Membranous Organelles PDF

# Lecture 3: Membranous Organelles ## Functions: - Lipid synthesis: Especially in steroid-secreting cells such as adrenal cortex cells. - Glycogen metabolism: Enzymes involved in regulating glycogen metabolism are associated with the SER membrane, for example, in the liver cells. - Regulation of m...

# Lecture 3: Membranous Organelles ## Functions: - Lipid synthesis: Especially in steroid-secreting cells such as adrenal cortex cells. - Glycogen metabolism: Enzymes involved in regulating glycogen metabolism are associated with the SER membrane, for example, in the liver cells. - Regulation of mineral metabolism, e.g., HCL production in the stomach. - Calcium storage: In the skeletal and cardiac muscle fibers to control muscle contraction. - Drug detoxification: Due to the presence of cytochrome P450 enzymes in the SER membrane especially in the liver cells. ## Medical Application: - Underdeveloped SER in liver cells in newborn infants causes jaundice. - Jaundice is caused by the accumulation of bilirubin which is normally metabolized by SER enzymes in liver cells and excreted as bile. ## (4) Golgi Complex (Apparatus): - **Definition**: It is a membraneous organelle that processes, packages, and sorts macromolecules such as proteins and lipids after their synthesis. - **Light Microscopy (LM)**: - Cannot be seen with H&E. - In cells with deeply basophilic cytoplasm of the protein-secreting cells, as plasma cell, its position appears as a non-stained area called Negative Golgi Image. - It can be visualized only by using special stains. - **Electron Microscopy (EM)** - **Golgi stack**: It is composed of a variable number, typically 3-6, of flattened sacs called cisternae. - Each stack has a convex immature surface facing the nucleus called (Cis Face) and a concave mature surface towards the cell membrane called (Trans Face). - **Transfer (microvesicles):** They derive from rough endoplasmic reticula and fuse with the convex surface. - **Secretory (macrovesicles):** - Formed by budding from the mature Golgi surface. - Exocytosed out-side the cell or remain inside as lysosomes. ## Function: - Protein modification by the adding of carbohydrates and phosphate to proteins. - Lysosomes formation, share with RER. - Maintaining and renewal of cell membrane and cell coat through exocytosis. ## Types of Lysosomes: - **Primary Lysosomes**: Lysosomes that have not entered into a digestive process appear as a homogeneous vesicle. - **Secondary Lysosomes**: Lysosomes that have entered into a digestive process include different subtypes. - **Heterophagosome**: Result when the secondary lysosome fuse with solid particles (phagosome). - **Multi-vesicular body**: Result when the secondary lysosome fuse with pinocytotic vesicle. - **Autophagosome**: If the digested material is one of cytoplasmic organelles. ## Fate of the Digested Material: - **Digested nutrients**: Diffuse back to the cytoplasm for cellular reuse. - **Indigestible compounds**: Residual bodies extruded outside by exocytosis. - **Note**: In some long-lived cells (e.g., neurons, heart muscle), large quantities of residual bodies accumulate and are referred to as lipofuscin, or age pigment. ## Function of Lysosomes: Digestion: - Digestion of material engulfed from outside the cell, for example, bacteria. - Digestion of material from inside the cell such as old mitochondria. Keep the cell healthy. - Digestion of completely breakdown cells that have died (autolysis). - Release enzymes outside to destroy material around the cell such as in osteoclast. ## Lysosomal Storage Disorders: - In these diseases, a specific lysosomal enzyme is absent or inactive, and certain molecules (ex: glycogen) are not digested. - As a result, these substances accumulate in the cells, interfering with their normal functions. ## (5) Lysosomes: - **Definition**: Membrane bound vesicles containing hydrolytic enzymes increase in cells with phagocytic activity such as macrophage. - **LM**: They may be demonstrated in cell sections by immunohistochemical techniques. - **EM**: - Primary lysosomes appear spherical homogenous electron-dense vesicles. - Secondary lysosomes appear spherical heterogenous electron-dense vesicles as they contain digested elements. ## Formation of Lysosomes: - Lysosomal enzymes are synthesized and segregated in the RER. - Then they are transferred to the Golgi complex as transfer vesicles. - In the Golgi complex, enzymes are modified and packed as lysosomes. ## (6) Peroxisomes (Microbodies): - **Definition**: Small (0.2-1.0 µm), membrane-bound organelles that contain oxidative enzymes. - **Functions**: - Peroxisomes oxidize specific organic substrates producing hydrogen peroxide (H2O2) that is very damaging to the cell. - H2O2 is eliminated by the enzyme catalase, which is present in peroxisomes. - Catalase degrades several toxic molecules and drugs, particularly in liver and kidney peroxisomes. - Peroxisomes contain enzymes involved in lipid metabolism. ## (7) Secretory Granules: - They are originally granules derived from RER and containing secretory proteins. - These are transported to the immature face of Golgi stack, then to the mature face to bud off from them as secretory granules. These granules migrate to the apical pole of the secretory cells to exocytosis. - **LM**: They appear as an apical acidophilic area. - **EM**: They appear as membrane-bound electron dense granules in the apical pole of secretory cells. - **Function**: Addition of new cell membrane to replace the apical membranes lost in the process of endocytosis. ## (2) Mitochondria (The powerhouse of the cell) : - **Definition**: - Membranous organelle, concerned with energy production. - Their number varies from one thousand in liver cells (active) to few mitochondria in fat cells (inactive). - **LM**: Cannot be seen by H&E. Visualized by special stains e.g. iron hematoxylin. - **EM**: Two membranes and two spaces - **Two Membranes**: - **Outer smooth membrane** - **Inner membrane**: - Folded into cristae to increase surface area. - Cristae show elementary particles, which carry respiratory enzymes responsible for energy production. - **Two spaces**: - **Outer (intermembrane) space**: It is a narrow space between the inner and outer membrane. - **Inner space (Inter-cristae, matrix space)**: - The interior of the mitochondria. - This space is filled with a matrix rich in protein and contains lipids, mitochondrial RNA& circular DNA, ribosomes, enzymes, and dense granules rich in Ca2+. - **Note**: New mitochondria originate from preexisting mitochondria by growth and subsequent division (binary fission) of the organelle itself. ## Function: - Providing the cell by energy by generation of ATP. - Maintain calcium levels of the cell. - Play important role in the process of apoptosis (program cell death). ## Clinical Application: - **Mitochondrial Disease**: - Mitochondrial dysfunction reduces energy production by the mitochondria. - Mitochondrial disease causes damage to the cells of brain, heart, liver, muscles and kidney. - Symptoms include muscle weakness, loss of motor control, and pain. - Other symptoms include gastro-intestinal disorders, poor growth, cardiac disease and liver disease. ## (3) Endoplasmic Reticulum: - **Endoplasmic Reticulum**: Helps move substances within cells. Network of tubules (cisternae) - **Rough (Granular) tubules** - **Smooth (Non granular) tubules** - **LM**: - **Basophilia** - **Acidophilia** - **EM**: - **Tubules + Ribosomes** - **Tubules** - **Function**: - **Rough Endoplasmic Reticulum (RER)**: - **Definition**: RER is a series of connected parallel flattened tubules (cisternae) with ribosomes attached to its surface. - **Site**: Increases near the nucleus and the Golgi apparatus. - **LM**: By H&E, if increased in the cell shows cytoplasmic basophili due to the presence of ribosomes. - **EM**: connected parallel flattened tubules (cisternae) with ribosomes on the surface. - **Function**: - Protein synthesis - Lysosomal enzymes synthesis and origin of lysosomes - **Smooth Endoplasmic Reticulum (SER)**: - **Definition**: - Branching anastomosing smooth tubules. - In muscle SER is well-developed specialized and called Sarcoplasmic Reticulum. - **LM**: By H&E, if increased in the cell shows cytoplasmic acidophilia. - **EM**: - The SER appears as branching anastomosing tubules or flattened vesicles with smooth wall. - There are no ribosomes on the surface.