Cell: The Unit of Life
Welcome to HSLC Guru! In this chapter we explore the cell — the basic structural and functional unit of all living organisms. Following the ASSEB Class 11 Biology syllabus, this lesson takes you from the discovery of cells and the formulation of the cell theory to the detailed structure of prokaryotic and eukaryotic cells, including their membranes, organelles and the nucleus. The notes, exam-style questions, MCQs, fill-in-the-blanks, true/false items and glossary are designed to help you prepare confidently for your ASSEB Class 11 Biology examination.
Summary
Discovery of the Cell and Cell Theory: The cell was first observed by Robert Hooke in 1665 while examining a thin slice of cork under a self-designed microscope. He saw tiny box-like compartments and named them “cellulae” (Latin for little rooms), giving rise to the word cell. A few years later, Anton von Leeuwenhoek (1674) was the first to observe living cells such as bacteria, protozoa and red blood cells using improved microscopes. Robert Brown (1831) discovered the nucleus, and later Purkinje coined the term protoplasm for the living substance of the cell. The Cell Theory was proposed by the German botanist M. J. Schleiden (1838) and the zoologist Theodore Schwann (1839); it states that all plants and animals are composed of cells, and the cell is the basic unit of life. Rudolf Virchow (1855) added the important principle Omnis cellula-e cellula — all cells arise from pre-existing cells.
Prokaryotic Cells: Prokaryotes (bacteria, blue-green algae, mycoplasma and PPLO) are smaller and multiply more rapidly than eukaryotes. They lack a true membrane-bound nucleus; instead the genetic material lies free in the cytoplasm in a region called the nucleoid. Many bacteria also possess small circular DNA called plasmids. The bacterial cell envelope is a three-layered structure consisting of an outermost glycocalyx (which may be a loose slime layer or a tough capsule), a rigid cell wall made of peptidoglycan, and the innermost plasma membrane. On the basis of cell-wall staining, bacteria are classified as Gram-positive (purple) or Gram-negative (pink). The plasma membrane often invaginates to form a mesosome, which helps in respiration, DNA replication and cell-wall formation. Bacterial cells contain 70S ribosomes, reserve materials in inclusion bodies (e.g., glycogen granules, gas vacuoles, cyanophycean granules), and surface appendages such as flagella for movement, pili for conjugation and fimbriae for attachment.
Eukaryotic Cells: Found in protists, fungi, plants and animals, eukaryotic cells possess a true nucleus and well-developed membrane-bound organelles. The plasma membrane is described by the fluid mosaic model (Singer and Nicolson, 1972) — a phospholipid bilayer with embedded proteins that show lateral movement. Plant cells additionally have a cell wall made of cellulose, hemicellulose, pectin and proteins; the middle lamella holds adjacent cells together. The endomembrane system includes the endoplasmic reticulum (RER with ribosomes for protein synthesis, SER for lipid synthesis), the Golgi apparatus for packaging, modification and secretion, lysosomes as digestive sacs, and vacuoles for storage and turgor maintenance.
Other Organelles and the Nucleus: Mitochondria are double-membraned organelles with inner folds (cristae) and a matrix; they are the powerhouses of the cell because they synthesise ATP. Plastids, especially chloroplasts, carry out photosynthesis using thylakoid stacks (grana) embedded in a stroma. Peroxisomes handle photorespiration and detoxification, while glyoxysomes in oilseed seedlings convert stored fats into sugars. The cytoskeleton (microtubules, microfilaments and intermediate filaments) provides shape, mechanical support and intracellular transport. Cilia and flagella are hair-like cell-surface projections with a 9 + 2 microtubule arrangement that bring about movement. The centrosome, present near the nucleus of animal cells, contains two centrioles that organise the spindle apparatus during cell division. The nucleus is enclosed by a double nuclear envelope with pores; it contains the nucleolus (site of rRNA synthesis) and chromatin (DNA + histones), which condenses into chromosomes during division. Each chromosome has a centromere, two chromatids and a kinetochore; based on the centromere position chromosomes are classified as metacentric, sub-metacentric, acrocentric or telocentric. Humans normally possess 46 chromosomes (23 pairs).
Question and Answers
1-Mark Questions
Q1. Who discovered the cell?
Answer: Robert Hooke discovered the cell in 1665.
Q2. Who first observed living cells?
Answer: Anton von Leeuwenhoek first observed living cells in 1674.
Q3. Who proposed the cell theory?
Answer: The cell theory was proposed by M. J. Schleiden (1838) and Theodore Schwann (1839).
Q4. Who modified the cell theory and how?
Answer: Rudolf Virchow (1855) modified it by stating that all cells arise from pre-existing cells (Omnis cellula-e cellula).
Q5. What is a nucleoid?
Answer: The region of the bacterial cytoplasm in which the naked DNA is found is called the nucleoid.
Q6. What is a mesosome?
Answer: A mesosome is an in-folding of the bacterial plasma membrane that helps in respiration, DNA replication and secretion.
Q7. Which is the smallest cell known?
Answer: Mycoplasma (PPLO) is the smallest known cell, about 0.3 µm in length.
Q8. Name the powerhouse of the cell.
Answer: Mitochondrion is called the powerhouse of the cell because it produces ATP.
Q9. What is the function of the nucleolus?
Answer: The nucleolus is the site of ribosomal RNA (rRNA) synthesis.
Q10. How many chromosomes are present in a normal human cell?
Answer: A normal human somatic cell contains 46 chromosomes arranged in 23 pairs.
2-3 Marks Questions
Q1. Differentiate between prokaryotic and eukaryotic cells.
Answer: Prokaryotic cells are smaller (1–10 µm), lack a true membrane-bound nucleus and membrane-bound organelles, and possess 70S ribosomes; their genetic material is a single circular DNA in the nucleoid. Eukaryotic cells are larger (10–100 µm), possess a well-defined nucleus enclosed by a nuclear envelope and have membrane-bound organelles such as mitochondria, ER and Golgi; their ribosomes are 80S in the cytoplasm.
Q2. Describe the bacterial cell envelope.
Answer: The bacterial cell envelope is a tightly bound three-layered structure. The outermost layer is the glycocalyx, which may be a loose slime layer or a thick capsule. Beneath it lies the rigid cell wall made of peptidoglycan, which gives shape and prevents the cell from bursting. The innermost layer is the plasma membrane that controls the movement of substances and bears mesosomes that aid respiration and DNA replication.
Q3. Distinguish between flagella, pili and fimbriae.
Answer: Flagella are long, thin filamentous appendages made of flagellin that bring about bacterial locomotion. Pili are longer tubular structures made of pilin protein and play a role in conjugation (transfer of DNA between bacteria). Fimbriae are short bristle-like fibres that help bacteria attach to surfaces and host tissues but are not involved in motility.
Q4. Write a short note on the fluid mosaic model.
Answer: Proposed by Singer and Nicolson (1972), the fluid mosaic model describes the plasma membrane as a phospholipid bilayer in which proteins are embedded — some peripheral and some integral. The lipid bilayer is “fluid” because both lipids and proteins can move laterally, allowing the membrane to perform functions such as transport, cell signalling, growth and endocytosis.
Q5. What is the endomembrane system? Name its components.
Answer: The endomembrane system is a group of membrane-bound organelles whose functions are coordinated. It includes the endoplasmic reticulum, the Golgi apparatus, lysosomes and vacuoles. Mitochondria, chloroplasts and peroxisomes are not part of this system because they function independently.
Q6. Describe peroxisomes and glyoxysomes.
Answer: Peroxisomes are small, single-membrane bound organelles that contain enzymes such as catalase and oxidase; they break down hydrogen peroxide and participate in photorespiration in plants. Glyoxysomes are similar organelles found in fat-storing tissues of germinating seeds and convert stored lipids into carbohydrates by the glyoxylate cycle to fuel seedling growth.
5-7 Marks Questions
Q1. Describe the structure of a typical bacterial cell with the help of a labelled diagram.
Answer: A typical bacterial cell is enclosed by a three-layered cell envelope consisting of an outer glycocalyx (capsule or slime), a peptidoglycan cell wall and an inner plasma membrane. The plasma membrane invaginates inwards to form mesosomes, which assist in respiration, DNA replication and the formation of cell wall. The cytoplasm is dense and granular and lacks membrane-bound organelles. Within it lies the nucleoid — a region containing a single circular naked DNA molecule. Smaller circular DNA pieces called plasmids may be present and confer additional characters such as antibiotic resistance. The cytoplasm also contains 70S ribosomes for protein synthesis and inclusion bodies that store reserve materials such as glycogen, sulphur or gas vacuoles. On the outside, bacteria may possess flagella for locomotion, pili for conjugation and fimbriae for adhesion. The bacterial cell, therefore, although structurally simple, is a complete metabolic unit.
Q2. Explain the structure and functions of mitochondria.
Answer: Mitochondria are double-membraned, sausage-shaped organelles ranging from 1.0 to 4.1 µm in length. The outer membrane is smooth and permeable, while the inner membrane is folded inwards into finger-like projections called cristae which increase the surface area for chemical reactions. The space enclosed by the inner membrane is called the matrix; it contains a single circular DNA molecule, 70S ribosomes, RNA and the enzymes of the Krebs cycle. The inner membrane bears small stalked particles called F1 particles or oxysomes that are the actual sites of ATP synthesis. Functions: (i) Mitochondria are the principal sites of aerobic respiration where pyruvate is oxidised to CO2 and H2O. (ii) They synthesise ATP by oxidative phosphorylation and hence are called the powerhouses of the cell. (iii) They synthesise certain proteins of their own due to the presence of DNA and ribosomes, supporting the endosymbiotic theory of their origin.
Q3. Describe the structure of a chloroplast and explain its role in photosynthesis.
Answer: Chloroplasts are double-membraned plastids found in green plant cells, generally lens-shaped, 5–10 µm in length and 2–4 µm in width. The space enclosed by the inner membrane is called the stroma, in which a number of flattened, membranous sacs called thylakoids are arranged in stacks named grana (singular: granum). Adjacent grana are connected by stroma lamellae or fret membranes. The thylakoid membranes contain chlorophyll and other photosynthetic pigments. The stroma also contains circular DNA, 70S ribosomes and the enzymes for the Calvin cycle. Functions: the light reactions of photosynthesis occur on the thylakoid membranes, where light energy is absorbed and converted into ATP and NADPH; the dark (Calvin) reactions take place in the stroma, fixing CO2 into carbohydrates. Because they are self-replicating with their own DNA, chloroplasts also support the endosymbiotic theory.
Q4. Describe the nucleus and its components.
Answer: The nucleus is the largest organelle in a eukaryotic cell and was first described by Robert Brown in 1831. It is generally spherical or oval and is bounded by a double-layered nuclear envelope in which the outer membrane is continuous with the rough endoplasmic reticulum. The envelope bears nuclear pores through which materials such as RNA and proteins are exchanged between the nucleus and the cytoplasm. Inside lies a clear, semi-fluid nucleoplasm containing the chromatin network — a complex of DNA and histone proteins which condenses into rod-like chromosomes at the time of cell division. One or more dense, rounded bodies called nucleoli are also present; they are the sites of rRNA synthesis and ribosome assembly. Functions: the nucleus controls all metabolic activities of the cell, contains the hereditary material, regulates gene expression and directs cell division.
Q5. Discuss the structure of a chromosome and classify chromosomes on the basis of centromere position.
Answer: A chromosome is a thread-like, deeply staining body that becomes visible during cell division. Each chromosome consists of two identical longitudinal halves called chromatids, joined together at a constricted region called the centromere. On both sides of the centromere lie disc-shaped structures called kinetochores to which the spindle fibres attach. Some chromosomes have an additional secondary constriction that bears a small projection called the satellite, and such chromosomes are called SAT-chromosomes. Based on the position of the centromere chromosomes are classified into four types: (i) Metacentric — centromere in the middle, two equal arms; (ii) Sub-metacentric — centromere slightly away from the middle, arms of unequal length; (iii) Acrocentric — centromere near the end, one very short and one very long arm; (iv) Telocentric — centromere at the terminal end, only one arm. Each chromosome carries genes arranged in a linear fashion that determine the heredity of the organism.
Multiple Choice Questions (MCQs)
Q1. Who discovered the cell?
a) Schleiden b) Schwann c) Robert Hooke d) Virchow
Answer: c) Robert Hooke
Q2. The cell theory was proposed by —
a) Hooke and Brown b) Schleiden and Schwann c) Virchow and Hooke d) Mendel and Darwin
Answer: b) Schleiden and Schwann
Q3. Omnis cellula-e cellula was given by —
a) Robert Brown b) Schwann c) Virchow d) Leeuwenhoek
Answer: c) Virchow
Q4. The smallest known cell is —
a) Bacterium b) Mycoplasma (PPLO) c) Yeast d) Amoeba
Answer: b) Mycoplasma (PPLO)
Q5. Mesosomes are formed by the in-folding of —
a) Cell wall b) Nuclear membrane c) Plasma membrane d) Glycocalyx
Answer: c) Plasma membrane
Q6. The fluid mosaic model of the plasma membrane was proposed by —
a) Robertson b) Davson and Danielli c) Singer and Nicolson d) Schleiden
Answer: c) Singer and Nicolson
Q7. Which organelle is called the suicide bag of the cell?
a) Mitochondrion b) Lysosome c) Ribosome d) Golgi body
Answer: b) Lysosome
Q8. Cristae are found in —
a) Chloroplast b) Golgi apparatus c) Mitochondrion d) ER
Answer: c) Mitochondrion
Q9. Stacks of thylakoids in a chloroplast are called —
a) Stroma b) Grana c) Cristae d) Matrix
Answer: b) Grana
Q10. The nucleolus is the site of —
a) DNA replication b) Protein synthesis c) rRNA synthesis d) Lipid synthesis
Answer: c) rRNA synthesis
Fill in the Blanks
Q1. The cell was first observed by ________ in 1665.
Answer: Robert Hooke
Q2. The bacterial cell wall is mainly made up of ________.
Answer: peptidoglycan
Q3. The site of aerobic respiration in the cell is the ________.
Answer: mitochondrion
Q4. The cytoskeleton is composed of microtubules, microfilaments and ________ filaments.
Answer: intermediate
Q5. A normal human cell contains ________ chromosomes.
Answer: 46
True or False
Q1. All living cells arise from pre-existing cells.
Answer: True
Q2. Prokaryotic cells contain membrane-bound organelles.
Answer: False
Q3. The plant cell wall is mainly composed of cellulose.
Answer: True
Q4. Centrioles are present in the cells of higher plants.
Answer: False
Q5. Ribosomes of prokaryotic cells are of the 70S type.
Answer: True
Glossary
| Term | Meaning |
|---|---|
| Cell | The basic structural and functional unit of all living organisms. |
| Cell Theory | A theory that states all living organisms are made up of cells and cells arise from pre-existing cells. |
| Prokaryote | An organism whose cells lack a true membrane-bound nucleus and organelles. |
| Eukaryote | An organism whose cells possess a true nucleus and membrane-bound organelles. |
| Nucleoid | The region of cytoplasm in prokaryotes where the genetic material lies. |
| Glycocalyx | The outermost covering of a bacterial cell — capsule or slime layer. |
| Mesosome | An infolding of the bacterial plasma membrane that helps in respiration and DNA replication. |
| Plasmid | An extra-chromosomal small circular DNA in bacteria carrying additional genes. |
| Plasma Membrane | The selectively permeable lipid-protein boundary of every cell. |
| Fluid Mosaic Model | The widely accepted model of the plasma membrane proposed by Singer and Nicolson (1972). |
| Endomembrane System | Coordinated group of membrane-bound organelles — ER, Golgi, lysosomes and vacuoles. |
| Endoplasmic Reticulum | A network of membranes inside the cell; rough ER bears ribosomes, smooth ER does not. |
| Golgi Apparatus | Stacks of cisternae that modify, package and dispatch cellular products. |
| Lysosome | A single-membrane organelle containing hydrolytic enzymes; called the suicide bag. |
| Vacuole | Membrane-bound sac storing water, food or waste; large central vacuole in plants. |
| Mitochondrion | Double-membraned organelle that produces ATP through aerobic respiration. |
| Chloroplast | Plastid containing chlorophyll where photosynthesis occurs. |
| Peroxisome | Organelle that breaks down hydrogen peroxide and participates in photorespiration. |
| Glyoxysome | Specialised peroxisome in oilseeds that converts fats to sugars. |
| Cytoskeleton | Protein filament network — microtubules, microfilaments, intermediate filaments — giving shape and support. |
| Cilia and Flagella | Hair-like surface projections with a 9 + 2 microtubular structure used for movement. |
| Centrosome | Organelle with two centrioles that organises spindle fibres in animal cells. |
| Nuclear Envelope | Double membrane surrounding the nucleus, perforated by nuclear pores. |
| Chromatin | The relaxed network of DNA and histone proteins in the nucleus. |
| Nucleolus | Dense body within the nucleus that synthesises rRNA. |
| Chromosome | Condensed thread-like body of DNA and protein that carries genes. |
| Centromere | Constricted region of a chromosome where chromatids are joined and spindle fibres attach. |
| Metacentric | Chromosome with the centromere exactly in the middle. |
| Acrocentric | Chromosome with the centromere near one end giving very unequal arms. |
| Telocentric | Chromosome with the centromere at the terminal end. |