1. Introduction: The Cell as the Basics of Life

Cells are the basic structural and functional units of all living organisms. The Cell Theory states:

1. All living organisms consist of one or more cells,
2. The cell is the fundamental unit of life,
3. All cells arise only from pre-existing cells

Robert Hooke first coined the term ‘cell’ in 1665 after observing cork under a microscope. Schleiden and Schwann further developed the idea that both plants and animals are composed of cells, while Virchow later asserted that all cells derive from existing cells

2. Types of Cells: Prokaryotes vs Eukaryotes

Prokaryotic cells (bacteria, cyanobacteria) lack a nucleus and membrane-bound organelles. They possess a cell wall (except Mycoplasma), a circular DNA molecule, ribosomes, and sometimes flagella and pili .

Eukaryotic cells (plants, animals, fungi, protists) feature a true nucleus and numerous membrane-bound organelles, supported by a cytoskeleton .

3. Structural Organization of an Eukaryotic Cell

Most eukaryotic cells have three fundamental components

• Plasma membrane
• Cytoplasm (cytosol + organelles)
• Nucleus

3.1 Plasma (Cell) Membrane

A flexible, semi‑permeable barrier made of a lipid bilayer embedded with proteins, cholesterol, and glycolipids. It separates the cytoplasm from the external environment and regulates transport, signaling, adhesion, and structural support .

3.2 Cytoplasm

The fluid content of the cell (cytosol) includes various organelles:

• Ribosomes: Non-membrane structures responsible for protein synthesis (70S in prokaryotes, 80S in eukaryotes)
• Endoplasmic Reticulum (ER):
• Rough ER (with ribosomes): synthesizes glycoproteins and phospholipids.
• Smooth ER: synthesizes lipids, steroids, and detoxifies chemicals
• .Golgi Apparatus: Series of flattened cisternae involved in protein/glycolipid modification, packaging, and secretion. Cisternae mature from cis to trans face
• .Lysosomes: Membranous vesicles containing hydrolytic enzymes; function in intracellular digestion, autophagy, and recycling of cell materials
• .Vacuoles: Prominent in plant cells, bounded by tonoplast, store sap, pigments, waste, and maintain turgor pressure; in protists and animals, vacuoles play roles like contractile osmoregulation
• Mitochondria: Powerhouses of the cell; double-membrane bound; site of ATP production and apoptosis. Inner membrane has cristae with enzymes, matrix contains circular DNA and ribosomes .
• Plastids (in plant cells):
• Chloroplasts (photosynthesis),
• Chromoplasts (pigment),
• Leucoplasts (storage)
• Cytoskeleton: Internal framework of microtubules, microfilaments, and intermediate filaments; maintains cell shape, anchors organelles, facilitates movement
• Centrosome & Centrioles: Non-membrane organelles; centrioles help organize microtubules and form spindle fibers and basal bodies for cilia/flagella
• Cilia & Flagella: Hair-like projections with “9+2” microtubule arrangement for movement; emerge from basal bodies.

3.3 Nucleus

Membrane-bound organelle containing DNA. It consists of:

• Nuclear envelope: Double‑membrane with nuclear pores.
• Nucleoplasm: Matrix inside the nucleus.
• Chromatin: DNA-protein complex that condenses into chromosomes during division.
• Nucleolus: Site of ribosomal RNA synthesis and ribosome assembly

Functions:

• Regulates cell activities via gene expression
• Ensures integrity and transmission of genetic material
• Controls cell cycle and division processes.

4. Cell Wall & Plasmodesmata (Plant-Specific)

Plants, fungi, and some prokaryotes have a rigid cell wall outside the cell membrane:

• Plant cell wall: Composed of cellulose, hemicellulose, pectin, and proteins; primary wall in young cells, secondary wall in mature cells
• .Plasmodesmata: Channels through cell walls that connect cytoplasm of neighboring cells, enabling communication and transport .

5. Endomembrane System

Includes nucleus, ER, Golgi apparatus, lysosomes, vacuoles, and plasma membrane — responsible for synthesis, packaging, transport, and degradation of biomolecules .

6. Conceptual Summary Table

Component Type Key Features & Functions

Cell Theory – Universal principles of cellular structure and generation

Prokaryotic Simple Cell cellLacks nucleus; with cell wall; 70S ribosomes

Eukaryotic CellComplex cellMembrane-bound organelles; 80S ribosomes

Plasma Membrane Membrane Lipid bilayer with proteins; transport and signaling

Ribosomes Organelle 70S/80S; protein synthesis

ER (Rough, Smooth) Organelle Protein/lipid synthesis

Golgi Apparatus Organelle Modifies & packages proteins

Lysosomes Organelle Digestive enzymes; recycling

Vacuoles Organelle Storage; turgor pressure

Mitochondria Organelle ATP production; contains DNA

Plastids Organelle Photosynthesis, storage, pigment

Cytoskeleton Proteinage Shape, support, movement

Centrosome/Centriole Organelle Microtubule organizing center

Cilia & Flagella Locomotion Extensions(9+2 structure)

Nucleus Organelle Genetic info, control center

Cell Wall (plants) Structure Rigid support; cellulose

Plasmodesmata Channels Cell-to-cell transport

🧠 Important Concepts & Quick Recap

• Cell Theory is foundational.
• Prokaryotes are simpler and lack organelles.
• Eukaryotes are structurally complex and compartmentalized.
• Organelle functions are specialized—from energy (mitochondria) to waste disposal (lysosomes).
• Cell wall & plasmodesmata are unique features of plant cells.
• The endomembrane system enables efficient handling of biomolecules.