UNIT I - DIGESTION Chapter 21

OBJECTIVES

Core:• I can describe the structure and function of the digestive tract.

• I can outline the chemical digestion of the 3 main macromolecules.

• I can explain the absorption process and refer to relevant structures (villi).

• I can explain how the liver supports the digestive system.

Advanced:• I can explain homeostasis of blood sugar regulation.

INTRODUCTION

Getting Their Fill of Krill

• Whales are one of the largest animals in the world, it takes an enormous amount of food to support a 72-ton humpback whale

These marine mammals eat small fishes and crustaceans called krill

Humpback whales strain their food from seawater using large plates called baleen, which they have on each side of their upper jaw act like a sieve

In a typical day, the whale’s digestive system will process as much as 2 tons of fish and krill

OBTAINING AND PROCESSING FOODS Animals ingest their food in a variety of ways

Animal diets are highly varied, and include

- Herbivores, plant-eaters (cattle, gorillas, snails, sea urchins)

- Carnivores, meat-eaters (lions, hawks, spiders, snakes)

- Omnivores, eating both plants and other animals (crows, cockroaches, raccoons, humans!)

ANIMAL FEEDING MECHANISMS

Suspension feeders: extract food particles suspended in the surrounding water (whales, clams, oysters, sponges!)

Substrate feeders: live on their food source and eat their way through it (caterpillars and worms)

Fluid feeders: obtain food by sucking nutrient-rich fluids from a living host, either plant or animals (Aphids, mosquitoes (female only) – males live off plant nectar!) Hummingbirds – benefit for their host

Bulk feeders: meaning they ingest relatively large pieces of food.

21.2 – FOOD PROCESSING OCCURS IN FOUR STAGES

1. Ingestion

2. Digestions

3. Absorption

4. Elimination

FOUR STAGES

Ingestion: The act of eating

Digestion: is the breaking down of food into molecules small enough for the body to absorb Typically occurs in two stages: mechanical

digestion and chemical digestion

Hydrolysis for chemical digestion, where water and enzymes are used to catalyze the reaction

Proteins, fats, and carbohydrates are too big to pass through the cell membrane therefore it must be broken down.

See chart polymers are broken down into monomers!

FOUR STAGES

Absorption: the cells lining the digestive tract take up (absorb) the products of digestion like amino acids and simple sugars.

These nutrients enter into the blood to the body cells where they can be joined to make macromolecules of the cells or broken down further for energy.

Elimination: undigested material passes out of the digestive tract

21.3 DIGESTION OCCURS IN SPECIALIZED COMPARTMENTS

Food vacuoles are digested in compartments which house hydrolytic enzymes

Animals have a variety of ways to digest their foods;

Sponges digest their food entirely in food vacuoles

Cnidarians and flatworms have gastrovascular cavities with a single opening

Most animals have a alimentary canal, a digestive tube with two openings

ALIMENTARY CANAL

Food enters the mouth, pharynx, esophagus, crop, stomach(s), gizzards, intestine, and anus

HUMAN DIGESTIVE SYSTEM

21.4 - The human digestive system consists of an alimentary canal and accessory glands The rhythmic muscle contractions of peristalsis which squeeze food toward the stomach along the alimentary canal

The pyloric sphincter: regulates the passage of food from the stomach to the small intestine

DIGESTION OCCURS IN THE ORAL CAVITY

The teeth break up food, saliva moistens, and salivary enzymes begin the hydrolysis of starch

The tongue pushes the bolus of food into the pharynx

THE PHARYNX

21.6 The food and breathing passages both open into the pharynx The swallowing reflex

Moves food from the pharynx into the esophagus, while keeping it out of the trachea

CONNECTION

The Heimlich maneuver can save lives

The Heimlich maneuver: can dislodge food from the pharynx or trachea during choking

PERISTALSIS

The esophagus squeezes food along to the stomach by peristalsis Peristalsis in the esophagus moves food into the stomach by using

muscles contractions from smooth muscle to create a wave and push the bolus to the stomach

GASTRIC JUICES

The stomach stores food and breaks it down with acid and enzymes

Pepsin in the gastric juice begins the hydrolysis of protein

Gastic juice is made up of mucus and enzymes and strong acid

mixes with food to produce acid chyme

GASTRIC JUICES

The gastric glands have three types of cells that secrete different components of the gastric juice

1. Mucous cells (dark pink) secrete mucous, which lubricates and protects the stomach lining

2. Parietal cells (yellow) secrete hydrochloric acid (HCl)

3. Chief cells (tan) secrete pepsinogen, an inactive form of the enzyme pepsin.

PEPSIN

1. Pepsinogen and HCl are secreted into the lumen (cavity) of the stomach

2. The HCl converts the pepsinogen to pepsin

3. Pepsin then activates more pepsinogen in a chain reaction as a positive feedback loop

REGULATION

Secreting pepsin in the inactive form, pepsinogen, protects the cells of the gastric glands

Mucus helps protect the stomach lining from both pepsin and acid

Cells in our gastric gland do not secrete gastric juice constantly, their activity is regulated by a combination of nerve signals and hormones

When you see, smell, or taste food, your brain signals your stomach to secrete gastric juice

When food is in your stomach, gastrin hormones are released in your bloodstream to secrete gastric juices

REGULATION CONT’DA negative-feedback mechanism inhibits the secretion of gastric juices when the stomach contents become too acidic.

The acid inhibits the secretion of gastrin, thus gastric juices

Your stomach churns every 20s producing the acid chime which is a nutrient broth of digested food

GERD – gastroesophageal reflux disease (chronic heartburn)

ACID CHYME

Pyloric sphincter helps regulate the passage of acid chime in squirts into the small intestine

This can take 2-6 hours to empty after a meal

An acid chime rich in fats slows the squirting down to allow for slower digestion and more digestive enzymes to be released

As you will see, there are other enzymes released from your gall bladder and pancreas to aid in digestion

ULCERS

Bacterial infections in the stomach and duodenum are associated with ulcers from the bacteria Helicobacter pylori

They are tolerant of the low pH of the stomach and eat away and burrow into stomach linings.

Your white blood cells cause inflammation and your body can’t keep up with the loss of cells and replace them before a hole develops

This hole, or ulcer, can become very dangerous.

21.11 – THE SMALL INTESTINE

The small intestine is the major organ of chemical digestion and nutrient absorption

The small intestine has a length over 6m and about 2.5cm diameter

Nutrients enter into the bloodstream through the small intestine with the aid of two glandular organs

The pancreas

The liver

THE PANCREAS The pancreas produces a mixture of digestive enzymes and an alkaline solution rich in bicarbonate

The alkaline solution neutralizes the very acid chyme from the stomach

The major enzymes produced in the pancreas include;

Pancreatic amylase (maltose and other dissacharides)

Trypsin & Chymotrypsin

Nucleases & nucleosidases

Lipase

These enzymes are secreted into the small intestine, specifically the duodenum (first 25 cm)

THE LIVERThe liver performs a wide variety of functions, including the production of bile.

Bile contains bile salts that emulsify fats

Increase surface area, which allows digestive enzymes to attack fat droplets easier

The gallbladder stores bile is until it is needed

ENZYMES AND DIGESTION

Know the enzymes in the chart!

Lactase – digests milk sugar, children>adults

PROTEIN DIGESTIONThe small intestine completes the digestion of proteins that began in the stomach.

The pancreas and the duodenum secrete hydrolytic enzymes that completely dismantle polypeptides into amino acids

The enzyme trypsin and chymotrypsin break polypeptides into small polypeptides

Aminopeptidases and carboxypeptidase split off one amino acid at a time

Dipeptidases hydrolyzes fragments of two or three a.a long

THE SMALL INTESTINE STRUCTURE

Structurally, the small intestine is well suited for its task of absorbing nutrients.

Its surface area is huge ~300m2 (size of a tennis court)

It has a series of folds and projections that are circular in structure

These finger-like projections are called villi

Each of the epithelial cells lining a villus has many tiny surface projections called microvilli

These extend into the lumen of the small intestine which contributes to the high surface area

THE SMALL INTESTINE CONT’D

Some nutrients are absorbed by simple diffusion; others are pumped against their concentration gradient into the epithelial cells

The small lymph vessel (yellow) and network of capillaries (red, purple, and blue) penetrate the core of each villus.

Fatty acids and glycerol are transported into the lymph vessel

Other absorbed nutrients, like amino acids and sugars pass into the capillaries.

THE SMALL INTESTINE CONT’D

The capillaries that drain away nutrients from the villi converge into larger veins and eventually into a main vessel, the hepatic portal vein, that leads directly to the liver

The liver converts many of these nutrients into substances the body needs, like glycogen.

The liver also detoxifies any toxins ingested

THE LARGE INTESTINE

The large intestine, also known as the colon, is 1.5m long and 5cm in diameter

There is a small ‘blind pouch’ called the cecum which is the start of the large intestine

There is also a finger-like extension called the appendix which houses white blood cells

THE LARGE INTESTINE CONT’D

The main job of the large intestine is to absorb water

About 7L of fluid enters the lumen and approx. 90% is absorbed back into the blood and tissue fluids.

As the water is reabsorbed, the remains of the digested food become more solid and are called feces

Feces is mainly indigestible plant fibers and prokaryotes that live in the colon.

These bacteria, like E. coli are important as they help in digestion but produce important vitamins like biotin, folic acid, several B vitamins and vitamin K.

21.17 – EIGHT ESSENTIAL AMINO ACIDS

21.18

Vitamins and minerals

Are essential in the human diet

Most of these vitamins function as coenzymes

A healthy diet includes these 13 vitamins

21.19 21.19 ESSENTIAL MINERALS ARE REQUIRED FOR MANY BODY FUNCTIONS

Minerals are inorganic nutrients that play a variety of roles