Unit 1 Part 2b (Synthesis And Transport Of Proteins)

Question 1

(I) Why do eukaryotic cells have a system of internal membranes?

Answer 1

The plasma membrane is too small an area to carry out all the vital functions carried out by membranes

Question 2

(I)What does the endoplasmic reticulum form?

Answer 2

A network of membrane tubules continuous with the nuclear membrane

Question 3

(I) What is the Golgi apparatus?

Answer 3

Series of flattened membrane discs

Question 4

(I) What are lysosomes?

Answer 4

Membrane-bound organelles containing a variety of hydrolases that digest proteins, lipids, nucleic acids and carbohydrates

Question 5

(II) Where are lipids synthesised?

Answer 5

The smooth endoplasmic reticulum and inserted into its membrane

Question 6

(II) Where are proteins synthesised?

Answer 6

Synthesis of proteins begins in cytosolic ribosomes

Question 7

(II) Describe the synthesis of cytosolic proteins?

Answer 7

Synthesis is completed in cytosolic ribosomes
Transmembrane proteins carry a signal sequence (short stretch of amino acids at one end of the polypeptide that determines the eventual location of the protein). This halts translation and directs the ribosome synthesising the protein to dock with the ER, forming the RER.
Translation continues after docking, and the protein is inserted into the membrane of the ER.

Question 8

(I) What do vesicles do?

Answer 8

Transport materials between membrane compartments

Question 9

(III) What happens to proteins once they’re in the ER?

Answer 9

They are transported by vesicles that bud off from the ER and fuse with the Golgi apparatus.

Question 10

(III) Describe how proteins move through the Golgi apparatus?

Answer 10

Molecules move through the Golgi discs in vesicles that bud off from one disc and fuse to the next one in the stack.
During this they undergo post-translational modification: enzymes catalyse the addition of various sugars in multiple steps to form the carbohydrates.

Question 11

(III) What happens to vesicles that leave the Golgi apparatus?

Answer 11

They take proteins to the plasma membrane and lysosomes.
They move along microtubules to other membranes and fuse with them within the cell.

Question 12

(IV) Describe the secretory pathway?

Answer 12

1. Secreted proteins are translated in ribosomes on the RER and enter its lumen.
2. The proteins move through the Golgi apparatus and are then packaged into secretory vesicles.
3. These vesicles move to and fuse with the plasma membrane, releasing the proteins out of the cell.
4. Many secreted proteins (such as digestive enzymes) are synthesised as inactive precursors and require proteolytic cleavage to produce active proteins.