Digestion - ch28

Heterotrophic Nutrition

Heterotrophic nutrition involves five stages:

  1. Ingestion: Taking food into the system.

  2. Digestion: Breaking food into small soluble pieces.

  3. Absorption: Digested food is passed into the bloodstream.

  4. Assimilation: Digested food is used for energy and growth by cells.

  5. Egestion: Getting rid of undigested, unabsorbed waste from the body through the rectum and the anus. This is not the same as excretion.

The Digestive System

You must be able to draw and label this diagram

Final Products of Digestion


Food Type

Final Product of Digestion

Starch

Glucose

Protein

Amino acids

Lipid

Fatty acids and glycerol

Mechanical Digestion

Mechanically breaking down food occurs in the following ways:

  • The teeth.

  • Peristalsis.

  • Action of stomach muscles.

Chemical Digestion

Chemical digestion involves the breakdown of food using enzymes.

Reasons for Digestion:

  • To make nutrients soluble.

  • For transport of nutrients.

  • For absorption of nutrients.

Digestion in the Mouth

  • Teeth: Physically break down the food.

  • Saliva: Softens the food and contains enzymes, water, salts, and mucin;

    • Amylase: Breaks starch into maltose.

  • Tongue: Pushes food to the back of the mouth.

Swallowing

  • Food is pushed to the back of the pharynx and into the oesophagus.

  • Epiglottis: Prevents food from going into the trachea.

  • Peristalsis: Contractions of the muscular wall of the alimentary canal, pushing food along and mechanically breaking it down.

Digestion in the Stomach

  • Food enters through the cardiac sphincter.

  • Mixed with gastric juices:

    • Contains water, hydrochloric acid, mucous, and the enzyme pepsin.

    • Pepsin is produced as pepsinogen (inactive) and activated by hydrochloric acid.

    • Converts protein into peptides.

  • Hydrochloric acid kills pathogens, activates pepsinogen, and denatures salivary amylase.

  • Food is churned and mixed, converting it into chyme.

  • Leaves stomach through the pyloric sphincter.

Digestion and Absorption in the Small Intestine

Duodenum

  • The first part of the small intestine where pancreatic juice and bile are introduced.

Gall Bladder and Bile

  • Bile is produced in the liver and stored in the gall bladder, then released into the duodenum.

  • Functions of bile:

    • Emulsifies fats.

    • Neutralizes acidic chyme from the stomach.

    • Contains water, bile salts, and bile pigments.

Pancreas

  • Releases pancreatic juice containing lipase, amylase, and sodium bicarbonate.

  • Lipase: Breaks fats into fatty acids and glycerol.

  • Amylase: Breaks starch into maltose.

The Liver

  • Largest organ in the body:

    • Produces bile, converts excess amino acids into urea, detoxifies alcohol, stores iron, copper, zinc, vitamins A and D.

    • Converts glucose to glycogen.

The Ileum

  • Absorption takes place here:

    • Folds (villi) increase the surface area for absorption.

Absorption Adaptations

  • Long length for prolonged contact.

  • Villi and microvilli increase the surface area.

  • Thin walls for easy diffusion.

  • Lined with blood capillaries and each villus has a lacteal for fat absorption.

The Large Intestine

  • Contains vestigial organs (caecum and appendix).

  • Absorbs water back into the blood from waste.

  • Egestes faeces through the rectum and anus.

Functions of Fiber

  1. Absorbs and stores water, creating bulk.

  2. Stimulates peristalsis.

  3. Lowers blood cholesterol.

Functions of Symbiotic Bacteria in the Large Intestine

  1. Produce vitamins K and B.

  2. Break down cellulose.

  3. Compete with harmful microorganisms.

A Balanced Diet

Contains the correct amounts of:

  • Nutrients

  • Lipids

  • Carbohydrates

  • Proteins

  • Vitamins

  • Minerals

  • Water The total nutrient requirement depends on gender, age.