Edexcel IGCSE Biology 复习笔记 2.5.8 Humans: The Digestive System

• The digestive system is an example of an organ system in which several organs work together to digest and absorb food
• Digestion is a process in which relatively large, insoluble molecules in food (such as starch, proteins) are broken down into smaller, soluble molecules that can be absorbed into the bloodstream and delivered to cells in the body
• These small soluble molecules (such as glucose and amino acids) are used either to provide cells with energy (via respiration), or with materials with which they can build other molecules to grow, repair and function
• The human digestive system is made up of the organs that form the alimentary canal and accessory organs
  • The alimentary canal is the channel or passage through which food flows through the body, starting at the mouth and ending at the anus
  • Digestion occurs within the alimentary canal
  • Accessory organs produce substances that are needed for digestion to occur (such as enzymes and bile) but food does not pass directly through these organs

   

The human digestive system includes the organs of the alimentary canal and accessory organs that work together to break large insoluble molecules into small soluble molecules

Alimentary Canal and Accessory Structures Table

The Stages of Food Breakdown

• Food taken into the body goes through 5 different stages during its passage through the alimentary canal (the gut):
  • Ingestion - the taking in of substances, e.g. food and drink, into the body through the mouth
  • Mechanical digestion - the breakdown of food into smaller pieces without chemical change to the food molecules
  • Chemical digestion - the breakdown of large, insoluble molecules into small, soluble molecules
  • Absorption - the movement of small food molecules and ions through the wall of the intestine into the blood
  • Assimilation - the movement of digested food molecules into the cells of the body where they are used, becoming part of the cells
  • Egestion - the passing out of food that has not been digested or absorbed (as faeces) through the anus

   

• Peristalsis is a mechanism that helps moves food along the alimentary canal
• Firstly, muscles in the walls of the oesophagus create waves of contractions which force the bolus along
• Once the bolus has reached the stomach, it is churned into a less solid form, called chyme, which continues on to the small intestine
• Peristalsis is controlled by circular and longitudinal muscles
  • Circular muscles contract to reduce the diameter of the lumen of the oesophagus or small intestine
  • Longitudinal muscles contract to reduce the length of that section the oesophagus or the small intestine

   

• Mucus is produced to continually lubricate the food mass and reduce friction
• Dietary fibre provides the roughage required for the muscles to push against during peristalsis

Circular and longitudinal muscles in the alimentary canal contract rhythmically to move the partially digested food mass along in a wave-like action

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