Cell: The Unit of Life Class 11 Biology — Revision Notes

TL;DR: The cell is the basic unit of life and all living organisms are composed of one or more cells. Understanding cell structure and function is fundamenta…

The cell is the basic unit of life and all living organisms are composed of one or more cells. Understanding cell structure and function is fundamental to

Cell Theory

• All living organisms are composed of cells
• Cell is the basic unit of life
• All cells arise from pre-existing cells through cell division
• Established by scientists like Schleiden, Schwann, and Virchow
• Theory explains how life originates and propagates

Prokaryotic Cells

• Lack membrane-bound nucleus; genetic material in nucleoid region
• Smaller than eukaryotic cells, typically 0.5 to 5 micrometers
• Include bacteria and archaea
• Have cell membrane but lack membrane-bound organelles
• Ribosomes are 70S type and smaller than eukaryotic ribosomes

Eukaryotic Cell Structure

• Nucleus contains genetic material and controls cell functions
• Mitochondria produces energy through aerobic respiration
• Endoplasmic reticulum synthesizes proteins and lipids
• Golgi apparatus modifies and packages proteins
• Lysosomes contain enzymes for breaking down waste

Cell Membrane Structure and Function

• Selectively permeable barrier controlling substance movement
• Phospholipid bilayer provides basic structure
• Proteins embedded in membrane perform various functions
• Allows nutrient entry and waste removal
• Maintains ion concentrations and cell homeostasis

Cell Division Processes

• Mitosis produces two identical daughter cells for growth and repair
• Meiosis produces four haploid gametes for sexual reproduction
• Binary fission in prokaryotes produces two identical cells
• Cell cycle has G1, S, G2, and M phases
• Checkpoints control progression through cell cycle

Key Terms

• Cell: Basic unit of life and smallest living entity
• Mitochondria: Organelle that produces energy through cellular respiration
• Nucleus: Membrane-bound organelle containing genetic material
• Enzyme: Protein that catalyzes biochemical reactions in cells

Cell theory, and the one clause that took seventeen more years

Robert Hooke, in 1665, looked at a slice of cork through his own microscope and saw a honeycomb of empty compartments. He called them cells, after the small rooms of a monastery. What he actually saw were the dead cell walls left behind — not living cells at all. Anton von Leeuwenhoek was the first to observe and describe living cells.

In 1838 Matthias Schleiden concluded from his study of plants that all plants are composed of cells. In 1839 Theodor Schwann examined animal tissues, reported that animal cells have a thin outer layer now called the plasma membrane, and concluded that the bodies of animals and plants are composed of cells and their products. Together they proposed the cell theory.

That theory had a gap: it did not explain where new cells come from. Rudolf Virchow closed it in 1855 with the statement omnis cellula-e cellula — all cells arise from pre-existing cells. The modern cell theory is therefore three clauses: all living organisms are composed of cells and their products, the cell is the basic unit of life, and all cells arise from pre-existing cells. Examiners very often ask specifically for Virchow's contribution, because it is the one that turns a description into an explanation.

Prokaryotic and eukaryotic — where the real line falls

The dividing line is not size, and it is not complexity. It is whether the genetic material is enclosed by a nuclear membrane. Prokaryotes — bacteria, blue-green algae, mycoplasma and PPLO — have genetic material that is naked, not enveloped by a nuclear membrane, occupying a region called the nucleoid. Eukaryotes have a true nucleus bounded by a nuclear envelope.

That single difference brings others with it. Prokaryotes have no membrane-bound organelles at all: no mitochondria, no plastids, no endoplasmic reticulum, no Golgi. Their ribosomes are 70S, made of a 50S and a 30S subunit, while eukaryotic cytoplasmic ribosomes are 80S, made of 60S and 40S.

The exception worth remembering is that mitochondria and chloroplasts contain their own circular DNA and their own 70S ribosomes, and divide by fission. That prokaryote-like character inside a eukaryotic cell is the observation the endosymbiotic hypothesis was built to explain, and it is a favourite short-answer question.

The prokaryotic cell in detail

Bacteria occur in four basic shapes — bacillus (rod), coccus (spherical), vibrio (comma-shaped) and spirillum (spiral). Beneath that variety the organisation is consistent.

• The cell envelope has three layers acting as a unit: an outermost glycocalyx, then the cell wall, then the plasma membrane. The glycocalyx may be a loose slime layer or a thick, tough capsule.
• Gram staining separates bacteria into two groups by how the envelope responds: those that retain the stain are Gram positive and those that do not are Gram negative.
• The genetic material is a single circular DNA in the nucleoid. Many bacteria additionally carry small circular DNA outside the genomic DNA called plasmids, which confer characters such as resistance to antibiotics.
• Mesosomes are infoldings of the plasma membrane. They help in cell wall formation, DNA replication and its distribution to daughter cells, and also in respiration and secretion. In cyanobacteria, membranous extensions called chromatophores contain pigments.
• Bacterial flagella have three parts — filament, hook and basal body — and are thinner and structurally quite different from eukaryotic flagella. Pili and fimbriae are surface structures not involved in motility.
• Ribosomes are 70S and associate with messenger RNA to form polysomes, or polyribosomes, so that several ribosomes translate the same message at once. Reserve material is stored in inclusion bodies, which lie free in the cytoplasm and are not membrane bound.

The plasma membrane and how substances cross it

The fluid mosaic model was proposed by Singer and Nicolson in 1972 and is the model CBSE expects you to name and describe.

• Lipids are arranged as a bilayer, with the polar heads facing outwards on both surfaces and the hydrophobic tails facing inwards. This arrangement forms spontaneously because the tails are kept away from water.
• Proteins are of two kinds: integral proteins, partially or totally buried in the membrane, and peripheral proteins lying on its surface.
• The quasi-fluid nature of the lipid allows lateral movement within the bilayer. That fluidity is what enables growth, formation of intercellular junctions, secretion, endocytosis and cell division.
• Passive transport needs no energy: neutral solutes move down a concentration gradient by simple diffusion, and water moves in the same way by osmosis.
• Active transport is carrier-protein mediated, moves substances against a concentration gradient and requires energy in the form of ATP. Polar molecules cannot pass the non-polar bilayer freely and need carrier proteins.

The endomembrane system

The endoplasmic reticulum, Golgi complex, lysosomes and vacuoles are grouped together as the endomembrane system because their functions are coordinated. Mitochondria, chloroplasts and peroxisomes are deliberately excluded from this grouping.

• Rough endoplasmic reticulum bears ribosomes on its surface and is the site of protein synthesis and secretion; it is extensive in cells actively engaged in protein synthesis.
• Smooth endoplasmic reticulum lacks ribosomes and is the principal site of lipid synthesis. In animal cells steroidal hormones are synthesised here.
• The Golgi apparatus, discovered by Camillo Golgi in 1898, consists of flat disc-shaped cisternae stacked in parallel. It has a convex cis or forming face and a concave trans or maturing face, which are distinct in both morphology and function.
• Its principal function is packaging materials for delivery within the cell or for secretion outside. Proteins arriving from the endoplasmic reticulum are modified here, and glycoproteins and glycolipids are formed.
• Lysosomes are membrane-bound vesicles formed from the Golgi. They are rich in hydrolytic enzymes — lipases, proteases and carbohydrases — that are optimally active at acidic pH, which is why they are called the suicide bags of the cell.
• Vacuoles are bounded by a single membrane called the tonoplast, and in plant cells may occupy up to ninety per cent of the cell volume. In Amoeba the contractile vacuole is important in excretion and osmoregulation.

Mitochondria, plastids and the nucleus

These three carry the organelle questions most often set, and the numerical details in them are precisely what the marks are given for.

• Mitochondria have a double membrane. The outer membrane is smooth; the inner membrane forms infoldings called cristae which increase the surface area, and encloses the matrix. They are the sites of aerobic respiration and produce ATP, which is why they are called the power houses of the cell.
• Plastids are of three kinds: chloroplasts containing chlorophyll and carotenoids, chromoplasts containing fat-soluble carotenoid pigments, and colourless leucoplasts. Leucoplasts are further named for what they store — amyloplasts store starch, elaioplasts store oils and fats, aleuroplasts store proteins.
• Inside a chloroplast, the space enclosed by the inner membrane is the stroma. Flattened membranous sacs called thylakoids are arranged in stacks called grana, connected by stroma lamellae. Chlorophyll is in the thylakoid membranes, and the enzymes for sugar synthesis are in the stroma.
• Both mitochondria and chloroplasts contain circular DNA, 70S ribosomes and divide by fission, so they are described as semi-autonomous organelles.
• The nucleus was first described by Robert Brown in 1831. Its envelope consists of two membranes with a perinuclear space between them, interrupted by nuclear pores through which RNA and protein molecules move in both directions.
• The nucleolus is not membrane bound and is the site of active ribosomal RNA synthesis. Chromatin exists as loosely packed, transcriptionally active euchromatin and densely packed, inactive heterochromatin.

Worked Examples

• Mycoplasma is the smallest cell — At about 0.3 micrometre in length, Mycoplasma is the smallest cell known. Bacteria are larger, at roughly 3 to 5 micrometres.
• The largest isolated single cell is the egg of an ostrich — A standard one-mark question. Note the phrasing 'isolated single cell' — the longest cells are nerve cells, which is a different superlative.
• A human red blood cell is about 7 micrometres across — Useful as a reference scale, and it is also the standard example of a cell that has lost its nucleus at maturity.
• Centriole is 9+0, cilium is 9+2 — The centriole has nine evenly spaced peripheral fibrils of tubulin and no central tubule. The axoneme of a cilium or flagellum has nine peripheral doublets plus a central pair. Confusing these two arrays is the single most common error in this chapter.

Common Mistakes Students Make

• Saying prokaryotes have no genetic material or no ribosomes. They have both — the DNA is simply not enclosed by a nuclear membrane, and their ribosomes are 70S rather than 80S.
• Writing that the 9+2 array is found in centrioles. Centrioles have a 9+0 cartwheel organisation; the 9+2 array belongs to the axoneme of cilia and flagella.
• Including mitochondria and chloroplasts in the endomembrane system. The system is the endoplasmic reticulum, Golgi complex, lysosomes and vacuoles only; the others are excluded because their functions are not coordinated with these.
• Confusing the cis and trans faces of the Golgi. The cis or forming face is convex and receives material; the trans or maturing face is concave and dispatches it. They differ in morphology as well as function.
• Calling the nucleolus a membrane-bound organelle. It is not bounded by any membrane; it is a naked region of the nucleus where ribosomal RNA is synthesised.
• Attributing the whole of cell theory to Schleiden and Schwann. Their formulation could not explain the origin of new cells; Virchow supplied that in 1855.
• Assuming ribosomes are membrane bound. Ribosomes have no membrane at all — they are composed of ribonucleic acid and proteins.

Where This Shows Up in Real Life

Why some antibiotics harm bacteria but not us

Bacterial ribosomes are 70S while human cytoplasmic ribosomes are 80S, and several antibiotics act by binding specifically to the 70S ribosome and blocking bacterial protein synthesis. The structural difference described in this chapter is what gives such a drug its selective action. It also explains why antibiotics are useless against viruses, which have no ribosomes of their own at all.

Plasmids, antibiotic resistance and genetic engineering

Plasmids are small circular DNA molecules outside the genomic DNA that confer characters such as resistance to antibiotics, and bacteria can pass them between one another. That is one route by which resistance spreads through a bacterial population. The same property makes plasmids the standard vehicle for carrying a chosen gene into a bacterial cell, which is the foundation of the biotechnology studied in Class 12.

Lysosomes and cellular clean-up

Lysosomal enzymes are hydrolases that work best at acidic pH and can digest carbohydrates, proteins, lipids and nucleic acids. This lets the cell break down worn-out organelles and material taken in from outside. If the membrane ruptures, those enzymes are released into the cytoplasm and digest the cell itself, which is the reason for the name suicide bags.

Why plant cells do not burst in pure water

The rigid cell wall outside the plasma membrane resists the inward movement of water once the cell is full, so a plant cell placed in a hypotonic solution becomes turgid rather than bursting. An animal cell without a wall has no such protection. This single structural difference explains turgidity, plasmolysis and much of the water relations work in plant physiology.

Board-Style Questions with Model Answers

State the three postulates of the modern cell theory and name the scientist who contributed the third. (3 marks)

The postulates are that all living organisms are composed of cells and the products of cells, that the cell is the basic structural and functional unit of life, and that all cells arise from pre-existing cells. Schleiden and Schwann proposed the first formulation in 1838 and 1839 from their studies of plants and animals respectively, but it could not explain how new cells are formed. Rudolf Virchow supplied the third postulate in 1855 with the statement omnis cellula-e cellula.

Distinguish between prokaryotic and eukaryotic cells on any four points. (4 marks)

First, prokaryotes have no nuclear membrane and their genetic material lies naked in a region called the nucleoid, whereas eukaryotes have a true nucleus enclosed by a nuclear envelope. Second, prokaryotes lack membrane-bound organelles such as mitochondria, plastids, endoplasmic reticulum and Golgi complex, all of which are present in eukaryotes. Third, prokaryotic ribosomes are 70S, made of 50S and 30S subunits, while eukaryotic cytoplasmic ribosomes are 80S, made of 60S and 40S subunits. Fourth, prokaryotes usually possess a single circular DNA molecule and may carry plasmids, whereas eukaryotes have several linear chromosomes associated with histone proteins.

Describe the fluid mosaic model of the plasma membrane. (5 marks)

The model was proposed by Singer and Nicolson in 1972. According to it the membrane consists of a bilayer of lipids in which the polar heads face the aqueous surfaces on either side and the non-polar hydrophobic tails face inward, away from water. Protein molecules are distributed in this lipid bilayer as a mosaic: integral proteins are partially or totally buried within the bilayer, while peripheral proteins lie on its surface. The lipid is quasi-fluid, so proteins and lipids can move laterally within the bilayer. This fluidity is significant because it enables cell growth, the formation of intercellular junctions, secretion, endocytosis and cell division. The membrane is selectively permeable, allowing some molecules to cross by simple diffusion or osmosis without energy and moving others against a concentration gradient by active transport using ATP.

Why are mitochondria and chloroplasts called semi-autonomous organelles? (3 marks)

Both organelles contain their own DNA, which is circular, and their own ribosomes, which are of the 70S type rather than the 80S type found in the surrounding eukaryotic cytoplasm. They are therefore able to synthesise some of their own proteins, and they divide by fission independently of the division of the cell. However they still depend on the nuclear genome for most of their proteins, so their independence is only partial, and they are described as semi-autonomous rather than autonomous.

Give the structure and function of the Golgi apparatus. (5 marks)

The Golgi apparatus was first observed by Camillo Golgi in 1898. It consists of many flat, disc-shaped sacs or cisternae, generally about half a micrometre to one micrometre in diameter, stacked parallel to one another and held together concentrically near the nucleus. The stack has two distinct faces: the convex cis or forming face and the concave trans or maturing face, which differ in morphology as well as in function. Its main function is the packaging of materials, to be delivered either to intracellular targets or secreted outside the cell. Materials to be packaged are received from the endoplasmic reticulum, modified within the cisternae, and then dispatched. The Golgi is also the site where glycoproteins and glycolipids are formed, and it is an important site for the formation of lysosomes.

Explain why lysosomes are called suicide bags of the cell. (2 marks)

Lysosomes are membrane-bound vesicles containing a rich collection of hydrolytic enzymes — lipases, proteases and carbohydrases — which are optimally active at acidic pH and are capable of digesting carbohydrates, proteins, lipids and nucleic acids. So long as these enzymes remain enclosed the cell is safe, but if the lysosomal membrane ruptures the enzymes are released into the cytoplasm and digest the cell's own contents, destroying it. Because the organelle can thus bring about the destruction of the very cell containing it, it is called a suicide bag.

Assertion–Reason Questions (CBSE Format)

For each question choose: (a) both A and R are true and R is the correct explanation of A; (b) both are true but R is not the correct explanation; (c) A is true but R is false; (d) A is false but R is true.

Quick Self-Check

Key Terms Glossary

Cell theory

The principle that all living organisms are composed of cells and their products, that the cell is the basic unit of life, and that all cells arise from pre-existing cells.

Nucleoid

The region of a prokaryotic cell in which the genetic material lies, not enclosed by any nuclear membrane.

Plasmid

Small circular DNA outside the genomic DNA of a bacterium, which may confer characters such as antibiotic resistance.

Mesosome

An infolding of the plasma membrane in a prokaryote, helping in cell wall formation, DNA replication and distribution, respiration and secretion.

Fluid mosaic model

The model of membrane structure proposed by Singer and Nicolson in 1972, describing proteins distributed in a quasi-fluid lipid bilayer.

Active transport

Carrier-mediated movement of a substance across a membrane against its concentration gradient, requiring energy as ATP.

Endomembrane system

The endoplasmic reticulum, Golgi complex, lysosomes and vacuoles, grouped together because their functions are coordinated.

Cisternae

The flat, disc-shaped membranous sacs stacked in parallel that make up the Golgi apparatus.

Cristae

Infoldings of the inner mitochondrial membrane that increase surface area for the reactions of aerobic respiration.

Grana

Stacks of flattened thylakoid sacs inside a chloroplast; the thylakoid membranes contain chlorophyll.

Stroma

The ground substance enclosed by the inner membrane of a chloroplast, containing the enzymes for sugar synthesis.

Euchromatin

Loosely packed chromatin that stains lightly and is transcriptionally active.

Heterochromatin

Densely packed chromatin that stains darkly and is transcriptionally inactive.

Axoneme

The core of a cilium or flagellum, containing nine peripheral doublets of microtubules and a central pair, described as the 9+2 array.

One-Minute Revision

• Hooke saw dead cork walls and named cells; Leeuwenhoek first saw living cells; Virchow added that cells come from pre-existing cells.
• The prokaryote–eukaryote line is the nuclear membrane, not size or complexity.
• Prokaryote ribosomes 70S (50S + 30S); eukaryote cytoplasmic ribosomes 80S (60S + 40S).
• Mitochondria and chloroplasts hold circular DNA and 70S ribosomes and divide by fission — semi-autonomous.
• Fluid mosaic model: Singer and Nicolson, 1972. Polar heads out, hydrophobic tails in, proteins integral or peripheral.
• Endomembrane system = ER + Golgi + lysosomes + vacuoles. Mitochondria, chloroplasts and peroxisomes are excluded.
• Golgi: cis face convex and receiving, trans face concave and dispatching. Packaging, glycoproteins, glycolipids.
• Cristae belong to mitochondria; grana and thylakoids to chloroplasts; mesosomes to prokaryotes.
• Centriole is 9+0; the axoneme of cilia and flagella is 9+2.
• Nucleolus has no membrane and is the site of ribosomal RNA synthesis.

Syllabus reference: NCERT Class 11 Biology — Cell: The Unit of Life. Leads directly into Cell Cycle and Cell Division, and underpins Biomolecules and the whole of Plant and Human Physiology.

Frequently Asked Questions

Why are mitochondria called the powerhouse of the cell?

Mitochondria produce ATP through aerobic respiration, which is the primary energy currency of cells. ATP powers nearly all cellular activities.

What is the main difference between mitosis and meiosis?

Mitosis produces two identical diploid daughter cells for body growth. Meiosis produces four non-identical haploid cells for sexual reproduction.

Why is Robert Hooke not credited with discovering living cells?

Hooke examined a thin slice of cork in 1665, which is dead plant tissue. What he observed were the empty compartments left by the cell walls of cells that had already died, and he named them cells after the small rooms of a monastery. He therefore discovered and named the structure but never saw a living cell. Anton von Leeuwenhoek was the first to observe and describe living cells.

What exactly does the S in 70S and 80S stand for?

The S refers to the Svedberg unit, which is a measure of sedimentation rate during centrifugation rather than a measure of mass. Because sedimentation depends on shape as well as size, the values do not add in the ordinary way — this is why a 50S and a 30S subunit combine to give a 70S ribosome rather than an 80S one. The same reasoning explains why 60S and 40S subunits form an 80S ribosome.

How is a bacterial flagellum different from a eukaryotic flagellum?

They are different structures that happen to share a name. A bacterial flagellum is composed of three parts — filament, hook and basal body — and is very thin, with the filament being the longest portion extending from the cell surface. A eukaryotic flagellum is an extension of the plasma membrane containing an axoneme with nine peripheral doublets of microtubules and a central pair, the 9+2 arrangement, and arises from a basal body. They also differ in how they generate movement.

Do plant cells have lysosomes and centrioles?

Plant cells generally lack centrioles, which is why the spindle formed during their cell division is described as anastral. Their functions of storage and degradation are largely taken over by the large central vacuole, which is bounded by the tonoplast and can occupy up to ninety per cent of the cell volume. Animal cells have centrioles within a centrosome and rely on lysosomes for intracellular digestion.

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