NEET Biology: Cell – The Unit of Life

Cells are the basic structural and functional units of life. Every living organism is made up of one or more cells. Cell biology explains how cells are structured, how their organelles perform different functions, and how cells carry out essential life processes.

The study of cells also includes the differences between prokaryotic and eukaryotic cells and the roles of different cell organelles. Cell division through mitosis and meiosis is another important part of cell biology, as it helps in growth, repair, reproduction, and the transfer of genetic information.

Historical Discoveries in Cell Biology

The study of cell biology developed through several important discoveries made by scientists over time. These discoveries helped scientists understand the cell, its nucleus, and its internal structures. 

Overview of a Cell

A cell contains different structures that work together to perform essential life processes. Some components are present in all cells, while structures such as the nucleus are found in eukaryotic cells. Plant and animal cells also differ in some important structures and organelles.

Cell Theory

Cell theory is a fundamental biological principle developed by three scientists:

1. Matthias Schleiden (German Botanist)

In 1838, Schleiden observed various plants, concluding that all plants are composed of different types of cells, which further form tissues.

2. Theodor Schwann (German Zoologist)

In 1839, Schwann studied animal cells, observing a thin outer layer, the cell membrane. He extended this to plant cells, finding an outer cell wall in addition to the cell membrane. He proposed that all plants and animals are composed of cells and products of cells. 

3. Collaborative Postulate (Schleiden & Schwann)

Their combined observations led to the first postulate of cell theory: All living organisms are composed of cells and products of cells. However, they did not explain how new cells were formed.

4. Rudolf Virchow (German Physician)

In 1855, Virchow explained the origin of new cells with "Omnis cellula e cellula" (all cells arise from pre-existing cells). This completed cell theory, adding the understanding of cell reproduction.

Cell Sizes and Shapes

Cells differ in their size and shape depending on their structure and function. Some cells are very small, while others are long or large, making these examples important to remember for NEET Biology. 

Plant Cell and Animal Cell Differences

Plant and animal cells share several common structures, but they also have some important differences. These differences are commonly tested in NEET Biology and are useful for quick revision.

What are Ribosomes: Structure, Types and Functions

Ribosomes are non-membrane-bound structures made up of ribosomal RNA (rRNA) and proteins. They are the site of protein synthesis, so they are commonly called the protein factories of the cell.

Ribosomes are classified based on their sedimentation coefficient, expressed in Svedberg units (S). The S value indicates how a particle sediments during ultracentrifugation and does not represent its weight.

The S values of the two subunits do not add up directly to the S value of the complete ribosome because the Svedberg unit depends on the sedimentation behaviour of the particle.

Prokaryotic Cell

Prokaryotic cells are generally small and simple in structure compared with eukaryotic cells. They do not have a well-defined, membrane-bound nucleus or other membrane-bound cell organelles.

Examples include bacteria, cyanobacteria, archaea, and Mycoplasma. A typical prokaryotic cell is about 1–2 µm in size, while a typical eukaryotic cell is generally about 10–20 µm.

Types and Shapes of Bacteria

Bacteria show different shapes depending on their structure. Four common bacterial shapes are:

Structure of a Prokaryotic Cell

The major structures of a prokaryotic cell and their functions are summarised below:

Cell Envelope of Prokaryotes

The cell envelope provides protection and helps maintain the shape and integrity of the prokaryotic cell. It consists of three main layers arranged from outside to inside.

Gram Staining

Christian Gram developed the Gram staining method, which is used to differentiate bacteria based on their ability to retain the stain.

• Gram-positive bacteria retain the primary stain and appear purple or violet.

• Gram-negative bacteria do not retain the primary stain and appear pink or red after counterstaining.

Eukaryotic Cell

Eukaryotic cells have a well-defined, membrane-bound nucleus and several specialised cell organelles. They are generally more complex and larger than prokaryotic cells.

Important features of eukaryotic cells include:

• A membrane-bound nucleus containing genetic material.

• Several membrane-bound organelles that perform specialised functions.

• A cytoskeleton made of microtubules, intermediate filaments and microfilaments.

• Cilia and flagella containing microtubules and a specialised internal arrangement.

Eukaryotic Cell Membrane

The cell membrane forms the boundary of the cell and controls the movement of substances between the cell and its surroundings. It is mainly made up of lipids and proteins, with carbohydrates present on its outer surface.

The structure and arrangement of these components are explained by the Fluid Mosaic Model, proposed by Singer and Nicolson in 1972.

Fluid Mosaic Model

The membrane is called fluid because its lipid molecules can move within the bilayer. It is called mosaic because different proteins are arranged within or on the lipid bilayer.

Membrane proteins also help in the movement of substances across the membrane.

Eukaryotic Cell Wall

The cell wall is a rigid, non-living structure present outside the plasma membrane in plants, fungi and many algae. It provides shape, mechanical support and protection and is generally permeable to many substances.

Its composition differs among organisms:

In plant cells, the primary cell wall is relatively flexible. A secondary cell wall may develop inside it as the cell matures.

The middle lamella, mainly composed of calcium pectate, helps cement adjacent plant cells together. Plasmodesmata are cytoplasmic connections between adjacent plant cells that allow communication and exchange of materials.

Endomembrane System

The endomembrane system consists of organelles that work together in the synthesis, modification, packaging, transport and digestion of materials inside the cell. The main components are the endoplasmic reticulum, Golgi apparatus, lysosomes and vacuoles.

Mitochondria, chloroplasts and peroxisomes are not included in the endomembrane system.

Endoplasmic Reticulum

The endoplasmic reticulum (ER) is a network of membranous tubules and sacs. It is continuous with the outer membrane of the nuclear envelope.

Golgi Apparatus

The Golgi apparatus was discovered by Camillo Golgi. It consists of flattened membrane-bound sacs called cisternae and plays an important role in the modification, packaging and transport of proteins and lipids.

It has two main faces:

• Cis face: The forming face that receives materials from the ER.

• Trans face: The maturing face that releases processed and packaged materials.

The Golgi apparatus also performs glycosylation, in which carbohydrate groups are added to certain proteins and lipids to form glycoproteins and glycolipids.

Lysosomes, Vacuoles and Microbodies

Several small membrane-bound organelles perform important functions such as digestion, storage and detoxification.

Mitochondria

Mitochondria are double membrane-bound organelles that play a major role in the production of ATP through cellular respiration. They are often called the powerhouses of the cell.

The inner mitochondrial membrane is folded into structures called cristae, which increase the surface area available for important reactions. The internal region enclosed by the inner membrane is called the matrix.

Mitochondria are semi-autonomous organelles because they contain their own double-stranded circular DNA and 70S ribosomes.

Plastids

Plastids are double membrane-bound organelles found in plant cells and euglenoids. They are involved in photosynthesis, colour formation and storage.

Chloroplasts are also semi-autonomous organelles because they contain their own circular DNA and 70S ribosomes.

Cilia and Flagella

Cilia and flagella are hair-like structures involved mainly in movement. Both have a similar internal arrangement of microtubules.

The axoneme is the central core of cilia and eukaryotic flagella and has a characteristic 9+2 microtubule arrangement.

Centrioles and Centrosome

Centrioles are non-membrane-bound structures found mainly in animal cells. They are involved in the formation of spindle fibres during cell division and also form the basal bodies of cilia and flagella.

A centriole has a 9+0 arrangement of microtubule triplets. A centrosome generally contains two centrioles arranged perpendicular to each other.

Nucleus

The nucleus is a double membrane-bound organelle that contains the genetic material and plays a major role in controlling cellular activities. The nuclear envelope contains nuclear pores that allow the movement of substances between the nucleus and cytoplasm.

The main components of the nucleus are:

Chromosomes

Chromosomes are highly condensed structures made mainly of DNA and proteins. They become clearly visible during cell division.

Important chromosome structures include:

• Centromere: The primary constriction of a chromosome.

• Kinetochore: A protein structure associated with the centromere where spindle fibres attach.

• Sister Chromatids: Two identical copies of a chromosome joined at the centromere after DNA replication.

Types of Chromosomes Based on Centromere Position

The position of the centromere determines the relative length of the chromosome arms and its appearance during anaphase.

Cell biology and cell division are important areas for NEET Biology preparation, covering everything from cell structure and organelles to chromosomes, the cell cycle, mitosis and meiosis. A clear understanding of these concepts, along with regular NCERT-based revision and practice, can help students strengthen their fundamentals and approach NEET 2026 with greater confidence.