ER and Secretory Pathway

Question 1

Describe the structure of the endoplasmic reticulum (ER)

Answer 1

• ER is a network of membranous tubules and sacs called cisternae that account for more than half of the total membrane of eukaryotic cells.
• Cisternae are interconnected by a single continuous membrane called the cisternae space or lumen that can take up to more than 10 percent of the total volume of the cell.

*The ER can be divided into rough ER and smooth ER

Question 2

What is the structure of the rough endoplasmic reticulum?

Answer 2

• The rough ER is covered with ribosomes on the cytosolic side of the continuous, flattened sacs giving the rough appearance when viewed by EM.
• The lumen of rough ER is connected to the nuclear envelope, the double membrane that surrounds the cell’s nucleus.

Question 3

What is the function of the rough ER

Answer 3

• The main function of the rough ER is to serve as a port of entry for secretory, lysosomal, and integral membrane proteins.
• The rough ER functions as the location of protein synthesis for excreted, sorted, or transmembrane proteins.
• It is also the location of many post-translational modifications of proteins

Question 4

What is the function of the smooth ER?

Answer 4

The Smooth ER consists of tubules and vesicles that branch to allow increased surface area for the action or storage of key enzymes and the enzyme products. It is involved in the synthesis of lipids and the metabolism of carbohydrates. It contains enzymes for detoxification. It plays a role in the glucose release from the liver and the storage of calcium ions in muscle cells.

Question 5

Describe the process of lipid synthesis in animals

Answer 5

• In animals, fatty acids are synthesized in the Cytosol by the fatty acid synthase complex (C16 or C18).
• Elongation, modification, and desaturation as well as lipid synthesis are catalyzed by enzymes localized in the ER.
• FA are delivered to the ER by a Fatty acid transport protein (FATP).

Question 6

Describe the process of lipid synthesis in plants

Answer 6

• In plants, fatty acids are synthesized in the chloroplast stroma.
• Animals are unable to synthesize polyunsaturated fatty acids. They lack the fatty acid desaturases necessary to convert oleic acid (18:1n-9) into linoleic acid (18:2n-6) and α-linolenic acid

Question 7

What is the purpose of the Sarcoplasmic Reticulum (SR)?

Answer 7

• The Sarcoplasmic reticulum (SR) is a subset of the smooth ER.
• In muscle cells, the SR tubules serve as a storage of calcium which is released as one step in the contraction process of muscle.

*Calcium pumps serve to move the calcium.

Question 8

What two types of proteins are synthesized into the ER?

Answer 8

1. Soluble proteins
2. Transmembrane proteins

Question 9

Describe the process of protein folding

Answer 9

• Protein folding begins while the protein is being translated by the ribosome (Co-translational folding). Chaperones mediate proper folding and assembly
• Disulfide-bond formation is catalyzed by PDI, a protein disulfide isomerase

Question 10

What types of modifications can be made to proteins?

Answer 10

• Proteins can be post-translationally modified: glycosylation (N or O-linked) or lipidation (N-myristoylation, S-prenylation, S-acylation, GPI anchor) or phosphorylation.
• Acylation occurs mostly in the Golgi