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Cells The Unit of Life NCERT Textbook With Solutions PDF Free Download
Chapter 8: Cells The Unit of Life
8.1 What Is A Cell?
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, the cell is the fundamental structural and functional unit of all living organisms.
Anton Von Leeuwenhoek first saw and described a live cell. Robert Brown later discovered the nucleus.
The invention of the microscope and its improvement leading to the electron microscope revealed all the structural details of the cell.
8.2 Cell Theory
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.
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
He also concluded, based on his studies on plant tissues, that the presence of cell walls is a unique character of the plant cells.
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 theory, however, did not explain how new cells were formed.
Rudolf Virchow (1855) first explained that cells are 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. 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.
8.3 An Overview of Cell
You have earlier observed cells in an onion peel and/or human cheek cells under the microscope.
Let us recollect their structure. The onion cell which is a typical plant cell has a distinct cell wall as its outer boundary and just within it is the cell membrane.
The cells of the human cheek have an outer membrane as the delimiting structure of the cell.
Inside each cell is a dense membrane-bound structure called the nucleus. This nucleus contains the chromosomes which in turn contain the genetic material, DNA.
Cells that have membrane-bound nuclei are called eukaryotic whereas cells that lack a membrane-bound nucleus are prokaryotic.
In both prokaryotic and eukaryotic cells, a semi-fluid matrix called cytoplasm occupies the volume of the cell.
The cytoplasm is the main arena of cellular activities in both plant and animal cells. Various
chemical reactions occur in it to keep the cell in the ‘living state’.
Besides the nucleus, the eukaryotic cells have other membrane-bound distinct structures called organelles like the endoplasmic reticulum (ER), the Golgi complex, lysosomes, mitochondria, microbodies, and vacuoles. The prokaryotic cells lack such membrane-bound organelles.
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NCERT Solutions Class 11 Biology Chapter 8 Cells The Unit of Life
1. Which of the following is correct?
(a) Cells of all living organisms have a nucleus.
(b) Both animal and plant cells have a well-defined cell wall.
(c) In prokaryotes, there are no membrane-bound organelles.
(d) Cells are formed de novo from abiotic materials
2. What is a mesosome in a prokaryotic cell? Mention the functions that it performs.
A mesosome is a unique membranous structure formed by the extensions of the plasma membrane into the cell. If it is found attached to the nucleoid, it is known as a septal mesosome and is known as a lateral mesosome if the mesosome is free from the nucleoid.
The functions of mesosomes are as follows
- They help in cell-wall formation
- They help in DNA replication and distribution to daughter cells.
- They also help in respiration, and secretion processes, to increase the surface area of the plasma membrane and enzymatic content.
3. What are the characteristics of prokaryotic cells?
- The nuclear membrane is absent and the genetic material is not enclosed by an envelope. The naked DNA is found to be coiled in the cytoplasm and is referred to as a genophore or nucleoid
- They lack membrane-bound organelles and a membrane-bound nucleus
- The cell lumen is filled with a fluid known as cytoplasm
- Prokaryotic DNA is situated in the nucleoid of the cell
- The prokaryotic cell wall acts as a layer of protection and helps in maintaining cell shape
- The ribosome is the only cytoplasmic organelle that is found in prokaryotic cells
4. Multicellular organisms have a division of labor. Explain.
Cells are organized to form tissues that make organs and organ systems. A cell is an autonomous structure that is capable of carrying out functions on its own.
Hence the division of labor is essential to carry out different functions for increased efficiency and higher survival.
5. The cell is the basic unit of life. Discuss in brief
Every plant and animal cell are made of organ systems and organ systems are made up of organs.
Organs are constructed by tissues, and tissues are made up of a cluster of cells. Since cells are autonomous structures capable of carrying out functions on their own, it is said that cell is the basic unit of life.
6. What are nuclear pores? State their function.
In several places, the nuclear membrane is surrounded by minute pores formed by the fusion of two membranes called nuclear pores. They are simple perforations on the nuclear envelope.
- Retains the shape of the nucleus
- Known to preserve the stability of the genetic material by safeguarding it from respiratory breakdown occurring in the cytoplasm
- Responsible for the movement of RNA and protein molecules in both directions between the nucleus and the cytoplasm.
NCERT Class 11 Biology Textbook Chapter 8 Cells The Unit of Life With Answer PDF Free Download