Cell Organelles

A cell has to perform different functions with the help of its various membrane-bound organelles.

• It has to synthesize substances, e.g., protein synthesis by ribosomes, lipid synthesis on the surface of smooth endoplasmic reticulum (SER), photosynthesis of food (e.g., glucose, starch) by6 chloroplasts.
• It has to secrete cell products, e.g., enzymes, hormones, mucus, etc.
• it has to digest those substance which are taken up by the cell during endocytosis. Such intracellular digestion is done by enzymes of lysosomes.
• It has to generate energy, e.g., synthesis of energy- rich ATP (adenosine triphosphate) by mitochondria.

Every cell has a membrane around it to keep its own contents separate from the external environment. Large and complex cells, including cells from multicellular organisms, need a lot of chemical activities to support their complicated structure and function. To keep these activities of different kinds separate from each other, these cells use membrane-bound little structures (or ‘organelles’) within themselves. This is one of the features of the eukaryotic cells that distinguish them from prokaryotic cells. Some of these organelles are visible only with an electron microscope.

Some important examples of cell organelles are as follows:- Endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, plastids and vacuoles.

ENDOPLASMIC RETICULUM:-

• The endoplasmic reticulum (ER) is a large network of membrane- bound tubes and sheets. It looks like long tubules or round or oblong bags (vesicles). The ER membrane is similar in structure to the plasma membrane. There are two types of ER- rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER).

1. RER(Rough endoplasmic reticulum):- It looks rough under a microscope because it has particles called ribosomes, which are present in all active cells, are the sites of protein manufacture. The manufactured proteins are then sent to various places in the cell depending on nee, using the ER.
2. SER(Smooth endoplasmic reticulum):- The SER helps in the manufacture of fat molecules, or lipids, important for cell function. some of these proteins and lipids help in building the cell membrane biogenesis.

Some other proteins and lipids function as enzymes and hormones. Although the ER varies greatly in appearance in different cells, it always forms a network system. Thus, one function of the ER is to serve as channels for the transport of materials (especially proteins) between various regions of the cytoplasm or between the cytoplasm and the nucleus. The ER also functions as a cytoplasmic framework providing a surface for some of the biological activities of the cell. In the liver cells of the group of animals called vertebrates. SER plays a crucial role in detoxifying many poisons and drugs.

GOLGI APPARATUS

The Golgi apparatus, first described by Camillo Golgi, consists of a system of membrane-bound vesicles arranged approximately parallel to each other in stacks called cisterns. These membranes often have connections with the membranes of ER and therefore constitute another portion of a complex cellular membrane system.

The material synthesized near the ER is packaged and dispatched to various targets inside and outside the cell through the Golgi apparatus. Its function include the storage, modification and packaging of products in vesicles. In some cases, complex sugars may be made from simple sugars in the Golgi apparatus. The Golgi apparatus is also involved cin the formation of lysosomes.

LYSOSOMES

Lysosomes are a kind of waste disposal system of the cell. Lysosomes help to keep the cell clean by digesting any foreign material as well as worn-out cell organelles. Foreign material entering the cell, such as bacteria or food, as well as old organelles end up in the lysosomes, which break them up into small pieces. Lysosomes are able to do this because they contain powerful digestive enzymes capable of breaking down all organic material. During the disturbance in cellular metabolism, fir example, when the cell gets damaged, lysosomes may burst and the enzymes digest their own cell. Therefore, lysosomes are known as the ‘suicidal bags’ of a cell. Structurally, lysosomes are membrane-bound sacs filled with digestive enzymes. These enzymes are made by RER.

MITOCHONDRIA

Mitochondria are known as the powerhouse of the cell. The energy required for various chemical activities needed for life is released by mitochondria in the form of ATP (Adenosine triphosphate) molecules. ATP is known as the energy currency of the cell. The body uses energy stored in ATP for making new chemical compounds and for mechanical work. Mitochondria have two membrane coverings instead of just one. The outer membrane is very porous while the inner membrane is deeply folded. These folds create a large surface area for ATP-generating chemical reactions.

Mitochondria are strange organelles in the sense that have their own DNA and ribosomes. Therefore, mitochondria are able to make some of their own proteins.

PLASTIDS

Plastids are present only in plant cells. There are two types of plastids-chromoplasts (coloured plastids) and leucuplasts (white or colourless plastids). Plastids containing the pigment chlorophyll are known as chloroplasts. Chloroplasts also contain various yellow or orange pigments in addition to chlorophyll. leucoplasts are primarily organelles in which materials such as starch, oils and protein granuels are stored.

The internal organisation of the plastids consists of numerous membrane layers embedded in a material called the stroma. Plastids are similar to mitochondria in external structure. Like the mitochondria, plastids also have their own DNA and ribosomes.

VACUOLES

Vacuoles are storage sacs for solid or liquid contents. Vacuoles are small sized in animal cells while olant cells have very large vacuoles. The central vacuole of some plant cells may occupy 50-90% of the cell volume.

In plant cells vacuoles are full of cell sap and provide turgidity and rigidity to the cell. Many substances of importance in the life of the plant cell are stored in vacuoles. These include amino acid, sugars, various organic acids and some proteins. In single-celled organisms like Amoeba, the food vacuole contains the food items that the Amoeba has consumed. In some unicellular organisms, specialised vacuoles also play important roles in expelling excess water and some wastes from the cell.

Each cell thus acquires its structure and ability to function because of the organisation of its membrane and organelles in specific ways. The cell thus has a basic structural organisation. This helps the cells to perform functions like respiration, obtaining nutrition, and clearing of waste material, or forming new proteins.

Thus, the cell is the fundamental structural unit of living organisms. It is also the basic functional unit of life.