Cell: The Unit of Life Notes: Class 11 Biology Chapter 8
Class 11 Biology Chapter 8 Cell: The Unit of Life covers cell theory, the differences between prokaryotic and eukaryotic cells, and the structure and function of various cell organelles.
Overview of Topics & Subtopics (Click to Jump)
1. Cell Theory
Short Note:
• Cell Theory (Schleiden, Schwann, later modified by Virchow) states: all living organisms are composed of cells; the cell is the basic structural and functional unit of life; and all cells arise from pre-existing cells (Omnis cellula-e cellula).
2. Prokaryotic vs Eukaryotic Cells
Short Note:
• Prokaryotic cells lack a true, membrane-bound nucleus and membrane-bound organelles (e.g., bacteria); genetic material lies in a region called the nucleoid.
• Eukaryotic cells have a well-defined, membrane-bound nucleus and various membrane-bound organelles (e.g., plant and animal cells).
3. Cell Organelles
Short Note:
• Nucleus: control centre, contains chromatin/chromosomes (DNA + protein). Mitochondria: 'powerhouse of the cell', site of aerobic respiration and ATP production.
• Endoplasmic Reticulum (ER): rough ER (has ribosomes, protein synthesis) and smooth ER (lipid synthesis). Golgi Apparatus: packaging and secretion of materials.
• Ribosomes: site of protein synthesis. Lysosomes: 'suicide bags', contain digestive enzymes. Plastids (in plants): chloroplasts (photosynthesis), chromoplasts, leucoplasts. Vacuole: storage; large central vacuole in plant cells maintains turgidity.
NCERT Textbook Exercise Questions
Which cell organelle is known as the powerhouse of the cell? Why?
Answer: The mitochondrion is known as the powerhouse of the cell because it is the site of aerobic cellular respiration, where energy released from the oxidative breakdown of food molecules is trapped in the form of ATP, the energy currency of the cell.
What are the characteristics of prokaryotic cells?
Answer: Prokaryotic cells lack a nuclear membrane (genetic material lies free in the cytoplasm as a nucleoid), lack membrane-bound organelles, are generally smaller and simpler in structure, and possess ribosomes (70S type) and, in some, a cell wall and flagella.
Multicellular organisms have division of labour. Explain.
Answer: In multicellular organisms, different groups of cells perform specific, specialised functions (e.g., muscle cells for movement, nerve cells for coordination, gland cells for secretion), rather than each cell performing all functions independently. This division of labour increases efficiency, as specialised cells/tissues/organs work together in a coordinated manner for the survival of the whole organism.
How do plant cells differ from animal cells?
Answer: Plant cells have a rigid cell wall made of cellulose, plastids (chloroplasts), and a large central vacuole, which are absent in animal cells. Animal cells have centrioles (generally absent in most plant cells) and lack a cell wall, having only a flexible plasma membrane.
Discuss the role of the plasma membrane in transportation of molecules across it.
Answer: The plasma membrane is selectively permeable, allowing certain molecules to pass through while restricting others. Small, non-polar molecules (like O2, CO2) diffuse freely across it; other molecules pass through via facilitated diffusion (using carrier/channel proteins, without energy) or active transport (using energy, against the concentration gradient, via specific pumps like the Na+-K+ pump).
