AP Biology 1.3-1.4: Carbohydrates and Lipids

Carbohydrate

An organic macromolecule commonly represented by the empirical formula $(CH_2O)_n$, giving an approximate C:H:O ratio of 1:2:1. Carbohydrates function in energy storage, energy release, and structural materials.

How are carbohydrates classified by the number of sugar units they contain?

Monosaccharides contain one sugar unit, disaccharides contain two covalently linked sugar units, and polysaccharides contain many monosaccharide units linked by glycosidic bonds.

Monosaccharide

A single sugar unit that cannot be hydrolyzed into a simpler carbohydrate. Common monosaccharides contain three to seven carbons and often have names ending in $-ose$.

How do aldoses and ketoses differ structurally?

An aldose contains an aldehyde functional group, typically at the end of the carbon chain. A ketose contains a ketone functional group, typically within the carbon chain.

How do the prefixes triose, pentose, and hexose classify monosaccharides?

They indicate the number of carbon atoms: a triose has 3, a pentose has 5, and a hexose has 6.

Glucose

A six-carbon aldose with molecular formula $C_6H_{12}O_6$. It is a major cellular fuel whose oxidation provides energy for ATP production and is also the monomer in starch, glycogen, and cellulose.

Why are glucose, galactose, and fructose isomers?

They have the same molecular formula, $C_6H_{12}O_6$, but different arrangements of atoms and functional groups. Glucose and galactose are aldoses, whereas fructose is a ketose.

Which common monosaccharides are hexoses, and how do they differ by functional group?

Glucose, galactose, and fructose are all hexoses. Glucose and galactose are aldoses; fructose is a ketose.

What structural forms can monosaccharides adopt in aqueous solution?

They can exist as open-chain molecules or cyclic rings, with ring forms generally predominating in water. Ring formation creates an anomeric carbon that can have different stereochemical configurations.

Anomeric carbon

The carbon that was part of the carbonyl group in the open-chain form and becomes a new stereocenter when a monosaccharide cyclizes. In glucose, this is carbon 1.

How do alpha and beta anomers of a cyclic sugar differ?

They differ in the orientation of the hydroxyl group attached to the anomeric carbon. For the convention described for glucose, an OH below the ring plane is the alpha form, while one above the plane is the beta form.

Disaccharide

A carbohydrate formed when two monosaccharides join through a glycosidic bond. The linkage forms in a dehydration reaction that releases $H_2O$.

Glycosidic bond

A covalent bond linking monosaccharide units in a disaccharide or polysaccharide. It forms when an OH group and an H are removed as water during condensation.

What happens chemically when two monosaccharides form a disaccharide?

A dehydration synthesis reaction removes an OH group from one sugar and an H from the other, producing $H_2O$ and a covalent glycosidic linkage.

What monosaccharides compose lactose, maltose, and sucrose?

Lactose consists of glucose and galactose, maltose consists of two glucose units, and sucrose consists of glucose and fructose.

How is a glycosidic linkage such as $\alpha(1\rightarrow4)$ interpreted?

The bond uses the alpha configuration at carbon 1 of one monosaccharide and connects it to carbon 4 of the other monosaccharide. The numbers identify the carbon atoms involved in the linkage.

Polysaccharide

A polymer made of many monosaccharides connected by glycosidic bonds. Polysaccharide chains may be linear or branched and can serve storage or structural roles.

How do amylose and amylopectin differ?

Both are glucose polymers in plant starch. Amylose is primarily unbranched with $\alpha(1\rightarrow4)$ linkages, whereas amylopectin contains $\alpha(1\rightarrow6)$ branch points in addition to $\alpha(1\rightarrow4)$ linkages.

Starch

The principal storage polysaccharide in plants, composed of glucose polymers called amylose and amylopectin. Digestion can hydrolyze starch into smaller sugars such as maltose and glucose.

Glycogen

A highly branched glucose-storage polysaccharide in animals, especially in liver and muscle cells. Its branching permits stored glucose to be mobilized relatively rapidly when blood glucose decreases.

How do starch and glycogen function similarly and differently?

Both store glucose through predominantly alpha glycosidic linkages. Plants store glucose as starch, while animals store it as the more highly branched glycogen.

Cellulose

A structural polysaccharide composed of glucose units joined by $\beta(1\rightarrow4)$ glycosidic bonds. Its straight chains hydrogen-bond to neighboring chains, forming strong plant cell-wall fibers.

Why can humans not digest cellulose efficiently?

Human digestive enzymes do not hydrolyze cellulose's $\beta(1\rightarrow4)$ glycosidic bonds. Some herbivores and termites rely on symbiotic microorganisms that produce cellulase.

How does the alpha-versus-beta configuration affect polysaccharide structure and function?

Alpha linkages favor coiled or helical storage polymers such as starch, whereas beta linkages in cellulose produce extended chains that hydrogen-bond together for structural strength.

Chitin

A nitrogen-containing polysaccharide made from repeating modified glucose units, specifically $N$-acetyl-$\beta$-$D$-glucosamine. It provides structural support in arthropod exoskeletons and fungal cell walls.

What are important biological roles of dietary carbohydrate and fiber?

Glucose supplies readily available energy for ATP production, while insoluble fiber adds bulk, slows glucose uptake, can bind some cholesterol in the intestine, and promotes digestive regularity.

What is the approximate energy yield of carbohydrates compared with fats?

Carbohydrates provide approximately $4.3\ \text{kcal g}^{-1}$, whereas fats provide approximately $9\ \text{kcal g}^{-1}$.

Lipid

A diverse group of mostly nonpolar, hydrophobic molecules containing many carbon-carbon and carbon-hydrogen bonds. Major lipid types include fats, oils, waxes, phospholipids, and steroids.

Why are most lipids hydrophobic?

Their structures contain extensive nonpolar hydrocarbon regions, which cannot form favorable interactions with polar water molecules. As a result, most lipids are poorly soluble in water.

What major functions do lipids perform in organisms?

They provide long-term energy storage, insulation and water resistance, membrane structure, and precursors for hormones and other biologically important molecules.

What are the structural components of a triacylglycerol?

A triacylglycerol contains one glycerol molecule esterified to three fatty acids. Forming its three ester bonds through condensation releases three water molecules.

Ester bond

A covalent linkage formed between a carboxylic acid and an alcohol, typically with loss of water. In fats, ester bonds connect fatty acids to the hydroxyl groups of glycerol.

Fatty acid

A molecule with a long hydrocarbon chain and a terminal carboxylic acid group. Fatty acids may be saturated or unsaturated depending on the presence of carbon-carbon double bonds.

How do saturated and unsaturated fatty acids differ?

Saturated fatty acids contain only carbon-carbon single bonds and therefore have the maximum possible number of hydrogens. Unsaturated fatty acids contain one or more carbon-carbon double bonds.

Monounsaturated versus polyunsaturated fatty acid

A monounsaturated fatty acid has one carbon-carbon double bond. A polyunsaturated fatty acid has two or more carbon-carbon double bonds.

Why are many saturated fats solid while many cis-unsaturated fats are liquid at room temperature?

Straight saturated chains pack closely and experience stronger intermolecular attractions. Cis double bonds introduce bends that disrupt packing, lowering the melting point and often producing liquids.

How do cis and trans double-bond configurations affect fatty-acid shape?

In a cis configuration, relevant hydrogens lie on the same side of the double bond, producing a kink in the chain. In a trans configuration, they lie on opposite sides, making the chain more linear.

What chemical change occurs during hydrogenation of an unsaturated oil?

Hydrogen is added across some carbon-carbon double bonds, decreasing unsaturation and making the material more solid. The process can also convert some cis double bonds into trans configurations.

Why are trans fats a health concern?

Dietary trans fats are associated with increased LDL cholesterol and plaque formation in arteries, which can elevate cardiovascular disease risk.

Omega-3 fatty acid

A polyunsaturated fatty acid with a double bond beginning at the third carbon from the methyl, or omega, end of the chain. Some omega-3 fatty acids are essential because humans cannot synthesize them.

Essential fatty acid

A fatty acid required by the body but not synthesized in sufficient amounts, so it must be obtained through food. Omega-3 and omega-6 fatty acids are important examples.

Why are some fatty acids described as essential nutrients?

Humans require them for normal biological functions but lack the enzymes needed to synthesize them adequately. They must therefore be supplied by the diet.

Wax

A lipid formed from long-chain fatty acids esterified to long-chain alcohols. Waxes create water-repellent coatings on plant surfaces and on structures such as aquatic birds' feathers.

How does a phospholipid differ from a triacylglycerol?

A phospholipid has two fatty acid tails and a phosphate-containing polar group attached to a glycerol backbone. A triacylglycerol has three fatty acid tails and is primarily used for energy storage.

Amphipathic molecule

A molecule containing both hydrophilic and hydrophobic regions. Phospholipids are amphipathic because their phosphate-containing heads interact with water while their hydrocarbon tails avoid it.

Why do phospholipids spontaneously form bilayers in water?

Their hydrophilic heads orient toward the surrounding water, while their hydrophobic tails cluster away from water. This arrangement minimizes unfavorable water-hydrocarbon interactions and creates a membrane bilayer.

What is the orientation of phospholipids in a biological membrane?

The polar, hydrophilic phosphate-containing heads face the aqueous environments on both sides of the membrane. The nonpolar fatty acid tails face inward toward one another.

Micelle

A spherical aggregate in which amphipathic molecules arrange their hydrophilic portions toward water and their hydrophobic portions toward the interior. Micelles can form when phospholipids or similar molecules are dispersed in water.

Steroid

A lipid characterized by four fused carbon rings. Steroids are hydrophobic or largely hydrophobic despite having a structure distinct from fats and phospholipids.

What is the role of cholesterol in animal cells?

Cholesterol is a sterol embedded within the phospholipid bilayer that helps regulate membrane fluidity. It is also a precursor for steroid hormones, vitamin D, and bile salts.

How does cholesterol help maintain membrane fluidity?

Cholesterol limits excessive phospholipid movement at higher temperatures and prevents tight packing and solidification at lower temperatures. Thus, it buffers temperature-dependent changes in membrane fluidity.

What are the principal biological sources and storage sites of fats?

Animals store fats mainly in adipocytes, where fat droplets provide long-term energy reserves and insulation. Plants commonly store fats or oils in seeds for use during seedling development.