Cell: The Unit of LifeNEET Botany · Class 11 · NCERT Chapter 6

Introduction and Cell Theory

The cell is the fundamental structural and functional unit of all living organisms. Whether you look at a single bacterium or a human being with 37 trillion cells, everything starts with the cell.

Discovery of the cell

Robert Hooke (1665) was the first to observe and describe cells. Using a self-built microscope, he examined thin slices of cork (dead bark) and saw tiny box-like compartments he called "cellulae" (Latin for small rooms). He published his observations in Micrographia (1665). Anton van Leeuwenhoek later observed living cells (bacteria, protozoa) using his superior microscopes.

Cell theory

Matthias Schleiden (1838) proposed that all plants are made of cells. Theodor Schwann (1839) extended this to animals. Together they formulated the cell theory: all living organisms are composed of cells, and the cell is the basic unit of life.

Rudolf Virchow (1855) made the crucial addition: "Omnis cellula e cellula"(every cell arises from a pre-existing cell). This is the third postulate.

1. All living organisms are composed of one or more cells.
2. The cell is the basic structural and functional unit of life.
3. All cells arise from pre-existing cells (Virchow, 1855).

Exceptions to cell theory: Viruses are not made of cells. They are nucleoprotein particles with no cytoplasm, no membranes of their own, and no ribosomes. They can only replicate inside a host cell.

Prokaryotic Cell

The term "prokaryote" comes from Greek: pro = before, karyon = nucleus. Prokaryotic cells lack a membrane-bound nucleus and membrane-bound organelles. They are the simplest and oldest cells, appearing about 3.5 billion years ago.

Cell envelope and surface structures

Most bacteria have three layers surrounding the cytoplasm, from outside in:

1. Glycocalyx: Outermost layer. If organized and firmly attached: a capsule (protects from phagocytosis, helps in pathogenicity). If loosely attached: a slime layer.
2. Cell wall: Made of peptidoglycan (murein) in bacteria (eubacteria). Archaebacteria have a different cell wall composition. Gram-positive bacteria: thick peptidoglycan. Gram-negative bacteria: thin peptidoglycan + outer lipopolysaccharide membrane.
3. Plasma membrane: Phospholipid bilayer with proteins. Selectively permeable.

Surface appendages: Pili (sex pili) allow DNA transfer between bacteria (conjugation). Fimbriae are shorter, help bacteria attach to surfaces.Flagella (prokaryotic): made of flagellin protein, solid structure (NOT the 9+2 arrangement of eukaryotic flagella).

Cytoplasm and genetic material

The cytoplasm contains: 70S ribosomes (50S + 30S subunits), a nucleoid region (naked circular DNA, no nuclear membrane, no histones),plasmids (extra-chromosomal circular DNA, carry antibiotic resistance genes, self-replicating), mesosomes (infoldings of plasma membrane involved in cell wall synthesis, DNA replication, and respiration), and inclusion bodies(granules of stored materials: PHB, polyphosphate, etc.).

Mycoplasma is the smallest known prokaryotic cell and the simplest organism capable of independent growth. It has NO cell wall (pleomorphic = no fixed shape). Also called PPLO (Pleuro-Pneumonia-Like Organism).

Eukaryotic Cell: Overview

Eukaryotic cells have a true membrane-bound nucleus and membrane-bound organelles. They are larger (10-100 µm) than prokaryotic cells (1-10 µm) and structurally more complex. All organisms in Kingdoms Protista, Fungi, Plantae, and Animalia have eukaryotic cells.

Plant cell vs animal cell: key differences

Plasma Membrane: Fluid Mosaic Model

The plasma membrane (cell membrane) is the outermost boundary of the cell in animal cells (plant cells have a cell wall outside it). It is selectively permeable: allows some substances to cross freely and controls others.

Fluid mosaic model

Singer and Nicolson (1972) proposed the fluid mosaic model. Key features:

• Phospholipid bilayer: Two layers of phospholipid molecules. Each has a hydrophilic (water-loving) head and two hydrophobic (water-fearing) fatty acid tails. Heads face outward; tails face inward. Creates a stable barrier.
• Integral (intrinsic) proteins: Embedded throughout the bilayer (transmembrane). Include ion channels, carriers, receptors.
• Peripheral (extrinsic) proteins: Attached to the surface (inner or outer) of the bilayer but do not penetrate it.
• Cholesterol (in animal cells only): Intercalated between phospholipids. Acts as a fluidity buffer.
• Glycoproteins and glycolipids: On the outer face only. Form the glycocalyx. Involved in cell recognition and signalling.

"Fluid" means the phospholipid molecules can move laterally within each layer. "Mosaic" means proteins are scattered throughout the bilayer like tiles in a mosaic. The earlier model (Danielli-Davson, 1935) proposed a protein-lipid-protein sandwich, which was incorrect.

Cell Wall

The cell wall is a rigid layer outside the plasma membrane in plant cells (and also in fungi, bacteria, and algae, but with different compositions).

Layers of the plant cell wall

1. Middle lamella: The first layer to form during cell division (as the cell plate). Made of pectin (calcium pectate). It cements adjacent cells together. Shared between neighbouring cells.
2. Primary cell wall: Formed inside the middle lamella. Made of cellulose microfibrils in a matrix of pectin, hemicellulose, and glycoproteins. Present in all dividing cells. Thin and flexible.
3. Secondary cell wall: Formed inside the primary wall in some specialized cells (xylem, sclerenchyma). Additional cellulose layers plus lignin (in wood-forming cells). Provides rigidity.

Plasmodesmata are cytoplasmic channels that pass through the cell wall, connecting adjacent plant cells. They allow communication and transport between cells. In animal cells, the equivalent communication channels are gap junctions.

Endomembrane System

The endomembrane system is a coordinated network of membrane-bound compartments inside eukaryotic cells. All components are connected by vesicle transport. The system includes: rough ER, smooth ER, Golgi apparatus, lysosomes, and vacuoles. Mitochondria and chloroplasts are NOT part of the endomembrane system(they have their own DNA and ribosomes).

Endoplasmic reticulum (ER)

The ER is a network of flattened membrane tubes and sacs (cisternae) distributed throughout the cytoplasm. It is continuous with the outer nuclear envelope.

• Rough ER (RER): Has ribosomes on its outer (cytoplasmic) surface. Synthesizes proteins destined for secretion, the plasma membrane, or the Golgi. Proteins enter the ER lumen as they are made.
• Smooth ER (SER): No ribosomes. Synthesizes lipids (phospholipids, steroids, steroid hormones). Abundant in liver cells (detoxification), adrenal cortex (steroid synthesis), and muscle cells (as sarcoplasmic reticulum, stores Ca2+).

Golgi apparatus

Discovered by Camillo Golgi (1898). The Golgi apparatus consists of 3-8 stacked, flattened membrane-bound sacs (cisternae). It has two faces:

• Cis face (forming face): Faces the ER. Receives vesicles from the ER containing newly synthesized proteins.
• Trans face (maturing face): Faces the plasma membrane. Releases vesicles to the cell surface, plasma membrane, or forms lysosomes.

As proteins move from the cis to the trans side, they are modified by the addition of sugars (glycosylation), phosphate groups, or sulphate groups. The Golgi "sorts" proteins to their correct destinations.

Lysosomes

Lysosomes are single-membrane-bound vesicles formed by the trans face of the Golgi apparatus. They contain about 50 types of acid hydrolase enzymes (proteases, lipases, nucleases, glycosidases) at an acidic pH (4.5-5). They function in:

• Autophagy: Digesting the cell's own worn-out organelles (self-eating).
• Phagocytosis: Digesting foreign bacteria or particles engulfed by the cell.
• Autolysis: If the lysosomal membrane ruptures, enzymes are released and digest the entire cell. This is why they are called "suicidal bags of the cell." (Coined by Christian de Duve, who also won the Nobel Prize for discovering lysosomes.)

Vacuoles

In plant cells, the large central vacuole (surrounded by the tonoplastmembrane) can occupy 70-90% of the cell volume. It stores water, ions, sugars, organic acids, pigments (anthocyanins), and waste products. Water enters the vacuole by osmosis, creating turgor pressure that keeps plant cells firm. In protists, contractile vacuolespump out excess water (osmoregulation) in Amoeba and Paramecium.

Mitochondria

Mitochondria (singular: mitochondrion) are the "powerhouses of the cell," producing ATP by cellular respiration (oxidative phosphorylation). They are found in all eukaryotic cells (plant and animal). Size: 0.5-10 µm. They can change shape and move within the cell.

Structure

• Outer membrane: Smooth, continuous. Permeable to small molecules via protein channels called porins.
• Inner membrane: Folded into cristae (shelf-like projections). The cristae greatly increase the surface area. The inner membrane is the site of the electron transport chain (ETC) and ATP synthesis.
• Intermembrane space: Between the outer and inner membranes. Protons (H+) accumulate here during the ETC, creating a proton gradient (the chemiosmotic gradient).
• Matrix: The fluid enclosed by the inner membrane. Contains Krebs cycle enzymes, circular mitochondrial DNA (naked, no histones), 70S ribosomes, and metabolites.
• F0-F1 particles (ATP synthase/oxysomes): Located on the inner membrane (on the cristae). F0 = proton channel in the membrane. F1 = catalytic knob protruding into the matrix. Protons flow from intermembrane space through F0 into the matrix, driving ATP synthesis at F1.

Semi-autonomous organelle

Mitochondria have their own circular DNA (similar to bacterial DNA), their own 70S ribosomes, and can synthesize some of their own proteins. However, most of their proteins (~90-95%) are encoded by nuclear DNA. This is why they are called semi-autonomous. Their bacterial-like features support the endosymbiotic theory: they evolved from aerobic bacteria engulfed by ancestral eukaryotic cells.

Plastids: Chloroplast and Others

Plastids are double-membrane-bound organelles found only in plant cells and algae. All plastids have their own circular DNA and 70S ribosomes (semi-autonomous, like mitochondria). Plastids can interconvert: for example, a chloroplast can become a chromoplast as a fruit ripens from green to orange/red.

Types of plastids

• Chloroplasts: Green plastids. Site of photosynthesis. Contain chlorophyll a, chlorophyll b, and carotenoids. Light reactions in thylakoid membranes (grana). Calvin cycle (dark reactions) in the stroma.
• Chromoplasts: Contain carotenoid pigments (yellow, orange, red). Found in flowers (petals), ripe fruits, and autumn leaves. Not for photosynthesis; attract pollinators and seed dispersers.
• Leucoplasts: Colourless plastids for storage. Amyloplasts store starch (potato), elaioplasts (lipidoplasts) store oils/fats, aleuroplasts store proteins.

Chloroplast structure

• Outer membrane: Permeable to small molecules.
• Inner membrane: Less permeable; specific transporters.
• Thylakoids: Flattened, disc-like membrane sacs. Contain chlorophyll and photosystems I and II. Site of light reactions.
• Grana: Stacks of thylakoid discs (up to 100 per granum). Connected by stroma lamellae (intergranal lamellae).
• Stroma: Fluid around thylakoids. Contains Calvin cycle enzymes (including RuBisCO), circular chloroplast DNA, 70S ribosomes, and starch grains.

Ribosomes: 70S vs 80S

Ribosomes are the sites of protein synthesis (translation). They are found in all cells (prokaryote and eukaryote) and have no membrane of their own. They are made of ribosomal RNA (rRNA) and proteins.

Types

• 70S ribosomes (prokaryotes + organelles): Large subunit = 50S (23S + 5S rRNA, ~34 proteins). Small subunit = 30S (16S rRNA, ~21 proteins). Found in bacteria, mitochondria, chloroplasts.
• 80S ribosomes (eukaryotic cytoplasm): Large subunit = 60S (28S + 5.8S + 5S rRNA, ~49 proteins). Small subunit = 40S (18S rRNA, ~33 proteins). Found in cytoplasm of all eukaryotic cells, and on the surface of rough ER.

Important: Svedberg units (S) are NOT additive. 50S + 30S = 70S (not 80S) and 60S + 40S = 80S (not 100S). This is because S values measure sedimentation rate, which depends on shape and density as well as mass.

The fact that mitochondria and chloroplasts have 70S ribosomes(same as bacteria) is a key piece of evidence for the endosymbiotic theory. It is also the reason why antibiotics like streptomycin and chloramphenicol can kill bacteria (target 70S) without harming human cells (which have 80S in the cytoplasm).

Cytoskeleton, Cilia, Flagella, Centriole

Cytoskeleton

The cytoskeleton is a network of protein fibres in the cytoplasm of eukaryotic cells. It provides shape, mechanical support, and enables cell movement. Three main types:

• Microtubules: Hollow tubes of tubulin protein. Largest type (25 nm diameter). Form spindle fibres during cell division, cilia, flagella, and the cytoskeleton of centrioles.
• Microfilaments (actin filaments): Made of actin protein. Smallest type (7 nm). Involved in muscle contraction, cell motility, and cytokinesis.
• Intermediate filaments: 10 nm diameter. Made of various proteins (keratin, vimentin). Provide mechanical strength.

Cilia and flagella (eukaryotic)

Cilia are short, numerous projections. Flagella are longer and fewer. Both have the same internal structure: an axoneme with the 9+2 arrangement: 9 doublets of peripheral microtubules surrounding 2 central single microtubules. Dynein arms (motor protein, uses ATP) between adjacent doublets generate the sliding force that causes bending. The basal body anchoring the cilium/flagellum has a 9+0 arrangement (same as centriole: 9 triplets, no central pair).

Note: prokaryotic flagella are completely different: solid, made of flagellin protein, NO 9+2 arrangement, rotate like a propeller.

Centriole

Centrioles are cylindrical structures with a 9+0 arrangement: 9 sets of triplet microtubules arranged in a ring with NO central microtubule pair. They are found in animal cells and lower plant cells (algae, mosses, ferns) but are absent in higher (vascular) plant cells and prokaryotes. Two centrioles perpendicular to each other form the centrosome, which organizes the spindle apparatus during cell division and forms the basal body of cilia/flagella.

Nucleus

The nucleus is the control centre of the eukaryotic cell. It contains the cell's genetic material (DNA) and directs all cellular activities through gene expression. Most cells have one nucleus; some have more (multinucleate: Plasmodium, skeletal muscle fibres). Mature mammalian red blood cells (RBCs) have no nucleus.

Components

• Nuclear envelope: Double membrane (outer + inner membrane) with a perinuclear space between them. Outer membrane is continuous with rough ER (also has ribosomes). Present in all eukaryotes; absent in prokaryotes.
• Nuclear pores: Octagonal channels in the nuclear envelope. Allow selective bidirectional transport: mRNA and ribosomal subunits EXIT; proteins (transcription factors, histones) and nucleotides ENTER.
• Nucleoplasm: Gel-like fluid filling the nucleus. Contains enzymes, nucleotides, and other molecules for DNA replication and transcription.
• Nucleolus: Dense, non-membrane-bound structure. Site of rRNA gene transcription and ribosome assembly (ribosome biogenesis). Disappears during prophase; reappears in telophase. Cells that synthesize many proteins have a large nucleolus.
• Chromatin: Complex of DNA + histone proteins. Euchromatin: loosely packed, light staining, transcriptionally active. Heterochromatin: tightly packed, dark staining, transcriptionally inactive. During cell division, chromatin condenses into visible chromosomes (most condensed in metaphase).

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