Cell Organelles – Structure and Functions

Key Concepts

• Ribosome
• Lysosomes
• Mitochondria
• Charloplast
• Endoplasmic reticulum
• Golgi apparatus
• Nucleus

Introduction: 

Cytoplasm – It is a semi-liquid substance that is present in most of the parts of the cell within the cell membrane. Many chemical reactions take place here. It contains both living and non-living parts. The living cytoplasm is always in a state of some movement. 

Explanation: 

Cell organelles 

These are little organs present inside the cell which have a definite shape, a definite structure, and a definite function. The organelles are living parts present in the entire body performing specific functions such as maintaining the shape of the cell to reproduction, energy production, movement, protein synthesis and transport of substances. The cell organelle includes ribosomes, lysosomes, mitochondria, chloroplast, endoplasmic reticulum, Golgi apparatus, vacuoles, and nucleus. Animal cells have a centrosome and lysosome, which is not present in plant cells. Among the cell organelles, chloroplast, cell wall, plastids, and large central vacuole are present only in plant cells. 

Ribosome 

They are numerous small granules scattered freely in the cytoplasm or attached to the membranes of the endoplasmic reticulum. These are single-walled dense spherical bodies composed mainly of RNA. It consists of two major components, small and large ribosomal subunits. In which each subunit consists of one or more ribosomal RNA (rRNA) molecules and many ribosomal proteins (r-proteins). rRNA carries out protein synthesis in ribosomes. These are known as the factories for the synthesis of proteins. 

Lysosomes 

Lysosomes are intracellular digestive centers. These are small vesicles of different shapes containing some digestive enzymes. It contains 40 different types of enzymes. These enzymes destroy and digest foreign substances around them. They digest the stored food during starvation of the cell. These are also known as “suicide bags” since many damaged cells are rapidly destroyed or dissolved by their own lysosomes. The formations of bones are by digesting cartilages. 

Mitochondria 

Mitochondria are where the cell produces energy. These are spherical, rod-shaped, or thread-like bodies, but usually, they look like sausages. These are sites where cellular respiration occurs, and energy is released. This energy is stored in the form of an energy-rich compound called ATP (adenosine triphosphate) and is used in various metabolic functions of the body. Here the synthesis of respiratory enzymes takes place. It is also known as “power houses of the cell.” It has its own DNA containing several genes and its own ribosomes. 

Cristae:

Minute double-walled bags with their inner walls produced into finger-like processes projecting inwards. 

Plastids :

These are special organelles only found in plant cells. These are double membrane, proteinaceous matrix that contains DNA. Based on the color they impart, there are different plastids. Chloroplast is one of the plastids. 

Chloroplast :

Chloro means green; hence, these are green colored plastids. The green colored pigment present in them is called chlorophyll. The parts which are exposed to light are abundant in chloroplasts. Example: Leaves. 

There are other pigments such as orange and yellow that are present in plants, but they are masked by large quantities of chlorophyll. Its function is to trap solar energy and absorb carbon dioxide for the manufacture of starch and sugar during photosynthesis. It has DNA and has the capacity to divide. Raw tomatoes and unripe chilies are green due to chlorophyll, but during ripening, the chlorophyll degenerates and the masked red (carotene) takes over. 

Endoplasmic reticulum:

It is present all over the cytoplasm as an irregular network of double membranes. Its outer end is connected with the cell membrane, and the inner end is connected with the nuclear membrane. It acts as the supporting framework of the cell and also a pathway for the distribution of materials from one part to another. It is smooth, but when it has a particle like ribosome attached, it appears to be rough. Also, it helps in the synthesis and transport of proteins and fat. 

Golgi apparatus :

Golgi apparatus is supposed to be originated from the endoplasmic reticulum in the form of granules, filaments or rods. Very small vesicles of different shapes are located near the nucleus. These are known as the delivery system of the cell. It consists of many small groups of hollow tubular structures with membranous walls and is associated with some minute vesicles and vacuoles. The function of Golgi apparatus is the secretion of the cell including enzymes, hormones, etc. 

Granules and Vacuoles :

Granules and Vacuoles are non-living substances or cell inclusions. Granules are small particles in the cytoplasm, whereas vacuoles are clear spaces in the cytoplasm. Granules contain food materials such as starch, glycogen, and fats. Vacuoles are filled with water and various substances in solution.  

Nucleus :

The basic unit of a cell, which is a small spherical mass located nearly at the center of the cytoplasm, is called nucleus. 

• It is an important part of the cell that regulates and coordinates various life processes of the cell.  
• It contains both the living and non-living parts.  

• It is the largest cell organelle.  
• A nucleus is mostly spherical and has a nuclear membrane that is filled with a dense fluid called nucleoplasm. 
• The nucleoplasm contains certain thread-like structures called chromatin fibers. 
• The nucleus plays a vital role in cell division. During cell division, the chromatin fibers become thick and ribbon-like structures called chromosomes. 

• The chromosomes carry genetic characters from the parents to offspring.  
• Chromosomes are made of chromatin.  
• The chromatin is composed of hereditary units called genes. 
• Genes are made of a complex chemical substance called DNA (deoxyribonucleic acid).