is located at the base of the internode or just above the node.
• The node is the point on a plant stem where branches or leaves arise.
• The internode is the part of the stem between two nodes.

Fig 3.5: New branches arising from the node of a stem

Fig 3.6: Lawn mowing

• When a garden hedge is cut, more branches appear after some time, giving the hedge a bushy appearance.
• Grass also reappears after being mowed and/or grazed by animals, because of the intercalary meristem present at the nodes of its stem.

Plants have three types of meristematic tissues:

• Apical meristem — at root and shoot tips; increases length
• Lateral meristem — along the circumference of stems; increases girth
• Intercalary meristem — at the base of certain plants such as grasses; helps regeneration after cutting

Characteristics of meristematic cells:

• Small, with thin cell walls
• Large and prominent nucleus
• Dense cytoplasm with many organelles
• Vacuoles generally absent
• Cells tightly packed with little or no intercellular space

• Due to continuous cell division, meristematic tissue adds new cells to the plant body.
• Some newly formed cells remain meristematic; others lose the ability to divide and undergo changes in structure and function to become permanent tissues.
• This process — meristematic tissue becoming specialised to perform specific functions — is called differentiation.

3.2.4 Permanent Tissues

• A T.S. of a sunflower stem under a microscope shows different groups of cells, each specialised to perform a specific function — these are permanent tissues.
• Permanent tissues can be:
  • Simple — composed of only one type of cell
  • Complex — composed of more than one type of cell

Fig 3.7: Internal structure of a sunflower stem showing epidermis, cuticle, collenchyma, parenchyma, sclerenchyma, phloem, lateral meristem and xylem

(i) Protective tissue — Epidermis

• The epidermis forms the outermost layer of the plant body — a tightly packed, single layer of flat and rectangular cells.
• It protects all parts of the plant; cells are covered with a waxy layer of cutin called cuticle.
• In plants living in dry habitats, the epidermis may have a thick cuticle layer to reduce water loss during transpiration.
• Hair-like projections arise from epidermal cells:
  • In roots — called root hair, which increase surface area for absorption of water and minerals
  • In leaves — the epidermis contains pores called stomata, which help in gaseous exchange and transpiration
• Transpiration helps water transportation by creating a transpiration pull in xylem, and helps eliminate wastes from the plant body.

(ii) Supporting tissue — Simple permanent tissues

There are three types of simple permanent tissues — parenchyma, collenchyma and sclerenchyma.

Fig 3.8a: Parenchyma — thin walls

Fig 3.8b: Collenchyma — thick walls

Fig 3.8c: Sclerenchyma — thick lignified walls

• a. Parenchyma:
  • Living cells with thin walls, loosely packed with intercellular spaces
  • Mainly stores food; also performs photosynthesis in green parts of plants
  • In aquatic plants, specialised parenchyma forms air spaces that help plants float
• b. Collenchyma:
  • Living cells with unevenly thickened corners due to pectin deposition (gives flexibility like rubber)
  • Provides support and flexibility — allows stems and tendrils to bend without breaking
• c. Sclerenchyma:
  • Thick walls due to lignin deposition — hard and strong (forms woody structure)
  • Most cells are dead
  • Found in stems, leaf veins, and hard coverings of seeds/nuts (e.g., coconut husk, walnut shell)

(iii) Conducting tissues — Complex permanent tissues

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

Fig 3.9a: Xylem — tracheid, vessel, xylem fibre, xylem parenchyma

Fig 3.9b: Phloem — sieve tube, sieve pore, companion cell, phloem parenchyma

• Xylem:
  • Transports water and minerals from roots to other parts of the plant; provides strength
  • Consists of tracheids, vessels, xylem parenchyma and xylem fibres
  • Tracheids and vessels are tubular and thick-walled
  • Xylem parenchyma is the only living component; tracheids, vessels and xylem fibres are primarily sclerenchymatous
• Phloem:
  • Mostly made up of living cells; consists of sieve tubes, companion cells, phloem parenchyma and phloem fibres
  • Sieve tubes transport food from leaves to other parts of the plant
  • Companion cells regulate the cellular functions of sieve tube cells, monitoring loading/unloading of sugars
  • Phloem parenchyma stores food materials, resin, tannins and latex
  • Phloem fibres support sieve tubes and provide strength

Plant tissues are organised into three tissue systems:

1. Dermal tissue system: forms the outer covering of the plant; protects inner parts and reduces water loss
2. Ground tissue system: forms the main body of a plant between dermal and conducting tissues; includes parenchyma, collenchyma and sclerenchyma
3. Vascular tissue system: consists of conducting tissues — xylem and phloem

Fig 3.10: Tissue systems in plants — whole plant view

Fig 3.10: T.S. of leaf, stem and root showing tissue systems

3.3 Animal Tissues

• Like plants, animal cells also group together and specialise in performing different functions — these groups form animal tissues.
• Animal tissues are mainly of four types: epithelial, connective, muscular and nervous.

3.3.1 Epithelial tissues — Structure and functions

• Epithelial tissue forms the outer covering of the body (skin) and also lines internal organs such as the mouth, lungs, blood vessels and intestine.
• Composed of closely packed cells with very little space between them.
• Functions: prevents entry of germs, reduces water loss, helps in absorption, secretion and movement of substances.

Fig 3.11: Locations of different epithelial tissues in the body

(a) Exchange — thin flat cells

(b) Protection — many layers

(c) Secretion — cuboidal/columnar

(d) Sensory — receptor cells with cilia

(e) Absorption — tall pillar-like cells

3.3.2 How are various parts connected in our body?

• Blood connects different parts of the body by transporting nutrients, gases, hormones, etc.
• 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 bone are connective tissues, but differ in composition — blood is fluid, bone is hard — due to differences in their matrix:
  • Blood: matrix (plasma) is watery, soft, jelly-like
  • Bone: matrix is hard, solid, rigid

Plasma (55%) & formed elements (45%)

RBCs

WBC

Platelets

Fig 3.12a: Components of blood

Fig 3.12b: Types of bones

Fig 3.12c: Tendon, cartilage, ligament at a joint

• Bones have a rigid matrix containing calcium and phosphorus compounds, giving strength and rigidity.
• Cartilage has a soft, jelly-like matrix, providing flexibility and cushioning.
• Tendons connect muscles to bones; ligaments connect bones to bones and prevent excessive movement.

3.3.3 Can we control movement in our body?

• Voluntary movements (running, writing, lifting) are under conscious control, carried out by skeletal muscles attached to the skeleton:
  • Made up of bundles of long, cylindrical cells called muscle fibres
  • Unbranched, multinucleate (many nuclei) and striated (light/dark bands)

Skeletal muscle (arm)

Skeletal muscle fibres (striated)

• Involuntary movements occur automatically without conscious control (e.g., movement of food in the intestine, beating of the heart).
• Smooth muscles — found in organs like the stomach and intestines:
  • Spindle-shaped cells, single nucleus, lack striations
  • Help in slow, continuous movements like digestion

Smooth muscle

Found in stomach & intestines

• Cardiac muscles — found only in the heart:
  • Fibres are cylindrical and branched, with a single nucleus, and faint striations
  • Work tirelessly and rhythmically, enabling the heart to beat throughout life without fatigue

Cardiac muscle (branched)

Found only in the heart

3.3.4 How does the body sense, communicate and respond?

• Quick reactions (like pulling your hand from something hot) and memory are controlled by nervous tissue, the body’s control and coordination network.
• The brain acts as the control centre, coordinating activities, memory and responses across the body.
• The cells of nervous tissue are called neurons or nerve cells, specialised to receive, process and transmit messages.
• Each neuron has three main parts:
  • Cell body — contains the nucleus and controls cell activities
  • Dendrites — receive signals from other neurons
  • Axon — a long fibre that carries messages from the cell and ends at axon terminals

Fig 3.14: Structure of a neuron — cell body, dendrites, axon, axon terminals

3.4 The Musculoskeletal System

• The musculoskeletal system is made up of bones, muscles, joints, cartilage, tendons and ligaments.
• Helps us stand upright, move, maintain posture and protect delicate organs.
• Functions under the control of the nervous system.
• Muscles pull on bones to produce movement; 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.
• The adult human skeleton makes up about 12–15 per cent of body weight on average, varying with age, gender and body composition.

Fig 3.15: Musculoskeletal system in human body

3.4.1 The musculoskeletal system in action

3.5 Types of Joints

Fig 3.16: Types of joints in the human skeleton

3.5.1 Ball and socket joint

• The shoulder joint allows free movement of the arm — 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, connecting the arm to the skeleton.
• Allows forward, backward, sideways and circular movements.

3.5.2 Hinge joint

• The elbow bends and straightens in one direction only, like a door hinge — this is called a hinge joint.
• A similar hinge joint is present in the knee, where the kneecap protects the joint.

3.5.3 Pivot joint

• Shaking your head ‘no’ shows the gentle movement at the back of the neck.
• The skull is connected to the backbone through a pivot joint, allowing the head to move side to side like a doorknob turning in its socket.

3.5.4 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 — the bones cannot move, keeping the brain safe even when the body moves.

Fig 3.17: Poses of classical and folk dances of India — observe which joints and movements are involved

3.6 Skeletal System

• The skeletal system consists of a framework of bones that provides strength and protects delicate internal organs — includes the skull, vertebral column and rib cage.
• The backbone or vertebral column (spine) extends from the base of the skull, made up of a series of small bones called vertebrae.
• Between each vertebra is a cartilage disc, acting as a cushion and allowing flexibility to bend and twist without injuring the spinal cord.
• You have 12 pairs of ribs forming the rib cage, which protects vital organs such as the heart and lungs.
• Ribs are attached to the spine at the back and to the breast bone (sternum) in the front, joined by flexible cartilage.
• This flexibility allows the rib cage to expand and contract during breathing, changing chest space to move air in and out of the lungs.

🌍 Bridging Science and Society

• Plant pathologists have observed a disease in plants called crown gall disease — tumour-like swellings develop on stems due to rapid, uncontrolled cell division, caused by a bacterium called Agrobacterium tumefaciens.

Fig 3.20: Crown gall disease

• Scientists studied how this bacterium transfers genetic material into plant cells — this knowledge was later used in plant tissue culture and genetic engineering.
• Today, Agrobacterium is used as a tool to introduce useful genes into plants, producing valuable phytochemicals, improved crops, and disease-resistant varieties.

🔬 Think as a Scientist — From one cell to an organism: Totipotency

• In 1958, F. C. Steward demonstrated that even single cells from vascular phloem of carrot retain the ability to regenerate whole plants — the first person to do so!
• He grew phloem cells in a nutrient medium containing simple sugars and hormones; these cells divided to form a mass of cells, which gradually divided and differentiated into a complete plant.
• Cells of phloem first dedifferentiated (regained the ability to divide) to form an undifferentiated mass.
• These, under appropriate conditions, further divided and redifferentiated to form roots, shoot, and eventually the complete plant.
• This ability of mature plant cells to undifferentiate, divide and redifferentiate into a new plant is known as totipotency; such cells are called totipotent cells — similar to a zygote’s ability to develop into an entire organism.

Revise, Reflect, Refine

Exercise

4. You can perform a straight-leg jump (knees and ankles stiff) and a normal jump (knees and ankles bend naturally). How did your ankle, knee and hip positions differ between the two jumps?

Show Answer

In the straight-leg jump, the ankle, knee and hip joints remain stiff and barely bend, so the force of landing is transmitted directly through the rigid limb, creating more impact stress. In the normal jump, the ankle, knee and hip joints flex naturally, absorbing and distributing the impact force gradually, making the landing smoother and reducing stress on the joints.

Exercise

8. An elephant severely debarked a tree to access nutrients. (i) Which function(s) of the tree is/are hampered by debarking? (ii) Which plant tissue would be affected by further damage even after debarking? (iii) Which function would be hampered if tissues beneath the bark were severely damaged? (iv) What assumptions are you making?

Show Answer

(i) Protection (loss of epidermis/bark) and transport of food (if phloem just beneath the bark is removed) are hampered. (ii) Further damage would affect the vascular tissues (xylem and phloem) beneath the bark. (iii) If tissues beneath the bark (phloem, cambium) are severely damaged, the transport of food (and possibly water if xylem is reached) would be hampered, potentially killing the tree. (iv) Assumption: the debarking did not go deep enough to damage the xylem/phloem directly — if it did, water transport would also be affected, making the damage more severe.

Exercise

10. Sohan used two types of sugarcane cuttings, type A and type B. Type B sprouted into plants; type A did not. (i) Why could type B grow but not type A? (ii) What difference was present? (iii) What observation determined the effect? (iv) What parameters should be kept the same for a fair comparison?

Type A — no node

Type B — with node

Show Answer

(i) Type B cuttings could grow because they contained a node, which has meristematic tissue capable of producing new shoots/roots; Type A lacked a node, so it had no actively dividing cells to regenerate. (ii) Type B had a visible node/bud; Type A was a section without any node. (iii) Sprouting (or failure to sprout) after a few weeks was the key observation. (iv) Same cutting length, same soil/growing conditions, same water and light exposure, and same age/health of the parent plant should be kept constant for a fair comparison.

🚀 The Journey Beyond

• Visit a doctor and find out what happens in ligament rupture, cartilage rupture and fracture of bones. How can we reduce the risk by changing our lifestyle and nutritional balance?
• Sit with feet flat on the floor, place fingers on the back of your ankle just above the heel, point toes down and up — feel the tendon moving. Explore other tendons around different joints.
• Reflect on physical practices like yoga or kabaddi — how would they support bone and muscle health?
• Reflect on gardening methods like pruning, grafting, irrigation or crop rotation — how does each support healthy functioning of plant tissues like meristems, conducting tissues or supporting tissues?
• Study various dance forms of tribal communities across the country. Observe the joint movements involved and develop a dance or drama on the concept of joint movements.

Fig 3.24 (ankle tendon) & Fig 3.25 (tribal dance poses)

The Quest Continues… Will it be possible to obtain a complete animal from an animal cell like plants? If yes, what would be the advantages and challenges of this development?

Take the Quiz

Test what you’ve learned with this 25-question quiz covering the entire chapter: