Definition:
Cell is the smallest unit of life capable of performing all living functions.
Properties of cell
- It is the smallest living unit of life.
- It is so small that it is not visible to the naked eye.
- The shape varies in different organisms and within an organism.
- Size also differs.
- All living cells exhibit certain basic properties like respiration, growth,
- metabolism etc.
- Originate from a pre-existing cell. A mother cell divides to produce daughter cells.
Invention of Microscope
- The first, simple microscope was constructed by Antony van Leeuwenhoek.
- He used biconvex lens to make the microscope.
- The invention of compound microscope was done by Robert Hooke.
- These compound microscopes consisted of concave lens.
- The modern compound microscopes are a modification of one made by Robert Hooke. Their magnification power has been improved up to 2,000 times.
- Electron microscopes are the one that use electronic beams to magnify the objects. Their magnification power is over 2,00,000 times.
Cell theory
- All plants and animals are composed of cells.
- It is the basic unit of life.
- This was further expanded by Virchow by suggesting that all cells arise from pre-existing cells.
Classification of Cell on the basis of their cellular complexity
Prokaryotic Cells
- The nuclear region is poorly defined;
- membrane-bound organelles are absent.
- The undefined nuclear region containing only nucleic acid called nucleoid.
- Prokaryotes are unicellular organisms.
Eukaryotic Cells
- Eukaryotes are usually multicellular organisms.
- Yeast is exceptionally a unicellular eukaryote.
- Nuclear region is well defined, bounded by nuclear membrane.
- Other membrane-bound organelles are also present.
- Also read Structure of Chromosomes
Components of the cell
Cell membrane
- It is the protective layer that surrounds the cell.
- Cell membrane selectively allows the entry of only some substances.
- prevents the movement of other materials. Hence, it checks the transport of substances in and out of the cell.
Cell wall
- In plants, an extra protective covering of a polysaccharide, cellulose is present.
- It is called cell wall that protects plant cells from environmental variations.
Cytoplasm
- It is a jelly-like substance present between cell membrane and nucleus.
- It contains various cell organelles such as mitochondria, Golgi bodies, lysosomes etc.
Nucleus
- It is a dense spherical body located at the centre of the cell.
- It is surrounded by porous nuclear membrane.
- It contains spherical body called nucleolus.
- It also contains thread-like structures called chromosomes.
- Chromosomes are the structures that carry genes and play an important role in inheritance.
- Genes are the structural and functional unit of inheritance.
- The entire living substance in a cell is known as protoplast.
Vacuoles
- 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.
- Also Read Genetics
Plastids
- They are present only in plant cells.
- Plastids that contain green colour pigment chlorophyll are known as chloroplasts. It is the chlorophyll that gives green colour to the leaves.
Vacuoles
- 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.
- The membrane of the vacuole is called tonoplast. This membrane encloses a
fluid called cell sap.
Plastids
- They are present in plant cells.
- Chloroplast is a plastid containing green pigment called chlorophyll that is required in photosynthesis.
- Plastids are of two types – leucoplasts and chromoplasts
- Leucoplasts are colourless and are used to store food while chromoplasts are plastids containing pigments. Chloroplasts are a type of chromoplasts.
- Chloroplasts consist of two regions – grana (stacks of sac like membrane bound structures that contain pigment chlorophyll) and stroma (ground substance containing enzymes and starch grains)
Endoplasmic Reticulum (ER)
They are of two types:
- Rough Endoplasmic Reticulum (RER) is important for synthesis and packaging of proteins.
- Smooth Endoplasmic Reticulum (SER) acts as storage organelle. It also helps in lipid (fat) synthesis.
Golgi Apparatus
- It is made up of parallel arranged membrane-bound vesicles called cisternae.
- It helps in storage, modification, and packaging of products in vesicles.
- It helps in formation of glycoproteins and glycolipids.
Lysosomes
- It is a membrane-bound structure that holds variety of enzymes.
- Rich in all types of hydrolytic enzymes, which are active at acidic pH.
- It is involved in the digestion of carbohydrates, proteins, lipids, and nucleic acids.
Mitochondria
- It is a double membrane-bound structure.
- The inner membrane of mitochondria is deeply folded to form cristae.
- Cristae increase the surface area in the organelle.
- It is the site of cellular respiration and hence known as ‘power house of cell’.
- They have their own circular DNA.
- They divide by fission.
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