EduAiTutors BlogMay 6, 2026•23 minutes

Cell Biology for NEET 2026: Complete Chapter Guide with PYQs, Common Mistakes and Topper Strategy

anilgupta

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Every year, Cell Biology gives NEET aspirants between 8 and 10 marks across three chapters. Students who understand exactly which concepts get tested, and why, collect these marks with barely any effort in the exam hall. Students who treat Cell Biology as a memory exercise lose marks they had every right to score.

This guide covers the complete Cell Biology unit for NEET 2026: Cell Structure and Function, Biomolecules, and Cell Division. It is built on 15 years of classroom experience and a thorough analysis of every NEET question asked from these chapters from 2010 to 2025. You will find complete theory, PYQ tables, common mistakes and a topper strategy in one place. Read it once with your NCERT open. Revise it twice more in the weeks before your exam.

How Much Cell Biology Actually Matters in NEET

The three chapters that make up the Cell Biology unit contribute the following to your NEET Biology score:

Chapter	NEET Weightage	Avg Questions Per Year	Total Questions (2010-2024)	Difficulty
Cell: The Unit of Life	5.17%	1 to 2	62	Medium
Biomolecules	4.08%	1 to 2	49	Medium
Cell Cycle and Cell Division	8.95%	4 to 5	54	Medium
Cell Biology Unit Total	~18%	7 to 9	165+	Medium

This is not a chapter you can afford to skip or skim. At 7 to 9 questions contributing 28 to 36 marks, Cell Biology is the single largest unit in Class 11 Biology for NEET. The good news is this: the question types are predictable, the concepts repeat year after year, and a student who prepares this unit systematically can expect to score 80 to 90 percent of available marks from it.

Here is another fact most students do not know: 70 percent of all NEET Cell Biology questions are rated medium difficulty. There are almost no hard questions from this unit. The marks are sitting there for any student who reads NCERT carefully and practises PYQs. What follows is exactly how you do that.

Cell Theory: The Foundation Every Examiner Tests

Cell Theory is one of the most under-revised topics in Cell Biology despite generating direct NEET questions in multiple years. Students assume it is too basic to appear. It is not.

The three postulates of Cell Theory as stated in NCERT:

All living organisms are composed of cells
The cell is the basic structural and functional unit of life
All cells arise from pre-existing cells

The scientists behind Cell Theory:

Robert Hooke observed dead cork cells in 1665 and coined the word “cell.” His observation was structural, not functional. He saw empty compartments in cork under his microscope and named them after small rooms in a monastery.

Matthias Schleiden (1838) stated that all plants are made of cells. Theodore Schwann (1839) extended this to animals. Rudolf Virchow (1855) added the third postulate: cells come only from pre-existing cells (Omnis cellula e cellula). This final addition by Virchow is what transforms Cell Theory from an observation into a scientific principle. The year and scientist behind each postulate are directly tested in NEET.

The one NEET trap in Cell Theory:

NEET has asked: “Who proposed the cell theory?” Many students write Schleiden and Schwann. The technically correct answer, depending on how the question is framed, includes Virchow as the person who completed the modern cell theory by adding the third postulate. Read the question carefully. If it asks “who completed” or “who added to” cell theory, the answer is Virchow. If it asks “who proposed the original cell theory,” the answer is Schleiden and Schwann.

Prokaryotic vs Eukaryotic Cells: The Most Tested Distinction in NEET Cell Biology

The comparison between prokaryotic and eukaryotic cells is the single most reliable source of questions in the Cell: The Unit of Life chapter. It appears almost every year in some form, either as a direct comparison question, an organelle identification question, or a statement-based true or false question.

The Complete Comparison Table for NEET:

Feature	Prokaryotic Cell	Eukaryotic Cell
Size	1 to 10 micrometers	10 to 100 micrometers
Nuclear membrane	Absent	Present
Nucleoid	Present (no membrane)	Absent
Membrane-bound organelles	Absent	Present
Ribosome size	70S (50S + 30S)	80S (60S + 40S)
Cell wall	Present (peptidoglycan in bacteria)	Present in plants (cellulose), absent in animals
Plasmid	Present	Rare (absent in most)
Mitosis / Meiosis	Absent	Present
DNA type	Circular, no histone	Linear, with histone
Examples	Bacteria, Cyanobacteria, Mycoplasma	Fungi, Plants, Animals, Protists

Three facts from this table that NEET tests most often:

Fact 1: Ribosome size in prokaryotes is 70S, not 80S. The ribosomes inside mitochondria and chloroplasts are also 70S, which is why these organelles are believed to have originated from ancient prokaryotes (endosymbiotic theory). NEET has asked about the ribosome size of mitochondria specifically to test whether students know this connection.

Fact 2: Mycoplasma is the smallest and simplest known cell. It is prokaryotic and has no cell wall. NEET has used Mycoplasma in questions asking for the smallest living organism and in questions about organisms without a cell wall.

Fact 3: The nucleoid region of prokaryotes. The DNA in prokaryotes is not enclosed by a membrane. It exists in a region called the nucleoid. This region is not bounded by any membrane but is still clearly visible under an electron microscope. Students frequently confuse nucleoid (prokaryotic) with nucleus (eukaryotic) and lose marks.

Common Mistake in PYQs:

NEET 2023 asked whether prokaryotic cells have membrane-bound organelles. Many students answered “yes” because they confused the cell membrane (which prokaryotes do have) with membrane-bound internal organelles (which they do not). The cell membrane is not the same as membrane-bound organelles. Prokaryotes have the former but not the latter.

Cell Membrane: The Fluid Mosaic Model You Must Know at Every Level of Detail

The fluid mosaic model of the cell membrane is one of the highest-density topics in NEET Cell Biology. It generates questions every two to three years and the details tested go well beyond a surface-level understanding.

The model was proposed by S.J. Singer and G.L. Nicolson in 1972. This year and both names appear in NEET questions. Do not memorise one name and forget the other.

The four components of the fluid mosaic model:

Phospholipid Bilayer
The basic structure is a bilayer of phospholipid molecules. Each phospholipid has a hydrophilic (water-loving) head that faces outward toward the aqueous environment, and two hydrophobic (water-fearing) fatty acid tails that face inward away from water. This arrangement is thermodynamically stable and does not require energy to maintain.
Integral (Intrinsic) Proteins
These proteins are embedded within the phospholipid bilayer. Some span the entire width of the membrane (transmembrane proteins). They are responsible for transport of molecules that cannot cross the lipid bilayer on their own, including ions, glucose and large polar molecules. Channel proteins, carrier proteins and pump proteins are all integral proteins.
Peripheral (Extrinsic) Proteins
These proteins sit on the surface of the membrane, either on the outer face or the inner face. They are not embedded in the lipid bilayer. They function in signalling, structural support and cell-to-cell recognition.
Cholesterol
Cholesterol molecules are interspersed within the phospholipid bilayer of animal cells. Cholesterol modulates membrane fluidity: it prevents the membrane from becoming too rigid at low temperatures and too fluid at high temperatures. Plant cells do not have cholesterol in their membranes. This difference between animal and plant cell membranes is tested directly in NEET.

What “fluid” means in the model

The lipid molecules can move laterally within their own leaflet of the bilayer. This lateral movement is what makes the membrane “fluid.” The proteins embedded in it can also move laterally. However, a phospholipid molecule flipping from one leaflet of the bilayer to the other (called “flip-flop”) is extremely rare and requires energy. Lateral diffusion requires no energy. NEET has tested the distinction between lateral diffusion (energy-independent, common) and flip-flop movement (energy-dependent, rare).

What “mosaic” means in the model

The proteins are distributed unevenly across the surface of the membrane, creating a patchwork or mosaic pattern when viewed from above. This is the origin of the “mosaic” part of the name.

Membrane Carbohydrates

Carbohydrate chains are attached to the outer surface of the membrane, either to lipids (forming glycolipids) or to proteins (forming glycoproteins). These carbohydrate chains form what is called the glycocalyx. The glycocalyx is responsible for cell recognition and cell-to-cell communication. Blood group antigens (A, B, O) are glycoproteins on the surface of red blood cells. NEET has asked about the role of glycocalyx and the ABO blood group connection in the same question.

The NEET Question Pattern for Fluid Mosaic Model

NEET typically asks one of three things about the fluid mosaic model in any given year:

First: Who proposed the model and in what year (Singer and Nicolson, 1972).
Second: A structural feature question such as “Which component of the cell membrane prevents excessive rigidity?” (Answer: Cholesterol in animal cells).
Third: A function question such as “Which component of the cell membrane is responsible for cell recognition?” (Answer: Glycoproteins or carbohydrate chains of the glycocalyx).

Quick Revision Table: Fluid Mosaic Model Components

Component	Type	Function	NEET Frequency
Phospholipid bilayer	Lipid	Basic structural framework	Every related question
Integral proteins	Protein	Transport, channel formation	High
Peripheral proteins	Protein	Signalling, structural support	Medium
Cholesterol	Lipid	Membrane fluidity regulation (absent in plants)	High
Glycolipids	Lipid + Carbohydrate	Cell recognition	Medium
Glycoproteins	Protein + Carbohydrate	Cell recognition, ABO antigens	High

Cell Wall: Structure, Composition and the Plant-Fungi-Bacteria Distinction

The cell wall is one of those topics where students lose marks not because they do not know the concept, but because they mix up which organism has which type of cell wall. NEET tests this distinction precisely.

Cell Wall Composition by Organism

Organism	Cell Wall Composition
Plants	Cellulose (primary wall) + Lignin or Suberin (secondary wall in some)
Fungi	Chitin
Bacteria	Peptidoglycan (also called Murein)
Algae	Cellulose (most), CaCO3 (diatoms have silica)
Animals	No cell wall

The three layers of a plant cell wall

The middle lamella is the outermost layer, shared between adjacent cells. It is made of calcium pectate and holds cells together. When fruit ripens and becomes soft, the middle lamella breaks down due to pectinase enzyme activity. This is why overripe fruit feels mushy.

The primary cell wall is thin, flexible and present in growing cells. It consists of cellulose microfibrils embedded in a matrix of hemicellulose and pectin.

The secondary cell wall forms inside the primary cell wall in cells that need extra strength (wood cells, fibres). It is thick, rigid and contains lignin in addition to cellulose. Lignin is what makes wood hard.

Plasmodesmata

are cytoplasmic channels that pass through the cell walls of adjacent plant cells. They allow direct communication between cells without crossing the cell membrane. The cytoplasm of adjacent cells is directly continuous through plasmodesmata. This pathway is called the symplast pathway. Plasmodesmata are absent in animal cells because animals have no cell wall to pass through.

NEET has asked “which structure allows direct cytoplasmic connection between plant cells?” The answer is plasmodesmata, not gap junctions (which are the animal cell equivalent) and not the middle lamella (which is a structural layer, not a channel).

Turgor Pressure and Plasmolysis

When a plant cell is placed in a hypotonic solution (more dilute than the cell), water enters by osmosis, the vacuole swells and presses the cytoplasm against the cell wall. This pressure exerted by the cell contents on the cell wall is called turgor pressure. The rigid cell wall exerts an equal and opposite wall pressure. A plant cell that is fully turgid is firm and upright. This is what keeps plant stems rigid without a skeleton.

When a plant cell is placed in a hypertonic solution (more concentrated than the cell), water leaves by osmosis, the vacuole shrinks and the cell membrane pulls away from the cell wall. This is called plasmolysis. The space between the cell wall and the shrunken cell membrane fills with the external hypertonic solution. Plasmolysis is reversible: if the plasmolysed cell is returned to a hypotonic solution, water re-enters and the membrane presses back against the cell wall (deplasmolysis).

NEET Trap on Plasmolysis

A plasmolysed cell is alive. Plasmolysis does not kill the cell unless it is extreme and prolonged. NEET has used this as a distractor in questions asking “which statement about plasmolysis is correct?” Students who think plasmolysis kills the cell select the wrong option.

Cell Organelles: Complete NEET-Ready Notes with PYQ Data

This is where the bulk of NEET marks from Cell Biology sit. Every organelle in this section has appeared in at least one NEET question between 2016 and 2024. The structure you need for each organelle is the same: location, number of membranes, key structural features, specific function, and the one or two details the exam actually tests. Anything beyond that is interesting but not mark-producing.

The Nucleus: Structure and Function Every NEET Student Must Know

The nucleus is the control centre of the cell and the most detailed organelle in the NEET syllabus. NCERT covers it at the highest depth compared to any other organelle, which is exactly why it generates the most questions.

Number of membranes: 2 (double membrane-bound organelle)

The nucleus has an outer membrane and an inner membrane. The outer nuclear membrane is continuous with the Rough Endoplasmic Reticulum (RER). This is a direct structural connection that NEET has tested. Students who treat the nucleus and the RER as completely separate compartments miss this question every time.

The Four Components of the Nucleus:

1. Nuclear Envelope
The double membrane surrounding the nucleus. The space between the outer and inner nuclear membranes is called the perinuclear space. The nuclear envelope is perforated by nuclear pores. Nuclear pores are not simple holes. They are regulated channels lined with proteins called nucleoporins, and they control what enters and exits the nucleus. mRNA moves out of the nucleus through nuclear pores. Ribosomal subunits (assembled in the nucleolus) also exit through nuclear pores.

2. Nucleoplasm
The fluid matrix inside the nucleus. It contains the chromosomes, nucleolus, nuclear proteins and dissolved ions. The nucleoplasm is analogous to the cytoplasm of the cell, but contained within the nuclear envelope.

3. Chromatin
DNA in the nucleus does not exist as bare strands. It is tightly wound around proteins called histones to form a complex called chromatin. The basic structural unit of chromatin is the nucleosome. One nucleosome consists of a core of 8 histone proteins (an octamer: 2 copies each of H2A, H2B, H3 and H4) around which 146 base pairs of DNA are wound 1.65 times. The histone H1 is a linker histone that sits outside the nucleosome core and helps in further compaction.

This is extremely important for NEET: histones are basic (positively charged) proteins. DNA is acidic (negatively charged) due to its phosphate groups. The electrostatic attraction between positively charged histones and negatively charged DNA is what holds the nucleosome together. NEET has asked “what type of protein is histone?” and “why does DNA bind to histone?” and the answer to both trace back to this charge relationship.

Chromatin exists in two states:

Euchromatin: loosely packed, lighter staining, transcriptionally active DNA
Heterochromatin: tightly packed, darker staining, transcriptionally inactive DNA

4. Nucleolus
The nucleolus is a dark-staining, spherical structure inside the nucleus. It has no membrane of its own. It is the site of rRNA synthesis and ribosome assembly. Ribosomal subunits are assembled in the nucleolus and then exported to the cytoplasm through nuclear pores.

The nucleolus disappears during cell division (prophase) because the genes coding for rRNA stop being transcribed when chromosomes condense. It reappears during telophase when chromosome decondensation begins. NEET has tested “when does the nucleolus disappear?” and “what is synthesised in the nucleolus?” as direct MCQ questions.

Number of nucleoli per cell varies. Human cells typically have one to four nucleoli. Cells that are very active in protein synthesis have larger and more prominent nucleoli because they need more ribosomes.

Critical NEET Fact:

Prokaryotes have no nucleus, no nuclear envelope, no nucleolus and no histone proteins. Their DNA is associated with histone-like proteins (not true histones) and exists as a naked circular chromosome in the nucleoid region. This distinction between prokaryotic nucleoid and eukaryotic nucleus appears as a direct NEET question pattern in reasoning-based questions.

Mitochondria: The Most Tested Organelle in NEET Cell Biology

Mitochondria generate more NEET questions than any other organelle. The reason is that they have a complex structure with multiple sub-compartments, their own DNA and ribosomes, a unique evolutionary history, and a central role in the energy metabolism topics covered in Respiration in Plants and Breathing and Exchange of Gases. Every structural detail below has appeared in a NEET question.

Number of membranes: 2 (double membrane-bound organelle)

Structure of Mitochondria:

The outer mitochondrial membrane is smooth and permeable to small molecules and ions due to the presence of protein channels called porins.

The inner mitochondrial membrane is impermeable to most molecules. This impermeability is essential for the proton gradient used in ATP synthesis. The inner membrane is folded into structures called cristae (singular: crista). Cristae dramatically increase the surface area of the inner membrane. This increased surface area allows more copies of the ATP synthase enzyme and the electron transport chain complexes to be embedded in the membrane, which means more ATP can be produced.

The space inside the inner membrane is the matrix. The matrix contains:

Mitochondrial DNA (circular, like prokaryotic DNA)
70S ribosomes
Enzymes of the Krebs (TCA) cycle
Enzymes of the fatty acid oxidation pathway

The space between the outer and inner membranes is the intermembrane space. This is where protons (H+) are pumped during the electron transport chain, creating the proton gradient that drives ATP synthesis.

Why Mitochondria Are Semi-Autonomous:

Mitochondria are described as semi-autonomous organelles because they have their own DNA and their own ribosomes (70S, same size as prokaryotic ribosomes). This means they can synthesise some of their own proteins without depending on the cell’s cytoplasmic ribosomes. However, they are not fully autonomous because most of their proteins are still encoded by nuclear DNA and imported from the cytoplasm.

The Endosymbiotic Theory:

The endosymbiotic theory (proposed by Lynn Margulis) states that mitochondria and chloroplasts evolved from ancient free-living prokaryotes that were engulfed by a larger ancestral eukaryotic cell. The evidence includes:

Both have double membranes (outer from the host cell, inner from the ancient prokaryote)
Both have circular DNA like prokaryotes
Both have 70S ribosomes like prokaryotes
Both divide by binary fission, not mitosis

NEET 2024 asked a direct statement-based question comparing mitochondria and chloroplasts on their membrane permeability. The correct understanding: the inner membrane of the mitochondrion is relatively LESS permeable than the inner membrane of the chloroplast. This is because mitochondrial ATP synthesis absolutely requires maintaining a steep proton gradient across the impermeable inner membrane. Chloroplast thylakoid membranes work differently.

NEET 2019 Direct PYQ:
“Which of the following pair of organelles does not contain DNA?” Options included Mitochondria-Lysosomes, Chloroplast-Vacuoles, Lysosomes-Vacuoles, Nuclear envelope-Mitochondria. The answer is Lysosomes and Vacuoles. Both lysosomes and vacuoles have no DNA. Mitochondria and chloroplasts both have DNA.

Chloroplasts: Photosynthesis Factory with NEET-Level Structural Detail

Chloroplasts are found only in plant cells and photosynthetic protists. They are the site of photosynthesis. Like mitochondria, they have their own circular DNA and 70S ribosomes and are considered semi-autonomous organelles.

Number of membranes: 2 (double membrane-bound organelle)

Structure of Chloroplasts:

The outer membrane is permeable. The inner membrane is selectively permeable.

Between the outer and inner membranes is the intermembrane space.

Inside the inner membrane is the stroma. The stroma is the fluid matrix and is the site of the Calvin cycle (dark reactions / light-independent reactions). It contains:

Chloroplast DNA (circular)
70S ribosomes
Enzymes of the Calvin cycle (including RuBisCO, the most abundant enzyme on Earth)
Starch granules

Within the stroma are flattened membranous sacs called thylakoids. Thylakoids are arranged in stacks called grana (singular: granum). A single stack of thylakoids is a granum. Individual thylakoids in one granum are connected to thylakoids in another granum by flat membranous channels called stroma lamellae (also called intergranal lamellae or frets).

The light-dependent reactions (Light Reactions) of photosynthesis occur in the thylakoid membranes. Photosystems I and II, the electron transport chain of photosynthesis, and the ATP synthase of photosynthesis are all embedded in the thylakoid membranes.

The NEET Comparison Table: Mitochondria vs Chloroplast

Feature	Mitochondria	Chloroplast
Membranes	2 (outer + inner)	2 (outer + inner) + thylakoid membranes
Matrix equivalent	Matrix	Stroma
Membrane folds	Cristae (inner membrane)	Thylakoids (in stroma)
Ribosome size	70S	70S
DNA	Circular	Circular
Division type	Binary fission-like	Binary fission-like
Function	ATP synthesis (respiration)	ATP + sugar synthesis (photosynthesis)
Present in	All eukaryotic cells	Plant cells and algae only

NEET PYQ (2016 and 2024 both tested):
“Mitochondria and chloroplasts are both double membrane-bound organelles” — TRUE.
“Inner membrane of mitochondria is relatively less permeable compared to chloroplast” — TRUE.

Endoplasmic Reticulum: SER vs RER Every Student Confuses

The endoplasmic reticulum (ER) is a vast network of membrane-enclosed tubes and flattened sacs (cisternae) extending throughout the cytoplasm. It is continuous with the outer nuclear membrane at one end.

Rough Endoplasmic Reticulum (RER)

RER has ribosomes attached on its cytoplasmic surface, which gives it a rough or granular appearance under the electron microscope. The ribosomes on the RER synthesise proteins that are destined for:

Secretion outside the cell
Insertion into the cell membrane
Transport to lysosomes

After synthesis, these proteins enter the lumen of the RER, where they undergo folding and initial processing before being transported to the Golgi apparatus.

Cells that secrete large amounts of protein (like pancreatic acinar cells producing digestive enzymes, or plasma cells producing antibodies) have extensive RER.

Smooth Endoplasmic Reticulum (SER)

SER has no ribosomes on its surface, which is why it appears smooth. Its functions are:

Lipid synthesis: including phospholipids for membrane formation and steroid hormones
Detoxification: particularly in liver cells, where SER contains enzymes that detoxify drugs, alcohol and other toxins
Calcium storage: in muscle cells, the SER (called sarcoplasmic reticulum) stores calcium ions that are released to trigger muscle contraction

Cells that produce large amounts of lipids or steroids (like adrenal cortex cells producing steroid hormones, or liver cells) have extensive SER.

The NEET Trap on ER:

NEET 2021 and multiple mock-test questions have asked: “In prokaryotes, which type of ER is present?” The correct answer is neither. Prokaryotes have NO endoplasmic reticulum at all — not RER, not SER. Students who read that “prokaryotes have ribosomes” sometimes incorrectly infer that they must have RER. They do not. Ribosomes in prokaryotes are free in the cytoplasm.

Quick Revision: RER vs SER

Feature	RER	SER
Ribosomes	Present	Absent
Appearance	Rough / granular	Smooth
Primary function	Protein synthesis + processing	Lipid and steroid synthesis
Detoxification	No	Yes (liver cells)
Calcium storage	No	Yes (muscle cells)
Present in	Protein-secreting cells	Lipid-secreting cells

Golgi Apparatus: The Packaging and Dispatch Centre

The Golgi apparatus (also called Golgi complex or Golgi body) is a stack of flattened membranous sacs called cisternae. It is the cell’s post office: it receives proteins from the ER, modifies them, packages them, and sends them to their correct destinations.

Structure of the Golgi Apparatus:

The Golgi apparatus has two distinct faces:

The cis face (forming face) is the receiving side. It faces the nucleus and the endoplasmic reticulum. Vesicles from the RER fuse with the cis face to deliver proteins for processing.

The trans face (maturing face) is the shipping side. Processed and packaged proteins leave from the trans face in vesicles. These vesicles travel to lysosomes, the plasma membrane (for secretion) or other cellular destinations.

The cis and trans faces look different from each other and are NOT interchangeable. NEET has tested “which face of the Golgi apparatus receives vesicles from the ER?” (Answer: cis face) and “which face releases vesicles?” (Answer: trans face). A classic wrong answer in NEET is selecting “trans face” for receiving vesicles from ER.

Functions of the Golgi Apparatus:

Glycosylation: adding sugar chains to proteins (forming glycoproteins) and lipids (forming glycolipids). This process is completed in the Golgi apparatus, not in the ER. NEET has directly stated that glycosylation is completed in the Golgi.
Sulfation: adding sulfate groups to proteins and lipids
Packaging: forming secretory vesicles (for proteins to be secreted), lysosomal vesicles and membrane vesicles
Cell plate formation: during plant cell division, Golgi vesicles contribute to forming the cell plate (early cell wall)

Acrosome of Sperm:
The acrosome is the cap-like structure on the head of a sperm cell. It contains hydrolytic enzymes that help the sperm penetrate the egg during fertilisation. The acrosome is formed from the Golgi apparatus during spermatogenesis. This is a direct NEET question that links Golgi structure to reproduction: “Which organelle is responsible for acrosome formation in sperm?” Answer: Golgi apparatus.

Lysosomes: The Suicidal Bags of the Cell

Lysosomes are membrane-bound vesicles containing hydrolytic (digestive) enzymes. They are produced by the Golgi apparatus.

Number of membranes: 1 (single membrane-bound organelle)

This is the most tested fact about lysosomes in NEET. The question “Which organelle is enclosed by a single membrane?” was asked in NEET 2016 (Phase I) with options Chloroplasts, Lysosomes, Nuclei, and Mitochondria. The answer is Lysosomes. All three others (nucleus, mitochondria, chloroplasts) are double membrane-bound.

Lysosomal Enzymes:

Lysosomes contain approximately 40 different hydrolytic enzymes including proteases, lipases, nucleases and carbohydrases. All of these enzymes work at an acidic pH of around 4.5 to 5.0. The lysosome actively maintains this acidic interior by pumping protons in from the cytoplasm.

This acidic pH serves as a safety mechanism. If a lysosome ruptures and releases its enzymes into the neutral cytoplasm (pH ~7.2), the enzymes become inactive because they are far from their optimal pH. This prevents the enzymes from digesting the cell’s own cytoplasm under normal circumstances.

Functions of Lysosomes:

Intracellular digestion: breaking down foreign material engulfed by phagocytosis (bacteria, viruses)
Autophagy: digesting the cell’s own damaged organelles and proteins. This is a cellular quality control system.
Autolysis: under certain conditions (programmed cell death or injury), lysosomes release their enzymes and digest the entire cell. This is why de Duve (who discovered lysosomes) called them “suicidal bags.”

Autolysis in Development:

Autolysis is not just cell destruction. It is a programmed, necessary process during development. The disappearance of a tadpole’s tail during metamorphosis into a frog involves lysosomal autolysis of the tail cells. NEET has tested this connection between lysosomes and metamorphosis as an application-based question.

Vacuoles: Different Roles in Plants vs Animals

Vacuoles are membrane-bound sacs filled with cell sap or other materials. Their membrane is called the tonoplast.

Number of membranes: 1 (single membrane-bound organelle)

Plant Cell Vacuoles:

In mature plant cells, the central vacuole can occupy up to 90 percent of the cell volume. It maintains turgor pressure (by controlling water entry through osmosis), stores pigments (anthocyanins that give flowers their red, blue and purple colours), stores metabolic waste products that the plant cannot excrete, and stores nutrients like sucrose and ions.

Animal Cell Vacuoles:

Animal cells have smaller, temporary vacuoles used for:

Food vacuoles (formed during phagocytosis in Amoeba and WBCs)
Contractile vacuoles in freshwater protists like Amoeba: regulate water content by actively pumping out excess water (osmoregulation)

NEET Fact:

Vacuoles do not contain DNA. Neither do lysosomes. NEET 2019 directly tested “which pair of organelles does NOT contain DNA?” — the answer was lysosomes and vacuoles.

Ribosomes: Size, Type and Where They Are Found

Ribosomes are the sites of protein synthesis. They are not membrane-bound. They are made of rRNA and proteins.

Two types by location:

Free ribosomes float in the cytoplasm and synthesise proteins for use within the cell (cytoplasmic proteins, enzymes, etc.)

Membrane-bound ribosomes are attached to the RER and synthesise proteins for secretion, membrane insertion, or lysosomal delivery.

Ribosome sizes — this is tested almost every year:

Ribosome Type	Overall Size	Large Subunit	Small Subunit	Found In
Prokaryotic	70S	50S	30S	Bacteria, Cyanobacteria
Eukaryotic	80S	60S	40S	Cytoplasm of eukaryotes
Mitochondrial	70S	50S	30S	Inside mitochondria
Chloroplast	70S	50S	30S	Inside chloroplasts

Critical NEET Point: “S” stands for Svedberg unit, a measure of sedimentation rate during centrifugation. It is NOT a size unit and the subunits do not add up arithmetically. 50S + 30S gives a 70S ribosome, not an 80S ribosome, because the two subunits interact and the combined particle sediments differently from either subunit alone. NEET has tested whether students know that S values are not additive.

Centrosome and Centrioles: Animal Cell Division Structures

The centrosome is the microtubule-organising centre of animal cells. It is located near the nucleus. Each centrosome contains two centrioles arranged at right angles to each other.

Structure of a Centriole:

A centriole is made of 9 triplets of microtubules arranged in a ring (9 + 0 arrangement, meaning 9 peripheral triplets and no central microtubules). This is different from the arrangement in cilia and flagella (9 + 2 arrangement: 9 peripheral doublets plus 2 central microtubules).

Functions of Centrosome:

During cell division, the centrosome duplicates and the two centrosomes move to opposite poles of the cell. From each centrosome, microtubules extend to form the spindle apparatus that separates chromosomes during mitosis and meiosis.

NEET Column Matching PYQ (2018):
NEET asked students to match organelles with their locations or components. One pair tested was: Centriole — Basal body of cilia or flagella. The basal body of every cilium and flagellum is a centriole-like structure (9 triplet arrangement) located at the base of the cilium. This evolutionary relationship between centrioles and basal bodies is tested in NEET directly.

Plant cells DO NOT have centrosomes or centrioles. Plants manage cell division using a different type of microtubule-organising structure. This absence of centrioles in plant cells is a direct NEET question type.

Peroxisomes and Glyoxysomes: The Organelles Most Students Forget

These two organelles appear in NEET less frequently but enough to cost students who skip them one mark.

Peroxisomes are single membrane-bound organelles containing oxidative enzymes, particularly catalase. Their name comes from the fact that they produce and then break down hydrogen peroxide (H2O2) as part of their metabolic activity. In animal cells, peroxisomes are involved in the oxidation of very long-chain fatty acids. In plant cells, they participate in photorespiration (the wasteful process by which RuBisCO fixes O2 instead of CO2 during photosynthesis).

Glyoxysomes are present in germinating fatty seeds (like castor, groundnut). They contain enzymes of the glyoxylate cycle, which allows the plant to convert stored fats into sugars during germination when photosynthesis has not yet started. The Golgi apparatus is associated with glyoxysomes in certain NEET questions because Golgi functions in lipid processing.