Ch. 5 - The Path of Food

• Main steps in getting and using food

  1. Ingestion: taking in food

  2. Digestion: breaking down food

  3. Absorption: taking digested food from our intestines into our blood

  4. Assimilation: using the food in the cells of the body

  5. Egestion: discharge of undigested waste
• Alimentary canal

  A tube that runs from the mouth to the anus
• Food is one of our most basic needs
• Reasons we need food

  • To give us energy
  • To provide the raw materials for growth
  • To allow us to repair the body
• Digestive system

  Consists of the alimentary canal and some other glands, allows us to convert food into a form that the body can use easily
• Digestive system

  1. Breaks our food into smaller molecules

  2. Allows the molecules to pass from our intestines into our blood
• Blood

  Carries the food molecules to all parts of the body
• Any food that does not pass into our blood is removed (or egested) from our bodies as waste called faeces
• Mouth

  The function of the mouth is to take in and digest food
• Two types of digestion take place in the mouth:

  • Physical digestion
  • Chemical digestion
• Physical digestion

  the breakdown of food using mechanical or physical methods
• Example of Physical Digestion


  Occurs in the mouth when the teeth cut and chew food into smaller pieces. The smaller pieces of food have a larger surface area and can be more easily broken down later in the rest of the digestive system.
• Teeth

  there are four types of teeth in the adult human jaw. The sequence of teeth from the front to the back of the jaw is as follows: incisors, canine, premolars, molars
• Incisors

  • Sharp edges, like a chisel
  • Used to cut, slice and nibble food
• Canines

  • Long and pointed
  • Used to grip and tear food
• Premolars

  • large with flat surfaces
  • Used for chewing, crushing and grinding food
• Molars

  • larger than premolars
  • Used for chewing, crushing and grinding food
• Chemical Digestion

  The breakdown of food using enzymes (or hydrochloric acid in the stomach)
• Enzyme

  A chemical (made of protein) produced by the body that speeds up chemical reactions in the body without the enzyme being used up
• Enzymes

  • Scientists give the name catalyst to substances that speed up reactions
  • Enzymes are called biological catalysts
• Salivary glands

  • Located in the cheeks and under the tongue
  • Produce liquid called saliva
  • Saliva helps to soften and moisten food
• Salivary amylase


  • An enzyme in saliva
  • Breaks down starch and turns it into a simple sugar called maltose
• Our digestive system produces many enzymes, each one breaking down a different type of food
• Amylase breaks down starch


  Turns it into a simple sugar called maltose
• Maltose


  Another enzyme then breaks it down into an even simpler sugar (called glucose)
• Oesophagus (or food pipe) 


  a muscular tube. It forces food down from the mouth to the stomach using a wave of muscular action called peristalsis. Peristalsis also pushes food through the rest of our intestines.

  This muscular action is stimulated by fibre (also called roughage) in our diet. Fibre is material that is not broken down or digested by enzymes in our digestive system.

  Good examples of fibre include: fruit, vegetables, cereal grains, brown bread, muesli and porridge.
• Stomach


  • a muscular bag that holds food for a few hours.
  • Hydrochloric acid in the stomach kills bacteria and also softens food.
  • The stomach produces enzymes that chemically digest food.
  • The stomach also churns and physically digests and mixes the food.
  • The stomach is an organ.
• Organ


  a structure in a living thing that carries out a particular function (or functions) e.g. the stomach
• Small Intestine

  • a long tube (about 6 metres) but it has a narrow diameter (about 2.5 cm)
  • it has 2 functions:

  1. Digestion
  2. Absorption

  • produces many enzymes that complete the breakdown of food
  • Most digestion takes place in the part of the small intestine below the stomach
  • in the rest of the small intestine, the food is absorbed from the intestine into the bloodstream. Digested food is then transported all over the body by the blood.
  • Two organs, the liver and the pancreas, help to digest food. They pass their products into the small intestine.
• Liver

  • a complex organ that carries out a range of functions
  • one of its many functions is to produce a liquid called bile
  • bile passes from the liver into the small intestine
  • bile helps to digest fat in the small intestine
  • the liver is the largest organ inside the body
• Pancreas

  • produces many digestive enzymes
  • These enzymes pass from the pancreas into the small intestine, where they help to digest food
  • The salivary glands, the liver and the pancreas are glands attached to the digestive system
  • They are called associated glands
• Large Intestine

  • About 1.5 metres long
  • About twice the diameter of the small intestine
• Material entering the large intestine

  Contains a lot of liquid along with unabsorbed waste material
• Large intestine

  1. Takes water back into the bloodstream (reabsorbs water)

  2. Semi-solid waste material left is called faeces
• Too much water is taken back


  Waste becomes too solid, in a condition called constipation
• Too little water is taken back


  Waste becomes liquid and we suffer from diarrhoea
• Faeces


  1. Stored in the rectum

  2. Pass out of the intestine through the anus