Cell — Structure and Functions
Welcome to HSLC Guru! In this lesson we present complete English-medium notes for Class 8 General Science Chapter 8 — Cell — Structure and Functions as per the ASSEB (Assam State School Education Board) syllabus. You will find a clear summary of the chapter, all textbook questions and answers, additional MCQs, fill-in-the-blanks, true/false statements, and a useful glossary table. This material is designed to help students understand the basic unit of life, learn the structure of plant and animal cells, and prepare confidently for class tests, half-yearly and final examinations.
Chapter Summary
The cell is the basic structural and functional unit of life. The cell was first discovered by Robert Hooke in 1665, while he was examining a thin slice of cork under his self-made microscope. He observed tiny box-like compartments and named them “cells” (from the Latin word cellula, meaning a small room). Later, Anton van Leeuwenhoek observed living cells under an improved microscope. The Cell Theory was proposed by two German scientists — Matthias Schleiden (1838) for plants and Theodor Schwann (1839) for animals — which states that all living organisms are made up of cells, and the cell is the basic unit of life. Later, Rudolf Virchow (1855) added that all cells arise from pre-existing cells.
Cells show a great variety of shape, size, and number. They may be spherical (like blood cells), rectangular (plant cells), spindle-shaped (muscle cells), branched (nerve cells), or irregular (Amoeba). The smallest cell is the bacterium Mycoplasma (about 0.1 micrometre), while the largest cell is the egg of an ostrich. Most cells are microscopic and require a microscope to be seen. Based on number of cells, organisms are classified as unicellular (single-celled — Amoeba, Paramoecium, bacteria, yeast) and multicellular (many-celled — plants, animals, humans). In multicellular organisms, similar cells group together to form tissues, tissues form organs, and organs work together as organ systems to perform life processes.
Every cell has three basic parts — cell membrane (plasma membrane), cytoplasm, and nucleus. The cell membrane is a thin, living, selectively permeable covering that controls movement of substances. In plant cells an additional cell wall made of cellulose lies outside the membrane. The cytoplasm is the jelly-like substance where various cell organelles are present — mitochondria (the powerhouse of the cell, releases energy), endoplasmic reticulum (ER) (transport channel), Golgi apparatus (packaging and secretion), ribosomes (protein synthesis), vacuoles (storage; large central vacuole in plants), and plastids (only in plant cells; chloroplasts contain chlorophyll for photosynthesis). The nucleus is the control centre of the cell. It is bounded by a nuclear membrane and contains chromosomes (made of DNA, carry hereditary information) and a dense body called the nucleolus.
Plant cells differ from animal cells in having a rigid cell wall, plastids (chloroplasts), and a large central vacuole; animal cells have only a cell membrane, no plastids, and small vacuoles, but they possess centrosomes. Based on the type of nucleus, cells are of two types — prokaryotic cells (without a true nucleus and membrane-bound organelles, e.g., bacteria, blue-green algae) and eukaryotic cells (with a true nucleus enclosed by a nuclear membrane and well-developed organelles, e.g., plant cells, animal cells, fungi, protists). Understanding cell structure is the foundation of biology, since all life processes — respiration, growth, reproduction, and response — finally take place at the cellular level.
Textbook Questions and Answers
A. Very Short Answer Type Questions (1 Mark)
Q1. Who discovered the cell?
Answer: The cell was discovered by Robert Hooke in the year 1665.
Q2. What is the basic structural and functional unit of life?
Answer: The cell is the basic structural and functional unit of life.
Q3. Name the smallest and the largest cell.
Answer: The smallest cell is Mycoplasma (a bacterium), and the largest cell is the egg of an ostrich.
Q4. Which organelle is called the “powerhouse of the cell”?
Answer: Mitochondrion is called the powerhouse of the cell because it produces energy in the form of ATP.
Q5. Give one example each of unicellular and multicellular organism.
Answer: Unicellular — Amoeba; Multicellular — Human being / Mango plant.
Q6. What is the jelly-like substance found inside a cell called?
Answer: The jelly-like substance found inside a cell is called cytoplasm.
Q7. Which organelle is found only in plant cells?
Answer: Plastids (especially chloroplasts) are found only in plant cells.
Q8. Name the green-coloured plastid that helps in photosynthesis.
Answer: Chloroplast is the green-coloured plastid that helps in photosynthesis.
Q9. Who proposed the cell theory?
Answer: The cell theory was proposed by Matthias Schleiden (1838) and Theodor Schwann (1839).
Q10. What is the control centre of the cell?
Answer: The nucleus is the control centre of the cell.
B. Short Answer Type Questions (2-3 Marks)
Q1. Why is the cell called the structural and functional unit of life?
Answer: The cell is called the structural unit of life because all living organisms — from a tiny bacterium to a huge whale — are made up of one or more cells. It is called the functional unit because all life activities such as respiration, digestion, growth, reproduction, and excretion finally take place inside the cell. Without cells, life cannot exist.
Q2. What is cell theory? State its main points.
Answer: The cell theory was given by Schleiden and Schwann (and later modified by Virchow). Its main points are:
- All living organisms are composed of one or more cells.
- The cell is the basic structural and functional unit of life.
- All cells arise from pre-existing cells (added by Virchow).
Q3. Differentiate between a unicellular and a multicellular organism.
| Unicellular Organism | Multicellular Organism |
|---|---|
| Body is made up of a single cell. | Body is made up of many cells. |
| The single cell performs all life functions. | Different cells perform different functions. |
| Examples: Amoeba, Paramoecium, bacteria. | Examples: Mango tree, frog, human being. |
Q4. What is the function of the cell membrane?
Answer: The cell membrane (also called plasma membrane) is the thin, living, outer covering of an animal cell. Its main functions are:
- It gives shape and protection to the cell.
- It is selectively permeable — it allows only certain substances to enter or leave the cell.
- It separates the contents of one cell from another and from the surroundings.
Q5. Write any three differences between a plant cell and an animal cell.
| Plant Cell | Animal Cell |
|---|---|
| Has a cell wall made of cellulose. | No cell wall; only cell membrane is present. |
| Plastids (chloroplasts) are present. | Plastids are absent. |
| Has a single large central vacuole. | Has many small vacuoles or none. |
Q6. What are organelles? Name any three.
Answer: The tiny living structures present inside the cytoplasm of a cell, each performing a specific function, are called cell organelles. Examples — mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus, plastids, vacuoles, and lysosomes.
C. Long Answer Type Questions (5-6 Marks)
Q1. Describe the structure of a typical cell with the functions of its main parts.
Answer: A typical cell has three main parts:
- Cell Membrane (Plasma Membrane): A thin, living, selectively permeable covering that protects the cell and controls the movement of substances in and out of it.
- Cytoplasm: A jelly-like fluid in which all the cell organelles such as mitochondria, ribosomes, ER, Golgi apparatus, vacuoles, and plastids are suspended. All metabolic activities occur here.
- Nucleus: The largest, dense, spherical body in the centre of the cell. It is bounded by the nuclear membrane and contains the nucleolus and chromosomes (carry hereditary information). The nucleus controls all the activities of the cell.
In plant cells, an extra layer called the cell wall made of cellulose lies outside the cell membrane and provides rigidity and shape.
Q2. Explain the functions of the following cell organelles: (a) Mitochondria (b) Endoplasmic Reticulum (c) Golgi Apparatus (d) Ribosomes (e) Plastids.
Answer:
- Mitochondria: Called the “powerhouse of the cell”; release energy in the form of ATP through cellular respiration.
- Endoplasmic Reticulum (ER): A network of tubes and sheets that helps in the transport of materials inside the cell. Rough ER bears ribosomes (protein synthesis); Smooth ER helps in fat synthesis.
- Golgi Apparatus: Packs, modifies, and sends materials (proteins and other substances) to different parts of the cell or out of it. It is the secretion centre of the cell.
- Ribosomes: Tiny granular structures present on the ER or free in cytoplasm; they synthesise proteins.
- Plastids: Found only in plant cells. Chloroplasts (green) help in photosynthesis; chromoplasts give colour to flowers and fruits; leucoplasts store food.
Q3. Differentiate between a prokaryotic cell and a eukaryotic cell.
| Prokaryotic Cell | Eukaryotic Cell |
|---|---|
| True nucleus is absent; nuclear material lies free in cytoplasm. | True nucleus is present, bounded by a nuclear membrane. |
| Membrane-bound organelles are absent. | Membrane-bound organelles like mitochondria, ER, Golgi are present. |
| Cell is generally small (1-10 μm). | Cell is comparatively large (10-100 μm). |
| Ribosomes are smaller (70S type). | Ribosomes are larger (80S type). |
| Examples: Bacteria, blue-green algae. | Examples: Plant cells, animal cells, fungi. |
Q4. Draw and describe a well-labelled diagram of an animal cell. (Description only.)
Answer: An animal cell, when seen under a microscope, appears more or less rounded or irregular in shape. Its main features are:
- The outermost covering is the cell membrane (no cell wall).
- Inside is the jelly-like cytoplasm containing organelles — mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus, lysosomes, and small vacuoles.
- A well-defined nucleus with nuclear membrane, chromosomes, and nucleolus is located at or near the centre.
- A centrosome with two centrioles is present near the nucleus, which helps in cell division.
- Plastids are absent and vacuoles, if present, are small and many.
Q5. What are tissues, organs, and organ systems? Give one example of each.
Answer:
- Tissue: A group of similar cells working together to perform a particular function. Example — muscular tissue helps in movement; xylem tissue conducts water in plants.
- Organ: A structure made up of two or more types of tissues that performs a definite function. Example — heart, kidney, leaf, root.
- Organ System: A group of organs working together to perform a major life activity. Example — digestive system (mouth, stomach, intestine, liver, etc.) which digests food.
Additional Multiple Choice Questions (MCQ)
Q1. Who discovered the cell?
(a) Schleiden (b) Schwann (c) Robert Hooke (d) Virchow
Q2. The cell was discovered in the year:
(a) 1655 (b) 1665 (c) 1675 (d) 1865
Q3. Which one of the following is a unicellular organism?
(a) Mango (b) Frog (c) Amoeba (d) Hibiscus
Q4. The control centre of a cell is:
(a) Cytoplasm (b) Mitochondria (c) Nucleus (d) Ribosomes
Q5. Powerhouse of the cell is:
(a) Ribosome (b) Mitochondrion (c) Golgi apparatus (d) Nucleolus
Q6. Which organelle is present only in plant cells?
(a) Mitochondria (b) Ribosomes (c) Plastids (d) Nucleus
Q7. Cell wall in plant cells is made up of:
(a) Protein (b) Fat (c) Cellulose (d) Starch
Q8. Bacteria are examples of:
(a) Prokaryotic cells (b) Eukaryotic cells (c) Plant cells (d) Animal cells
Q9. The largest cell is:
(a) Nerve cell (b) Mycoplasma (c) Egg of ostrich (d) Red blood cell
Q10. Chromosomes are made up of:
(a) Protein only (b) Fat (c) DNA and protein (d) Cellulose
Q11. Which scientist first observed living cells?
(a) Robert Hooke (b) Anton van Leeuwenhoek (c) Schwann (d) Virchow
Q12. The statement “All cells arise from pre-existing cells” was given by:
(a) Schleiden (b) Schwann (c) Virchow (d) Hooke
Q13. The longest cell in the human body is:
(a) Red blood cell (b) Muscle cell (c) Nerve cell (d) Skin cell
Q14. Which of the following is NOT a cell organelle?
(a) Mitochondria (b) Ribosomes (c) Chromosome (d) Golgi apparatus
Q15. A group of similar cells performing a particular function is called:
(a) Organ (b) Organ system (c) Tissue (d) Organism
Q16. Which scientist proposed the cell theory for plants?
(a) Schwann (b) Schleiden (c) Virchow (d) Hooke
Q17. The selectively permeable layer of a cell is the:
(a) Cell wall (b) Cell membrane (c) Nuclear membrane (d) Cytoplasm
Q18. Which one of the following carries hereditary characters?
(a) Cytoplasm (b) Cell membrane (c) Chromosomes (d) Mitochondria
Fill in the Blanks
- The cell was discovered by Robert Hooke in 1665.
- The basic structural and functional unit of life is the cell.
- Mitochondrion is known as the powerhouse of the cell.
- The green-coloured plastid that helps in photosynthesis is called chloroplast.
- Bacteria are examples of prokaryotic cells.
- The cell wall in plant cells is mainly made up of cellulose.
- The jelly-like substance present inside a cell is called cytoplasm.
- The thread-like structures inside the nucleus that carry hereditary information are called chromosomes.
True or False
- Plant cells have a cell wall made of cellulose. — True
- Animal cells contain plastids. — False
- The nucleus controls all the activities of the cell. — True
- Amoeba is a multicellular organism. — False
- Ribosomes help in the synthesis of proteins. — True
- Mitochondria are absent in eukaryotic cells. — False
- Bacteria are prokaryotic organisms. — True
Glossary
| Term | Meaning |
|---|---|
| Cell | The basic structural and functional unit of all living organisms. |
| Cell Membrane | Thin, selectively permeable outer covering of a cell. |
| Cell Wall | Rigid outer layer of cellulose found in plant cells. |
| Cytoplasm | Jelly-like fluid inside the cell where organelles are suspended. |
| Nucleus | Spherical, dense body that controls all cell activities. |
| Nucleolus | Small dense body inside the nucleus. |
| Chromosomes | Thread-like structures inside the nucleus carrying genes (DNA). |
| Mitochondria | Organelle that produces energy; powerhouse of the cell. |
| Ribosomes | Tiny organelles that synthesise proteins. |
| Endoplasmic Reticulum | Network of tubes for transporting materials inside the cell. |
| Golgi Apparatus | Organelle that packages and secretes substances. |
| Vacuole | Membrane-bound sac for storage; large in plant cells. |
| Plastids | Plant-cell organelles; chloroplasts perform photosynthesis. |
| Chloroplast | Green plastid containing chlorophyll for photosynthesis. |
| Unicellular | Organism made up of a single cell, e.g., Amoeba. |
| Multicellular | Organism made up of many cells, e.g., humans. |
| Tissue | Group of similar cells performing a particular function. |
| Organ | Structure of different tissues performing a definite function. |
| Organ System | Group of organs working together for a major life activity. |
| Prokaryotic Cell | Cell without a true nucleus, e.g., bacteria. |
| Eukaryotic Cell | Cell with a true nucleus, e.g., plant and animal cells. |
| Cell Theory | Theory by Schleiden and Schwann stating life is made of cells. |
| Selectively Permeable | Property of allowing only certain substances to pass through. |
| Centrosome | Organelle in animal cells that helps in cell division. |
Match the Following
| Column A | Column B |
|---|---|
| 1. Mitochondria | (a) Photosynthesis |
| 2. Chloroplast | (b) Powerhouse of the cell |
| 3. Ribosomes | (c) Hereditary material |
| 4. Chromosomes | (d) Protein synthesis |
| 5. Cell wall | (e) Made of cellulose |
Answers: 1 — (b), 2 — (a), 3 — (d), 4 — (c), 5 — (e).
Quick Revision Points
- Cell — discovered by Robert Hooke (1665) using a self-made microscope on a slice of cork.
- Living cells were first observed by Anton van Leeuwenhoek.
- Cell theory — proposed by Schleiden (1838) and Schwann (1839); modified by Virchow (1855).
- Smallest cell — Mycoplasma (~0.1 μm); largest cell — egg of an ostrich.
- Longest cell — nerve cell.
- Three basic parts of every cell — cell membrane, cytoplasm, nucleus.
- Cell wall — found in plant cells, made of cellulose.
- Mitochondria — powerhouse of the cell; ribosomes — protein factory.
- Plastids — only in plant cells; chloroplast performs photosynthesis.
- Two types of cells based on nucleus — prokaryotic and eukaryotic.
- Cells → Tissues → Organs → Organ Systems → Organism (organisational hierarchy).
- Selectively permeable — cell membrane allows only certain substances to pass.
- Centrosome is found only in animal cells; large central vacuole is found in plant cells.
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