Cell: The Unit of Life

JKBOSE/CBSE
Cell The Unit of Life Class 11 Notes
Cell: The Unit of Life - Master Notes

Cell: The Unit of Life

[PREMIUM NCERT MODULE • CYTOLOGY BASICS]

I. Cell Theory & Overview
  • Discovery: Robert Hooke (Cells); Leeuwenhoek (Live cells); Robert Brown (Nucleus).
  • Cell Theory: Schleiden & Schwann. Modified by Rudolf Virchow (Omnis cellula-e cellula).
  • Size: Mycoplasma (Smallest, 0.3µm); Ostrich Egg (Largest); Nerve Cell (Longest).
II. Prokaryotic Cell
  • No membrane-bound nucleus or organelles.
  • Cell Envelope: Glycocalyx (Slime layer/Capsule) -> Cell Wall -> Plasma Membrane.
  • Mesosomes: Infoldings of plasma membrane (for DNA replication, respiration).
  • Ribosomes: 70S (50S + 30S). Polysomes are multiple ribosomes on one mRNA.
  • Plasmids: Extrachromosomal circular DNA (Antibiotic resistance).
III. Eukaryotic Cell: Plasma Membrane

Fluid Mosaic Model (Singer & Nicolson, 1972):

  • Lipid bilayer (Phospholipids) with proteins embedded in it.
  • Proteins: Peripheral and Integral.
  • Fluid nature allows lateral movement of proteins and membrane growth.
[i] Transport: Passive (Diffusion/Osmosis) vs Active (Na+/K+ pump - uses ATP).
IV. Organelles - The Endomembrane System

Functions are coordinated: ER -> Golgi -> Lysosome -> Vacuole.

  • ER: RER (Protein synthesis); SER (Lipid/Steroid synthesis).
  • Golgi: Packaging and modification. Formation of Glycoproteins/Glycolipids.
  • Lysosomes: Hydrolytic enzymes active at acidic pH (Suicide bags).
  • Vacuoles: Tonoplast membrane. High concentration of ions/materials inside.
V. Mitochondria & Plastids
  • Mitochondria: Powerhouse (ATP synthesis). Double membraned; Cristae increase surface area.
  • Plastids: Chloroplasts (Chlorophyll), Chromoplasts (Colors), Leucoplasts (Storage).
    --> Amyloplast: Starch; Elaioplast: Oil/Fat; Aleuroplast: Protein.
[!] Semi-autonomous: Both have their own ds-circular DNA and 70S ribosomes.
VI. Nucleus & Cytoskeleton
  • Ribosomes: 80S (Eukaryotic); 70S (Organelles/Bacteria).
  • Centrosome: 9+0 arrangement of triplets. Basal body of cilia/flagella.
  • Cilia/Flagella: 9+2 arrangement of doublets (Axoneme).
  • Nucleus: Nuclear envelope with pores. Nucleolus is the site of rRNA synthesis.

Cell HOTS Questions

[ ORGANELLE LOGIC & ULTRASTRUCTURE ]

[Q] Why are Mitochondria and Chloroplasts called Semi-autonomous organelles?
[A] Explanation:

They are "semi-autonomous" because:
1. They have their own Double-stranded circular DNA.
2. They have their own 70S Ribosomes.
3. They can synthesize some of their own proteins.
However, they still depend on the nuclear DNA and cytoplasmic machinery for many essential functions and replication enzymes.

[Q] Explain the difference between 70S and 80S ribosomes in a Eukaryotic cell.
[A] Detail:

In a eukaryotic cell, 80S ribosomes are found in the Cytoplasm and on the Rough ER. However, 70S ribosomes are found inside Mitochondria and Plastids. This supports the endosymbiotic theory (that these organelles evolved from prokaryotic ancestors).

[Q] What happens to a protein after it is synthesized on the RER?
[A] Pathway:

1. The protein enters the ER lumen for folding.
2. It is packed into vesicles and sent to the Cis-face of the Golgi.
3. In Golgi, it undergoes modification (e.g., Glycosylation).
4. It exits from the Trans-face to reach its final destination (secreted out or to lysosomes).

[Q] Why are Lysosomes active only at acidic pH?
[A] Explanation:

Lysosomes contain Hydrolytic enzymes (hydrolases) that require an acidic environment (pH 4.5-5.0) for optimal activity. This is a safety mechanism; if a lysosome bursts, the enzymes will be inactivated in the neutral pH (7.2) of the cytoplasm, preventing accidental digestion of the cell.

[Q] Differentiate between Cilia/Flagella (9+2) and Centrioles (9+0).
[A] Comparison:

Axoneme (Cilia/Flagella): Has 9 peripheral doublets and 2 central singlets (9+2).
Centriole/Basal Body: Has 9 peripheral triplets and 0 central microtubules (9+0). Centrioles lack the central singlets found in the axoneme.

[Q6] Function of Mesosomes.

Cell wall formation, DNA replication, and respiration in bacteria.

[Q7] Site of rRNA synthesis.

Nucleolus (inside the nucleus).

[Q8] Tonoplast role.

Membrane of the vacuole; facilitates uptake of ions against concentration gradient.

[Q9] Middle lamella composition.

Mainly Calcium pectate.

[Q10] George Palade discovery.

Ribosomes (in 1953).

[Q11] Cis vs Trans face of Golgi.

Cis is "Forming" (convex); Trans is "Maturing" (concave).

[Q12] Primary function of SER.

Synthesis of lipids and steroid hormones.

[Q13] Algal cell wall components.

Cellulose, galactans, mannans, and calcium carbonate.

[Q14] Omnis cellula-e cellula meaning.

All cells arise from pre-existing cells (Rudolf Virchow).

[Q15] Smallest cell name.

Mycoplasma (PPLO).

[Q16] Gas vacuoles found in?

Blue green and Purple/Green photosynthetic bacteria.

[Q17] Secondary cell wall location.

Formed on the inner side (towards membrane) of the primary wall.

[Q18] Chromatin contains?

DNA, some basic proteins (histones), non-histone proteins, and RNA.

[Q19] Kinetochore position.

Disc-shaped structures on the sides of the centromere.

[Q20] Acrocentric vs Telocentric.

Acro: Centromere close to end. Telo: Centromere at the very end.

Cell - 50 Premium Facts

Cell: 50 Mastery Facts

Premium Rapid Revision

01
Cell: Basic structural/functional unit of all life.
02
Cell Theory: Schleiden (Botanist) and Schwann (Zoologist).
03
Robert Brown: Discovered the nucleus (1831).
04
Omnis cellula-e cellula: Rudolf Virchow (1855).
05
Mycoplasma: Smallest cell (0.3 µm); lacks cell wall.
06
Fluid Mosaic Model: Proposed by Singer and Nicolson.
07
Phospholipids: Arranged in a bilayer with heads outward.
08
Na+/K+ pump: Example of active transport (uses ATP).
09
Middle lamella: Glues cells together; contains Calcium pectate.
10
Plasmodesmata: Cytoplasmic bridges between plant cells.
11
Mesosome: Functional analogue of mitochondria in prokaryotes.
12
70S Ribosomes: Found in Prokaryotes, Mitochondria, Chloroplasts.
13
80S Ribosomes: Found in Eukaryotic cytoplasm.
14
Endomembrane system: ER, Golgi, Lysosomes, Vacuoles.
15
Rough ER: Site of protein synthesis and secretion.
16
Smooth ER: Site of lipid and steroid hormone synthesis.
17
Camillo Golgi: Discovered Golgi bodies (1898).
18
Cis face Golgi: Receiving face from ER.
19
Trans face Golgi: Releasing face for vesicles.
20
Glycosylation: Modification of proteins/lipids in Golgi.
21
Lysosomes: Polymorphic organelles containing hydrolases.
22
Tonoplast: Selectively permeable membrane of the vacuole.
23
Mitochondria: Only semi-autonomous organelle with Kreb's enzymes.
24
Cristae: Infoldings of inner mitochondrial membrane.
25
Plastids: Found in all plant cells and Euglenoids.
26
Chlorophyll: Green pigment for capturing light energy.
27
Thylakoids: Arranged in stacks called Grana.
28
Amyloplasts: Store carbohydrates (Potato starch).
29
Elaioplasts: Store fats and oils.
30
Aleuroplasts: Store proteins.
31
George Palade: Discovered ribosomes (1953).
32
Cytoskeleton: Network of filaments (support/motility).
33
Cilia vs Flagella: Cilia are shorter and numerous.
34
Axoneme: Core of cilia/flagella consisting of microtubules.
35
9+2 array: Microtubule arrangement in axoneme.
36
9+0 array: Microtubule arrangement in centrioles.
37
Centrosome: Contains two non-membrane bound centrioles.
38
Hub: Central part of the centriole.
39
Nuclear envelope: Double membrane with nuclear pores.
40
Perinuclear space: Gap between two nuclear membranes.
41
Nucleolus: Non-membrane bound site of rRNA synthesis.
42
Chromatin: Contains DNA, Histones, and RNA.
43
Centromere: Primary constriction of the chromosome.
44
Metacentric: Centromere in the middle (V-shape).
45
Sub-metacentric: One arm slightly shorter (L-shape).
46
Acrocentric: Centromere close to end (J-shape).
47
Telocentric: Terminal centromere (I-shape).
48
Satellite: Small fragment beyond secondary constriction.
49
Microbodies: Peroxisomes, Glyoxysomes (in plants).
50
Kinetochore: Disc on centromere for spindle attachment.
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