Carbohydrates (2)

Glucagon

When blood glucose falls (as occurs between meals), other special cells of the pancreas respond by secreting glucagon into the blood. Glucagon raises blood glucose by signaling the liver to break down its glycogen stores and release glucose into the blood for use by all the other body cells.

Glycogen molecule

A glycogen molecule contains hundreds of glucose units in highly branched chains. Each new glycogen molecule needs a special protein for the attachment of the first glucose.

Condensation reaction

A reaction in which two molecules become covalently bonded to each other through the loss of a small molecule, usually water; also called dehydration reaction.

Starch molecule

A starch molecule contains hundreds of glucose molecules in either occasionally branched chains, some of which are flexible (amylopectin) or unbranched chains (amylose).

Adrenaline

Another hormone that signals the liver cells to release glucose is the "fightor-flight" hormone, adrenaline. When a person experiences stress, adrenaline acts quickly to ensure that all the body cells have energy fuel in emergencies. Among its many roles in the body, epinephrine works to release glucose from liver glycogen to the blood.

Hydrolysis

Breaking down complex molecules by the chemical addition of water

When blood glucose falls below normal _______________________________________

food can replenish it, or in the absence of food, glucagon is released by the pancreas, it then signals the liver to break down glycogen stores.

Lactate

hydrolyses lactose into one glucose and one galactose molecule.

Cellulose

Human enzymes can digest starch but they cannot digest cellulose because the bonds that link the glucose molecules together are different

Carbohydrate digestion: in mouth

In the mouth, thoroughly chewing high-fiber foods slows eating and stimulates the flow of saliva. The salivary enzyme amylase starts to work, hydrolysing starch to shorter polysaccharides and to the disaccharide maltose. Because food is in the mouth for a relatively short time, very little carbohydrate digestion takes place there; it begins again in the small intestine.

Blood glucose homeostasis is regulated primarily by two hormones: ____________________, which moves glucose from the blood into the cells, and _____________________, which brings glucose out of storage when necessary.

Insulin and glucagon

When blood glucose rises above normal __________________________________

Insulin can signal the cells to take in glucose for energy (in the liver and muscle stored as glycogen). Eating balanced meals that provide abundant carbohydrates, including fibers, and a little fat help to slow down the digestion and absorption of carbohydrate so that glucose enters the blood gradually. Eating at regular intervals also helps the body maintain a balance between the extremes

Microvilli

Monosaccharides, the end products of carbohydrate digestion, enter the capillaries of the intestinal villi.

Nutrient absorption largely takes place in the ___________________________

Small intestine; glucose and galactose enter the cells lining the small intestine by active transport (use energy, fast); fructose is absorbed by facilitated diffusion (no energy, slower). As the blood from the small intestine circulates through the liver, cells there take up fructose and galactose and most often convert them to glucose.

Lactose

The combination of galactose and glucose makes the disaccharide lactose (joined by a Bterm-3-(1,4,)-glycosidic bond), the principal carbohydrate of milk. Known as milk sugar, lactose contributes half of the energy provided by fat-free milk (4-5% in animal milk) Digested by intestinal lactase to yield one D-galactose and one D-glucose

Carbohydrate digestion: in small intestine

The small intestine performs most of the work of carbohydrate digestion. A major carbohydrate-digesting enzyme, pancreatic a-amylase, enters the intestine via the pancreatic duct and continues breaking down the polysaccharides to shorter glucose chains and maltose (hydrolysed starch). The final step takes place on the outer membranes (brush border) of the intestinal cells. There specific enzymes (maltase, sucrase, lactase; anchored onto the brush border cells and are not free) break down specific disaccharides

Glucose homeostasis

To function optimally, the body must maintain blood glucose within limits that permit the cells to nourish themselves (~ 5 mM). If blood glucose falls below normal (hypoglycaemia), a person may become dizzy and weak; if it rises above normal (hyperglycaemia; characteristic of diabetes), a person may become fatigued. Left untreated, fluctuations to the extremes—either high or low—can be fatal.

Maltase

breaks maltose (disaccharide) into two glucose molecules.

Sucrase

breaks sucrose into one glucose and one fructose molecule.

Carbohydrate digestion: in large intestine

Within 1 to 4 hours after a meal, all the sugars (monosaccharides) and most of the starches have been digested. Only the fibers remain in the digestive tract. Fibers in the large intestine attract water, which softens the stools for passage without straining. Also, bacteria in the GI tract ferment some fibers. This process generates water, gas, and short-chain fatty acids

Sucrose

a disaccharide composed of glucose and fructose; commonly known as table sugar, beet sugar, or cane sugar. Sucrose also occurs in many fruits and some vegetables and grains. The a-D-glucose and B-D-fructose are joined by an a(1,2,)B-glycosidic bond Digested by intestinal sucrase

Carbohydrate digestion: in stomach

Carbohydrate digestion ceases in the stomach. The activity of salivary amylase diminishes as the stomach's acid (HCl) and protein-digesting enzymes inactivate the enzyme. The stomach's digestive juices contain no enzymes to break down carbohydrates. HCl can hydrolyse starch to release very limited amounts of glucose Fibers are not digested, but because they linger in the stomach, they delay gastric emptying, thereby providing a feeling of fullness (satiety).

Maltose

The disaccharide maltose consists of two glucose units; 2 units of D-glucose joined by an a-(1,4)-glycosidic bond Maltose is produced whenever starch breaks down—as happens in human beings during carbohydrate digestion; formed by the digestion of starch by pancreatic enzyme alpha-amylase in the intestine. Maltose is only a minor constituent of a few foods, most notably barley.

Starch

The human body stores glucose as glycogen, but plant cells store glucose as starches—long, branched or unbranched chains of hundreds or thousands of glucose molecules linked together. When you eat the plant, your body hydrolyzes the starch to glucose and uses the glucose for its own energy purposes. All starchy foods come from plants. Grains are the richest food source of starch, also found in tubers (potatoes) and legumes

Glycogen

a high branched animal polysaccharide composed of glucose; a storage form of glucose manufactured and stored in the liver and muscles. Provides a rapid source of glucose (energy) when needed. Glycogen is not a significant food source of carbohydrate and is not counted as a dietary carbohydrate in foods.

Insulin

a hormone secreted by special cells in the pancreas in response to (among other things) elevated blood glucose concentration. Insulin controls the transport of glucose from the bloodstream into the liver, muscle and fat cells.

Polysaccharide

compounds composed of many monosaccharides linked together (> 20 units). An intermediate string of 3 to 20 monosaccharides is an oligosaccharide.