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Short Notes Cell - The Unit of Life - Short Notes for NEET
Cell Theory
- Schleiden (1838): All plants are made of cells
- Schwann (1839): All animals are made of cells
- Virchow (1855): Omnis cellula-e-cellula (all cells arise from pre-existing cells)
- Modern Cell Theory:
- All living organisms are composed of cells and products of cells
- All cells arise from pre-existing cells
- Cell is the structural and functional unit of life
Prokaryotic vs Eukaryotic Cells
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Size | 0.1-5.0 μm (smaller) | 5-100 μm (larger) |
| Nuclear membrane | Absent | Present |
| Nucleolus | Absent | Present |
| Chromosome | Single, circular, not associated with histones | Multiple, linear, associated with histones |
| Membrane-bound organelles | Absent (no ER, Golgi, mitochondria, lysosomes) | Present |
| Ribosomes | 70S (50S + 30S) | 80S (60S + 40S) in cytoplasm; 70S in mitochondria & chloroplast |
| Cell wall | Non-cellulosic (peptidoglycan/murein) | Cellulosic in plants; absent in animals |
| Cell division | Binary fission, no mitosis/meiosis | Mitosis and meiosis |
| Examples | Bacteria, Cyanobacteria (blue-green algae), Mycoplasma, PPLO | Plants, Animals, Fungi, Protists |
Plant Cell vs Animal Cell
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Cell wall | Present (cellulose) | Absent |
| Plasma membrane | Present | Present |
| Plastids | Present | Absent |
| Vacuoles | Large central vacuole | Small, many or absent |
| Centrioles | Absent (except in lower plants) | Present |
| Lysosomes | Rare | Common |
| Glyoxysomes | Present | Absent |
| Nucleus | Peripheral (due to large vacuole) | Central |
| Reserve food | Starch | Glycogen |
Cell Wall (Plant Cells)
- Composition: Cellulose, hemicellulose, pectin, and proteins
- Layers:
- Primary wall: Thin, flexible, capable of growth; formed first
- Middle lamella: Outermost layer, made of calcium pectate; holds adjacent cells together
- Secondary wall: Thick, rigid, formed inside primary wall after cell stops growing; contains lignin
- Plasmodesmata: Cytoplasmic connections through cell wall between adjacent cells
- Pits: Thin areas in secondary wall for exchange of substances
- Functions: Shape, protection, mechanical strength, prevents over-expansion
Cell Membrane (Plasma Membrane)
- Model: Fluid Mosaic Model (Singer & Nicolson, 1972)
- Composition:
- Lipids: Phospholipids (main component), cholesterol, glycolipids
- Proteins: Integral (intrinsic) and peripheral (extrinsic)
- Carbohydrates: As glycoproteins and glycolipids (form glycocalyx)
- Phospholipid bilayer: Hydrophilic heads outward, hydrophobic tails inward
- Properties:
- Quasi-fluid nature (lipids and proteins can move laterally)
- Selectively permeable
- Functions:
- Selective permeability and transport
- Cell recognition (glycocalyx)
- Receptor sites for hormones
- Cell signaling
Cell Organelles
Endomembrane System
Endomembrane System includes: Endoplasmic Reticulum, Golgi apparatus, Lysosomes, Vacuoles
Not included: Mitochondria, Chloroplasts, Peroxisomes
Not included: Mitochondria, Chloroplasts, Peroxisomes
(i) Endoplasmic Reticulum (ER)
| Feature | Rough ER (RER) | Smooth ER (SER) |
|---|---|---|
| Ribosomes | Present on surface (80S) | Absent |
| Structure | Flattened sacs (cisternae) | Tubular network |
| Main function | Protein synthesis and secretion | Lipid and steroid synthesis |
| Abundant in | Cells secreting proteins (pancreatic cells, plasma cells) | Adipocytes, interstitial cells, liver cells |
| Additional functions | • Membrane biogenesis • Formation of lysosomes | • Synthesis of glycogen, lipids, steroids • Detoxification in liver • Provides enzymes for glycogenolysis • Forms Sarcoplasmic reticulum in muscles (Ca²⁺ storage) |
(ii) Golgi Apparatus (Golgi Complex/Golgi Body)
- Discovered by: Camillo Golgi (1898)
- Structure:
- Cisternae: Flattened, stacked membranous sacs (4-8 in number)
- Cis face (forming face): Receives vesicles from ER
- Trans face (maturing face): Secretes vesicles
- Tubules and vesicles: At periphery
- In plant cells: Called Dictyosomes (many scattered units)
- Functions:
- Packaging and dispatching of materials
- Modification of proteins (glycosylation)
- Formation of glycoproteins and glycolipids
- Secretion of enzymes and hormones
- Formation of lysosomes
- Formation of cell plate during plant cell division
- Synthesis of cell wall materials (pectin, hemicellulose)
- Acrosome formation in sperms
(iii) Lysosomes
- Discovered by: Christian de Duve (1955)
- Called: "Suicidal bags" or "Disposal units" of cell
- Structure: Single membrane-bound vesicles (0.2-0.5 μm)
- Content: Hydrolytic enzymes (acidic hydrolases) - about 40 types
- Lipases, proteases, carbohydrases, nucleases
- Active at acidic pH (~5)
- Formation: From Golgi apparatus and ER
- Types/Functions:
- Autophagy: Digestion of own cell's worn-out organelles
- Heterophagy: Digestion of extracellular materials taken in by phagocytosis
- Autolysis: Complete digestion of cell (cell death)
- Intracellular digestion
- Sperm acrosome contains lysosomes for fertilization
- Bone remodeling by osteoclasts
- Abundant in: WBCs, macrophages, liver cells, kidney cells
(iv) Vacuoles
- Structure: Membrane-bound sacs; membrane called Tonoplast
- In plant cells:
- Large central vacuole (occupies 90% of cell volume in mature cells)
- Contains cell sap (water, minerals, sugars, amino acids, proteins, waste products)
- Provides turgidity and rigidity to cell
- Storage of substances (anthocyanin pigments, toxic metabolites)
- In animal cells:
- Small and many
- Contractile vacuoles: In protozoans (osmoregulation, excretion)
- Food vacuoles: In Amoeba (digestion)
- Functions:
- Storage of water, minerals, nutrients
- Maintains turgor pressure
- Storage of waste products and secondary metabolites
- Provides color to flowers and fruits (anthocyanins)
- In some cells, acts like lysosomes
(v) Mitochondria
- Discovered by: Kolliker (1880)
- Term coined by: C. Benda (1898)
- Called: "Powerhouse of the cell"
- Structure:
- Double membrane-bound organelle
- Outer membrane: Smooth, permeable (has porins)
- Inner membrane: Folded into cristae(increase surface area)
- Contains oxysomes/F₁ particles/ATP synthase (elementary particles)
- Contains enzymes of electron transport chain
- Intermembrane space: Between outer and inner membranes
- Matrix: Inner compartment containing:
- Enzymes for Krebs cycle
- Circular DNA (mtDNA)
- 70S ribosomes
- RNA
- Shape: Sausage-shaped or cylindrical (0.5-1.0 μm diameter)
- Number: Varies (500-2000 per cell); more in metabolically active cells
- Functions:
- ATP production through aerobic respiration (Krebs cycle, ETC, oxidative phosphorylation)
- Site of cellular respiration
- Provides intermediates for biosynthesis
- Semi-autonomous organelle: Has own DNA and ribosomes, can synthesize some proteins
- Endosymbiotic theory: Mitochondria originated from bacteria
- Maternal inheritance: mtDNA inherited from mother only
(vi) Plastids
- Found in: Plant cells and some protists only
- Term coined by: Schimper
- Structure: Double membrane-bound organelles
- Semi-autonomous: Have own DNA (circular) and 70S ribosomes
| Type | Color | Pigment | Location & Function |
|---|---|---|---|
| Chloroplasts | Green | Chlorophyll (a and b) | Location: Leaves and green stems Structure: • Outer membrane: Smooth, permeable • Inner membrane: Smooth • Stroma: Matrix containing enzymes, DNA, 70S ribosomes, starch granules • Thylakoids: Flattened sacs containing chlorophyll • Grana: Stacks of thylakoids (site of light reaction) • Stroma lamellae: Connect grana Function: Photosynthesis (light and dark reactions) |
| Chromoplasts | Yellow, orange, red | Carotenoids (carotene, xanthophyll) | Location: Petals, fruits, old leaves Function: Provide color; attract pollinators and seed dispersers |
| Leucoplasts | Colorless | No pigment | Location: Non-green parts (roots, underground stems) Types & Function: • Amyloplasts: Store starch • Elaioplasts/Oleoplasts: Store oils and fats • Aleuroplasts: Store proteins |
(vii) Ribosomes
- Discovered by: George Palade (1953)
- Called: "Protein factories"
- Composition: rRNA (60%) + Proteins (40%)
- Not membrane-bound
- Structure: Two subunits (large and small)
| Feature | 70S Ribosomes | 80S Ribosomes |
|---|---|---|
| Subunits | 50S + 30S | 60S + 40S |
| Location | • Prokaryotic cells • Mitochondria • Chloroplasts | • Eukaryotic cytoplasm • Attached to ER (bound ribosomes) • Free in cytoplasm (free ribosomes) |
| Function | Protein synthesis (translation) | |
- Polyribosomes/Polysomes: Several ribosomes attached to a single mRNA
(viii) Microbodies (Peroxisomes and Glyoxysomes)
- Structure: Single membrane-bound, spherical organelles
- Peroxisomes:
- Contain enzymes for β-oxidation of fatty acids
- Contain catalase: Breaks down H₂O₂ (toxic) to H₂O + O₂
- Present in both plant and animal cells
- In liver and kidney cells: Detoxification
- In plant leaf cells: Photorespiration
- Glyoxysomes:
- Found in germinating seeds (especially oily seeds)
- Contain enzymes for glyoxylate cycle
- Convert fats to carbohydrates during germination
(ix) Cytoskeleton
- Definition: Network of protein filaments in cytoplasm
- Components:
- Microfilaments (Actin filaments):
- Thinnest (6 nm diameter)
- Made of actin protein
- Function: Cell shape, muscle contraction, cell motility, cytokinesis
- Intermediate filaments:
- Medium size (8-11 nm diameter)
- Made of various proteins (keratin, vimentin, etc.)
- Function: Mechanical strength, maintain cell shape
- Microtubules:
- Thickest (25 nm diameter)
- Made of tubulin protein (α and β tubulin dimers)
- Function: Cell shape, chromosome movement, cilia & flagella structure, intracellular transport
- Microfilaments (Actin filaments):
- Functions:
- Mechanical support and cell shape
- Cell motility
- Intracellular transport
- Chromosome segregation during cell division
(x) Cilia and Flagella
| Feature | Cilia | Flagella |
|---|---|---|
| Length | Short (5-10 μm) | Long (150 μm) |
| Number | Many per cell (hundreds to thousands) | Few per cell (1-4, usually 1-2) |
| Movement | Rowing/oar-like movement | Whip-like/undulating movement |
| Examples | • Paramecium (locomotion) • Respiratory tract epithelium (move mucus) • Fallopian tube (move ovum) | • Sperm cells • Euglena • Chlamydomonas |
- Ultrastructure(both cilia and flagella):
- 9+2 arrangement: 9 peripheral doublet microtubules + 2 central single microtubules
- Axoneme: Core structure with 9+2 arrangement
- Enclosed by plasma membrane
- Radial spokes: Connect peripheral doublets to central sheath
- Dynein arms: Motor proteins on peripheral doublets (cause movement, use ATP)
- Nexin: Links peripheral doublets together
- Basal body/Kinetosome:
- Structure at base of cilia/flagella
- 9+0 arrangement: 9 peripheral triplet microtubules, no central microtubules
- Anchors cilia/flagella to cell
- Similar to centriole in structure
- Functions:
- Locomotion
- Feeding (create water currents)
- Movement of substances over cell surface
(xi) Centrioles
- Found in: Animal cells and some lower plants (absent in higher plants)
- Structure:
- Cylindrical structures made of microtubules
- 9+0 arrangement: 9 peripheral triplet microtubules, no central microtubules
- Length: 0.3-0.5 μm; Diameter: 0.15 μm
- Two centrioles form Centrosome/MTOC (Microtubule Organizing Center)
- Oriented at right angles to each other
- Functions:
- Form spindle apparatus during cell division
- Form basal bodies of cilia and flagella
- Organize microtubule network
- Replication: Before cell division, each centriole duplicates
(xii) Nucleus
- Discovered by: Robert Brown (1831)
- Present in: All eukaryotic cells (except mature RBCs and sieve tube cells)
- Usually: One per cell (uninucleate)
- Binucleate: Paramecium (macro & micronucleus), some liver cells
- Multinucleate: Syncytial cells (skeletal muscle fibers, some fungi)
- Anucleate: Mature mammalian RBCs, mature sieve tube cells
Nuclear Envelope/Nuclear Membrane
- Structure:
- Double membrane structure
- Outer membrane: Continuous with rough ER, may have ribosomes
- Inner membrane: Smooth
- Perinuclear space: Space between outer and inner membranes (10-50 nm)
- Nuclear pores: Openings (about 3000-4000 pores)
- Formed by fusion of inner and outer membranes
- Have nuclear pore complex (made of proteins called nucleoporins)
- Allow transport of RNA, proteins, ribosomes between nucleus and cytoplasm
- Diameter: ~100 nm
- Functions:
- Separates nucleoplasm from cytoplasm
- Regulates exchange of materials
- Provides shape to nucleus
Nucleoplasm
- Nuclear sap/Karyolymph/Nuclear matrix
- Transparent, semi-solid, granular substance inside nucleus
- Contains: Nucleotides, enzymes, proteins, ions, chromatin, nucleolus
Chromatin
- Composition: DNA + Histone proteins + Non-histone proteins + RNA
- Structure:
- Thread-like, intertwined network in non-dividing cells
- Condenses to form chromosomes during cell division
- Nucleosome: Basic unit of chromatin
- DNA (146 bp) wrapped around histone octamer
- Histone octamer = 2 molecules each of H2A, H2B, H3, H4
- H1 histone: Linker histone (binds to linker DNA between nucleosomes)
- Resembles "beads-on-string" structure
- Types:
- Euchromatin: Loosely packed, light staining, transcriptionally active
- Heterochromatin: Densely packed, darkly staining, transcriptionally inactive
- Functions:
- Contains genetic information (DNA)
- Regulates gene expression
Chromosomes
- Term coined by: Waldeyer (1888)
- Visible during: Cell division (condensed chromatin)
- Structure(at metaphase):
- Chromatid: Each chromosome has two sister chromatids joined at centromere
- Centromere/Primary constriction/Kinetochore:
- Point where sister chromatids join
- Site of attachment for spindle fibers (at kinetochore disc)
- Contains specific DNA sequence
- Arms: Parts on either side of centromere
- Secondary constriction: May be present; contains NOR (Nucleolar Organizer Region)
- Satellite: Small fragment beyond secondary constriction; such chromosomes called SAT chromosomes
- Telomeres: Terminal ends of chromosomes; protect chromosome from degradation
- Types based on centromere position:
- Metacentric: Centromere in middle; V-shaped; arms equal
- Sub-metacentric: Centromere slightly away from middle; L-shaped; arms unequal
- Acrocentric: Centromere near one end; J-shaped; one arm very short
- Telocentric: Centromere at terminal end; I-shaped; one arm only (absent in humans)
- Number:
- Constant for a species
- Humans: 46 (23 pairs); 44 autosomes + 2 sex chromosomes
- Housefly: 12; Rat: 42; Dog: 78; Cat: 38
- Functions:
- Carry genetic information
- Inheritance of traits
- Control cellular activities
Nucleolus
- Not membrane-bound
- Spherical structure inside nucleus
- Usually one or more per nucleus
- Composition: rRNA (ribosomal RNA) + Proteins + DNA
- Site of: rRNA synthesis and ribosome biogenesis
- NOR (Nucleolar Organizer Region):
- Specific chromosomal region associated with nucleolus
- Contains rRNA genes
- Present at secondary constriction of SAT chromosomes
- Disappears during cell division and reappears in telophase
- Functions:
- Synthesis of rRNA
- Assembly of ribosomal subunits
- Called "Factory of ribosomes"
Key Points to Remember:
- Endomembrane system: ER, Golgi, Lysosomes, Vacuoles (NOT mitochondria/plastids)
- Semi-autonomous organelles: Mitochondria, Chloroplasts (have own DNA & ribosomes)
- 9+2 arrangement: Cilia and flagella axoneme
- 9+0 arrangement: Basal body and centrioles
- 70S ribosomes: Prokaryotes, mitochondria, chloroplasts
- 80S ribosomes: Eukaryotic cytoplasm
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FAQs on Short Notes Cell - The Unit of Life - Short Notes for NEET
| 1. What is a cell and why is it considered the basic unit of life? |  |
Ans. A cell is the smallest structural and functional unit of an organism. It is considered the basic unit of life because all living organisms are composed of cells, which perform essential life functions. Cells can exist as independent organisms, like bacteria, or as part of multicellular organisms, where they work together to maintain the overall function and health of the organism.
| 2. What are the main differences between prokaryotic and eukaryotic cells? | |
Ans. Prokaryotic cells are simple, unicellular organisms that lack a true nucleus and membrane-bound organelles. Their genetic material is free-floating within the cell. In contrast, eukaryotic cells are more complex, can be unicellular or multicellular, and contain a defined nucleus as well as various membrane-bound organelles, such as mitochondria and endoplasmic reticulum, which compartmentalise cellular functions.
| 3. What are the key components of a plant cell that differentiate it from an animal cell? | |
Ans. Key components of a plant cell that differentiate it from an animal cell include the presence of a rigid cell wall made of cellulose, chloroplasts for photosynthesis, and large central vacuoles that store water and maintain turgor pressure. Animal cells do not have these structures, which are crucial for the plant's structure and function.
| 4. How do cells obtain energy to carry out their functions? | |
Ans. Cells obtain energy primarily through cellular respiration, a process that converts glucose and oxygen into ATP (adenosine triphosphate), the energy currency of the cell. In plants, energy is also obtained from photosynthesis, where sunlight is used to synthesise glucose from carbon dioxide and water, providing both energy and organic materials for growth.
| 5. What role do ribosomes play in the cell? | |
Ans. Ribosomes are the cellular structures responsible for protein synthesis. They translate messenger RNA (mRNA) into polypeptide chains, which then fold into functional proteins. Ribosomes can be found either free-floating in the cytoplasm or attached to the endoplasmic reticulum, influencing the type of proteins they produce and their destination within or outside the cell.
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