Carbohydrates

Types of Carbohydrates

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically with the formula Cn(H2O)n. They are classified based on their structure and complexity:

• Monosaccharides: Simple sugars (e.g., glucose, fructose).

• Disaccharides: Two monosaccharides linked (e.g., maltose, lactose, sucrose).

• Oligosaccharides: Short chains of monosaccharides.

• Polysaccharides: Long chains (e.g., starch, glycogen, cellulose).

Ketose vs. Aldose

Monosaccharides are further classified based on the type of carbonyl group present:

• Aldose: Contains an aldehyde group at the end (e.g., glucose).

• Ketose: Contains a ketone group, usually at the second carbon (e.g., fructose).

Naming System: The names reflect the number of carbons and the type of carbonyl group (e.g., aldohexose, ketohexose).

Enediol Intermediate: During isomerization, an enediol intermediate forms, allowing conversion between aldose and ketose forms.

Stereoisomer Key Terms

Stereoisomers are molecules with the same formula but different spatial arrangements:

• Enantiomers: Non-superimposable mirror images.

• Diastereomers: Stereoisomers that are not mirror images.

• Meso Structures: Molecules with multiple stereocenters but are optically inactive due to internal symmetry.

Example: D- and L-glyceraldehyde are enantiomers.

Stereoisomers of Carbohydrates

• D vs. L: Refers to the configuration of the chiral center furthest from the carbonyl group. D is most common in nature.

• Optical Activity: D and L forms rotate plane-polarized light in opposite directions.

• Determining D vs. L: If the hydroxyl group on the reference carbon is on the right, it is D; left is L.

• Alpha vs. Beta Anomers: Anomers differ at the anomeric carbon (C-1 in aldoses). Alpha: OH down; Beta: OH up (in Haworth projection).

Heteropolysaccharides vs. Homopolysaccharides

• Homopolysaccharides: Composed of one type of monosaccharide (e.g., cellulose, starch).

• Heteropolysaccharides: Composed of two or more types of monosaccharides (e.g., hyaluronic acid).

Identification of Key Structures

• Glyceraldehyde: Simplest aldose (triose).

• Glucose: Aldohexose, main energy source.

• Fructose: Ketohexose, found in fruits.

• Galactose: Aldohexose, component of lactose.

• Ribose vs. Deoxyribose: Ribose (RNA), deoxyribose (DNA; lacks one oxygen).

• Maltose: Glucose + glucose (α-1,4 linkage).

• Lactose: Glucose + galactose (β-1,4 linkage).

• Sucrose: Glucose + fructose (α-1,2 linkage).

• Amylose vs. Amylopectin: Both are starch; amylose is linear (α-1,4), amylopectin is branched (α-1,6).

• Glycogen: Highly branched storage polysaccharide in animals.

• Cellulose: Linear, β-1,4 linked glucose; structural in plants.

Lipids and Their Functions in Biological Systems

Types of Lipids

Lipids are hydrophobic biomolecules with diverse structures and functions. The four main types are:

• Fatty Acids

• Glycerides (mono-, di-, tri-)

• Nonglyceride Lipids (e.g., sphingolipids, steroids, waxes)

• Complex Lipids (e.g., lipoproteins)

Common Functions of Lipids

• Energy storage (e.g., triglycerides)

• Structural components of membranes (e.g., phospholipids, cholesterol)

• Signaling molecules (e.g., eicosanoids, steroids)

• Insulation and protection

Fatty Acids

• Saturated vs. Unsaturated: Saturated have no double bonds; unsaturated have one or more double bonds.

• Chemical and Physical Differences: Saturated fats are solid at room temperature; unsaturated are liquid. Double bonds cause kinks, reducing packing.

Common Fatty Acids: See Table 17.1 for chain length and saturation (e.g., palmitic acid, oleic acid).

Omega Fatty Acids: Named for the position of the first double bond from the methyl end (omega-3, omega-6).

Eicosanoids

• Precursors: Derived from arachidonic acid.

• Types: Prostaglandins, thromboxanes, leukotrienes.

• Prostaglandin Structure: Cyclopentane ring with two side chains.

• Function: Involved in inflammation, pain, and regulation of blood flow.

Glycerides

• Mono-, Di-, Tri-Glycerides: One, two, or three fatty acids esterified to glycerol.

• Esterification: Formation of ester bonds between fatty acids and glycerol.

• Hydrogenation: Addition of hydrogen to unsaturated bonds, making fats more saturated.

• Acid Hydrolysis: Breaking ester bonds with acid.

• Saponification: Hydrolysis of triglycerides with base to produce soap and glycerol.

Phosphoglycerides: Glycerol backbone, two fatty acids, and a phosphate group (phosphatidate structure).

Common Membrane Phospholipids: Phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine (see Fig. 17.7).

Nonglyceride Lipids

• Sphingolipids: Based on sphingosine backbone.

• Sphingosine Structure: Long-chain amino alcohol.

• Types:

  • Sphingomyelin: Contains phosphocholine.

  • Ceramide: Sphingosine + fatty acid.

  • Glucocerebroside: Sphingosine + glucose.

  • Galactocerebroside: Sphingosine + galactose.

• Steroids: Four fused rings (steroid nucleus). Functions include hormones, membrane structure.

• Waxes: Esters of long-chain fatty acids and alcohols. Protective, water-repellent properties.

Complex Lipids

• Definition: Lipids with additional components (e.g., proteins, carbohydrates).

• Plasma Lipoproteins: Transport lipids in blood.

Ranking of Densities: Chylomicrons < VLDL < LDL < HDL.

Process of Endocytosis of LDL: LDL binds to receptors, is internalized, and cholesterol is released for cellular use.

Structure of Biological Membranes

• Fluid Mosaic Model: Membranes are dynamic, with proteins and lipids moving laterally.

• Lipid Bilayer Structure: Two layers of phospholipids with hydrophobic tails inward, hydrophilic heads outward.

• Components: Phospholipids, cholesterol, proteins.

• Peripheral vs. Transmembrane Proteins: Peripheral proteins are loosely attached to membrane surface; transmembrane proteins span the bilayer.