Biology

1. The ability of ice to float because of the expansion of water as it solidifies is an important factor in the fitness of the environment. If ice sank, then eventually all ponds, lakes, and even oceans would freeze solid, making life as we know it impossible on earth. During the summer, only the upper few inches of the ocean would thaw. Instead, when a deep body of water cools, the floating ice insulates the liquid water below, preventing it from freezing and allowing life to exist under the frozen surface.

2. Carbon atoms are the most versatile building blocks of molecules. A covalent bonding capacity of four contributes to carbon's ability to form diverse molecules. Carbon can bond to a variety of atoms, including oxygen, hydrogen, nitrogen, and sulfur. Carbon atoms can also bond to other carbons, forming the carbon skeletons of organic compounds.

3. Most macromolecules are polymers. Carbohydrates, lipids proteins, and nucleic acids are the four major classes of organic compounds in cells. Some of these compounds are very large and are called macromolecules. Most macromolecules are polymers, chains of identical or similar building blocks called monomers. Monomers form larger molecules by condensation reactions in which water molecules are released, dehydration. Polymers can disassemble by the reverse process, hydrolysis.

4. Monosaccharides are the simplest carbohydrates. They are used directly for fuel, converted to other types of organic molecules, or used as monomers for polymers. Disaccharides consist of two monosaccharides connected by a glycosidic linkage. Fats are constructed by joining a glycerol molecule to three fatty acids by dehydration reactions. Saturated fatty acids have the maximum number of hydrogen atoms. Unsaturated fatty acids have one or more double bonds between their carbons. The primary structure of a protein is its unique sequence of amino acids. Secondary structure is the folding or coiling of the polypeptide into repeating configurations, such as the a helix and the pleated sheet, which result from hydrogen bonding between parts of the polypeptide backbone. Tertiary structure is the overall three-dimensional shape of a polypeptide and results from interactions between amino acid side chains. Proteins made of more than one polypeptide chain have a quaternary level of structure. The structure and function of a protein are sensitive to physical and chemical conditions. Protein shape is ultimately determined by its primary structure, but in the cell chaperone proteins may help the folding process. Each nucleotide monomer consists of a pentose covalently bonded to a phosphate group and to one of four different nitrogenous bases. RNA has ribose as its pentose; DNA has deoxyribose. RNA has U and DNA T. In making a polynucleotide, nucleotides join to form a sugar-phosphate backbone from which the nitrogenous bases project. The sequence of bases along a gene specifies the amino acid sequence of a particular protein.

5. Prokaryotic cells have no nuclei or other membrane-enclosed organelles. Eukaryotic cells have membrane-enclosed nuclei and other specialized organelles in their cytoplasm. However, both prokaryotic and eukaryotic cells are bounded by a plasma membrane.

6. The evolutionary relationships between prokaryotic and eukaryotic cells are that the nucleus contains most of the genes that control the eukaryotic cell. Many of the different membranes of the eukaryotic cell are part of an endomembrane system. These membranes are related either through direct physical continuity or by the transfer of membrane segments as tiny vesicles. These relationships, however, do not mean that the various membranes are alike in structure and function. The endomembrane system includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, various kinds of vacuoles, and the plasma membrane.

7. The current model of the molecular architecture of membranes is the fluid mosaic model. S. J. Slinger and G. Nicolson advocated a revised membrane model that placed the proteins in a location compatible with their amphipathic character. They proposed that membrane proteins are dispersed and individually inserted into the phospholipid bilayer to be exposed to water. This molecular arrangement would maximize contact of hydrophilic regions of proteins and phospholipids with water while providing their hydrophobic parts with a non-aqueous environment. According to this model, the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids; hence the term fluid mosaic model.

8. Membranes have distinct inside and outside faces. The two lipid layers may differ in specific lipid composition, and each protein has directional orientation in the membrane. The plasma membrane also has carbohydrates, which are restricted to the exterior surface. This asymmetrical distribution of proteins, lipids, and carbohydrates is determined as the membrane is being built by the endoplasmic reticulum. Molecules that start out on the inside face of the endoplasmic reticulum end up on the outside face of the plasma membrane. A single cell may have membrane proteins performing several functions, and a single protein may have multiple functions.

9. Membrane carbohydrates are usually branched oligosaccharides with fewer than fifteen sugar units. Some of these oligosaccharides are covalently bonded to lipids, forming molecules called glycolipids. Most, however, are covalently bonded to proteins, which are thereby glycoproteins. The oligosaccharides on the external side of the plasma membrane vary from species to species, among individuals of the same species, and even from one cell type to another in a single individual. The diversity of the molecules and their location on the cell's surface enable oligosaccharides to function as markers that distinguish one cell from another.

10. One mechanism by which substances cross membranes is diffusion. Diffusion is the tendency for molecules of any substance to spread out into the available space. Each molecule moves randomly, yet diffusion of a population of molecules may be directional. The diffusion of a substance across a biological membrane is called passive transport, because the cell does not have to expend energy to make it happen. The diffusion of water across a selectively permeable membrane is a special case of passive transport called osmosis. The direction of osmosis is determined only by a difference in total solute concentration. Water moves from a hypotonic, low concentration, solution to a hypertonic, high concentration, solution even if the hypotonic solution has more kinds of solutes. Some transport proteins can move solutes against their concentration gradients,