3.1 The Cell — Structure of Cell and Cell Organelles

Short Introduction Earth is a living planet, home to a huge variety of life. Irrespective of their size and shape, all life forms are made up of units called cells. The functions performed by living organisms are also performed at the cell level. Therefore, the cell is the basic unit of structure and function of all living organisms.

Main Idea

The cell is the basic unit of structure and function of all living organisms. In 1665, Robert Hooke discovered the cell. Since then, with improved microscopes, many organelles have been discovered. A typical eukaryotic cell contains various membrane-bound and non-membrane-bound organelles, each with a specific function, including the cell wall, cell membrane, cytoplasm, endoplasmic reticulum, ribosomes, Golgi apparatus, lysosomes, mitochondria, plastids, vacuoles, centrioles, cytoskeleton, cilia and flagella, and the nucleus.

Core Concept

Discovery of the Cell generated by DeepSeek to PDF

In 1665, Robert Hooke discovered the cell when he examined a thin slice of cork tissue under a compound microscope. He observed cells as empty chambers with thick outer coverings. The quality of microscope lenses improved greatly in the nineteenth century, leading to the discovery of the cell nucleus in 1831 and many cytoplasmic organelles in the coming years. Cell Structure and Organelles 1. Cell Wall Surrounds the plasma membrane of plant cells. It is a rigid, inert covering secreted and deposited outside the cell membrane. Layers: Consists of three layers: Middle lamella: Made up of magnesium and calcium salts of pectin. It is sticky and holds neighbouring cell walls together. Primary wall: Contains cellulose fibres arranged in a crisscross fashion. It is thin and flexible. Secondary wall: Formed inside the primary wall in some plant cells (e.g., xylem vessels). It is very thick and rigid due to lignin. Plasmodesmata: Tiny pores through which neighbouring cells form cytoplasmic connections. Composition: Algae have cellulose in their cell wall. Fungal cell wall is made up of chitin. Prokaryotes possess a cell wall made up of peptidoglycan. Cell wall is absent in animals and animal-like protists (protozoa). Functions: Supports the structure of individual cells and the plant as a whole. It protects and gives shape to the cell. Plant cells can develop turgor pressure due to the presence of the cell wall. 2. Cell Membrane Thin sheet-like covering of the cell. Chemical composition: Proteins 60–80%, phospholipids 20–40%, and traces of carbohydrates. Fluid Mosaic Model: Postulates that the cell membrane consists of a double layer of phospholipids in which proteins are incorporated in a mosaic fashion. Protein molecules float like icebergs in a sea-like fluid of phospholipids. Cholesterol: Cell membranes of eukaryotes also contain cholesterol, which prevents stiffening of the cell membrane and is required for the fusion of secretory vesicles with the membrane. Carbohydrates: Either linked with proteins or lipids. Functions: Acts as a barrier and gatekeeper for the cell. It is semipermeable, maintains a fixed environment inside the cell, and regulates the exchange of materials between the cell and its environment. 3. Cytoplasm Aqueous substance between the cell membrane and nucleus. It is about 90% water with many dissolved and suspended materials. Functions: Site for many biochemical processes, stores food granules and waste materials, and is home to a variety of cell organelles. 4. Endoplasmic Reticulum (ER) A system of membranes present throughout the cytoplasm of eukaryotic cells. Flattened sacs are called cisternae. Rough ER (RER): Covered with ribosomes. Involved in the synthesis of some proteins. Smooth ER (SER): Ribosomes absent. Synthesizes lipids including steroids, detoxifies harmful substances, and in muscle cells, has an important role in the contraction process. generated by DeepSeek to PDF

Functions: Provides mechanical support, involved in transport of substances within the cell. 5. Ribosomes Tiny granular structures found in both prokaryotic and eukaryotic cells. They are not bound by any membrane. Composed of roughly equal amounts of proteins and ribosomal RNA (rRNA). Prokaryotic ribosomes are smaller in size. Structure: Each ribosome consists of two subunits (one small and one large) that join when the ribosome has to perform its function. Function: Site of protein synthesis. A large number of ribosomes are scattered in the cytoplasm; in eukaryotes, many are attached to the surface of RER. 6. Golgi Apparatus Discovered by Camillo Golgi. Present in all eukaryotic cells. A collection of flattened sacs called cisternae, stacked over each other. They are constantly formed at one end and break up into vesicles at the other end. Functions: Stores and modifies materials into a finished form before packing them into vesicles. Some vesicles settle in the cytoplasm as organelles like lysosomes. Others fuse with the cell membrane to release packed material as cell secretion (e.g., enzymes, hormones, mucus). 7. Lysosomes Single membrane-bound small sac-like structures. Contain a variety of digestive enzymes. The enzymes are synthesized on RER and transported to the Golgi apparatus. Lysosomes then bud off from the Golgi apparatus with their processed enzymes. Functions: Intracellular digestion (digest materials taken up by the cell from outside as a food vacuole). They also engulf and digest unwanted cell organelles. This process is termed autophagy. 8. Mitochondria Found in all aerobic eukaryotic cells. Double membrane-bound structures. The outer membrane is smooth, and the inner membrane forms finger-like projections called cristae, which increase the surface area for respiration. The fluid inside the mitochondrion is called the matrix. Mitochondria have their own DNA and ribosomes. They can multiply within the cell at their own. Function: Produce energy in the form of ATP, that is why they are called the powerhouse of the cell. Site of aerobic respiration. 9. Plastids Double membrane-bound organelles found in plants and algae. Three types: Chloroplast: Usually oval in structure. Two membranes form the chloroplast envelope. They have their own DNA and ribosomes and can multiply. They have a system of membranes containing chlorophyll and other photosynthetic pigments. This system consists of hollow coin- like membranous structures called thylakoids. Many thylakoids stack to form a granum. Some thylakoids of adjacent grana fuse to form intergrana. The fluid part is called stroma. Chloroplasts synthesize food by photosynthesis (light-dependent phase in thylakoid membranes; light-independent phase in stroma). Chromoplast: Coloured other than green (red, pink, yellow, blue, purple, etc.). Found in flower petals to attract insects for pollination. Present in the wall of ripened fruits where they attract birds and other animals which help in seed dispersal. generated by DeepSeek to PDF

Leucoplast: Non-pigmented plastids. Food-storing organelles usually found in roots, bulbs, and stem tubers. Store carbohydrates, proteins, or lipids. 10. Vacuole A membrane-bound fluid-filled sac. Animal cell: May have many small vacuoles which exist temporarily. Contain water and food substances. Some freshwater organisms like Amoeba and sponges have contractile vacuoles which collect and pump out extra water and other wastes. Some cells ingest food by forming food vacuoles. Food vacuoles also store food. Plant cell: Have a large central vacuole formed by joining small vacuoles. The membrane is called the tonoplast. It contains liquid called cell sap (dissolved materials like mineral salts, sugars, and amino acids). It provides support and helps in growth. Its primary role is to maintain turgor pressure. 11. Centrioles Hollow, open-ended cylinder-like structures. Found in animal cells. Exist in pairs near the nuclear envelope. Each centriole consists of nine triplets of microtubules. At the start of cell division, centrioles duplicate, and two pairs move to opposite poles, helping in the formation of the spindle apparatus. Also involved in the formation of cilia and flagella. 12. Cytoskeleton A system of a variety of fibrous proteins throughout the cytoplasm. Three types of cytoskeletal fibres: Microtubules: Made up of tubulin protein. Unbranched, hollow tube-like structures. Give rise to spindle fibres, cilia, and flagella. Microfilaments: Very thin protein fibres consisting of contractile proteins mainly actin. Responsible for the streaming movements of the cytoplasm and regulate overall cell movement. Intermediate filaments: Composed of a variety of proteins including keratin and vimentin. Form a branching network, maintain cell structure, and fix cells with each other in tissues. 13. Cilia and Flagella Hair-like extensions are called cilia. Whip-like extensions are called flagella. Consist of nine pairs of microtubules which surround a single central pair of microtubules. Connected to the basal body, which anchors a cilium or flagellum to the cell. Function: Movement. 14. Nucleus Acts as the control centre of the cell because it contains hereditary material DNA. Surrounded by two membranes which collectively form the nuclear envelope. Bears nuclear pores at points where both membranes fuse. Through nuclear pores, the nucleus communicates with the cytoplasm. Some nutrients and proteins enter the nucleus, and ribosomes and mRNA leave the nucleus. Contains a fluid called nucleoplasm.

Detailed Explanation

The cell is the fundamental unit of life. It is a complex structure with many parts, each specialized for a particular function. The cell wall is a rigid outer layer in plant cells, made of cellulose, providing support and shape. The cell membrane is a semipermeable barrier composed of a phospholipid bilayer with embedded proteins (fluid mosaic generated by DeepSeek to PDF

model). It controls what enters and exits the cell. The cytoplasm is a jelly-like substance where many biochemical reactions occur. The endoplasmic reticulum is a network of membranes; rough ER (with ribosomes) makes proteins, and smooth ER makes lipids and detoxifies. Ribosomes are the sites of protein synthesis. The Golgi apparatus modifies, sorts, and packages proteins and lipids. Lysosomes are digestive sacs containing enzymes that break down waste and cellular debris (autophagy). Mitochondria are the powerhouses of the cell, generating ATP through aerobic respiration, and have their own DNA. Plastids (chloroplasts, chromoplasts, leucoplasts) are found in plants; chloroplasts carry out photosynthesis. Vacuoles are storage sacs; plant cells have a large central vacuole for turgor pressure. Centrioles help in cell division and form cilia/flagella. The cytoskeleton provides structural support and movement. Cilia and flagella are involved in movement. The nucleus is the control centre, containing DNA and directing cell activities.

Intermediate Level

The cell is the basic structural and functional unit of all living organisms. Discovered by Robert Hooke in 1665, the cell contains various organelles. The cell wall is a rigid layer outside the cell membrane in plant cells, fungi, and bacteria, made of cellulose, chitin, or peptidoglycan. The cell membrane is a phospholipid bilayer with embedded proteins (fluid mosaic model) that regulates the exchange of materials. The cytoplasm is the aqueous medium where organelles are suspended. The endoplasmic reticulum (rough and smooth) is involved in protein and lipid synthesis and transport. Ribosomes synthesize proteins. The Golgi apparatus modifies and packages proteins. Lysosomes contain digestive enzymes for intracellular digestion and autophagy. Mitochondria are double- membrane-bound organelles that produce ATP (powerhouse of the cell). Plastids (chloroplasts, chromoplasts, leucoplasts) are found in plants, with chloroplasts carrying out photosynthesis. Vacuoles are storage organelles, with plant cells having a large central vacuole. Centrioles help in cell division. The cytoskeleton provides mechanical support and is involved in movement. Cilia and flagella are involved in movement. The nucleus is the control centre, containing DNA and nucleoplasm.

Important Points

• 1. Robert Hooke discovered the cell in 1665.
• 2. The cell nucleus was discovered in 1831.
• 3. The cell is the basic unit of structure and function of all living organisms.
• 4. Cell wall: Rigid, inert covering outside the cell membrane in plant cells. Made of cellulose (plants), chitin
• (fungi), or peptidoglycan (prokaryotes). Absent in animals.
• 5. Middle lamella: Made of magnesium and calcium salts of pectin; sticky.
• 6. Primary wall: Cellulose fibres in crisscross fashion; thin and flexible.
• 7. Secondary wall: Thick and rigid due to lignin.
• 8. Plasmodesmata: Tiny pores in cell wall for cytoplasmic connections.
• 9. Cell membrane: Proteins 60–80%, phospholipids 20–40%, traces of carbohydrates.
• 10. Fluid mosaic model: Phospholipid bilayer with proteins in mosaic fashion.
• 11. Cholesterol: Prevents stiffening of the cell membrane.
• 12. Cytoplasm: About 90% water.
• 13. RER: Covered with ribosomes; protein synthesis.
• 14. SER: No ribosomes; lipid synthesis, detoxification, muscle contraction.
• 15. Ribosomes: Not bound by any membrane; site of protein synthesis; composed of proteins and rRNA; two
• subunits.
• 16. Golgi apparatus: Discovered by Camillo Golgi; modifies and packages materials; forms lysosomes.
• 17. Lysosomes: Single membrane-bound; contain digestive enzymes; intracellular digestion and autophagy.
• 18. Mitochondria: Double membrane-bound; cristae; matrix; own DNA and ribosomes; powerhouse of the
• cell; aerobic respiration.
• 19. Chloroplast: Double membrane; thylakoids, grana, stroma; own DNA and ribosomes; photosynthesis.
• 20. Chromoplast: Coloured plastids; attract insects and birds.
• 21. Leucoplast: Non-pigmented; store food.
• 22. Vacuole: Membrane-bound; large central vacuole in plants; tonoplast; cell sap; turgor pressure.
• 23. Centrioles: Animal cells; nine triplets of microtubules; spindle apparatus; cilia and flagella.
• 24. Cytoskeleton: Microtubules (tubulin), microfilaments (actin), intermediate filaments (keratin, vimentin).
• 25. Cilia and flagella: Nine pairs of microtubules surrounding a central pair; movement; basal body.
• 26. Nucleus: Control centre; nuclear envelope; nuclear pores; nucleoplasm; contains DNA.

Remember

• Cell = basic unit of structure and function.
• Robert Hooke = discovered cell (1665).
• Cell wall = cellulose (plants), chitin (fungi), peptidoglycan (prokaryotes).
• Middle lamella = pectin.
• Plasmodesmata = pores in cell wall.
• Cell membrane = fluid mosaic model.
• Cholesterol = prevents stiffening.
• RER = protein synthesis; SER = lipid synthesis.
• generated by DeepSeek to PDF
• Ribosomes = protein synthesis.
• Golgi = modifies and packages.
• Lysosomes = digestion, autophagy.
• Mitochondria = powerhouse, ATP.
• Chloroplast = photosynthesis.
• Chromoplast = coloured, attracts pollinators.
• Leucoplast = stores food.
• Vacuole = tonoplast, cell sap, turgor pressure.
• Centrioles = spindle apparatus, 9 triplets.
• Cytoskeleton = microtubules, microfilaments, intermediate filaments.
• Cilia/flagella = movement, 9+2 arrangement.
• Nucleus = control centre, DNA.

Common Mistakes

• 1. Thinking all cells have a cell wall: Cell walls are absent in animals and animal-like protists (protozoa).
• 2. Confusing RER and SER: RER has ribosomes and makes proteins; SER lacks ribosomes and makes lipids,
• detoxifies, and helps in muscle contraction.
• 3. Thinking ribosomes are membrane-bound: Ribosomes are not bound by any membrane.
• 4. Confusing lysosomes and peroxisomes: The text only discusses lysosomes; they contain digestive
• enzymes and are involved in autophagy.
• 5. Thinking mitochondria are only in animal cells: Mitochondria are found in all aerobic eukaryotic cells,
• including plants.
• 6. Confusing chloroplasts, chromoplasts, and leucoplasts: Chloroplasts are green and photosynthetic;
• chromoplasts are coloured (non-green) and attract pollinators; leucoplasts are non-pigmented and store
• food.
• 7. Thinking centrioles are in plant cells: Centrioles are found in animal cells; plant cells form spindle
• apparatus without centrioles.
• 8. Forgetting the 9+2 arrangement of cilia/flagella: They consist of nine pairs of microtubules
• surrounding a single central pair.
• 9. Thinking the nucleus is the only organelle with DNA: Mitochondria and chloroplasts also have their
• own DNA.
• 10. Confusing tonoplast and cell sap: The tonoplast is the membrane of the plant vacuole; cell sap is the
• liquid inside.

Quick Revision

• The cell is the basic unit of structure and function of all living organisms, discovered by Robert Hooke in 1665. The
• cell wall is a rigid outer layer in plant cells (cellulose), fungi (chitin), and prokaryotes (peptidoglycan), absent in
• animals. The cell membrane is a fluid mosaic of phospholipids and proteins that regulates exchange. The cytoplasm
• is the aqueous medium for biochemical processes. The endoplasmic reticulum (RER with ribosomes for protein
• synthesis; SER for lipid synthesis and detoxification) is a membrane network. Ribosomes (not membrane-bound)
• synthesize proteins. The Golgi apparatus modifies and packages materials. Lysosomes contain digestive enzymes for
• intracellular digestion and autophagy. Mitochondria are double-membrane-bound organelles that produce ATP
• (powerhouse) and have their own DNA. Plastids include chloroplasts (photosynthesis), chromoplasts (coloured,
• attract pollinators), and leucoplasts (store food). Vacuoles are storage sacs; plant cells have a large central vacuole
• with tonoplast and cell sap for turgor pressure. Centrioles (animal cells) help in spindle formation. The cytoskeleton
• (microtubules, microfilaments, intermediate filaments) provides support and movement. Cilia and flagella (9+2
• arrangement) are for movement. The nucleus is the control centre, containing DNA, nuclear pores, and nucleoplasm.

60-Second Revision

• Cell = basic unit of life (Hooke, 1665).
• Cell wall = cellulose/chitin/peptidoglycan.
• Cell membrane = fluid mosaic model.
• RER = protein; SER = lipid.
• Ribosomes = protein synthesis.
• Golgi = packaging.
• Lysosomes = digestion/autophagy.
• Mitochondria = ATP/powerhouse.
• Chloroplast = photosynthesis.
• Chromoplast = colour/attract.
• Leucoplast = store food.
• Vacuole = tonoplast/turgor.
• Centrioles = spindle/9 triplets.
• Cytoskeleton = support/movement.
• Cilia/flagella = movement/9+2.
• Nucleus = control centre/DNA.
• Frequently Asked Questions (FAQs)
• Q1. Who discovered the cell and when? Answer: Robert Hooke discovered the cell in 1665 when he examined a
• thin slice of cork tissue under a compound microscope.
• generated by DeepSeek to PDF

• Q2. What is the basic unit of structure and function of all living organisms? Answer: The cell is the basic unit of
• structure and function of all living organisms.
• Q3. What is the composition of the cell wall in plants, fungi, and prokaryotes? Answer: In plants, the cell wall is
• made up of cellulose. In fungi, it is made up of chitin. In prokaryotes, it is made up of peptidoglycan. The cell wall is
• absent in animals and animal-like protists.
• Q4. What are the layers of the plant cell wall? Answer: The plant cell wall consists of three layers: the middle
• lamella (made of magnesium and calcium salts of pectin), the primary wall (cellulose fibres in a crisscross fashion),
• and the secondary wall (thick and rigid due to lignin).
• Q5. What is the fluid mosaic model? Answer: The fluid mosaic model postulates that the cell membrane consists
• of a double layer of phospholipids in which proteins are incorporated in a mosaic fashion. Protein molecules float
• like icebergs in a sea-like fluid of phospholipids.
• Q6. What is the function of cholesterol in the cell membrane? Answer: Cholesterol prevents stiffening of the cell
• membrane and is required for the fusion of secretory vesicles with the membrane.
• Q7. Differentiate between RER and SER. Answer: Rough Endoplasmic Reticulum (RER) is covered with ribosomes
• and is involved in the synthesis of some proteins. Smooth Endoplasmic Reticulum (SER) lacks ribosomes and
• synthesizes lipids including steroids, detoxifies harmful substances, and in muscle cells, has an important role in the
• contraction process.
• Q8. What are the functions of the Golgi apparatus? Answer: The Golgi apparatus stores and modifies materials
• into a finished form before packing them into vesicles. Some vesicles settle in the cytoplasm as organelles like
• lysosomes. Others fuse with the cell membrane to release packed material as cell secretion (e.g., enzymes,
• hormones, mucus).
• Q9. What is autophagy? Answer: Autophagy is the process in which lysosomes engulf and digest unwanted cell
• organelles.
• Q10. Why are mitochondria called the powerhouse of the cell? Answer: Mitochondria are called the powerhouse
• of the cell because they produce energy in the form of ATP through aerobic respiration.
• Q11. What are the three types of plastids? Answer: The three types of plastids are chloroplasts (photosynthetic,
• green), chromoplasts (coloured other than green, attract pollinators), and leucoplasts (non-pigmented, store food).
• Q12. What is the function of the large central vacuole in plant cells? Answer: The primary role of the central
• vacuole in a plant cell is to maintain turgor pressure within the plant cell. It also provides support and helps in
• growth.
• Q13. What is the structure of centrioles? Answer: Centrioles are hollow, open-ended cylinder-like structures,
• found in animal cells. Each centriole consists of nine triplets of microtubules.
• Q14. What are the three types of cytoskeletal fibres? Answer: The three types of cytoskeletal fibres are
• microtubules (made of tubulin), microfilaments (made of actin), and intermediate filaments (made of keratin and
• vimentin).
• Q15. What is the function of the nucleus? Answer: The nucleus acts as the control centre of the cell because it
• contains hereditary material DNA. It regulates cell activities like metabolism, growth, and reproduction.
• generated by DeepSeek to PDF

• Multiple Choice Questions (MCQs)
• Q1. Who discovered the cell? A. Robert Hooke B. Camillo Golgi C. Robert Brown D. Anton van Leeuwenhoek Correct
• Answer: A Explanation: Robert Hooke discovered the cell in 1665.
• Q2. The cell wall of fungi is made up of: A. Cellulose B. Chitin C. Peptidoglycan D. Lignin Correct Answer: B
• Explanation: The fungal cell wall is made up of chitin.
• Q3. The middle lamella is made up of: A. Cellulose B. Lignin C. Magnesium and calcium salts of pectin D. Chitin
• Correct Answer: C Explanation: The middle lamella is made up of magnesium and calcium salts of pectin.
• Q4. Which model explains the structure of the cell membrane? A. Lock and key model B. Fluid mosaic model C.
• Induced fit model D. Unit membrane model Correct Answer: B Explanation: The fluid mosaic model explains the
• structure of the cell membrane.
• Q5. Which organelle is the site of protein synthesis? A. Lysosome B. Ribosome C. Golgi apparatus D. Mitochondria
• Correct Answer: B Explanation: Ribosomes are the site of protein synthesis.
• Q6. Which organelle is known as the powerhouse of the cell? A. Chloroplast B. Mitochondria C. Ribosome D. Golgi
• apparatus Correct Answer: B Explanation: Mitochondria are called the powerhouse of the cell because they produce
• ATP.
• Q7. Which plastid is responsible for photosynthesis? A. Chromoplast B. Leucoplast C. Chloroplast D. Tonoplast
• Correct Answer: C Explanation: Chloroplasts synthesize food by photosynthesis.
• Q8. The membrane of the plant vacuole is called: A. Tonoplast B. Plasmalemma C. Cristae D. Thylakoid Correct
• Answer: A Explanation: The membrane of the plant vacuole is called the tonoplast.
• Q9. Centrioles are composed of: A. Nine triplets of microtubules B. Nine pairs of microtubules C. Eight triplets of
• microfilaments D. Tubulin and actin Correct Answer: A Explanation: Each centriole consists of nine triplets of
• microtubules.
• Q10. Which organelle contains its own DNA and ribosomes? A. Golgi apparatus B. Lysosome C. Mitochondria D.
• Ribosome Correct Answer: C Explanation: Mitochondria have their own DNA and ribosomes.
• Q11. The process by which lysosomes digest unwanted cell organelles is called: A. Autophagy B. Phagocytosis C.
• Exocytosis D. Osmosis Correct Answer: A Explanation: Autophagy is the process where lysosomes engulf and digest
• unwanted cell organelles.
• Q12. Cilia and flagella consist of: A. Nine pairs of microtubules surrounding a single central pair B. Nine triplets of
• microtubules C. Actin filaments D. Keratin filaments Correct Answer: A Explanation: Cilia and flagella consist of nine
• pairs of microtubules surrounding a single central pair.
• Q13. The control centre of the cell is the: A. Cytoplasm B. Nucleus C. Ribosome D. Golgi apparatus Correct Answer: B
• Explanation: The nucleus acts as the control centre of the cell because it contains DNA.
• Q14. Which of the following is NOT a function of the cell membrane? A. Acts as a barrier B. Regulates exchange of
• materials C. Synthesizes proteins D. Maintains a fixed environment inside the cell Correct Answer: C Explanation:
• Protein synthesis is the function of ribosomes, not the cell membrane.
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• Q15. Chromoplasts are found in: A. Roots and bulbs B. Flower petals and ripened fruits C. Stems and leaves D. Seeds
• and tubers Correct Answer: B Explanation: Chromoplasts are found in flower petals to attract insects and in ripened
• fruits to attract birds and animals.
• Subtopic 3.1 complete. Reply "next" to approve and I will produce Subtopic 2: 3.2 Structural Advantages of
• Animal and Plant Cell.