Digestion and Nutrition

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Digestion and Nutrition

• Chapter 32

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Obtaining Energy

• All organisms require energy to maintain their
  complex structure.
• The ultimate source of energy is the sun.
• Green plants utilize energy in sunlight to make
  glucose.
• Autotrophs (phototrophs)
• A few autotrophs are chemotrophs, gaining energy
  from inorganic chemical reactions.

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Animals are Heterotrophs!

• Animals are heterotrophs, depending on other
  organisms for food.
• Animals fall into one of three dietary
  categories
• Herbivores eat mainly autotrophs (plants and
  algae).
• Carnivores eat other animals.
• Omnivores regularly consume animals as well as
  plants or algal matter.
• Saprophagous animals feed on decaying organic
  matter.

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Why We Eat

• Regardless of what an animal eats, an adequate
  diet must satisfy three nutritional needs
• Fuel for all cellular work.
• The organic raw materials for biosynthesis.
• Essential nutrients, substances such as vitamins
  that the animal cannot make for itself.

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The Main Stages of Food Processing

• Ingestion is the act of eating.
• Digestion is the process of breaking food down
  into molecules small enough to absorb.
• Involves enzymatic hydrolysis of polymers into
  their monomers.

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The Main Stages of Food Processing

• Absorption is the uptake of nutrients by body
  cells.
• Elimination occurs as undigested material passes
  out of the digestive system.

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Intracellular Digestion

• In intracellular digestion, food particles are
  engulfed by endocytosis and digested within food
  vacuoles.
• Protozoa, sponges.

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Extracellular Digestion

• Extracellular digestion is the breakdown of food
  particles outside cells.
• Digestion occurs in the alimentary canal.
• Cells lining the lumen of the alimentary canal
  are specialized for secreting enzymes or
  absorbing nutrients.

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Digestive Systems

• Animals with simple body plans have a
  gastrovascular cavity that functions in both
  digestion and distribution of nutrients.

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Digestive Systems

• Animals with a more complex body plan have a
  digestive tube with two openings, a mouth and an
  anus.
• This digestive tube is called a complete
  digestive tract or an alimentary canal.

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Digestive Systems

• The digestive tube can be organized into
  specialized regions that carry out digestion and
  nutrient absorption in a stepwise fashion.

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Mammalian Digestive System

• The mammalian digestive system consists of the
  alimentary canal and various accessory glands
  that secrete digestive juices through ducts.

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Digestive Enzymes

• Enzymes are essential in the breakdown of food
  into small, absorbable units.
• Digestive enzymes are hydrolytic enzymes.
• Food molecules are split by hydrolysis.
• R-R H2O digestive enzyme R-OH H-R

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Digestive Enzymes

• Proteins are broken down into individual amino
  acids.
• Complex carbohydrates are broken down into simple
  sugars.
• Fats are reduced to glycerol, fatty acids, and
  monoglycerides.

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Motility in Alimentary Canal

• Food moves through the alimentary canal by cilia,
  specialized musculature, or both.
• Gut musculature is present in coelomates.

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Motility in Alimentary Canal

• The gut is lined with opposing layers of smooth
  muscle a circular layer and a longitudinal layer.

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Motility in Alimentary Canal

• Two types of gut movement
• Segmentation involves alternate constriction of
  rings of smooth muscle that move the contents
  around, mixing with enzymes.
• Peristalsis involves waves of contraction behind
  the food mass that move it through the gut.

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Organization - Five Major Regions

• Reception
• Conduction Storage
• Grinding early digestion
• Terminal digestion and absorption
• Water absorption and concentration of solids.

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Receiving Region

• The receiving region consists of devices for
  feeding and swallowing.
• Mouthparts mandibles, jaws, teeth, radula,
  bills.
• Buccal cavity mouth
• Muscular pharynx throat
• Salivary glands produce lubricating secretions
  that may also contain toxic enzymes or salivary
  enzymes to begin digestion.
• Amylase begins hydrolysis of starches.

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Receiving Region

• The vertebrate tongue assists in food
  manipulation and swallowing.
• Also used as a chemosensor.

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Conduction and Storage Region

• The esophagus transfers food to the digestive
  region.
• In many invertebrates (annelids, insects,
  octopods) the esophagus is expanded into a crop
  used for storage.
• Birds also have a crop that serves to store and
  soften food.

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Region of Grinding Early Digestion

• The stomach provides initial digestion as well as
  storing and mixing food with gastric juice.
• For further grinding of food, terrestrial
  oligochaetes and birds have a muscular gizzard
  that is assisted by stones or grit swallowed with
  food.

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The Stomach

• The lining of the stomach is coated with mucus,
  which prevents the gastric juice from destroying
  the cells.
• Pepsin is a protease that splits specific peptide
  bonds.

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Region of Terminal Digestion and Absorption

• The small intestine is the longest section of the
  alimentary canal.
• It is the major organ of digestion and absorption.

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Region of Terminal Digestion and Absorption

• Increasing the surface area of the intestine
  increases the area available for absorption.
• Longer intestine
• Villi fingerlike projections of intestinal
  tissue in birds and mammals
• Microvilli tiny processes on intestinal cells.

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The Small Intestine

• The first portion of the small intestine is the
  duodenum, where acid chyme from the stomach mixes
  with digestive juices from the pancreas, liver,
  gallbladder, and intestine itself.

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The Small Intestine

• The pancreas produces
• Proteases, protein-digesting enzymes.
• Lipases for breaking up fat.
• Amylase for hydrolyzing starches.
• Nucleases which degrade RNA DNA into
  nucleotides.

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The Small Intestine

• The liver secretes bile into the bile duct which
  drains into the duodenum.
• Bile is stored in the gallbladder between meals.
• Bile salts are important for digestion of fats.

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The Small Intestine

• Enzymatic digestion is completed as peristalsis
  moves the mixture of chyme and digestive juices
  along the small intestine.

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Absorption of Nutrients

• The small intestine has a huge surface area due
  to the presence of villi and microvilli that are
  exposed to the intestinal lumen.

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Absorption of Nutrients

• The enormous microvillar surface is an adaptation
  that greatly increases the rate of nutrient
  absorption.

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Absorption of Nutrients

• The core of each villus contains a network of
  blood vessels and a small vessel of the lymphatic
  system called a lacteal.

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Absorption of Nutrients

• Amino acids and simple sugars pass through the
  epithelium of the small intestine and enter the
  bloodstream.
• Initial absorption occurs by facilitated
  transport, later by active transport.

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Absorption of Nutrients

• Fats are emulsified by bile salts.
• Micelles are tiny droplets consisting of fatty
  acids and monoglycerides complexed with bile
  salts.
• Micelles diffuse into epithelial cells.
• Resynthesized into triglycerides and pass into
  the lacteals of the lymphatic system.

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Region of Water Absorption

• The large intestine, or colon is connected to the
  small intestine.

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Region of Water Absorption

• A major function of the colon is to recover water
  that has entered the alimentary canal.
• The wastes of the digestive tract, the feces,
  become more solid as they move through the colon.
• The terminal portion of the colon is the rectum
  where feces are stored until they can be
  eliminated.

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Region of Water Absorption

• The colon houses various strains of the bacterium
  Escherichia coli.
• Some produce various vitamins.

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Regulation of Food Intake

• Hunger centers in the brain regulate food intake.
• A drop in blood glucose level stimulates a
  craving for food.
• Homeostatic mechanisms control the bodys storage
  and metabolism of fat.

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Regulation of Food Intake

• Undernourishment occurs in animals when their
  diets are chronically deficient in calories.
• Can have detrimental effects on an animal.
• Overnourishment results from excessive food
  intake.
• Leads to the storage of excess calories as fat.

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Regulation of Digestion

• Hormones help coordinate the secretion of
  digestive juices into the alimentary canal.

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Glucose Regulation as an Example of Homeostasis

• Animals store excess calories as glycogen in the
  liver and muscle.
• Glycogen is made up of many glucose subunits.
• Glucose is a major fuel for cells.

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Glucose Regulation as an Example of Homeostasis

• Blood glucose levels rise, the pancreas produces
  insulin.
• Insulin enhances transport of glucose into body
  cells and stimulates storage of glucose as
  glycogen.
• Results in lower blood glucose levels.

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Glucose Regulation as an Example of Homeostasis

• Lower blood glucose levels stimulates the
  pancreas to secrete glucagon.
• Glucagon promotes breakdown of glycogen in the
  liver back into glucose which is released into
  the blood.

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Nutritional Requirements

• An animal must obtain organic carbon (from
  glucose) and organic nitrogen (from amino acids
  obtained during digestion of protein) in order to
  build organic molecules such as carbohydrates,
  lipids and proteins.

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Nutritional Requirements

• An animals diet must also supply essential
  nutrients in preassembled form.
• An animal that is malnourished is missing one or
  more essential nutrients in its diet.

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Nutritional Requirements

• Herbivorous animals may suffer mineral
  deficiencies if they graze on plants in soil
  lacking key minerals.

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Vitamins

• Vitamins are organic molecules required in the
  diet in small amounts.
• To date, 13 vitamins essential to humans have
  been identified.
• Vitamins are grouped into two categories
• Fat-soluble
• Water-soluble

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Minerals

• Minerals are simple inorganic nutrients that are
  usually required in small amounts.

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Essential Fatty Acids

• Animals can synthesize most of the fatty acids
  they need.
• The essential fatty acids are certain unsaturated
  fatty acids.
• Deficiencies in fatty acids are rare.

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Essential Amino Acids

• Animals require 20 amino acids and can synthesize
  about half of them from the other molecules they
  obtain from their diet.
• The remaining amino acids, the essential amino
  acids, must be obtained from food in preassembled
  form.

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Essential Amino Acids

• A diet that provides insufficient amounts of one
  or more essential amino acids causes a form of
  malnutrition called protein deficiency.
• Malnutrition is much more common than
  undernutrition in human populations.

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Essential Amino Acids

• Most plant proteins are incomplete in amino acid
  makeup.
• Individuals who eat only plant proteins need to
  eat a variety to ensure that they get all the
  essential amino acids.