Membranes_Protein Trafficking

In the fluid mosaic model of membranes, both the phospholipid and proteins can move transversely and laterally.

• False

• True

Answer explanation

Correct ans:

In the fluid mosaic model of membranes, both the phospholipid and proteins can move laterally (sideways) within the phospholipid layer while phospholipid molecules are able to move transversely (180 deg) between the phospholipid layers.

Transmembrane proteins in cell surface membrane are involved in regulating fluidity and transport of substances across the cell surface membrane.

• False

• True

Answer explanation

Correct ans:

Transmembrane proteins have no role in membrane fluidity. Cholesterol molecules are the main one playing a role in regulating fluidity at different temperature.  At low temperature, cholesterol disrupt the close packing of phospholipid molecules but at higher temperature, cholesterol reduce the movements of phospholipid molecules.

 

Phospholipid with saturated or unsaturated hydrocarbon chain (with kinks) contribute to membrane fluidity but do not play a role in regulation of membrane fluidity.  The kinks (due to unsaturated hydrocarbon chain in phospholipid) prevent close packing of phospholipid molecules, hence membrane with more phospholipid having unsaturated hydrocarbon chains is more fluid, regardless of temperature change.

All membrane-bound proteins are transport proteins.

• False

• True

Answer explanation

Correct ans:

Membrane bound proteins can be both integral and peripheral.

 

Only integral transmembrane CHANNEL PROTEINS, CARRIER PROTEINS and PROTEIN PUMPS can transport substances across the membrane.

 

Several integral and peripheral proteins function to relay signals or catalyse enzymatic reactions at the membranes etc which are not transport functions.

Cholesterol in the membrane helps cells to bind together.

• False

• True

Answer explanation

Correct ans:

Cholesterol in the membrane helps to regulate membrane fluidity.

Carbohydrate chains of both proteins and lipids (glycoproteins and glycolipids) of the membrane helps cells to bind together. 

Proteins and glucose can cross membranes by facilitated diffusion.

• False

• True

Answer explanation

Correct ans:

Proteins cross membranes via exocytosis/endocytosis (bulk transport) due to their large molecular size.

 

Glucose can cross membranes via facilitated diffusion as glucose molecules are polar molecules that cannot pass through the hydrophobic core of the membrane.  Glucose molecules require glucose transporter channel proteins to enter/exit the cell.

Water can only cross the membrane through channel proteins as the water molecules are polar.

• False

• True

Answer explanation

Correct ans:

Some water molecules can also cross the membrane directly via osmosis due to their small size and because the membrane is fluid.

 

Majority of the water molecules pass through the membrane via aquaporin (channel protein) which provide a water-filled hydrophilic channel for polar water molecules to enter or leave the cells.  This is facilitated diffusion because water move from high to low water potential via a (channel) protein.

Active transport process transports material across a cell surface membrane against a concentration gradient.

• False

• True

Answer explanation

Note that active transport process require energy, usually obtained from hydrolysis of ATP to form ADP & inorganic P & energy is released.

Facilitated diffusion and active transport require energy while diffusion and osmosis do not.

• False

• True

Answer explanation

Correct ans:

Active transport requires energy obtained from ATP hydrolysis and move molecules against a concentration gradient. 

 

Facilitated diffusion, diffusion and osmosis do not require energy and move molecules down a concentration gradient. 

Secretory vesicles, Golgi body and mitochondria have membranes that can become part of the cell surface membrane.

• False

• True

Answer explanation

Correct ans:

Golgi body forms secretory vesicles.  Membrane of secretory vesicles fuse with cell surface membrane via exocytosis and thus secretory vesicles’ membranes become incorporated into the cell surface membrane directly. 

 

Golgi body membrane become part of Golgi vesicle’s membrane when the vesicle bud off the trans face of Golgi body.  Cannot state the Golgi body membrane become part of the cell surface membrane as Golgi vesicles can be secretory vesicles (whose membrane become part of the cell surface membrane) and lysosomes (whose membrane does not become part of the cell surface membranes but can become membrane of endosome when lysosome fused with endosome to release lysosome’s hydrolytic enzymes).

 

Mitochondria do not contribute to part of the cell surface membrane. Mitochondria produce ATP which upon hydrolysis releasing energy.  Energy released can be used in many activities e.g the protein trafficking process.

Secretory vesicles containing the modified proteins bud off from the rough endoplasmic reticulum and move towards the Golgi body.

• False

• True

Answer explanation

Correct ans: 

Transport vesicles containing the modified proteins bud off from the rough endoplasmic reticulum and move towards the cis face of the Golgi body.