Cell Structure and Organization
Short Questions
1. Who first observed cells under a microscope?
Answer: Robert Hooke in 1665.
2. What is the smallest cell mentioned in the chapter?
Answer: Mycoplasma (0.3 µm).
3. What is the largest cell described in the document?
Answer: Ostrich egg (~15 cm).
4. Who proposed the fluid mosaic model of the plasma membrane?
Answer: Singer and Nicolson in 1972.
5. What is the main function of the Golgi complex?
Answer: Modifies and packages secretions.
6. What is the site of ribosome biogenesis in a eukaryotic cell?
Answer: Nucleolus.
7. What type of ribosomes are found in mitochondria?
Answer: 70S ribosomes.
8. What is the primary function of lysosomes?
Answer: Digestion of macromolecules.
9. What is the main pigment in chloroplasts?
Answer: Chlorophyll.
10. What is the term for cytoplasmic bridges in plant cells?
Answer: Plasmodesmata.
11. What is the basal body of bacterial flagella called the smallest motor?
Answer: It drives rotary movement.
12. What is the composition of the bacterial cell wall in Gram-positive bacteria?
Answer: Peptidoglycan.
13. What organelle is absent in mature human RBCs?
Answer: Mitochondria.
14. What is the role of peroxisomes in cells?
Answer: Detoxify hydrogen peroxide.
15. What is the Svedberg unit used to measure?
Answer:Ribosome sedimentation rate.
Long Questions
1. Explain why mitochondria are called the powerhouse of the cell.
Answer: Mitochondria produce ATP, the cell’s energy currency, through aerobic respiration. The inner membrane’s cristae house oxysomes (ATP synthase) that facilitate the Krebs cycle and electron transport chain. This ATP powers cellular activities, earning mitochondria the title of the cell’s powerhouse.
2. Describe the structure and function of the plasma membrane.
Answer: The plasma membrane is a selectively permeable phospholipid bilayer with embedded proteins and carbohydrates, as per the fluid mosaic model. It regulates molecule transport via passive (e.g., diffusion) and active (e.g., Na+/K+ pump) mechanisms. Its quasifluid nature allows protein movement, maintaining cell integrity and communication.
3. What are the differences between prokaryotic and eukaryotic cells?
Answer: Prokaryotic cells lack a defined nucleus and membrane-bound organelles, with a single circular DNA and 70S ribosomes. Eukaryotic cells have a nucleus, membrane-bound organelles, linear DNA with histones, and 80S ribosomes. Examples include bacteria (prokaryotes) and plant/animal cells (eukaryotes).
4. How does the endoplasmic reticulum contribute to cellular functions?
Answer: The endoplasmic reticulum (ER) is a network of tubules and cisternae, divided into smooth ER (lipid synthesis, detoxification) and rough ER (protein synthesis). Rough ER, studded with ribosomes, produces secretory proteins, while smooth ER stores calcium and detoxifies drugs. It also supports organelle positioning and membrane formation.
5. What is the role of the Golgi complex in a cell?
Answer: The Golgi complex modifies, packages, and sorts secretions from the ER in its cisternae, with cis and trans faces. It produces its own secretions (e.g., pectin in plants) and forms transport vesicles for targeted delivery. The cisternal maturation model explains its dynamic processing of cellular products.
6. Explain the structure and function of lysosomes.
Answer: Lysosomes are membrane-bound vesicles containing hydrolytic enzymes (e.g., proteases, lipases) active in acidic pH, arising from Golgi-associated ER. They digest macromolecules intracellularly (e.g., in autophagic vesicles) and extracellularly (e.g., in sperm acrosome). Polymorphic forms include primary, secondary, and residual bodies.
7. Describe the types and functions of plastids in plant cells.
Answer: Plastids are double-membraned organelles classified as leucoplasts (store nutrients like starch), chromoplasts (contain pigments for flower/fruit color), and chloroplasts (perform photosynthesis). Chloroplasts contain chlorophyll in thylakoids, while leucoplasts and chromoplasts lack photosynthetic apparatus. They contain DNA and ribosomes, supporting their functions.
8. What is the significance of the nucleus in a eukaryotic cell?
Answer: The nucleus, with a double-membraned envelope, contains chromatin (DNA, histones) and the nucleolus for ribosome synthesis. It controls heredity, variation, and protein synthesis by regulating DNA and RNA production. Its constant chromosome number is vital for phylogenetic studies and cell division.
9. How do cilia and flagella contribute to cell movement?
Answer: Cilia (short, numerous) and flagella (long, few) are hair-like, membrane-bound outgrowths with a 9+2 microtubule axoneme. They generate fluid currents for locomotion and material passage, driven by basal bodies. In eukaryotes, they differ structurally from prokaryotic flagella, aiding coordinated movement.
10. Explain the role of the cytoskeleton in maintaining cell structure.
Answer: The cytoskeleton, composed of microtubules (tubulin), microfilaments (actin), and intermediate filaments (fibrous proteins), forms a fibril network. It maintains cell shape, enables contraction, and supports organelle mobility and cell division. Its dynamic structure adapts to cellular needs, ensuring structural integrity.
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