Movement Through Membranes
Hydrocarbons, carbon dioxide, and oxygen are hydrophobic substances that can pass easily across the cell membrane.
Ions and polar molecules cannot pass easily across the membrane. The former substances move across the membrane through passive diffusion. In passive diffusion, the substance will travel from where it is more concentrated to where it is less concentrated, diffusing down its concentration gradient. This type of diffusion requires that no work be done, and it relies only on the thermal motion energy intrinsic to the molecule in question. The term passive diffusion is used because the cell expends no energy in moving the substance.
The word for the passive transport of water is osmosis. In osmosis, water flows from the hypotonic solution (the solution with lower solute concentration) to a hypertonic solution (one with a higher solute concentration). Another form is an isotonic solution where both concentrations are equal. Animal cells want to be in an isotonic solution whereas plant cells prefer a slightly hypotonic solution to keep their cell walls rigid.
Hydrophilic substances get across the membrance through transport proteins. Transport proteins work in two ways:
1) They provide a hydrophilic channel through which the molecules can pass
2) They bind loosely to the molecule and carry them through the membrane
The process by which ions and hydrophilic substances diffuse across the cell membrance with the help of transport proteins is called facilitated diffusion. Transport proteins are specific for the substances they transport.
In active transport, substances can be moved against their concentration gradient. Not surprisingly, in this type of transport, the cell must expend energy. This type of transport is crucial for cells to be able to maintain sufficient quantities of substances that are relatively rare in their enviornment.
Specific tansmembrance proteins are responsible for active transport and ATP supplies the energy for this type of transport. ATP transfers one of its phospates to the transport protein, which might be responsible for making the protein change its shape to allow for the passage of the substance.
Things to Look Forward To Tomorrow:
2) More on Active Transport
3) What a sodium potassium pump actually is
4) Last day before our fabulous 5 day weekend!!!