Site author Richard Steane
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The following section is in order to get the following processes into perspective:
ingestion, digestion, egestion, absorption, assimilation, respiration, excretion, defaecation

ingestion, digestion, egestion etc Much of our food is taken into the body ( ingested ) in solid form, but it must be converted into a liquid form [like a solution] before it can be taken into the bloodstream (absorbed ) and passed around to various parts inside it. Although we usually take in plenty of water at the same time, it is unlikely that most food will simply dissolve, so it must be broken down chemically and physically (digested ) into simpler substances [with smaller molecules] which will dissolve. The simpler substances can then be taken to the body's cells and used either as fuel for energy (respired ) or as raw material to be built up again for growth and repair (assimilated), but anything which is not used by the body is got rid of when the gut is emptied (defaecation or egestion ).
This must not be confused with excretion which is the removal of waste products formed inside cells.

What is the name for the [whole] tube in which these processes take place?

the gut or alimentary canal

What are the main causes for:

a) the physical breakdown of food? [clue - there are usually 20-32 of them]

> teeth

b) the chemical breakdown of food into simpler substances?

> enzymes

What carries the simpler substances around the body?

> blood / bloodstream

What is the biological name for the final waste product of the process?

> faeces

Why is it likely that dietary fibre will be part of this?

> it is not soluble / unable to be digested

Name some classes of food substances or ingredients which do not need to be broken down (into simpler subunits) before they can be taken in:

? > minerals / vitamins / (simple) sugars

Measure the distance between your seat and your mouth, using a metre rule: ?70 cm Using this and the figure from the book, make an estimate of how many times your gut is coiled.

> about 10 times

Explain why even this is likely to be an underestimate.

[clue: What main regions of the body does the gut pass through? Is there much space there?]

> oesophagus (gullet) goes straight through the chest

enzymes explained


In the different parts of the gut, various digestive juices are released. These contain several different sorts of enzymes, each with a specific purpose: the breaking down of a particular type of major food component (carbohydrates, fats, proteins) into simpler substances that can be easily absorbed.

Complete the table below, summarising the effects of different sorts of enzymes. These terms can be general(*) or specific(@) - see the examples already added to the top 2 lines below.
It should be useful to refer back to work on the main classes of foods.

Class of FOOD: broken down into SIMPLER SUBUNITS called : by ENZYMES called:
@ e.g. starch
- simple sugars e.g. glucose
- maltose (2 glucose units, joined)
amylases (see later work)
Fats and oils ? >fatty acids and glycerol >lipases
Proteins ?>amino acids >proteases

Enzymes themselves are proteins, and they need special conditions in which to work: warm temperature, plenty of water, and a certain pH - some need neutral conditions ( pH value near to 7), whilst others need acidic conditions ( pH values below 7), and yet others need alkaline conditions ( pH values above 7). Each part of the gut normally provides just the right conditions for the enzymes that are found there. However it is important to realise that digestive juices that contain acids and alkalies, and emulsifying agents merely provide ideal conditions for enzymes to act. For example, acid found in the stomach does not of itself break down (protein) food; it is the enzyme pepsin that breaks down protein in the acid conditions of the stomach.

Enzymes are sometimes called biological catalysts because they speed up reactions, and only small amounts are required. Digestive enzymes speed up the breakdown of complex food substances into simpler subunits. The breakdown of food in digestion is actually caused by water chemically splitting the complex food molecules, a process called hydrolysis (but this is not the same as simply dissolving).

Please note that digestive enzymes do not break foods down into the elements they are made of, and they do not release any energy from the food. This happens when the simpler food substances are respired in the individual cells.

Name some food substances which are almost entirely composed of starch:

> rice > bread > potatoes, pasta etc

Molecules of starch are built up from molecules of what simpler substances? (2 possible answers )

> glucose > maltose

What 2 structures will small molecules have to pass through in order to enter the blood stream?

> gut wall > blood capillary

Demonstration: the digestion of QuaversClick for a photo

This snack, consisting mainly of starch, is placed in 2 liquids at 2 different temperatures, and left for a while.

Digestion is shown if the product disintegrates. This is more than just getting soggy!

Fill in the results at the appropriate time in the table below:

Final results of demonstration - appearance of Quavers placed in
amylase solution water
totally broken down just extremely soggy!
slightly broken down just soggy!

What does this tell you about the combination of factors that are needed for the digestion of quavers?

> amylase and warmth must both be present

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