Synthesis of Lipids and Proteins

Question 1

Two parts of the Cystoplasm

Answer 1

Cytosol - liquid component

Ribsomes and the membrane-bound organelles (ER, Golgi apparatus, mitochondria and chloroplast) which are suspended in the cystosol

Question 2

Synthesis of proteins

Answer 2

The synthesis of all proteins begins in

Cytosolic Ribosomes

Question 3

Endoplasmic Reticulum

Answer 3

Lipids and proteins are synthesised in the ER

Rough ER (RER) has ribosomes on its
cytosolic face 

Smooth ER (SER) lacks ribosomes.

Question 4

Synthesis of Lipids

Oils, Phospholipids, Steroid Hormones

Answer 4

Lipids are synthesised in the smooth
endoplasmic reticulum (SER) and inserted
into its membrane

Question 5

Synthesis of Cytosolic Proteins

Answer 5

The synthesis of cytosolic proteins is completed at cytosolic ribosomes, and these proteins remain in the cytosol

Question 6

Synthesis of Transmembrane Proteins

Answer 6

Synthesis of Transmembrane Proteins begins at cytosolic ribosomes but is not completed there.

Transmembrane proteins carry a signal
sequence, which halts translation and directs
the ribosome synthesising the protein to dock
with the ER, forming RER.

Translation continues after docking, and the
protein is inserted into the membrane of the
ER.

The ribosome is released back into the cytosol once translation is completed.

Question 7

Signal Sequence

Answer 7

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.

Question 8

Synthesis of other proteins

Answer 8

Proteins that are destined for other areas are synthesised entirely by cytosolic ribosomes and have other signal sequences which are used to direct them to their correct organelle.

Question 9

Proteins destined to be lysosome enzyme or a secreted protein

Answer 9

Proteins that are destined to be lysosome enzymes or a secreted protein are synthesised entirely by cytosolic ribosomes and have other signal sequences which are used to take the ribosome to the the RER

They are released directly into the lumen of the RER instead of being inserted into the membrane

Question 10

Movement of proteins between

membranes

Answer 10

Once the proteins are in the ER, they are transported by vesicles that bud off from the ER and fuse with the Golgi apparatus

As proteins move through the Golgi apparatus they undergo post-translational modification (The addition of carbohydrate groups is the major modification)

Molecules move through the Golgi discs in vesicles that bud off from one disc and fuse to the next one in the stack. Enzymes catalyse the addition of various sugars in multiple steps to form the carbohydrates

Vesicles that leave the Golgi apparatus take
proteins to the plasma membrane and
lysosomes

Vesicles move along microtubules to other
membranes and fuse with them within the
cell

Question 11

The Secretory Pathway

Answer 11

Secreted proteins (Peptide hormones and digestive enzymes) are translated in ribosomeson the RER and enter its lumen.

These vesicles move to and fuse with the plasma membrane, releasing the proteins out of the cell

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

(Proteolytic cleavage is another type of posttranslational modification. Digestive enzymes are one example of secreted proteins that require proteolytic cleavage to become active.)