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UNIT 1:  CELLULAR FUNCTIONS

C. LIPIDS

·         this category of molecules includes fats (energy storage), phospholipids (construction of membranes), waxes (coatings to prevent water loss), and steroids (messenger molecules/hormones)

 Fats

• over 100 C-H bonds found in fat molecules make them excellent long-term energy storage molecules
• they are a very concentrated source of energy – a gram of fat contains more than twice as much energy as a grain of carbohydrate – therefore, more energy in less mass
• since fats contain mostly water, it gives them an insulation property
• made up of two types of molecules:  fatty acids and glycerol
• fatty acids are long-chained hydrocarbons with a carboxyl group at one end of the chain
• they may be saturated, or unsaturated
• saturated fatty acids have the maximum number of single bonds on each carbon on the chain
• unsaturated fatty acids have at least one double bond in the chain (between two successive carbons)
• saturated fatty acids are linear in shape, whereas unsaturated fatty acids are bent at the regions of the double bonds
• this difference in shape accounts for the fact that saturated fatty acids are sold at room temperature (butter, animal lard), whereas unsaturated fatty acids are liquid at room temperature (canola oil)
• the kinks prevent the unsaturated acid chains from packing close together, which would result in the solid form
• lipids that are made from saturated fatty acids are known as saturated fats (animal lard, butter)
• lipids that are made from unsaturated fatty acids are known as unsaturated fats (canola oil, olive oil, etc.)
• a fat molecule is formed when one glycerol combines with three fatty acid chains
• the result is a triglyceride (Figure 1.21, p. 18)
• humans store fat in fat cells called adipose tissue
• fat cells just keep growing as a person gains weight – more and more triglycerides are added to existing cells
• even though it is popular to have a low fat diet, it doesn’t mean that fat is completely bad for you
• lipids have an important role in maintaining good health
• some fatty acids are essential since the body cannot make them
• linoleic, linolenic, and arachidonic fatty acids must be taken in from food

Phospholipids

• consist of a hydrophilic (water “liking”) phosphate molecule, bonded to hydrophobic (water “fearing”) fatty acid tails
• the phosphate part of the lipid is polar, and the hydrocarbon tail of the fatty acid is non-polar
• the cell membrane of living cells is constructed of this molecule (see Figure 2.6, pp. 42-43)
• it is a dual-natured molecule where the polar head bonds with water (the outside and inside of the cell), and the non-polar tails remain on the inside of the membrane to help dissolve non-polar proteins and other essential membrane components – makes for a very effective barrier

Steroids

• steroids are a very important group of lipids that consist of 4 fused hydrocarbon rings
• each different type of steroid is characterized by a special functional group attachment
• note the differences between the steroid molecules testosterone, estrogen, and cholesterol (Figure 1.24, p. 20)
• steroid molecules testosterone and progesterone are hormones that control the development of sexual characteristics
• cholesterol has a very important role in maintaining the fluidity of cell membranes (see Figure 2.6, pp. 42-43)

Waxes

• these lipids possess a waxy feel to them
• they are molecules that are mostly used in biology to help organisms in the prevention of water loss
• cutin is an example – used as a coating on stems, fruit, and leaves, to prevent water loss
• also seen in beeswax and honeycombs

Homework:  p. 22, 1-5.