Cell: The Unit of Life

Introduction

The cell is the basic structural and functional unit of life. All living organisms are composed of cells, and all new cells arise from pre-existing cells (cell theory). A thorough understanding of cell structure is the foundation of all biological sciences.

Cell Theory

1. 1.Proposed by Matthias Schleiden (botanist, 1838) and Theodor Schwann (zoologist, 1839):
2. 2.All living organisms are composed of cells.
3. 3.The cell is the basic unit of structure and function.

Prokaryotic vs Eukaryotic Cells

| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| Nucleus | No membrane-bound nucleus; nucleoid only | True membrane-bound nucleus |
| Size | 1–10 micrometres | 10–100 micrometres |
| Organelles | No membrane-bound organelles | Membrane-bound organelles present |
| Ribosomes | 70S (50S + 30S) | 80S (60S + 40S) in cytoplasm; 70S in mitochondria/chloroplasts |
| Examples | Bacteria, Cyanobacteria | Plants, Animals, Fungi, Protists |

Prokaryotic Cell Structure

• Cell wall: made of peptidoglycan (murein); absent in Mycoplasma
• Plasma membrane: semi-permeable bilayer
• Mesosome: infolding of plasma membrane; helps in cell wall formation, DNA replication, respiration (site of enzymes in bacteria)
• Nucleoid: region of DNA (double-stranded, circular, naked — no histone proteins). May have plasmids (extra-chromosomal DNA)
• Ribosomes: 70S; attached to mesosomes and in cytoplasm
• Capsule: polysaccharide layer; virulence factor (e.g., · Streptococcus pneumoniae · )
• Flagella: made of flagellin protein; for motility
• Pili/Fimbriae: attachment to surfaces; sex pili for conjugation (gene transfer)

Eukaryotic Cell: The Plasma Membrane

• The Fluid Mosaic Model (Singer and Nicolson, 1972):
• Bilayer of phospholipids (hydrophilic heads outward, hydrophobic tails inward)
• Integral proteins: span the membrane; channel/carrier proteins for transport
• Peripheral proteins: on surface; structural/enzymatic roles
• Cholesterol: in animal cells; regulates fluidity
• Membrane is fluid — lipids and proteins can move laterally (fluid); proteins are embedded like mosaics

Cell Organelles

Endoplasmic Reticulum (ER)

• Network of membrane-bound tubules and sacs (cisternae)
• Rough ER (RER): has ribosomes; synthesises secretory proteins and membrane proteins; forms glycoproteins; continuous with nuclear envelope
• Smooth ER (SER): no ribosomes; lipid and steroid synthesis; detoxification (liver); Ca++ storage (muscle SER → sarcoplasmic reticulum)

Golgi Apparatus (Golgi Complex)

• Stacks of flattened membranous cisternae
• Cis face (receives from ER) → Trans face (forms secretory vesicles)
• Functions: processing, sorting, packaging, and secretion of proteins and lipids; forms lysosomes; produces cell wall components in plants

Lysosomes

• Membrane-bound vesicles with hydrolytic enzymes (acid hydrolases, work at pH 5)
• Formed by Golgi apparatus
• Functions: intracellular digestion; autophagy (self-digestion of damaged organelles); autolysis (self-destruction — "suicide bags" of cell)

Mitochondria

• Double membrane-bound; inner membrane folded into cristae (increases surface area for ATP synthesis)
• Matrix: contains circular DNA, 70S ribosomes, enzymes of Krebs cycle
• Inner membrane: site of Electron Transport Chain (ETC) and ATP synthase (oxidative phosphorylation)
• Powerhouse of the cell — site of cellular respiration (aerobic)
• Semi-autonomous organelles (own DNA + ribosomes) → support endosymbiotic theory

Plastids (only in plant cells and algae)

• Double membrane-bound; contain their own DNA and 70S ribosomes
• Chloroplasts: contain chlorophyll; grana (stacks of thylakoids — site of light reactions) and stroma (fluid — site of dark reactions/Calvin cycle)
• Chromoplasts: coloured plastids (lycopene in tomato, carotene in carrot); attract pollinators/seed dispersers
• Leucoplasts: colourless; store starch (amyloplasts), oils (elaioplasts), proteins (aleuroplasts)
• Plastids can interconvert (e.g., chloroplast → chromoplast as fruit ripens)

Ribosomes

• Non-membrane-bound; site of protein synthesis (translation)
• Eukaryotic cytoplasmic ribosomes: 80S (60S large + 40S small subunit)
• Mitochondria and chloroplast ribosomes: 70S (50S + 30S)
• Prokaryotic ribosomes: 70S — this difference is exploited by antibiotics (e.g., streptomycin, erythromycin target 70S)

Nucleus

• Bounded by nuclear envelope (double membrane with nuclear pores)
• Nucleoplasm: contains chromatin (DNA + histone proteins) and nucleolus
• Nucleolus: site of rRNA synthesis and ribosome assembly; disappears during cell division
• Chromatin: euchromatin (loosely packed, active) vs heterochromatin (tightly packed, inactive)
• During cell division: chromatin condenses into chromosomes. Human somatic cells: 46 chromosomes (23 pairs)

Centrosome and Centrioles

• Present in animal cells and lower plants; absent in higher plants
• Two centrioles at right angles (barrel-shaped, 9+0 arrangement of microtubule triplets)
• Form spindle fibres during cell division (aster formation)

Vacuoles

- Membrane-bound (tonoplast); in plant cells: large central vacuole (maintains turgor, stores pigments, waste); in animal cells: small, temporary (food vacuoles, contractile vacuoles in Amoeba)

Cell Wall (Plant cells)

• Primary cell wall: cellulose, pectin, hemicellulose; flexible; present in growing cells
• Secondary cell wall: inside primary wall; lignin; in xylem tracheids and vessels for rigidity
• Middle lamella: calcium pectate; cement between adjacent cells
• Plasmodesmata: cytoplasmic connections through cell walls; symplastic pathway