Based on the provided sources, here is a summary of the chapter "The Fundamental Unit of Life: Cell":

The chapter introduces the cell as the basic structural and functional unit of living organisms. It holds a central position in biology similar to the
atom in physical sciences. All living organisms, from archaebacteria and eubacteria to protists, fungi, plants, and animals, are composed of
microscopic units called cells.

Present-day cells share fundamental properties, including using DNA as genetic material, being surrounded by a plasma membrane, and
employing basic mechanisms for energy metabolism. However, they also exhibit great diversity and have evolved different lifestyles. Some
organisms are unicellular, consisting of a single cell capable of independent self-replication, such as bacteria, protozoa (like amoeba), and
yeasts. Other organisms are multicellular, composed of collections of cells that function in a coordinated manner, with different cells
specialized for particular tasks. For instance, the human body has over 200 different kinds of specialized cells. While a single cell performs all
activities in unicellular organisms, multicellular organisms exhibit a division of labour, where specialized cells perform one or a few specific
activities. Reproduction involves only the single cell in unicellular organisms, whereas in multicellular organisms, only specific germ cells
participate in reproduction, with somatic cells remaining intact. Unicellular organisms generally have a short life span compared to multicellular
organisms.

The historical discovery of the cell began in 1665 when Robert Hooke observed the honeycomb-like structure of cork slices through his self-
designed microscope and called the little compartments "cells," meaning "little room" in Latin. This indicated for the first time that living
organisms consisted of smaller structures. Hooke had primarily seen the thickened cell walls. In 1674, Anton von Leeuwenhoek, using an
improved microscope, discovered free-living cells in pond water. Robert Brown discovered and named the nucleus in plant cells in 1831. J.E.
Purkinje gave the term protoplasm for the living fluid substance inside the cell in 1839. Haeckel established in 1866 that the nucleus was
responsible for storing and transmitting hereditary characters.

The Cell Theory was first proposed by Jakob Matthias Schleiden in 1838, stating that all plants consist of cells. Theodor Schwann
independently asserted in 1839 that all animals and plants are made up of cells, forming the basis of the cell theory. This theory was further
refined by R. Virchow in 1855 with the idea that all cells arise only from pre-existing cells (omnis cellulae a cellula). The key postulates of the
cell theory are:
1. All organisms are composed of cells and cell products.
2. All metabolic reactions take place in cells; thus, cells are structural and functional units of life.
3. All cells arise from pre-existing cells only.
4. Every organism starts life as a single cell, though viruses are an exception to cell theory.

Studying cells requires microscopes due to their small size. Common types are the light (or compound) microscope and the electron
microscope. Light microscopes use light and multiple lenses, with magnification ranging from 300 to 1500 times. An electron microscope is a
large instrument using electromagnets for magnification and electrons for illumination. Developed in 1932, it allows observation of subcellular
structures not visible with a compound microscope, with magnification from 100,000 to 500,000 times. Key differences include the type of
lenses (glass vs. electromagnets), illumination source (light vs. electron beam), and the necessity of an internal vacuum in the electron
microscope.

Various activities, such as preparing temporary mounts of onion peels, Tradescantia leaves, root tips, or even cheek cells and observing human
red blood cells or processed eggs in different solutions, demonstrate the presence of cells and processes like osmosis. These observations
show that cells, even from different sources or sizes, are the basic building units. While cell shape and size can vary between organisms and
different parts of a multicellular organism, all cells of higher organisms share basic similarities: a plasma membrane, cytoplasm with
organelles, and a nucleus.

Organisms can be classified as non-cellular (viruses) or cellular (bacteria, plants, animals). Cellular organisms are further divided into
prokaryotes and eukaryotes. Prokaryotic cells are primitive and incomplete, lacking a nuclear membrane around their genetic material (DNA).
Their nuclear material, a single chromosome, is in direct contact with the cytoplasm in an undefined region called the nucleoid. Membrane-
bound organelles are absent, but ribosomes are present. Prokaryotes include archaebacteria, bacteria, and cyanobacteria. Eukaryotic cells are
advanced and complete, containing membrane-bound nuclei and other cellular organelles. They are found in unicellular and multicellular plants
and animals and contain a plasma membrane, nucleus, DNA, cytoplasm, ribosomes, and organelles like mitochondria. Key differences between
prokaryotic and eukaryotic cells include size (prokaryotic smaller), nucleus presence (absent/nucleoid vs. present with nuclear membrane),
chromosome number (single vs. more than one), nucleolus presence (absent vs. present), and presence of membrane-bound organelles
(absent vs. present). A nucleoid is a smaller, free-lying region in the cytoplasm without a membrane, while a nucleus is larger, covered by a
double membrane, contains a nucleolus, and has more DNA associated with histone proteins.

Within multicellular organisms, there is a division of labour among different parts (organs), e.g., the heart pumps blood, the stomach digests
food. Similarly, the cell itself has a division of labour; specific components called cell organelles perform special functions, such as protein
synthesis by ribosomes or clearing waste by lysosomes. These organelles collectively form the basic building blocks, the cells.

Cell shape is ultimately determined by its specific function. Some cells, like Amoeba and white blood cells, have variable shapes, while others, in
most plants and animals, have fixed shapes. Cell shape can be influenced by factors like surface tension, protoplasm viscosity, mechanical
action of adjoining cells, and the rigidity of the cell membrane or cell wall. Cells exhibit diverse shapes, including polyhedral, spherical, spindle-
shaped, elongated, branched, and discoidal.

Cell size varies widely. Bacterial cells are typically 0.2 to 5.0 µm, while eukaryotic cells are larger, mostly 10 to 100 µm. Some cells, like ostrich
eggs (18 cm) or nerve cells with metre-long axons, are visible to the naked eye. The smallest cells are Mycoplasma gallisepticum (about 0.1
µm). Cell volume is fairly constant for a specific cell type regardless of the organism's size. Differences in organ or organism mass depend on
cell number, not volume; larger organisms have more cells. The number of cells in most multicellular organisms is indefinite, but in some, like
nematodes, it is fixed (eutely). Humans are estimated to have about 100 trillion (10^14) cells.

The structure of a cell includes three major functional regions: the cell membrane (or plasma membrane), the nucleus, and the cytoplasm.

1.
The Plasma Membrane is the outer covering of every cell, present in plant, animal, and microbial cells. It is a living, thin, elastic, and
selectively permeable membrane. According to the fluid mosaic model, it is primarily composed of a bilayer of phospholipids with proteins
floating within it. These proteins act as enzymes, transport proteins (permeases), pumps, and receptor proteins. The flexibility of the
membrane allows processes like endocytosis. Selective permeability ensures useful molecules enter, metabolic intermediates remain inside,
and wastes/secretions leave, helping maintain homeostasis. Substances move across the plasma membrane through processes like
diffusion (spontaneous movement of molecules from high to low concentration, e.g., O2, CO2), and osmosis (spontaneous movement of
water through a selectively permeable membrane from high to low water concentration). Cells placed in hypotonic solutions swell due to
water entering (endosmosis), in isotonic solutions maintain size, and in hypertonic solutions shrink due to water leaving (exosmosis). Other
transport methods include mediated transport via specific carrier proteins (facilitated diffusion and active transport). Active transport
requires energy (ATP) to move substances against the concentration gradient. Endocytosis is the ingestion of material through the plasma
membrane, including phagocytosis ("cell eating" of solid particles), potocytosis ("cell drinking" of small molecules/ions), and receptor-
mediated endocytosis. Exocytosis is the extrusion of cellular contents by fusion of a vesicle membrane with the plasma membrane, used for
 removing waste or secreting substances.

2.
The Cell Wall is a rigid, non-living layer outside the plasma membrane in plant cells. It is freely permeable and provides protection, determines
cell shape, and prevents desiccation. It is made mainly of cellulose fibers (microfibrils). The cell wall allows the plant cell to become turgid by
resisting expansion when water enters, provides mechanical strength, is freely permeable, has pores (pits) allowing cytoplasmic connections
(plasmodesmata) to adjacent cells, and adjacent cell walls are glued by a middle lamella. Plasmolysis is the shrinkage of protoplasm away
from the cell wall when a living plant cell loses water in a hypertonic solution. This process occurs only in living cells with a selectively
permeable membrane.

3.
The Nucleus is a large, generally centrally located, spherical component bounded by a double membrane (nuclear envelope). The nuclear
envelope has pores allowing material transfer between the nucleus and cytoplasm. Inside is the nucleoplasm, containing the nucleolus (site
of ribosome formation) and chromatin material. Chromatin is made of DNA (storing genetic information) and proteins (histones). Distinct
DNA segments are genes. During cell division, chromatin condenses into thick, ribbon-like chromosomes. Chromosomes contain hereditary
information in genes and are composed of DNA and proteins. Before division, chromosomes duplicate, forming sister chromatids attached at
the centromere. Eukaryotic species have a fixed number of chromosomes. Body cells (somatic cells) are diploid, containing paired
chromosomes (e.g., 46 in humans). Gametes (sex cells) are haploid, containing half the number (e.g., 23 in humans). The nucleus controls
metabolic activities, regulates the cell cycle, and transmits hereditary traits. Some cells, like human RBCs and phloem sieve tubes, lose their
nuclei.

4.
The Cytoplasm is the part of the cell between the plasma membrane and the nuclear envelope. It consists of the aqueous cytosol and
various cell organelles and inclusions suspended within it. The cytosol is the soluble ground substance (about 90% water) containing
dissolved substances and large molecules forming a colloidal solution. It contains a protein fiber network called the cytoskeleton, which
helps maintain cell shape and assists in movement. Cytosol stores chemicals and is the site of metabolic pathways like glycolysis. Cell
organelles are small, membrane-bound structures within the cytoplasm that perform specific functions, enabling the cell to live and perform
its activities. These include:
 Endoplasmic Reticulum (ER): A membranous network connected to the nuclear envelope and plasma membrane. It has rough ER (RER)
with ribosomes for protein synthesis and smooth ER (SER) without ribosomes for lipid synthesis. Functions include forming a skeletal
framework, material transport, synthesis of lipids, steroids, cholesterol, and hormones, detoxification, enzyme synthesis, and membrane
biogenesis. RER synthesizes secretory proteins.
Ribosomes: Dense, granular particles in the cytosol or attached to RER, made of RNA and protein. They are not membrane-bound and are
present in both prokaryotes and eukaryotes (except mammalian RBCs). Their function is protein synthesis.
Golgi Apparatus: A stack of flattened sacs (cisternae), vesicles, and vacuoles. It is involved in processing, packaging, and transporting
cellular secretions and synthesizing components of the cell wall, plasma membrane, and lysosomes.
Lysosomes: Small, single-membrane vesicles containing digestive enzymes made by RER. They act as intracellular digestive systems,
destroying foreign material and worn-out organelles, hence called "digestive bags," "demolition squads," or "cellular housekeepers". They
can burst and digest the cell itself, earning the name "suicide bags". Lysosomes are significant in processes like defense (WBCs),
autophagy (digesting stored food), metamorphosis (digesting embryonic tissues), and fertilization (digesting ovum membrane).
Mitochondria: Double-membrane bounded organelles with inner folds called cristae. They are the sites of cellular respiration, where food
is oxidized to release energy stored in ATP. Thus, they are known as the "power house" of the cell. ATP is the energy currency used for
cellular activities. Mitochondria contain their own DNA and ribosomes, making them semiautonomous.
Plastids: Occur in most plant cells, containing their own DNA and ribosomes, and are self-replicating. Types include chromoplasts
(coloured, non-green), chloroplasts (green), and leucoplasts (colourless). Chloroplasts contain chlorophyll and are the sites of
photosynthesis, converting light energy into chemical energy (food). They have a double membrane, grana (stacks of thylakoids
containing chlorophyll), and stroma (matrix with photosynthetic enzymes, DNA, ribosomes). Chromoplasts give colour to organs to attract
pollinators/disseminators, and leucoplasts store food (carbohydrates, fats, proteins).
Vacuoles: Membrane-bounded spaces that store fluids or solids. In animal cells, they are small and temporary; in mature plant cells, they
are large and central, bounded by a tonoplast and filled with cell sap. Vacuoles help maintain osmotic pressure and turgidity in plant cells
and store metabolic wastes.
Peroxisomes: Small, single-membrane organelles containing oxidative enzymes like catalase, which breaks down toxic hydrogen
peroxide. They perform oxidative reactions and detoxification.
Centrosome: Found only in animal cells, it consists of two centrioles and helps in cell division by forming the spindle. Plant cells have
polar caps for this function.

The organization of membranes and organelles gives each cell its distinct structure and function. This organization allows cells to perform
basic life functions like respiration, nutrition, waste clearing, and protein formation. The conclusion that the cell is the fundamental structural
and functional unit reinforces the cell theory.