Class 9 Science : NCERT Solutions
The Fundamental Unit of Life

Feature	Plant Cell	Animal Cell
Cell Wall	Present (cellulose)	Absent
Plastids	Present (chloroplasts, etc.)	Absent
Vacuole	Large, central vacuole	Small, temporary vacuoles
Shape	Usually rectangular/fixed	Usually round/irregular
Centrioles	Absent in higher plants	Present
Lysosomes	Rare	Present
Storage	Starch grains	Glycogen granules
Plasmolysis	Occurs in hypertonic solution	Cells shrink/burst (no plasmolysis)

Feature	Prokaryotic Cell	Eukaryotic Cell
Nucleus	Absent (nucleoid region)	Present (membrane-bound)
Size	Small (1-10 μm)	Large (5-100 μm)
Membrane-bound organelles	Absent	Present (mitochondria, ER, etc.)
Chromosome	Single, circular DNA	Multiple, linear chromosomes
Cell division	Binary fission	Mitosis/Meiosis
Ribosomes	70S (smaller)	80S (larger)
Examples	Bacteria, Archaea	Plants, Animals, Fungi

If the plasma membrane ruptures or breaks down:

1. Loss of selective permeability – The cell will lose control over the movement of substances in and out.

2. Cellular contents will leak out – Important molecules like proteins, nucleic acids, and organelles may escape.

3. Entry of harmful substances – Toxic materials from outside can enter freely.

4. Disruption of homeostasis – The internal environment will not be maintained.

5. Cell death – Ultimately, the cell will die because it cannot perform essential functions like respiration, nutrition, and waste removal.

Example: In hypotonic solution, animal cells may burst (lysis) if membrane is damaged.

Without the Golgi apparatus:

1. No packaging and sorting – Proteins and lipids synthesized by ER would not be processed, modified, or packaged.

2. No formation of lysosomes – Waste disposal system of the cell would be absent.

3. No secretion – Important substances like enzymes, hormones, and cell wall materials (in plants) would not be secreted.

4. Disruption of membrane biogenesis – New membrane material would not be formed properly.

5. Accumulation of substances – Unprocessed materials would accumulate in the cell, disrupting metabolism.

6. Cell death – Eventually, the cell would die due to inability to perform essential functions.

Organelle: Mitochondrion

Why called "powerhouse"?
1. ATP production – Mitochondria produce ATP (Adenosine Triphosphate) through cellular respiration.

2. Energy currency – ATP is the energy currency of the cell used for various metabolic activities.

3. Site of Krebs cycle and electron transport chain – Major energy-yielding biochemical pathways occur here.

4. Double membrane structure – Inner membrane has folds (cristae) that increase surface area for ATP production.

Analogy: Like a power plant that generates electricity for a city, mitochondria generate energy (ATP) for the cell.

Lipids: Synthesized in the Smooth Endoplasmic Reticulum (SER).

Proteins: Synthesized by ribosomes attached to the Rough Endoplasmic Reticulum (RER).

Process:
1. Proteins are made by ribosomes on RER and enter the ER lumen for processing.
2. Lipids are synthesized in SER.
3. Both are transported to the Golgi apparatus for modification, packaging, and distribution.
4. Finally, they are used in membrane biogenesis to form new plasma membrane or organelle membranes.

Amoeba obtains food through endocytosis, specifically phagocytosis ("cell eating"):

Steps:
1. Detection – Amoeba senses food particles (like bacteria, algae) using pseudopodia.

2. Engulfment – Pseudopodia extend and surround the food particle, forming a food vacuole.

3. Internalization – The food vacuole is engulfed into the cytoplasm.

4. Digestion – Lysosomes fuse with the food vacuole, releasing digestive enzymes to break down food.

5. Absorption – Nutrients are absorbed into the cytoplasm.

6. Egestion – Undigested material is expelled out by exocytosis.

Key feature: Flexibility of plasma membrane enables this process.

Definition: Osmosis is the movement of water molecules from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution) through a semi-permeable membrane.

Key points:
1. Involves only solvent (water) movement, not solute.

2. Requires a semi-permeable membrane that allows water but not solutes to pass.

3. Driven by concentration gradient of water.

Examples:
• Absorption of water by plant roots.
• Swelling of raisins in water.
• Preservation of food in concentrated sugar/salt solutions (prevents microbial growth).

Setup:
• Potato acts as semi-permeable membrane (living cells).
• Water moves by osmosis.

(i) Water gathers in B and C because:
Sugar/salt in the hollow creates a hypertonic solution inside the cup. Water from potato cells (higher water concentration) moves into the hollow by osmosis, accumulating there.

(ii) Potato A (empty) is the control:
It shows that without solute, no osmosis occurs. Helps compare and confirm that water movement in B and C is due to solute concentration, not other factors.

(iii) Water does not gather in:
• A (empty): No concentration gradient – both sides have similar water concentration.
• D (boiled potato with sugar): Boiling kills cells, destroying the semi-permeable membrane. No osmosis occurs even with sugar inside.

Function	Type of Cell Division	Key Features
Growth and repair	Mitosis	• Produces two identical daughter cells • Maintains chromosome number (diploid) • Occurs in somatic cells
Formation of gametes	Meiosis	• Produces four genetically different cells • Reduces chromosome number by half (haploid) • Occurs in reproductive cells

Tissue Type	Cell Wall Characteristics	Function
Parenchyma	• Thin, primary cell wall • Made of cellulose • Living cells	Photosynthesis, storage, secretion
Collenchyma	• Unevenly thickened cell wall • Thickened at corners • Contains pectin and cellulose • Living cells	Mechanical support in growing parts
Sclerenchyma	• Uniformly thick, lignified cell wall • Often dead at maturity • Provides rigidity	Mechanical support, protection