Chemistry 1 Solo 2 Lecture 1

Carbohydrates

Aldehyde or ketone derivatives of polyhydroxy alcohols composed of carbon, hydrogen, and oxygen in a 1:2:1 ratio

Carbohydrate Functions

Provide a source of energy (e.g., glucose)

Structural components of RNA and DNA (e.g., ribose and deoxyribose sugars)

Carbohydrate dietary sources

Bread, pasta, beans, fruit, soft drinks, rice, and potatoes

More about carbohydrates

Various types of sugar are derived from different sources

Simple sugars are called monosaccharides and include glucose (aka dextrose), fructose, and galactose.

Table sugar refers to sucrose, a disaccharide of glucose and fructose.

Three main groups of carbohydrates exist

Monosaccharides

Disaccharides

Polysaccharides

Monosaccharides

Simple sugars that consist of a single polyhydroxy aldehyde or ketone unit

Unable to be hydrolyzed to a simpler form

Formed from the breakdown of starches and disaccharides within the small intestine

The backbone is composed of carbon atoms

Monosaccharides are termed __________ according to the position of the carbonyl group

Aldoses and ketoses

Disaccharides

Composed of two monosaccharides joined covalently by an O-glycosidic bond with the loss of a molecule of water

The most common disaccharides

Maltose (glucose + glucose)

Lactose (glucose + galactose)

Sucrose (glucose + fructose)

Polysaccharides

Large numbers of monosaccharides linked together

i.e., Starch, glycogen, and cellulose

Starch

Major carbohydrate storage in plants

Composed of amyloses and amylopectins that contain glucose residues

Glycogen

Major carbohydrate storage in animals

A heavily branched polysaccharide containing many glucose residues

Most abundant in the liver and skeletal muscle

Cellulose

Provides structural support in plants

Glycoproteins

Proteins with oligosaccharides covalently attached to the extracellular region

Most proteins that are secreted are glycoproteins

Antibodies

Hormones

Coagulation factors

Functions of carbohydrates in glycoproteins include

Regulation of cell lifespan; loss of sialic acid residues from the ends of oligosaccharide chains of RBCs results in removal of RBCs from circulation

Cell-to-cell recognition

Secretion

Glycolysis

Metabolism of glucose molecule to pyruvate or lactate for production of energy

Gluconeogenesis

Formation of glucose-6-phosphate from noncarbohydrate sources

Glycogenolysis

Breakdown of glycogen to glucose for use as energy

Glycogensis

Conversion of glucose to glycogen for storage

Lipogenesis

Conversion of carbohydrates to fatty acids

Lipolysis

Decomposition of fat

Carbohydrate metabolism

Glucose is the primary energy source for the human body

A six-carbon monosaccharide

Derived

• The breakdown of carbohydrates in the diet or body stores (glycogen) or

• Endogenously synthesized from protein or the glycerol part of triglycerides

Glucose transporters are found on cell surfaces, which promote glucose uptake in cells

Glucose and some other sugars like galactose have the ability to reduce cupric ions (Cu³+) to cuprous ions (Cu²+) in alkaline solution; these are referred to as reducing sugars

Step by Step

• Ingestion of food and salivary amylase leads to hydrolyzing starch. Swallowing of food and gastric enzymes continue to digest food as it flows in the intestinal tract

• Pancreatic enzymes are released into the intestinal tract until monosaccharides are formed; a requirement that must occur before being absorbed by the intestinal mucosal cells

• Absorbed monosaccharides are transported to the liver where fructose and galactose are converted to glucose

According to the body’s requirement glucose is either:

• Released into circulation for energy production in the tissues

• Converted to glycogen in the liver for storage

• Utilized in skeletal muscle for protein synthesis

• Used in adipose tissue for triglyceride synthesis

When calorie intake exceeds use, the excess is converted to fat and glycogen for storage in adipose tissue and liver or muscle, respectively

When energy exceeds calorie intake, glucose formation occurs from the breakdown of carbohydrate stores and from non-carbohydrate sources

An example of a disaccharide is

Lactose

The formation of glucose from non-carbohydrate sources occurs mostly in the liver and is referred to as

Gluconeogenesis

The conversion of glucose into its storage form is referred to as

Glycogenesis

Regulation of blood glucose concentration

Concentration of glucose in the blood is maintained at steady levels even under various conditions of feeding, fasting, and exercise. How?

Maintenance of glucose concentration occurs through the action of hormones

• Insulin

Hormones that oppose insulin:

• Glucagon

• Epinephrine

• Cortisol

• Growth hormone

Other hormones influencing glucose metabolism:

• Thyroxine (FT4)

• Somatostatin

Insulin, glucagon, and epinephrine control plasma glucose concentration by regulating glycogenolysis

Cortisol and growth hormone increase gluconeogenesis

Regulation of Blood Glucose Concentration - Insulin

A protein produced and secreted by the beta cells of the Islets of Langerhans in the pancreas

Decreases blood glucose levels by stimulating the uptake of glucose into fat and muscle and stimulates glycolysis

Promotes the conversion of glucose to glycogen or fat for storage

Inhibits glucose production by the liver

Stimulates protein synthesis and inhibits protein breakdown

Regulation of Blood Glucose Concentration - Glucagon

Protein hormone secreted by the alpha cells of the pancreas that Increases blood glucose through glycogenolysis and gluconeogenesis in the liver

A minor target organ for glucagon is adipose tissue; glucagon increase lipolysis and enhances ketogenesis

Levels are regulated by plasma glucose concentrations; levels become increased during stress and exercise and in hypoglycemic episodes

Long term diabetes mellitus leads to an impaired glucagon response to hypoglycemia

Extremely high levels of glucagon are seen in patients with glucagonomas: tumors in the alpha cells of the pancreas

Regulation of Blood Glucose Concentration - Epinephrine (adrenaline)

Epinephrine is a catecholamine secreted by the adrenal gland that stimulates glucagon secretion and inhibits insulin secretion

Increased production is seen in physical or emotional stress (fight or flight) and in pheochromocytomas (adrenal medulla tumor)

Regulation of Blood Glucose Concentration - Growth Hormone

Secreted by the anterior pituitary gland

Stimulates gluconeogenesis

Enhances lipolysis

Opposes insulin-stimulated glucose uptake

Regulation of Blood Glucose Concentration - Cortisol

Cortisol is often called the “stress hormone” because of its connection to the stress response

Cortisol is one of the steroid hormones and is made in the adrenal glands; secretion of the hormone is controlled by the hypothalamus, the pituitary gland, and the adrenal gland; cortisol is secreted in response to adrenocorticotropic hormone (ACTH)

Most cells within the body have cortisol receptors

Cortisol can help control blood sugar levels by stimulating gluconeogenesis

Cortisol also helps regulate metabolism, reduce inflammation, and even assist with memory formulation. It has a controlling effect on salt and water balance and helps control blood pressure. In women, cortisol also supports the developing fetus during pregnancy

Regulation of Blood Glucose Concentration - Other hormones influencing glucose metabolism

Thyroxine (FT4)

• Secreted by the thyroid gland

• Indirectly involved in glucose homeostasis

• Stimulates glycogenolysis and gluconeogenesis

• Increases rate of intestinal glucose absorption

Somatostatin

• Found in GI tract, hypothalamus, and delta cells of the pancreatic islets

• Known as growth hormone inhibiting hormone

• Inhibits secretion of insulin, glucagon, and growth hormone

Regulation of Blood Glucose Concentration - Figure 33-1

Which hormone decreases blood glucose levels?

Insulin

Which hormone produces hyperglycemia

All of the above (Epinephrine, glucagon, and thyroid hormone)

Which of the following hormones has an indirect effect on carbohydrate metabolism by inhibiting the release of growth hormone for the pituitary and inhibiting secretion of glucagon and insulin

Somatostatin