لخّصلي

Structure of the Endoplasmic Reticulum endoplasmic reticulum network of interconnected membranes Figure 1: ER morphology - diagram: the endoplasmic reticulum is shown as a complex interconnected membrane system.For metabolism and detoxification of drugs, toxins, and carcinogens, the lumen will likely contain enzymes such as cytochrome P450s (CYPs) When engaged in calcium ion regulation, calcium ions are stored in the lumen of the endoplasmic reticulum (specifically referred to as the sarcoplasmic reticulum)However, unlike the nucleus, chloroplasts, and mitochondria which have two lipid bilayer membranes (resulting in distinct outer and inner membranes), the endoplasmic reticulum has only one lipid bilayer membrane, similar to the plasma membrane.Nevertheless, the luminal contents vary depending on what is being synthesized (protein or lipid) or carried out (e.g., whether calcium ion regulation or drug/toxin detoxification).For lipid syntheses, such as phospholipids (membrane lipids), phosphatidylcholine, and phosphatidylethanolamine, the lumen will likely contain greater amounts of enzymes involved in the synthesis of such lipids.Similar to other biological membranes, the membrane of the endoplasmic reticulum is a lipid bilayer, which means it has two layers of phospholipids, with various membrane proteins and carbohydrates.When engaged in protein synthesis, the lumen will contain enzymes and chaperone proteins for protein folding, modifications, and initial stages of transport.Image Credit: OpenStax.

Structure of the Endoplasmic Reticulum
endoplasmic reticulum network of interconnected membranes
Figure 1: ER morphology – diagram: the endoplasmic reticulum is shown as a complex interconnected membrane system. Image Credit: eLifeSciences.org.
What is the endoplasmic reticulum? What does it look like and what is it made up of?
The endoplasmic reticulum or ER is one of the most prominent organelles in the cytoplasm of a eukaryotic cell. This organelle occurs as an interconnected network of flattened sacs and tubules. The flattened sacs that are stacked on each other are called cisternae.

Now that we know the ER is a membrane-bound organelle, the next question would is this:

Is the endoplasmic reticulum single- or double-membraned?
The endoplasmic reticulum membrane is a single membrane connected to the outer nuclear envelope (i.e., the membrane of the nucleus). Similar to other biological membranes, the membrane of the endoplasmic reticulum is a lipid bilayer, which means it has two layers of phospholipids, with various membrane proteins and carbohydrates. However, unlike the nucleus, chloroplasts, and mitochondria which have two lipid bilayer membranes (resulting in distinct outer and inner membranes), the endoplasmic reticulum has only one lipid bilayer membrane, similar to the plasma membrane.

fluid mosaic model of cell membrane
Figure 2: The plasma membrane, a typical biological membrane consisting of a lipid bilayer, cholesterol, carbohydrates, membrane proteins, and cholesterol in the case of animal cells. Image Credit: OpenStax.
Are ribosomes part of the endoplasmic reticulum?
No, ribosomes are cellular structures with distinct characteristics and functions. They may be seen attached to the surface of the endoplasmic reticulum where they mark the sites (locations) of protein synthesis.

What is in the lumen of the endoplasmic reticulum?
The lumen of the endoplasmic reticulum contains various enzymes and molecular chaperones. Nevertheless, the luminal contents vary depending on what is being synthesized (protein or lipid) or carried out (e.g., whether calcium ion regulation or drug/toxin detoxification).

When engaged in protein synthesis, the lumen will contain enzymes and chaperone proteins for protein folding, modifications, and initial stages of transport.
For lipid syntheses, such as phospholipids (membrane lipids), phosphatidylcholine, and phosphatidylethanolamine, the lumen will likely contain greater amounts of enzymes involved in the synthesis of such lipids. To name a few, such enzymes include acyl-CoA synthetases, acyltransferases, phosphatidate phosphatase, diacylglycerol acyltransferase, and phosphatidylserine synthase.
For metabolism and detoxification of drugs, toxins, and carcinogens, the lumen will likely contain enzymes such as cytochrome P450s (CYPs)
When engaged in calcium ion regulation, calcium ions are stored in the lumen of the endoplasmic reticulum (specifically referred to as the sarcoplasmic reticulum)