6 Lipids, Membranes and Cells

Table 3.1 The Building Blocks of Organisms 15% 7% 2% 26% 2% Water 70%

6 3 Lipids: Water-Insoluble Molecules Lipids are insoluble in water. This insolubility results from the many nonpolar covalent bonds of hydrogen and carbon in lipids. Lipids aggregate away from water, which is polar, and are attracted to each other via weak, but additive, van der Waals forces.

6 4 Lipids: Water-Insoluble Molecules Roles for lipids in organisms include:  Energy storage (fats and oils)  Cell membranes (phospholipids)  Capture of light energy (carotinoids)  Hormones and vitamins (steroids and modified fatty acids)  Thermal insulation  Electrical insulation of nerves  Water repellency (waxes and oils)

6 5 Fatty acids Fatty acids are long chains of hydrocarbons with a carboxyl group (—COOH) at one end.

6 6 Saturated fatty acids Saturated fatty acids have only single carbon-to-carbon bonds and are said to be saturated with hydrogens.

6 7 Unsaturated fatty acids Unsaturated fatty acids have at least one double- bonded carbon in one of the chains —the chain is not completely saturated with hydrogen atoms. Saturated Unsaturated

6 8 Lipids: Water-Insoluble Molecules Saturated fatty acids are rigid and straight, and solid at room temperature. Animal fats are saturated. The double bonds cause kinks that prevent easy packing. Unsaturated fatty acids are liquid at room temperature. Plants commonly have unsaturated fatty acids. Saturated Unsaturated

6 9 Triglycerides Fats and oils are triglycerides, composed of three fatty acid molecules and one glycerol molecule. Glycerol is a three-carbon molecule with three hydroxyl (—OH) groups, one for each carbon.

6 10 Triglycerides store energy Fats and oils store energy (solid or liquid)

6 11 Lipids: Pigments & vitamins Carotenoids are light-absorbing pigments found in plants and animals.  -carotene, is a plant pigment used to trap light in photosynthesis. Chlorophyll = Green Carotenes = Orange Xanthophylls = Red

6 12 Lipids: Pigments & vitamins Vitamins are small molecules that are not synthesized by the body, but are necessary for its normal functioning. Vitamins must be acquired by food. Vitamin A which is important for vision and bone development. Rhodopsin

6 13 Lipids: Pigments & vitamins  -carotene, is the Source of Vitamin A In animals, this pigment, when broken into two identical pieces, becomes vitamin A which is important for vision and bone development.

6 14 Lipids: Cholesterol & steroid hormones Steroids are signaling molecules (Hormones are secreted in one site and act in a different site). Steroids are organic compounds with a series of fused rings. The steroid cholesterol is a common part of animal cell membranes. Cholesterol is also is an initial substrate for synthesis of the hormones testosterone and estrogen.

Figure 3.23 All Steroids Have the Same Ring Structure Cholesterol is also is an initial substrate for synthesis of the hormones testosterone and estrogen.

6 16 Lipids: Cholesterol & steroid hormones Cholesterol is not soluble in water and precipitates in blood vessels. Atherosclerosis

6 17 Lipids: Wax Waxy coatings repel water and prevent water loss from structures such as hair, feathers, and leaves. Waxes are highly nonpolar molecules consisting of saturated long fatty acids bonded to long fatty alcohols via an ester linkage.

6 18 Lipids: Wax Waxes are highly nonpolar molecules consisting of saturated long fatty acids bonded to long fatty alcohols via an ester linkage. A fatty alcohol is similar to a fatty acid, except for the last carbon, which has an —OH group instead of a —COOH group. Bees wax: O H 3 C-(CH 2 ) 14 -C-OH H 3 C-(CH 2 ) 14 -CH 2 -OH fatty acid fatty alcohol

6 19 Phospholipids Phospholipids have two hydrophobic fatty acid tails and one hydrophilic phosphate group attached to the glycerol. As a result, phospholipids orient themselves so that the phosphate group faces water and the tail faces away. In aqueous environments, these lipids form bilayers, with heads facing outward, tails facing inward. Cell membranes are structured this way.

Figure 3.20 Phospholipid Structure

Figure 3.21 Phospholipids Form a Bilayer

6 23 Functions of cell membrane The cell’s membrane is a lipid bilayer The main functions of membrane: Separates between the cytoplasm and the external environment - selective barrier. Cell-cell and cell-substratum ( extra cellular matrix - ECM ) interactions. Transmission of signals from the external environment into the cell. Cell “eating drinking and secretion” of biomaterials.

6 24 Functions of membrane Separates between the cytoplasm and the external environment - selective barrier.

6 25 Lipid Bilayer The lipid bilayer is a two dimensional fluid 2  m/sec 1/month/unit 30,000 rpm

6 26 Lipid Bilayer Membrane fluidity and integrity are affected by: Fatty acid composition and length (longer = more stable) Degree of saturation (saturation = less kinks = less fluid) Cholesterol (more= less fluid)

6 27 Functions of membrane Other functions are provided by membrane proteins

6 28 Functions of membrane Membrane proteins associate with the lipid bilayer in various ways

6 29 Functions of membrane Proteins often cross the bilayer as an alpha helix

6 30 Functions of membrane Beta Barrel

6 31 Lipid Bilayer The cell’s lipid bilayer is asymmetrical

6 32 Photobleaching techniques Fluorescence Recovery After Photobleaching

6 33 Prokaryotic cell