NCERT Class 9 Science Exploration Chapter 02 Cell The Building Block of Life PDF Download

Chapter 2: Cell: The Building Block of Life

It is widely accepted amongst the scientific community that life originated in water. Some researchers believe that life may have originated in small water pools with changing environmental conditions rather than in the oceans. Hot springs are examples of such environments. In India, the hot springs of Puga Valley in Ladakh maintain very high temperatures (nearly at the boiling point of water) even in a cold climate. These environmental conditions seem to be similar to those on the early Earth, about 3.5 billion years ago. The organisms living in these hot springs are mostly heat-loving bacteria called thermophiles, which are unicellular.

Scientists from the Birbal Sahni Institute of Palaeosciences, Lucknow, studied these hot springs and found that calcium carbonate formed rapidly around them. These deposits may have protected early organic molecules from harmful radiation and extreme conditions, and they may have also helped in the formation of the first protective membrane - the barrier that defines a cell.

All living organisms are made up of cells. The cell represents the basic level at which life exists. Some organisms, such as bacteria or yeast consist of only one cell (unicellular), while others like plants, fish, birds or humans are made up of millions of cells (multicellular) that work together. A group of similar cells performing similar functions forms tissues. Different tissues are organised to form an organ and several organs work together to form organ systems. Such as, nasal pores, nasal cavity, trachea and lungs form respiratory system.

Even when cells are organised into tissues, organs, and organ systems the cell remains the fundamental unit of structure and function in all living organisms. This makes us wonder how such tiny cells perform so many different activities. What are the different components of a cell? How do cells in our body communicate with each other? Do cells live forever, or do they die? In this chapter, we will explore the answers to these questions as we enter the fascinating world of cells!

2.1 How to Study Cells?

What do we call the ability of the human eye to see two very close objects as separate and distinct? Imagine two tiny dots drawn on a piece of paper. As the dots are moved closer, there comes a point at which they can no longer be seen as separate. When viewed from about 25 cm (the near point of human eye), two points separated by about 0.1 mm can be seen as distinct; otherwise, they appear as a single point. This is called the limit of resolution of the human eye, which is 0.1 mm.

A cell is usually too small to be seen by the unaided eye (Fig. 2.1). This raises an important question - how have cell biologists studied the structure and function of cells that are much smaller than the limit of resolution of the human eye?

You have learnt about convex lens. A convex lens or a combination of lenses, i.e., an objective lens and an eyepiece are used for the magnification of an object (Fig. 2.1) to make it appear larger.

[Figure 2.1: Size of the objects and its visibility through unaided to aided eye, See in your textbook]

Robert Hooke was the first person to observe a cell in 1665 using a self-designed microscope (capable of about 200-300X magnification). While examining a thin slice of cork, he observed small box-like compartments and named them 'cells'. In school laboratories, light microscopes are used to observe objects using different objective lenses (e.g., 10X, 40X) to achieve better magnification and resolution under visible light. Explore different parts of a microscope (Fig. 2.2) in your school laboratory and use it to observe fine structures of various materials. Under the microscope, you will see a magnified image of an object. Can you estimate its actual size?

Teacher's Note

The limit of resolution is 0.1 mm, but a typical plant cell is about 0.1 mm, and animal cells are smaller (around 0.01 mm). This is why you cannot see individual cell details without a microscope - the cells are right at or below the eye's limit. A light microscope magnifies them enough to see the structures inside.

Activity 2.1: Let us estimate the size of a cell

1. Take a transparent ruler with millimetre (mm) markings.
2. Place the ruler on the stage of microscope, focus on it using the adjustment knob and observe the diameter of the circular field of view through the eyepiece and measure it in mm.
3. Convert the diameter from mm to micrometre (µm). Suppose the diameter of the visible field is 5 mm, meaning \( 5 \times 1000 = 5000 \) µm.
4. Remove the ruler and place an onion peel slide on the stage of the microscope.
5. Focus on the slide and count the number of cells present along the diameter of the field of view in one straight line.
6. Estimate the real size of the cell using the formula:

Unit conversion:
1 millimetre (mm) = 1000 micrometre (µm)

Estimated size of the onion peel cell =

Suppose, 25 cells are seen along the diameter. In that case, the size of one onion cell would be \( 5000 \text{ µm}/25 = 200 \text{ µm} \).

If the estimated size of an onion peel cell is 200 µm, how much does a light microscope magnify this cell? The total magnification of a microscope depends on its magnifying power of the eyepiece and the objective lens. If both the eyepiece and the objective lens have the magnifying power of 10X, then the total magnification will be 100X. This means that a cell with an estimated size of 200 µm will appear 100 times larger.

[Figure 2.2: Structure of a light microscope, See in your textbook]

Thus, a microscope allows us to see very small structures clearly and is an essential tool for studying the cell structure. Over the years, scientists have improved the microscope by improving its three main features - resolution (measure of clarity), contrast (the difference in brightness between various

Teacher's Note

When you perform Activity 2.1, remember that you are counting cells in a straight line across the field of view. If you count 25 cells and the diameter is 5000 µm, each cell is \( 5000 \div 25 = 200 \) µm. Do not confuse the magnification (how many times bigger it looks) with the actual size (the real measurement in µm).

Ready to Go Beyond

Apart from light microscopes, scientists also use powerful electron microscopes that reveal the fine details of a cell structure. These instruments use a beam of electrons instead of light to produce highly magnified images, allowing us to see cell structure at the nanometre scale with remarkable clarity (a nanometre is one-billionth of a metre).

[Figure 2.3: An electron microscope, See in your textbook]

Key Points

• The human eye can only see objects down to about 0.1 mm in size, so cells (which are much smaller) need a microscope to be studied.
• A light microscope magnifies cells using lenses and visible light, allowing us to see their general structure clearly.
• You can estimate the actual size of a cell by measuring how many cells fit across the visible field of view and dividing the field diameter by that number.
• Electron microscopes use a beam of electrons instead of light and can reveal even finer details of cell structure at the nanometre scale.

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