Cell: The Unit of Life

CELL THEORY AND BASIC CONCEPTS

**Definition of Cell**: The cell is the **fundamental structural and functional unit of all living organisms**. It is the smallest unit of life capable of independent existence and performing all essential functions of life.

**Historical Development**:

**Antonie Von Leeuwenhoek** (1670s) – First person to see and describe a live cell using an improved microscope

**Robert Brown** (1831) – Discovered the nucleus in plant cells while studying orchids

**Microscopy advancement** – Invention and improvement of electron microscope revealed complete structural details of cells

**Cell Theory** (formulated by Schleiden, Schwann, and Virchow):

**Statement 1: Cellular Composition**

In 1838, **Matthias Schleiden** (German botanist) examined numerous plants and concluded that **all plants are composed of cells which form plant tissues**

In 1839, **Theodore Schwann** (German zoologist) studied animal cells and discovered the **plasma membrane** (thin outer layer)

Schwann observed that **cell wall is unique to plant cells** (absent in animals)

**Combined Hypothesis**: Bodies of all animals and plants are composed of cells and products of cells

**Statement 2: Cell Division and Origin**

The original cell theory did not explain how new cells formed

In 1855, **Rudolf Virchow** proposed the principle: **"Omnis cellula-e cellula"** (Every cell arises from a pre-existing cell)

This explained that **cells divide and new cells are formed from pre-existing cells**

**Final Cell Theory (as understood today)**:

1. All living organisms are composed of cells and products of cells

2. All cells arise from pre-existing cells

3. Cell is the basic unit of life (implicit)

---

OVERVIEW OF CELL STRUCTURE AND ORGANIZATION

**Unicellular vs Multicellular Organisms**:

**Unicellular organisms** – Single cell performs all life functions; capable of independent existence (e.g., Amoeba, Paramecium, bacteria)

**Multicellular organisms** – Many specialized cells working together (e.g., humans, plants, animals)

**Basic Components Present in All Cells**:

**Plasma Membrane** – Outermost boundary; selectively permeable; controls entry and exit of substances

**Cytoplasm** – Semi-fluid matrix filling the cell; site of most cellular activities; contains chemicals and organelles

**Genetic Material (DNA)** – Contains hereditary information; controls cell functions

**Two Main Types of Cells**:

| Feature | Prokaryotic Cells | Eukaryotic Cells |

|---------|------------------|-----------------|

| **Nucleus** | Absent; DNA naked in nucleoid region | Present; DNA enclosed in nuclear membrane |

| **Membrane-bound organelles** | Absent | Present (ER, Golgi, lysosomes, mitochondria, chloroplasts) |

| **Size** | Smaller (1-2 µm) | Larger (10-20 µm) |

| **Examples** | Bacteria, cyanobacteria, PPLO | Plants, animals, protists, fungi |

| **Ribosomes** | 70S (50S + 30S subunits) | 80S (60S + 40S subunits) |

| **Cell Wall** | Present in most (except Mycoplasma) | Present in plants and fungi; absent in animals |

| **Reproduction** | Asexual (binary fission) | Sexual and asexual |

**Variation in Cell Size and Shape**:

**Smallest cells**: Mycoplasmas (0.3 µm); PPLO (about 0.1 µm)

**Typical bacteria**: 3-5 µm

**Largest isolated cell**: Ostrich egg

**Human RBCs**: 7.0 µm diameter

**Longest cells**: Nerve cells (can extend several feet)

**Shapes**: Disc-like (RBCs), polygonal, columnar, cuboid, thread-like, irregular (amoeboid)

**Shape variation reason**: Shape correlates with function (e.g., nerve cells elongated for conduction; columnar cells for absorption)

---

PROKARYOTIC CELLS: STRUCTURE AND FEATURES

**Definition**: Prokaryotic cells are cells **lacking a membrane-bound nucleus and other membrane-bound organelles**. Genetic material is naked and located in a region called the nucleoid.

**Representatives of Prokaryotes**:

Bacteria (most common)

Blue-green algae (Cyanobacteria)

Mycoplasma

PPLO (Pleuro-Pneumonia-Like Organisms)

**General Characteristics**:

Generally smaller and multiply more rapidly than eukaryotic cells

Display variety in shape and size

Basic bacterial shapes: **Bacillus** (rod-like), **Coccus** (spherical), **Vibrio** (comma-shaped), **Spirillum** (spiral)

**8.4.1: CELL ENVELOPE AND ITS MODIFICATIONS**

**Cell Envelope Structure**: The prokaryotic cell envelope is a **tightly bound three-layered structure** serving as a protective unit:

1. **Outermost Layer – Glycocalyx**

Composition and thickness vary among bacteria

**Slime layer** – Loose, unorganized coating; provides adherence

**Capsule** – Thick, tough, organized layer; provides protection against drying and phagocytosis

Function: Protection, attachment, water retention

2. **Middle Layer – Cell Wall**

Determines cell shape

Provides strong structural support

Prevents cell from bursting (maintains turgor pressure) or collapsing

Composition: Peptidoglycan (in bacteria)

**Gram staining classification**:

**Gram-positive bacteria** – Thick peptidoglycan layer; stain purple (retain crystal violet dye)

**Gram-negative bacteria** – Thin peptidoglycan layer with outer lipid membrane; stain pink (retain safranin dye)

3. **Inner Layer – Plasma Membrane**

Selectively permeable (semi-permeable)

Similar structure to eukaryotic membrane

Interacts with outside environment

Composed of phospholipid bilayer with embedded proteins

**Mesosome** (Special Prokaryotic Structure):

**Definition**: Specialized membranous structure formed by **infoldings of plasma membrane**

**Structure**: Extensions into cytoplasm in form of vesicles, tubules, and lamellae

**Functions**:

Cell wall formation

DNA replication and distribution to daughter cells

Respiration

Secretion processes

Increases surface area of plasma membrane for enzymatic reactions

Provides increased enzymatic content

**Chromatophores** (in Cyanobacteria):

Membranous extensions into cytoplasm

Contain photosynthetic pigments

Enable photosynthesis in prokaryotes

**Flagella** (Motility Structures):

**Definition**: **Thin filamentous extensions from cell wall** enabling cell movement

**Structure** (three parts):

1. **Filament** – Longest portion; extends from cell to outside

2. **Hook** – Curved transitional region

3. **Basal body** – Embedded in cell wall and membrane; acts as motor

**Arrangement**: Bacteria show variety – monotrichous (one), atrichous (none), lophotrichous (few at one end), peritrichous (distributed)

**Movement mechanism**: Rotation of basal body causes filament rotation

**Pili and Fimbriae** (Surface Structures):

**Pili** – Elongated tubular structures made of special protein; longer than fimbriae; role in bacterial conjugation (DNA transfer)

**Fimbriae** – Small bristle-like fibers sprouting from cell surface; shorter than pili; aid attachment to rocks in streams and to host tissues

**Note**: Both do NOT play role in motility (unlike flagella)

**8.4.2: RIBOSOMES AND INCLUSION BODIES**

**Prokaryotic Ribosomes**:

**Location**: Associated with plasma membrane in prokaryotes

**Size**: Approximately 15 nm × 20 nm

**Composition**: Two subunits – **50S and 30S units** forming **70S prokaryotic ribosomes** (compared to 80S in eukaryotes)

**Function**: **Site of protein synthesis**; translate mRNA into proteins

**Polyribosomes (Polysomes)**: Multiple ribosomes attach to single mRNA molecule; collectively translate mRNA simultaneously, increasing protein synthesis efficiency

**Inclusion Bodies**:

**Definition**: **Reserve materials stored in prokaryotic cytoplasm, not membrane-bound**; lie freely in cytoplasm

**Types**:

**Phosphate granules** – Store phosphorus; energy source

**Cyanophycean granules** – Found in cyanobacteria; starch-like compounds

**Glycogen granules** – Store glucose; energy reserves

**Gas vacuoles** – Found in photosynthetic bacteria; provide buoyancy; enable floating in water layers

**Function**: Serve as reserve food material during unfavorable conditions

**Genetic Material in Prokaryotes**:

**Genomic DNA** – Single, circular, chromosome (main DNA)

**Plasmids** – Small circular DNA molecules outside genomic DNA

Confer unique phenotypic characters to bacteria

Example: **Antibiotic resistance** plasmids enable survival in antibiotic-containing environments

Used in genetic engineering for bacterial transformation

Enable horizontal gene transfer (conjugation)

---

EUKARYOTIC CELLS: GENERAL FEATURES

**Definition**: Eukaryotic cells are cells with **membrane-bound nucleus** containing DNA and **numerous membrane-bound organelles** enabling compartmentalization.

**Representatives**:

All protists

All plants

All animals

All fungi

**Key Characteristics**:

**Extensive compartmentalization** – Membrane-bound organelles divide cytoplasm into functional compartments

**Organized nucleus** – Nuclear envelope encloses genetic material organized into chromosomes

**Complex structures** – Possess locomotory and cytoskeletal structures (microtubules, microfilaments)

**Genetic organization** – DNA organized into multiple chromosomes (not naked)

**Larger size** – Generally 10-20 µm or larger

**Slower reproduction** – Slower than prokaryotes due to complex division process

**Differences between Plant and Animal Cells**:

| Feature | Plant Cell | Animal Cell |

|---------|-----------|------------|

| **Cell Wall** | Present (cellulose-based) | Absent |

| **Plastids** | Present (chloroplasts) | Absent |

| **Central Vacuole** | Large, prominent (up to 90% of cell) | Small or absent |

| **Centrioles** | Absent (in most) | Present; help in cell division |

| **Shape** | Fixed (due to cell wall) | Rounded/variable |

| **Plasmodesmata** | Present (cell-to-cell connections) | Absent |

---

CELL MEMBRANE (PLASMA MEMBRANE): STRUCTURE AND FUNCTION

**Historical Development of Membrane Models**:

**Early studies** – Chemical analysis of RBCs (red blood cells) showed membrane composed of lipids and proteins

**1950s-1960s** – Electron microscopy revealed detailed membrane structure

**1972** – **Singer and Nicolson** proposed the **Fluid Mosaic Model** (currently accepted)

**Fluid Mosaic Model of Plasma Membrane**

**Structure and Composition**:

**1. Phospholipid Bilayer** (Framework):

**Basic structure** – Two layers of phospholipid molecules arranged tail-to-tail

**Orientation**:

**Polar hydrophilic heads** – Face outward toward aqueous environment (water-loving)

**Nonpolar hydrophobic tails** – Face inward, protected from aqueous environment (water-repelling)

**Function** – Forms selectively permeable barrier

**Fluidity** – Quasi-fluid nature enables lateral movement of components

**2. Cholesterol**:

Present in animal cell membranes

Embedded between phospholipid molecules

Function: Regulates membrane fluidity; provides structural support

**3. Proteins**:

**Percentage composition** – In human erythrocytes: ~52% protein, ~40% lipids

**Types based on location**:

**Integral proteins** – Partially or totally buried in membrane; span entire bilayer; transport proteins

**Peripheral proteins** – Lie on membrane surface; easily extracted

**Functions** – Transport, recognition, enzymatic activity, structural support

**Ratio variation** – Protein-to-lipid ratio varies greatly in different cell types

**4. Carbohydrates**:

Present as glycoproteins and glycolipids

Located on cell surface (outer layer only)

Form glycoprotein chains and molecules

Functions: Cell recognition, antigen presentation, immune response

**Key Principles of Fluid Mosaic Model**:

**Fluidity** – Lipids and proteins move laterally within membrane (not flip-flop vertically)

**Mosaic nature** – Diverse components (lipids, proteins, carbohydrates) embedded in fluid lipid matrix

**Dynamic structure** – Not rigid; allows flexibility for cellular functions

**Selective permeability** – Some substances pass easily; others require carrier proteins

**Importance of Membrane Fluidity** (for cellular functions):

Cell growth and expansion

Formation of intercellular junctions

Secretion of substances

Endocytosis and exocytosis

Cell division

Cell movement

**Membrane Transport Mechanisms**

**1. Passive Transport** (No energy required; ATP not used):

**a) Simple Diffusion**:

**Definition** – **Movement of neutral solutes across membrane along concentration gradient** (high to low concentration)

**Mechanism** – Direct passage through lipid bilayer

**Examples** – O₂, CO₂, lipid-soluble substances

**Characteristics** – Spontaneous; no carrier protein needed; faster with larger concentration difference

**b) Osmosis**:

**Definition** – **Movement of water molecules across semipermeable membrane from region of high water concentration (low solute) to low water concentration (high solute)**

**Mechanism** – Water diffuses to equalize solute concentration

**Direction** – Water moves toward hypertonic solution (more solute)

**Importance** – Maintains cell turgor; prevents cell lysis or plasmolysis

**Examples** – Plant cells in hypotonic solution (turgid); animal cells in hypertonic solution (crenated)

**c) Facilitated Diffusion**:

**Definition** – **Movement of polar molecules across membrane with help of carrier proteins; along concentration gradient (no energy)**

**Mechanism** – Carrier protein changes shape to transport molecule

**Examples** – Glucose transport into RBCs, ions into muscle cells

**Characteristics** – Selective; saturatable (limited by protein number); faster than simple diffusion

**2. Active Transport** (Energy required; ATP used):

**Definition** – **Movement of molecules against concentration gradient (low to high) requiring energy in form of ATP**

**Mechanism** – Carrier protein pumps molecule against gradient

**Energy source** – ATP hydrolysis provides energy for conformational change of carrier protein

**Examples**:

**Na⁺/K⁺ ATPase pump** – Expels 3 Na⁺ ions outward; imports 2 K⁺ ions inward; maintains concentration gradients

Absorption of glucose in intestinal epithelial cells (against concentration gradient)

Nerve cell potential maintenance

**Characteristics** – Highly selective; not saturated easily; can move large molecules; essential for nutrient absorption and ion homeostasis

---

CELL WALL (PLANT CELLS AND FUNGI)

**Definition**: The cell wall is a **non-living rigid structure forming outer covering outside plasma membrane** in plants and fungi.

**Structure and Composition**:

**In Algae**:

**Cellulose** – Main component

**Galactans and mannans** – Polysaccharides

**Minerals** – Calcium carbonate (CaCO₃) in some species

**In Higher Plants**:

**Cellulose** – Rigid framework (60-70%)

**Hemicellulose** – Cross-linking polysaccharides

**Pectins** – Adhesive compounds

**Proteins** – Glycoproteins for strength

**Water** – 20-30% water content enables flexibility

**In Fungi**:

**Chitin** – Main structural component (instead of cellulose)

**Types of Cell Walls in Plants**

**1. Primary Cell Wall**:

**Location** – Present in young, actively growing cells

**Structure** – Thinner; more flexible

**Composition** – Cellulose microfibrils loosely arranged in pectin matrix

**Function** – Capable of growth; expands with cell growth

**Presence** – All plant cells have primary wall

**2. Secondary Cell Wall**:

**Location** – Formed on inner side (toward plasma membrane) after primary wall matures

**Formation** – Deposited after cell stops growing

**Structure** – Thicker; more rigid; heavily lignified

**Composition** – More cellulose (80%), less pectin; contains lignin (in woody plants)

**Function** – Provides strength and rigidity; reduced permeability

**Presence** – Mainly in mature, specialized cells (xylem vessels, fibers)

**Lignin** – Phenolic compound impregnating cellulose; provides hardness and water resistance

**Middle Lamella**:

**Location** – Layer between neighboring cell walls; glues cells together

**Composition** – Mainly **calcium pectate** (calcium salt of pectin)

**Function** – **Holds/glues different neighboring cells together** enabling tissue cohesion

**Solubility** – Dissolves in dilute acid (used to separate cells in maceration)

**Plasmodesmata** (Cytoplasmic Connections):

**Definition** – **Microscopic channels traversing cell wall and connecting cytoplasm of neighboring cells**

**Structure** – Tubular extensions of plasma membrane; contain endoplasmic reticulum (desmotubule)

**Diameter** – About 40-50 nm

**Number** – Multiple plasmodesmata connect adjacent cells

**Function** – **Enable communication and transport between plant cells** (cell-to-cell transport of ions, molecules, proteins, RNA)

**Biological significance** – Make plant cells highly interconnected functioning as syncytium

**Functions of Cell Wall**:

Gives definite shape to cell (plant cells have fixed shape due to cell wall)

Provides mechanical support and rigidity to plant tissues

Protects cell from mechanical damage

Prevents pathogenic infection

Provides barrier to undesirable macromolecules

Enables cell-to-cell interaction

Prevents cell lysis in hypotonic solutions (turgor support)

---

ENDOMEMBRANE SYSTEM: COORDINATED FUNCTIONS

**Definition**: The **endomembrane system comprises membrane-bound organelles whose functions are coordinated** to work as an integrated system.

**Components of Endomembrane System**:

1. **Endoplasmic Reticulum (ER)** – Rough and Smooth

2. **Golgi Complex (Golgi Apparatus)**

3. **Lysosomes**

4. **Vacuoles**

**NOT Part of Endomembrane System** (function independently):

Mitochondria

Chloroplasts

Peroxisomes

**Coordinated Functions**: These organelles work together in protein synthesis, packaging, modification, transport, and secretion pathways.

**ENDOPLASMIC RETICULUM (ER)**

**Definition**: The **ER is a network of tiny tubular membrane structures scattered throughout cytoplasm**, forming an extensive reticulum (network).

**Structure and Organization**:

**Appearance under EM** – Interconnected tubules and flattened sacs (cisternae)

**Continuity** – Often continuous with outer membrane of nuclear envelope

**Space division** – Divides intracellular space into:

**Luminal compartment** – Inside ER cavity (lumen)

**Extra-luminal compartment** – Cytoplasm outside ER

**Two Types of ER**:

**1. Rough Endoplasmic Reticulum (RER)**:

**Structure** – ER bearing ribosomes on outer surface

**Appearance** – Rough due to ribosome attachment

**Location of ribosomes** – Only on cytoplasmic (outer) surface

**Ribosomes present** – 80S eukaryotic ribosomes

**Location in cell** – More extensive in protein-synthesizing cells

**Continuity** – Continuous with outer nuclear membrane

**Function**:

**Primary function** – Synthesis of proteins destined for secretion (secretory proteins)

Synthesis of proteins to be incorporated into membranes

Synthesis of lysosomal proteins

Synthesis of peroxisomal proteins

Translation of mRNA into polypeptide chains

**Cells rich in RER** – Pancreatic acinar cells (digestive enzymes), plasma cells (antibodies), salivary gland cells

**Connection to Golgi** – RER products transported to Golgi for further processing

**2. Smooth Endoplasmic Reticulum (SER)**:

**Structure** – ER lacking ribosomes on surface; appears smooth

**Appearance** – Smooth, tubular network

**Ribosome attachment** – No ribosomes attached

**Location in cell** – Abundant in steroid-synthesizing and detoxifying cells

**Functions**:

**Synthesis of lipids** – Phospholipids, steroids, triglycerides

**Synthesis of cholesterol** – In animal cells

**Metabolism of carbohydrates** – Glucose synthesis (gluconeogenesis) in liver

**Detoxification** – Breaking down toxic substances (alcohol, drugs) in liver cells

**Storage of calcium ions** – In muscle cells; released during contraction

**Synthesis of steroid hormones** – In endocrine glands

**Synthesis of hydrophobic molecules**

**Cells rich in SER** – Liver cells (detoxification), steroid-producing cells (hormones), muscle cells (calcium storage)

**Note**: Both RER and SER synthesize lipids; RER additionally synthesizes proteins for export, while SER specializes in lipid and carbohydrate metabolism.

---

GOLGI APPARATUS (GOLGI COMPLEX)

**Definition**: The **Golgi apparatus is a membranous organelle comprising stack of flattened, disk-shaped cisternae** functioning in protein and lipid modification and packaging.

**Structure**:

**Appearance** – Stack of flattened membranous sacs (cisternae) arranged like stack of coins

**Number of cisternae** – 4-6 cisternae per stack in animal cells; up to 10-20 in plant cells

**Sides of stack**:

**Cis face (cis-Golgi)** – Convex surface facing ER; receiving side

**Trans face (trans-Golgi)** – Concave surface opposite to ER; shipping side

**Central region** – Central cisternae (medial-Golgi)

**Size** – Approximately 0.5-1 µm in diameter

**Associated vesicles** – Transport vesicles bud from cis face and fuse with trans face

**Location** – Near nucleus; particularly well-developed in secretory cells

**Functions** (remember as CAMP):

**1. C – Chemical Modification**:

**Glycosylation** – Addition of carbohydrate groups to proteins (forming glycoproteins) and lipids (forming glycolipids)

**N-glycosylation** – Addition of oligosaccharides to asparagine (N)

**O-glycosylation** – Addition of oligosaccharides to serine (S) or threonine (T)

**Phosphorylation** – Addition of phosphate groups

**Sulfation** – Addition of sulfate groups

**Proteolysis** – Enzyme-mediated cleavage of proteins

**Purpose** – These modifications activate proteins and lipids, determine their destination, and affect their function

**2. A – Assembly**:

Formation of large molecules from smaller subunits

Assembly of proteoglycans

Cross-linking of complex carbohydrates

**3. M – Movement (Transport) and Modification**:

Progressive modification of glycans on oligosaccharide chains

Addition and removal of sugar groups creating diversity

Movement of molecules through cisternae by vesicular transport

**4. P – Packing and Processing**:

**Sorting** – Separation of proteins/lipids by destination

Some to lysosomes (marked with mannose-6-phosphate)

Some to plasma membrane (exocytosis)

Some to different cellular locations

**Packaging** – Enclosure of modified molecules in transport vesicles

**Concentration** – Modification for concentration during transport

**Overall Secretory Pathway** (Connection to ER):

1. **Proteins synthesized in RER** – Nascent proteins with signal sequences

2. **Transport to Golgi** – Via COPII-coated vesicles budding from ER

3. **Processing in Golgi** – Cisterna-to-cisterna transport; progressive modifications

4. **Packaging** – Into secretory vesicles at trans-Golgi network (TGN)

5. **Secretion** – Vesicles fuse with plasma membrane; exocytosis release contents

**Note**: Golgi apparatus is more developed in animal cells than plant cells. Plant cells have dictyosomes (equivalent structures).

---

LYSOSOMES

**Definition**: **Lysosomes are membrane-bound vesicular organelles containing powerful digestive (hydrolytic) enzymes** that break down various cellular materials.

**Structure**:

**Membrane** – Single phospholipid membrane envelope

**Contents** – Acidic cytoplasm (pH 4.8) containing ~40-50 different hydrolytic enzymes

**Size** – 0.2-0.5 µm diameter (variable)

**Appearance** – Dense, dark-staining organelles under EM

**Distribution** – Randomly distributed in cytoplasm

**Composition of Lysosomal Enzymes**:

**Proteases** – Break down proteins into amino acids

**Lipases** – Break down lipids into glycerol and fatty acids

**Carbohydrases** – Break down carbohydrates into monosaccharides (includes:

Amylase – breaks starch

Glucosidase – breaks glucose polymers

Hyaluronidase – breaks hyaluronic acid)

**Nucleases** – Break down DNA and RNA into nucleotides

**Phosphatases** – Remove phosphate groups

**Sulfatases** – Remove sulfate groups

**Acid phosphatase** – Generic hydrolytic enzyme

**Important Feature**: All enzymes are **acid hydrolases** (work optimally at acidic pH ~4.8); inactive if released into cytoplasm (neutral pH~7.2) – providing safety mechanism.

**Functions**:

**1. Autophagy** (Self-digestion):

**Definition** – **Breakdown of old, damaged, or non-functional organelles within same cell**

**Process** – Damaged organelle enclosed in membrane vesicle; lysosome fuses with vesicle; enzymes digest organelle

**Example** – Breakdown of mitochondria in muscle cells

**Significance** – Cellular renewal; recycling of cellular materials

**2. Intracellular Digestion**:

Breakdown of materials brought into cell by endocytosis

Digestion of phagocytosed microorganisms (bacteria, viruses)

Breakdown of worn-out organelles

**3. Cell Death (Programmed Cell Death)**:

**Apoptosis** – Controlled cell death; lysosomes rupture releasing enzymes

**Example** – Loss of tadpole tail during metamorphosis; removal of webbing between fingers in fetus

**Significance** – Controlled elimination of damaged or unnecessary cells

**4. Tissue Remodeling**:

Breakdown of extracellular materials

Example – Bone resorption; cartilage breakdown

**Special Types of Lysosomes**:

**Primary lysosomes** – Lysosomes with enzymes, not yet fused with vesicles

**Secondary lysosomes** – Lysosomes having fused with vesicles; actively digesting

**Tertiary lysosomes (Residual bodies)** – Lysosomes containing indigestible materials

**Lysosomal Origin**:

Derived from **Golgi apparatus**

Marked with **mannose-6-phosphate** signal sequence

Transport via vesicles from Golgi to lysosomes

**Clinical Significance**:

**Lysosomal storage diseases** – Defective lysosomal enzymes cause accumulation (e.g., Tay-Sachs disease – accumulation of gangliosides)

**Non-functioning lysosomes** – Lead to cellular accumulation of waste; cell dysfunction and death

---

VACUOLES

**Definition**: **Vacuoles are large membrane-bound vesicular organelles containing aqueous solution and serving storage and maintenance functions.**

**Structure**:

**Membrane** – Single membrane called tonoplast (similar to plasma membrane)

**Contents** – Cell sap (aqueous solution containing sugars, salts, ions, pigments, alkaloids)

**Size** – Highly variable; up to 90% of plant cell volume

**Types of Vacuoles**:

**1. In Plant Cells**:

**a) Central Vacuole**:

**Size** – Large; occupies 80-90% of mature plant cell volume

**Location** – Central position; displaces nucleus to periphery

**Contents** – Cell sap (water, dissolved sugars, proteins, salts, pigments, alkaloids)

**Functions**:

**Maintains cell turgidity** – Turgor pressure keeps plant tissues firm and rigid

**Storage** – Accumulates sugars, minerals, proteins, pigments

**Growth** – Cell expansion mainly through vacuole enlargement (less energy than cytoplasm synthesis)

**Pigment storage** – Anthocyanin (red/blue colors), carotenoids (yellow/orange)

**Osmotic regulation** – Maintains osmotic potential; controls water movement

**Provides structural support** – Turgor prevents wilting

**Herbivore protection** – Toxic compounds (alkaloids) deter herbivores

**Tonoplast** – Selectively permeable; controls solute movement

**Relation to shape** – Central vacuole enforces cell shape

**b) Vacuoles in Young Plant Cells**:

**Appearance** – Multiple small vacuoles

**Maturation** – Fuse together forming single large central vacuole

**c) Contractile Vacuoles**:

**Location** – Protists (Paramecium, Amoeba)

**Function** – Osmoregulation; removes excess water by contraction and expulsion

**Process** – Contracts periodically; expels water through pore preventing cell lysis in hypotonic environments

**2. In Animal Cells**:

**Size** – Small or absent in most animal cells

**Exception** – Plant-eating protists may have contractile vacuoles

**Reason** – Animal cells maintain isotonic environment; excess water excluded by kidney system and regulation

**3. Specialized Vacuoles**:

**Food vacuoles** – Digest ingested food (in protists)

**Gas vacuoles** – Store gases; provide buoyancy (in aquatic microorganisms)

**Secretory vacuoles** – Store and transport secretory products (in some cells)

**Relation to Cell Wall**:

**Plant cells** – Large vacuole pushes cytoplasm against cell wall; maintains cell shape

**Plasmolysis** – In hypertonic solution, vacuole shrinks; cytoplasm detaches from cell wall; cell loses turgidity (reversible)

**Turgidity** – Normal state; cell full of water; firm and rigid

---

MITOCHONDRIA (POWERHOUSE OF CELL)

**Definition**: **Mitochondria are double-membrane bound organelles responsible for ATP production through cellular respiration** (aerobic respiration).

**Discovery**:

Early observation: Dark-staining granules in cells

**R. Altmann (1886)** – Named them "bioplasts"

**Term "mitochondria"** coined for their thread-like or grain-like appearance under light microscopy

**Structure** (Two Membranes):

**1. Outer Membrane**:

**Permeable** – Allows passage of molecules up to 5000 molecular weight

**Contains porins** – Protein channels enabling passage of ions and small molecules