Cell: The Building Block of Life — CBSE Class 9 Science Important Questions

Answer: (c) Mitochondrion.

Mitochondria carry out cellular respiration, releasing energy stored in food as ATP, so they are called the powerhouse of the cell.

Answer: (b) Cell wall.

A rigid cell wall made of cellulose surrounds the plant cell membrane and gives shape and support. Animal cells have only a flexible cell membrane.

Answer: (c) DNA lying free in the cytoplasm.

Prokaryotes (e.g. bacteria) have no true nucleus; their genetic material lies free in the cytoplasm in a region called the nucleoid, and they lack membrane-bound organelles.

Answer: (b) Out of the cell by osmosis.

The salt solution is hypertonic (more concentrated than the cell's contents). Water moves from the region of higher water concentration (inside the cell) to lower (outside) across the membrane, so the cell loses water and shrinks (plasmolysis in plant cells).

Answer: (a) Both are true and R explains A. The compartments in cork reminded Hooke of small rooms, so he called them cells. (They were the walls of dead cells; Leeuwenhoek later saw living cells.)

The cell theory states that:

1. All living organisms are composed of one or more cells.
2. The cell is the basic structural and functional unit of life.
3. All cells arise from pre-existing cells (given by Virchow).

1. The cell wall is a rigid, non-living outer covering made of cellulose (in plants) and is found only in plant cells, bacteria and fungi; the cell membrane is a thin, living, flexible layer present in all cells.

2. The cell wall is fully permeable, letting most substances pass, whereas the cell membrane is selectively permeable, controlling what enters and leaves the cell.

Cell wall: Plant cells have a rigid cellulose wall giving fixed shape; animal cells have only a membrane and so an irregular, flexible shape.

Chloroplasts: Plant cells contain chloroplasts for photosynthesis; animal cells cannot make their own food.

Vacuole: A plant cell has a single large central vacuole for turgidity; animal cells have small, temporary vacuoles if any.

Structure: The nucleus is a large, usually spherical organelle bounded by a double nuclear membrane with tiny nuclear pores that allow exchange of materials with the cytoplasm. Inside lies the nucleoplasm containing a dense nucleolus and thread-like chromatin, which condenses into chromosomes carrying DNA during cell division.

Functions:

1. It acts as the control centre, directing all cell activities.
2. It stores the hereditary information (DNA/genes) and passes it to daughter cells.
3. The nucleolus helps make ribosomes.

Functions of four organelles:

1. Nucleus — the control centre; stores DNA and directs all cell activities including reproduction.
2. Mitochondria — the powerhouse; carry out respiration to release energy as ATP.
3. Endoplasmic reticulum (ER) — a network of membranes that transports materials; rough ER helps make proteins, smooth ER makes lipids.
4. Lysosomes — contain digestive enzymes that break down worn-out parts and foreign material ('suicide bags').

(The cell membrane additionally controls the entry and exit of substances.)

(a) Cells divide so that an organism can grow, repair damaged tissues and replace worn-out cells, and reproduce (gametes are formed by division).

(b)

(c) Cancer is a disease in which some cells keep dividing without control, forming a mass of cells (a tumour) that can spread to other parts of the body. Normally cell division is tightly regulated; in cancer this control is lost.

(i) The process is osmosis — the movement of water across a selectively permeable membrane from a region of higher water concentration to lower.

(ii) Plain water is more dilute (higher water concentration) than the raisin's contents, so water moves into the raisin cells by osmosis. The cells gain water, become turgid, and the raisin swells and softens.

(iii) Strong sugar syrup is hypertonic (lower water concentration than the raisin), so water moves out of the raisin cells into the syrup. Losing water, the cells shrink and the raisin shrivels further.

(iv) This is possible because the cell membrane is selectively (semi-) permeable — it lets water molecules pass through freely while restricting the dissolved sugar, so a net flow of water occurs.