AH Biology - Unit 1 - KA2(b) - The Synthesis and Transport of Proteins

What is the ratio between surface area and volume of eukaryotic cells, and what is a result of this?

Eukrayotic cells have a relatively small surface area to volume ratio. As a result of this, the plasma (cell) membrane is too small an area to carry out all the vital functions carried out by membranes.

What do eukaryortic cells have in order to overcome the problem of the plasma (cell) membrane being too small an area to carry out all the vital functions carried out by membranes?

To overcome the problem of the plasma (cell) membrane being too small an area to carry out all the vital functions carried out by membranes, eukaryotic cells have a system of internal membranes which increase the total area of membrane.

What are the key membraned organelles?

The key membraned organelles are:

• The endoplasmic reticulum

• Golgi apparatus

• Lysosomes

• Vesicles

What is the endoplasmic reticulum?

The endoplasmic reticulum is an organelle that forms a network of membrane tubules that are continuous with the nuclear membrane.

What does the smooth endoplasmic reticulum do, and what does it contain?

The smooth endoplasmic reticulum synthesises lipids and contains no ribosomes.

What does the rough endoplasmic reticulum do, and what does it contain?

The rough endoplasmic reticulum synthesises membrane proteins, and proteins for secretion and contains ribosomes.

What is the golgi apparatus?

The golgi apparatus is a series of flattened membrane discs. It is very important for preparing proteins for secretion. Post translational modification of these proteins takes place here.

What are lysosomes?

Lysosomes are membrane bound organelles that contain a variety of hydrolase enzymes that digest proteins, lipids, nucleic acids, and carbohydrates.

What are vesicles?

Vesicles are membraned compartments that transport materials from one membraned compartment to another.

Where are lipids and proteins synthesised?

Lipids and proteins are synthesised in the endoplasmic reticulum.

What does the rough endoplasmic reticulum have on its cystolic face?

The rough endoplasmic reticulum has ribosomes on its cystolic face and completes the synthesis of membrane proteins.

What does the smooth endoplasmic reticulum not have?

The smooth endoplasmic reticulum does not have ribosomes and synthesises lipids which are inserted into its membrane.

Where does the synthesis of all proteins begin?

The synthesis of all proteins begins in the cystolic ribosomes. Cystolic ribosomes are ribosomes found free in the cytoplasm. For proteins that remain in the cytosol (cytoplasm), the synthesis is completed in these cystolic ribosomes.

What is the process of translation for transmembrane proteins (proteins that span the membrane)?

The process of translation for transmembrane proteins (proteins that span the membrane) is:

1. Translation begins in the cystolic ribosomes.

2. During translation the transmembrane protein reaches a signal sequence on the protein. A signal sequence is a short stretch of amino acids at one end of the polypeptide that determines the eventual location of a protein in a cell.

3. The signal sequence halts translation and directs the cystolic ribosome synthesising the protein to move and dock with the endoplasmic reticulum, forming rough endoplasmic reticulum.

4. Translation continues after docking.

5. Once completed, the protein is inserted into the membrane of the endoplasmic reticulum.

What is stage 1 for the translation of transmembrane proteins?

Stage 1 for translation of transmembrane proteins is:

• Translation begins in the cystolic ribosomes.

What is stage 2 for the translation of transmembrane proteins?

Stage 2 for the translation of transmembrane proteins is:

• During translation the transmembrane protein reaches a signal sequence on the protein. A signal sequence is a short stretch of amino acids at one end of the polypeptide that determines the eventual location of a protein in a cell.

What is stage 3 for the translation of transmembrane proteins?

Stage 3 for the translation of transmembrane proteins is:

• The signal sequence halts translation and directs the cystolic ribosome synthesising the protein to move and dock with the endoplasmic reticulum, forming rough endoplasmic reticulum.

What is stage 4 for the translation of transmembrane proteins?

Stage 4 for the translation of transmembrane proteins is:

• Translation continues after docking.

What is stage 5 for the translation of transmembrane proteins?

Stage 5 for the translation of transmembrane proteins is:

• Once completed, the protein is inserted into the membrane of the endoplasmic reticulum.

What is a signal sequence?

A signal sequence is a short stretch of amino acids at one end of the polypeptide that determines the eventual location of a protein in a cell.

Once proteins are in the endoplasmic reticulum, what happens?

Once proteins are in the endoplasmic reticulum, they are transported by vesicles that bud off from the endoplasmic reticulum and fuse with the golgi apparatus.

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What is the process of movement of proteins between membranes?

The process of movement of proteins between membranes is:

1. Once the proteins are in the endoplasmic reticulum, they are transported by vesicles that bud off from the endoplasmic reticulum and fuse with the golgi apparatus.

2. Once in the golgi apparatus, the proteins move from one flattened disc to the next in vesicles that bud off from one disc and fuse to the next one in the stack.

3. During this time, post-translational modification takes place in the golgi apparatus. One of the post-translational modifications that takes place here is the addition of a carbohydrate group to them. This is controlled by enzymes that catalyse the addition of various sugars in multiple steps to form the carbohydrates.

4. After this, the vesicles leave the golgi apparatus and take these proteins to the plasma membrane, lysosomes, and other membranes in the cell.

5. Vesicles move along microtubules to these other membranes and fuse with them.

What is stage 1 of the movement of proteins between membranes?

Stage 1 of the movement of proteins between membranes is “Once the proteins are in the endoplasmic reticulum, they are transported by vesicles that bud off from the endoplasmic reticulum and fuse with the golgi apparatus.”

What is stage 2 of the movement of proteins between membranes?

Stage 2 of the movement of proteins between membranes is “Once in the golgi apparatus, the proteins move from one flattened disc to the next in vesicles that bud off from one disc and fuse to the next one in the stack.”

What is stage 3 of the movement of proteins between membranes?

Stage 3 of the movement of proteins between membranes is “During this time, post-translational modification takes place in the golgi apparatus. One of the post-translational modifications that takes place here is the addition of a carbohydrate group to them. This is controlled by enzymes that catalyse the addition of various sugars in multiple steps to form the carbohydrates.”

What is stage 4 of the movement of proteins between membranes?

Stage 4 of the movement of proteins between membranes is “After this, the vesicles leave the golgi apparatus and take these proteins to the plasma membrane, lysosomes, and other membranes in the cell.”

What is stage 5 of the movement of proteins between membranes?

Stage 5 of the movement of proteins between membranes is “Vesicles move along microtubules to these other membranes and fuse with them.”

What will some proteins made inside the cell be?

Some proteins made inside the cell will be secreted by the cell. For example, peptide hormones and digestive hormones.

What is the process of the secretory pathway?

The process of the secretory pathway is:

1. The proteins made inside the cell are translated in ribosomes attached to the rough endoplasmic reticulum.

2. They enter the lumen.

3. Vesicles bud off and transport proteins to the golgi apparatus.

4. The vesicles fuse with the golgi apparatus and release proteins inside the golgi apparatus.

5. As they pass through the golgi apparatus discs, post-translational modification takes place.

6. They are then packaged into secretory vesicles.

7. The vesicles move to the plasma membrane along microtubules.

8. They fuse with the plasma membrane and release the proteins out of the cell.

What is stage 1 of the secretory pathway?

Stage 1 of the secretory pathway is “The proteins made inside the cell are translated in ribosomes attached to the rough endoplasmic reticulum.”

What is stage 2 of the secretory pathway?

Stage 2 of the secretory pathway is “They enter the lumen”

What is stage 3 of the secretory pathway?

Stage 3 of the secretory pathway is “Vesicles bud off and transport proteins to the golgi apparatus.”

What is stage 4 of the secretory pathway?

Stage 4 of the secretory pathway is “The vesicles fuse with the golgi apparatus and release proteins inside the golgi apparatus.”

What is stage 5 of the secretory pathway?

Stage 5 of the secretory pathway is “As they pass through the golgi apparatus discs, post-translational modification takes place.”

What is stage 6 of the secretory pathway?

Stage 6 of the secretory pathway is “They are then packaged into secretory vesicles”

What is stage 7 of the secretory pathway?

Stage 7 of the secretory pathway is “The vesicles move to the plasma membrane along microtubules.”

What is stage 8 of the secretory pathway?

Stage 8 of the secretory pathway is “They fuse with the plasma membrane and release the proteins out of the cell.”

What are many secreted proteins synthesised as?

Many secreted proteins are synthesised as inactive precursors and require proteolytic cleavage to produce active proteins.

What is proteolytic cleavage and where is this process common?

Proteolytic cleavage is a type of post-translational modification. This process is common in digestive enzymes, many of which must be produced as inactive precursors to stop them from digesting the cells that produce them.