BIO 172January 30 and February 1, 2019Lipids, Carbs, and MembranesReadings 2.5 and 5.1-5.21. Organic molecules include proteins, nucleic acids, carbohydrates, and lipids, each of which is built from simpler units. 2. Cell membranes are composed of lipids, proteins, and carbohydrates. 3. The plasma membrane is a selective barrier that controls the movement of molecules between the inside and the outside of the cell.2.5 Summary2.5 Organic molecules include proteins, nucleic acids, carbohydrates, and lipids, each of which is built from simpler units.o Amino acids are linked by covalent bonds to form proteins. o An amino acid consists of a carbon atom (the α carbon) attached to a carboxyl group, an amino group, a hydrogen atom, and a side chain. o The side chain determines the properties of an amino acid. o Nucleotides assemble to form nucleic acids, which store and transmit genetic information. o Nucleotides are composed of a 5-carbon sugar, a nitrogen-containing base, and a phosphate group. o Nucleotides in DNA incorporate the sugar deoxyribose, and nucleotides in RNA incorporate the sugar ribose. o The bases are pyrimidines (cytosine, thymine, and uracil) and purines (guanine and adenine). o Sugars are carbohydrates, molecules composed of C, H, and O atoms, usually in the ratio 1:2:1, and are a source of energy. o Monosaccharides assemble to form disaccharides or longer polymers called complex carbohydrates. o Lipids are hydrophobic. o Triacylglycerols store energy and are made up of glycerol and fatty acids. o Fatty acids consist of a linear hydrocarbon chain of variable length with a carboxyl groupat one end.o Fatty acids are either saturated (no carbon–carbon double bonds) or unsaturated (one or more carbon–carbon double bonds).o The tight packing of fatty acids in lipids is the result of van der Waals forces, a type of weak, noncovalent bond.  Cells were discovered in 1665 by Robert Hooke. Cell Theory: the theory that the cell is the fundamental unit of life in all organisms and that cells come only from preexisting cells. 5.1 Structure of Cell Membranes Lipids are the main component of cell membranes.  They have properties that allow them to form a barrier in an aqueous (watery) environment.  Proteins are often embedded in or associated with the membrane, where they perform important functions such as transporting molecules.  Carbohydrates can also be found in cell membranes, usually attached to lipids (glycolipids) and proteins (glycoproteins). Cell membranes are composed of two layers of lipids. Major type: phospholipids- Made up of a glycerol backbone attached to a phosphate group and two fatty acids.- The phosphate head group is hydrophilic (“water-loving”) because it is polar, enabling it to form hydrogen bonds with water. - By contrast, the two fatty acid tails are hydrophobic (“water-fearing”) because they are nonpolar and do not form hydrogen bonds with water.  Molecules with both hydrophilic and hydrophobic regions in a single molecule are termed amphipathic. In an aqueous environment, amphipathic molecules spontaneously arrangethemselves into various structures in which the polar head groups on theoutside interact with water and the nonpolar tail groups come together on the inside away from water.  This arrangement results from the tendency of polar molecules like water to exclude nonpolar molecules or nonpolar groups of molecules. The shape of the structure is determined by the bulkiness of the head group relative tothe hydrophobic tails. - For example, lipids with bulky heads and a single hydrophobic fatty acid tail are wedge-shaped and pack into spherical structures called micelles. - By contrast, lipids with less bulky head groups and two hydrophobic tails form a bilayer.  A lipid bilayer is a structure formed of two layers of lipids in which the hydrophilic heads are the outside surfaces of the bilayer and the hydrophobic tails are sandwichedin between, isolated from contact with the aqueous environment.- The bilayers form closed structures with an inner space since free edges would expose the hydrophobic chains to the aqueous environment.  This organization in part explains why bilayers are effective cell membranes. It also explains why membranes are self-healing.  Small tears in a membrane are rapidly sealed by the spontaneous rearrangement of the lipids surrounding the damaged region because of the tendency of water to exclude nonpolar molecules. How did the first cell membranes form? What are the consequences of the ability of phospholipids to form a bilayer when placed in water? - The bilayer structure forms spontaneously, dependent solely on the properties of the phospholipid and without the action of an enzyme, as long as the concentration of free phospholipids is high enough and the pH of the solution is similar to that of a cell.  The pH is important because it ensures that the head groups are in their ionized (charged) form and thus suitably hydrophilic.  Thus, if phospholipids are added to a test tube of water at neutral pH, they spontaneously form spherical bilayer structures called liposomes that surround a central space. As the liposomes form, they may capture macromolecules present in solution. Early evolution of life on Earth Experiments show that liposomes can form, break, and re-form in environments like tidal flats that are repeatedly dried and flooded with water. The liposomes can even grow, incorporating more and more lipids from the environment, and capture nucleic acids and other molecules in their interiors.  Depending on their chemical composition, early membranes might have been either leaky or almost impervious to the molecules of life. Over time, they evolved in such a way as to allow at least limited molecular traffic between the environment and cell interior. - At some point, new lipids no longer had to be incorporated from the environment.  Instead, proteins guided lipid synthesis within the cell, although how this switch to protein-mediated synthesis happened remains uncertain. All evidence suggests that membranes formed originally by straightforward physical processes, but that their composition and functionevolved over time.  Cell membranes are dynamic. Lipids freely associate with one another because of extensive van der Waals forces between their fatty acid tails. - These weak interactions are easily broken