Tissues in Action — CBSE Class 9 Science Important Questions

Answer: (b) Apical meristem.

Apical meristem is located at the tips of roots and shoots and its division increases the length (primary growth) of the plant. Lateral meristem increases girth.

Answer: (b) Ligament. Ligaments are strong, slightly elastic connective tissue joining bone to bone; overstretching them causes a sprain. Tendons join muscle to bone.

Answer: (b) Are made of more than one type of cell working together.

A complex tissue contains different cell types performing a common function. Xylem has tracheids, vessels, parenchyma and fibres; phloem has sieve tubes, companion cells, parenchyma and fibres.

Answer: (c) Sclerenchyma.

The hard, gritty stone cells (sclereids) and fibres of coconut husk and pear are sclerenchyma — dead cells with thick, lignified walls that provide hardness and mechanical strength.

Answer: (a) Both A and R are true and R is the correct explanation of A. The deposition of lignin thickens the walls so much that the cell cavity is obliterated and the protoplasm dies, which is why mature sclerenchyma cells are dead yet still provide strength.

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

Order: cell $\rightarrow$→ tissue $\rightarrow$→ organ $\rightarrow$→ organ system $\rightarrow$→ organism. Example: muscle cells form muscle tissue, which with other tissues forms the stomach (an organ), part of the digestive system.

Any two:

1. Good posture while sitting, standing and carrying bags, to avoid strain on the back and neck.

2. A balanced diet with calcium, vitamin D and protein for strong bones and muscles.

3. Regular exercise, with a warm-up first to prevent sprains and muscle injuries.

Parenchyma: Cells are living with thin cellulose walls. Main function is storage of food and water, and (as chlorenchyma) photosynthesis; it fills spaces and provides turgidity.

Collenchyma: Living cells with walls irregularly thickened at the corners with cellulose and pectin. Main function is to give flexibility and mechanical support, allowing bending without breaking (e.g. in leaf stalks).

Sclerenchyma: Dead cells with thick, lignified walls. Main function is to give rigidity, hardness and mechanical strength (e.g. husk, seed coats).

Striated (skeletal) muscle: Located attached to bones/skeleton. It is voluntary (under our conscious control) and shows light and dark bands (striations); cells are long, cylindrical and multinucleate.

Smooth (unstriated) muscle: Located in the walls of internal organs such as the stomach, intestines and blood vessels. It is involuntary, working without our control; cells are spindle-shaped with a single nucleus and no striations.

Cardiac muscle: Found only in the wall of the heart. It is involuntary but striated, made of branched cells that contract rhythmically and tirelessly throughout life.

Removing a ring of bark takes away the phloem, which is located in the bark, while the xylem in the inner wood remains intact.

• Water and minerals can still travel up from the roots through the xylem, so the leaves keep functioning for a while.
• However, the food (sugars) made in the leaves can no longer be carried down to the roots, because that downward transport needs phloem.

As a result the roots are gradually starved of food. Over time they weaken and die, and once the roots fail the whole tree dies. This practice is called girdling or ringing and confirms that phloem transports food downward while xylem carries water upward.

Meristematic tissue — actively dividing cells that cause growth. Apical increases length at root/shoot tips, lateral increases girth, and intercalary is found at the base of leaves or internodes.

Permanent tissue — cells that have stopped dividing and taken up fixed functions.

• Simple (one cell type): Parenchyma stores food and helps photosynthesis; collenchyma gives flexible support; sclerenchyma gives rigidity and strength.
• Complex (many cell types): Xylem conducts water and minerals upward and supports the plant; phloem transports food from leaves to the rest of the plant.

The four main animal tissues each specialise in a task:

1. Epithelial tissue — Structure: tightly packed cells forming a continuous sheet with little intercellular space, resting on a basement membrane. Location and function: lines the inner surface of the intestine (columnar epithelium) where it protects and helps absorb digested food; skin epithelium protects underlying tissues.

2. Connective tissue — Structure: cells are loosely spaced in an abundant matrix (which may be fluid, jelly-like or solid). Location and function: bone has a hard matrix of calcium salts and forms the framework that supports the body and protects organs; blood, cartilage and adipose tissue are other examples.

3. Muscular tissue — Structure: elongated cells (muscle fibres) containing contractile proteins. Location and function: skeletal muscle attached to bones contracts to produce voluntary movement of body parts.

4. Nervous tissue — Structure: made of neurons, cells with a cell body and long processes (dendrites and axon). Location and function: forms the brain, spinal cord and nerves, and rapidly conducts electrical impulses to coordinate the body's activities.

Together these tissues cover and line, support and connect, move, and control the body — a clear division of labour.

(i) The apical meristem at the tips of the branches divides to increase their length each year.

(ii) The hardness of the inner wood is mainly due to xylem (specifically its lignified xylem fibres and vessels), which forms the tough woody core and also conducts water.

(iii) The food-carrying tissue in the soft outer bark is the phloem, which transports sugars made in the leaves to the rest of the tree.

(iv) The lateral meristem (cambium) divides to add new xylem and phloem, increasing the trunk's girth and producing the annual rings.