Nutrition, Digestion, and Absorption

• Nutrition, digestion, and absorption are key processes for sustaining life

Why We Eat

• The primary reason to eat is to stay alive, fulfilling hunger
• Food provides raw materials and energy essential for bodily functions

Metabolic Reactions

• Assimilation involves the synthesis of organic molecules
• Assimilation builds complex organic compounds like glucose from CO2 and H2O, and glycogen from glucose, which requires energy
• Dissimilation breaks down organic molecules to produce energy, primarily in the form of ATP

Energy Sources

• Animals are heterotrophs, deriving nutrition from consuming other organisms
• Autotrophs can synthesize necessary nutrients through:
  • Sunlight (photosynthesis)
  • Chemical energy (chemosynthesis)
• Heterotrophs rely on the synthesis of autotrophs, adapting to utilize these synthesized nutrients.

Types of Heterotrophs

• Saprobes acquire nutrients by absorbing them from dead organic matter
• Detritivores actively consume dead organic matter
• Predators obtain nutrition by feeding on living organisms

Types of Predators

• Herbivores consume plants
• Carnivores prey on animals
• Omnivores prey on both plants and animals
• Filter feeders filter small organisms from aquatic environments
• Fluid feeders, such as mosquitoes, feed on fluids

Energy Balance

• Energy balance is achieved when energy input equals energy output (use and loss)
• Energy needs can be measured using heat energy:
• A calorie is the heat needed to raise 1 gram of water by 1°C
• A kilocalorie (kcal) equals 1,000 calories
• A Calorie (Cal) is equivalent to a kilocalorie, equaling 1,000 calories
• The joule is the International System of Units measure of energy
  • One joule equals 0.239 calories

Metabolism

• Metabolic rate measures an animal's energy needs met through food intake and digestion
• Fats, carbohydrates, and proteins are foods that provide energy
• Energy consumption is needed for movements, synthesis of molecules, and transport processes
• Basal metabolic rate is the metabolic rate resulting from essential physiological functions during rest
• Physical activity increases the basal energy requirement

Energy Sources

• Carbohydrates, specifically glucose (C6H12O6 + 6O2), convert to 6CO2 + 6H2O
  • Carbohydrates provide an energy equivalent of 17 kJ/g
• Fats provide an energy equivalent of 40 kJ/g
• Proteins (amino acids) provide an energy equivalent of 17 kJ/g

Energy Stores

• Animals store energy between meals
• Carbohydrates are stored as glycogen in liver and muscle cells, which is enough for about a day's needs
• Fat stores more energy per gram and is compact due to little water content
• Protein is not a primary energy storage, but can be metabolized as a last resort

Energy Imbalance

• Calorie intake not equal to calorie output
• If calorie intake is greater than output, increased storage occurs
• If calorie intake is less than output, utilization of stores occurs