Carbohydrates and Lipids

• Carbohydrates and lipids are relatively simple biochemical molecules composed mainly of carbon, hydrogen, and oxygen.
• Despite their simple structure, they perform complex functions in cells, serving as energy sources, providing structural support, and facilitating cell-to-cell communication.
• Hormones, certain vitamins, and defence mechanisms are based on lipids
• Lipids provide insulation for nerve cells and have waterproofing qualities

Carbohydrates

• Carbohydrates, also known as simple and complex sugar molecules, are common biological molecules
• The term "carbohydrate" comes from "carbo" (carbon) and "hydrate" (water)
• In photosynthesizing organisms, carbon dioxide and water are used to create carbohydrates
• Carbohydrates serve as an energy source, building material, and aid in cell communication
• Carbohydrates are present in fruits, vegetables, and grains
• Nutritionists study the carbohydrate composition of different foods

Monosaccharides

• Monosaccharides are the simplest type of carbohydrate containing a single sugar unit
• They consist of carbon, hydrogen, and oxygen in a 1:2:1 ratio, represented by the formula (CH2O)n
• Carbohydrates exist as monosaccharides or as multiple monosaccharide units linked together
• Glucose is a widely used monosaccharide produced by plants during photosynthesis, providing energy for plants and animals
• Triose (3 carbons), pentose (5 carbons), and hexose (6 carbons) monosaccharides are prevalent in living organisms
• Monosaccharides with five or more carbon atoms form a ring structure when in water through a reaction between two functional groups in the same molecule
• In a glucose ring, the -OH group on carbon 1 can have two arrangements: α-glucose and β-glucose
• Isomers are molecules with the same chemical formulas but different atomic arrangements
• Different arrangements of the –OH group on glucose give chemicals different properties
• Humans can digest starches from α-glucose easier, but cellulose from β-glucose is indigestible
• Glucose, fructose, and galactose are isomers of each other

Disaccharides

• Disaccharides are made of two monosaccharides joined via dehydration synthesis
• Maltose forms from the linkage of two α-glucose molecules, with oxygen as a bridge between the 1-carbon of one glucose and the 4-carbon of the second glucose
• A glycosidic bond is the bond that links monosaccharides into larger carbohydrates
• A glycosidic bond forms between α-glucose and fructose yields sucrose
• Lactose is the milk sugar disaccharide when galactose and β-glucose bond
• The chemical shorthand for glycosidic bonds between a 1-carbon and a 4-carbon is 1→ 4, and other linkages like 1→ 2, 1→ 3, and 1→ 6 are common
• Linkages are designated as α or β, depending on the orientation of the -OH group bonded to the 1-carbon
• The linkage in maltose and sucrose is an a-linkage, but the linkage in lactose is a β-linkage
• Disaccharides are easily dissolved in water due to the same functional groups that make monosaccharides hydrophilic
• Maple syrup, derived from maple tree sap, contains sucrose dissolved in water.

Complex Carbohydrates: Polysaccharides

• Complex carbohydrates (polysaccharides) form when hundreds to thousands of monosaccharides link together
• Some polysaccharides store energy in cells, while others provide structural support
• Starch and glycogen are storage carbohydrates, whereas cellulose and chitin are structural carbohydrates
• A polysaccharide is a chain of monosaccharides linked by glycosidic linkages
• A polysaccharide is a macromolecule, meaning a large molecule assembled from covalently linked smaller subunits
• Polysaccharide assembly through dehydration synthesis is polymerization
• Polymerization is the process of linking identical or variable subunits (monomers) into a long chain to form a larger molecule (polymer)
• The linkage of non-identical subunits creates diverse biological molecules like polysaccharides and DNA
• Plant starches, glycogen, and cellulose are among the most common polysaccharides, assembling from glucose
• Cellulose, the main component of plant cell walls, is the most abundant organic molecule on Earth
• Cellulose molecules are long, straight, and contain polar OH groups, enabling them to form numbers hydrogen bonds side by side
• These many hydrogen bonds give cellulose fibers their great strength
• Polysaccharides can be linear unbranched molecules or branched with side chains of sugar units attached to a main chain.
• Polysaccharides are polar and hydrophilic but attract water without dissolving due to their size

Lipids

• Lipids are a diverse group of non-polar biological molecules composed mostly of hydrogen, carbon, and lesser amounts of oxygen
• Lipids are smaller than complex carbohydrates, are not macromolecules, and are not polymers of defined monomeric subunits
• Lipids' insolubility contributes to their ability to form cell membranes
• Some lipids are stored as an energy source and lipids serve as hormones to regulate cellular activities as well as being vitamins
• Lipids in living organisms fall into five main categories: fatty acids, fats, phospholipids, steroids, and waxes.

Fatty Acids

• Fatty acids are the structural backbone of most lipids

• A fatty acid consists of a hydrocarbon chain with a carboxyl functional group (-COOH) at one end, giving it acidic properties

• Fatty acids in living organisms composed of hydrocarbon chains with our or more carbons

• The most common fatty acids have even-numbered chains of 14 to 22 carbons

• As chain length increases, fatty acids become progressively less water-soluble

• If a fatty acid's hydrocarbon chain binds the maximum possible number of hydrogen atoms and all carbons are linked with single bonds, it is a saturated fatty acid

• If double bonds exist in the chain, it is said to be unsaturated, meaning more hydrogen can be bonded

• Fatty acids with one double bond are monounsaturated, and those with multiple double bonds are polyunsaturated

• A double bond in an unsaturated fatty acid creates a kink in the molecule, causing it to bend

Fats

• A fat is a lipid made from two molecules: fatty acid and a glycerol

• One to three fatty acid chains join to a single glycerol molecule via dehydration synthesis between -OH groups on the glycerol and carboxyl groups of the fatty acids

• Triglycerides are well-known fats containing three fatty acid chains linked to a glycerol

• A fat molecule can have identical or different fatty acid chains linked to the glycerol

• Saturated fats from animals (e.g., butter, lard) contain saturated fatty acids

• Unsaturated fats (oils) mainly contain unsaturated and polyunsaturated fatty acids

• As their fatty acid chain increases, triglycerides generally become less fluid

• Shorter chains remain liquid as oils at room temperature

• Saturated fats (e.g., butter) are solids because their chains are long, straight, and pack closely together

• Unsaturated fatty acid chains are bent, preventing them from packing as tightly as saturated fats, which stay more fluid and are liquid at room temperature

• Organisms need flexible cells to move easily and warm-blooded mammals/birds have more saturated fats

• Plant seeds contain unsaturated fats and cold-water fish contain unsaturated fish "oils."

• Triglycerides yield twice as much energy as carbohydrates

• A layer of fatty tissue under the skin serves as insulation in mammals and birds

• Plant fats are generally unsaturated and considered healthier than saturated animal fats

• Diets rich in saturated fats can lead to heart disease, whereas unsaturated fats can improve your health

Phospholipids

• Phospholipids are phosphate-containing lipids essential for cell membranes
• A glycerol backbone links to two to fatty acids and the third site links to a charged phosphate group, which often binds a polar unit
• Thus a phospholipid has fatty acids (hydrophobic) and a hydrophilic polar group (head group)
• Amphipathic molecules contain hydrophobic and hydrophilic regions
• Phospholipids form the lipid bilayer of cell membranes where the hydrophilic end faces outward toward water, and the hydrophobic tails face inward

Steriods

• Steroids are a group of lipids with structures based on four fused carbon rings
• Small differences in the side groups attached to the rings distinguish one steroid from another
• Sterols, the most abundant steroids, have a single polar -OH group and a complex, non-polar hydrocarbon chain
• Sterols have dual solubility properties and assume positions in cells that satisfy these properties
• Cholesterol, a steroid in animal plasma membranes, has similar sterols, called phytosterols, in plant cell membranes
• Cholesterol converts into compounds like vitamin D but too much dietary cholesterol is harmful
• High cholesterol and saturated fats can develop atherosclerosis, where fat deposits form on blood vessel linings, which can lead to a heart attack
• Sex hormones like testosterone, estrogens, and progesterone are steroids that control the development of sexual traits and sex cells
• Anabolic steroids mimic testosterone to build muscle mass that are banned by all major sporting bodies because many are unsafe

Waxes

• Waxes are large lipid molecules made of long fatty acid chains linked to alcohols or carbon rings
• Waxes are hydrophobic, extremely non-polar soft solids over a wide range of temperatures for waterproof coatings
• Cutin, a cutin secreted by plant cells, forms a water-resistant coating on stems, leaves, and fruit for water conservation and protection from infections and diseases
• Birds secrete waxy material for dry feathers and bees produce beeswax for honeycombs