Chapter 21 Nutrition and Digestion

I. Obtaining and Processing Food

1. Heterotrophs cannot make their own food, so they must eat or absorb it, including all animals, fungi, and many protists and bacteria. Heterotrophs may consume autotrophs, other heterotrophs, or organic molecules from other organisms.

2. Animals obtain and ingest food in a variety of ways. There are 3 categories:

(1)  Herbivores

(2)  Carnivores

(3)  Omnivores

3. Animals have evolved various feeding mechanisms:

(1)  Filter feeders

(2)  Substrate feeders

(3)  Fluid feeders

(4)  Bulk feeders

4. There are 4 main stages of food processing:

(1)  Ingestion

(2)  Digestion

(3)  Absorption

(4)  Elimination

5. Digestion occurs in specialized compartments.

(1)  Animals with simple body plans digest food within a gastrovascular cavity with a single opening.

(2)  Most animals have an alimentary canal, a digestive tube extending between two openings, a mouth at one end and an anus at the other. Specialized regions of the alimentary canal can digest and absorb nutrients in a stepwise fashion.

(3)  Food --> mouth --> pharynx (throat) --> esophagus --> crop --> stomach --> gizzard --> intestine --> anus

II. The Human Digestion System

1. The human digestive system contains an alimentary canal (gut) and accessory glands (salivary glands, gallbladder, liver, and pancreas).

2. The glands secrete digestive chemicals that enter the alimentary canal through ducts (exocrine).

3. Muscles move the food through the alimentary canal by peristalsis, alternating waves of contraction and relaxation of the smooth muscles.

4. Sphincters are muscular ring-like valves regulate the passage of food into (cardia) and out (pylorus) of the stomach.

5. Three pairs of salivary glands (parotid, sublingual, and mandibular glands) secrete more than a liter/day of saliva through ducts to the oral cavity. Saliva contains a glycoprotein, buffers, antibacterial agents, and the digestive enzyme amylase.

6. The swallowing reflex includes the movement of the epiglottis cartilage. To save someone who is choking, the assistance can come through the use of the Heimlich maneuver, invented by Dr. Henry Heimlich in the 1970s.

7. The stomach stores food, and gastric glands secrete the gastric juice (pH=2), gastrin, and digestive enzymes. The gastric glands have 3 types of cells:

(1)  Mucous cells secrete mucus

(2)  Parietal cells secrete hydrochloric acid (HCl)

(3)  Chief cells secrete pepsinogen

8. The small intestine (duodenum) is the major organ of chemical digestion and nutrient absorption. Sources of digestive enzymes and bile:

(1)  The pancreas produces pancreatic juice, a mixture of digestive enzymes and an alkaline solution.

(2)  The liver produces bile, containing bile salts to break up fats.

(3)  The gallbladder stores bile until it is needed in the small intestine.

9. Most digestion is completed in the duodenum. The remaining regions of the small intestine, the jejunum and ileum, are the major site for absorption of nutrients.

10. The inner wall of the small intestine has large folds with villi (singular form villus). The surface of a villus contains many epithelial cells with microvilli. The combination of folds and projections greatly increases the surface area to 300 m², about the size of a tennis court.

11. Some nutrients are absorbed by simple diffusion, while others are pumped by active transportation. Most absorbed nutrients, such as amino acids and sugars, pass directly across the thin walls of the capillaries into the blood. Fatty acids and glycerol are transported into a lymph vessel.

12. The large intestine reabsorbs water and compacts the feces. The appendix in human cecum contains a mass of white blood cells. The main portion of the large intestine is the colon, which is to complete the reabsorption of water.

III. Nutrition

1. An animalꞌs diet must provide sufficient energy. The small molecules absorbed from food are needed to provide:

(1)  Organic building blocks for macromolecules

(2)  Chemical energy to power cellular work

(3)  Essential nutrients to maintain health

2. The Food and Drug Administration (FDA) requires 2 blocks of information on food labels:

(1)  Ingredients from the greatest amount (by weight) to the least.

(2)  Key nutrients along with amounts contained per serving and as a percentage of a daily value based on a 2,000-kcal (= 2,000 Calories = 2,000,000 calories) diet.

3. Metabolic rate is the rate of energy consumption by an animal (the sum of all energy used over a given time interval).

4. Basal metabolic rate (BMR) is the number of kilocalories to fuel essential processes (such as cell maintenance, breathing, heart beating, and maintenance of body temperature) for a resting animal in a given time.

(1)  The BMR for humans averages 1,300-1,500 kcal per day for adult females, and about 1,600-1,800 kcal per day for adult males.

(2)  In other words, an adult human performing absolutely no activity still requires about 1,500 kcal per day just to keep the body alive.

5. Body mass index (BMI) is a ratio of weight (pounds) to height (feet).

(1)  Obesity is defined as a too-high BMI. A BMI of 25-29 is considered overweight, and above 30 is obese.

(2)  About 1/3 of all Americans are obese, and another 1/3 are overweight.

(3)  Obesity contributes to health problems, including type II diabetes, cancer of colon and breast, and cardiovascular diseases.

(4)  Obesity is estimated to be a factor in 300,000 deaths per year in US.