Cell Transport: Passive Transport Notes

What will spontaneously happen to phospholipids surrounded by water?
Explain what the fluid mosaic model of cell membranes refers to.
What trend in a cell’s surface area to volume ratio do we see as a cell increases in size? Show this mathematically.

The cell membrane is a selectively permeable, hydrophobic barrier referred to as a phospholipid bilayer.
The membrane is the first line of defense for the cell, protecting the biochemical environment that maintains homeostasis.
There are a variety of molecules embedded in the membrane, and we can visualize the membrane as a fluid mosaic.
Cells maintain a high surface area to volume ratio by staying small in size and having folded cell membranes.
The lesson will examine the physical laws that dictate how cells transport certain materials through the membrane.

Diffusion is a type of passive transport.
Particles move from a greater concentration to a lesser concentration until equilibrium is reached.
Equilibrium is when particles are evenly distributed.

Diffusion is a natural, spontaneous process.
It does NOT require the expenditure of energy.
In diffusion, molecules move down a concentration gradient.
Concentration gradient: Occurs when there is a difference in the concentration of a solute in a given volume.
Down a concentration gradient = From high concentration to low concentration.
If there is more than one solute, each moves down its own concentration gradient.

When there is a concentration gradient for a solute, potential energy is stored in the gradient.
As particles diffuse down their gradient, the stored energy is released.

Diffusion ends in dynamic equilibrium.
- Dynamic equilibrium: when particles continue to move but the overall concentration does not change

There are two types of diffusion: Simple and Facilitated.
Simple diffusion = small, non-polar molecules diffuse directly through the cell membrane.
This is passive transport, so no ATP is required, and the process is spontaneous.

Many small non-polar molecules can diffuse directly into the cell.
- Example: $O_2$
  - As long as the mitochondria are using the oxygen for cellular respiration, $O_2$ will continue to diffuse into a cell.
  - If the cell is NOT doing cellular respiration, the diffusion of oxygen into the cell will slow or stop.

Facilitated Diffusion: small, polar molecules diffuse through protein channels.
Hydrophilic, polar molecules and ions cannot go directly through the membrane (they need help).
- Examples: Water, Glucose, Sodium ions, Chloride ions

Facilitated diffusion always involves protein channels – but it is still diffusion.
Polar molecules and ions, which are hydrophilic, must use proteins to diffuse through the membrane.
The membrane is hydrophobic and will not allow hydrophilic substances through.
They still go down a concentration gradient – from high to low concentration – the definition of diffusion.
Since diffusion is passive transport, facilitated diffusion is passive transport.

Two types:
- Carrier proteins: Change shape to allow passage of diffusing molecule.
- Channel proteins: Provide a tunnel for diffusing molecule to pass through.
The specificity of membrane transport protein is due to the unique amino acid sequence of the polypeptide chain.