Overview of how nutrients are handled in animal bodies after absorption.
Diagram summarizing the biochemistry and physiology across various animals' bodies.
Concept of nutrient pools: Protein pool, glucose/carb pool, fat pool.
Flow of nutrients through metabolism culminating in ATP production (burned up, eventually producing CO₂ and water).
Nutrient Pools
Carbohydrate Pool
Carbohydrates are broken down into glucose.
Stored as glycogen.
Glycolysis: Process by which glucose is converted to pyruvate, producing ATP.
Anaerobic metabolism possible in the absence of oxygen leads to the formation of lactic acid.
Pyruvate Dehydrogenase (PDH):
Converts pyruvate to acetyl CoA.
This conversion is irreversible (a committed step in metabolism).
Acetyl CoA serves as a fundamental link among all three nutrient pools.
Cannot revert back to glucose, which is crucial for maintaining blood glucose levels.
Excess glucose/pyruvate can be converted into fat, contributing to body fat accumulation.
Protein Pool
Dietary proteins are converted to amino acids.
These can be stored as body protein or converted to acetyl CoA.
Some amino acids can be converted to glucose when in a state of starvation.
Muscle or organ tissue utilized to maintain blood glucose levels during starvation.
Fat Pool
Dietary fats, when metabolized, yield acetyl CoA.
Fat cannot be converted back to glucose, restricting the ability to maintain blood glucose during prolonged fasting (e.g., in certain animal species that primarily rely on fat metabolism).
Overview of Herbivory
Challenges of Eating Plants
Plants, as primary producers, provide nutrients but pose challenges due to:
Low protein content in some plant materials.
Low essential amino acids.
Indigestible fibers (cellulose) due to lack of cellulase enzymes in animals.
Toxic compounds present in many plants targeted to deter herbivory.
Role of microbial symbiosis in aiding herbivores to digest cellulose and detoxify harmful compounds.
Definition of Fiber
Fiber includes cellulose and is defined through neutral detergent solubility (NDS) and neutral detergent fiber (NDF):
Neutral Detergent Fiber (NDF): Remaining portion has components such as hemicellulose (digestible), cellulose (tough), and lignin (indigestible).
Lignin: A phenolic polymer; adds toughness to plant material and complicates digestion.
Strategies for Digesting Fiber
Non-Digesting Herbivores
Some herbivores (e.g., caterpillars, pandas) focus on digesting soluble components of plants and may not digest fibrous plant materials effectively.
Fermentation Process
Fermentation: Anaerobic process converting glucose into short-chain fatty acids (SCFAs).
Microbes break down cellulose and ferment it to produce SCFAs (i.e., acetic acid, propionic acid, butyric acid).
SCFAs utilized by herbivores for energy.
Microbial cells themselves become a source of nutrition when consumed by the herbivore.
Recycling nitrogen compounds to synthesize amino acids leads to lower protein dietary requirements during microbial activity.
Fermentation Chambers Requirements
Enlarged gut chambers with specific conditions: neutral pH, anaerobic environment, constant temperature.
Sufficient time for fermentation to occur (may take days depending on size and metabolic rate).
Ability to selectively retain substrates for fermentation.
Types of Herbivores
Mammalian Herbivores
Foregut Fermenters (e.g., cows, deer):
Have specialized stomach chambers, including the rumen, reticulum, omasum, and abomasum.
Utilize fermentation in the foregut, leading to improved nutrient absorption, especially of proteins and vitamins.
Hindgut Fermenters (e.g., rabbits, horses):
Utilize enlarged cecum or colon for fermentation, which allows for digestion post-small intestine absorption of soluble nutrients.
May employ cecotrophy to re-consume fecal pellets enriched with nutrients from fermentation.
Size and Digestive Strategy Correlation
Smaller mammals generally rely on hindgut fermentation due to limited gut capacity and higher metabolic rates.
As size increases, foregut fermentation becomes favorable for efficient nutrient extraction.
Very large mammals (elephants, rhinos) revert to hindgut fermentation as they cannot be selective in their intake of fibrous material.
Specific Cases Among Avian and Other Herbivores
Limited avian herbivores exist (often small sizes); the Huatzin is an example of a foregut fermenter among birds.
Other avian examples (ducks, geese) utilize some fermentation but have higher energy utilization through their digestive systems.
Unique cases of herbivores across various taxonomies, including reptiles and insects (e.g., termites) adapting to digest fibrous materials through symbiotic relationships with their gut microbiota.