Down below are notes that I have taken from reading about the membrane structure also there are quiz questions and answers that i have posted below.
Notes:
¨ The cell is composed of two distinctive environments: the hydrophilic aqueous cytoplasm and the hydrophobic lipid membranes
¨ The lipid environment is defined by the family of molecules that are characterized by their hydrophobic nature and their common metabolic origin.
¨ Three members of the lipid family
o fats (triacylglcerol), phospholipids, and steroids.
¨ Lipids are hydrophobic
o The molecules consist of long, 16-18 carbon, hydrocarbon backbones
§ only have a small amount of oxygen containing groups.
¨ Lipids have many functions in organisms
o waxes, pigments, steroid hormones, and cell membranes
¨ Fats, steroids, and phospholipids are very important to the functioning of membranes in cells
Fats (triacylglycerols, triglycerides)
¨ To construct a fat, or triacylglycerol
o three fatty acid molecules are attached to the glycerol through an ester bond between the carboxyl group of the fatty acids and the three alcohol groups of a glycerol molecule.
¨ A fat molecule can be composed of one, two, or three different types of fatty acids each of which can be saturated or unsaturated.
¨ Double bonds of the unsaturated fatty acids introduce kinks in the hydrocarbon backbone, unsaturated fats will not pack into a regular structure and thus remain fluid at lower temperatures
¨ Fats are mainly energy storage and insulating molecules.
¨ Per gram, fats contain twice as much energy as carbohydrates.
o Layers of fat also surround the vital organs of animals to cushion them, and layers of fat under the skin of animals provide insulation.
¨ Phospholipids contain only two fatty acids attached to a glycerol head.
o This occurs by a condensation reaction
¨ the third alcohol of the glycerol forms an ester bond through reaction with phosphoric acid.
¨ As a triprotic acid the phosphate group attached to the glycerol has the potential to form ester links with a variety of other molecules
o carbohydrates, choline, inositol and amino acids
¨ The phosphate group along with the glycerol group make the head of the phospholipid hydrophilic, whereas the fatty acid tail is hydrophobic
¨ phospholipids are amphipathic:a molecule with a polar end and a hydrophobic end
¨ Phospholipids serve a major function in the cells of all organisms:
o they form the phospholipid membranes that surround the cell and intracellular organelles such as the mitochondria
¨ the cell membrane is a fluid, semi-permeable bilayer that separates the cell's contents from the environment,
¨
¨ The membrane is fluid at physiological temperatures and allows cells to change shape due to physical constraints or changing cellular volumes.
¨ The phospholipid membrane allows free diffusion of some small molecules
o oxygen, carbon dioxide, and small hydrocarbons
§ not charged ions, polar molecules or other larger molecules such as glucose.
o This semi-permeable nature of the membrane allows the cell to maintain the composition of the cytoplasm independent of the external environment.
¨ The steroids are a family of lipids based on a molecule with four fused carbon rings.
o includes many hormones and cholesterol.
§ Cholesterol is a component of the cell membrane in animals and functions to moderate membrane fluidity because it restricts the motion of the fatty acid tails
Quiz Questions and Answers:
1. Lipids are the primary determinants of membrane structure, while proteins carry out membrane function.
2. List the molecular components common to all phosphoglycerides.
a. Phosphate, glycerol, and two fatty acids
3. Name the three classes of membrane lipid. (Also stae which is the most common class.)
a. Glycolipids, cholesterol, phospholipids (phospholipids)
4. What type of bonding dominates interactions between lipid tails and limits fluidity?
a. Van Der Waals forces
5. How is asymmetry preserved? Why don’t lipids and proteins flip-flop through the bilayer?
a. Lipid heads are hyrophilic, as are exposed portions of proteins. In other words, on either side of the mebrane, protiens and lipids interact via hydrogen bonding with water. In order to cross the membran, these molecules would have to first relinquish their hydrogen bonds with water.
i. In fact, ANY hydrophilic or partially hydrophilic molecule, whether a metabolite, a drug, or an enzyme, must relinquish favorable interactions with water in order to cross a membrane. The more hydrophilic (and the larger) a molecule, the less likely it is to cross via simple diffussion.
6. How could you identify a transmembrane helix just by examining the amino acid sequence of a protein?
a. Thransmebrane helices can often be indentified from a protein’s sequence as characteristic stretches of two-dozen or so hyrophobic amino acids )or more appropriately, amino acids with hydrophobic side chains; these are glycine, alanime, phenylalanine, leucine, valine, and isoleucine.
