xix.4. Cell Organelles

About	A cell organelle is a subcellular structure that, like an organ in the body, has one or more specific tasks to carry out within the cell. The nucleus, mitochondria, and ribosomes are a few of the more significant cell organelles. These cell organelles comprise both membrane and non-membrane-bound organelles that exist within cells and have diverse shapes and functions. They collaborate effectively and work together to support the cell’s normal operation. The cell has a variety of organelles that are divided into three groups based on whether or not they have a membrane. Organelles without membranes: Non-membrane-bound cell organelles include the cell wall, ribosomes, and cytoskeleton. They can be found in both eukaryotic and prokaryotic cells. Organelles with a single membrane-bound: vacuole, lysosome, Golgi apparatus, and endoplasmic reticulum are single membrane-bound organelles Double membrane-bound organelles: the nucleus, mitochondria, and chloroplast, which are double membrane-bound organelles, which are present in the eukaryotic cell only. Let’s now examine specific cell organelles to learn more about their structure and functions.
Cell Membrane	The plasma membrane is another name for the cell membrane. The cell’s plasma membrane is its outermost layer, separating its interior from the outside world. The cell membrane is made up of lipids organized in a bilayer. The membrane’s lipid component is primarily composed of phosphoglycerides. The movement of molecules through the plasma membrane is one of its key activities. The membrane is selectively permeable to some molecules present on either side of it. Neutral solutes can pass across the membrane through the process of simple diffusion down the concentration gradient, or from a higher concentration to a lower concentration.
Cell Wall	The cell wall is a non-living hard structure that produces an exterior coating for the plasma membrane of fungi and plants. The cell wall not only gives the cell structure and safeguards it from mechanical harm and infection, but it also facilitates cell-to-cell communication and acts as a barrier to unwanted macromolecules. The cell wall is absent in animals.
Cytoplasm	It is the jelly-like material situated between the nucleus and the cell membrane. The liquid substance that makes up the plasma membrane is called cytoplasm. Several specialized cell organelles are also present.
Cytoskeleton	The term “cytoskeleton” refers to an intricate web of proteinaceous filaments that are found in the cytoplasm. A cell’s cytoskeleton performs a variety of tasks, including movement, mechanical support, and maintaining the shape of the cell.
Nucleus	Every eukaryotic cell has a double-membraned organelle called the nucleus. It is the largest organelle and serves as the command center for cellular operations as well as the Genetic repository for the cell. By structure, the nucleus is dark, spherical, and surrounded by a nuclear membrane. It is a porous membrane-like cell membrane and forms a boundary between the cytoplasm and nucleus. Nucleolus, or tiny spherical entities, are found inside the nucleus. It also contains chromosomes, a crucial component. Genes are vital biological structures that are carried by chromosomes, which are delicate, thread-like structures. In organisms, genes function as a genetic unit, assisting in the transmission of features from one generation to another. As a result, the nucleus manages the traits and operations of our body’s cells. Using the genetic information contained in DNA, the nucleus’ main job is to keep track of cellular functions like metabolism and growth. RNA and protein synthesis in the nucleus is carried out by nucleoli. Nucleoli are spherical structures found in the nucleoplasm.
Mitochondria	Mitochondria are the sites of aerobic respiration. They are referred to as the “powerhouses” of the cell since they generate cellular energy in the form of ATP. The mitochondrion is a double membrane-bound structure with the outside and inner membranes distinctively partitioning its lumen into two aqueous compartments, namely the outer and inner compartments. The matrix is the name of the inside compartment. The continuous limiting boundary of the organelle is formed by the outer membrane. The cristae are a series of infoldings that the inner membrane forms as it moves toward the matrix. The surface area is increased by the cristae. Each of the two membranes has unique enzymes related to mitochondrial activity.
The Endoplasmic Reticulum	The endoplasmic reticulum (ER) is a network or reticulum of minute tubular structures dispersed throughout the cytoplasm. They are the cell’s transport system and are responsible for moving materials around the cell. Hence, the ER separates the intracellular space into two different compartments, namely the luminal (inside the ER) and extraluminal (cytoplasm) compartments. The endoplasmic reticulum’s variations include: Rough Endoplasmic Reticulum (RER): Rough endoplasmic reticulum refers to endoplasmic reticulum with ribosomes on its surfaces. RER is typically seen in cells that are actively producing and secreting proteins. They overlap the outer membrane of the nucleus and are thick and continuous. Smooth Endoplasmic Reticulum (SER): Smooth endoplasmic reticulum seems smooth surface when there are no ribosomes present. The principal location for lipid synthesis is the smooth endoplasmic reticulum. Steroid hormones that resemble lipids are produced in animal cells by the SER.
Ribosomes	Ribosomes are significant cytoplasmic organelles that are not membrane-bound and are found in close proximity to the endoplasmic reticulum. Either the endoplasmic reticulum encloses ribosomes or they are freely dispersed throughout the cytoplasm of the cell. They are made of proteins and ribonucleic acid (RNA). Protein synthesis, which ensures the life of the cell, is the primary function of the ribosomes in all live cells.
Golgi Apparatus	It is a membrane-bound organelle made up primarily of cisternae, which are a series of flattened, stacked pouches. The number of cisternae in a Golgi complex varies. Transporting, altering, and packing proteins and lipids to specific locations are the main duties of this cell organelle. The Golgi apparatus is a key location of glycoprotein and glycolipid synthesis.
Lysosomes	They are membrane-bound vesicular structures that are the result of the Golgi apparatus’s packaging process. It has been discovered that the isolated lysosomal vesicles are extremely rich in nearly all hydrolytic enzyme types (hydrolases – lipases, proteases, and carbohydrates), which are most active at the acidic pH. These enzymes can break down lipids, proteins, carbohydrates, and nucleic acids. It helps to digest dead and damaged cells, aids in digestion, and eliminates waste. As a result, it is also known as “suicidal bags”.
Vacuoles	The membrane-bound area in the cytoplasm is known as the vacuole. It contains water, sap, excretory product, and other components that are not beneficial to the cell. Vacuoles eventually dispose of the waste products. As a result, the rest of the cell is safe from contamination. Tonoplast is the name of the solitary membrane that surrounds the vacuole. The number and size of vacuoles differ between animal and plant cells. The vacuoles in plant cells are larger than those in animal cells. In plant cells, vacuoles have the potential to occupy up to 90% of the total cell volume.
Plastids	Every plant cell, including euglenoids, contains plastids. They may be seen clearly under a microscope due to their size. They carry particular pigments, which give the plants particular colors. Chloroplasts, chromoplasts, and leucoplasts are three different types of plastids. Chloroplasts: Chlorophyll and carotenoids are pigments found in chloroplasts, which are in charge of absorbing light energy necessary for photosynthesis. Moreover, the chloroplasts are double membrane-bound. The inner chloroplast membrane is considerably less permeable than the outer membrane. The stroma is the area of the chloroplast that is constrained by its inner membrane. A group of organized flattened membrane sacs are called thylakoids. Thylakoids are stacked like collections of coins, which are called grana, or intergranular thylakoids. Moreover, stroma lamellae are flat membranous tubules that connect the thylakoids of the various grana. A region known as the lumen is enclosed by the thylakoids’ membrane. The stroma of the chloroplast includes enzymes essential for glucose and protein synthesis. Ribosomes and tiny, circular, double-stranded DNA molecules are also present in chloroplast. In the thylakoids, chlorophyll pigments are found. Chromoplasts: The chromoplasts include fat-soluble carotenoid pigments like carotene, xanthophylls, and others. This imparts a yellow, orange, or red color to the plant portion. Leucoplasts: The leucoplasts are nutrient-stored, colorless plastids of various sizes and forms. Amyloplasts store carbs (starch), such as potatoes, whereas elaioplasts and aleuroplasts store oils and fats, and proteins, respectively.
Cilia and Flagella	Cilia and flagella are outgrowths of the cell membrane that resemble hair. The movement of the cell or the surrounding fluid is induced by cilia. Comparatively longer flagella are in charge of cell motility. Although prokaryotic bacteria have flagella, they differ structurally from eukaryotic flagella. The axoneme, the core of the Cilia and flagella, has many microtubules that run parallel to the long axis. Nine doublets of radially oriented peripheral microtubules and two centrally positioned microtubules typically make up an axoneme. The peripheral doublets’ tubules are connected to one another by radial spokes, and the central tubules are connected by bridges and surrounded by a central sheath.. The cilium and flagellum both emerge from basal bodies, which are centriole-like structures.
Centrosome and Centrioles	The centrosome organelle is made up of two centrioles, which are mutually perpendicular structures. Each centriole is made up of nine tubulin protein peripheral fibrils that are evenly spaced apart and are made up of interconnected triplets. The hub, or central portion of the centriole, is made of protein. The hub joins the peripheral fibrils via a protein-based radial spoke. During cell division, the centrioles from the basal bodies of the cilia and flagella give rise to spindle fibres.
Microbodies	Both plant and animal cells contain numerous membrane-bound tiny vesicles called microbodies that contain various enzymes.