BIOL 191 | Ch. 37 - Digestion

Nutrients

Any organic or inorganic substances required for survival, growth, development, tissue repair, or reproduction (i.e., chemical building blocks & energy)

Food processing in animals occurs in four phases:

Ingestion, digestion, absorption, egestion

Macronutrients

Organic; carbohydrates, proteins, lipids

Micronutrients

Organic; nucleic acids and vitamins

Glycogen

Extra energy stores in muscle and liver cells (short-term)

Triglycerides

Extra energy stored in fat cells (long-term)

• Most long-term energy is stored as lipids

Catabolism

The subunits released by digestion are taken into the circulation and carried to the body’s cells

Anabolism

Cells use these subunits as the building blocks to assemble the macromolecules necessary for cellular structure and function

Animal cells and nutrient synthesis

Can synthesize many, but not all, of the organic molecules they need

Essential nutrients

Must be obtained in the diet in their complete form

Classified into 4 groups;

• Essential amino acids

• Essential fatty acids

• Vitamins

• Minerals

Essential amino acids

Only 9 are essential for humans, all are found in the ‘super-grain’ quinoa

Essential fatty acids

Humans require linoleic acid and α-linoleic acid, and must get these or similar molecules in their diet — the omega-3 and omega-6 fatty acids

Vitamins

Organic molecules needed in tiny amounts, often function as coenzymes

Water-soluble vitamins

Biotin, folic acid, niacin, pantothenic acid, vitamin B’s, and vitamin C

Fat (lipid) - soluble vitamins

A (retinol), D (cholecalciferol), E (tocopherol), K (menadione)

Minerals

Some are required large amounts, such as calcium — needed for bones and teeth, and nerve and muscle function|

Others are needed in very small amounts, such as iron — found in hemoglobin and myoglobin and other enzymes

Digestion

Usually occurs extracellularly

Intracellular digestion

Occurs in simple invertebrates (e.g., protists); following phagocytosis

Extracellular digestion

Occurs in most animals in a cavity (e.g., a gastrovascular cavity in Cnidarians & others is the simplest form)

Alimentary canal

Tubes with specialized regions and openings at opposite ends, containing:

• Smooth muscle, which helps churn food

• Epithelial cells line the canal, synthesize and secret digestive enzymes and hormones

• Specialized regions; acidic environments can be separated from non-acidic environments and undigested foods can be stored

Once digested, absorption of food may be

passive or active; by means of simple or facilitated diffusion, or active transport

Alimentary Canal Needs

Can use some nutrients for their own needs, but most are transported into blood for the rest of the body

Vertebrate digestive system

Consist of the alimentary canal (aka the gastrointestinal (GI) tract) organs plus several accessory structures

• Structure and function change along its length

• Digestive systems differ (e.g., birds lack a gallbladder, some fish lack a stomach)

Accessory structures

Extra parts that support the bigger system, but aren’t apart of the main part themselves.

• Tongue, teeth

• Salivary glands

• Liver

• Gallbladder

• Pancrease

Salivary glands

In terrestrial vertebrates, food stimulates these glands to produce saliva

Functions of saliva include;

• Moisten and lubricate food to facilitate swallowing

• Dissolve food particles to facilitate taste

• Kill ingested bacteria
  initiate carbohydrate digestion with salivary amylase (digestion is typically the least important function)

Food after the mouth

Moves to into the pharynx, a voluntary process, then into the esophagus, an involuntary process

Peristalsis

Rhythmic waves of smooth muscle contraction that propel a bolus of food along the tube

Crop

Found in some animals, instead of food moving directly from the esophagus to the stomach, it instead first enters a storage organ called the ____

• Found in most birds and many invertebrates

Stomach’s Four Functions

• Stores & breaks up food

• Secrets hydrochloric acid (HCl): kills most bacteria

• Secretes pepsinogen + HCl → pepsin, begins protein digestion, unfolding

• Squeezes and mixes food with the acid and digestive enzyme → chyme

Gastric cell digestion prevention

Enzymes are secreted in inactive forms, such as pepsinogen. The acid in the stomach changes it to pepsin

Chyme → release of Gastrin → HCl secretion

Stomach digestion

No significant digestion of other organic molecules occurs in the stomach; little to no absorption occurs in the stomach

Bird Stomach

Divided into two parts;

• The proventriculus is the glandular portion that secrets acid and pepsinogen

• The gizzard a rough muscular structure that grinds food into small fragments

  • Contains sand or tiny stones that help grind ingested food

Ruminant mammals (cows, sheep, deer, etc.)

Have evolved a complex stomach that consists of four chambers.

• This allows digestion of cellulose that vertebrates cannot break down by themselves

Rumen and Reticulum

Act as storage and processing sites

Omasum

Absorbs some water and ions

Abomasum

Is the “true stomach” where acid and protease secretion occurs

Small Intestine

Most digestion & absorption occurs here

Each villus contains a capillary and a lacteal for nutrient transport

Small intestine features that increase surface area

• The mucosa is folded

• Finger-like projections called villi

• Membranes of epithelial cells have microvilli

Lacteal

Absorbs most digested fat

Capillaries

Absorb most food molecules, including some small lipids

Duodenum

Acidic (first 25 cm)

Pancrease

Regulated by the arrival of chyme, which triggers the release of two hormones

Cholecystokinin (CCK)

Stimulates the pancreas & liver to secrete a mix of digestive enzymes into the intestine

Secretin

Stimulates the pancreas to secrete bicarbonate ions (HCO₃^-) into the small intestine to neutralize the acidic chyme

Carbs in omnivores

Most are starch, cellulose, and glycogen (polysaccharides)

Pancreatic Amylase

Digests nearly all starch and glycogen in the small intestine

Chyme

The thick, soupy mix of partially digested food and digestive juices that forms in your stomach.

Proteins & the small intestine

Partially digested protein in chyme is further digested here by many protease enzymes, including typsin → polypeptide fragments

Liver

Secretes bile salts that emulsify lipids into tiny droplets, increasing surface area for digestive enzymes to work on

Ingested Lipids

Most are in the form of triglycerides; triglyceride breakdown occurs via the actions of pancreatic lipase

Bile

Formation of micelles, which allow lipids to diffuse into intestinal cells

There they can be incorporated into chylomicrons, which are released by exocytosis and enter lacteals

The Large Intestine

Main functions are to absorb some of the remaining water and ions to store and concentrate waste

• Cecum — a small pouch with the appendix

• Colon — where limited absorption occurs

• Rectum — stores feces prior to defacation

Large intestine size

Varies greatly among different vertebrates; absent in many animals, notably fishes

Absorptive state

After a meal when food is in the gut and nutrients are absorbed

Post-absorptive state

Stomach and small intestine are empty and metabolism runs on stored nutrients

Blood glucose concentration increases in two ways

glycogenolysis and gluconeogenesis

Glycogenolysis

Breaking down glycogen into glucose

Gluconeogenesis

Making new glucose from non-carbohydrate sources, e.g., glycerol

Insulin and Glucagon

Regulate blood nutrient levels. Both are hormones secreted by pancreatic cells in the islets of Langerhans.

• When blood glucose rises, insulin is released and stimulates cells to take up glucose

Glucose permeability

Glucose can’t cross plasma membranes without the aid of a transport protein; insulin signaling leads to increased glucose transporters (GLUTs) being available in the membrane

Glucose increases in the absorptive state

The hormone insulin lowers the concentration of glucose in the blood to maintain the normal range

Glucose decreases in the post-absorptive state

Insulin secretion stops, neurons release norepinephrine & stimulate the pancreas to release glucagon. This promotes glycogenolysis & gluconeogenesis → increased glucose