Cell The Unit of Life Notes for NEET Exam, Check Short Notes Here
NEET Short Notes
Check here the short notes on the Cell The Unit of Life which is an important chapter for NEET 2020. Cell The Unit of Life is an important section in Unit Structure and Function as every year 2-3 questions are asked from this chapter. This chapter is completely theoretical and needs a lot of revision. In this article, we have covered short notes of brief points for the revision purpose. Let's begin with a Cell introduction.
Cell The Unit of Life Notes
The fundamental functional and structural unit of life is known as a Cell.
- Anton von Leeuwenhoek was the one who first saw cell and described it in a living cell.
- In 1838, German botanist named Matthias Schleiden observed many different types of plants which contained different types of cells and form tissues.
- Animal cells were studied by Schwann in 1839. He was a British Zoologist and according to studies he reported that cells have an outer layer which is known as Plasma membrane. He later concluded that the presence of the cell wall is uniquely present in plant cells.
- Later, both scientists worked together and formulated the Cell Theory. They both did not explain the origin of the new cell.
- Rudolf Virchow explained the origin of the new cell. He proposed that the new cells exist from pre-existing cells and he modified the Schwann and Schleiden hypothesis to give the theory a final shape.
Today, the Cell theory says:
(i) All living organisms are composed of cells and products of cells.
(ii) All cells arise from pre-existing cells.
There are different types of the plant as well as animal cells. Inside each cell is there is a dense membrane-bound structure called Nucleus.
- The Nucleus contains genetic material in the form of chromosomes.
- Cells without a well-defined nucleus are known as Prokaryotic cells whereas the ones with a membrane bound nucleus are the eukaryotic cells.
- There is a fluid which makes the volume of the cell and is known as cytoplasm and it is present in both animal and plant cells.
- There are various types of membrane-bound structures which are known as Cell Organelles. Some of the cell organelles are Endoplasmic Reticulum(ER), Golgi complex, Lysosomes, Mitochondria etc.
- Cells differ in their shape, size and activity.
- Mycoplasmas are the smallest cells known. They are 0.3 micrometres in length and the largest cell known is the single isolated cell of the egg of an ostrich.
- Nerve cell i.e. neurons are the longest cells.
- Prokaryotic Cells:
- These are the cells which lack a nucleus and are mostly represented by blue-green algae, mycoplasma, bacteria and PPLO (Pleuro Pneumonia like organisms).
- These cells multiply very fast and range from 0.2 microns to 10 microns.
- These are single-celled and are found everywhere i.e. in rocks, soil, water, ocean, lakes etc.
- Based on the shape there are four types of bacteria-
- Bacillus (Rod-like)
- Coccus (Spherical)
- Spirillum (Spiral)
- Vibrio (Comma- shaped)
- There is a covering on prokaryotic cells which protects them from injuries and shocks. The covering is known as the cell envelope. It is made up of different types of layers-
- Glycocalyx: The layer could be thick or could be loose. If it is thick it is called capsule and if it is loose it is called slime layer. Glycocalyx is made up of macromolecules.
- Cell wall: It lies below glycocalyx and is made up of peptidoglycan. Cell wall provides shape and structural support to the cell and protects bacteria from collapsing or bursting. There are two types of bacteria-
- Gram-Positive Bacteria ( they can take up gram stain)
- Gram-Negative Bacteria ( they cannot take up gram stain)
- Plasma Membrane: This is the innermost layer and it is semi-permeable. It prevents leakage from the cell and allows the exchange of material inside and outside of the cell.
- Cytoplasm: It is a semi-fluid structure wherein all the parts of the cell float. It makes up the volume of the cell. There are different types of cell organelles found in the cytoplasm. Some of them are:
- Mesosomes: When plasma membrane extends into the cell in the form of tubules, it is called mesosome. They help the cell in performing various functions- cell wall formation, cellular respiration and secretion and increase the surface area of the plasma membrane.
- Bacterial cells can be motile as well as non-motile. If they are motile their filamentous flagella are thin.
- Bacterium flagella are composed of three parts- basal body, hook and filament.
- There are structures which are non-motile and are known as pili and fimbriae.
- Pili are made up of special protein and are elongated tubular structures.
- Fimbriae are bristle-like structures which are spike out of the cell. They attach to other bacteria and host tissues as well.
- Chromatophores: These are the membrane extensions with photosynthetic pigments. These are seen in cyanobacteria.
- Ribosomes: Small structures which are associated with plasma membrane are called ribosomes. Prokaryotes have 70s ribosomes which are made up of 30 s smaller subunits and 50 s larger subunits. They generally occur in groups which are known as polysomes.
- Inclusion Bodies: These are membrane fewer bodies and exist in cytoplasm freely. They function as the reserve for storing materials. Some of the examples of inclusion bodies are phosphate granules, glycogen granules. Gas vacuoles are present in green and blue-green and purple photosynthetic bacteria.
- Eukaryotic Cell:
- These cells include all protists, animals, fungi and plants.
- These cells contain an organized nucleus and another membrane-bound organelle.
- Their genetic material is well organized into chromosomes
- . Some of the important cell organelles are Endoplasmic Reticulum, Mitochondria, Ribosomes, Golgi apparatus etc.
- Eukaryotic cells have a variety of locomotory and cytoskeletal structures.
- All these cells are non-identical and possess cell wall, plastids and large central vacuole which is usually absent in all other plant cells.
- Plant cell and Animal cells are different from each other in many ways. The differences between them are tabulated below.
Cell: Shape and Size
They are usually rectangular in shape and their shape is fixed.
They are circular in shape and irregular sometimes.
Present but very rare
Present but lies on the one side of the cell
Present and lies in the centre of the cell
Usually present with chloroplast
Large and few centre vacuoles
Small and numerous centre vacuoles
Present but fewer
Present and numerous
Prokaryotic and Eukaryotic cell are different from each other in many ways. The differences between prokaryotic and eukaryotic cells are tabulated below.
BASIS OF COMPARISON
Kind of cell
Presence of Nucleus
The well-defined nucleus is absent, nucleoid is present
Shape of DNA
Mode of Reproduction
Lysosomes and Peroxisomes
Present in plants
Transcription and Translation
Number of chromosomes
More than 1
Plants and Animals
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