Introductory Topics - Biology

Unlike carbohydrates and proteins, lipids are mainly nonpolar molecules. As a result, they are unable to be dissolved in aqueous solutions, such as blood. This makes them require a lipoprotein in order to be transported through the bloodstream. Without these proteins, lipids would be insoluble in the blood and cause clots.

Amphipathic molecules have both polar and nonpolar sections. For phospholipids, the phosphate region carries a negative charge, making it polar, while the lipid tail is a nonpolar hydrocarbon. Being amphipathic is a key characteristic that allows phospholipids to form the plasma membrane. The polar ends are able to face the aqueous environment while the nonpolar ends are faced towards each other. This creates an effective nonpolar barrier around the cell, while still remaining stable by having polar regions facing the aqueous environments.

Cellulose is a polysaccharide in which the monomers are linked by a beta linkage. This linkage requires a specialized enzyme that is not found in human beings. As a result, humans cannot break down cellulose.

Glycogen is used to store glucose in the body, and can be broken down to release energy. Starch is used to store glucose in plants, but can also be broken down by humans. Amylose is a component of starch.

There are four classes of biological molecules: fats, carbohydrates, proteins, and nucleic acids. Of these, fats produce the most energy per gram at a whopping nine calories per gram. Carbohydrates and proteins produce less than half of this, at only four calories per gram.

Lipids, or fats, are formed from hydrocarbon chains. The carbon atoms link together and bond to hydrogen to fill their orbitals. When all the bonds in the molecule are single bonds, the lipid is saturated. There is no such thing as a "di-saturated fat."

When there is a single double bond in the chain, the lipid is mono-unsaturated. When there are multiple double bonds in the chain, the lipid is unsaturated. Trans fats have a double bond with the adjacent hydrogen atoms on opposite sides of the carbon chain, maintaining a linear structure.

Most fats in the body are triglycerides. Their primary function is long-term energy storage.

Carbohydrates are known as sugars or saccharides. Virtually all carbohydrates end in the suffix "-ose," making them easy to identify. Ribose is a five-carbon sugar found in RNA. Some other common carbohydrates include deoxyribose, glucose, cellulose, and lactose. Sucrose is the technical name of table sugar, which is also a carbohydrate.

The other classes of molecules are fats, proteins, and nucleic acids. Fats, or lipids, can be further broken down into other categories, such as sterols and glycerolipids. Sterol lipids generally have the suffix "-ol" and include cholesterol and estradiol (estrogen). Glycerolipids include triglycerides. Proteins are formed from amino acids. Enzymes are a class of protein and generally have the suffix "-ase," such as helicase. Other proteins often have the suffix "-in" or "-en," such as insulin.

Lipids are hydrophobic, which means that they are insoluble in water, a polar solvent. Lipids can store abundant energy. Lipids contain large nonpolar regions, which means that they are soluble in nonpolar solvents. Triglycerides are only one type of lipids. Other types include, but are not limited to: monoglycerides, diglycerides, phospholipids, and fatty acids.

Saturated fatty acids do not have double bonds between carbon atoms. Therefore, the maximum amount of hydrogen atoms are bonded to the carbon skeleton chain. Saturated fatty acids are solid at room temperature, like butter. Unsaturated fatty acids have at lease one double bond. These double bonds create kinks in the carbon skeleton, preventing hydrogen atoms from packing onto the skeleton. Unsaturated fatty acids are therefore more fluid and are liquid at room temperature, like oil and natural peanut butter.

Because lipids do not include true polymers, they are not big enough to be considered macromolecules. However, like fats, lipids are hydrophobic. Fats are considered macromolecules and are composed of glycerol and fatty acid molecules. The glycerol and fatty acid molecules are joined via an ester linkage, which is formed by a dehydration reaction.

ATP stands for adenosine triphosphate. Ribose is the five-carbon sugar, which is bonded to a phosphate group and adenine group in a molecule of ATP.

Polymers are molecules that are made of two or more repeating units of the same molecule or atom. For example, a polysaccharide is made of two or more monosaccharides bonded by dehydration synthesis. Lipids, on the other hand, are not. For example, triglycerides are composed of a glycerol molecule covalently bonded to three fatty acids via dehydration synthesis; however, fatty acids are elongated by the addition of methyl groups to the fatty acid chain.

Carbohydrates are composed of carbon, hydrogen, and oxygen. Carbohydrates include sugars, starches, cellulose, and chitin. Carbohydrates can be organized into four chemical groups including: monosaccharides, disaccharides, oligosaccharides, and polysaccharides. The small carbohydrates called monosaccharides and disaccharides are typically referred to as sugars. Carbohydrates perform many roles in living organisms most notably energy and structural components (e.g. cellulose and chitin).

Lipids are a diverse group of organic compounds containing 4-28 carbons. Lipids include fats, cholesterols, and vitamins. Lipids have a hydrophilic region with a polar carboxyl group and a hydrophobic region with a non-polar hydrocarbon chain. Lipids can be either saturated fatty acids (i.e. no double bonds) or unsaturated (i.e. one or more double bonds). The double bonds add a kink and reduce the melting point. Glycogen is a polysaccharide that is used for energy storage in animals.

Glycogen is a polysaccharide made by animals for energy storage. If it is not used, then it will be converted to fat for more permanent storage. Starch is a polysaccharide that functions as plant storage. The polysaccharide cellulose provides structure to plants. Glucose is a ubiquitous fuel source used by most organisms in some of the following ways: aerobic respiration, fermentation, or anaerobic respiration. Glucose is also the product of photosynthesis.

Maltose is a type of disaccharide that consists of two molecules of the monosaccharide glucose. Monosaccharides come together to produce a disaccharide and a molecule of water: condensation reaction. Polysaccharides include starch, glycogen, and cellulose. Starch is responsible for plant energy storage and includes amylose and amylopectin. Glycogen is used in animals for energy storage. Cellulose is a structural molecule utilized in plants.

Monosaccharides come together to produce a disaccharide and a molecule of water. Lactose is a disaccharide that consists of a molecule of the monosaccharides glucose and galactose. Maltose is a type of disaccharide that consists of two molecules of the monosaccharide glucose. Sucrose is a disaccharide that consists of a molecule of both the monosaccharides glucose and fructose.

Vitamins A, D, E, K are all fat-soluble vitamins. Fat-soluble vitamins are absorbed through the intestinal tract with the help of fat (i.e. lipids). Because these vitamins are not water-soluble they are not excreted in urine and thus are more likely to accumulate in the body, leading to hypervitaminosis (i.e. excess amounts of vitamins A, D, E, and K can lead to health consequences.). Vitamin C is water-soluble and is excreted in urine; therefore, regular intake is important.

Humans can obtain vitamins from a variety of mechanisms. Diet—consumption of food—is an intuitive resource that provides humans with vitamins; however, there are certain vitamins that are obtained by other means. Intestinal microorganisms help us produce both vitamin K (a fat soluble vitamin) and biotin. After exposure to sunlight’s natural ultraviolet light vitamin D is synthesized within the skin. Vitamin deficiencies can be very serious and include the following disorders: rickets, beriberi, scurvy, and night-blindness.

Cholesterol is the precursor to all steroid hormones. Cholesterol is used as a structural component in cell membranes. It is also a precursor for many hormones—including testosterone and progesterone—and vitamins. Cholesterol is involved in cell communication including: intracellular transport, cell signaling, and nerve conduction. There are two primary types of cholesterol: LDL and HDL.

Phospholipids are a type of complex lipid. They consist of a glycerol, two fatty acids, and a polar phosphate group with an alcohol. Their function is to create cell membranes, specifically the fluid mosaic model. They have a polar "head" and a non-polar "tail." The head is hydrophilic ("water loving"), and the tail is hydrophobic ("water hating"), making it perfect for making membranes.