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Lysosomes are small membrane-bound organelles found in most animal cells. They are absent in most plant cells, although plant cells do have similar structures like vacuoles that perform some of the same functions. Lysosomes are an important part of the eukaryotic endomembrane system. These organelles were first discovered by the Belgian scientist  Christian de Duve  in 1955. Lysosomes are known as the  digestive system of the cell,  because they contain hydrolytic enzymes that help in the breakdown of biological substances like proteins, lipids, carbohydrates and nucleic acids. Lysosomes are also called the  "suicidal bags"  of the cell. This is because under certain conditions like cellular damage, stress, or ageing, lysosomes may rupture and release their enzymes into the cytoplasm. These enzymes digest the cell's own components, leading to  autolysis or self-destruction . Types of Lysosomes Functionally, lysosomes can be divided into three main type...

Mitochondria and chloroplasts are essential organelles in eukaryotic cells. Mitochondria are involved in aerobic respiration and ATP production, while chloroplasts are the site of photosynthesis in plant and algal cells. The widely accepted explanation for their origin is the  Endosymbiotic Theory. The earliest idea related to the symbiotic origin of plastids was proposed by  Konstantin Mereschkowski  in 1905, who suggested that chloroplasts evolved from  autotrophic cyanobacteria  like ancestors through a process of symbiogenesis. Later in the 1920s,  Ivan Wallin,  an American biologist, extended this idea to mitochondria, proposing that they originated from  aerobic bacteria.  However, these early ideas received little acceptance at that time. A major revival and refinement of the endosymbiotic theory came with  Lynn Margulis  in 1967, who in her seminal paper  "On the Origin of Mitosing Cells"  provided detailed evidenc...

In eukaryotic cells, the DNA is extremely long and must be efficiently packed to fit within the confines of the nucleus. This packaging is achieved without tangling or damaging the genetic material with the help of the nucleosome, which is the basic unit responsible for this organization. It is not only responsible for compacting the DNA but also plays an essential role in regulating access to genetic information during processes such as transcription, replication and DNA repair. The nucleosome acts as the  first level  of chromatin organization and serves as a structural and functional framework for further compaction. Core Structure of the Nucleosome The nucleosome consists of two main components:  the core particle and the linker DNA.  The core particle is built around a  histone octamer,  which is made up of two molecules each of histones  H2A, H2B, H3 and H4.  These histones are small, positively charged proteins that interact with the negati...

The plasma membrane, also called the  cell membrane,  is a  selectively permeable  biological barrier that surrounds the cytoplasm of all living cells. It plays a crucial role in maintaining cellular integrity and homeostasis by regulating the movement of substances in and out of the cell. It is mainly composed of a phospholipid bilayer with embedded proteins, cholesterol and carbohydrates, which together allow the membrane to carry out several specialized functions. Main Functions of Plasma Membrane 1. Selective Permeability: One of the most important functions of the plasma membrane is to regulate the entry and exit of substances. It allows only certain molecules (like oxygen, water and glucose) to enter, while waste materials and unwanted substances are removed. This selective permeability is due to the lipid bilayer and specific transport proteins. 2. Transport of Materials: The membrane facilitates both  passive transport  (like diffusion and osmosis) ...

Fibroblasts  are a type of connective tissue cell that originate from  mesenchymal stem cells.  They are specifically classified as  non-epithelial, non-immune and non-muscle cells.  This means that fibroblasts do not belong to the epithelial cells (which form linings of organs and body surfaces), they are not part of the immune cell system (like macrophages or lymphocytes) and they are not muscle cells (like smooth or skeletal muscle). Instead, fibroblasts form a distinct population of structural cells that are found in the  interstitial spaces,  which are the spaces between functional cells of almost all organs. In these spaces, fibroblasts play a key role in maintaining the extracellular matrix (ECM) and supporting tissue architecture. Their main identity is based on their ability to synthesize and remodel the extracellular matrix (ECM), especially collagen and fibronectin. They are classified as: Connective tissue cells because they are the princip...

In all primary plant organs such as roots, stems, leaves, flowers and fruits, the outermost protective covering is formed by the  epidermal tissue system.  This system is a collective term used to describe the outer cell layers that protect the internal tissues of the plant. The  epidermis  is the main and most prominent component of this system. In simpler terms, we can say that epidermal tissue is the entire tissue system, while the epidermis is the actual single outer layer of cells that we directly observe in plant organs. The epidermal tissue system not only includes the epidermis, but also contains various specialized structures like stomata (with guard cells), trichomes (hair-like projections) and root hairs, each of which plays a functional role in plant physiology. It forms the first line of defence of the plant body. Structure of Epidermis The epidermis generally consists of a  single  layer of  parenchymatous cells,  which are flat, com...

Sclerenchyma is a type of simple permanent tissue found in higher plants, mainly responsible for providing mechanical strength and support. These cells are specially adapted to offer rigidity to plant parts that are no longer elongating or growing. The word "sclerenchyma" is derived from the Greek word scleros, meaning  "hard",  which reflects the tough nature of these cells. The following are the key characteristics of sclerenchyma cells: 1. Dead at Maturity: Sclerenchyma cells are non-living when they reach maturity. They lose their protoplasm, making them metabolically inactive. This feature is important because their primary role is to provide support, not participate in physiological processes. 2. Highly Thickened Cell Walls: One of the most important features of sclerenchyma is the presence of very thick secondary cell walls. These thick walls are uniformly thickened and are rich in  lignin,  a complex and hard organic substance that adds rigidity and impermea...

Sclerenchyma and collenchyma are two important types of simple permanent tissues in plants. Both are specialized for support, but they differ in their structure, composition and functional roles. These differences help the plant maintain its shape, stand upright and survive in various environmental conditions. Let us understand their differences based on structure and function. Structural Differences: Sclerenchyma cells are  dead at maturity  and have extremely  thick  cell walls due to the uniform deposition of lignin, a complex organic polymer. These cells lack protoplasm and usually have very narrow lumens. They are rigid and occur as either fibres (long and narrow) or sclereids (short and irregular). Because of their hardness, they often make plant parts like seed coats and nut shells hard and tough. On the other hand, collenchyma cells are  living  and have unevenly thickened cell walls. These thickenings are mainly due to the deposition of cellulose...

The nucleolus is a dense, spherical structure found inside the nucleus of eukaryotic cells. It is not surrounded by a membrane, yet it is one of the most prominent sub-nuclear structures visible under a microscope. The nucleolus plays a very important role in gene expression and cellular metabolism. Understanding its functions is essential because of its direct connection to protein synthesis, cell growth and cell cycle regulation. Function of the Nucleolus The primary function of the nucleolus is ribosome biogenesis, which involves the synthesis and assembly of ribosomal RNA (rRNA) and ribosomal proteins to form ribosomes. These ribosomes are essential for protein synthesis in the cell. Besides this primary role, the nucleolus also performs several additional functions that contribute to cellular processes like stress response, cell cycle regulation, and RNA modification. Primary Function : 1. Ribosome Biogenesis: The primary and most well-known function of the nucleolus is the synth...

The solenoid model is a well-known hypothesis that explains the  secondary level of DNA packaging  in eukaryotic cells. After the  primary level of compaction,  where DNA is wrapped around histone proteins to form nucleosomes, further folding is required to fit the large eukaryotic genome inside the nucleus. In 1976, scientists  Finch and Klug  proposed the solenoid model to describe how nucleosomes organize into a higher-order helical structure known as the  30 nm chromatin fiber.  This model plays a crucial role in making chromatin more compact, yet still accessible for essential processes like replication and transcription. Structure In the solenoid model, nucleosomes are arranged in a spiral or helical fashion forming a hollow tube-like fiber. About  six  nucleosomes are present per turn of the solenoid. The linker DNA, which connects each nucleosome, bends in such a way that it helps the nucleosomes to pack closely. Histone  H1...

To understand nucleosome properly, it is important to know that eukaryotic DNA is very long and cannot fit inside the nucleus unless it is packed in an organised way. This organised packaging starts with a basic unit known as the  nucleosome.  A nucleosome is the basic structural and functional unit of  chromatin  in eukaryotic cells. It plays a crucial role in  DNA packaging  by enabling long DNA strands to be compacted within the nucleus in a highly organised way. The term "nucleosome" was first introduced by  Roger Kornberg  in 1974, who also explained its  "bead-like"  appearance under the electron microscope. A nucleosome consists of DNA wrapped around histone proteins. This structure is essential for compacting DNA within the nucleus, ensuring its proper organisation for processes like transcription and replication. Structure of Nucleosome The nucleosome consists of a core particle and linker DNA. The core particle is an octam...

Chromosomes are thread-like structures made up of  DNA and histone proteins.  They are found inside the nucleus of eukaryotic cells and are responsible for the storage, expression and transfer of genetic information from one generation to the next. During cell division, chromatin condenses to form visible chromosomes, ensuring proper distribution of genetic material. Structure of Chromosomes Each chromosome consists of two identical sister  chromatids  joined at a point called the  centromere.  These chromatids carry the same genetic information. Chromosomes ensure the equal distribution of genetic material during cell division and maintain hereditary continuity. Types of Chromosomes There are two major bases for the classification of chromosomes: 1. Based on Centromere Position This classification depends on where the centromere is located along the length of the chromosome. Metacentric:  Centromere is exactly in the middle, forming two equal arms. Su...

In the nucleus of eukaryotic cells, chromatin refers to the complex of  DNA and histone proteins.  It is the molecular material that carries genetic information in a packed form. When observed under a microscope during the  interphase  of the cell cycle, this chromatin appears as a  loose, thread-like network,  which is called the  chromatin network.  Hence,  chromatin  is the substance made of DNA and proteins, while the  chromatin network  refers to its visible, mesh-like arrangement inside the nucleus when the cell is not dividing. Structure and Components of the Chromatin Network The chromatin network consists of multiple  nucleosomes,  where  DNA is wrapped around histone protein cores.  This arrangement allows the extremely long DNA strands to be compacted efficiently and organized in a way that maintains accessibility for transcription, replication and DNA repair. This network is finely distributed an...

To understand the importance of peroxisomes in plant cells, it is necessary to know that these are small, membrane-bound organelles found in the cytoplasm. Although they are present in both animal and plant cells, in plant cells, they are particularly important due to their involvement in  photorespiration and lipid metabolism.  These organelles contain oxidative enzymes such as catalase and oxidase, which are responsible for removing harmful substances like hydrogen peroxide. Role in Photorespiration This is the most specific and unique function of peroxisomes in plant cells. During photosynthesis, under certain conditions like high oxygen and low carbon dioxide levels, a side process called photorespiration occurs. In this process, the chloroplasts produce glycolate, which is transported to peroxisomes. Inside the peroxisomes, glycolate is converted into glycine. This reaction releases hydrogen peroxide (H₂O₂) as a by-product, which is highly toxic to the cell. The enzyme ca...

To understand the advantages of the extensive network of the endoplasmic reticulum (ER), it is important to know that the ER is a large, interconnected membranous structure found throughout the cytoplasm of eukaryotic cells. It is divided into two types:  rough ER (with ribosomes)  and  smooth ER (without ribosomes).  The broad and interconnected nature of the ER offers several benefits. Out of these, two key advantages are explained below: 1. Efficient Intracellular Transport The extended network of tubules and cisternae in the ER allows for smooth and efficient transport of proteins, lipids and other molecules within the cell. Since the ER extends throughout the cytoplasm and connects directly with the nuclear envelope and sometimes the plasma membrane, it forms a highway-like system that helps in the rapid movement of substances. This is particularly useful for proteins synthesized by the rough ER, as they are quickly delivered to the Golgi apparatus or other dest...

The term semi-autonomous means that an organelle has some level of independence, but not complete independence, in performing its functions and maintaining itself. In eukaryotic cells,  mitochondria and chloroplasts  are called semi-autonomous organelles because they possess certain unique features that allow them to perform some of their own functions independently from the nucleus, but still depend on the nucleus for many essential components. Introduction to the Concept of Semi-Autonomy Before we understand why mitochondria and chloroplasts are called semi-autonomous, it is important to know that most organelles in a eukaryotic cell depend completely on the  nucleus  for their proteins, enzymes and replication. However, mitochondria and chloroplasts are exceptions to this. They show several prokaryotic-like features which support the idea of their partial independence. These organelles are also central to energy-related functions: mitochondria in aerobic respirati...

The organelle that contains the enzymes for cellular respiration is the  mitochondrion.  This organelle plays a central role in energy metabolism by housing the enzymes and components necessary for the complete breakdown of glucose and other molecules to produce ATP (adenosine triphosphate), which is the primary energy currency of the cell. Before directly entering into the details of the enzymes, it is helpful to understand how the mitochondrion is structurally designed to perform this role efficiently. Mitochondria are double-membrane-bound organelles found in almost all eukaryotic cells. The outer membrane serves as a protective layer, while the inner membrane is extensively folded into structures known as  cristae,  which increase the surface area for housing respiratory enzymes. The matrix, the innermost compartment, contains the enzymes for the Krebs cycle, while the inner membrane houses the electron transport chain (ETC) and ATP synthase, which are essential ...

The mitochondrion is commonly called the  "powerhouse of the cell"  because it is the main site for the production of  adenosine triphosphate (ATP),  which is the energy currency of cells. This label reflects the central role of mitochondria in cellular respiration, where they convert energy stored in food molecules into usable chemical energy. Before explaining how this happens, it is important to understand the background of this concept and how ATP generation is connected to the structure and function of mitochondria. ATP is required for almost all cellular processes such as muscle contraction, active transport across membranes, cell division and biosynthesis. However, ATP cannot be stored in large amounts, so it must be constantly produced. Mitochondria carry out aerobic respiration, which is a highly efficient process that yields a large amount of ATP compared to anaerobic pathways. In fact, one molecule of glucose can yield up to 36 to 38 molecules of ATP in th...

Photosynthesis is a highly specialised biological process in which light energy is converted into chemical energy by autotrophic organisms like plants, algae and some protists. To understand which organelles are involved in  photosynthesis,  it is important to first clarify that the core reactions take place within specific compartments of the cell. However, some other organelles also support or regulate this process indirectly. Therefore, before discussing each organelle, it is essential to know that the primary site of photosynthesis is the chloroplast, but other organelles such as the  endoplasmic reticulum, Golgi apparatus,  and even the  peroxisomes  and  vacuoles  have supplementary roles in supporting photosynthesis-related functions. 1. Chloroplasts Chloroplasts are the main organelles  directly involved  in photosynthesis. They are double-membrane-bound structures found in the cytoplasm of plant and algal cells. These organelles...

The fluid mosaic model is one of the most widely accepted and foundational concepts that explains the structural organization of the plasma membrane in living cells. It was first proposed by  S.J. Singer and Garth L. Nicolson in 1972  and remains relevant today, though slightly refined with modern findings. Before we move into the components and structure, it is important to understand that this model helps explain how the membrane remains flexible, selectively permeable, and functionally active for processes like transport, cell signaling and communication. Basic Concept of the Fluid Mosaic Model According to the fluid mosaic model, the plasma membrane is viewed as a dynamic, semi-fluid bilayer of lipids with proteins embedded in it, much like boats floating in a sea. The word  "fluid"  refers to the lateral movement of lipids and some proteins within the membrane, while  "mosaic"  refers to the patchwork arrangement of various proteins that float freely o...