Class 8 Science Chapter 8 Notes Cell Structure And Function Notes
In this post, we will discuss Class 8 Science Chapter 8 notes pdf Cell Structure And Function Notes. Class 8th science ch 8 notes help students in revision. Cell Structure And Function notes class 8, we will learn about Cell, Parts of the Cell and Comparison of Plant and Animal Cells. You can also download the Cell Structure And Function notes class 8 pdf for further study. NCERT notes for the Class 8 Science Chapter 8 are designed by our subject expert team.
In this Class 8th Science, Ch 8 Notes pdf, we will learn about the Cell Structure And Function
Class 8 Science Chapter 8 notes Overview
you have already learnt that things around us are either living or non-living. All living organisms carry out certain basic functions. In this chapter, you shall learn about the basic structural unit of an organ, which is the cell.
Cells are the fundamental structural unit of the living organism and the basic unit of life. Cells may be compared to bricks. Bricks are assembled to make a building. Similarly, cells are assembled to make the body of every organism.
- As bricks in a building and cells in living organisms are basic structural units.
- Cells are of different shapes and sizes.
Discovery of the Cell
Robert Hooke in 1665 observed slices of cork under a simple magnifying device. Cork is a part of the bark of a tree. He took thin slices of cork and observed them under a microscope. He noticed partitioned boxes or compartments in the cork slice. He also noticed that one box was separated from the other by a wall or partition. Hooke coined the term ‘cell’ for each box.
Cells of living organisms could be observed only after the discovery of improved microscopes.
Type of Organism on the Basis of Number of Cells
The human body has trillions of cells which vary in shape and size.
- multicellular (multi: many; cellular: cell):- Organisms made of more than one cell are called multicellular. For example, human, cow and rose.
- multicellular organisms are carried out by groups of specialised cells forming different tissues. Tissues, in turn, form organs.
- unicellular (uni: one; cellular: cell):- Organisms are made up of a single cell. The single-celled organisms are called unicellular. for example amoeba and paramecium.
- All the basic functions like digestion, respiration and excretion etc are performed by a single cell.
Shape of Cells
The change in shape is due to the formation of pseudopodia which facilitates movement and helps in capturing food. The membrane provides shape to the cells of plants and animals. A cell wall is an additional covering over the cell membrane in plant cells. It gives shape and rigidity to these cells.
Pseudopodia (pseudo: false; podia: feet):- Organisms which have false feet mean an organism which keeps on changing its shape is called pseudopodia. A white blood cell (WBC) in human blood is another example of a single cell which can change its shape.
Fig. (a, b, c) shows different cells such as blood, muscle and nerve of human beings. The different shapes are related to their specific functions.
Generally, cells are round, spherical or elongated [Fig. (a)]. Some cells are long and pointed at both ends. They exhibit a spindle shape [Fig.(b)]. Cells sometimes are quite long. Some are branched like the nerve cell or a neuron [Fig.(c)]. The nerve cell receives and transfers messages, thereby helping to control and coordinate the working of different parts of the body.
Size of Cells
The size of cells in living organisms may be small as a micron or may be as large as a few centimetres. However, most of the cells are microscopic in size and are not visible to the unaided eye. They can see by a microscope.
- The smallest cell is 0.1 to 0.5 micrometre in bacteria.
- The largest cell measuring 170 mm ×130 mm, is the egg of an ostrich.
- The size of the cell is related to its function. For example, nerve cells, both in the elephant and rat, are long and branched. They perform the same function, that of transferring messages.
Cell Structure and Function
- In a multicellular organism, each organ is made up of serval organs.
- Each organ is further made up of smaller parts called tissues.
- A tissue is a group of similar cells performing a specific function.
- An organ is made up of tissues which in turn, are made up of cells. The cell is the basic structural unit in a living organism.
Parts of the Cell
The basic components of a cell are the cell membrane, cytoplasm and nucleus.
- Cell Membrane
- The cell membrane gives shape to the cell.
- The cytoplasm and nucleus are enclosed within the cell membrane, also called the plasma membrane.
- The membrane separates cells from one another and also the cell from the surrounding medium. The plasma membrane is porous and allows the movement of substances or materials both inward and outward.
The boundary of the onion cell is the cell membrane covered by another thick covering called the cell wall.
- It is the jelly-like substance present between the cell membrane and the nucleus.
- cells are present in the cytoplasm.
- The nucleus is an important component of the living cell. It is generally spherical and located in the centre of the cell.
- It can be seen easily with the help of a microscope.
- The nucleus is separated from the cytoplasm by a membrane called the nuclear membrane. This membrane is also porous and allows the movement of materials between the cytoplasm and the inside of the nucleus.
- The nucleus contains thread-like structures called chromosomes. These carry genes and help in the inheritance or transfer of characters from the parents to the offspring.
Type of Organism on the Basis of Nucleus Membrane
Prokaryotes:- The cells having nuclear material without a nuclear membrane are termed prokaryotic cells. The organisms with these kinds of cells are called prokaryotes. Examples are bacteria and blue-green algae.
Eukaryotes:- The cells, like onion cells and cheek cells having a well-organised nucleus with a nuclear membrane, are designated as eukaryotic cells. All organisms other than bacteria and blue-green algae are called eukaryotes.
- Vacuoles are fluid-filled membrane-bound structures in the cell.
- In-plant cells, a single large vacuole is present.
- In animal cells, numerous small vacuoles are present.
- They are present only in plant cells.
- They are of different colours.
- Some of them contain a green pigment called chlorophyll. Green-coloured plastids are called chloroplasts.
- Chlorophyll in the chloroplasts of leaves is essential for photosynthesis.
Comparison of Plant and Animal Cells
The similarities and distinguishing features of plant and animal cells are given below.
What We Learn Class 8 Science Chapter 8 Notes Cell Structure And Function Notes
In Class 8th Science Ch 8 Notes pdf / Cell Structure And Function Notes
- All organisms are made of smaller parts called organs.
- Organs are made of still smaller parts. The smallest living part of an organism is a ‘cell’.
- Cells were first observed in cork by Robert Hooke in 1665.
- Cells exhibit a variety of shapes and sizes.
- The number of cells also varies from organism to organism.
- Some cells are big enough to be seen with the unaided eye. Hen’s egg is an example.
- Some organisms are single-celled, while others contain a large number of cells.
- The single cell of unicellular organisms performs all the basic functions performed by a variety of cells in multicellular organisms.
- The cell has three main parts: (i) the cell membrane, (ii) cytoplasm which contains smaller components called organelles, and (iii) the nucleus.
- The nucleus is separated from the cytoplasm by a nuclear membrane.
- Cells without a well-organised nucleus, i.e. lacking a nuclear membrane, are called prokaryotic cells.
- Plant cells differ from animal cells in having an additional layer around the cell membrane termed the cell wall.
- Coloured bodies called plastids are found in plant cells only. Green plastids containing chlorophyll are called chloroplasts.
- A plant cell has a big central vacuole unlike a number of small vacuoles in animal cells.
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