Cell: The Building Block of Life | Class 9 Science Notes
By ConceptScroll Team · Published on 17 July 2026 · 4 min read
Cell: The Building Block of Life – this guide gives you a concise, exam-ready overview of Cell: The Building Block of Life from Class 9 Science, written by ConceptScroll editors and reviewed against the latest NCERT textbook.
2.3.1 Why do eukaryotic cells need these organelles?
Eukaryotic cells perform various life processes simultaneously in different organelles, each specialized for specific functions such as building materials, removing waste, and providing energy. The cell acts like a tiny living factory with each part performing a specific job. The nucleus, surrounded by a double-layered nuclear membrane with pores, contains chromosomes made of DNA and proteins. DNA carries genetic information in functional segments called genes. The nucleolus inside the nucleus synthesizes ribosomal subunits, which assemble into ribosomes in the cytoplasm. Ribosomes, either free or attached to the rough endoplasmic reticulum (RER), are the sites of protein synthesis. The endoplasmic reticulum (ER) is a network continuous with the nuclear envelope and comes in two types: RER with ribosomes for protein synthesis, and smooth ER (SER) without ribosomes involved in lipid and hormone synthesis. The Golgi apparatus, made of flattened sacs, modifies, sorts, and packages proteins and lipids for transport or secretion. Lysosomes are membrane-bound sacs containing enzymes that digest unwanted materials and damaged organelles, keeping the cell clean. Mitochondria, the powerhouse of the cell, have a double membrane with inner folds called cristae and perform cellular respiration to produce energy stored as ATP. Plastids in plant cells, such as chloroplasts containing chlorophyll, perform photosynthesis to synthesize food. Other plastids include chromoplasts with pigments for colors in flowers and fruits, and leucoplasts that store food materials like starch. Vacuoles are storage organelles; in mature plant cells, a large central vacuole stores water and maintains cell firmness. Animal cells may have small vacuoles for temporary storage. The chapter also notes that mitochondria and plastids have their own DNA and ribosomes, suggesting an evolutionary link with bacteria.
📊 Diagram: Fig. 2.12: From cell to DNA; Fig. 2.13: Endoplasmic reticulum and Golgi apparatus—pathway for protein processing and secretion; Fig. 2.14: Structure of a mitochondrion; Fig. 2.15: Structure of a chloroplast.
🔗 Connection: Leads to the section on cell growth and division.
Frequently asked questions
Two students, Renu and Rohit, were having a discussion on the plastids. Renu emphasised that all parts of the plants, even roots, contain plastids. However, Rohit did not agree with the statement and told her that plastids are absent in plant roots since the roots are underground and do not need to perform photosynthesis. Who is correct? Justify your answer.
Renu is correct. Plastids are present in all parts of the plant including roots. Although roots do not perform photosynthesis, they contain leucoplasts, a type of plastid involved in storage of starch, oils, and proteins. Hence, plastids are not absent in roots; they just differ in type and function compared to chloroplasts found in green parts.
Mitochondria and chloroplasts are two important organelles in a plant cell. Discuss how these two organelles are structurally and functionally similar to each other, and different from each other.
Similarities:
- Both have double membranes.
- Both contain their own DNA and ribosomes.
- Both are involved in energy transformations.
Differences:
- Mitochondria are the site of cellular respiration, producing ATP by breaking down glucose.
- Chloroplasts are the site of photosynthesis, converting light energy into chemical energy.
- Chloroplasts contain chlorophyll pigment; mitochondria do not.
- Mitochondria are found in almost all eukaryotic cells; chloroplasts are found only in plant cells a
Which of the following pairs of cell organelles contains DNA? (i) Chloroplasts, Ribosomes (ii) Mitochondria, Nucleus (iii) Golgi bodies, Ribosomes (iv) Nucleus, Lysosomes
(ii) Mitochondria, Nucleus Explanation: Both mitochondria and nucleus contain DNA. Chloroplasts also contain DNA but ribosomes do not. Golgi bodies and lysosomes do not contain DNA.
A researcher carried out an experiment in which she took two carrots of similar size. She placed one carrot in plain water and the other carrot in concentrated salt solution (Fig. 2.21). After 24 hours she recorded her observations. (i) What hypothesis does she want to test through this experiment? (ii) What would you suggest for the improvement of this experiment? (iii) Why does the carrot in plain water stay stiff and crunchy, but the carrot in concentrated salt solution become rubbery and limp?
(i) Hypothesis: Water moves from a region of higher water potential to a region of lower water potential through a semi-permeable membrane (osmosis).
(ii) Improvement: Use more replicates for each condition, measure the change in weight or firmness quantitatively, and control temperature.
(iii) Explanation: The carrot in plain water gains water by osmosis, making it stiff and crunchy due to turgidity. The carrot in salt solution loses water to the solution (which has lower water potential), be
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Clear NCERT-aligned notes on खाद्य संसाधनों में सुधार for Class 9 Science.