Carbohydrates and Lipids: Structure, Classification, and Biological Functions

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.

Type	Structure	Physical State
Saturated	No double bonds	Solid
Unsaturated	One or more double bonds	Liquid

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.

Class	Density	Main Function/Cargo
Chylomicrons	Lowest	Transport dietary triglycerides
VLDL	Very Low	Transport endogenous triglycerides
LDL	Low	Deliver cholesterol to tissues
HDL	High	Reverse cholesterol transport

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.