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CELL – THE BASIC UNIT OF LIFE




Protoplasm:

  • All cells contain living substance called protoplasm. It is jelly-like, viscous, colorless semi-fluid substance in which various cell organelles and inclusion remains in colloidal form.
  • Protoplasm can be distinguished in two forms:
    1. Cytoplasm: it is that part of protoplasm which surrounds the nucleus.
    2. Nucleoplasm: it is that part of protoplasm which is located inside the nucleus.                                            Protoplasm = cytoplasm + nucleoplasm

Cytoplasm:

The part of the cell which occurs between the plasma membrane and nucleus envelop is called the cytoplasm.
  • Cytoplasm consists of an aqueous ground substance, the cytosol, containing a variety of cell organelles and other inclusions such as insoluble waste and storage product (starch, lipid, etc.).
  • Chemically cytoplasm contains about 90% water,, 7% proteins, 2% carbohydrates and lipids and 1% inorganic minerals, minerals vitamins, etc.

Cell organelles:

  • A cell has to perform different functions with the help of its various membrane- bound cell organelles.
  • Cell organelles are “small organs” of the cell and are found embedded in the cytosol. They form living part of the cell and each of them has a definite shape, structure and function.

Endoplasmic reticulum (ER):

  • The endoplasmic reticulum is a complex network of membranous system in the cytoplasm of eukaryotic cells.
  • It is absent in prokaryotic cells and matured RBCs of mammals.
  • ER occurs in three forms:

      Cisternae (i.e., closed, fluid-filled sacs), vesicles and tubules.

  • ER is of two types:
    1. Rough endoplasmic reticulum (RER) with ribosomes attached on its surface for synthesizing proteins.
    2. Smooth endoplasmic reticulum (SER) which is without ribosomes and is meant for secreting lipids.
Functions:
  1. ER provides mechanical support to the cell.
  2. It forms intracellular transporting system.
  3. It forms new nuclear membrane after every cell division.
  4. SER takes part in the synthesis of lipids.
  5. RER is concerned with the transport of proteins which are synthesized by ribosomes on their surface.

Golgi complex (Golgi apparatus or Golgi body):

  • Golgi complex was discovered by Caomillo Golgi (1898). Golgi complex occurs in almost all eukaryotic cells. It is not found in prokaryotic cells.
  • Golgi apparatus consists of a set of membrane-bounded, fluid-filled vesicls, vacuoles and flattened cisternae (closed sacs). Cisternae are usually stacked together in parallel rows.
  • The plant cells contain many freely distributed subunits of Golgi apparatus, called dictyosomes.
Functions:
  1. Golgi complex is primarily associated with the secretory activities of the cell.
  2. It is involved in the synthesis of cell wall, plasma membrane and lysosomes.
  3. It is involved in the formation of cell plate during cell dividion.
  4. Dictyosomes in plant serve as temporary storage place for proteins and other synthesized products.

Ribosomes:

  • Ribosomes are dense, spherical and granular particles which occur freely in cytoplasm or remain attached to the endoplasmic reticulum (RER).
  • Chemically ribosomes are made up of ribonucleic acid (RNA) and proteins.
  • They are present both in prokaryotic and eukaryotic cells.
Function: Ribosomes help in the process of protein synthesis. They are, therefore, called the protein factories of the cell.

Mitochondria:

  • Mitochondria were first seen by Kolliker (1880) in muscle cells but Benda (1898) named them as mitochondria.
  • They are present in eukaryotic cell but absent in prokaryotic cells.
  • Mitochondria exist in variable shapes (cylindrical, rod-shaped, and spherical) and size (0.2mm-2mm).
  • Each mitochondrion is enclosed by a double membrane of lipoprotein. The outer membrane is smooth whereas inner membrane forms irregular series of folds called cristae.
  • The inner cavity of mitochondria is filled with gel-like proteinaceous matrix which contains a few small-sized ribosomes, respiratory enzymes, and a circular DNA molecule.
Functions:
  1. Mitochondria are the sites of cellular respiration, where energy in the form of ATP (adenosine triphosphate) is formed. That is why they are called the powerhouses of the cell.
  2. Since mitochondria have their own ribosomes and DNA, they can synthesize their own proteins, and they are self-duplicating units. So, they are regarded as semiautonomous organelles.

Lysosomes:

  • Lysosomes are dark, spherical, single membrane bound sacs containing several digestive enzymes. These enzymes are capable of digesting or breaking down all organic materials.
  • Lysosomes are formed by Golgi complex. They are found in the cytoplasm of eukaryotic cell, mostly in animal cells.
Functions:
  1. Lysosomes are involved in intracellular digestion.
  2. During starvation or aging, the cell digests its own organelles through lysosomal enzymes, a process called autophagy.
  3. Lysosomes help to keep the cell clean by digesting any foreign material as well as worn out cell organelles.
  4. When a cell is destined to die, the lysosomal enzymes digest the whole cell, a process called autolysis. Therefore lysosomes are also known as ‘suicide bags’ of the cell.

Plastids:

  • The term ‘plastid’ was given by Haeckel in 1866. Plastids occur in most plant cells and are absent in animal cells.

Chloroplasts:

  • They are spherical or discoidal in shape and are enclosed in double membrane. Inside a plastid, two definite regions are clearly visible- Grana and Stroma.
  • Grana are stacks of membrane-bounded, flatterened discoid sacs containing the molecules of chlorophyll.
  • Stroma is the homogeneous matrix in which grana are embedded.
  • Plastids contain their own DNA and ribosomes i.e., they have their own protein synthesizing machinery. They are also self-replicating organelles.
On the basis of color, plastids are of following three types:
  1. Chloroplasts: Green-colored plastids containing chlorophyll.
  2. Leucoplasts: Colorless plastids.
  3. Chromoplasts: Colored plastids (except green color).
Functions:
  1. Chloroplasts are photosynthetic organelles. The chlorophyll present in them trap solar energy for the purpose of synthesizing food for the plant. So chloroplasts are the ‘Kitchens of the cells’.
  2. Leucoplasts help in storing food products like starch, protein and lipids.
  3. Chromoplasts provide color to flower which in turn attracts insects for pollination.

Centrosome:

  • Centrosome is found only in animal cells.
  • It is not bounded by any membrane but consists of two granule-like centrioles. Centrioles are hollow cylindrical structures which are made up of microtubules.

Functions:

  1. Centrosome helps in cell division in animal cells. During cell division centrioles migrate to the poles of animal cells and are involved in the formation of the spindle.
  2. They produce basal bodies from which cilia and flagella arise.

Vacuoles:

  • Vacuoles are fluid-filled or solid-filled and membrane bound spaces in the cytoplasm.
  • Vacuoles are small sized in animal cells while plant cells have very large vacuoles.
  • The vacuole is bounded by a membrane, called tonoplast. The vacuole is filled with cell sap which is watery solution rich in sugar, amino acids, proteins, mineralsand metabolic wastes.
Functions:
  1. Vacuoles are meant for the storage of food, water and other substances.
  2. Vacuoles help to maintain the osmotic pressure in a cell (osmoregulation).
  3. Vacuoles provide turgidity and rigidity to the plant cells.

Peroxisomes:

  • Peroxisomes are small and spherical organelles containing powerful oxidative enzymes.
  • They are bounded by a single membrane.
Function: Peroxisomes are specialized for carrying out some oxidative reactions, such as detoxification or removal of toxic substances from the cell.


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