The cell is the basic structural and functional unit of all living organisms. Robert Hooke discovered the cell in 1665 when he observed a thin slice of cork under a microscope and saw box-like structures which he called "cells". Anton van Leeuwenhoek was the first to observe living cells. The cell theory, proposed by Schleiden and Schwann in 1839, states: (1) All living organisms are made of cells. (2) The cell is the basic unit of life. (3) All cells arise from pre-existing cells (added by Virchow).

Organisms made of a single cell are called unicellular organisms. Examples: Amoeba, Paramecium, bacteria, Chlamydomonas. A single cell performs all life functions — nutrition, respiration, excretion, reproduction. Organisms made of many cells are called multicellular organisms. Examples: humans, plants, animals, fungi. In multicellular organisms, cells are specialised to perform specific functions. A group of similar cells performing a specific function forms a tissue.

Prokaryotic cells have no membrane-bound nucleus; the genetic material (DNA) lies in the cytoplasm in a region called the nucleoid. Examples: bacteria, blue-green algae (cyanobacteria). Eukaryotic cells have a well-defined nucleus enclosed by a nuclear membrane. Examples: plant cells, animal cells, fungi, protists. Prokaryotic cells are generally smaller and simpler. Eukaryotic cells contain membrane-bound organelles like mitochondria, endoplasmic reticulum, and Golgi apparatus.

The cell membrane is the outermost boundary of the cell in animal cells; in plant cells it lies just inside the cell wall. It is a thin, flexible, and selectively permeable membrane made of lipids and proteins (fluid mosaic model). Functions: (1) Gives shape to the cell. (2) Controls movement of substances in and out of the cell. (3) Protects internal organelles. Movement of substances across the membrane occurs by diffusion, osmosis, and active transport.

The cell wall is a rigid outer covering found in plant cells, fungal cells, and prokaryotes — absent in animal cells. In plants, it is made of cellulose. Functions: (1) Provides structural strength and rigidity to the cell. (2) Protects the cell from mechanical damage. (3) Prevents excessive osmotic uptake of water. (4) Allows free passage of materials, hence it is fully permeable. The cell wall lies outside the plasma membrane and gives the plant its firm structure.

The nucleus is the control centre of the cell. It is surrounded by a double-layered nuclear membrane with pores (nuclear pores) that allow exchange of materials. The nucleus contains: (1) Nucleoplasm — the fluid inside the nucleus. (2) Chromatin — a network of DNA and proteins that condenses into chromosomes during cell division. (3) Nucleolus — a dense spherical body inside the nucleus that produces ribosomes. The nucleus controls all cell activities and carries hereditary information in the form of genes (DNA).

Cytoplasm is the jelly-like fluid (cytosol) that fills the cell between the nucleus and the cell membrane. It contains various organelles: (1) Mitochondria — powerhouse of the cell, site of aerobic respiration, produces ATP. (2) Endoplasmic Reticulum (ER) — network of membranes; Rough ER has ribosomes and makes proteins; Smooth ER makes lipids. (3) Ribosomes — tiny organelles that synthesize proteins; found on rough ER and free in cytoplasm. (4) Golgi Apparatus — packages and dispatches proteins and lipids.

Plastids are found only in plant cells and some algae. Types: (1) Chloroplasts — contain the green pigment chlorophyll, the site of photosynthesis. (2) Chromoplasts — contain coloured pigments (red, yellow, orange) responsible for colours in flowers and fruits. (3) Leucoplasts — colourless plastids that store food in the form of starch, oils, or proteins. Chloroplasts have their own DNA and ribosomes, similar to mitochondria, supporting the endosymbiotic theory.

Vacuoles are membrane-bound storage sacs within the cell. In plant cells, a large central vacuole (tonoplast) occupies up to 90% of the cell volume and maintains cell turgidity (osmotic pressure). Functions: (1) Store water, food, waste, and other materials. (2) Maintain osmotic pressure. (3) Provide rigidity to plant cells. In animal cells, vacuoles are small and temporary. In unicellular organisms like Amoeba, food vacuoles digest food, and contractile vacuoles expel excess water.

Diffusion is the movement of molecules from a region of high concentration to low concentration without any energy. Example: CO2 and O2 exchange in cells. Osmosis is the movement of water molecules across a selectively permeable membrane from a dilute solution (low solute) to a concentrated solution (high solute). Plasmolysis: when a plant cell is placed in a hypertonic solution, water moves out, the vacuole shrinks, and the cell membrane pulls away from the cell wall — this is plasmolysis. The cell becomes flaccid (limp).

Plant Cell: has cell wall (cellulose), has chloroplasts, has large central vacuole, has plastids, no centrioles (except lower plants). Animal Cell: no cell wall, no chloroplasts, vacuoles are small and temporary, no plastids, has centrioles (for cell division). Both have: cell membrane, nucleus, mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus. Cell wall and chloroplasts are the two most important features that distinguish plant cells from animal cells.

Lysosomes are membrane-bound organelles containing powerful digestive enzymes. They are produced by the Golgi apparatus. Functions: (1) Digest foreign matter, bacteria, and old/damaged organelles — hence called the "suicidal bags" of the cell (term by Christian de Duve). (2) Break down cellular waste. (3) During starvation, lysosomes digest the cell's own organelles to provide energy (autophagy). If the lysosomal membrane bursts, the enzymes digest the entire cell.

The endoplasmic reticulum (ER) is a network of membrane-bound tubules and sheets extending from the nuclear envelope throughout the cytoplasm. There are two types: Rough ER (RER) — studded with ribosomes, involved in protein synthesis and transport; and Smooth ER (SER) — lacks ribosomes, involved in lipid synthesis, detoxification and calcium storage. RER produces membrane proteins and secretory proteins. SER is abundant in liver cells and muscle cells. ER also acts as a transport system within the cell.

Mitochondria are double membrane-bound organelles found in all eukaryotic cells. The outer membrane is smooth while the inner membrane is folded into finger-like projections called cristae, which increase surface area for ATP production. The matrix (inner fluid) contains its own circular DNA, ribosomes and enzymes. Mitochondria perform cellular respiration, breaking down glucose to produce ATP — the energy currency of the cell. Because they have their own DNA and ribosomes, they are believed to have originated from ancient bacteria (endosymbiotic theory). Number of mitochondria is highest in cells with high energy demand like muscle cells.

The Golgi apparatus (Golgi body/Golgi complex) is a stack of flattened, membrane-bound sacs called cisternae. It was discovered by Camillo Golgi in 1898. It acts as the post office or processing and packaging center of the cell. Functions: (1) Processes and modifies proteins received from RER, (2) Packages materials into vesicles for transport, (3) Forms lysosomes, (4) Produces the cell plate during plant cell division. Materials enter from the cis face (near ER) and exit from the trans face (toward plasma membrane). It is especially prominent in secretory cells like pancreatic cells.

Ribosomes are the smallest cell organelles with no membrane. They are the site of protein synthesis in the cell. Composition: made of rRNA (ribosomal RNA) and proteins. Found in two locations — free in cytoplasm (make proteins for cell's own use) and attached to RER (make proteins for export/membrane use). In prokaryotes: 70S type (50S + 30S subunits). In eukaryotes: 80S type (60S + 40S subunits). Mitochondria and chloroplasts have their own 70S ribosomes, supporting their bacterial origin. Cells with high protein production (like pancreatic acinar cells, liver cells) have very large numbers of ribosomes.

Osmosis is the movement of water molecules through a selectively permeable membrane from a region of higher water concentration (lower solute concentration) to a region of lower water concentration (higher solute concentration). Three types of solutions based on osmotic concentration: (1) Hypotonic — less solute than cell; water enters cell → cell swells (turgid in plants, may burst in animals = lysis). (2) Hypertonic — more solute than cell; water leaves cell → cell shrinks (plasmolysis in plants, crenation in animals). (3) Isotonic — equal solute as cell; no net movement of water. Osmosis is critical for water absorption by roots, maintaining cell turgidity, and kidney function.

Prokaryotic cells: No true nucleus (nucleoid region instead), no membrane-bound organelles, smaller (1–10 µm), 70S ribosomes, cell wall always present (peptidoglycan in bacteria), single circular DNA, reproduce by binary fission. Examples: bacteria, blue-green algae (cyanobacteria). Eukaryotic cells: True membrane-bound nucleus, membrane-bound organelles (mitochondria, ER, Golgi, etc.), larger (10–100 µm), 80S ribosomes, cell wall present in plants and fungi only, linear DNA associated with histones, reproduce by mitosis/meiosis. Examples: plant cells, animal cells, fungi, protists. Key similarity: both have plasma membrane, ribosomes and DNA.

Cell division is the process by which a parent cell divides into two or more daughter cells. It is essential for growth, repair, reproduction and replacement of worn-out cells. Types: (1) Amitosis — direct division without spindle formation; seen in primitive organisms like Amoeba. (2) Mitosis — produces two genetically identical diploid daughter cells; occurs in somatic (body) cells; responsible for growth and repair. (3) Meiosis — produces four haploid daughter cells genetically different from parent; occurs in reproductive organs; responsible for formation of gametes. Each cycle of cell division is called the cell cycle, consisting of Interphase (preparation) and Mitotic phase (actual division).

Key organelles and their one-line functions: Cell membrane — controls entry/exit of substances. Cell wall — provides rigidity and protection (plant cells only). Nucleus — control centre; contains hereditary material DNA. Mitochondria — ATP production (cellular respiration). Chloroplast — photosynthesis (plant cells only). ER (Rough) — protein synthesis and transport. ER (Smooth) — lipid synthesis, detoxification. Golgi apparatus — processing, packaging and secretion. Ribosomes — protein synthesis. Lysosomes — intracellular digestion (suicide bags). Vacuoles — storage; large central vacuole maintains turgor in plants. Centrosome — cell division in animal cells. Remember: Plant cells have cell wall, chloroplasts and large vacuole. Animal cells have centrioles and lysosomes.