Make Connections: The Plasma Membrane and Phospholipid Structure

Because the C-H bonds in the fatty acid tails are relatively nonpolar, the phospholipid tails are excluded from water, meaning they are what?

Hydrophobic

Is cholesterol hydrophobic or hydrophilic?

Hydrophobic

What type of molecules do not cross the plasma membrane easily?

- Polar molecules and ions are hydrophilic, so they cannot very easily cross the hydrophobic portion of the plasma membrane (formed by the phospholipid tails). - - Water is an unusual molecule because, despite the fact that it is polar, it is small enough to pass directly through the hydrophobic interior of the lipid bilayer, albeit slowly. - Polar molecules and ions generally cross the plasma membrane with the help of transport proteins. For example, water crosses the bilayer rapidly via transport proteins called aquaporins. - Larger Molecules*

What is specific to just endocytosis (and not exocytosis)?

- forms vesicles from inward folding of the plasma membrane - decreases the surface area of the plasma membrane In endocytosis, substances are taken into the cell by folding in of the plasma membrane and pinching off of the membrane to form a vesicle.

What do endocytosis and exocytosis have in common?

- requires cellular energy - transported substances never physically cross the plasma membrane - They all use vesicles to transport substances from one structure to another Notice that in both exocytosis and endocytosis, the transported substances never actually cross the plasma membrane as they leave or enter the cell.

What is specific to just exocytosis (and not endocytosis)?

- secretes large molecules out of the cell - requires fusion of vesicles with the plasma membrane -increases the surface area of the plasma membrane In exocytosis, substances are transported to the plasma membrane in vesicles derived from the endomembrane system. These vesicles fuse with the plasma membrane, releasing the enclosed substances outside the cell.

Where is cholesterol found in the plasma membrane?

Among the hydrophobic tails

A phospholipid has two "tails" made up of what two molecules, which consist of a carboxyl group with a long hydrocarbon chain attached?

Fatty Acid

Because the phosphate group and its attachments are either charged or polar, the phospholipid head has an affinity for water meaning it is what?

Hydrophilic

Is the cytoplasm hydrophobic or hydrophilic?

Hydrophilic

Is the extracellular fluid hydrophobic or hydrophilic?

Hydrophilic

What is a key feature of the plasma membrane?

It is semi-permeable

A phospholipid has a "head" made up of a glycerol molecule attached to what other small molecule?

Phosphate Group

Do polar/charged or nonpolar/uncharged substances have a hard time crossing the plasma membrane?

Polar/Charged Ions

What is the function of a sodium-potassium pump?

The function of the sodium-potassium pump in animal cells (and the proton pump in bacteria and plant cells) prevents chemical and electrical gradients across the plasma membrane from reaching equilibrium (at which point the cell would be dead) and powers many types of active transport across the plasma membrane.

Describe a phospholipid head

The hydrophilic head includes a glycerol molecule attached to a single phosphate group, which is then attached to another small molecule. The phosphate group and its attachments are either polar or charged. Water molecules are polar and therefore are attracted to these charged regions of the phospholipid head.

Describe a phospholipid tail

The hydrophobic tails are two fatty acids attached to the glycerol molecule of the head. Water molecules are not attracted to the tails because C-H bonds are relatively nonpolar and therefore do not result in charged regions in the tails.

What do phospholipids form?

The main fabric of the plasma membrane

What do phospholipids do in an aqueous solution?

They will self-assemble into a double layer (bilayer) that resembles the bilayer of the plasma membrane. This self-assembly occurs because phospholipids are hydrophilic at one end (the phospholipid head) and hydrophobic at the other end (the phospholipid tails).