Discovery Of The Cell

All living organisms are made up of fundamental structural units. These units are analogous to the bricks used to build a house; just as bricks are assembled to create a building, these units are assembled to form the body of an organism. This basic structural unit of life is called a cell.

The cell was first observed by Robert Hooke in 1665. Using a simple magnifying device, he examined thin slices of cork, which is part of the outer bark of a tree. He noticed that the cork tissue was made up of many small, box-like compartments separated by walls. These compartments resembled the structure of a honeycomb. Hooke called each of these boxes a 'cell'. What he observed were actually the dead cells of the cork tissue.

Understanding of living cells progressed significantly much later, after the development of more advanced microscopes with higher magnification capabilities. Today, thanks to these improved tools, we have extensive knowledge about the detailed structure and functions of cells.

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The Cell

Cells are considered the basic structural units of living organisms, similar to how bricks are the basic units of a building. While buildings made from identical bricks can have diverse designs, shapes, and sizes, living organisms also exhibit great variety, but are fundamentally composed of cells.

Unlike non-living bricks, cells in living organisms are complex, intricate living structures. A common example is a hen's egg. It is large enough to be seen without a microscope and represents a single cell.

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Organisms Show Variety In Cell Number, Shape And Size

Scientists study living cells using microscopes, which magnify tiny structures. Stains or dyes are often used to colour different parts of the cell, making their detailed structures more visible under the microscope.

Living organisms display immense diversity in their shapes and sizes, and consequently, the cells within them, as well as the number of cells, also vary.

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Number Of Cells

Organisms can differ greatly in the number of cells they are composed of. Large organisms like tall trees or elephants have trillions of cells in their bodies. The human body, for instance, contains trillions of cells, varying in shapes and sizes, with different groups of cells performing specific functions.

Organisms made up of more than one cell are called multicellular organisms ('multi' means many, 'cellular' means cell). Even organisms with billions of cells start their life as a single cell, a fertilised egg. This single cell divides repeatedly, and the number of cells increases as the organism develops.

Some organisms, however, consist of only a single cell throughout their life. These are called unicellular organisms ('uni' means one). Examples include Amoeba and Paramecium.

Surprisingly, a single-celled organism can perform all the essential life functions necessary for survival that are carried out by multiple specialised cells, tissues, and organs in multicellular organisms. For example, a single-celled Amoeba can capture and digest food, respire, excrete waste, grow, and reproduce.

In multicellular organisms, specialised groups of cells form tissues, which perform specific functions. Tissues, in turn, combine to form organs, and organs work together in organ systems.

The number of cells in smaller multicellular organisms doesn't necessarily affect their functioning compared to larger ones; it's the organisation and specialisation of cells that matter.

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Shape Of Cells

Cells exhibit a variety of shapes, which are often related to their specific functions.

• Amoeba: A unicellular organism that has no fixed shape. It constantly changes its shape by extending temporary projections called pseudopodia ('pseudo' meaning false, 'podia' meaning feet). These pseudopodia are used for movement and capturing food.
• White Blood Cells (WBCs): Found in human blood, these are single cells that can also change their shape. However, unlike Amoeba which is a complete organism, a WBC is just one type of cell within a multicellular body.
• Other Cells: In multicellular organisms, cells have more defined shapes. They can be round, spherical, or elongated, like red blood cells. Muscle cells are often long and pointed at both ends, giving them a spindle shape. Nerve cells (neurons) are long and branched, which allows them to transmit messages over long distances, coordinating body functions.

The shape of a cell is primarily determined by its outer boundary. In both plant and animal cells, the cell membrane provides shape. Plant cells have an additional rigid outer layer called the cell wall, which gives them a fixed shape and provides structural support and protection. Bacterial cells also have a cell wall.

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Size Of Cells

The size of cells varies enormously. Some are microscopic, measured in micrometres (millionths of a metre), ranging from 0.1 to 0.5 micrometres in bacteria (the smallest known cells). Most cells require a microscope to be seen. However, some cells are large enough to be visible with the unaided eye, such as the egg of an ostrich, which is one of the largest single cells, measuring about 170 mm by 130 mm.

The size of an organism's body is related to the *number* of cells, not necessarily the *size* of its cells. For example, the nerve cells in an elephant and a rat perform the same function (transmitting messages) and have similar long, branched structures, even though an elephant is much larger than a rat. This indicates that the size of a cell is more related to its specific function than to the overall size of the organism.

A hen's egg, seen without magnification, is a good example of a large single cell. The yellow yolk part is the actual cell, surrounded by the white albumin which solidifies upon boiling.

Cell Structure And Function

Living organisms are organised into various levels. From simple to complex, these levels are cells, tissues, organs, and organ systems. Different organs in an organism perform specific functions; for instance, the digestive system consists of organs like the stomach and intestines, each with roles in digestion, absorption, and assimilation of food.

Similarly, in plants, roots absorb water and minerals, while leaves are responsible for producing food through photosynthesis. Each organ is composed of smaller parts called tissues. A tissue is a group of similar cells that work together to perform a specific function. Therefore, a cell is the basic structural unit that makes up tissues, which in turn form organs in a living organism.

Parts Of The Cell

Cells have several key components, or structures, each performing specific roles necessary for the cell's life and function. The fundamental parts of most cells are the cell membrane, cytoplasm, and nucleus.

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Cell Membrane

The outermost boundary of the cell (in animal cells) or the layer just inside the cell wall (in plant cells) is the cell membrane, also known as the plasma membrane. This membrane encloses the cytoplasm and nucleus. It separates the contents of one cell from another and also separates the cell from its external environment.

The plasma membrane is porous, meaning it has tiny openings, which allows it to control the movement of certain substances into and out of the cell, but prevents others from passing through. This selective permeability helps regulate the cell's internal environment.

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Cytoplasm

The cytoplasm is a jelly-like substance that fills the space between the cell membrane and the nucleus. It is where most of the cell's metabolic activities occur. Suspended within the cytoplasm are various smaller structures called organelles. These organelles are specialised compartments that perform specific functions necessary for the cell's survival, such as energy production (mitochondria), protein synthesis (ribosomes), and processing/packaging materials (Golgi bodies). You will learn more about specific organelles in higher classes.

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Nucleus

The nucleus is a prominent, usually spherical structure, often located near the center of the cell. It is a vital component that acts as the control center for all cellular activities. The nucleus is typically larger than other organelles and can be easily seen when stained and viewed under a microscope.

The nucleus is enclosed by a double-layered membrane called the nuclear membrane. Like the cell membrane, the nuclear membrane is also porous, regulating the passage of substances between the cytoplasm and the inside of the nucleus.

Inside the nucleus, there is a smaller, dense spherical body called the nucleolus. The nucleus also contains thread-like structures called chromosomes. Chromosomes are composed of DNA and proteins and carry the genes.

A gene is the basic unit of inheritance in living organisms. Genes are responsible for controlling and transferring hereditary characteristics from parents to their offspring. This is why offspring inherit traits (like eye colour or hair texture) from their parents. Chromosomes are visible only when the cell is preparing to divide.

The entire living content of a cell, including the cytoplasm and the nucleus, is collectively called protoplasm. Protoplasm is often referred to as the 'living substance' of the cell.

Based on the organisation of the nucleus, cells are classified into two types:

• Prokaryotic Cells: These cells have nuclear material that is not enclosed within a nuclear membrane. The genetic material is located in a region in the cytoplasm but without a defined nucleus. Organisms composed of prokaryotic cells are called prokaryotes ('pro' meaning primitive, 'karyon' meaning nucleus). Examples are bacteria and blue-green algae.
• Eukaryotic Cells: These cells have a well-organised nucleus that is surrounded by a nuclear membrane. The genetic material is contained within this defined nucleus. Organisms composed of eukaryotic cells are called eukaryotes ('eu' meaning true, 'karyon' meaning nucleus). All organisms other than bacteria and blue-green algae are eukaryotes.

Other structures found within the cytoplasm include vacuoles, which are fluid-filled sacs. Plant cells often have a single, large central vacuole, while animal cells may have several much smaller vacuoles.

Plastids are another type of organelle, primarily found in plant cells. They are small, coloured bodies scattered in the cytoplasm. Some plastids contain the green pigment chlorophyll and are called chloroplasts. Chloroplasts are responsible for capturing light energy for photosynthesis and give leaves their green colour. Chlorophyll is essential for the process of photosynthesis.

Comparison Of Plant And Animal Cells

Plant cells and animal cells share many similarities but also have key differences:

Sl. No.	Part	Plant Cell	Animal Cell
1.	Cell membrane	Present	Present
2.	Cell wall	Present (Outside cell membrane, rigid)	Absent
3.	Nucleus	Present (Usually towards the periphery due to large vacuole)	Present (Usually in the center)
4.	Nuclear membrane	Present	Present
5.	Cytoplasm	Present	Present
6.	Plastids	Present (Includes chloroplasts)	Absent
7.	Vacuole	Usually one large central vacuole	Usually several small vacuoles (or absent)

These differences, particularly the presence of a cell wall, large central vacuole, and chloroplasts in plant cells, are adapted to the specific needs of plants, such as structural support and photosynthesis.

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