Cell Membrane

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The cellular membrane is essential for separating the contents of the cell (intracellular fluid) from everything around it (interstitial fluid or plasma, etc.). It serves as a boundary that only allows for certain components to enter: this is called selective permeability. 

  

The cell membrane is composed of a phospholipid bilayer and many other proteins, glycolipids and cholesterol. 
The phosphate head groups of the phospholipids are hydrophilic, and therefore face the aqueous environment inside and outside the cell. 
The lipid tails are hydrophobic and face one another, away from the aqueous environment. 
Molecules can only enter the cell either through the lipid bilayer or through channels (proteins).
  
Proteins can be part of the membrane itself (integral) or only be on the outer edges of the lipid bilayer (peripheral). 
Integral proteins go through the lipid bilayer so the transmembrane portion is not charged
Peripheral proteins are typically located on the cell interior and can also be involved in cell signaling
These proteins have many important functions, which include:
Catalyzing reactions (enzymes)
Transporting molecules through the lipid bilayer
Receptors for signaling between internal and external environment
Anchor internal structures to the cell membrane
Attach adjacent cells to one another
Serve as a marker for cell identification

Cholesterol is a hydrophobic molecule which is inserted in the lipid portion of the phospholipid bilayer. It provides fluidity and rigidity to the membrane.  
20% by weight of the membrane
Sensitive to temperature changes
Reduces membrane permeability
  
Glycoproteins are proteins modified with carbohydrates (prefix "glyco-" refers to sugars). Sometimes, the carbohydrates are attached directly to the lipids: these are glycolipids. Together, these form the glycocalix of a cell.

Cholesterol:

A. Is a hydrophilic molecule

B. Can be inserted in the lipid bilayer of a cell

C. Provides fluidity and rigidity to the cell membrane 

D. Helps transport molecules across the membrane 

E. All of the above are correct

I don't know

Cell membrane proteins can have any of the following functions, EXCEPT:

A. Speed up the rate of biochemical reactions

B. Facilitate the transport of molecules across the cell membrane

C. Serve as cellular identifiers

D. Directly cause changes in protein expression

E. Interact with other cells to enable adhesion

I don't know

Wize University Physiology Textbook > Cellular and Membrane Physiology

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