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Chapter 11 - Digestion
Transcript
Digestion
Overview of Digestion
Assimilation – processes of nutrient acquisition, digestion, and absorption
Takes place along the gastrointestinal (GI) tract
Contiguous with the external environment
Composed of many cell types
Secretory cells
Absorptive cells
Muscle cells
Neurons
Egestion – Excretion of undigested food
Diets Provide Energy
Energy content of diet must match the metabolic demands of the animal
Caloric content – energy content of a marcomolecule, e.g., protein and carbohydrates – 4 kcal, fat – 9 kcal
Not all food is digestible
Energy is lost as feces
Specific Dynamic Action (SDA) or heat increment – increase in metabolic rate during the digestive process; important source of thermal energy
Figure 11.2
Nutrients are the external molecules that allow an animal to maintain and build cells.
Vitamins and Minerals
Group of unrelated molecules with diverse functions
Many participate in catalysis as cofactors for enzymes
Usually categorized based on solubility
Fat-soluble – vitamins A, D, E, K
Water soluble – vitamins B, C
Obtained in diet or from bacteria living in the GI tract
Metallic elements that participate in protein structure
Calcium, Phosphorus, Iron, Copper, Zinc
Most are absorbed along the GI tract by specific transporters
Table 11.1
Vitamins
Amino Acids
Animals use 20 amino acids to build proteins
Most can be produced by the animal
Eight essential amino acids must be obtained in the diet
Diets deficient in any essential amino acid lead to developmental defects and slow growth
Protein quality
The amino acid profile of dietary protein
Animal tissue provides higher protein quality than plant tissue
Some plants lack specific amino acids
Fatty Acids
Animals can make almost all fatty acids from acetyl CoA
Animals cannot produce sufficient amounts of
omega-3 or omega-6 fatty acids
Omega-3 fatty acids must be ingested as linolenic acid
Found in plant seeds
Omega-6 fatty acids must be ingested as linoleic
Found in cold-water fish
Digestive Enzymes
Enzymes convert macromolecules to forms that can be absorbed and processed
Lipases
Break down triglycerides and phospholipids into fatty acids
Proteases
Break down proteins into shorter polypeptides
Amylases
Break down polysaccharides into oligosaccharides
Nucleases
Break down DNA into nucleotides
Most digestion takes place extracellularly
For example, in the lumen of the GI tract
Symbiotic Organisms and Digestion
Digestion in many animals benefits from the assistance of symbiotic organisms
For example, bacteria, fungi, and photosynthetic organisms
Three main types of symbionts participate in digestion
Enterosymbionts
Live within the lumen of the GI tract
Often in an enlarged region called the cecum
Exosymbionts
Actively cultivated outside the body
Endosymbionts
Grow in interstitial spaces or within host cells
Nutrient Transport Across Plasma Membranes
Some nutrients are transported by protein carriers
Polar molecules require specific protein transporters
Transport down a concentration gradient
Facilitated diffusion
Transport against a concentration gradient
Active transport
Via Na+-dependent cotransporters
Some nutrients are transported in vesicles
Uptake
Pinocytosis or Phagocytosis
Expulsion
Exocytosis
Carbohydrate Breakdown and Absorption
Main types consumed by animals
Polysaccharides
Glycogen, starch, cellulose, chitin
Disaccharides (sucrose, lactose, maltose)
Poly- and disaccharides are hydrolyzed to monosaccharides
For example, glucose, fructose, galactose
Monosaccharides are absorbed by epithelial cells in the intestine (enterocytes)
Active transport and facilitated diffusion
Figure 11.4
Carbohydrate Transport
Combination of active transport and facilitated diffusion
Intestinal absorptive cells (enterocytes)
Proteins
Extracellular hydrolysis, starting in the stomach, to dipeptides and amino acids
Amino acids are absorbed into epithelial cells by amino acid-Na+ cotransporters
Some proteins are also carried into cells intact via endocytosis/exocytosis
i.e. Infant mammalian antibodies
Transferring immunoprotection
Lipids
Digestion and import of lipids is complicated by their hydrophobicity
GI tract secretes bile that emulsifies lipids into small droplets (micelles)
Dietary fats are broken down into fatty acids and monoglycerides
Lipids diffuse across cell membrane into enterocyte
Transport of lipids depend on physical properties
Short chain fatty acids and glycerol in blood
Triglycerides in the lymph as chylomicrons
Table 11.2
Chylomicrons and Lipoproteins in the Blood
Sensing, Finding, Consuming Food
Basic dietary strategies
Carnivory
Herbivory
Omnivory
Physiology of digestion is matched to the chemical and physical nature of the diet
Animals detect food with different sensory receptors
Gustatory and olfactory receptors detect chemical stimuli
Various receptors detect energy emitted by, or reflected from, the food source
For example, light, sound, heat, or electricity
Figure 11.8
Feeding in Simple Animals: Sponges
Simple animals (e.g., sponges) ingest food by phagocytosis
Digestion occurs intracellularly in endocytic vacuoles
Feeding Structures
Most animals have specialized mouthparts for obtaining and processing food
Siphons
Attachment organs
Found in many parasitic organisms
Tongues
Radula – rasping tongue
Proboscis – long tubelike tongue
Beaks
Jaws
Bird Beaks
Are composed of bone covered by overlapping epidermal scales
Beak morphology is diverse and reflects various feeding strategies
Evolutionary adaptation
Serve purposes other than feeding
Vocalization
Defense
Grooming
Courtship
Figure 11.12a
Teeth
Many vertebrates have toothlike structures
Mammals have structurally unique bony teeth
Three main parts: crown, neck, root
Three main layers: enamel, dentin, pulp
Four types of teeth: incisors, canines, premolars, molars
The shape of the teeth reflects the type of diet
Digestive Systems
Evolutionary history shows increasing anatomical and functional specialization
Two-way gut
Simple internal sac
Sac may have diverticula to increase surface area
Food enters and wastes leave
via the same opening
Digestive Systems
One-way gut (gastrointestinal tract)
Specialized regions
Mouth, pharynx, esophagus
Mechanical breakdown of food
Stomach
Acidic compartment
Upper or small intestines
Digestion and absorption
Lower or large intestines
Absorption of water
Anus
Release of indigestible material
Figure 11.14
Gastrointestinal Tract
The Coelom
Internal cavity between layers of mesoderm
Linked to evolutionary and developmental origins of the one-way gut
Acoelomates - No coelom
Pseudocoelomates - Cavity between endoderm and mesoderm
Coelomates - Cavity within mesoderm
Figure 11.16
Coelom Formation
Blastopore appears early in development, during gastrulation
Protostomes
Most invertebrates
Blastopore becomes mouth
Coelom forms when mesoderm splits (schizocoelous)
Deuterostomes
Chordates, hemichordates, echinoderms
Blastopore becomes anus
Coelom forms when mesoderm pinches off from the gut (enterocoelous)
Schizocoelous process in chordates
Gut Formation
Gut is derived from endoderm
Three regions
Foregut
Esophagus, stomach, and the anterior section of the duodenum
Forms buds that become the pancreas and liver
Midgut
Posterior part of duodenum, jejunum, ileum, and large intestine
Hindgut
Colon and rectum
Surface Area
In most animals, nutrients are hydrolyzed in the lumen of the GI tract
Nutrients are taken up by cells lining the gut
Nutrient uptake is improved by increasing surface area in two ways
Increasing gut length
Increasing surface undulations
Circular folds, Villi, and Microvilli
Specialized Compartments
Specialized compartments increase efficiency of digestion
Compartments have functional specializations
pH
Enzymes
Types of secretory and absorptive cells
Muscular valves (sphincters) control passage of food from one compartment to the next
Complexity of gut morphology varies across taxa
Reflects complexity of the diet and ease of digestion
Figure 11.20
Ruminants
Some mammals possess modifications that improve the digestion of plant material (e.g., ruminants)
Fermentative Bacteria
Salivary Glands
Multicellular exocrine glands
Ducts open into mouth
Saliva
Lubricates food
Dissolves food so nutrients can bind to gustatory receptors
Cleanses the mouth with antimicrobial properties
Contain enzymes that initiate digestion
Salivation is controlled by nerve signals
Parasympathetic nerves stimulate salivation
Sympathetic nerves inhibit salivation
Figure 11.22
Stomach
Surface is composed of columnar epithelial cells
Tight junctions
Prevent leakage across epithelium
Mucous neck cells
Secrete mucus
Parietal cells
Secrete hydrochloric acid (HCl)
Chief cells
Secrete the protease pepsin
Enteroendrocrine cells
Secrete hormones into the blood
Intestines
Most nutrients are absorbed in the intestines
Four main layers
Mucosa, Submucosa (Blood and lymphatic vessels, nerves), Circular smooth muscle, Longitudinal smooth muscle
Mucosal cells
Enterocytes
Absorptive cells with microvilli
Goblet cells
Secrete mucus
Enteroendocrine cells
Secrete hormones
Paneth cells
Secrete antimicrobial molecules
Figure 11.24
Exocrine Secretions into the Intestine
Bile
Solution of digestive chemicals and liver waste products
Produced in the liver and stored in the gallbladder
Phospholipids
Aid in the uptake of lipids
Bile salts
Emulsify fats
Bile duct opens into small intestine
Exocrine Secretions into the Intestine
Enzymes secreted by the pancreas
Proteases break down protein
Secreted as inactive proenzymes
Activated in the intestine
Amylase breaks down glycogen and starch
Lipases break down triglycerides
Nucleases break down nucleic acids
Exocrine Secretions of the Pancreas
Regulating Feeding and Digestion
Digestive functions are controlled by nerve and hormone signals
Nerve signals
From central nervous system (CNS)
Within the gastrointestinal tract
Hormone signals
Wide variety of endocrine tissues and hormones
Control of Appetite
Three hormones control appetite by binding to receptors in the hypothalamus
Leptin
Secreted by white adipose tissue when lipid content is high
Suppresses appetite
Ghrelin
Secreted by stomach when empty
Stimulates appetite
Peptide YY
Secreted by colon when full
Suppresses appetite
Control of Appetite
Hypothalamic neurons release neurotransmitters in response to hormones from the gut
Some neurotransmitters stimulate appetite
Neuropeptide Y (NPY)
Agouti-related peptide
Gamma amino butyric acid (GABA)
Some neurotransmitters inhibit appetite
Proopiomelanocortin (POMC)
Figure 11.26
Hormonal Control of Appetite
Control of Secretions
Secretions of saliva, acid, mucus, bile, bicarbonate, digestive enzymes
Respond to both the anticipation of food and its presence in the digestive system
Nerve and hormonal signals
Figure 11.28
Control of Intestinal Secretions
Control of Gut Motility
Food moves along the GI tract by contractions of smooth muscle
Controlled by nerves and hormones
Optimal speed
Fast enough to minimize amount of indigestible material in the GI tract
Slow enough to allow time for digestion and assimilation
Rate will vary according to diet
Control of Gut Motility
Smooth muscle layers
Longitudinal – controls length
Circular – controls diameter
Between muscle layers is a nerve network
Myenteric plexus
Receives signals from CNS
Peristalsis
Contractile waves that move food down the GI tract
Controlled by intrinsic myogenic activity and interstitial cells of Cajal that act as pacemaker cells
Resting muscle tone
Controls lumen diameter and is regulated by intrinsic pathways (myogenic) and neurotransmitter release
Figure 11.30
Control of Smooth Muscle Motility
Metabolic Transitions Between Meals
Postprandial period
Period immediately after feeding
Duration can vary from seconds to months
Nutrients are absorbed into the blood
Some nutrients are utilized and others are stored
Hormones control postprandial levels of nutrients
Insulin from ?-cells of the pancreas
Stimulates glucose uptake and storage
Promotes conversion of glucose to fat
Glucagon from ?-cells of the pancreas
Stimulates glucose release by liver
Glucocorticoids (e.g., cortisol) from adrenal cortex
Stimulates gluconeogenesis by the liver
Mobilize triglycerides from adipose tissue
Starvation Response
Reorganization of metabolism to ensure long-term survival
Conserve glucose to protect glucose-dependent tissues, for example, nervous tissue
Muscles shift to lipid metabolism
After lipid and glucose stores are depleted protein breakdown accelerates
Amino acids are converted to fatty acids and carbohydrates
Structural degradation occurs because there is no protein store in the body
Degradation of skeletal muscle
Figure 11.32
Starvation Response
Digestive System Degradation
and Rebuilding
Reduce energetic costs in between meals
Pythons that eat infrequently
Mucosa and submucosa degrade
Gut becomes thinner and the brush border decreases
Smooth muscles and nerves are maintained
After a meal, the snake rebuilds the GI tract just ahead of the bolus of food
High SDA; ? body temperature
Figure 11.33
Dormancy
Hypometabolism
Reduction in metabolic rate
Allows the animal to survive adverse environmental conditions
Types of hypometabolism
Torpor
Hibernation
Estivation
Most dormant mammals accumulate urea and urine because of protein breakdown
Bears recycle the urea nitrogen to regenerate amino acids and proteins
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