1. The membrane has a lipid bilayer — two layers of fat (lipid) molecules arranged with their water-attracting (hydrophilic) heads pointing outward and their water-repelling (hydrophobic) tails pointing inward toward each other.
2. Various protein molecules are embedded throughout this lipid bilayer, scattered like tiles in a mosaic — hence the name “mosaic”.
3. The lipid and protein molecules can move sideways, flip, and rotate within the layer — making the membrane fluid.
4. The embedded proteins act as gatekeepers — they form channels or pumps that help specific substances (like glucose, ions, or large molecules) pass through the membrane in a controlled way.
5. The membrane is selectively permeable — small, non-polar molecules like oxygen and carbon dioxide can pass freely; water passes through special protein channels called aquaporins; large or charged molecules can only enter with the help of transport proteins.

This selective control allows the cell to take in nutrients, remove waste, and maintain a stable internal environment (homeostasis).

1. Materials needed: Two potato pieces of equal size, Beaker A with plain water, Beaker B with 20% salt solution, a weighing balance.
2. Procedure: Measure and record the initial weight of both potato pieces. Place one piece in Beaker A (plain water) and the other in Beaker B (salt solution). Leave undisturbed for about one hour.
3. Observation — Beaker A: The potato piece swells and its final weight is greater than the initial weight. Water has moved into the potato cells.
4. Observation — Beaker B: The potato piece shrinks and its final weight is less than the initial weight. Water has moved out of the potato cells.
5. Conclusion: The cell membrane is selectively permeable — it allows water to pass but not sugar or salt. Water moves by osmosis from a region of higher water concentration (dilute solution) to lower water concentration (concentrated solution). Plain water is hypotonic to the cell sap → water enters. Salt solution is hypertonic to the cell sap → water exits.

Why a plant cell needs both:

• The cell wall gives rigidity and structural support — allowing plants to stand upright, resist wind, and maintain leaf/flower shape.
• The cell membrane (inside the cell wall) regulates the movement of specific molecules, controlling metabolism.
• Together, both membranes work to allow selective absorption of water and nutrients while maintaining the cell’s shape under varying environmental conditions.

1. Isotonic solution (समपरासारी): The solute concentration of the external solution equals the solute concentration inside the cell. No net movement of water occurs. The cell remains unchanged in size.
2. Hypotonic solution (अल्पपरासारी): The solute concentration of the external solution is less than the inside of the cell (i.e., the external solution is more dilute). Water moves into the cell by osmosis. An animal cell swells and may eventually burst (lyse) because it has no rigid cell wall to limit expansion.
3. Hypertonic solution (अतिपरासारी): The solute concentration of the external solution is greater than the inside of the cell (i.e., the external solution is more concentrated). Water moves out of the cell by osmosis. An animal cell shrinks (crenation) as it loses water.

Note: In plant cells placed in hypertonic solution, the cell does not shrink because the rigid cell wall maintains its outer shape — only the inner content (cytoplasm) shrinks away from the wall (plasmolysis).

1. Structure: Each mitochondrion has two membranes — an outer smooth membrane and an inner membrane that is deeply folded into finger-like projections called cristae.
2. The cristae increase the surface area available for chemical reactions, which greatly improves the efficiency of energy production.
3. The space enclosed by the inner membrane is filled with a semi-fluid matrix containing enzymes, its own DNA, and its own ribosomes.
4. Function — Cellular Respiration: In mitochondria, glucose and other food molecules are broken down using oxygen in a process called cellular respiration. The energy released is stored in the form of ATP (Adenosine Triphosphate) molecules.
5. ATP acts as the energy currency of the cell — it is used to power all cellular activities including muscle contraction, protein synthesis, and active transport.

1. (a) Endoplasmic Reticulum (ER) — अन्तर्द्रव्यी जालिका: The ER is a large network of membranes that spreads throughout the cytoplasm and is connected to the outer nuclear membrane. It has two types:
   • Rough ER (RER): Has ribosomes on its surface (looks rough); involved in protein synthesis and transport. Example: pancreatic cells that make digestive enzymes.
   • Smooth ER (SER): Lacks ribosomes (looks smooth); involved in synthesis of lipids and hormones.
2. (b) Golgi Apparatus — गॉल्जी उपकरण: Discovered by Camillo Golgi in 1898, this organelle consists of stacks of flattened, sac-like structures. It is functionally linked to the ER. Its key functions are:
   • Modifies, sorts, and packages proteins and lipids received from the ER.
   • Sends them to their destination — either outside the cell (secretion), to the cell membrane, or to form lysosomes.
   • Called the cell’s “post office” because it addresses and dispatches cellular materials.
3. (c) Lysosomes — लाइसोसोम: These are small, single membrane-bound sacs filled with powerful digestive enzymes. They serve as the cell’s “clean-up system”:
   • Break down old, worn-out organelles and cellular waste materials.
   • Digest foreign particles (bacteria) that enter the cell.
   • The breakdown products are recycled back into the cytoplasm for reuse.

Structural Similarities: Both are double membrane-bound organelles. Both contain their own circular DNA and ribosomes, meaning they can partially make their own proteins. These features suggest that both organelles evolved from ancient free-living bacteria through a process called endosymbiosis.

1. Matthias Schleiden (1838) — a German botanist — proposed that all plants are made up of cells.
2. Theodor Schwann (1839) — a German zoologist — extended this to animals, proposing that all animals are also made up of cells.
3. Rudolf Virchow (1855) — a German scientist — added the third and most important principle: “Omnis cellula e cellula” — all new cells arise only from pre-existing cells.
4. Together, these contributions form the Classical Cell Theory, which states:
   • All living organisms are made up of one or more cells.
   • The cell is the basic structural and functional unit of all living beings.
   • All cells arise from pre-existing cells (through cell division).
5. Unifying power of Cell Theory: It applies to every living organism — from a tiny bacterium (1 cell) to a giant blue whale (trillions of cells). It explains how life continues from generation to generation through cell division. It ties together biology, medicine, evolution, and genetics under one common principle.

Importance of meiosis in sexual reproduction:

• Meiosis produces gametes with half the chromosome number. When two gametes (sperm + egg) fuse during fertilisation, the original chromosome number is restored in the offspring.
• Meiosis creates genetic diversity — children resemble their parents but are not identical to them, because different combinations of chromosomes end up in different gametes.
• Without meiosis, chromosome numbers would double with every generation, which would be fatal.

1. When a large amount of sugar is added to the amla, the surrounding solution becomes very concentrated (hypertonic) — it has a very high solute (sugar) concentration and very low water concentration.
2. If bacteria land on the murabba, the high sugar concentration outside the bacterial cells is greater than the concentration inside the bacterial cells.
3. By the process of osmosis, water moves from the region of higher water concentration (inside the bacterial cell) to the region of lower water concentration (outside, i.e., the sugar solution).
4. The bacteria rapidly lose water from their cells. Their cell contents shrink — this is called plasmolysis in plant/bacterial cells. The bacteria are essentially dehydrated.
5. Without water, the bacteria cannot carry out metabolic activities and die or become inactive — they cannot cause spoilage. This is why the murabba stays preserved for a long time.