Ans: Cell theory was proposed by Matthias Schleiden and Theodor Schwann in 1839.

Ans: Lysosome is known as the suicidal bag of the cell.

Ans: Unicellular organism: composed of a single cell that performs all life functions. Example: Amoeba. Multicellular organism: composed of many cells, each specialised for specific functions. Example: Human beings.

Ans: The plant cell will undergo plasmolysis. Water from the cell vacuole will move out by osmosis into the hypertonic salt solution. The protoplasm will shrink and pull away from the cell wall. The cell will become flaccid and eventually die if kept in this solution for long.

Ans: The cell membrane is made up of a double layer of lipid molecules (phospholipid bilayer) with protein molecules embedded in it (fluid mosaic model). It is flexible and thin. It is called selectively permeable because it allows only certain molecules to pass through freely (like water, O2, CO2) while restricting the movement of other molecules. This selective permeability allows the cell to control its internal environment.

Ans: (a) Mitochondria: site of cellular respiration; produces energy in the form of ATP; has its own DNA and ribosomes. (b) Ribosomes: site of protein synthesis; found on rough ER and free in cytoplasm; smallest organelle with no membrane. (c) Golgi apparatus: packages and dispatches proteins and lipids secreted by the cell; forms lysosomes; involved in formation of cell plate during cell division.

Ans: Substances move in and out of cells by: (1) Diffusion — passive movement of molecules from high to low concentration, no energy required. Example: CO2 and O2 exchange. (2) Osmosis — movement of water through selectively permeable membrane from dilute to concentrated solution. (3) Active transport — movement of molecules against the concentration gradient using energy (ATP). It requires carrier proteins and is essential for uptake of minerals and amino acids against the gradient.

Ans: Structure: The nucleus is spherical and bounded by a double-layered nuclear membrane with nuclear pores. Inside is nucleoplasm containing chromatin (DNA + protein) and one or more nucleoli. Functions: (1) Controls all metabolic activities of the cell. (2) Carries genetic information (genes/DNA) passed to daughter cells during division. (3) Nucleolus produces ribosomes. (4) Directs protein synthesis by sending mRNA to ribosomes. It is called the control centre because without the nucleus, cells cannot survive or reproduce — it directs every activity of the cell.

Ans: Mitochondria is called the powerhouse of the cell because it produces ATP (Adenosine Triphosphate) through the process of cellular respiration. ATP is the main energy currency used by the cell for all its activities.

Ans: The plant cell will undergo plasmolysis. Water will move out of the cell by osmosis (as the external solution is hypertonic), causing the plasma membrane to shrink away from the cell wall. The cell becomes flaccid.

Ans: Cell theory was proposed by Schleiden and Schwann in 1838–39 and later modified by Rudolf Virchow (1855). It states: (1) All living organisms are made up 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 (Omnis cellula e cellula — added by Virchow).

Ans: Diffusion: (1) Movement of any substance (solid, liquid or gas) from high to low concentration. (2) Does not require a membrane. (3) Net movement stops at equilibrium. Osmosis: (1) Movement of water/solvent only. (2) Requires a selectively permeable membrane. (3) Movement is from dilute to concentrated solution. Both are passive processes (no energy required).

Ans: Structure of nucleus: (1) Nuclear envelope — double membrane boundary with nuclear pores for transport of materials. (2) Nucleoplasm — fluid-filled interior containing chromatin and nucleolus. (3) Chromatin — DNA + protein complex; condenses into chromosomes during cell division. (4) Nucleolus — dense region involved in rRNA synthesis and ribosome assembly. Functions: (1) Controls all metabolic activities of the cell. (2) Contains hereditary information (DNA) passed to daughter cells during division. (3) Directs protein synthesis by producing mRNA. (4) Regulates cell division.

Ans: Endocytosis is the process by which a cell engulfs external material by folding its plasma membrane around the substance to form a vesicle inside the cell. Types: (1) Phagocytosis — engulfing of solid particles (cell eating); e.g., WBCs engulfing bacteria. (2) Pinocytosis — engulfing of liquid droplets (cell drinking). Example: Amoeba captures its food (bacteria, algae) by wrapping its pseudopodia around the prey — a form of phagocytosis. Endocytosis is most common in: WBCs (neutrophils, macrophages) for immune defense, and Amoeba for obtaining nutrition. It is only possible in animal cells since plant cells have a rigid cell wall that prevents membrane folding.

Ans: Plant cell structure: A plant cell is surrounded by a rigid cell wall (cellulose) outside the plasma membrane. It has a large central vacuole, chloroplasts (containing chlorophyll), nucleus, mitochondria, ER, Golgi apparatus, ribosomes and plasmodesmata (cytoplasmic connections through cell wall). Differences from animal cell: (1) Cell wall — present in plant cell (cellulose); absent in animal cell. (2) Chloroplasts — present in plant cell; absent in animal cell. (3) Vacuole — large central vacuole in plant cell; small or no vacuoles in animal cell. (4) Centrioles — absent in most plant cells; present in animal cells for cell division. (5) Shape — plant cells are more rigid and box-like; animal cells are irregular in shape. (6) Lysosomes — rare in plant cells; more common in animal cells.