Digestion

Learning objectives

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WALT To be able to relate the generalised structure of the gut wall in the oesophagus, stomach, duodenum and ileum to the functions of these organs.

To be able to describe mechanisms for the absorption of food by the ileum, including the roles of diffusion, facilitated diffusion and active transport.

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WILF To be able to explain how the structure of the digestive tract is adapted for absorption

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Digestion

Humans, like all animals, use holozoic nutrition, that is obtaining nourishment as animals do by ingesting complex organic matter Holozoic nutrition consists of these stages:

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ingestion- taking large pieces of food into the body

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digestion- breaking down the food by mechanical and chemical means

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absorption- taking up the soluble digestion products into the body's cells

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assimilation- using the absorbed materials egestion- eliminating the undigested material

How is the human gut adapted for these functions?

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a long tube, the alimentary canal or digestive tract (or simply gut) which extends from the mouth to the anus.

a number of associated glands.

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The lining wall of the alimentary canal appears different in different parts of the gut, reflecting their different roles, but always has these four basic layers:

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The mucosa,

This secretes digestive juices and absorbs digested food. It is often folded to increase its surface area. On the inside, next to the lumen (the space inside the gut) is a thin layer of cells called the epithelium. Mucosa cells are constantly worn away by friction with food moving through the gut, so are constantly being replaced.

The submucosa,

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This contains blood vessels, lymph vessels and nerves to control the muscles. It may also contain secretory glands.

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The muscle layer

This is made of smooth muscle, under involuntary control. It can be subdivided into circular muscle (which squeezes the gut when it contracts) and longitudinal muscle (which shortens the gut when it contracts). The combination of these two muscles allows a variety of different movements.

The serosa

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This is a tough layer of connective tissue that holds the gut together, and attaches it to the abdomen

Parts of the Alimentary Canal

tongue salivary gland oesophagus liver gall bladder bile duct duodenum jejunum ileum caecum epiglottis stomach pancreas transverse colon ascending colon descending colon rectum anus

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The internal surface area is increased enormously by three levels of folding: large folds of the mucosa,

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Villi - large structures composed of many cells that can clearly be seen with a light microscope

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Microvilli - small sub-cellular structures formed by the folding of the plasma membrane of individual cells.

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Can only be seen clearly with an electron microscope, and appear as a fuzzy brush border under the light microscope.

Circular and longitudinal muscles bi-directional peristalsis. This also improves absorption.

Questions

1) (i) Describe how each of the following helps the oesophagus to carry out its function.

Stratified epithelium allows for cells damaged by passage of food / resists damage as food passes; Muscle layers move food along / peristalsis ;

(2 marks)

(ii) Stratified epithelium is described as a tissue.

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Explain what is meant by a tissue.

cells similar (in structure); with same / particular / specific / single / one function or job or process;

(2 marks)

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2) (a) Name the part labelled A. Mucosa / gastric gland (accept: gastric pit)

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(b) Give two ways in which the structure of the wall of the stomach differs from the wall of the oesophagus 3 layers of muscle v 2 layers / oblique layer; (Gastric) glands present; (accept 'glands that secrete enzymes’ / equiv) Specific cells present, e.g. oxyntic cells that secrete HCl; Columnar epithelial cells v stratified; All smooth muscle v some striated.

(differences must be structural, not functional - e.g. enzymes) . (c) Give one way in which the stomach wall is adapted

(1 mark) (2 marks)

(i) to churn food; Muscle layers arranged in all directions / oblique, or 3rd, layer

(1 mark)

(ii) to make food poisoning by bacteria less likely (Hydrochloric) acid / low pH / (endo)peptidase (kills bacteria)

(1 mark)

(iii) to prevent enzymes produced in the stomach wall digesting the surface of the stomach.

Mucus secretion / enzymes produced in inactive state

(1 mark)