Endoplasmic Reticulum
The endoplasmic reticulum (ER) is an extensive network of interconnected membrane-bound tubules and flattened sacs (cisternae) that extends throughout the cytoplasm of eukaryotic cells and is continuous with the outer nuclear envelope. It exists in two forms: rough ER (RER), which is studded with ribosomes and specializes in the synthesis, folding, and quality control of proteins destined for secretion or membrane insertion; and smooth ER (SER), which lacks ribosomes and is involved in lipid synthesis, calcium storage, and detoxification of drugs and toxins. The ER works closely with the Golgi apparatus to process and sort proteins and lipids to their correct cellular destinations.
Rough ER vs Smooth ER — Structure and Function Comparison
| Feature | Rough ER | Smooth ER |
|---|---|---|
| Ribosomes | Present on membrane surface | Absent |
| Appearance | Rough/granular under microscope | Smooth under microscope |
| Primary function | Protein synthesis and folding | Lipid synthesis, detoxification, calcium storage |
| Products | Secretory proteins, membrane proteins | Steroids, phospholipids, fatty acids |
| Abundance in | Pancreatic cells, plasma B cells | Liver cells, muscle cells, steroid-producing cells |
Interactive Tools
Wikimedia Commons, CC BY-SA
Related Terms
Ribosome
Ribosomes are small, dense ribonucleoprotein complexes found in all living cells that carry out protein synthesis (translation) by decoding messenger RNA (mRNA) sequences into polypeptide chains. Each ribosome consists of two subunits — a large subunit and a small subunit — composed of ribosomal RNA (rRNA) and proteins, with prokaryotic ribosomes being 70S and eukaryotic cytoplasmic ribosomes being 80S. Ribosomes can be found free in the cytoplasm, where they synthesize cytosolic proteins, or bound to the rough endoplasmic reticulum, where they produce proteins destined for secretion or membrane insertion.
Golgi Apparatus
The Golgi apparatus is a membrane-bound organelle in eukaryotic cells consisting of a series of flattened, stacked membranous sacs (cisternae) that receive proteins and lipids from the endoplasmic reticulum and process, modify, package, and sort them for delivery to their final destinations within or outside the cell. Often called the "post office" or "processing plant" of the cell, it modifies molecules through glycosylation, phosphorylation, and sulfation before packaging them into transport vesicles. It was discovered by Italian physician Camillo Golgi in 1898, for which he shared the Nobel Prize in Physiology or Medicine in 1906.
Cell Membrane
The cell membrane, also called the plasma membrane, is a thin, flexible bilayer of phospholipid molecules embedded with proteins that surrounds every cell and forms its outermost boundary. It is selectively permeable, meaning it regulates the passage of ions, nutrients, and waste products into and out of the cell through passive diffusion, facilitated diffusion, and active transport. The fluid mosaic model, proposed by Singer and Nicolson in 1972, describes its structure as a dynamic, two-dimensional fluid in which proteins float within or are attached to the phospholipid bilayer.
From Greek "endon" meaning within, Latin "plasma" meaning something molded, and Latin "reticulum" meaning little net, referring to its net-like appearance inside the cell. The structure was first observed and named by Keith Porter, Albert Claude, and Ernest Fullam in 1945 using electron microscopy.