respiration experiment

Respiration Experiment

Experiment 1

To determine whether energy is released during respiration ( 呼吸作用 ).

Method
Divide a quantity of germinating seeds (e.g. bean seeds) into two equal amounts. Boil one amount of seeds in water to kill them and sterilise the boiled seeds with formalin (if bacterial decomposition is allowed to take place, heat will be released).

Set up the apparatus as in the diagram below.

Fig Apparatus to demonstrate heat production by germinating seeds

The cotton wool plug allows the passage of oxygen into the flasks.

Observation
The temperature in the vacuum flask was seen to rise; there was no increase in the control vacuum flask.

Note
An increase in temperature is an indication that energy is being released.

Conclusion
Because there has been an increase in temperature, energy is released during respiration.

Experiment 2
To determine whether carbon dioxide is released during respiration.

Method
Set up the apparatus as in the diagram below. Place both test tubes in a warm, dark place for at least 12 hours.

Note
Bromothymol blue solution may be used in place of clear limewater: In contact with CO₂, a dilute solution of bromothymol blue will turn lime-green, because of the formation of carbonic acid.

The foam rubber (cotton wool may be used as well) allows any CO₂ produced to reach the limewater, but prevents the seeds from becoming immersed.

Observation
The limewater in the experiment test tube became milky thus showing the presence of carbon dioxide. There was no change in the control test tube.

Conclusion
Carbon dioxide is released during respiration.

Experiment 3

Alternative experiment

Examine the diagram below,

When air is drawn through the apparatus and flask A contains a chemical to remove all CO₂ (sodium hydroxide), the limewater in flask D turns cloudy after a minute or two. This shows the presence of carbon dioxide. The limewater in flask B stays clear. This shows that the snails must be producing the carbon dioxide.

The experiments can be carried out using different living things in flask B. If plants are used:

flask C must be covered so that the plant will not photosynthesise and use up any carbon dioxide which may be produced.
the apparatus should be set up without the pump working and left for 24 hours. Plants respire more slowly than animals. After 24 hours, the pump is turned on gently. The limewater in flask D turns milky if it has not already done so.

Anaerobic respiration ( 需氧呼吸 )

If oxygen is not present cellular respiration will still occur.

Anaerobic respiration ( 需氧呼吸 ) in plants

Alcoholic fermentation

Carbon dioxide is released and ethanol (ethyl alcohol) is produced.
Very little energy is produced.
This process is known as fermentation and allows fungi such as yeast, and bacteria to live without oxygen.

Experiment 4

To illustrate fermentation
Fermentation is the incomplete breakdown of glucose in plants, resulting in the formation of alcohol and carbon dioxide, and the release of a little energy).

Method
Set up the apparatus as in the diagram below:

Note
Any CO₂ produced will turn the clear limewater milky. An increase in temperature - registered by means of the thermometer - will be an indication that respiration is taking place.

Observation
Three changes take place in the experiment thermos flask:

the temperature increases a few degrees
the clear limewater turns milky
the glucose / Pasteur's solution has lost its sweet taste (but don't taste it to find out!)
No change takes place in the control thermos.

Conclusion
Heat is produced during fermentation, and glucose is converted to alcohol and CO₂.

Brewing ( 釀啤酒 ) and bread-making

Due to the formation of carbon dioxide this process is economically important for making bread and alcoholic beverages.

Brewing and Wine-making

Alcoholic drinks are made by fermenting a sugary solution. Wine is usually made from sugars present in grapes. The grapes are crushed to extract the juice which contains sugar and wild yeast. The yeast ferments the sugar and gradually produces alcohol. Although the alcohol produced is always the same (ethanol), each wine has its own flavour. This depends mainly on the type of grapes used. Wines can also be made from a wide variety of flowers and fruits.

Yeast cannot live in solutions containing more than 14% alcohol ( 酒精 ), so fermentation stops when the alcohol reaches this concentration. Strong alcoholic drinks, like whisky and gin, which contain about 40% alcohol, are made by distilling wines and other weaker alcoholic solutions.

Beer is made from barley. Barley grain is mashed with warm water and hops. This produces a dilute sugary liquid to which yeast is added. Fermentation converts the sugar to alcohol.

Bread making

Brewers are interested in the alcohol produced by fermentation, but bakers are more interested in the carbon dioxide. Bakers add yeast to their bread dough. The yeast respires, producing bubbles of carbon dioxide which make the dough rise. When the dough is baked, heat kills the yeast and any alcohol evaporates. If dough is baked without using yeast, it is said to be unleavened. Unleavened bread does not rise, e.g. pitta bread.

Anaerobic respiration ( 缺氧呼吸 ) in animals-lactic acid fermentation ( 乳酸發酵 )

In animals anaerobic respiration produces lactic acid not ethanol and no CO₂ is produced.
Again only a very little energy is produced.
Our own muscle cells also respire anaerobically for short periods. This happens when our muscles have to work very hard for a short period, for example during a sprint race. Under these conditions we cannot breathe fast enough, nor pump our blood fast enough, to get sufficient oxygen to our muscles. So, there is an 'oxygen debt' in our muscles during vigorous exercise. Our muscles respire anaerobically and lactic acid is produced. Unfortunately lactic acid is a mild poison. It causes our muscles to ache and cramp. When we rest, the blood brings oxygen to the muscles which can then respire aerobically again. This uses up the lactic acid and relieves the pain.
We say the process is reversible.

　

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