Chapter 16

Carbohydrates

Carbohydrate

Nutrient and biomolecule made of carbon, hydrogen, and oxygen

• ex: sugar/starches, are polyhydroxy aldehydes or ketones, or compounds that can be hydrolyzed to them

• Carbohydrates can be classified as monosaccharides, disaccharides, and polysaccharides

• Carbohydrates are synthesized in green plants through photosynthesis

• In the body, they are used for burst of energy needed during exercise in the form of glucose (lots pf marathon runners hvae carbons beforehand)

Photosynthesis

process that uses the energy from the sun to convert carbon dioxide and water into glucose and oxygen

Monosaccharides

the simplest carbohydrates, generally have 3 to 6 carbon atoms in a chain with a carbonyl group and many -OH (hydroxyl) groups

• Monosaccharides are sweet tasting, but their relative sweetness varies a great deal

• they are polar compounds with high melting points

• The presence of so many polar functional groups capable of hydrogen bonding makes the monosaccharides very water soluble

• ALL MONOSACCHARIDES ARE CHIRAL EXCEPT HYDROXYACETONE

Aldoses (Aldehyde)

Monosaccharides with a carbonyl group at C1

Ketoses (Ketone)

Monosaccharides with carbonyl group at C2

Glyceraldehyde

Simplest aldose (C3H6O3)

• Constitutional isomer (same formula diff shape) of Dihdroxyacetone

• Has one chiral center can change the function of the molecule and two possible enantiomer (the l and d cuz they the same just diff on where the molecule is attached)

Dihdroxyacetone

Simplest ketose (C3H6O3)

• Constitutional isomer (same formula diff shape) of Glyceraldehyde

Chiral center

tetrahedral atom that have four different substituents

• The orientation of these chiral centers can change the function of the molecule

• To determine whether it is D or L (if OH is right or left), look at the arm that is farthest from the chiral center and see what side its on

• All naturally occurring sugars are D sugars

Number of carbons in its chain

• Triose → 3 carbon

• Tetrose → 4 carbon

• Pentose → 5 carbon

• Hexose → 6 carbon

  • ex: aldotriose

Fischer projection formula

• Prefix D is used when the -OH group is drawn on the right side of the carbon chain

• The prefix L is used when the OH group is drawn on the left side of the carbon chain

• the wedged and dashed line can be re-drawn in a Fischer projection formula

Glucose

• has four chirality centers

• Glucose is blood sugar and the most abundant monosaccharide

• Normal blood glucose levels are 70-110 mg/dL

• Insulin regulates blood glucose levels by stimulating the uptake of glucose into tissues or its conversion to glycogen

• Patients with diabetes type 1 produce insufficient insulin to adequately regulate blood sugar levels, so they must monitor their diet and/or inject insulin daily

Disaccharide lactose

Galactose + glucose

Disaccharide sucrose

fructose + glucose

Forming a ring

hydroxyl and carbonyl reacting together

Cyclic form of monosaccharides

The first step in cyclization is to rotate glucose 90 degrees

Second step, chain must be twisted around to form a six-membrane ring

• THE POINTY THING ON THE RIGHT IF OH IS BOTTOM IT IS ALPHA IF IT IS TOP IT IS BETA

Fischer Projection

A line shape

Haworth projection

the circular ring shape sugar molecule

Disaccharides

Carbohydrates composed of monosaccharides

• Formed when a hydroxyl group of one monosaccharide reacts with a hydroxyl group of a second monosaccharide

Glycosidic linkage

The bond that joins the two rings when this reaction occurs between two monosaccharides

• bonds can be alpha or beta

hydrolysis of maltose

yields 2 glucose molecules

Lactose

• disaccharide in milk; it consist of 1 galactose ring going 1→ 4 glycosidic bond (1 starts on the right pointy thing)

• the disaccharide bond is cleaved by the enzyme lactase in the body

• Individuals who are lactose intolerant no longer produce this enzyme

• Without the enzyme, lactose cannot be digested, causing abdominal cramps and diarrhea

Sucrose

disaccharide of 1 Glucose + 1 Fructose

• sweet but many many calories

• Tor educe calorie intake, many artificial sweeteners have been developed

Polysaccharides

contain three or more monosaccharides joined together

Cellulose

an unbranched polymer made up of repeating glucose units joined by 1→ 4 glycosidic linkages

• found in the cell walls of all plants, where it gives support and rigidity to wood, plant stems, and grass

• Humans do not possess the enzyme to hydrolyze cellulose and cannot digest it

• Cellulose makes up the insoluble fiber in our diets, which is important in adding bulk to waste to help eliminate it more easily

Starch

Polymer made up of repeating glucose units joined by alpha glycosidic linkages

• present in corn, rice, wheat, and potatoes

• two common forms of starch are amylose and amylopectin. Both starch molecules can be digested by humans using the enzymes and amylose

amylose

unbranched (linear) polymer

Amylopectin

branched (has side chains extending off main backbone) polymer

Glycogen

Major form of polysaccharide storage in animals, similar in structure to amylopectin

• stored mainly in the liver and in muscle cells

• when glucose is needed for energy, glucose units are hydrolyzed from the ends of the glycogen polymer

• Because glycogen is highly branched, there are many ends avilable for hydrolysis