Cytoplasm and Its Organelles

CYTOPLASM AND ITS ORGANELLES

1. ENDOPLASMIC RETICULUM (ER) 

It is a complex series of tubules whose walls are made up of unit membrane. Through this network of tubules, substances may be deli vered from the outer membrane of cell proper to the membrane of the nucleus or to other inclusion bodies of the cells e.g. mitochondria:

(i) Agranular or Smooth ER: Contains no granules.

(a) It is site of steroid (Adrenocortical hormone) synthesis in steroid secreting cells and the site of detoxification processes in other cells.

(b) As the sarcoplasmic reticulum, it plays important role in skeletal and cardiac muscles.

(ii) Granular or Rough ER or Ergastoplasm.

(a) Contains granules called ribo somes (diameter 15 nm; contains 65% RNA and 35% protein - Ribo nucleoprotein) which are attached to the cytoplasmic side of the membrane. 3-5 ribosomes clump together to form polyribosomes or polysomes.

(b) It is the site of protein synthesis e.g., hormones that are secreted by the cell; and proteins that are found in enzymes.

(c) Free ribosomes are also found in the cytoplasm, they synthesize cytoplasmic protein e.g., Haemoglobin.

2. GOLGI COMPLEX (OR GOLGI BODIES) 

It is a collection of membranous tubules and vesicles found always in the vicinity (neighbourhood) of the nucleus, prominent in actively secreting gland cells.

Functions

1. Wrapping and packaging department of the cell. 

2. Produces secretion granules i.e. membrane enclosed complexes, which stores hormones and enzymes in protein secreting cells; it packages proteins. 

3. Site of formation of lysosomes i.e. large irregular structures surrounded by membrane which are present in the cytoplasm. 

4. It adds certain carbohydrates to proteins to form glycoproteins, which play an important role in the association of the cells to form tissues.

3. MITOCHONDRIA 

Structure

(i) Length 5-12 µm; diameter 0.5-1 μm; filamentous or globular in shape; occur in variable numbers from a few hundred to few thousands in different cells.

(ii) Made up of outer membrane and inner membrane. Inner membrane folded to form cristae (shelves) which project into the interior of the mitochondrion.

(iii) Outer membrane 

(a) Studded with the enzymes concerned with                    biological oxidation (oxidation being catalyzed by enzymes).

(b) Provide raw materials for the reactions occurring inside the mitochondrion.

(iv) Interior (matrix) of mitochondrion con tains enzymes concerned with 'citric acid cycle' and 'respiratory chain oxidation', (sequence of reactions in which acetyl CoA is metabolised to CO₂ and H-atoms.

(v) Inner membrane is made up of repeat ing units each of which contains:

(a) Head piece 

(b) Stalk. It contains adenosine triphosphatase (ATPase) and other enzymes concerned with synthesis and metabolism of ATP.

(c) Base piece - contains the enzymes of electrons transfer chain. 

The ultimate purpose of these mechanisms is oxidative phosphorylation and synthesis of ATP

Functions

(i) Mitochondria are power generating units of the cells and are plentiful and best developed in parts of cells where energy requiring processes take place e.g. rapidly contracting skeletal muscles.

(ii) Also contains DNA and synthesize proteins.

4. LYSOSOMES

Structure

1. These are large irregular structures surrounded by unit membrane and are found in the cytoplasm; 250-750 um in diameter. A typical cell may contain several hundred lysosomes.

2. It is filled with large number of small granules, 5-8 nm in diameter which contains variety of enzymes, called lysozymes.

Functions

(i) Acts as a form of digestive (lytic) system for the cell, because enzymes present in it can digest essentially all macro molecules.

(ii) Engulf worn out components of the cells in which they are located. 

(iii) Engulf exogenous substances e.g. bacteria etc. and degrade them. 

(iv) When a cell dies, lysosomal enzymes causes autolysis of the remnant.

5. PEROXISOMES

(i) Its structure is similar to that of lyso somes but with a different chemical composition.

(ii) They consume oxygen in small amounts that is not used in the chemical reactions associated with ATP formation. 

(iii) They destroy certain products formed from oxygen, especially hydrogen peroxide, that can be toxic to the cells, hence the name peroxisomes.

6. CENTRIOLES OR CENTROSOMES

Structure

(i) These are two short cylinders called 'centrioles' visible only during cell division.

(ii) They are located near the nucleus and are so arranged such that they are at right angles to each other.

(iii) Tubules in group of three (triplets) run longitudinally in the walls of the centrioles. There are nine of these triplets spaced at regular intervals around the circumference.

Function

They are concerned with the movement of the chromosomes during cell division.

7. MICROTUBULES AND MICROFILAMENTS 

Microtubules are long hollow structures approx. 25 nm in diameter; make up structures or tracts on which chromosomes, mito-chondria and secretion granules move from one part of the cell to another. Microfilaments are solid fibers 4-6 nm in diameter. They comprise of the contractile protein actin and are responsible for the cell motion.

Functions

These are involved in the: 

(i) movements of the chromosomes;

(ii) cell movement; 

(iii) process that move secretion granules in the cell; and

(iv) movement of proteins within the cell membrane.