Cell and Cell Organelles

Habish Ribin Haneef
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A cell is the structural and functional unit of life. Cells are the building blocks of all living organisms. The human body is made up of trillions of cells. The cells provide structure for the body, take nutrients from the food and convert these nutrients into energy. The cells carry out specialized functions in our body. Cells are of different shapes and sizes so that our body is composed of cells of different shapes and sizes. The study of cell structure and function is termed as “Cell Biology”. The cell was discovered in 1665 by Robert Hooke, an English Scientist. Hooke observed a piece of bottle cork under a compound microscope and he saw minuscule structures that reminded him of small rooms. Soon, Hooke named these small rooms as “cells”. But, due to the limited magnification of his compound microscope, Hooke could not find more details regarding cells. Finally, he concluded that these were non-living entities. Later, Antonie Van Leeuwenhoek, a Dutch Scientist observed cells under a compound microscope with higher magnification and he noticed that the cells are displaying some sort of movement. This result helped Leeuwenhoek to recognize that these microscopic entities are actually alive. The discovery of cells was a milestone in the field of Science as it helped us realise that cells are the structural and functional unit of life and are carrying out several life processes.

Types of Cells:

Based on cellular structure, cells are classified into two- Eukaryotic cells and Prokaryotic cells.

cell type

Eukaryotic Cells are larger and more complex than prokaryotic cells. Eukaryotic organisms may be multicellular or unicellular. These cells are characterized by a true nucleus. The size of these cells ranges from 10-100  µm in diameter. Eukaryotic cells usually contain organelles that are absent in prokaryotic cells. Eukaryotic organisms reproduce sexually as well as asexually. The four types of eukaryotic cells are animal cells, plant cells, fungi cells, and protist cells. The animal cell contains membrane-bound organelles like the nucleus, mitochondria, Golgi apparatus, and endoplasmic reticulum and is surrounded by a plasma membrane. The plant cell contains all the structures that are present in eukaryotic cells, but they consist of some additional structures. For example, plant cells consist of chloroplast, central vacuoles, and other plastids that are absent in animal cells. Fungi cells consist of many structures found in both animal cells and plant cells like the nucleus, mitochondria, Golgi apparatus, cell membrane and endoplasmic reticulum. Chloroplast is absent in fungi cells and they do have a cell wall but it is mainly composed of “chitin”, a polysaccharide rather than cellulose in the case of plant cells. Protist cells contain all membrane-bound organelles found in animal cells and some of the cells also have chloroplasts.

Prokaryotic cells are smaller and have a simpler structure than eukaryotic cells as they do not have membrane-bound organelles. Prokaryotic organisms are always unicellular. The size of these cells ranges from 0.1-0.5  µm in diameter. It is generally understood that prokaryotic cells don’t have nuclei, but in some prokaryotic organisms like bacteria, there is a region within the cell where genes are suspended freely. This region is referred to as the nucleoid. Prokaryotic cells are usually seen in archaea, bacteria, and cyanobacteria. The hereditary material of these cells can either be DNA or RNA. Prokaryotic organisms reproduce through a method called binary fission, a mode of asexual reproduction.

Cell Organelles:

cell orgenalls

The different components of a cell are known as cell organelles. Each of these cell organelles plays a very crucial role in the survival of a cell. The main cell organelles are the cell membrane, cell wall, nucleus, cytoplasm, endoplasmic reticulum, mitochondria, plastids, ribosomes, Golgi apparatus, microbodies, cytoskeleton, cilia and flagella, centrosomes and centrioles, and vacuoles. Now, let’s have a look at each of these parts.

The cell membrane is also called plasma membrane. It is present in all the cells. The cell membrane encloses every cell in the body and it acts as a protective layer for the cell. The cell membrane is a double layer composed of phospholipid molecules. It controls the movement of substances in and out of the cells. The cell membrane separates the cell from the external environment. The cell membrane also protects the cellular component from damage and leakage.

The cell wall is a rigid and stiff structure surrounding the cell membrane. It is composed of cellulose, hemicellulose, and pectin. The functions of the cell wall are to provide shape and support to the cell and protect them from mechanical shocks and injuries. The cell wall is absent in animal cells but is the most prominent part of the plant cell. The cell wall is also the outermost layer of the plant cell.

The nucleus is the largest cell organelle and is found in all eukaryotic cells. It is a double-membraned organelle and contains DNA, the hereditary material of the cell. The nucleus functions as the control centre of cellular activities and it sends signals to the cells to grow, mature, divide, and die. The nucleus protects the DNA and is a vital part of any cell. It is surrounded by a nuclear membrane and forms a wall between the cytoplasm and the nucleus. There are some tiny spherical bodies within the nucleus called nucleoles. The nucleoles are responsible for the synthesis of protein and RNA.  Nucleus also contains another important structure called chromosomes. The chromosomes are thin and thread like structures and they carry gene, another imperative part. The genes helps in the inheritance of traits from one generation to another.

The cytoplasm is a jelly like substance and is present in both plant cell and animal cell. The cytoplasm is found between the cell membrane and nucleus. All the cell organelles are embedded in the cytoplasm. The cytoplasm is made up of water, organic, and inorganic compounds. Cytoplasm consist of enzymes that are responsible for controlling all metabolic activities in the cell. Most of the chemical reactions within the cell also takes place in the cytoplasm.

The endoplasmic reticulum is a corporation of membranous canals filled with liquid. This part of the cell is responsible for the transportation of substances throughout the cell. The endoplasmic reticulum also plays an important role in the metabolism of carbohydrates, synthesis of lipids, steroids and proteins. Endoplasmic reticulum is of two types- rough endoplasmic reticulum and smooth endoplasmic reticulum. The rough endoplasmic reticulum is made up of cisternae, tubules, and vesicles and is responsible for protein manufacture. On the other hand, smooth endoplasmic reticulum helps in the production of lipids and steroids and they also detoxifies the cell.

The mitochondria is a double membrane-bound, sausage-shaped organelle present in almost all eukaryotic cells. Mitochondria are known as the “powerhouses of the cell” as they produces ATP, the cell’s energy currency. The mitochondria consists of two distinct aqueous compartments. The inner compartment is folded into cristae and is called “matrix” and the outer membrane forms a continuous boundary with the cytoplasm. The process of aerobic respiration takes place in the mitochondria. It produces energy in the form of ATP and plays a crucial role in the transformation of molecules.

Plastids are large, membrane-bound organelles and contain pigments. Plastids are classified into three based on the types of pigments- chloroplasts, chromoplasts, and leucoplasts. 

The chloroplast is responsible for trapping the light energy for photosynthesis. A space called stroma is enclosed by the inner membrane. The green colour inducing pigment chlorophyll are found in flattened-disk like structures called thylakoids and these thylakoids are arranged like a pile of coins. Each pile is called granum and they are connected by flat membranous tubules called stromal lamella.

The chromoplasts consists of fat-soluble, carotenoid pigments such as xanthophylls and carotene and these pigments provides plants with their characteristic colour.

The leucoplasts stores nutrients and are colourless.

The ribosomes are seen in the form of tiny particles in a large number of cells. They are made up of 2/3rd of RNA and 1/3rd of protein. The ribosome is responsible for protein synthesis in all the living cells that ensure the survival of the cell. The ribosomes are known as the 70S in prokaryotes and 80S in eukaryotes. The letter “S” is known as Svedberg’s unit and it implies the size and density.

The Golgi Apparatus is a membrane-bound organelle and is made up of cisternae, a series of flattened, stacked pouches. The function of the Golgi apparatus includes transporting, modifying, and packaging proteins and lipids to targeted destinations. For this reason, the Golgi apparatus is called “the cell’s post office.” The Golgi Apparatus is present within the cytoplasm and is present in both plant cells and animal cells. It is also called the Golgi complex.

Microbodies are minute, membrane-bound organelles and are found in both plant cells and animal cells. They can be viewed only under an electron microscope. The microbodies consist of several enzymes and proteins.

The cytoskeleton is found in all living cells, especially in eukaryotes. The cytoskeleton is made up of different types of proteins that can divide rapidly or disassemble depending on the requirement of cells. The cytoskeleton is responsible for providing shape and mechanical resistance to the cell from deformation.

Cilia and flagella– Cilia are small structures and they are hair-like projections. The cilia are present outside the cell wall and they help in the movement of the cell or extracellular fluid. The flagella are slightly bigger than the cilia and they also help in cell movements. The core of the cilia and flagella is called axoneme. The axoneme consists of nine pairs of gradually arranged peripheral microtubules and a set of central microtubules running parallel to the axis. Cilia and flagella arise from centriole-like structures called basal bodies.

Centrosomes and centriolesCentrosomes are composed of two mutually perpendicular structures called centrioles. Each centriole is made up of 9 equally spaced peripheral fibrils of tubulin protein, and the fibril is a set of interlinked triplets. The core part of the centriole is called the hub and it is proteinaceous. The hub connects the peripheral fibrils through radial spoke, which is made up of proteins. During cell division, centrioles form spindle fibres from the basal bodies of cilia and flagella.

The vacuoles are liquid-filled organelles enclosed by a membrane. The vacuoles store food and many other nutrients that are essential for the survival of a cell. They also store waste products and ultimately eliminate them. Hence, the rest of the cell is saved from contamination. The number and size of vacuoles are different in plant cells and animal cells. Plant cells have larger vacuoles than animal cells.

Check your knowledge

Answer. Robert Hooke

Answer. Based on cellular structure, cells are classified into two- Eukaryotic cells and Prokaryotic cells.

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