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.

Cell Theory

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

1. All living organisms are composed of cells
2. The cell is the structural and functional unit of life
3. All cells arise from pre-existing cells (Virchow's addition: "Omnis cellula e cellula")

Prokaryotic vs Eukaryotic Cells

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

Plant Cell vs Animal Cell — Complete Comparison

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

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

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

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."

Practice Questions

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

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

Q3. Differentiate between chromatin and chromosome.

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

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

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

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

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

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.