Plasma Membrane

What is the general function of the plasma membrane?

To serve as a flexible boundary containing embedded proteins that act as transporters and/or channels to allow the passage of specific molecules between the extracellular space and the cytoplasm.

What is the structure of the plasma membrane?

An amphipathic^9.1, flexible/self-healing bilayer composed of phospholipids with hydrophilic, water-loving heads (phosphates) and hydrophobic, water-fearing tails (lipids).

Hydrophobic lipid tails, aka hydrocarbon tails, are typically fatty acids that have varying lengths and a ``kink'' from a cis-double bond. Because of the amphipathic nature of fatty acids, they form micelles around hydrophobic substances, isolating them from hydrophilic environments.

Proteins embedded in the plasma membrane function as transporters or channels for specific molecules, serve as adhesion molecules between cells, and function as receptors to bind extracellular molecules (See Figure 8.1 below).

Figure 8.1: The phospholipid bilayer.

What is the fluid mosaic model?

An accepted model of cell membrane composition whereby phospholipids and embedded proteins exist in a two-dimensional fluid plane that constantly glides over each other.

The ÒfluidityÓ of the cell's membrane is based on several factors:

• Shorter fatty acid chains allow for less interaction (steric interaction) between fatty acid chains and make the cell membrane more fluid.
• Presence of ``kinks'' secondary to $cis$-double bonds allows for less interaction and increases the fluidity of the cell membrane.
• Presence of cholesterol which decreases mobility of CH$_{2}$ on surrounding fatty acids, thereby making the membrane more rigid and less fluid.

Table 8.1: Determinants of plasma membrane fluidity.

	Increased Fluidity	Decreased Fluidity
Fatty acid tail length	Shorter	Longer
Number of cis-double bonds	Higher	Lower
Concentration of cell membrane cholesterol	Lower	Higher

What are five processes by which substances move across cell membranes?

1. Simple diffusion: the movement of dissolved particles from regions of higher concentration to areas of lower concentration (see Figure 8.2 below).

   Figure 8.2: Simple diffusion.

   

2. Facilitated diffusion: the movement of a substance that is coupled with a carrier molecule from a region of higher concentration to an area of lower concentration (see Figure 8.3 below).

   Figure 8.3: Facilitated diffusion.

   

3. Active transport: the movement of a substance against a concentration gradient, e.g. maintenance of membrane potentials in neurons (see Figure 8.4 on page ).
   • Energy is required to overcome the concentration gradient.

   Figure 8.4: Active transport.

   

4. Exocytosis: process by which a cell releases intracellular substances by intracellular vesicle fusion with the cell membrane, e.g. neurotransmitter release at pre-synaptic terminals of nerve axons (see Figure 8.5 on page ).
5. Endocytosis: process by which a cell ingests extracellular substances through cell membrane involutions and vesicle formation (see Figure 8.5 on page ). This occurs in two forms:
   • Pinocytosis (ingestion of small particles and fluids).
   • Phagocytosis (ingestion of large particles).

   Figure 8.5: Exocytosis and endocytosis.

   

Table 8.2: Five processes by which substances move across cell membranes.

	Particle Movement	Carrier Protein	Energy Required
Simple	High to low	No	No
Diffusion
Facilitated	High to low	Yes	No
Diffusion
Active	Low to high	Yes	Yes
Transport
Exocytosis	Intracellularly to extracellularly	No, occurs by fusion
		of vesicles with
		cell membrane.
Endocytosis	Extracellularly to intracellularly	No, occurs by involution
		of cell membrane.

What is osmosis?

The net movement of water from regions of lower solute concentration to areas of higher solute concentration which occurs when:

• a solute gradient exists across a membrane
• the solute cannot pass through the membrane

If the membrane is permeable to water, then the movement of water will compensate and rectify the disequilibrium of solute concentrations (see Figure 34.8 below):

Figure 8.6: Flow of water through a semipermeable membrane to balance solute concentrations: Before (left) and after (right) equilibration.

What is tonicity and how does it effect osmosis?

Tonicity refers to the amount of solute particles in a solution.

Therefore, a hypertonic solution outside of a cell is one in which the extracellular environment has a higher solute concentration than the intracellular environment. Conversely, a hypotonic solution outside of a cell is one in which the extracellular environment has a lower solute concentration than the intracellular environment. In an isotonic solution the extracellular environment has the same solute concentration as the intracellular environment and there is no net water flow.

Based on this and using the rules of osmosis, in a hypertonic environment water will flow out of the cell and in a hypotonic environment, water flow will into the cell (see Figure below).

Figure 8.7: In a hyperosmotic environment (top), water will flow out of the cell and the cell will shrink. In a hypoosmotic environment (bottom), water will flow into the cell.

What are membrane channels?

Proteins which extend across the lipid bilayer with unique polypeptide structures that typically come in two forms:

• Single pass $\alpha$ helices, responsible for transporting ions and small water soluble molecules
• Multiple pass $\beta$ sheets, responsible for $\beta$-barrels such as porins

What is the sodium-potassium (Na$^{+}$/K$^{+}$) pump?

A pump which swaps 3 Na$^{+}$ out of the cell with 2 K$^{+}$ into the cell, i.e. an antiport, in the setting of a strong electrochemical gradient for both ions.

Think ``Na$^{+}$-out my K$^{+}$-in'' pronounced ``Nought my kin''

As with other pumps that overcome electrochemical gradients, this requires energy by breaking down adenosine triphosphate (ATP) into adenosine diphosphate (ADP). Conversely, if the pump works in reverse, i.e. allows Na$^{+}$ into the cell and K$^{+}$ out of the cell, ADP is converted into ATP and the pump becomes a Òenergy-currencyÓ creating machine.

Because there are 3 Na$^{+}$ pumped for every 2 K$^{+}$, the pump creates an electrical imbalance and contributes to a membrane's electrical charge (membrane potential).

What is the role of the plasma membrane in maintaining membrane potentials?

Cells which have electrically excitable membranes, including neurons and myocytes, contain voltage-gated ion channels in their plasma membranes which control the flow of cations.

Na$^{+}$ and K$^{+}$ voltage-gated cation channels are the main ion channels found in neurons. Voltage-gated Ca$^{2+}$ cation channels are also found in myocytes.

What are three plasma membrane structures that allow for cell-cell communication and/or cellular adhesion?

1. Gap junctions, a continuous aqueous channel which connects two intracellular compartments and allows for cell-cell communication, e.g. neurotransmitter-gated ion channels (see Figure 8.8(a) on page ).
2. Desmosomes, button-like points that rivet cells together and anchor cells together (see Figure 8.8(b) on page ).
3. Tight junctions, a sealing attachment between cells that makes them ``water-proof'' and prevents the passage of almost anything, e.g. the junctions that connect epithelial cells in the intestine to maintain a barrier against the contents of the lumen of the small intestine (see Figure 8.8(c) on page ).

Figure 8.8: Three major types of cell junctions: (a) Gap junctions, (b) desmosomes and (c) tight junctions.