Summary
Highlights
The lesson introduces biomolecules, specifically nucleic acids like DNA and RNA, which give instructions for cell activities. Biomolecules are substances produced by cells and living organisms, performing various functions. The four major types are carbohydrates, lipids, nucleic acids, and proteins. Carbohydrates are the focus, known as 'go foods' that provide energy, and are widespread organic substances essential for all living things. They are composed of carbon, hydrogen, and oxygen, with a general formula often represented as C6H12O6, meaning 'watered carbon'. Carbohydrates are macronutrients, serving as one of the body's primary energy sources, found in fruits, vegetables, grains, and milk products.
Carbohydrates are classified into three main types: monosaccharides, disaccharides, and polysaccharides. Monosaccharides are simple sugars, with carbon counts typically ranging from three to seven, and their names usually end in '-ose'. They can be aldoses (with an aldehyde group) or ketoses (with a ketone group). Disaccharides are formed when two monosaccharides combine through a dehydration reaction (condensation reaction), releasing water and forming a covalent bond. Polysaccharides are long chains of many monosaccharides linked by glycosidic bonds, which can be branched or unbranched.
Common monosaccharides include glucose, a crucial energy source for cellular respiration and ATP production. Plants synthesize glucose and store excess as starch. Other examples are galactose (found in milk) and fructose (found in fruit sugars). Examples of disaccharides include sucrose (table sugar), formed from glucose and fructose monomers. Lactose, found in milk, is composed of glucose and galactose. Maltose (malt sugar) is another disaccharide, formed from two glucose molecules and found in grains and cereals.
Polysaccharides are long chains of monosaccharides linked by glycosidic bonds. Key examples include starch, the stored form of sugar in plants (a mixture of amylose and amylopectin), which provides food for embryos and humans/animals. Glycogen is the storage form of glucose in humans and other vertebrates, primarily stored in the liver and muscle cells. It is broken down into glucose during glycogenolysis when blood glucose levels decrease. Cellulose is the most abundant natural biopolymer, made of glucose monomers, and forms the cell walls of plants, providing structural support, as seen in wood and paper.
The presence of carbohydrates can be detected using specific tests. Sugars capable of being oxidized by mild oxidizing agents are called reducing sugars, possessing free aldehyde or ketone functional groups. All common monosaccharides are reducing sugars, as are the disaccharides maltose and lactose. Non-reducing sugars, like the disaccharide sucrose, are not oxidized by these agents. Common tests for reducing sugars include Benedict's solution (blue, changes to green/yellow, orange/red, or brick red depending on the amount) and Fehling's solution (blue, forms a reddish-brown precipitate if reducing sugar is present). The iodine test, typically yellowish, detects starch (a polysaccharide); it turns black or dark blue in the presence of starch.
Beyond energy, carbohydrates serve other vital functions. For example, arthropods like insects and crustaceans have exoskeletons made of chitin, a nitrogen-containing polysaccharide, which protects their internal body parts.