Topic 3: Protein transport

Question 1

Describe protein synthesis + protein transport

7 steps

Answer 1

1. DNA contains gene for making a protein
2. Transcription in nucleus forms mRNA
3. Translation on ribosome forms polypeptide
4. Polypeptide chain (primary structure) released
5. Polypeptide folds into 3D shape in rough ER lumen - protein is made
6. Protein is modified in the Golgi body
7. Protein transported through cytoplasm (then out of cell etc. if it is an extracellular enzyme)

Question 2

Define extracellular enzyme (2 points)

Answer 2

1. enzyme made on ribosomes inside the cell,
2. but which catalyses a chemical reaction outside the cell

e.g.
thrombin (in the blood)
amylase (in the mouth and small intestine)
pepsin (in the stomach)

Question 3

Key Summary: Explain the role of ER in {protein transport/forming extracellular enzymes}
(3 steps)

Answer 3

1. Polypeptide chain made on ribosomes passes through cisternae membrane and enters lumen of rough ER
2. POLYPEPTIDE FOLDING into 3D {shape/structure} in ER lumen (can describe this e.g. tertiary, globular structure + named bonds if an enzyme)
3. PACKAGES PROTEINS INTO AN ER VESICLE which pinches off ER and moves through cytoplasm

Question 4

Key Summary: Role of Golgi {body/apparatus} in {protein transport/forming extracellular enzymes}
(6 points)

Answer 4

1. MODIFIES PROTEINS
   eg. by adding carbohydrate if forming a glycoprotein
2. PACKAGES MODIFIED PROTEIN MOLECULES INTO GOLGI VESICLE which buds off the Golgi
3. • called a LYSOSOME if it contains {digestive/HYDROLYTIC} enzymes
   • called a SECRETORY vesicle if protein is to be secreted out of the cell
4. Some proteins LEAVE the cell by EXOCYTOSIS (vesicle fuses with the cell surface membrane) eg. DIGESTIVE ENZYMES
5. Some proteins remain in the cell
6. Some proteins become incorporated INTO the cell surface membrane (proteins become incorporated into the vesicle membrane first, then membranes fuse) eg. CHANNEL PROTEINS, GLYCOPROTEINS

Question 5

Key Summary: Describe protein (or named protein) {transport/route} taken by amino acids through the cell
(6 steps, 3 varieties of 6th step)

Answer 5

1. Amino acids incorporated into polypeptide chain as polypeptide made on ribosome attached to rough ER
2. Polypeptide passes through cisternae membrane and into LUMEN of rough ER where protein FOLDS INTO 3D SHAPE
3. ER packages proteins into vesicles which transport protein to Golgi body, where vesicle fuses with GOLGI BODY MEMBRANE to become part of the Golgi apparatus
4. Protein enters the Golgi body and is modified as protein moves through INSIDE THE GOLGI BODY eg. carbohydrate added to form glycoprotein (read question - if relevant)
5. Modified protein packaged into Golgi SECRETORY vesicle
6. Depending on the exam question, one of the 3 below will apply

IF THE PROTEIN IS TO BE {exported/SECRETED} FROM THE CELL (eg. EXTRACELLULAR ENZYMES and hormones) the Golgi secretory vesicle MOVES TO the cell surface membrane, FUSES WITH the membrane and protein molecules exit the cell by EXOCYTOSIS

OR

IF THE PROTEIN IS TO BECOME EMBEDDED IN THE CELL SURFACE MEMBRANE (eg. GLYCOPROTEIN), it first becomes incorporated into the Golgi vesicle membrane, then when the Golgi vesicle fuses with the cell surface membrane the protein becomes part of the cell surface membrane

OR

Some proteins (eg. intracellular enzymes), are delivered to specific sites WITHIN THE CELL - VESICLE MOVES THROUGH THE CELL CYTOPLASM, TRANSPORTING THE PROTEIN