Daily Newsletter

September 4, 2012 The Phospholipid Bilayer & Passive Transport

The basic structure of the cellular membrane is composed of Phospholipids. You will recall the introduction to phospholipids found in Daily Newsletter: August 29, 2012 - Carbohydrates and Lipids. The amphipathic nature of phospholipids means that they will naturally associate with one another. Specifically, the hydrophobic tails want to be with other hydrophobic compounds, and exclude polar compounds (Hydrophobic Exclusion-This is a good concise discussion of the topic by Stephen T. Abedon, Ph.D. at Ohio State). Below is a brief movie that shows what happens when phospholipids in water begin to interact.

Phospholipid Movie: Bilayer formation through molecular self-assembly

The resulting phospholipid bilayer is polar (hydrophilic) on the outside, while the middle is non-polar (hydrophobic). The sides interact with water, but the middle excludes polar substances. This creates a selectively permeable barrier, and is the basis of the membranes function. Changing phospholipids and adding sterols (like cholesterol) will change the integrity and stability of this basic membrane structure.
Selective permeability means that only certain classes of chemical can make it through the phsopholipid bilayer. For other chemicals, we need to provide a protein to serve as a pore, channel or transporter.
In general, there are two ways that a chemical can be moved across the membrane: Down the chemical's concentration gradient (diffusion), or Against the chemical's concentration gradient. When a chemical moves down it's concentration gradient, we do not need to add energy to the process. The concentration gradient and the kinetic energy of the molecule (Brownian Motion) provided the needed energy. We call this form of movement Passive Transport.

There are three basic forms of passive transport through the membrane:

Diffusion - Using the inherent Brownian motion of molecules, chemicals move from points of high concentration to points of low concentration.

Every chemical has a unique concentration gradient.
The concentration gradient of one molecule will not interfer with the concentration gradient of another molecule.
What interfers is the ability to move across the cellular membrane (phospholipid bilayer).
Molecules with high polarity, ions, and large molecules are excluded by the phospholipid tails, and thus can not cross.
Water*, CO₂, O₂, and nonpolar compounds (lipids) can cross though diffusion.
Water moves through very slowly; water moves across more readily due to porins (protein pores) possessed by cells to make sure water and small polar compounds can cross readily.

Osmosis - The movement of water across a selectively permeable membrane.

Water goes to where the party is, meaning it will move to a compartment that has a higher solute concentration.
Since we are talking about two fluid compartments on either side of a membrane, we are ultimately talking about relative solute concentrations.
REMEMBER: you are looking at two fluid compartments, so when we are looking at osmosis, we are discussing the movement of water across a selectively permeable membrane from one fluid compartment to a second fluid compartment.
In biology we always use the inside of the cell as our reference, so:
- An isotonic fluid has the same solute concentration as the inside of the cell.
- A hypertonic solution has a solute concentration that is higher than the solute concentration inside the cell.
- A hypotonic solution has a solute concentration that is lower than the solute concentration inside the cell.

Remember that we are looking at solute concentrations, not the concentration of a single chemical.
- Diffusion gradients are specific for each chemical.
- Osmosis is determined by total solute concentration on each side of a selectively permeable membrane.

Facilitated Diffusion - Ultimately, this is diffusion, but the cell has had to provide a passage for the chemical. So, this only applies to chemicals that can not normally pass through the phospholipid bilayer.

The cell provides a protein channel or pore for the chemical to pass through.
Each channel or pore is specific to a single chemical or set of chemicals.
Since this is an protein (enzyme) mediated action, the ability to transport follows standard enzyme kinetics.
The core idea about facilitated diffusion following enzyme kinetics is that diffusion becomes limited by the number of channels or pore available.
Note: You will generally build up a large concentration of a compound on one side of the membrane. We will refer to this as a steep gradient (high on one side, low on the other).
The facilitator proteins will allow molecules from the HIGH side to move toward the LOW side; hence the use of the word diffusion.
Rarely will the channel or pore (facilitator) allow transport in the reverse direction for the same ion.

In each case, the cell does not need to expend energy to move the chemical across the membrane. The motive force is built in to the chemical gradients. Remember, the cell membrane is going to provide an internal vs. an external space (compartment). Each side of the membrane will have a unique chemical profile. As such, there will be concentration gradients across the membrane.

Osmosis Movie

Important Memes

Occassionly, there will be a phrase that I want you to remember to help you as you move through biology.

Water goes to where the party is! (This is a great way to remember in what direction water will flow across a selectively permeable membrane in response to changes in osmolarity, aka, solute concentration).
The phospholipid bilayer establishes the structure of the membranes, but the proteins provide the function. (The proteins embedded in the membrane will determine the functional capabilities of the membrane).
When something is added to a protein, the protein changes shape.

Daily Challenge: Passive transport

In your own words, describe passive transport. Concentrate on osmosis and facilitated diffusion by providing an example that illustrates each action. Make sure you use an example that uses a cell (not beakers, flasks, etc...).