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 |