Cell: The Unit Of Life Complete Chapter Notes For Class 11 NEET by PW

A cell may be microscopic, but NEET questions can test it through small differences in structure, location and function. Distinguishing prokaryotic and eukaryotic cells, remembering which organelles are membrane-bound, and connecting structures such as cristae, thylakoids, ribosomes, centrioles and chromosomes with their roles is essential for this chapter.

The PW Cell: The Unit Of Life Complete Chapter One-Shot Revision Video For Class 11 NEET brings the chapter's key concepts together for quick revision. These notes can be used alongside the video to revise important structures, functions, comparisons and NEET-relevant facts before practising questions. 

What Is A Cell: Basic Overview

A cell is the fundamental structural and functional unit of life. A typical cell has:

• A boundary called the plasma membrane

• A semifluid substance called the cytoplasm

• Specialised structures called cell organelles

Cells are called the structural unit of life because all organisms contain cells. Unicellular organisms, such as bacteria, Amoeba, Paramecium, and Euglena, consist of one cell. Multicellular organisms, such as elephants and plants, contain many cells.

Cells are also the functional unit of life because they perform vital activities. In unicellular organisms, one cell performs all life processes. In multicellular organisms, groups of cells work together.

A unicellular organism has an independent existence and can perform all essential life functions. Anything less than a complete cell does not ensure independent living.

Viruses And Cellular Organisation

A virus contains genetic material, usually DNA or RNA, surrounded by a protein coat. However, it lacks:

• Plasma membrane

• Cytoplasm

• Cell organelles

A virus multiplies only after entering a host cell and using its machinery. Therefore, it is considered a link between living and non-living states, not an independent living cell.

The cell is the smallest unit of life. The organisation follows this sequence:

Cells → Tissues → Tissue Systems → Organs → Organ Systems → Complete Organism

Discovery Of The Cell And Cell Theory

Robert Hooke was the first person to use the term cell after observing dead cork cells. These cells had a honeycomb-like structure and retained only their cell walls.

Anton von Leeuwenhoek was the first to observe living cells, including bacteria, blood cells, sperm cells, Amoeba, and Paramecium.

Schwann observed that animal cells have a plasma membrane, while plant cells have an additional cell wall. He proposed that living organisms are made of cells and products of cells.

Schleiden and Schwann formulated the cell theory. In 1855, Virchow added:

Omnis cellula e cellula

This means new cells arise from pre-existing cells by cell division.

The modern cell theory states:

1. All living organisms are made of cells and products of cells.

2. New cells arise from pre-existing cells by cell division.

Cell Size And Shape

Cell size and shape vary according to the organism and the function performed.

• Mycoplasma is the smallest living cell, approximately 0.3 micrometres long.

• A typical eukaryotic cell is about 10–20 micrometres.

• A human red blood cell is about 7 micrometres in diameter.

• The ostrich egg is the largest isolated single cell.

• The neuron is the longest cell.

Examples of cell shapes include:

• Amoeboid white blood cells

• Long and narrow skin epithelial cells

• Elongated xylem tracheids

• Round or oval leaf mesophyll cells

The shape of a cell is related to its function. A neuron is long and branched because it transmits signals over long distances. The shape of a cell is also determined by the function it performs.

Basic Components Of A Cell

The main components of a plant or animal cell are:

1. Cell wall, where present

2. Plasma membrane

3. Cytoplasm

4. Cell organelles

Cell Wall

The cell wall is a rigid, non-living outer layer. It is present in plants, fungi, and bacteria but absent in animals, Amoeba, Paramecium, and Euglena.

The cell wall provides shape, support, and protection.

Plasma Membrane

The plasma membrane is a living, selectively permeable boundary present in all living cells, including prokaryotes and eukaryotes.

Cytoplasm

The cytoplasm is the semifluid matrix enclosed by the plasma membrane. Organelles remain suspended in it. It is the main arena of all cellular activities, and essential metabolic reactions occur in the cytoplasm.

Cell Organelles

Organelles are specialised structures that perform specific functions. They may be:

• Non-membrane-bound: Ribosomes and centrioles

• Double-membrane-bound: Mitochondria, chloroplasts, and nucleus

• Single-membrane-bound: Endoplasmic reticulum, Golgi body, vacuoles, lysosomes, and microbodies

Prokaryotic Cells

Bacteria, cyanobacteria, Mycoplasma, Rhizobium, Lactobacillus, and E. coli are prokaryotes. They are smaller and simpler than eukaryotic cells.

Prokaryotes lack:

• A true nucleus

• Nuclear membrane

• Membrane-bound organelles such as mitochondria, Golgi bodies, and endoplasmic reticulum

Their DNA lies directly in the cytoplasm and is called naked DNA. It is usually double-stranded, circular, and present as a single chromosome. Some bacteria also contain small extra-chromosomal circular DNA called plasmids, which may provide antibiotic resistance.

Bacterial Cell Envelope

From outside to inside, the bacterial cell envelope contains:

Glycocalyx → Cell Wall → Plasma Membrane

• Glycocalyx: Sticky outer layer made of mucopolysaccharides. A loose layer is a slime layer, while a thick layer is a capsule.

• Cell wall: Rigid, peptidoglycan-based layer that gives shape and prevents bursting. Mycoplasma lacks a cell wall.

• Plasma membrane: Living and selectively permeable inner layer.

The three layers of the cell envelope are remembered as Glycocalyx → Cell Wall → Plasma Membrane.

Bacteria are classified by Gram staining:

Bacterial surface structures include:

• Flagella: Motility

• Pili: Conjugation

• Fimbriae: Adhesion

A bacterial flagellum has a basal body, hook, and filament. Bacterial ribosomes are 70S, made of 50S and 30S subunits, and perform protein synthesis. Several ribosomes attached to one mRNA form a polyribosome or polysome.

Inclusion bodies are non-membrane-bound reserve materials. Gas vacuoles help some photosynthetic bacteria remain near the water surface.

Eukaryotic Cells And Organelles

Plants, fungi, protists, and animals are eukaryotes. They possess a true nucleus, nuclear envelope, membrane-bound organelles, and a cytoskeleton.

Plant cells have a cell wall, plastids, and a large central vacuole. Animal cells lack a cell wall and plastids but usually contain centrosomes with centrioles.

Endomembrane System

The endomembrane system includes:

• Endoplasmic reticulum

• Golgi apparatus

• Lysosomes

• Vacuoles

Mitochondria, chloroplasts, and peroxisomes are not included.

Endoplasmic Reticulum

The ER is continuous with the outer nuclear membrane. It contains cisternae and tubules.

Golgi Apparatus

The Golgi apparatus contains stacked cisternae. Its cis face receives transport vesicles from the ER, while its trans face forms secretory vesicles.

The Golgi modifies, packages, and distributes proteins and lipids:

• Protein + carbohydrate = Glycoprotein

• Lipid + carbohydrate = Glycolipid

The Golgi apparatus acts like a delivery service that modifies, packages, and sends cellular materials.

Lysosomes And Vacuoles

Lysosomes contain hydrolytic enzymes such as lipases, proteases, carbohydrases, and nucleases. Their acidic interior supports digestion. They are called suicidal bags because they can digest damaged cell parts.

Vacuoles include:

• Contractile vacuoles: Excretion and osmoregulation in protists

• Food vacuoles: Engulf and store food

• Sap vacuoles: Large central vacuoles in mature plant cells

The plant vacuole is surrounded by the tonoplast. Transport of ions into the vacuole against the concentration gradient requires active transport.

Mitochondria And Plastids

Mitochondria and plastids are semi-autonomous organelles. They contain their own circular DNA, 70S ribosomes, RNA, proteins, and replication machinery.

Mitochondria are the site of aerobic respiration:

• Krebs cycle: Matrix

• Electron transport chain: Inner membrane

• Oxidative phosphorylation: Inner membrane

Their inner membrane forms cristae, which increase surface area. Matrix → Krebs cycle; Inner membrane → ETC and ATP synthesis.

Plastids are of three types:

• Chloroplasts: Photosynthesis

• Chromoplasts: Colour due to carotenoids

• Leucoplasts: Storage

In chloroplasts:

• Calvin cycle and glucose formation occur in the stroma.

• Chlorophyll and the electron transport system occur on the thylakoid membrane.

• One thylakoid is a sac; one stack is a granum; many stacks form grana.

• Stroma lamellae connect grana.

Ribosomes, Centrosome And Cytoskeleton

Ribosomes are non-membrane-bound nucleoprotein particles made of rRNA and proteins. Eukaryotic cytoplasmic ribosomes are 80S, with 60S and 40S subunits. Mitochondria and chloroplasts contain 70S ribosomes.

The animal-cell centrosome contains two perpendicular centrioles. Each centriole has a 9 + 0 cartwheel arrangement: nine peripheral microtubule triplets and no central microtubules. Centrioles form basal bodies and assist in spindle-fibre formation.

The cytoskeleton contains:

• Microtubules: Tubulin; shape, spindle fibres, cilia, and flagella

• Microfilaments: Actin; movement, muscle contraction, and cytokinesis

• Intermediate filaments: Strength and nuclear support

Eukaryotic cilia and flagella have a plasma membrane-covered axoneme with a 9 + 2 arrangement: nine peripheral microtubule doublets and two central microtubules. Cilia are numerous and move surrounding fluid, while flagella are fewer and move the cell directly.

Nucleus And Chromosomes

The nucleus controls cellular activities and contains genetic material. It has a double nuclear envelope with a perinuclear space. Nuclear pores permit two-way exchange of RNA and proteins between the nucleus and cytoplasm.

The nucleoplasm contains chromatin and the nucleolus. The nucleolus is a non-membrane-bound site of active rRNA synthesis.

Chromatin contains DNA, histone proteins, non-histone proteins, and some RNA. It condenses into visible chromosomes during cell division.

A chromosome has two chromatids joined by a centromere, also called the primary constriction. The kinetochore is the spindle-fibre attachment site.

Satellite chromosomes possess a secondary constriction called the nucleolar organiser region, which assists in nucleolus formation.

 Cell: The Unit of Life forms the foundation for understanding cellular structure and functions in Class 11 Biology. Focus on the structure, functions and key differences between organelles, along with important comparisons such as prokaryotic and eukaryotic cells and 70S and 80S ribosomes. The PW Cell: The Unit Of Life Complete Chapter One-Shot Revision Video For Class 11 NEET can help you revise these concepts before attempting NEET questions. 

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