NCERT-CELL: THE UNIT OF LIFE-11th Class for NEET-8.1 WHAT IS A CELL? and 8.2 CELL THEORY

Cell Organelle Classroom Handouts

Sentence 1: When you look around, you see both living and non-living things. You must have wondered and asked yourself – ‘what is it that makes an organism living, or what is it that an inanimate thing does not have which a living thing has’ ?

This opening question ignites our curiosity about the essence of life. Unlike a rock or a chair, living organisms exhibit remarkable characteristics. They can grow, like a seedling sprouting into a towering tree; reproduce, ensuring the continuation of their species; and respond to their environment, such as a plant bending towards sunlight. These processes, absent in non-living things, are all underpinned by the intricate workings of cells.

Sentence 2: The answer to this is the presence of the basic unit of life – the cell in all living organisms. All organisms are composed of cells. Some are composed of a single cell and are called unicellular organisms while others, like us, composed of many cells, are called multicellular organisms.

Here, the key concept is revealed: cells are the fundamental building blocks of all living things. Imagine a house – bricks are the basic building blocks, and just like that, cells are the essential units that make up all living organisms. Some organisms, like the paramecium (a single-celled creature found in pond water), exist as a single, complex cell that carries out all vital functions. On the other hand, we humans are multicellular organisms, composed of trillions of cells working together in a harmonious symphony. Each cell type, like muscle cells for contraction or nerve cells for transmitting messages, has a specialized role to play in keeping us alive and functioning.

Sentence 3: Unicellular organisms are capable of (i) independent existence and (ii) performing the essential functions of life. Anything less than a complete structure of a cell does not ensure independent living. Hence, cell is the fundamental structural and functional unit of all living organisms.

This emphasizes the critical role of a complete cell for independent life. Unicellular organisms, like the baker’s yeast (Saccharomyces cerevisiae) used in bread making, demonstrate this beautifully. Within a single-celled yeast organism, all the essential functions of life occur – obtaining nutrients from its surroundings, carrying out respiration to generate energy, growing and reproducing to form new yeast cells. A virus, on the other hand, is not considered a living organism because it lacks the complete cellular machinery necessary for independent existence. It relies on the host cell to carry out vital functions, highlighting the importance of a fully functional cell for life.

Sentence 4: Anton Von Leeuwenhoek first saw and described a live cell. Robert Brown later discovered the nucleus.

This introduces us to the history of cell discovery. Pioneering scientist Anton van Leeuwenhoek, using a simple microscope he designed himself, was the first person to observe live microorganisms (single-celled organisms) in the 1670s. Imagine the awe he must have felt witnessing these tiny creatures moving and thriving in a drop of pond water! Later, in the 1800s, Robert Brown, a Scottish botanist, made another significant discovery. He identified the nucleus, a prominent structure within plant cells, which we now know houses the genetic material that governs an organism’s traits.

Sentence 5: The invention of the microscope and its improvement leading to the electron microscope revealed all the structural details of the cell.

This highlights the power of technological advancements in cell biology. The invention of the microscope was a game-changer, allowing scientists to peer into the previously unseen world of cells. Early microscopes were simple, but they opened a window to a universe of complexity within living organisms. As technology progressed, the development of the electron microscope in the 20th century revolutionized cell biology. With its much higher resolution, scientists could now observe the intricate details of cell structure, like the elaborate network of membranes and organelles within a cell.

Sentence 6: In 1838, Matthias Schleiden, a German botanist, examined a large number of plants and observed that all plants are composed of different kinds of cells which form the tissues of the plant.

This marks the beginning of the revolutionary cell theory. In 1838, botanist Matthias Schleiden embarked on a meticulous investigation of plant structures. Through his observations, he noted that all plant tissues, from the delicate petals of a flower to the sturdy trunk of a tree, were fundamentally composed of various types of cells [6]. This observation laid the groundwork for a unifying concept in biology.

Sentence 7: At about the same time, Theodore Schwann (1839), a British Zoologist, studied different types of animal cells and reported that cells had a thin outer layer which is today known as the ‘plasma membrane’. He also concluded, based on his studies on plant tissues, that the presence of a cell wall is a unique character of the plant cells.

This details the contributions of Theodor Schwann, a contemporary of Schleiden. Around the same time, zoologist Theodor Schwann turned his focus to animal cells. He observed that, similar to plants, animal tissues were also composed of cells. He also identified a thin outer layer surrounding these cells, which we now know as the plasma membrane, a critical structure that controls what enters and leaves the cell [7]. Interestingly, Schwann also noted a key difference between plant and animal cells – the presence of a rigid cell wall encasing plant cells, which is absent in animal cells.

Sentence 8: On the basis of this, Schwann proposed the hypothesis that the bodies of animals and plants are composed of cells and products of cells. Schleiden and Schwann together formulated the cell theory.

This explains the birth of cell theory. Based on their independent yet complementary observations, Schleiden and Schwann proposed a groundbreaking theory in 1838: the cell theory. This theory stated that all living organisms, from the towering redwood trees to the tiniest insects, are composed of cells. This fundamental principle unified the understanding of life across the vast diversity of the biological world.

Sentence 9: This theory however, did not explain as to how new cells were formed. Rudolf Virchow (1855) first explained that cells divided and new cells are formed from pre-existing cells (Omnis cellula-e cellula). He modified the hypothesis of Schleiden and Schwann to give the cell theory a final shape.

This highlights the refinement of cell theory. While the initial formulation of cell theory was a significant leap forward, it lacked a crucial piece of the puzzle – how new cells arise. In 1855, a German physician named Rudolf Virchow addressed this gap. Through his extensive research, he proposed the concept that new cells can only originate from pre-existing cells, a principle summarized in the famous Latin phrase “Omnis cellula e cellula” (meaning “all cells arise from cells”) [8]. This addition solidified the cell theory, providing a comprehensive explanation for the cellular basis of life and reproduction.

Sentence 10: Cell theory as understood today is: (i) all living organisms are composed of cells and products of cells. (ii) all cells arise from pre-existing cells.

This summarizes the core tenets of modern cell theory. The cell theory, as we understand it today, rests on two fundamental principles: All living organisms are composed of cells and products of cells: This states that every living thing, from a single-celled bacterium to a complex blue whale, is made up of cells or the products of cells (such as extracellular structures like bone in animals). All cells arise from pre-existing cells: This principle emphasizes that new cells don’t spontaneously appear but originate only from the division of existing cells. This process of cell division ensures the continuity of life and the transmission of genetic information from parent cells to daughter cells. By understanding these core principles, we gain a deeper appreciation for the cellular foundation of life and the remarkable processes that occur within these tiny building blocks of all living organisms.

NEET PLAY NCERT MCQ Quiz

MCQ Test

Assertion: Cells are considered the fundamental building blocks of all living organisms.

Reasoning: Living organisms, unlike non-living entities, exhibit characteristics such as growth, reproduction, and response to the environment, all of which are underpinned by the intricate workings of cells.

A) Both assertion and reasoning are true, and the reasoning is the correct explanation of the assertion.

B) Both assertion and reasoning are true, but the reasoning is not the correct explanation of the assertion.