Chapter 47 ~ Digestion

Lipoproteins: (high density) HDL and LDL (low density)

A large molecular complex consisting of lipids and protein; transports lipids in the blood. High-density lipoproteins (HDLs) transport cholesterol to the liver; low- density lipoproteins (LDLs) deliver cholesterol to many cells of the body.

What are the major categories of required nutrients and their general functions in the body?

Carbohydrates are digested to monosaccharides. Proteins are digested to amino acids. Fats are digested to fatty acids and monoacylglycerols. Nutrients are digested as they move through the digestive tract. Carbohydrates, proteins, and lipids. Carbs: Polysaccharides, Maltose and small polysaccharides (mouth); Action continues until acidic pH inactivates salivary amylase (stomach); and Undigested polysaccharides, Maltose and other disaccharides, Monosaccharides (small intestines). Protein: Short polypeptides (stomach); Polypeptides, Short peptides, Amino acids (small intestines). Lipids: Glob of fat, Emulsified fat droplets, Fatty acids and glycerol (small intestines)

Chief cells

Chief cells in the gastric glands secrete pepsinogen, an inactive enzyme precursor. When it comes in contact with the acidic gastric juice in the stomach, pepsinogen is converted to pepsin, the main digestive enzyme of the stomach. Pepsin hydrolyzes proteins, converting them to short polypeptides.

rugae

Folds, such as those in the lining of the stomach. are folds in the stomach wall that expand as the stomach fills with food. increase the surface area in the stomach; villi increase the surface area in the small intestine.

lacteal

One of the many lymphatic vessels in the intestinal villi that absorb fat.

Parietal cells

Parietal cells in the gastric glands secrete hydrochloric acid and intrinsic factor, a sub- stance needed for adequate absorption of vitamin B12.

predators

Relationship in which one organism (the predator, a secondary or higher level con- sumer) devours another organism (the prey).

Salivary amylase

Saliva contains an enzyme, salivary amylase, which begins the chemical diges- tion of starch into sugar. An enzyme in saliva that hydro- lyzes starch to the disaccharide maltose.

Chemical digestion

Saliva contains an enzyme, salivary amylase, which begins the chemical digestion of starch into sugar. Chemical digestion involves breaking down the food into simpler nutrients that can be used by the cells. Chemical digestion begins in the mouth when food mixes with saliva.

nutrients

The chemical substances in food that are used as components for synthesizing needed materials and/or as energy sources.

What is the benefit of a digestive tract with 2 openings?

You can ingest foods to obtain nutrients, as well as secrete waste products. Most invertebrates, and all vertebrates, have a tube-within- a-tube body plan. The body wall forms the outer tube. The inner tube is a digestive tract with two openings, sometimes referred to as a complete digestive system (FIG. 47-3). Food enters through the mouth, and undigested food is eliminated through the anus. The mixing and propulsive movements of the digestive tract are referred to as motility. The propulsive activity characteristic of most regions of the digestive tract is peristalsis, waves of muscular contraction that push the food in one direction. More food can be taken in while previously eaten food is being digested and absorbed farther down the digestive tract. In a digestive tract with two openings, various regions of the tube are adapted to perform specific functions.

pepsinogen

an inactive enzyme precursor. The precursor of pepsin; secreted by chief cells in the gastric glands of the stomach.

Trace elements

are minerals required in amounts less than 100 mg per day. minerals that we require minute amounts (less than 100 mg per day).

Mechanical digestion

begins as you bite, grind, and chew the meat and bun with your teeth. We must mechanically digest the meat and then chemically digest it by enzymatic hydrolysis. food is being mechanically disassembled by the teeth. Mechanical digestion involves physically breaking the food into smaller pieces. Mechanical digestion begins in the mouth as the food is chewed.

absorption

(1) The movement of nutrients and other substances through the wall of the digestive tract and into the blood or lymph. (2) The process by which chlorophyll takes up light for photosynthesis.

lumen

(1) The space enclosed by a membrane, such as the lumen of the endoplasmic reticulum or the thylakoid lumen. (2) The cavity or channel within a tube or tubular organ, such as a blood vessel or the digestive tract. (3) The space left within a plant cell after the cell's living material dies, as in tracheids.

Villi

(pl., villi) A multicellular, minute, elon- gated projection from the surface of an epithelial membrane, e.g., villi of the mucosa of the small intestine.

Hormone: Gastrin

1) Source (Organ that makes it): Stomach (mucosa) 2) Target (Organ it is secreted into): Stomach (gastric glands) 3) Function: Stimulates gastric glands to secrete pepsinogen and HCl; stimulates gastric motility

vitamin

A complex organic molecule required in very small amounts for normal metabolic functioning.

lipase

A fat-digesting enzyme.

trypsin

A pancreatic enzyme that digests polypeptides to short peptides.

mucosa

A type of epithelial membrane that lines a body cavity that opens to the outside of the body, e.g., the digestive and respiratory tracts; also called mucosa.

bolus

After being chewed and fashioned into a lump called a bolus, the bite of food is swallowed, moving through the pharynx into the esophagus. a small rounded mass of a substance, especially of chewed food at the moment of swallowing.

Essential amino acids

An amino acid that must be provided in the diet because the body cannot make it or cannot make it in sufficient quantities to meet nutritional needs.

omnivores

An animal that eats a variety of plant and animal materials.

Primary consumers

An animal that eats producers, e.g., plants or algae.

herbivores

An animal that feeds on plants or algae. Also called primary consumer.

carnivores

An animal that mainly feeds on other animals; a mammal belonging to order Carnivora, e.g., wolves or seals.

pepsin

An enzyme produced in the stomach that initiates digestion of protein.

heterotrophs

An organism that cannot synthesize its own food from inorganic raw materials and therefore must obtain body-building materials from other organisms. Also called consumer.

Deposit feeders

Animal that consumes nutrients by ingesting soil or sediments.

Bile is not an enzyme. What is its role in digestion? Include in your answer which organ makes it and where it is secreted.

Bile from the liver and enzymes from the pancreas are released into the duodenum and act on the chyme. The gallbladder stores bile from the liver. The liver secretes bile, which mechanically digests fats by a detergent- like action (discussed in a later section). Bile consists of water, bile salts, bile pigments, cholesterol, salts, and lecithin (a phospholipid). Because it contains no digestive enzymes, bile does not enzymatically digest food. The pear-shaped gallbladder stores and concentrates bile and releases it into the duodenum as needed.

elimination

Ejection of undigested food from the body.

Filter feeders

Suspension feeders that filter the suspended food are known as filter feeders.

digestion

The breakdown of food to small molecules.

bile

The fluid secreted by the liver; emulsifies fats.

What is the structural difference between a gastrovascular cavity and an alimentary canal?

The gastrovascular cavity is a central digestive cavity with a single opening, whereas the alimentary canal has two openings (the whole passage along which food passes through the body from mouth to anus. It includes the esophagus, stomach, and intestines).

motility

The mixing and propulsive movements of the digestive tract

ingestion

The process of taking food (or other material) into the body.

chyme

The stomach churns and chemically degrades the food so that it assumes the consistency of a thick soup; this partially digested food is called chyme.

peristalsis

waves of muscular contraction that push the food in one direction

Microvilli

(sing., microvillus) Minute projections of the plasma membrane that increase the surface area of the cell; found mainly in cells concerned with absorption or secretion, such as those lining the intestine or the kidney tubules.

Hormone: CCK

1) Source (Organ that makes it): Duodenum (mucosa) 2) Target (Organ it is secreted into): Pancreas, Gallbladder, Brain 3) Function: Stimulates release of digestive enzymes. Stimulates emptying of bile. Helps regulate food intake by signaling satiety

Hormone: Secretin

1) Source (Organ that makes it): Duodenum (mucosa) 2) Target (Organ it is secreted into): Pancreas, Liver, Stomach 3) Function: Stimulates pancreas to secrete sodium bicarbonate, which neutralizes acid in duodenum. Stimulates bile secretion Inhibits gastric secretion and gastric emptying

Enzyme: Lipase

1) Source (Organ that makes it): Pancreas (small intestine) 2) Target (Organ it is secreted into): Pancreas (small intestine) 3) Function: Pancreatic enzymes include trypsin and chymotrypsin, which digest polypeptides to short peptides; pancreatic lipase, which degrades fats; pancreatic amylase, which breaks down almost all types of complex carbo- hydrates, except cellulose, to disaccharides; and ribonuclease and deoxyribonuclease, which split the nucleic acids ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) to free nucleotides.

Enzyme: Trypsin

1) Source (Organ that makes it): pancreas 2) Target (Organ it is secreted into): pancreas 3) Function: Pancreatic enzymes include trypsin and chymotrypsin, which digest polypeptides to short peptides; pancreatic lipase, which degrades fats; pancreatic amylase, which breaks down almost all types of complex carbo- hydrates, except cellulose, to disaccharides; and ribonuclease and deoxyribonuclease, which split the nucleic acids ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) to free nucleotides. Trypsin, secreted in an inactive form by the pancreas, is activated by an enzyme called enterokinase. The trypsin then activates chymotrypsin and carboxypeptidase as well as addi- tional trypsin.

Enzyme: Pancreatic amylase

1) Source (Organ that makes it): pancreas (small intestines) 2) Target (Organ it is secreted into): pancreas (small intestines) 3) Function: The pancreas is an elongated gland that secretes both digestive enzymes and hormones that help regulate the level of glucose in the blood. The cells that line the pancreatic ducts secrete an alka- line solution rich in bicarbonate ions. This pancreatic juice neutral- izes the stomach acid in the duodenum and provides the optimal pH for action of the pancreatic enzymes. Pancreatic enzymes include trypsin and chymotrypsin, which digest polypeptides to short peptides; pancreatic lipase, which degrades fats; pancreatic amylase, which breaks down almost all types of complex carbo- hydrates, except cellulose, to disaccharides; and ribonuclease and deoxyribonuclease, which split the nucleic acids ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) to free nucleotides.

Enzyme: Pepsin

1) Source (Organ that makes it): stomach 2) Target (Organ it is secreted into): stomach 3) Function: Pepsin hydrolyzes proteins, converting them to short polypeptides. In the stomach food is mechanically digested by vigorous churn- ing, and proteins are enzymatically digested by the action of pepsin in the gastric juice.

Enzyme: Salivary amylase

1) Source (Organ that makes it): the mouth 2) Target (Organ it is secreted into): Stomach 3) Function: Saliva contains an enzyme, salivary amylase, which begins the chemical diges- tion of starch into sugar.

What is the function of glucose and essential amino acids?

Carbohydrates are used mainly as an energy source. Glucose concentration in the blood is carefully regulated. Excess glucose is stored as glycogen and can also be converted to fat. Proteins serve as enzymes and are essential structural components of cells. The best distribution of essential amino acids is found in the complete proteins of animal foods. Excess amino acids are deaminated by liver cells. Amino groups are converted to urea and excreted in urine; the remaining keto acids are converted to carbohydrate and used as fuel or are converted to lipid and stored in fat cells. Amino acids and glucose are transported directly to the liver by the hepatic portal vein.

Gastric glands

Glands in the wall of the stom- ach; contain cells that secrete hydrochloric acid, intrinsic factor, and pepsinogen, the precursor of the enzyme pepsin.

How are specialized teeth related to herbivores, omnivores and carnivores?

They are specialized according to the food that the organism consumes. The mouth is specialized for ingestion and for beginning the digestive process. Mechanical digestion begins as you bite, grind, and chew the meat and bun with your teeth. Unlike the simple, pointed teeth of fish, amphibians, and reptiles, the teeth of mammals vary in size and shape and are specialized to perform specific functions. The chisel-shaped incisors are used for biting and cutting food, whereas the long, pointed canines are adapted for piercing prey and tearing food (FIG. 47-6). The flattened surfaces of the premolars and molars are specialized for crushing and grinding.

How are LDLs (and HDLs) related to cardiovascular disease?

When glucose and insulin levels are high, two things happen: tri- acylglycerol levels increase, and high-density lipoprotein (HDL; the good cholesterol) concentration decreases. These metabolic events can lead to cardiovascular disease and increase the risk of type 2 diabetes Liver cells repackage cholesterol and triacylglycerols. These lipids are bound to proteins and transported as large molecular complexes, called lipoproteins. Some plasma cholesterol is transported by HDLs ("good" cholesterol), but most is transported by LDLs ("bad" cholesterol). LDLs deliver cholesterol to the cells. For cells to take in LDLs, a protein on the LDL surface must bind with a protein LDL receptor on the plasma membrane . After binding, the LDL enters the cell, and its cholesterol and other components are used. When cholesterol levels are high, HDLs collect the excess cholesterol and transport it to the liver. HDLs decrease risk for cardiovascular disease. What is the relationship between fat and cholesterol intake and cardiovascular disease? Lipids play a key role in the development of atherosclerosis, a progressive disease in which the arteries become clogged with fatty material. Low-density lipoproteins are the main source of the cholesterol that builds up in the walls of arteries. The type of fat consumed as well as other dietary and lifestyle factors is important. You can promote a healthy proportion of HDL to LDL by exercising regularly, following a healthy diet, maintaining appropriate body weight (obesity raises LDL and triacylglycerol levels), and not smoking cigarettes.

After studying each organ of the vertebrate digestive system (human), trace the pathway traveled by an ingested meal: describe the function of each organ and their specialized segments (if they have one), their enzymes and what product it breaks down (where appropriate). Don't forget the accessory organs and their roles. Table 47-1 may be helpful for some of it.

mouth ---> pharynx (throat) ---> esophagus ---> stomach ---> small intestine ---> large intestine ---> anus Most chemical digestion takes place in the duodenum. Carbohydrates are digested to monosaccharides, Proteins are digested to amino acids, and Fats are digested to fatty acids and monoacylglycerols. Nerves and hormones regulate digestion. Absorption takes place mainly through the villi of the small intestine. The large intestine eliminates waste. Undigested material, such as the cellulose of plant foods, along with unabsorbed chyme passes into the large intestine. Although only about 1.3 m (about 4 ft) long, this part of the digestive tract is referred to as "large" because its diameter is greater than that of the small intestine. The small intestine joins the large intestine about 7 cm (2.8 in.) from the end of the large intestine, forming a blind pouch, the cecum. From the cecum to the rectum (the last portion of the large intestine), the large intestine is known as the colon. The regions of the large intestine are the cecum; ascending colon; transverse colon; descending colon; sigmoid colon; rectum; and anus, the opening for the elimination of wastes. As chyme passes slowly through the large intestine, water and sodium are absorbed from it, and it gradually assumes the consistency of normal feces. Bacteria inhabiting the large intes- tine are nourished by the last remnants of the meal; in exchange, they benefit their host by producing vitamin K and certain B vitamins that can be absorbed and used.

ruminants

(cattle, sheep, deer, giraffes) are hoofed animals with a stomach divided into four chambers. Symbiotic bacteria and protists living in the first two cham- bers digest cellulose, splitting some of the molecule into sugars, which are then used by the host and by the bacteria themselves. The bacteria produce fatty acids during their metabolism, some of which are absorbed by the animal and serve as an important energy source.