Membranes
Review of Lipid and Membrane structure
the above structure is a phospholipid. Note the polar head (top) and hydrophobic tails (bottom). One of the tails is bent due a a double bond.
Lipids arrange themselves in bilayers as described here
Note the round heads oriented toward the outside (water) and the tails oriented toward each other (inside of membrane). This limits transport of many types of molecules (See below)
This image shows the lipid bilayer along with proteins carbohydrates
Functions for membrane proteins
Transport of substances across membranes: the following types of substances can be transported across lipid bilayers without the help of proteins
(1) Non-polar compounds including gases such as oxygen and carbon dioxide and larger compounds such as steroids
(2) Water and some very small polar compounds. Water travels slowly
Reception. Membrane proteins help detect internal signals (such as insulin) and external signals (such as sugar, for taste buds). Binding of the compound on the outside triggers a series of reactions that leads to a response.
Recognition For example self vs nonself immune reactions are determined by membrane proteins and or carbohydrates. The difference between blood types A,B AB and O is determined by different carbohydrates.
Formation of organs like liver and stomach can depend on membrane proteins sticking like cells together.
Junction formation Gap, Tight and Adhesion junctions are made of membrane proteins
Role of cholesterol
In cold temperatures, cholesterol breaks up phospholipid packing (in a manner similar to unsaturated fats. This makes the membranes more fluid)
At high temperature, cholesterol limits the lateral movement of phospholipids, thus making the membrane less fluid.
Be able determine whether an environment is hypotonic, isotonic or hypertonic to a cell.
Be able to predict what will happen if a plant or animal cell is put in a hypotonic, isotonic or hypertonic environment. .
Example:
- A cell with an internal NaCl concentration of . 1M is put into a solution of ..05 M NaCl. The salt can not cross the membrane. What happens?
This video covers membrane transport including endo and exocytosis
Crash Course, Membranes and Transport
Here is a video (also linked in the open stax text showing how receptor mediated endocytosis regulates cholesterol level
Test Your Self on Membrane transport
