1. Discovery of the Cell and Early History

While examining a thin slice of cork (a substance which comes from the bark of a tree), Robert Hooke in 1665 observed that the cork resembled the structure of a honeycomb consisting of many little compartments. He observed this using a self-designed, primitive microscope. He called these boxes "cells". The word cell is a Latin word for 'a little room'.

This was the very first time someone observed that living things consist of separate units, a milestone that completely changed biology.

2. What Are Living Organisms Made Up Of?

2.1 Unicellular vs. Multicellular Organisms

With the discovery of magnifying lenses, the microscopic world was revealed. Based on cellular composition, organisms are divided into:

• Unicellular Organisms: A single cell constitutes the entire living organism. Examples include Amoeba, Chlamydomonas, Paramecium, and bacteria.
• Multicellular Organisms: Many cells group together in a single body and assume different functions in it to form various body parts. Examples include some fungi, plants, and animals.

2.2 Cell Shape, Size, and Division of Labour

Every multi-cellular organism has come from a single cell (the fertilized egg divides to produce cells of its own kind). Cells vary greatly in shape and size, which is related to the specific function they perform.

• Some cells like Amoeba have changing shapes.
• In some cases, the cell shape is more or less fixed and peculiar for a particular type of cell (e.g., nerve cells have a typical long, branched shape to transmit messages).

Division of Labour: Multicellular organisms exhibit a division of labour. This means different parts of the human body perform different functions (e.g., the heart pumps blood, the stomach digests food). Similarly, division of labour is also seen within a single cell. Each cell has specific components called cell organelles. One organelle makes new material, another clears up waste, etc. A cell is able to live and perform all its functions because of these organelles.

3. Structural Organisation of a Cell

If we study a cell under a microscope, we encounter three core features in almost every cell: Plasma membrane, Nucleus, and Cytoplasm. All activities inside the cell and interactions with its environment are possible due to these features.

3.1 Plasma Membrane (or Cell Membrane)

This is the outermost covering of the cell that separates the contents of the cell from its external environment.

• Selectively Permeable: It allows or permits the entry and exit of some materials in and out of the cell. It also prevents the movement of some other materials. Therefore, it is called a selectively permeable membrane.
• Structure: It is flexible and made up of organic molecules called lipids and proteins. We can only observe its structure under an electron microscope.
• Endocytosis: The flexibility of the cell membrane enables the cell to engulf food and other material from its external environment. This process is known as endocytosis. Amoeba acquires its food through such processes.

Movement of Substances across the Plasma Membrane

Some substances like carbon dioxide or oxygen can move across the cell membrane by a process called diffusion. Diffusion is the spontaneous movement of a substance from a region of high concentration to a region of low concentration.

When CO₂ (cellular waste) accumulates in high concentrations inside the cell, and the concentration outside is low, CO₂ diffuses out of the cell. Similarly, O₂ enters the cell by diffusion.

3.2 Cell Wall

Plant cells, in addition to the plasma membrane, have another rigid outer covering called the cell wall. The cell wall lies outside the plasma membrane.

• The plant cell wall is mainly composed of cellulose. Cellulose is a complex carbohydrate that provides structural strength to plants.
• Because of the cell wall, plant, fungal, and bacterial cells can withstand very dilute (hypotonic) external media without bursting. The cell swells, building up pressure against the cell wall. The wall exerts an equal pressure against the swollen cell.

3.3 Nucleus

The nucleus is the control center of the cell. Remember the iodine/safranin solution put on the onion peel? It stains the cells so we can see the nucleus clearly.

• Nuclear Membrane: The nucleus has a double-layered covering called the nuclear membrane. It has pores that allow the transfer of material from inside the nucleus to its outside (to the cytoplasm).
• Chromosomes: The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide.
• DNA & Genes: Chromosomes contain information for the inheritance of characters from parents to the next generation in the form of DNA (Deoxyribo Nucleic Acid) molecules. Chromosomes are composed of DNA and protein. Functional segments of DNA are called genes.
• Chromatin: In a cell which is not dividing, this DNA is present as part of chromatin material, which appears as an entangled mass of thread-like structures. When the cell is about to divide, chromatin organizes into chromosomes.
• Function: The nucleus plays a central role in cellular reproduction (the process by which a single cell divides and forms two new cells). It also determines the way the cell will develop and what form it will exhibit at maturity.

Prokaryotic vs. Eukaryotic Cells

In some organisms like bacteria, the nuclear region of the cell may be poorly defined due to the absence of a nuclear membrane. Such an undefined nuclear region containing only nucleic acids is called a nucleoid.

Feature	Prokaryotic Cell	Eukaryotic Cell
Size	Generally small in size (1-10 µm) (1 µm = 10⁻⁶ m)	Generally large in size (5-100 µm)
Nuclear Region	Poorly defined due to the absence of a nuclear membrane; known as a nucleoid.	Well-defined and surrounded by a double-layered nuclear membrane.
Chromosome	Contains a single chromosome.	Contains more than one chromosome.
Organelles	Membrane-bound cell organelles are absent. (Even chlorophyll in photosynthetic bacteria is associated with membranous vesicles, not plastids).	Membrane-bound cell organelles are present (e.g., mitochondria, plastids, ER).

3.4 Cytoplasm

When we look at the temporary mounts of onion peel or human cheek cells, we see a large region of each cell enclosed by the cell membrane. This region takes up very little stain and is called the cytoplasm.

• The cytoplasm is the fluid content inside the plasma membrane. It also contains many specialised cell organelles.
• Each of these organelles performs a specific function for the cell.
• Note on Viruses: Viruses lack any membranes and hence do not show characteristics of life until they enter a living body and use its cell machinery to multiply. This highlights the significance of membranes!

4. Cell Organelles in Exhaustive Detail

Large, complex cells (especially from multicellular organisms) need a lot of chemical activities to support their complex structure and function. To keep these activities separate from each other, eukaryotic cells use membrane-bound little structures (organelles) within themselves.

4.1 Endoplasmic Reticulum (ER)

The ER is a large network of membrane-bound tubes and sheets. It looks like long tubules or round/oblong bags (vesicles). The ER membrane is similar in structure to the plasma membrane.

• Rough Endoplasmic Reticulum (RER): Looks rough under a microscope because it has particles called ribosomes attached to its surface. Ribosomes (present in all active cells) are the sites of protein manufacture. The manufactured proteins are then sent to various places in the cell via the ER.
• Smooth Endoplasmic Reticulum (SER): Appears smooth (no ribosomes). It helps in the manufacture of fat molecules (lipids), which are important for cell function.
• Membrane Biogenesis: Some of the proteins and lipids synthesized by the ER help in building the cell membrane. This process is known as membrane biogenesis. Some other proteins and lipids function as enzymes and hormones.
• Crucial Function of SER in Animals: In the liver cells of the group of animals called vertebrates, SER plays a crucial role in detoxifying many poisons and drugs.
• Transport: The ER serves as channels for the transport of materials (especially proteins) between various regions of the cytoplasm, or between the cytoplasm and the nucleus. It also acts as a cytoplasmic framework providing a surface for biochemical activities.

4.2 Golgi Apparatus

First described by Camillo Golgi, it consists of a system of membrane-bound vesicles (flattened sacs) arranged approximately parallel to each other in stacks called cisterns.

• These membranes often have connections with the membranes of the ER, forming another portion of a complex cellular membrane system.
• Function: The material synthesized near the ER is packaged and dispatched to various targets inside and outside the cell through the Golgi apparatus. Its primary functions include the storage, modification, and packaging of products in vesicles.
• In some cases, complex sugars may be made from simple sugars in the Golgi apparatus.
• The Golgi apparatus is also involved in the formation of lysosomes.

4.3 Lysosomes (The Suicide Bags)

Lysosomes are a kind of waste disposal system of the cell. They keep the cell clean by digesting any foreign material as well as worn-out cell organelles.

• Mechanism: Foreign materials entering the cell, such as bacteria or food, as well as old organelles, end up in the lysosomes, which break them into small pieces. Lysosomes are able to do this because they contain powerful digestive enzymes capable of breaking down all organic material.
• These enzymes are made by the Rough Endoplasmic Reticulum (RER).
• Why are they called Suicide Bags? During a disturbance in cellular metabolism—for example, when the cell gets damaged—lysosomes may burst and the enzymes digest their own cell. Therefore, lysosomes are known as the 'suicide bags' of a cell.

4.4 Mitochondria (The Powerhouse of the Cell)

Mitochondria are known as the powerhouses of the cell. The energy required for various chemical activities needed for life is released by mitochondria in the form of ATP (Adenosine Triphosphate) molecules.

• ATP: Known as the energy currency of the cell. The body uses energy stored in ATP for making new chemical compounds and for mechanical work.
• Structure: Mitochondria have two membrane coverings. The outer membrane is very porous while the inner membrane is deeply folded. These folds create a large surface area for ATP-generating chemical reactions.
• Semi-autonomous Nature: Mitochondria are strange organelles in the sense that they have their own DNA and ribosomes. Therefore, mitochondria are able to make some of their own proteins!

4.5 Plastids

Plastids are present only in plant cells. There are two main types of plastids based on pigments:

• Chromoplasts (coloured plastids): Plastids containing the pigment chlorophyll are known as chloroplasts. Chloroplasts are essential for photosynthesis in plants. Chloroplasts also contain various yellow or orange pigments in addition to chlorophyll.
• Leucoplasts (white or colourless plastids): These are primarily organelles in which materials such as starch, oils, and protein granules are stored.
• Internal Structure: The internal organisation of the plastid consists of numerous membrane layers embedded in a material called the stroma.
• Plastids are similar to mitochondria in external structure. Like mitochondria, plastids also have their own DNA and ribosomes.

4.6 Vacuoles

Vacuoles are storage sacs for solid or liquid contents.

• They are small sized in animal cells while plant cells have very large vacuoles.
• The central vacuole of some plant cells may occupy 50-90% of the cell volume.
• In plant cells, vacuoles are full of cell sap and provide turgidity and rigidity to the cell. Many substances of importance in the life of the plant cell are stored in vacuoles (e.g., amino acids, sugars, various organic acids, and some proteins).
• In single-celled organisms like Amoeba, the food vacuole contains the food items that the Amoeba has consumed. In some unicellular organisms, specialised vacuoles also play important roles in expelling excess water and some wastes from the cell.

5. Cell Division (Mitosis vs Meiosis)

New cells are formed in organisms in order to grow, to replace old, dead and injured cells, and to form gametes required for reproduction. The process by which new cells are made is called cell division. There are two main types:

• Mitosis: The process of cell division by which most of the cells divide for growth. In this process, each cell called mother cell divides to form two identical daughter cells. The daughter cells have the same number of chromosomes as the mother cell. It helps in growth and repair of tissues in organisms.
• Meiosis: Specific cells of reproductive organs or tissues in animals and plants divide to form gametes, which after fertilization give rise to offspring. They divide by a different process called meiosis which involves two consecutive divisions. When a cell divides by meiosis it produces four new cells instead of just two. The new cells only have half the number of chromosomes than that of the mother cell.