- 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:
- Cytoplasm: it is that part of protoplasm which surrounds the nucleus.
- Nucleoplasm: it is that part of protoplasm which is located inside the nucleus.
Protoplasm = cytoplasm + nucleoplasm
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.
- 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:
- Rough endoplasmic reticulum (RER) with ribosomes attached on its surface for synthesizing proteins.
- Smooth endoplasmic reticulum (SER) which is without ribosomes and is meant for secreting lipids.
- ER provides mechanical support to the cell.
- It forms intracellular transporting system.
- It forms new nuclear membrane after every cell division.
- SER takes part in the synthesis of lipids.
- 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.
- Golgi complex is primarily associated with the secretory activities of the cell.
- It is involved in the synthesis of cell wall, plasma membrane and lysosomes.
- It is involved in the formation of cell plate during cell dividion.
- Dictyosomes in plant serve as temporary storage place for proteins and other synthesized products.
- 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 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.
- 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.
- 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 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.
- Lysosomes are involved in intracellular digestion.
- During starvation or aging, the cell digests its own organelles through lysosomal enzymes, a process called autophagy.
- Lysosomes help to keep the cell clean by digesting any foreign material as well as worn out cell organelles.
- 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.
- The term ‘plastid’ was given by Haeckel in 1866. Plastids occur in most plant cells and are absent in animal cells.
- 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:
- Chloroplasts: Green-colored plastids containing chlorophyll.
- Leucoplasts: Colorless plastids.
- Chromoplasts: Colored plastids (except green color).
- 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’.
- Leucoplasts help in storing food products like starch, protein and lipids.
- Chromoplasts provide color to flower which in turn attracts insects for pollination.
- 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.
- 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.
- They produce basal bodies from which cilia and flagella arise.
- 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.
- Vacuoles are meant for the storage of food, water and other substances.
- Vacuoles help to maintain the osmotic pressure in a cell (osmoregulation).
- Vacuoles provide turgidity and rigidity to the plant cells.
- 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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