Cell: The Unit of Life
Objective & Matching
1. Which of the following is not correct?
(a) Robert Brown discovered the cell.
Explanation: This statement is incorrect. Robert Brown discovered the nucleus (in 1831). The cell was discovered by Robert Hooke (in 1665).
2. New cells generate from:
(c) Pre-existing cells
Explanation: According to the Cell Theory (modified by Rudolf Virchow in 1855), “Omnis cellula-e cellula”, which means new cells arise from pre-existing cells.
3. Match the following (Column I with Column II)
| Column I | Column II |
|---|---|
| (a) Cristae | (ii) Infoldings in mitochondria |
| (b) Cisternae | (iii) Disc-shaped sacs in Golgi apparatus |
| (c) Thylakoids | (i) Flat membranous sacs in stroma |
4. Which of the following is correct?
(c) In prokaryotes, there are no membrane bound organelles.
Explanation: Prokaryotic cells lack membrane-bound organelles like mitochondria, Golgi complex, ER, lysosomes, and a well-defined nucleus.
Structure & Function
5. What is a mesosome in a prokaryotic cell? Mention its functions.
Mesosome: A specialized differentiated form of the cell membrane in prokaryotes, formed by the infoldings of the plasma membrane into the cell in the form of vesicles, tubules, and lamellae.
1. Cell wall formation.
2. DNA replication and distribution to daughter cells.
3. Respiration (analogous to mitochondria).
4. Secretion processes.
5. Increasing the surface area of the plasma membrane and enzymatic content.
6. Transport across Plasma Membrane.
Neutral Solutes: Move across the membrane by the process of simple diffusion along the concentration gradient (from higher to lower concentration).
Polar Molecules: Cannot pass through the non-polar lipid bilayer in the same way because they are hydrophilic.
Transport of Polar Molecules: They require a carrier protein of the membrane to facilitate their transport across the membrane. This is called Facilitated Diffusion.
7. Double membrane bound organelles: Mitochondria & Chloroplasts.
– Structure: Outer membrane is smooth; Inner membrane folds into Cristae. Matrix contains single circular DNA, RNA, 70S ribosomes.
– Function: Site of aerobic respiration; produces ATP.
[Image of Chloroplast structure]
– Structure: Contains membranous sacs called Thylakoids (stacked as Grana) in a fluid Stroma. Stroma contains enzymes for carbohydrate synthesis, circular DNA, and 70S ribosomes.
– Function: Site of Photosynthesis; traps light energy to synthesize food.
8. What are the characteristics of prokaryotic cells?
- Nucleus: No well-defined nucleus; genetic material is naked (Nucleoid).
- Organelles: Lack membrane-bound organelles.
- Size: Generally smaller (1-10 µm) and multiply rapidly.
- Cell Wall: Present (except in Mycoplasma).
- Ribosomes: 70S type.
- Plasmids: Small circular extra-chromosomal DNA often present.
- Mesosomes: Special membrane infoldings present.
9. Multicellular organisms have division of labour. Explain.
In multicellular organisms, the body is made of billions of cells. Different groups of cells are specialized to perform specific functions. This specialization ensures efficiency.
- Muscle cells: Specialized for contraction and movement.
- Nerve cells: Specialized for conduction of impulses.
- Red blood cells: Transport oxygen.
Thus, functions are divided among different groups of cells, tissues, and organs, known as the division of labour.
10. Cell is the basic unit of life. Discuss.
The cell is the smallest unit capable of independent existence and performing the essential functions of life. Anything less than a complete structure of a cell does not ensure independent living.
- Structural Unit: All living organisms are composed of cells.
- Functional Unit: All metabolic reactions (respiration, digestion, synthesis) occur inside the cell.
- Hereditary Unit: Cells contain genetic material (DNA) passed from parents to offspring.
Organelles & Nucleus
11. What are nuclear pores? State their function.
Nuclear Pores: Minute pores formed by the fusion of the two membranes of the nuclear envelope.
Function: They act as passages through which the movement of RNA and protein molecules takes place in both directions between the nucleus and the cytoplasm.
12. Lysosomes vs Vacuoles (Endomembrane System).
| Lysosomes | Vacuoles |
|---|---|
| Membrane-bound vesicular structures formed by the Golgi apparatus. | Membrane-bound space found in the cytoplasm. |
| Rich in Hydrolytic Enzymes (lipases, proteases, carbohydrases). | Contains water, sap, excretory products, and other materials not useful for the cell. |
| Function: Intracellular digestion of foreign material or old organelles. | Function: Storage, maintenance of turgor pressure (in plants), and osmoregulation (in Amoeba). |
13. Structure of Nucleus and Centrosome.
– Nuclear Membrane: Double membrane with perinuclear space and nuclear pores.
– Nucleoplasm: The fluid matrix.
– Nucleolus: Spherical structure, site for active ribosomal RNA synthesis.
– Chromatin: Network of nucleoprotein fibres containing DNA and histones (forms chromosomes during division).
[Image of Centrosome and Centrioles]
– An organelle containing two cylindrical structures called centrioles.
– Centrioles lie perpendicular to each other.
– Organization like a cartwheel. Made of 9 evenly spaced peripheral fibrils of tubulin protein.
– Forms the basal body of cilia/flagella and spindle fibres during cell division.
14. Centromere and Classification of Chromosomes.
Centromere: A primary constriction on every chromosome where the disc-shaped structures called kinetochores are present.
Classification based on Centromere Position:
- Metacentric: Centromere in the middle; two equal arms.
- Sub-metacentric: Centromere slightly away from the middle; one shorter arm and one longer arm.
- Acrocentric: Centromere close to one end; one extremely short and one very long arm.
- Telocentric: Centromere at the terminal end.