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.
2.1 How to Study Cells?
The human eye has a limit of resolution, which is the ability to distinguish two close points as separate. At about 25 cm (the near point of the human eye), two points separated by 0.1 mm can be seen distinctly; closer than that, they appear as one. Since cells are much smaller than this limit, they cannot be seen by the naked eye. To study cells, scientists use microscopes that magnify objects. A convex lens or a combination of lenses (objective and eyepiece) is used to magnify objects. Robert Hooke was the first to observe cells in 1665 using a self-designed microscope with about 200-300X magnification. He observed small box-like compartments in thin cork slices and named them 'cells' because they resembled small rooms. In school laboratories, light microscopes with objective lenses of different powers (e.g., 10X, 40X) are used to observe cells under visible light. Electron microscopes, which use beams of electrons instead of light, provide much higher magnification and resolution, allowing observation of cell structures at the nanometre scale. The chapter includes an activity to estimate the size of a cell using a microscope and a transparent ruler. By measuring the diameter of the field of view and counting the number of cells along it, the actual size of a cell can be calculated. Improvements in microscopes over time have enhanced resolution, contrast, and magnification, making them powerful tools for studying cells. An electron micrograph of the lower surface of a Colocasia leaf showing stomata is also presented as an example of advanced microscopy.
📊 Diagram: Fig. 2.1: Size of the objects and its visibility through unaided to aided eye; Fig. 2.2: Structure of a light microscope; Fig. 2.3: An electron microscope; Fig. 2.4: Electron micrograph of lower surface of a Colocasia leaf showing stomata.
🧪 Activity: Activity 2.1: Estimating the size of a cell using a microscope and a transparent ruler by measuring the diameter of the field of view and counting the number of cells along it.
🔗 Connection: Leads to the section on the structure of a cell, introducing the cell membrane and other components.
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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- खाद्य संसाधनों में सुधार | Class 9 Science Notes
Clear NCERT-aligned notes on खाद्य संसाधनों में सुधार for Class 9 Science.