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 A and R are true and R is the correct explanation of A. Lysosomes hold hydrolytic (digestive) enzymes; when a cell is damaged, these may burst out and digest the cell itself, which is exactly why they are called suicide bags.

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.

Both mitochondria and chloroplasts are called semi-autonomous because they contain their own DNA and ribosomes and can make some of their own proteins and divide on their own, somewhat independently of the nucleus. This supports the idea that they were once free-living organisms taken into larger cells.

Key difference in role:

• Chloroplasts trap light energy and use it to make food (glucose) by photosynthesis — they store energy: $6CO_2+6H_2O\rightarrow C_6H_{12}O_6+6O_2$6CO_2+6H_2O→ C_6H_12O_6+6O_2.
• Mitochondria release energy from food during respiration, producing ATP: $C_6H_{12}O_6+6O_2\rightarrow6CO_2+6H_2O+\text{energy}$C_6H_12O_6+6O_2→6CO_2+6H_2O+energy.

So chloroplasts build energy-rich food while mitochondria break it down to power the cell.

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 secreted protein passes through several linked organelles that together form the endomembrane system:

1. Ribosome: Protein synthesis begins on ribosomes, which join amino acids in the order coded by the mRNA. Ribosomes making secreted proteins sit on the rough ER.

2. Rough endoplasmic reticulum: The new protein enters the rough ER, where it is folded and given an initial 'tag'. The ER then pinches off a small vesicle carrying the protein.

3. Golgi apparatus: The vesicle fuses with the Golgi, which modifies, sorts and packages the protein (for example adding sugar groups). This is the cell's 'post office'.

4. Secretory vesicle: The finished protein is packed into a vesicle that moves to the cell surface.

5. Cell membrane: The vesicle fuses with the membrane and releases the protein outside by exocytosis.

This coordinated pathway shows how organelles work as a team to produce and export proteins.

(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.