Animal Physiology: Food and Digestion

ANIMAL PHYSIOLOGY: FOOD AND DIGESTION

Learning Objectives

  • Identify chemical elements in carbohydrates, proteins, and lipids.
  • Describe the structure of carbohydrates, proteins, and lipids as large molecules made from smaller units.
  • Investigate food samples for glucose, starch, protein, and fat.
  • Understand a balanced diet including carbohydrates, protein, lipid, vitamins, minerals, water, and dietary fibre.
  • Identify sources and functions of carbohydrates, lipid, protein, vitamins A, C, and D, mineral ions calcium and iron, water, and dietary fibre.
  • Understand energy requirements variation.
  • Describe alimentary canal structure and function.
  • Understand peristalsis.
  • Understand digestive enzyme roles.
  • Understand bile production and role.
  • Understand small intestine adaptations for absorption.
  • BIOLOGY ONLY: Investigate energy content in food samples.

Food and Digestion Overview

  • Food and nutrients are used in various ways by the body.
  • The digestive system breaks down food, which is then absorbed into the blood.
  • The blood carries nutrients to the cells of the body.

Balanced Diet

  • A balanced diet contains the correct proportions of carbohydrates, lipids, proteins, minerals, vitamins, dietary fiber, and water.
  • Food is needed for: energy, growth/repair, and fighting disease/maintaining health.

Carbohydrates

  • Make up about 1% of body mass; the body's main energy source.
  • Cells oxidize glucose during cell respiration to release energy.
  • Glucose is found in sweet foods like fruits and vegetables.
  • Other sugars include fructose (fruit sugar) and lactose (milk sugar).
  • Sucrose (table sugar) is transported through plant stems.
  • Sugars taste sweet and are soluble in water.
  • Excessive consumption of processed foods can lead to high sugar intake.
  • Starch is a large, insoluble glucose polymer found in plants like potatoes, rice, wheat, and millet.
  • Starchy foods are the staple diets.
  • Glycogen is a similar carbohydrate to starch, found in animal cells (liver and muscle).
  • Large carbohydrates must be broken down into simple sugars for absorption.
  • Cellulose, a polymer of glucose, forms plant cell walls, but humans cannot digest it.
  • Cellulose functions as dietary fiber or 'roughage', which aids in intestinal muscle contraction to prevent constipation and diseases.

Carbohydrate Types

  • Monosaccharides: Simple sugars (e.g., glucose, fructose).
  • Disaccharides: Two monosaccharides joined (e.g., sucrose - glucose + fructose; lactose - glucose + galactose).
  • Polysaccharides: Polymers of sugars (e.g., starch, glycogen, cellulose).

Chemical Formula

  • The chemical formula for glucose is C6H{12}O_6.
  • All carbohydrates contain only carbon, hydrogen, and oxygen.
  • The hydrogen and oxygen atoms are in a 2:1 ratio, like water (H_2O).

Lipids (Fats and Oils)

  • Contain carbon, hydrogen, and oxygen, but with a lower proportion of oxygen compared to carbohydrates.
  • Animal lipids (fats) are solid at room temperature, while plant lipids (oils) are usually liquid.
  • Sources of animal fats: meat, butter, cheese, milk, eggs, oily fish.
  • Vegetable oils: olive oil, corn oil, rapeseed oil, margarine.
  • Lipids make up about 10% of body mass.
  • Essential for cell structure; stored as long-term energy reserves under the skin and around organs.
  • Fat under the skin provides insulation, and fat around organs protects them from mechanical damage.
  • Building blocks: glycerol and fatty acids. A glycerol molecule joins with three fatty acid molecules.
  • Too much saturated fat and cholesterol is unhealthy and linked to heart disease.

Saturated vs. Unsaturated Fats

  • Saturated fats are more common in animal sources and contain no double bonds in their fatty acids.
  • Unsaturated fats are more common in plant oils and contain double bonds in their fatty acids.
  • Unsaturated lipids are considered healthier.

Chemical Formula

  • Example: Tristearin (beef fat): C{51}H{98}O_6

Proteins

  • Make up about 18% of body mass; second largest percentage after water.
  • Found in all cells, needed for growth and tissue repair; used to make many compounds, including enzymes.
  • Rich sources: meat, fish, cheese, eggs (animal products).
  • Plant sources: beans, peas, and nuts.
  • Recommended daily intake: 70g.
  • Kwashiorkor is a protein-deficiency disease common in poorer countries.
  • Proteins are polymers made from 20 different amino acid subunits.
  • Amino acids contain carbon, hydrogen, oxygen, and nitrogen. Some also contain sulfur.
  • Amino acids link into long chains that fold or twist into spirals with cross-links.

Amino Acids

  • Humans can make about half of the 20 amino acids needed; the other 10 are essential amino acids and must be obtained from diet.
  • Meat, fish, eggs, and dairy products contain higher amounts of essential amino acids.
  • Vegetarians can obtain all essential amino acids by eating a varied diet.

Protein Structure and Function

  • The shape of a protein is determined by the order of amino acids.
  • There are thousands of different kinds of proteins with various structures and functions in organisms.
  • Examples: structural proteins (collagen, keratin), enzymes, and haemoglobin.

Minerals

  • Elements obtained from food as 'minerals' or 'mineral ions'.
  • Calcium is used for making teeth and bones.
  • The human body contains about 3g of iron, essential for oxygen transport in the blood.

Examples of Minerals and Their Roles

  • Calcium: 1000g; making teeth and bones; dairy products, fish, bread, vegetables.
  • Phosphorus: 650g; teeth/bones, DNA, ATP; most foods.
  • Sodium/Chlorine: 100g; body fluids; common salt, most foods.
  • Magnesium: 30g; making bones, found inside cells; green vegetables.
  • Iron: 3g; part of haemoglobin, carries oxygen; red meat, liver, eggs, spinach.

Mineral Deficiency Diseases

  • Lack of minerals leads to deficiency diseases.
  • Calcium deficiency leads to poor bone development, causing rickets.
  • Iron deficiency leads to anaemia.
Mineral Requirements
  • One-year-old child: 0.6g (600mg) calcium per day.
  • Sixteen-year-olds: 12mg iron per day.

Vitamins

  • Chemicals needed in small amounts to stay healthy.
  • Identified by the effect of a deficiency.
  • Vitamin D is needed for bones to take up calcium salts; deficiency can cause rickets.

Key Vitamins and Their Functions

  • Vitamin A: needed for light-sensitive chemicals in the retina; deficiency causes night blindness.
  • Vitamin C: needed to make connective tissue; deficiency causes scurvy.
  • Vitamin B group (B1, B2, B3): involved in cell respiration; deficiencies result in diseases like beri-beri.
  • Vitamin D: helps bones absorb calcium and phosphate; deficiency causes rickets, poor teeth.

Historical Context

  • James Lind discovered in 1753 that scurvy could be cured by eating fresh oranges and lemons.
  • Captain Cook kept his sailors healthy by ensuring they ate fresh fruit.
  • British sailors were called 'limeys' because they drank lime juice to prevent scurvy.

Recommended Daily Amounts (EU, 2012)

  • Vitamin A: 0.8mg
  • Vitamin B1: 1.1mg
  • Vitamin B2: 1.4mg
  • Vitamin B3: 16mg
  • Vitamin C: 80mg
  • Vitamin D: 5 μg

Food Tests

  • Simple tests can identify starch, glucose, protein, or lipid.
  • Foods need to be extracted with water to dissolve components.

Practical Tests

  • Starch: Iodine solution turns blue-black.
  • Glucose: Benedict's solution forms a brick-red precipitate when heated.
  • Protein: Biuret test produces a purple color.
  • Lipid: Ethanol dissolves lipids, forming a white cloudy emulsion in water.

Energy from Food

  • Foods have different energy content based on the proportions of carbohydrate, lipid, and protein.
  • Energy is measured in kilojoules (kJ).

Energy Yield per Gram

  • Carbohydrate: 17 kJ
  • Lipid: 39 kJ
  • Protein: 18 kJ

Energy Content Examples

  • High-lipid foods (butter, nuts) have high energy content.
  • Fruits and vegetables (mainly water) have lower energy content.

Measuring Energy in Food (Biology Only)

  • Food scientists use a calorimeter.
  • Electrical filament ignites the food in an oxygen-filled calorimeter.
  • Energy released heats water flowing through a coil.
  • Simplified method: burn food and measure heat to warm up water in a test tube.

Energy Calculation

  • Energy = (final\ temperature - starting\ temperature) \times mass\ of\ water \times 4.2 joules.

  • Divide by mass of food to find energy per gram.

  • Example: pasta (0.55g), water (20g) temperature rises. calculate the food's energy content.

    Energy = \frac{(43-21) \times 20 \times 4.2}{0.55}
    Energy = 3360J \ per \ g

Energy Requirements

  • Energy is needed even during sleep for body functions.
  • Total energy needs depend on age, body size, and activity levels.
  • The greater a person's weight, the more energy they need.
  • Men need more energy than women due to greater average body mass.
  • Pregnant women and manual workers need extra energy.

Factors Affecting Dietary Content

  • Pregnancy: extra iron or calcium needed.
  • Younger women: may need extra iron due to menstruation.

Digestion

  • Digestion breaks down food into building blocks for absorption.
  • Enzymes speed up digestion; digestive enzymes are made in the gut and act on food.
  • Mechanical digestion: physical breakdown of food (e.g., chewing).

Peristalsis

  • Muscles in the intestinal walls move food along the gut.
  • Circular muscles contract/longitudinal muscles relax to narrow the gut.
  • Longitudinal muscles contract/circular muscles relax to widen the gut.
  • Waves of muscle contraction push food along.

Digestive System

  • The gut is approximately 8m long in adults.
  • Mouth, stomach, and duodenum: break down food using enzymes.
  • Ileum: absorbs digested food.
  • Large intestine: absorbs water and stores waste.

Enzymes in Digestion

  • Carbohydrates are digested by carbohydrases like amylase and maltase.
  • Proteases break down proteins.
  • Lipases break down lipids.

Digestion Process

  • Mouth: Amylase in saliva starts starch breakdown.
  • Stomach: Hydrochloric acid kills bacteria; pepsin digests proteins.
  • Duodenum: Pancreatic enzymes digest starch, proteins, and lipids; bile emulsifies lipids.

Bile

  • Made in the liver, stored in the gall bladder.
  • Emulsifies large lipid globules into tiny droplets, increasing surface area for lipase enzymes.
  • Neutralizes acidic stomach contents.

Absorption in the Ileum

  • The ileum is adapted to absorb digested food with a large surface area.
  • Villi: tiny projections that increase the surface area of the lining.
  • Microvilli: projections on the surface cells of each villus, further increasing surface area.
  • Each villus contains blood capillaries and a lacteal for transporting fats.
  • Epithelium: Single layer of cells on the surface of each villus.

Surface Area Amplification

  • The total area of the lining of intenstine is ≈ 300m^2

Hepatic Portal Vein

  • Vessels from the ileum join to form the hepatic portal vein, which leads to the liver.
  • Liver processes and stores molecules.
  • Glucose is converted to glycogen and stored; glycogen is converted back to glucose when needed.

Assimilation

  • The absorption of the soluble food molecules from the blood into cells of tissues and used to build new parts of cells is called assimilation.

Large Intestine

  • Consists of cellulose, indigestible remains, water, bacteria, and cells.
  • Colon absorbs remaining water.
  • Faeces are stored in the rectum and expelled through the anus.
  • Egestion: process of removing waste (faeces) that has passed through the gut without entering the cells.