Lecture 6: Cell membranes Flashcards
How the body uses sugars
Glucose is often called “blood sugar” because it is the major monosaccharide found in the blood.
Glycogen exists in the body as a reservoir of available energy that is stored in the chemical bonds within individual glucose monomers.
Hydrolysis of glycogen, as occurs during periods of fasting, leads to release of the glucose monomers into the blood, thereby preventing blood glucose from decreasing to dangerously low levels
Lipids
Lipids (fats) are molecules composed predominantly (but not exclusively) of hydrogen and carbon atoms linked by nonpolar covalent bonds.
Structures are very diverse.
Roles of lipids in the body:
a major component of cell membranes.
provide a valuable source of energy.
important signaling molecules.
Lipids – steroids
Steroids have a basic 4 ring structure (ABCD, 6:6:6:5) with side-chains.
The different steroids arise due to functionalisation of the rings and changing the side-chains.
Important examples include cholesterol, oestrogen, testosterone, corticosteroids, bile acids
Lipids – fatty acids
Fatty acids are carboxylic acids with long alkyl side-chains (lipophilic).
Chain lengths can vary but 14, 16 and 18 carbons particularly common.
Diverse functions
Phospholipids are
amphipathic
can form micelles and liposomes lipid bilayer
Cell membrane structure
Cell membrane is a lipid bilayer (double layer).
Polar head-groups face water, lipophilic side-chains each other.
Also contains cholesterol and proteins (important for function).
Fluid structure (lipids and entities dissolved within layer can diffuse).
Membrane structure and function
Boundaries between cells and the environment.
A selective barrier to the passage of molecules.
Detect chemical signals from other cells.
Anchor cells to adjacent cells and to the extracellular matrix.
Highly specialized for the functions they perform.
Moving molecules across membranes
DIFFUSION – movement influenced by electrochemical gradient (i.e., concentration and charge) e.g., oxygen into capillaries.
OSMOSIS - Water molecules diffuse down concentration gradient until equilibrium is reached (= water diffusion).
PROTEIN TRANSPORTERS/ CARRIERS – membrane proteins can perform both passive and active transport of selected molecules.
ENDOCYTOSIS/ EXOCYTOSIS – vesicular bulk transport into/out of the cell, often receptor mediated.
Sugars can be
monosaccharides, oligosaccharides or polysaccharides.
Subunits of several different sizes by 5 and 6-carbon sugars are particularly important.
Polysaccharides
important for energy storage and formation of extracellular matrix.
Proteins embedded within cell membrane
control movement of polar molecules.
