Ans: Robert Hooke discovered the cell in 1665 while observing a thin slice of cork under a microscope.

Ans: The cell is the basic structural and functional unit of all living organisms.

Ans: Mitochondria is called the powerhouse of the cell because it produces energy in the form of ATP through cellular respiration.

Ans: Lysosomes are called "suicidal bags" because they contain digestive enzymes that can break down the entire cell if their membrane ruptures.

Ans: Prokaryotic cells: no membrane-bound nucleus, DNA lies freely in cytoplasm (nucleoid), smaller and simpler, e.g., bacteria. Eukaryotic cells: well-defined nucleus enclosed by nuclear membrane, contains membrane-bound organelles, larger and complex, e.g., plant and animal cells.

Ans: Osmosis is the movement of water molecules across a selectively permeable membrane from a region of low solute concentration to high solute concentration. Diffusion is the movement of any molecule from high to low concentration through any medium. Osmosis specifically involves water and a selectively permeable membrane; diffusion does not require a membrane.

Ans: Plasmolysis is the shrinkage of the cell membrane away from the cell wall when a plant cell is placed in a hypertonic solution. Water moves out of the cell by osmosis, causing the central vacuole to shrink and the protoplasm to contract. It occurs when the external solution has a higher concentration of solutes than the cell sap.

Ans: The nucleus is surrounded by a double-layered nuclear membrane with pores for material exchange. It contains: (1) Nucleoplasm — the fluid filling the nucleus. (2) Chromatin — network of DNA and protein that forms chromosomes during cell division. (3) Nucleolus — produces ribosomes. Functions: (1) Controls all metabolic activities of the cell. (2) Carries hereditary information in the form of genes. (3) Directs cell division and protein synthesis.

Ans: (1) Plant cells have a cell wall made of cellulose; animal cells do not. (2) Plant cells have chloroplasts for photosynthesis; animal cells lack chloroplasts. (3) Plant cells have a large central vacuole; animal cells have small, temporary vacuoles.

Ans: A plant cell has: cell wall, cell membrane, nucleus, chloroplasts, mitochondria, endoplasmic reticulum, Golgi apparatus, vacuole. Functions: (1) Mitochondria — produces ATP energy by aerobic respiration. (2) Chloroplasts — site of photosynthesis, contains chlorophyll. (3) Golgi apparatus — packages and secretes proteins and lipids. (4) Vacuole — stores water and maintains osmotic pressure, provides turgidity to the cell.

Ans: Robert Hooke discovered the cell in 1665 while observing a thin slice of cork under a microscope.

Ans: Lysosomes contain powerful digestive enzymes that can break down all cell components. When a cell is damaged or worn out, lysosomes burst and digest the entire cell, hence called suicide bags.

Ans: ATP (Adenosine Triphosphate) is the energy currency of the cell, produced mainly by mitochondria during cellular respiration.

Ans: 1. Plant cells have a rigid cell wall made of cellulose; animal cells lack a cell wall. 2. Plant cells have chloroplasts for photosynthesis; animal cells do not have chloroplasts. 3. Plant cells have a large central vacuole; animal cells have small or no vacuoles. 4. Animal cells have centrioles; plant cells generally lack them.

Ans: Plasmolysis is the shrinkage of the cell contents (protoplasm) away from the cell wall when a plant cell is placed in a hypertonic solution. In a hypertonic solution, the concentration of solutes outside is higher than inside the cell, so water moves out of the cell by osmosis, causing the plasma membrane to pull away from the cell wall.

Ans: Nucleus: (1) It controls all cellular activities as it contains the genetic material (DNA). (2) It is responsible for cell division and transmission of hereditary characters. Mitochondria: (1) They produce ATP through cellular respiration — hence called the powerhouse of the cell. (2) They have their own DNA and ribosomes, enabling them to synthesize some of their own proteins.

Ans: Structure: The Golgi apparatus consists of a stack of flattened membrane-bound sacs called cisternae arranged in a curved manner. It has a cis face (receiving side, near ER) and a trans face (secreting side, toward plasma membrane). Small vesicles bud off from the ER and fuse with the cis face. Function: (1) Processing and modification of proteins received from the RER (e.g., adding sugar chains — glycosylation). (2) Packaging of materials into vesicles for transport to the plasma membrane or lysosomes. (3) Formation of lysosomes. (4) Production of cell plate during plant cell division. It is especially active in secretory cells like goblet cells.

Ans: Osmosis is the movement of water molecules through a selectively permeable membrane from a region of high water concentration (dilute solution) to a region of low water concentration (concentrated solution) until equilibrium is reached. Example in plants: Root hair cells of plants absorb water from the soil by osmosis. The soil solution is more dilute than the cell sap of root cells, so water moves into the root hair cells. Importance: (1) Maintains turgidity of plant cells, keeping the plant upright. (2) Helps in absorption of water from soil. (3) Opening and closing of stomata depends on osmosis in guard cells. (4) Transport of water upward in the plant also involves osmotic processes.

Ans: Diagram: A plant cell should show — cell wall, plasma membrane, large central vacuole, nucleus with nucleolus, chloroplast, mitochondria, endoplasmic reticulum, Golgi apparatus, ribosomes. Functions of four organelles: (1) Chloroplast — contains chlorophyll, site of photosynthesis; converts light energy into chemical energy stored as food. (2) Mitochondria — site of cellular respiration; produces ATP; has own DNA and ribosomes. (3) Nucleus — control centre of cell; contains DNA (hereditary material); controls all metabolic activities; involved in cell division. (4) Endoplasmic Reticulum — RER helps in protein synthesis and transport; SER involved in lipid synthesis and detoxification; connects nucleus to plasma membrane serving as an internal transport system.

Ans: Prokaryotic vs Eukaryotic cells: (1) Nuclear material — Prokaryotic: no true nucleus, naked circular DNA in nucleoid; Eukaryotic: true membrane-bound nucleus with linear DNA. (2) Size — Prokaryotic: 1–10 µm; Eukaryotic: 10–100 µm. (3) Membrane-bound organelles — Prokaryotic: absent; Eukaryotic: present (mitochondria, ER, Golgi, etc.). (4) Ribosomes — Prokaryotic: 70S; Eukaryotic: 80S (mitochondria and chloroplasts have 70S). (5) Cell division — Prokaryotic: binary fission; Eukaryotic: mitosis/meiosis. Examples of prokaryotes: Bacteria (E. coli), Cyanobacteria (Nostoc). Examples of eukaryotes: Plant cell (Onion), Animal cell (Human RBC — though enucleated, other animal cells like cheek cells are fully eukaryotic).