Cell Structure and Function — Complete NCERT Guide

The cell is the basic unit of life. Every living thing — from bacteria to blue whales — is made of cells. Understanding cell structure is not optional for NEET or CBSE; it is foundational. Get this chapter right and you’ll have a framework for understanding photosynthesis, respiration, heredity, and biotechnology — all at once.

NEET asks 3–5 questions from cell structure and function every year. CBSE Class 9 and 11 boards have guaranteed 5–8 mark sections. This chapter has the highest return on investment for NEET biology — it’s detailed but very learnable.

Cell Theory

The cell theory, developed by Schleiden (1838), Schwann (1839), and later modified by Virchow (1855), has three main postulates:

All living organisms are composed of cells
The cell is the structural and functional unit of life
All cells arise from pre-existing cells (Virchow’s addition: “Omnis cellula e cellula”)

Viruses are NOT cells — they are acellular (non-cellular) entities. They need a host cell to replicate. This is a common NEET MCQ trick: “Is a virus a cell?” No — viruses are not covered by cell theory.

Prokaryotic vs Eukaryotic Cells

The most fundamental division in cell biology is between prokaryotes and eukaryotes.

Feature	Prokaryotic	Eukaryotic
Nucleus	Absent (nucleoid only)	Present (membrane-bound)
Membrane-bound organelles	Absent	Present
Size	1–10 μm (smaller)	10–100 μm (larger)
DNA	Circular, no histones	Linear, with histones
Ribosomes	70S (50S + 30S)	80S (60S + 40S)
Cell wall	Present (peptidoglycan in bacteria)	Present in plants (cellulose); absent in animals
Examples	Bacteria, Cyanobacteria (Blue-green algae)	Plants, animals, fungi, protists
Plasmids	Present (in bacteria)	Rare
Reproduction	Binary fission	Mitosis/meiosis

Mitochondria and chloroplasts have 70S ribosomes (like prokaryotes) and circular DNA — evidence for the endosymbiotic theory. These organelles were once free-living prokaryotes that were engulfed by early eukaryotic cells. This connection appears in NEET questions regularly.

Plant Cell vs Animal Cell — Complete Comparison

Feature	Plant Cell	Animal Cell
Cell wall	Present (cellulose)	Absent
Chloroplasts	Present (in green cells)	Absent
Central vacuole	Large (up to 90% of cell volume)	Small or absent
Centrioles	Absent (except lower plants)	Present
Plasmodesmata	Present	Absent
Glyoxysomes	Present	Absent
Tonoplast	Present (vacuole membrane)	Absent
Lysosomes	Rarely present	Present
Shape	Regular, rigid (cell wall)	Irregular, flexible
Storage	Starch (plastids)	Glycogen (cytoplasm)

Cell Organelles — Detailed Notes

The Nucleus

The nucleus is the control centre of the cell, containing the genetic information.

Structure:

Nuclear envelope: Double membrane (outer and inner) with nuclear pores
Nuclear pores: Protein-lined channels for transport of RNA, proteins, ribosomes in/out of nucleus. Around 3000–4000 pores per nucleus
Nucleoplasm: Fluid interior of nucleus containing chromatin and nucleolus
Chromatin: DNA + histone proteins (uncoiled during interphase). Condenses into chromosomes during cell division
Nucleolus: Dense, non-membrane-bound region inside nucleus. Site of rRNA synthesis and ribosome assembly. Disappears during cell division

NEET loves questions about the nucleolus: “Where does rRNA synthesis occur?” → Nucleolus. “What disappears during cell division?” → Nucleolus (and nuclear envelope in mitosis). Also remember: nucleolus is not bounded by a membrane — it’s just a dense region of active RNA synthesis.

Mitochondria — The Powerhouse

Mitochondria are the sites of aerobic cellular respiration, producing ATP.

Structure:

Outer membrane: Smooth, permeable (has porins). Encloses the entire organelle
Inner membrane: Highly folded into cristae (singular: crista). Impermeable — site of the electron transport chain and ATP synthase
Intermembrane space: Between outer and inner membranes. Important for chemiosmosis (H⁺ gradient)
Matrix: Interior of inner membrane. Contains:
- Mitochondrial DNA (circular, 70S ribosomes)
- Enzymes of Krebs cycle (TCA cycle)
- Pyruvate dehydrogenase complex

Why are cristae important? Folding of the inner membrane dramatically increases its surface area — more space for ATP synthase complexes → more ATP production per mitochondrion.

Mitochondria are semi-autonomous (have their own DNA and ribosomes) — further evidence for endosymbiotic origin.

Chloroplasts

Chloroplasts are the sites of photosynthesis, found only in plant cells and algae.

Structure:

Outer membrane: Permeable
Inner membrane: Semi-permeable
Intermembrane space: Between outer and inner membranes
Stroma: Fluid-filled interior of the inner membrane. Site of the Calvin cycle (dark reactions). Contains:
- Circular DNA, 70S ribosomes
- RuBisCO (most abundant enzyme on Earth)
Thylakoids: Flattened, disc-like membrane sacs inside the stroma
Grana (singular: granum): Stacks of thylakoids connected by stroma lamellae
Thylakoid membrane: Contains chlorophyll, carotenoids, and photosystems I and II. Site of the light reactions of photosynthesis

Endoplasmic Reticulum (ER)

The ER is a network of interconnected membrane-bound sacs and tubules continuous with the nuclear envelope.

Rough ER (RER):

Has ribosomes on its surface → appears “rough” under electron microscope
Function: Synthesis and processing of secretory proteins, membrane proteins, and lysosomal proteins
Newly synthesised proteins enter the RER lumen, get folded and modified (glycosylation), then packaged into vesicles for Golgi apparatus

Smooth ER (SER):

No ribosomes → appears “smooth”
Functions: Lipid synthesis, steroid hormone synthesis, detoxification of drugs and poisons (liver cells are rich in SER), calcium storage (in muscle cells: sarcoplasmic reticulum = specialised SER)

Golgi Apparatus (Golgi Complex)

The Golgi apparatus is the “post office” of the cell — it receives, processes, packages, and sends proteins and lipids to their final destinations.

Structure:

Stack of flattened, membrane-bound cisternae (4–8 per stack)
Cis face: Receives vesicles from the RER (faces the ER)
Trans face: Ships vesicles to their destinations (faces the plasma membrane or lysosomes)
Vesicles bud off from the trans face

Functions:

Modifies proteins (further glycosylation, sulphation, phosphorylation)
Sorts and packages proteins into secretory vesicles, lysosomes, or membrane vesicles
Synthesises certain polysaccharides (in plant cells: cell plate formation)

Lysosomes

Lysosomes are membrane-bound organelles containing hydrolytic (digestive) enzymes at a low pH (≈5).

Functions:

Intracellular digestion: Break down food particles taken in by phagocytosis (in macrophages, amoeba)
Autophagy: Digest damaged, old, or surplus organelles — recycling cellular components
Apoptosis: “Suicide bags of the cell” — in cell death, lysosomes release their enzymes to digest the cell from within
Extracellular digestion: In some organisms, enzymes released outside to digest food

The “suicide bags of the cell” description for lysosomes is one of the most famous NCERT lines in biology. During metamorphosis of tadpoles into frogs, lysosomes digest the tail cells — this is controlled autolysis. NEET expects you to know this specific example.

Ribosomes

Ribosomes are the sites of protein synthesis. They are NOT bound by a membrane.

Structure:

Two subunits: large and small
Prokaryotic ribosomes: 70S (50S large + 30S small)
Eukaryotic cytoplasmic ribosomes: 80S (60S large + 40S small)
Mitochondrial and chloroplast ribosomes: 70S (prokaryotic-type — endosymbiosis evidence)

Ribosomes can be free in cytoplasm (make proteins for internal use) or bound to RER (make secretory and membrane proteins).

Vacuoles

Plant central vacuole:

Can occupy up to 90% of a mature plant cell’s volume
Bounded by the tonoplast membrane
Contains cell sap (water, salts, sugars, organic acids, pigments)
Maintains turgor pressure — keeps plant cells firm (turgid)
When the vacuole loses water → cell becomes flaccid → plant wilts

Animal cell vacuoles:

Small, temporary, multiple (food vacuoles, contractile vacuoles in protists)
Contractile vacuoles in Amoeba/Paramecium pump out excess water (osmoregulation)

Centrioles and Centrosomes

Centrioles:

Present in animal cells (and lower plants)
Cylindrical structures made of 9 triplets of microtubules (9 × 3 arrangement, no central tubule)
Two centrioles at right angles form a centrosome
During cell division: centrosome organises the mitotic spindle
Basal bodies of cilia and flagella are also derived from centrioles (same structure)

Absent in: Higher plants and most fungi.

Peroxisomes

Peroxisomes are small, single-membrane-bound organelles containing oxidative enzymes (catalase, peroxidase). They break down hydrogen peroxide (H₂O₂), which is toxic, into water and oxygen:

2H₂O₂ → 2H₂O + O₂

Also involved in photorespiration in plants and fatty acid β-oxidation in liver and kidney cells.

Cell Membrane Structure — Fluid Mosaic Model

The cell membrane (plasma membrane) follows the fluid mosaic model proposed by Singer and Nicolson (1972).

Components:

Phospholipid bilayer: Two layers of phospholipids with hydrophilic heads facing outward (toward water) and hydrophobic tails facing inward. The bilayer is “fluid” — lipid molecules can move laterally
Proteins:
- Integral (transmembrane) proteins: Span the entire membrane. Include ion channels, carrier proteins, receptors
- Peripheral proteins: Attached to membrane surface. Include enzymes, structural proteins
Cholesterol: Inserts between phospholipids — regulates membrane fluidity (prevents it from becoming too rigid at low temperatures or too fluid at high temperatures)
Carbohydrates: Attached to proteins (glycoproteins) or lipids (glycolipids) on the outer surface → form the glycocalyx — important for cell recognition, immune responses, and adhesion

A very common NEET question: “Who proposed the fluid mosaic model?” → Singer and Nicolson. Another: “What is the glycocalyx?” → The sugar-coated outer surface of the cell membrane formed by glycoproteins and glycolipids. It’s involved in cell recognition and immune response.

5 Common Mistakes Students Make

Mistake 1: Saying plant cells have no lysosomes. Plant cells do have lysosomes, though they are less prominent. In plants, the central vacuole often performs lysosomal functions. The statement “only animal cells have lysosomes” is an oversimplification.

Mistake 2: Confusing the 70S and 80S ribosome assignments. 70S = prokaryotes (also mitochondria and chloroplasts). 80S = eukaryotic cytoplasm. The S stands for Svedberg unit (sedimentation rate), not size. 70S and 80S don’t add up (50S + 30S ≠ 80S) — Svedberg units don’t add linearly.

Mistake 3: Thinking the nucleolus is bounded by a membrane. The nucleolus has no membrane around it — it is a dense region of the nucleoplasm where rRNA genes are actively transcribed. It disappears during cell division when chromosomes condense.

Mistake 4: Saying plant cells have no centrioles. Higher plants (angiosperms, gymnosperms) lack centrioles. Lower plants (algae, mosses, ferns) DO have centrioles because they produce motile gametes. This distinction matters in NEET.

Mistake 5: Confusing the Golgi cis face and trans face. Cis face = receiving end (faces ER, receives vesicles from RER). Trans face = shipping end (faces plasma membrane, sends vesicles out). Think: “CIS = Close to ER; TRANS = Towards exit.”

Real-World Examples

Example 1: Your Leg Muscles During a 100m Sprint

Watch any 100m sprint at a school sports day — those quadriceps are firing 200 times a second. Every muscle contraction burns ATP, and mitochondria are the only organelles that can regenerate ATP fast enough to keep up. A single muscle cell can contain over 1,000 mitochondria, packed densest around the myofibrils where energy demand is highest. Elite sprinters like Neeraj Chopra’s training partners literally have more mitochondria per cell than untrained students — this is measurable, not metaphor.

Connect to the syllabus: Mitochondria are the “powerhouse of the cell” because they produce ATP via cellular respiration ( $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$ ) — a direct NEET MCQ favourite from the cell organelles section.

Example 2: The Cell Membrane as Mumbai’s Toll Naka

Every highway entering Mumbai has a toll naka — vehicles pay, get checked, and only authorised ones pass. The cell membrane works the same way. Glucose enters muscle cells through specific protein channels (GLUT transporters); sodium ions are actively pumped out even when they “want” to rush in. When you eat a Wagh Bakri chai with sugar, that glucose doesn’t just leak into your blood cells — it waits for the right transporter protein to open the gate. Damaged membranes, like a broken toll barrier, let everything flood in chaotically, which is exactly what happens in cell injury.

Connect to the syllabus: The cell membrane’s selective permeability — controlled by the fluid mosaic model (phospholipid bilayer + embedded proteins) — is a standard CBSE Class 11 and NEET definition question worth 2 marks reliably.

Example 3: Chloroplasts and ISRO’s Solar Panels — Same Logic, Different Scale

ISRO’s satellites run entirely on solar panels that convert sunlight to electricity. Chloroplasts in a single peepal leaf do something chemically more elegant — they convert photons into chemical energy stored as glucose, using nothing but water, $\text{CO}_2$ , and sunlight. A single leaf cell in a Ficus tree contains 20–100 chloroplasts, each stacked with thylakoid membranes (grana) that maximise surface area for light absorption — the same engineering principle ISRO uses by angling panels toward the sun.

Connect to the syllabus: Chloroplasts contain the green pigment chlorophyll, and the reaction $6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$ is photosynthesis — tested in CBSE Class 9 and appears as a NEET assertion-reason question.

Practice Questions

Q1. Why do cells in the pancreas have an abundance of rough endoplasmic reticulum?

Pancreatic cells (particularly acinar cells) produce large quantities of digestive enzymes (amylase, lipase, proteases) that are secretory proteins — they need to be packaged and exported. Rough ER is the site of synthesis and initial processing of secretory proteins. The more secretory protein a cell produces, the more RER it needs. Pancreatic acinar cells rank among the most active secretory cells in the body, hence their abundant RER.

Q2. What are cristae, and why are they important for ATP production?

Cristae are the shelf-like infoldings of the inner mitochondrial membrane. The inner membrane is impermeable to most ions, including H⁺ (protons). During the electron transport chain, protons are pumped from the matrix into the intermembrane space, creating a proton gradient. When H⁺ flows back into the matrix through ATP synthase (embedded in the inner membrane), ATP is synthesised (chemiosmosis).

Cristae massively increase the surface area of the inner membrane — more surface area = more ATP synthase complexes = more ATP produced per unit time. A mitochondrion with many well-developed cristae (like in muscle cells with high energy demand) can produce ATP faster than one with few cristae.

Q3. Differentiate between chromatin and chromosome.

Chromatin is the complex of DNA and histone proteins found in the nucleus of a non-dividing (interphase) cell. It exists as loose, extended fibres — allowing DNA to be accessible for transcription and replication.

Chromosomes are the condensed, compact form of chromatin that appear during cell division (mitosis/meiosis). The condensation is essential for equal distribution of genetic material to daughter cells — you can’t pull loose chromatin fibres apart neatly.

Same material, different states: chromatin (interphase, dispersed) → chromosome (cell division, condensed). The number of chromosomes in humans = 46, but the chromatin in the interphase nucleus is continuous with this genetic material.

Q4. What is the role of the Golgi apparatus in the cell?

The Golgi apparatus (Golgi complex) functions as the cell’s processing and distribution centre for proteins and lipids. Its key roles:

Receives proteins from the RER via vesicles at the cis face
Modifies proteins: adds/modifies sugar chains (glycosylation), adds sulphate groups, phosphorylates proteins
Sorts proteins and assigns them to correct destinations: secretory vesicles (for export), lysosomes (digestive enzymes), or plasma membrane components
Packages finished products into vesicles at the trans face for delivery

In plant cells: also participates in cell plate formation during cytokinesis and synthesises pectin and hemicellulose for the cell wall.

Q5. Why is the cell membrane described as “fluid” in the fluid mosaic model?

The “fluid” in fluid mosaic refers to the ability of phospholipid molecules (and most membrane proteins) to move laterally within each half of the bilayer. Phospholipid tails are mostly unsaturated fatty acids, which introduce kinks and prevent tight packing — this maintains membrane fluidity at physiological temperatures.

Cholesterol regulates this fluidity: at low temperatures, cholesterol prevents fatty acid tails from packing too tightly (prevents gelling); at high temperatures, cholesterol restrains excessive movement (prevents over-fluidity).

The membrane is not static — proteins and lipids are in constant motion, making the membrane a dynamic, self-sealing structure rather than a rigid boundary.

Q6. Why can antibiotics targeting 70S ribosomes kill bacteria without harming human cells?

Human cytoplasmic ribosomes are 80S (60S + 40S subunits), structurally different from bacterial 70S ribosomes (50S + 30S subunits). Antibiotics like streptomycin, tetracycline, and chloramphenicol bind specifically to the 30S or 50S subunits of bacterial ribosomes — they have low affinity for human 80S ribosomes and do not inhibit human protein synthesis at therapeutic doses.

This selective toxicity makes them clinically useful. However, mitochondria have 70S ribosomes — at high doses, some antibiotics can affect mitochondrial protein synthesis, which is a mechanism for some drug side effects (e.g., aminoglycoside-induced hearing loss relates to mitochondrial dysfunction in cochlear cells).

Q7. What is osmosis? How does the central vacuole help maintain turgor pressure in plant cells?

Osmosis is the movement of water molecules from a region of higher water potential (lower solute concentration) to a region of lower water potential (higher solute concentration) through a selectively permeable membrane.

The central vacuole of plant cells contains concentrated cell sap (dissolved salts, sugars, organic acids). When a plant cell is in a hypotonic solution (more dilute than cell sap), water enters the vacuole by osmosis → vacuole swells → pushes against the cell wall → creates turgor pressure. This pressure keeps the plant cell firm (turgid) and is what keeps non-woody plants upright.

When water is lost (drought, high salt soil), the vacuole shrinks → turgor pressure drops → cell becomes flaccid → the plant wilts. This is directly reversible by re-watering.

Q8. Explain why mitochondria and chloroplasts are described as semi-autonomous organelles.

Semi-autonomous means these organelles have some degree of independence in replication and protein synthesis, but are not fully independent — they still depend on nuclear genes for the majority of their proteins.

Evidence for semi-autonomy:

They have their own circular DNA (like prokaryotes)
They have their own 70S ribosomes (prokaryotic-type)
They can synthesise some of their own proteins (those encoded in organellar DNA)
They replicate by binary fission (like bacteria), not by de novo synthesis

Why “semi” and not fully autonomous:

They cannot make all their own proteins — the majority of mitochondrial/chloroplast proteins are encoded by nuclear DNA, synthesised on cytoplasmic ribosomes, and imported into the organelle
They cannot survive independently outside the cell

This semi-autonomy is evidence for the endosymbiotic theory: these organelles were once free-living bacteria that established a symbiotic relationship with ancestral eukaryotic cells.

Frequently Asked Questions

What is the difference between a cell wall and a cell membrane?

The cell membrane (plasma membrane) is present in ALL cells (both prokaryotic and eukaryotic). It is a selective barrier made of phospholipid bilayer + proteins. The cell wall is a rigid outer layer present in plants (cellulose), fungi (chitin), and bacteria (peptidoglycan) — but absent in animal cells. The cell wall provides structural support and shape, while the cell membrane controls what enters and exits the cell.

Why do plant cells not burst when placed in hypotonic solution?

The cellulose cell wall is rigid and inextensible. When water enters the plant cell by osmosis, the expanding vacuole pushes outward against the cell wall. The cell wall resists this pressure (wall pressure = turgor pressure at equilibrium). The inward pressure from the cell wall prevents the cell from bursting. Animal cells lack this wall — they burst (lyse) in hypotonic solutions because there’s nothing to oppose the expanding membrane.

What is the difference between plastids and mitochondria?

Both are double-membraned, semi-autonomous organelles. Plastids are plant-specific and are the site of photosynthesis (chloroplasts) or storage (leucoplasts for starch, chromoplasts for pigments). Mitochondria are present in both plant and animal cells and are the site of aerobic respiration (ATP production). Key exam distinction: chloroplasts have thylakoids/grana; mitochondria have cristae.

Why are liver cells rich in smooth ER?

The liver is the primary detoxification organ in the body. Smooth ER contains enzymes (cytochrome P450 family) that oxidise, reduce, or conjugate toxic compounds (drugs, alcohol, metabolic waste products) to make them water-soluble and excretable. Liver cells are therefore packed with SER to handle this constant detoxification load.

What is the difference between pinocytosis and phagocytosis?

Phagocytosis (“cell eating”): the cell engulfs large particles (bacteria, dead cells) by extending pseudopods. Used by macrophages and neutrophils for immunity, and by Amoeba for feeding. Pinocytosis (“cell drinking”): the cell takes in small droplets of extracellular fluid containing dissolved molecules. Both are forms of endocytosis — the plasma membrane folds inward to form a vesicle. Phagocytosis requires energy and receptor binding; pinocytosis is more constitutive.

How is the nuclear envelope different from other membranes?

The nuclear envelope is a double membrane (two lipid bilayers), unlike the single-bilayer membranes of the ER, Golgi, lysosomes, or plasma membrane. The outer nuclear membrane is continuous with the rough ER and may have ribosomes on it. The inner nuclear membrane faces the nuclear lamina (a structural protein network). The nuclear pores punctuate both membranes, creating regulated channels for macromolecular transport between nucleus and cytoplasm.