Nutrition, Digestion, Absorption, and Excretion

Transcription

1 ECOL Spring 2010 Nutrition, Digestion, Absorption, and Excretion Dr. Regis Ferriere Department of Ecology & Evolutionary Biology University of Arizona Lecture 2

2 Our main questions in this lecture Why do animals eat? How do animal digest food? How is food uptake regulated? How does the animal body deal with wastes? How does the animal body maintain water and salt balance in spite of excretion? What is the structure of the mammalian kidney? How does it function? How is its function regulated?

3 What do animals require from food? Animals are heterotrophs: derive energy & molecular building blocks, directly or indirectly, from. For many animals, food provides essential carbon skeletons that they cannot synthesize themselves. Humans require 8 essential from food. Animals cannot synthesize the acetyl group, but they ingest it in their food and use it to synthesize many molecules.

4 What do animals require from food? Animals need mineral elements for a variety of functions: Macronutrients, like Ca for bone growth, blood clotting, nerve and muscle action, enzyme activation (1.2kg in a 70kg body). Micronutrients, like Fe for enzymatic reactions, oxygen binding in blood (4g in a 70kg body). Animals must obtain vitamins from food. Carbon compounds functioning as for growth and metabolism. Humans require 13 vitamins.

5 Quiz check Food provides humans with the eight and thirteen that they need. like are needed in small or trace amounts. is a that is required for bone growth. A. Macronutrient B. Calcium C. Vitamins D. Micronutrients E. Essential amino acids F. Iron

6 F3: fig F2: fig How do animals digest food? The lining of the small intestine has extensive folds Cross section of small intestine Fold Villi Animals digest their food extracellularly. Food is ingested into a body cavity. Digestive are secreted into that cavity. They break down the food into nutrient molecules that can be absorbed by the cells lining the cavity. Absorptive areas of the gut are characterized by a large surface area produced by extensive folding and numerous villi (= folds) and microvilli (= cell projections). Epithelial cells Blood vessels Lacteal (lymph system) Muscle Three-dimensional view of fold Fold Villi Blood vessels Muscle Microvilli are extensions of epithelial cells in villi. Villus Microvilli of epithelial cells

7 How do animals avoid digesting themselves?! Most digestive enzymes are produced in inactive forms or zymogens. When secreted in the gut, a zymogen becomes activated by an other enzyme. Cells lining the gut are protected by. Tissue layers of the vertebrate gut. Mucosa is secretory and absorptive. Submucosa contains blood vessels and nerves. Muscle layers are separated by a nerve net that controls gut movements.

8 How does the gastrointestinal system function? Chemical digestion begins in the, where amylase is secreted with saliva and starts digestion of carbohydrates. The human stores and breaks down ingested food. Alcohol, caffeine, aspirin... can be absorbed through the stomach wall. Parietal cells secrete HCl, and chief cells secrete pepsinogen activated into pepsin by very low ph. Pepsin begins the digestion of protein.

9 How does the gastrointestinal system function? In the (3 sections), digestion of proteins and carbohydrates continues; digestion of lipids and absorption of nutrients begin. Most digestion occurs in duodenum (section 1). Entering the duodenum from the stomach, the acidic chyme is neutralized by pancreatic bicarbonate ions. Liver and pancreas provide many enzymes and other secretions. Bile aids in digesting lipids. 90% nutrient absorption occur in sections 2-3. All of the blood leaving the digestive tract flows to the liver. Liver cells absorb the nutrients and either store them or convert them to molecules the body needs.

10 F3: fig F2: fig How does the gastrointestinal system function? The digestive tract: 1. Mouth Mechanical and chemical processing (chewing reduces size of food; saliva digests carbohydrates) 2. Esophagus Transports food 3. Stomach Mechanical and chemical processing (digestion of proteins) Accessory organs: Salivary glands Secrete enzymes that digest carbohydrates; supply lubricating mucus Liver Secretes molecules required for digestion of fats 4. Small intestine Chemical processing and absorption (digestion of proteins, fats, carbohydrates; absorption of nutrients and water) 5. Large intestine Water absorption and feces formation 6. Rectum Holds feces Gallbladder Stores secretions from liver; empties into small intestine Pancreas Secretes enzymes and other materials into small intestine 7. Anus Feces elimination

11 Quiz check Food is broken down by which are secreted in inactive forms called. For example, results from the activation of (secreted by ) by low ph in the stomach. Low ph is caused by the release of HCl by. Pepsin initiates the digestion of in the stomach. The digestion of lipids start in the first section of the called. A. Chief cells E. Zymogen B. Proteins F. Pesinogen C. Small intestine G. Duodenum D. Digestive enzymes H. Pepsin I. Parietal cells

12 F3: fig How is the flow of nutrients regulated? When food is present in the gut, nutrients are absorbed. During the postabsorptive period (stomach and small intestine empty) energy metabolism and biosynthesis must run on internal reserves. The directs the traffic of the molecules that fuel metabolism: glucose and fat. Pancreatic hormones insulin and glucagon are key players of glucose homeostasis. Pancreas secretes INSULIN Pancreas secretes GLUCAGON Glucose If glucose level is too high If glucose level is too low Insulin causes cells in liver to synthesize glycogen; fat-storage cells synthesize triglycerides. Homeostasis (normal glucose levels in blood) Glycogen F2: fig Glucose level falls Glucose level rises Glucagon causes cells in liver to catabolize glycogen; fat-storage cells catabolize fatty acids. Glycogen Glucose

13 How do animals deal with ingested toxins? Toxins in food may come from natural sources, but many come from human activities such as the use of pesticides and the release of pollutants into the environment. Toxins such as PCBs (polychlorinated biphenyls, used as insulating fluid in electrical transformers) accumulate in the bodies of prey and are transferred and further concentrated in the bodies of their predators. This is bioaccumulation: produces high concentrations of toxins in animals high up the food chain.

14 What roles do excretory organs play in maintaining homeostasis? Excretory organs control the volume, concentration, and composition of the extracellular fluids of animals. Water enters or leaves cells by osmosis. Osmosis = movement of water across a membrane from high water content to low water content. Excretory organs control extracellular fluid osmolarity by filtration, secretion, and reabsorption. Osmolarity = concentration of osmotically active particles. The is the major excretory organ of vertebrates. is the output of excretory organs.

15 How do animals excrete toxic wastes from nitrogen metabolism? The end products of metabolism of carbohydrates and fats are water and CO2: Not difficult to eliminate. Metabolism of proteins and nucleic acids also produces nitrogenous wastes. Most common form: Ammonia. Highly toxic, must be excreted, or detoxified = converted into other molecules: urea or uric acid.

16 How do mammals maintain salt and water balance? The kidney is well adapted for water excretion or conservation. Mammals and birds have high body temperatures and high metabolism, hence potential for high rate of water loss. In mammals and birds, kidneys have evolved a unique adaptation: ability to reabsorb water from urine. As a result, mammals and birds can produce urine more concentrated than their extracellular fluids.

17 Quiz check The main excretory organ in mammals is the. Excretory organs control the composition and concentration of extracellular fluid by three mechanisms:. The composition is controlled so that toxic wastes like are eliminated. The concentration is controlled so that is minimized. A. Digestion, absorption, secretion B. Nutrition, filtration, secretion C. Filtration, secretion, reabsorption D. HCl E. Liver I. Pancreas F. Ammonia J. Heat gain G. Kidney K. CO2 H. Water loss L. Uric acid production

18 F3: fig F2: fig What is the structure of the kidney? Nephron: functional unit of the vertebrate kidney. Nephrons form a regular arrangement in the kidney. Each nephron consists of a glomerulus and tubules in cortex, and loop of Henle in medulla. Urinary system Kidney Nephron structure Cortex Nephron Nephron Kidney Medulla Renal vein Renal artery Ureter Bladder Ureter In some nephrons the loop of Henle is long and plunges into the medulla In most nephrons, the loop of Henle is relatively short and is located in the cortex Urethra Medulla Cortex

19 F3: fig F2: fig How does the mammalian kidney work? Blood is filtered in the glomerulus. Blood leaves glomerulus Glomerulus Pre-urine leaves Blood enters glomerulus.

20 How does the mammalian kidney work? Three involving regions the straight Permeability portions (limb) of loop of Henle F3: fig F2: fig (b) Urine is concentrated by countercurrent mechanisms Passive transport Active transport Thick ascending limb Descending limb Thin ascending limb Passive transport Descending limb is highly permeable to water but impermeable to solutes Ascending limb is impermeable to water but highly permeable to Na + and Cl

21 Quiz check Blood is filtered in the, which is part of the functional unit of the kidney called. After filtration pre-urine moves to the in the that form the. The is to water and to solutes, whereas the is to water and to Na+ and Cl-. A. impermeable E. glomerulus B. permeable F. ascending limb C. descending limb G. tubules D. loop of Henle H. nephron I. medulla

22 What mechanisms regulate kidney function? The kidney must maintain high glomerular filtration rate even if blood pressure or osmolarity vary. When blood pressure falls, the kidney release the enzyme renin. Renin converts a protein into hormone angiotensin. Angiotensin causes constriction of blood vessels. This triggers release of hormone aldosterone, which enhances water reabsorption, and stimulates thirst.

23 What mechanisms regulate kidney function? F3: fig F2: fig Changes in osmolarity influence the release of antidiuretic hormone (ADH) ADH stimulates the production and controls the intracellular location of aquaporin proteins. With ADH, aquaporins can fuse with membrane of collecting duct cells and serve as water channels: the cells become more permeable to water. ADH present Loop of Henle Solutes Distal tubule Cortex Collecting duct Aquaporins Medulla

24 Time for a beer Alcohol inhibits ADH release... This is why excessive beer drinking leads to even more urination and dehydration, which contributes to hangover symptoms...

25 Quiz check When blood pressure falls or osmolarity varies, are released. Lower blood pressure triggers production of which causes release of, resulting in greater and sensation. Change is osmolarity triggers secretion of, causing to fuse with of cells of. Greater also results. A. water reabsorption E. renin and angiotensin B. aquaporins F. ADH (antidiuretic hormone) C. hormones and enzymes G. collecting ducts D. membrane H. aldosterone I. thirst