Smooth ER, Organelles, and Mitochondria

 

To read more about Cell and it's function  check out Chapter 02🔗:

https://mednotesbyrazaullah.blogspot.com/2026/08/chapter-02-cell-and-its-functions.html

Agranular or Smooth Endoplasmic Reticulum

Agranular or smooth endoplasmic reticulum represents an essential structural variation of the cell’s internal membrane system, set apart from the rough endoplasmic reticulum by its complete absence of attached ribosomes. Because it lacks these protein-synthesizing complexes, the smooth endoplasmic reticulum does not participate directly in protein translation; instead, its specialized enzymatic machinery is primarily dedicated to the synthesis of lipid substances. It plays a central role in producing cellular phospholipids, cholesterol, and various steroid hormones, which are crucial both for maintaining the dynamic structure of all cellular membranes and for driving key systemic physiological signals.

Beyond its synthetic functions, the smooth endoplasmic reticulum operates as a vital hub for cellular detoxification and specialized metabolic storage. In liver cells, its extensive network of tubules houses enzyme complexes capable of chemically modifying, inactivating, and neutralizing potential cellular toxins, metabolic waste products, and lipid-soluble drugs, converting them into forms that can be safely excreted. Furthermore, in specialized tissues like muscle where it is referred to as the sarcoplasmic reticulum the smooth endoplasmic reticulum serves as a highly regulated storage reservoir for calcium ions. By rapidly releasing and reaccumulating in calcium in response to cellular signals, it directly triggers and controls muscle contraction, demonstrating its pivotal contribution to both structural maintenance and functional response within the cell.

• Agranular endoplasmic reticulum lacks attached surface ribosomes.
• Agranular endoplasmic reticulum is called smooth reticulum.
• Smooth reticulum synthesizes various cellular lipid substances.
• Intrareticular enzymes promote smooth reticulum chemical processes.

Structure and Function of Golgi Apparatus:

• Golgi apparatus relates closely to endoplasmic reticulum.
• Membranes resemble smooth endoplasmic reticulum lipid structures.
• Composed of four stacked thin enclosed vesicles.
• Positioned near one side of the cell nucleus.
• Prominent in highly active secretory cell structures.
• Located near secretory extrusion sites in cells.
• Functions in direct association with endoplasmic reticulum.
• Transport vesicles pinch off from endoplasmic reticulum.
• Transport vesicles fuse directly with Golgi apparatus.
• Entrapped ER substances move into Golgi apparatus.
• Golgi processes transported substances into functional lysosomes.
• Golgi forms secretory vesicles and cytoplasmic components.

Lysosomes and Intracellular Digestion:

• Lysosomes form by breaking off Golgi apparatus.
• Lysosomes disperse widely throughout cellular cytoplasm medium.
• Lysosomes provide a complete intracellular digestive system.
• Lysosomes digest damaged or old cellular structure.
• Lysosomes digest food particles ingested by cells.
• Lysosomes digest unwanted matter such as bacteria.
• Lysosome diameter ranges from 250 to 750 nanometers.
• Lysosomes are surrounded by lipid bilayer membranes.
• Filled with granules measuring 5 to 8 nanometers.
• Granules represent aggregates of 40 hydrolase enzymes.
• Hydrolase enzymes split compounds by adding water.

Peroxisomes and Detoxification Functions:

• Peroxisomes physically resemble cellular lysosome vesicular structures.
• Formed by self-replication or smooth ER budding.
• Peroxisomes contain cellular oxidase enzymes instead of hydrolases.
• Oxidases combine oxygen with hydrogen ions inside cells.
• Reaction forms hydrogen peroxide within peroxisome structures.
• Hydrogen peroxide acts as a highly oxidizing chemical compound.
• Peroxisomes contain large quantities of catalase enzymes.
• Catalase works with peroxide to detoxify poisonous chemicals.
• Liver peroxisomes detoxify half of ingested alcohol.

Secretory Vesicles and Enzyme Storage:

• Secretory cells function to release specialized chemical substances.
• Endoplasmic reticulum and Golgi form secretory substances.
• Golgi releases storage vesicles into cellular cytoplasm.
• Pancreatic acinar cells contain typical secretory vesicles.
• Secretory vesicles store inactive digestive protein proenzymes.
• Proenzymes release into pancreatic ducts and duodenum.
• Intestinal tract conditions activate stored digestive proenzymes.
• Counts range from hundreds to several thousand per cell.
• Concentrated in areas requiring major energy metabolism.
• Mitochondria vary widely in physical sizes and shapes.
• Globular shapes measure a few hundred nanometers wide.
• Elongated shapes reach 1 micrometer wide and 7 long.
• Branching and filamentous mitochondrial shapes exist naturally.
• Constructed of two lipid bilayer protein membranes.
• Inner membrane infoldings form specialized internal shelves.

Cytoskeleton, Nucleus, and Nucleoli:

• Tubulin molecules polymerize to form stiff microtubules.
• Microtubules construct strong tubular cellular structures.
• Sperm flagella contain typical specialized microtubule structures.
• Microtubules form central radiating structures in cilia.
• Centrioles consist of stiff microtubule protein structures.
• Mitotic spindles of dividing cells contain microtubules.
• Microtubules act as a rigid cell cytoskeleton.

Function and Structure of the Nucleus:

• Nucleus serves as the primary cell control center.
• The nucleus contains large DNA quantities forming genes.
• Genes determine structural and functional protein traits.
• Genes control intracellular cytoplasmic and nuclear enzymes.
• Genes control and promote cellular reproduction processes.
• Genes reproduce to create two identical sets.
• Cell splits via mitosis into daughter cells.
• Interphase nucleus reveals darkly staining chromatin material.
• Chromatin resides throughout an internal fluid called nucleoplasm.
• Chromatin organizes into distinct chromosomes during mitosis.

Nuclear Membrane Properties and Pores:

• A nuclear membrane is also called a nuclear envelope.
• The envelope consists of two separate bilayer membranes.
• The outer membrane connects with the cytoplasmic endoplasmic reticulum.
• Intermembrane space connects with internal reticulum spaces.
• Thousands of nuclear pores penetrate the nuclear membrane.
• Large protein complexes attach at pore edges.
• Central pore diameters measure about 9 nanometers.
• Pores pass molecules up to 44,000 weight.

Nucleoli and Ribosome Assembly:

• Nuclei contain highly staining structures called nucleoli.
• Nucleolus lacks a surrounding limiting membrane barrier.
• Represents accumulated ribosomal RNA and protein structures.
• Nucleolus enlarges during active protein synthesis periods.
• Nucleoli and ribosome formation begin inside the nucleus.
• Chromosomal DNA genes trigger cellular RNA synthesis.
• Synthesized RNA stores partially inside central nucleoli.
• Most RNA transports outward through nuclear pores.
• Cytoplasmic RNA combines with specific cellular proteins.
• Combined components assemble into mature functional ribosomes.
• Mature ribosomes form essential cytoplasmic protein molecules.