Tissues

Definition: A tissue is a group of cells that are similar in structure, origin, and function, and work together to perform a specific function in the body.

Example: Muscle tissue is made up of muscle cells that work together to bring about movement.

Given: Xylem is a complex permanent tissue in plants.

Answer: Xylem tissue is made up of four types of elements:
1. Tracheids – elongated cells with tapering ends that help in water conduction.
2. Vessels (Tracheae) – tube-like structures formed by a row of cells placed end to end; main water-conducting elements.
3. Xylem parenchyma – living cells that store food and help in lateral conduction of water.
4. Xylem fibres (Sclerenchymatous fibres) – dead cells that provide mechanical support.

Conclusion: Together, these four elements form the xylem tissue.

| Feature | Simple Tissues | Complex Tissues |
|---|---|---|
| Composition | Made up of only one type of cell. | Made up of more than one type of cell. |
| Function | Perform a single, uniform function. | Perform multiple functions together. |
| Examples | Parenchyma, Collenchyma, Sclerenchyma | Xylem, Phloem |
| Location | Found in pith, cortex, epidermis, etc. | Found in vascular bundles. |

Conclusion: Simple tissues are homogeneous (one cell type), while complex tissues are heterogeneous (multiple cell types) and perform more specialised functions like transport of water and food.

| Feature | Parenchyma | Collenchyma | Sclerenchyma |
|---|---|---|---|
| Cell wall composition | Thin cell wall made of cellulose. | Cell wall is unevenly thickened at corners; made of cellulose and pectin. | Cell wall is uniformly thick and heavily lignified (lignin deposits). |
| Nature of cell wall | Thin and uniform. | Thick at corners, thin elsewhere. | Very thick and hard throughout. |
| Living/Dead | Living cells. | Living cells. | Dead cells (at maturity). |

Conclusion: The degree of cell wall thickening increases from parenchyma → collenchyma → sclerenchyma, with sclerenchyma having the thickest, lignified walls.

Stomata are tiny pores present on the surface of leaves (epidermis). Their functions are:

1. Gaseous exchange: They allow the exchange of gases ($\text{CO}_2$ and $\text{O}_2$) between the plant and the atmosphere, which is essential for photosynthesis and respiration.
2. Transpiration: They allow the loss of excess water in the form of water vapour (transpiration), which helps in the upward movement of water and minerals from roots to leaves.
3. Regulation: Guard cells surrounding the stomata regulate the opening and closing of the stomatal pore, thus controlling water loss.

Note: Diagrams are described in detail below as required for board exams.

(a) Striated (Skeletal/Voluntary) Muscle Fibre:
- Draw a long, cylindrical, unbranched fibre.
- Show alternating dark (A-band) and light (I-band) bands → giving a striated appearance.
- Show multiple nuclei (multinucleate) at the periphery of the cell.
- Label: Striations, Nucleus, Muscle fibre.

(b) Unstriated (Smooth/Involuntary) Muscle Fibre:
- Draw a long, spindle-shaped (narrow at both ends) cell.
- No striations (smooth appearance).
- Show a single, centrally placed nucleus.
- Label: Spindle-shaped cell, Nucleus (central).

(c) Cardiac Muscle Fibre:
- Draw cylindrical, branched fibres.
- Show faint striations.
- Show a single, centrally placed nucleus.
- Show intercalated discs at junctions between cells.
- Label: Branching, Intercalated disc, Nucleus, Striations.

$$\text{Striated: Long, cylindrical, multinucleate, striated}$$
$$\text{Unstriated: Spindle-shaped, uninucleate, no striations}$$
$$\text{Cardiac: Cylindrical, branched, uninucleate, faint striations, intercalated discs}$$

Specific function of cardiac muscle:

Cardiac muscle is found exclusively in the walls of the heart. Its specific function is to contract and relax rhythmically and continuously throughout life (without fatigue) to pump blood to all parts of the body.

- It works involuntarily (not under conscious control).
- It contracts approximately 72 times per minute in a normal adult.
- The rhythmic contraction ensures a continuous supply of oxygenated blood to all organs.

| Feature | Striated Muscle | Unstriated Muscle | Cardiac Muscle |
|---|---|---|---|
| Shape | Long, cylindrical, unbranched fibres | Long, spindle-shaped (narrow at ends), unbranched | Cylindrical, branched fibres |
| Striations | Distinct alternating dark and light bands (striated) | No striations (smooth) | Faint striations present |
| Nucleus | Multiple nuclei (multinucleate), peripheral | Single nucleus, centrally placed | Single nucleus, centrally placed |
| Control | Voluntary (under conscious control) | Involuntary (not under conscious control) | Involuntary |
| Fatigue | Fatigues easily | Does not fatigue easily | Never fatigues |
| Intercalated discs | Absent | Absent | Present (at junctions between cells) |
| Location/Site | Attached to bones (limbs, tongue, etc.) | Walls of hollow organs — stomach, intestine, urinary bladder, blood vessels | Walls of the heart only |

Labelled Diagram of a Neuron:

Draw the following structure and label each part:

1. Cell body (Cyton/Soma): Large, roughly star-shaped structure containing the nucleus and cytoplasm.
2. Nucleus: Large nucleus inside the cell body.
3. Dendrites: Short, branched projections arising from the cell body — receive nerve impulses.
4. Axon: A single, long fibre arising from the cell body — conducts impulses away from the cell body.
5. Myelin sheath: Fatty covering around the axon (in myelinated neurons).
6. Nodes of Ranvier: Gaps in the myelin sheath at regular intervals.
7. Axon terminals (Synaptic knobs): Branched endings of the axon that pass impulses to the next neuron or effector organ.

$$\boxed{\text{Cell body} \xrightarrow{\text{Axon}} \text{Axon terminals (synapse)}}$$

*Key point:* The dendrites receive impulses and the axon transmits them. A neuron is the structural and functional unit of nervous tissue.

(a) Tissue that forms the inner lining of our mouth:
Squamous epithelial tissue (Simple squamous epithelium). The cells are flat, tile-like, and form a smooth lining.

(b) Tissue that connects muscle to bone in humans:
Tendon — a type of dense connective tissue made of white fibrous collagen fibres. It is strong but inelastic.

(c) Tissue that transports food in plants:
Phloem — a complex permanent tissue that transports food (sucrose and other organic solutes) from leaves to other parts of the plant.

(d) Tissue that stores fat in our body:
Adipose tissue — a type of loose connective tissue in which cells (adipocytes) are filled with fat globules. Found below the skin and around organs.

(e) Connective tissue with a fluid matrix:
Blood — the matrix of blood is called plasma (a liquid/fluid matrix) in which blood cells (RBCs, WBCs, platelets) are suspended.

(f) Tissue present in the brain:
Nervous tissue (Neural tissue) — made up of neurons and neuroglia. The brain is entirely composed of nervous tissue.

| Structure | Type of Tissue |
|---|---|
| Skin | Stratified squamous epithelial tissue (Epithelial tissue) |
| Bark of tree | Cork (Protective tissue) — dead cells with suberised walls; formed by cork cambium |
| Bone | Skeletal connective tissue — matrix is hard and rigid due to calcium and phosphorus salts |
| Lining of kidney tubule | Cuboidal epithelial tissue — single layer of cube-shaped cells that help in absorption and secretion |
| Vascular bundle | Complex permanent tissue — consists of Xylem (water conduction) and Phloem (food conduction) |

Parenchyma is the most common and widely distributed simple permanent tissue in plants. It is present in the following regions:

1. Epidermis — forms the outer protective layer of young stems and leaves.
2. Cortex — the region between epidermis and vascular bundle in stems and roots.
3. Pith (Medulla) — the central region of the stem.
4. Mesophyll of leaves — the green parenchyma (chlorenchyma) that performs photosynthesis.
5. Pericycle — the outermost layer of the stele.
6. Pulp of fruits — stores food and water.
7. Endosperm of seeds — stores food for the embryo.

Conclusion: Parenchyma is found in almost all soft parts of the plant — leaves, stems, roots, fruits, and seeds.

The epidermis is the outermost single layer of cells covering the entire plant body. Its roles are:

1. Protection: It protects the inner tissues from mechanical injury, desiccation (drying out), and infection by pathogens.
2. Regulation of water loss: The epidermis of aerial parts is covered with a waxy layer called cuticle (made of cutin) that prevents excessive water loss.
3. Gaseous exchange and transpiration: Stomata present in the epidermis of leaves allow exchange of gases ($\text{CO}_2$, $\text{O}_2$) and transpiration.
4. Absorption of water and minerals: In roots, the epidermis has root hair cells (unicellular extensions) that greatly increase the surface area for absorption of water and dissolved minerals from the soil.
5. Secretion: In some plants, epidermal cells form trichomes (hair-like structures) that may secrete oils or resins.

Cork (also called phellem) is a secondary protective tissue formed by the cork cambium in older stems and roots of plants. It acts as a protective tissue in the following ways:

1. Impermeable to water and gases: Cork cells are dead at maturity and their walls are heavily impregnated with suberin (a waxy substance), making them impermeable to water and gases. This prevents water loss from the plant.

2. Protection from mechanical injury: The multiple layers of tightly packed dead cork cells form a hard, thick covering that protects the inner living tissues from mechanical damage.

3. Protection from pathogens and insects: The tough, impermeable cork layer prevents the entry of disease-causing microorganisms and insects.

4. Insulation: Cork provides thermal insulation to the plant, protecting it from extreme temperatures.

5. Replaces epidermis: As the plant grows in girth, the epidermis is replaced by cork, which is a more durable protective covering.

Conclusion: Because cork cells are dead, compactly arranged, and suberised, they form an excellent protective covering for older plant parts.

Note: The image of the chart is not visible, but based on the standard NCERT chart for Class 9 Chapter — Tissues, the completed chart is as follows:

$$\textbf{TISSUES}$$

$$\downarrow \qquad\qquad\qquad\qquad \downarrow$$

$$\textbf{Plant Tissues} \qquad\qquad \textbf{Animal Tissues}$$

Plant Tissues:
$$\downarrow \qquad\qquad \downarrow$$
$$\textbf{Meristematic Tissues} \qquad \textbf{Permanent Tissues}$$

- Meristematic Tissues (based on location):
- Apical meristem
- Lateral meristem
- Intercalary meristem

- Permanent Tissues:
- Simple Permanent Tissues:
- Parenchyma
- Collenchyma
- Sclerenchyma
- Complex Permanent Tissues:
- Xylem (Tracheids, Vessels, Xylem parenchyma, Xylem fibres)
- Phloem (Sieve tubes, Companion cells, Phloem parenchyma, Phloem fibres)
- Protective Tissues:
- Epidermis
- Cork

Animal Tissues:
- Epithelial Tissue (Squamous, Cuboidal, Columnar, Ciliated, Glandular)
- Connective Tissue (Areolar, Adipose, Bone, Cartilage, Tendon, Ligament, Blood)
- Muscular Tissue (Striated/Skeletal, Unstriated/Smooth, Cardiac)
- Nervous Tissue (Neurons + Neuroglia)