Cell theory, formulated by Matthias Schleiden and Theodor Schwann comprises of three fundamental principles:
1. All organisms are composed of one or more cells: This means every living organism, whether unicellular like bacteria or multicellular like humans, is made up of cells or products of cells.
2. Cells are the basic structural and functional unit of living organisms: Cells are the smallest units of life that can perform all vital physiological functions.
3. All cells arise from pre-existing cells: This principle, proposed by Rudolf Virchow, states that new cells are formed only by the division of existing cells (omnis cellula e cellula).

Cells are broadly classified into two types based on their structural characteristics: prokaryotic cells and eukaryotic cells:
Prokaryotic Cells :
• Prokaryotic cells have an incipient nucleus that lacks a nuclear membrane.
• They do not contain membrane-bound organelles.
• Example: Bacteria.
Eukaryotic Cells
• Eukaryotic cells have a well-defined nucleus bounded by a nuclear membrane.
• They contain various membrane-bound organelles.
• Eukaryotic cells include all plant and animal cells.

Single-Celled Organisms: consist of a single, independent cell that carries out all essential functions of life.
Examples:
1. Bacteria: Prokaryotic organisms such as Escherichia coli.
2. Protists: Eukaryotic organisms like Amoeba and Paramecium.
3. Yeasts: Fungi that exist as single cells, such as Saccharomyces cerevisiae.
Few-Celled Organisms: are composed of a small number of cells that work together to perform specific functions.
Examples:
1. Colonial Protists: Some protists, like Volvox, form colonies of a few to several thousand cells.
2. Simple Multicellular Organisms: Certain algae and fungi that exist as small multicellular structures, such as Spirogyra (filamentous algae).
Multi-Celled Organisms: are composed of a large number of specialized cells organized into tissues, organs, and organ systems.
Examples:
1. Plants: Higher plants like Arabidopsis (a model plant) and Oryza sativa (rice).
2. Animals: Diverse animals ranging from simple organisms like Hydra to complex mammals like humans

Cells are structurally organized into three main categories, each contributing to their function and vitality:
1. Outer Covering
o Cell Wall: The cell wall is the outermost, rigid, and protective covering found in plant cells and fungi. It provides structural support and protection, helping to maintain the shape of the cell. The cell wall is primarily composed of cellulose in plants and chitin in fungi.
o Cell Membrane: The plasma membrane, also known as the cell membrane, is a selectively permeable boundary that surrounds the cytoplasm of the cell. It controls the movement of substances in and out of the cell, thereby maintaining the internal environment. This membrane is composed of a phospholipid bilayer with embedded proteins, allowing it to be flexible and dynamic.
2. Nucleus
o The nucleus is often referred to as the "brain or control room" of the cell because it controls and regulates the activities of the cell, including growth, metabolism, and reproduction.
o It is enclosed by a double-layered nuclear envelope, which contains nuclear pores to allow the exchange of materials between the nucleus and the cytoplasm.
o Inside the nucleus, the nucleoplasm contains chromatin (a complex of DNA and proteins) and the nucleolus, which is involved in the synthesis of ribosomes.
3. Cytoplasm
o It is the gel-like substance that fills the cell and surrounds the organelles. It is composed mainly of water, salts, and organic molecules, including proteins, lipids, carbohydrates, and nucleic acids.
o The cytoplasm also contains various organelles suspended within it, such as mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus, and more.
Protoplasm [NUCLEUS + CYTOPLASM]
o The protoplasm constitutes the internal fluid content of the cell where various biochemical reactions occur.
o It includes the cytoplasm, which is a jelly-like substance containing cell organelles and other cellular structures as well as the nucleus.

Cell organelles are specialized structures within cells that perform specific functions, contributing to the overall organization and function of the cell:
Endoplasmic Reticulum (ER)
• Structure: Network of membranes forming flattened sacs (cisternae) and tubules. Rough ER has ribosomes on its surface, while smooth ER lacks ribosomes.
• Function: Rough ER synthesizes and packages proteins for secretion or insertion into membranes. Smooth ER synthesizes lipids, detoxifies drugs and toxins, and stores calcium ions.
Ribosomes
• Structure: Small, non-membranous particles composed of RNA and proteins.
• Function: Site of protein synthesis where RNA is decoded into amino acids to form proteins.
Mitochondria
• Structure: Double-membrane structure with an outer membrane and an inner membrane folded into cristae. The inner membrane encloses the matrix, which contains mitochondrial DNA and enzymes for ATP production.
• Function: Site of cellular respiration and ATP production through aerobic metabolism.
Golgi Apparatus
• Structure: Stack of flattened membrane-bound sacs (cisternae).
• Function: Modifies, sorts, and packages proteins and lipids from the ER into vesicles for transport to other parts of the cell or secretion outside the cell.
Lysosomes • Structure: Membrane-bound vesicles containing digestive enzymes, budded off from Golgi bodies.
• Function: Break down macromolecules (proteins, lipids, carbohydrates) and are responsible for intracellular digestion. They also destroy foreign substances and hence called ‘suicide bags’.
Centrosomes and centrioles
• Structure: The centrosome is a small region of cytoplasm near the nucleus of animal cells. It consists of two cylindrical structures called centrioles which are composed of microtubules arranged in a cylindrical shape.
• Function: Involved in cellular organization, division, and structure in animal cells. They contribute significantly to the process of cell division by organizing spindle fibres, ensuring accurate chromosome segregation, and maintaining overall cellular function.
Plastids: Structure, Types, and Functions
Structure: Plastids are double-membrane-bound organelles found in the cells of plants and some algae. They have their own genetic material in the form of DNA and are semi-autonomous organelles. The outer membrane of plastids is smooth and continuous, while the inner membrane forms various internal structures depending on the type of plastid.
Types of Plastids:
1. Chloroplasts:
o Structure: Chloroplasts are a type of plastid that contain chlorophyll pigments, giving them a green colour. They have a double membrane with an inner membrane forming stacks of thylakoids called grana, surrounded by a fluid-filled stroma.
o Function: Chloroplasts are the sites of photosynthesis in plant cells. They capture light energy and convert it into chemical energy (glucose) through a series of biochemical reactions involving chlorophyll and other pigments.
2. Chromoplasts:
o Structure: Chromoplasts are plastids that contain pigments other than chlorophyll, such as carotenoids (yellow, orange, or red pigments).
o Function: Chromoplasts give colour to fruits, flowers, and other plant parts by producing and storing pigments that attract pollinators and seed dispersers. They also protect chlorophyll from excessive light.
3. Leucoplasts:
o Structure: Leucoplasts are colourless plastids without pigments.
o Function: Leucoplasts function primarily in storing starch, lipids, or proteins depending on the cell type. They are abundant in storage tissues like roots, tubers, and seeds, where they store energy reserves.
Functions of Plastids:
• Photosynthesis: Chloroplasts are the primary site of photosynthesis in plants, where they convert light energy into chemical energy in the form of glucose.
• Pigment Synthesis: Plastids synthesize and store pigments such as chlorophyll and carotenoids, which are essential for light absorption and photoprotection.
• Storage: Plastids, particularly leucoplasts, store various compounds like starch, lipids, and proteins. This stored energy and nutrients are crucial for plant growth, development, and reproduction.

Vacuoles
• Structure: Single membrane-bound, surrounded by a membrane called the tonoplast. Predominantly found in plant cells.
• Function: Store nutrients, waste products, and help maintain turgor pressure in plant cells.
Granules
• Structure: refer to small, often microscopic, particles that serve various specialized functions within the cell. They can be found in both prokaryotic and eukaryotic cells, playing crucial roles in cellular metabolism, storage, and structural support.
• Function: Storage of Energy and Nutrients: Some granules serve as storage depots for energy-rich molecules such as glycogen (in animals) or starch (in plants). These granules can be mobilized when the cell requires additional energy.

1. Cell Wall:
Plant Cells: Have a rigid cell wall made of cellulose, providing structural support and protection.
Animal Cells: Do not have a cell wall; they only have a flexible cell membrane.
2. Chloroplasts:
Plant Cells: Contain chloroplasts, which are the sites of photosynthesis. Chloroplasts contain chlorophyll, the pigment that captures light energy.
Animal Cells: Lack chloroplasts since they do not perform photosynthesis.
3. Vacuoles:
Plant Cells: Typically have a large central vacuole that maintains cell turgor, stores nutrients, and degrades waste products.
Animal Cells: Have small vacuoles but not a large central vacuole.
4. Energy Storage:
Plant Cells: Store energy in the form of starch.
Animal Cells: Store energy in the form of glycogen.
5. Centrioles:
Plant Cells: Do not contain centrioles.
Animal Cells: Contain centrioles that initiate the process of cell division.