Cell the unit of life neet notes

What is cell? 

The cell is the basic structural and functional unit of all living organisms. Whether an organism is made up of a single cell (unicellular) or trillions of cells (multicellular), it is the cell that forms the foundation of life. Discovered by Robert Hooke in 1665 through a simple microscope, cells have since been the center of biological studies.

Each cell performs vital activities such as metabolism, growth, reproduction, and response to stimuli, making it a self-sufficient living unit. Over time, advancements in microscopy have revealed the complex internal structure of cells, which includes organelles like the nucleus, mitochondria, endoplasmic reticulum, and others, each performing specific functions.

Understanding the cell is crucial because it helps us comprehend how life functions at the most fundamental level. From genetics to physiology, all life sciences stem from cellular biology.

Discovery of the Cell

The term "cell" was coined by Robert Hooke in 1665 when he observed cork under a microscope. 

Anton van Leeuwenhoek was the first to observe living cells (bacteria and protozoa). 

Cell theory was proposed by Schleiden and Schwann and later modified by Rudolf Virchow, who added that "All cells arise from pre-existing cells".

Types of Cells –

All living organisms are made up of cells, but not all cells are the same. Based on structure, complexity, and function, cells are broadly classified into two main types: prokaryotic and eukaryotic. This classification helps us understand the evolutionary relationships and biological processes in different organisms.

Prokaryotic Cells – 

Prokaryotic cells are the most basic and primitive form of life. The term "prokaryote" comes from the Greek words "pro" (before) and "karyon" (nucleus), meaning "before nucleus." These cells lack a true, membrane-bound nucleus and other membrane-bound organelles, which makes them structurally simpler than eukaryotic cells. 

Organisms with prokaryotic cells include bacteria and archaea. Their genetic material (DNA) is located in a region called the nucleoid, which is not enclosed by a membrane. Despite their simplicity, prokaryotic cells are highly efficient and capable of performing all essential life processes such as reproduction, metabolism, and growth. Prokaryotes usually have a cell wall, plasma membrane, cytoplasm, ribosomes, and sometimes flagella or pili for movement and attachment. Studying prokaryotic cells helps us understand the origin of life and the functions of simpler organisms that play crucial roles in ecosystems, biotechnology, and human health.

Eukaryotic Cells – 

Eukaryotic cells are complex, well-organized cells that contain a true nucleus enclosed by a nuclear membrane. The term "eukaryote" is derived from the Greek words "eu" (true) and "karyon" (nucleus), meaning "true nucleus." These cells also have various membrane-bound organelles that perform specialized functions. 

Eukaryotic cells are found in a wide range of organisms, including plants, animals, fungi, and protists. The presence of organelles such as mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and chloroplasts (in plants) allows these cells to carry out complex tasks efficiently.

Unlike prokaryotic cells, eukaryotic cells can be unicellular or multicellular and are generally larger in size. Their compartmentalized structure allows for better organization, division of labor, and higher levels of control over cellular activities.

Understanding eukaryotic cells is essential to studying human biology, plant physiology, and the vast diversity of multicellular life.

Cell structure: A microscopic marvel of life

Every living organism, from the tiniest bacteria to the largest whale, is made up of cells. These microscopic units are incredibly organized and efficient. Understanding the structure of a cell gives us insight into how life functions at its most basic level.

Main parts of a cell:

1. Cell Membrane (Plasma Membrane):

A thin, flexible boundary that surrounds the cell. It acts like a gatekeeper—allowing certain substances to enter or leave the cell while keeping others out.

2. Cytoplasm:

A jelly-like fluid inside the cell where all the organelles float. It provides a medium for chemical reactions and supports the cell's internal structure.

3. Nucleus:

The control center of the cell. It contains DNA (genetic material) and regulates cell growth, division, and metabolism. It's protected by a nuclear membrane.

Cell Organelles and Their Functions:

Organelle	Function
Mitochondria	Powerhouse of the cell – produces energy (ATP).
Endoplasmic Reticulum (ER)	Transport system. Rough ER has ribosomes (protein synthesis), Smooth ER helps in fat production.
Ribosomes	Build proteins from amino acids.
Golgi Apparatus	Packages and ships proteins and lipids.
Lysosomes	Break down waste and unwanted materials (only in animal cells).
Vacuoles	Storage for water, food, and waste. Larger in plant cells.
Chloroplasts	Found only in plant cells – perform photosynthesis.
Cell Wall	Present in plant cells – gives shape and support.

Mitochondria – The Powerhouse of the Cell

Mitochondria are double membrane-bound organelles found in almost all eukaryotic cells. They are often called the “powerhouse of the cell” because they generate ATP (adenosine triphosphate), the energy currency of the cell, through a process called cellular respiration.

Structure of Mitochondria

Mitochondria have a complex structure that suits their function:

1. Outer Membrane

• Smooth and permeable to small molecules
• Contains proteins called porins

2. Inner Membrane

• Highly folded into structures called cristae 
• Contains enzymes for ATP production
• Impermeable to most substances

3. Cristae

• Increase surface area for oxidative phosphorylation

4. Matrix

• Fluid inside the inner membrane
• Contains mitochondrial DNA, ribosomes, and enzymes for the Krebs cycle

Interesting Facts

Mitochondria are semi-autonomous – they can grow and divide on their own.

The number of mitochondria in a cell depends on its energy demand (e.g., muscle cells have more).

Believed to have originated from a symbiotic relationship with a prokaryote (endosymbiotic theory).

Endoplasmic Reticulum (ER) – The Cellular Network

The Endoplasmic Reticulum (ER) is a network of membranous tubules and flattened sacs found throughout the cytoplasm of eukaryotic cells. It plays a key role in synthesizing, folding, modifying, and transporting proteins and lipids.

Structure of ER

Membranous Network: ER is made up of cisternae (flattened sacs), vesicles, and tubules.

Continuous with Nuclear Envelope: The ER is directly connected to the outer membrane of the nucleus.

Dynamic: The structure and extent of ER change with the cell's functional state.

Ribosomes – The Protein Factories of the Cell

Ribosomes are tiny but powerful structures found in every living cell, playing a critical role in one of life’s most essential processes: protein synthesis. Often referred to as the protein factories of the cell, ribosomes read genetic instructions and assemble amino acids into proteins—molecules vital for virtually every function in the body.

Structure and Location

Ribosomes are made up of ribosomal RNA (rRNA) and proteins. Each ribosome has two subunits—a small one and a large one—that come together during protein synthesis. They can be found in two main places:

·        Floating freely in the cytoplasm, where they typically produce proteins used inside the cell.

·        Attached to the endoplasmic reticulum (ER), forming what is known as the rough ER, which makes proteins for export or use in cell membranes.

Ribosomes in Prokaryotes vs. Eukaryotes

Ribosomes exist in both simple prokaryotic cells and more complex eukaryotic cells. However, they differ slightly in size:

·        Prokaryotic ribosomes (like those in bacteria) are smaller (70S type).

·        Eukaryotic ribosomes (like those in human or plant cells) are larger (80S type).

Why Are Ribosomes So Important?

Without ribosomes, cells wouldn’t be able to make proteins, which means they couldn’t grow, repair themselves, or carry out most of their functions. From enzymes and hormones to structural components, proteins are the building blocks of life—and ribosomes are the machines that build them.

Golgi Body – The Packaging and Delivery Center of the Cell 

The Golgi body, also known as the Golgi apparatus or Golgi complex, is a membrane-bound organelle found in eukaryotic cells. It plays a central role in modifying, packaging, and transporting proteins and lipids produced by the endoplasmic reticulum (ER). 

Think of the Golgi body as the “post office” of the cell — it processes and ships products to their correct destination. 

Structure of Golgi Body

Composed of a stack of flattened, membrane-bound sacs called cisternae.

Has three main regions:

1. Cis face – the receiving side, located near the ER.

2. Medial cisternae – middle part where processing occurs.

3. Trans face – the shipping side, where modified products are sent to their destinations.

Points to Remember

Golgi bodies are absent in prokaryotic cells.

They work closely with the ER — proteins/lipids made in the ER are sent to the Golgi for modification.

In plant cells, the Golgi bodies are called dictyosomes.

NEET/Board Exam Tips

• Common question: Functions of Golgi body
• Understand cis vs trans face
• Know its role in lysosome formation and protein secretion

Lysosomes – The Waste Disposal System of the Cell

Lysosomes are membrane-bound organelles found in eukaryotic animal cells. They are often referred to as the "suicidal bags" or "clean-up crew" of the cell because they contain digestive enzymes that break down waste materials, damaged organelles, and invading pathogens.

Structure of Lysosomes

Single membrane-bound vesicles

Contain about 40+ hydrolytic enzymes (like proteases, lipases, nucleases)

Enzymes are active in an acidic pH (~5) maintained by proton pumps

Formed from the Golgi apparatus

Types of Lysosomes

1. Primary Lysosome – Newly formed, contains inactive enzymes

2. Secondary Lysosome – Formed by fusion with a vesicle, active in digestion

3. Residual Body – Contains undigested waste

4. Autophagic Vesicle – Digests the cell’s own organelles

Important Points to Remember

Lysosomes are prominent in white blood cells (WBCs) for destroying microbes.

They are not found in plant cells — plant cells use vacuoles for storage and waste management.

Enzymes in lysosomes are synthesized by RER and packaged by the Golgi body.

NEET/Board Exam Tips

Common question: “Why are lysosomes called suicidal bags?”

Know difference between autophagy and autolysis

Understand lysosome vs vacuole

Vacuoles – The Storage Compartments of the Cell

Vacuoles are membrane-bound organelles that act as storage sacs in cells. They are filled with water, enzymes, ions, and other substances. Vacuoles play different roles in plant and animal cells, and are especially large and important in plant cells.

Vacuoles in Plant Cells

• Occupy 50–90% of the cell's volume
• Maintain turgor pressure – helps the plant stand upright
• Store nutrients, pigments, and waste products
• Can contain toxic compounds to defend against herbivores
• Important in cell growth (by absorbing water)

Vacuoles in Animal Cells

• Smaller and temporary
• Help in storage, waste disposal, and transport of substances
• In some protists (like Amoeba):
• Contractile vacuole: Maintains water balance (osmoregulation)
• Food vacuole: Helps in digestion of engulfed food

Chloroplasts – The Green Energy Producers of the Cell

Chloroplasts are double membrane-bound organelles found in plant cells and some protists (like algae). They are responsible for photosynthesis – the process of converting light energy into chemical energy (glucose). Because of this, chloroplasts are often called the “kitchen of the cell.”

Facts About Chloroplasts

• Only in plant cells and algae
• Have their own DNA and 70S ribosomes → semi-autonomous
• Believed to have evolved through endosymbiosis (like mitochondria)
• Larger than mitochondria and usually lens-shaped

NEET/Board Exam Tips

• Labelled diagram of chloroplast is often asked
• Understand light reaction (in grana) and dark reaction (in stroma)
• Know that chloroplasts are absent in animal and prokaryotic cells

Cell Wall – The Protective Wall of the Cell

The cell wall is a rigid outer covering found in plant cells, fungi, bacteria, and some algae, but absent in animal cells. It provides structural support, protection, and helps maintain the shape of the cell. In plants, the cell wall is mainly made up of cellulose, a tough carbohydrate.

Points to Remember

• Present in: Plant cells, fungal cells, bacteria, some protists
• Absent in: Animal cells
• Not a living structure – it is permeable
• Cell membrane lies just beneath the cell wall

NEET/Board Exam Tips

• Know differences between plant and animal cells
• Remember components of cell walls in different organisms
• Understand functions and structure layers

Differnces between plant and animal cells:

Feature	Plant Cell	Animal Cell
1. Cell Wall	Present (made of cellulose) – gives shape and support	Absent
2. Shape	Usually rectangular or fixed shape	Usually round or irregular
3. Chloroplast	Present – used for photosynthesis	Absent
4. Vacuole	One large central vacuole (for storage and pressure)	Many small vacuoles (mainly for storage)
5. Lysosomes	Rare or absent	Present – help in digestion and waste removal
6. Centrioles	Absent in most plant cells	Present – help in cell division
7. Energy Production	Uses mitochondria and chloroplasts	Uses only mitochondria
8. Mode of Nutrition	Autotrophic (makes own food via photosynthesis)	Heterotrophic (depends on others for food)
9. Plasmodesmata	Present – for cell-to-cell communication	Absent
10. Cilia/Flagella	Rare in plant cells	Often present (for movement in some cells like sperm)

Plant Cell

A plant cell is a eukaryotic cell found in plants. It is unique because it contains organelles that help the plant make its own food through photosynthesis and maintain a rigid shape.

Animal Cell

An animal cell is also a eukaryotic cell, but it lacks structures needed for photosynthesis. Instead, animal cells get energy from food and are usually more flexible in shape.

Remember:

Both are eukaryotic and have organelles like nucleus, mitochondria, ER, Golgi, etc.

Main difference lies in presence of cell wall and chloroplast in plant cells.

Interesting Fact:

• The longest cell in the human body is the nerve cell.
• The largest cell (visible to the naked eye) is the ostrich egg.

Why is the Cell Called the Unit of Life?

Because all biological functions – from nutrition, respiration, reproduction to excretion – take place within the cell. It's where life begins.

Quick Tips for NEET/Board Exams:

• Nucleus = Brain of the cell
• Mitochondria = Powerhouse
• Ribosomes = Protein factory
• Golgi body = Post office of the cell
• Lysosomes = Clean-up crew

The Living World

Biological Classification

Plant Kingdom

Animal Kingdom Classification

Morphology of Flowering Plants class 11th

Anatomy of flowring plants

Structural Organisation in Animals class 11th

NEET Biology Full Syllabus

NEET Physics Full Syllabus

NEET Chemistry Full Syllabus

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