Respiration

Respiration is the process of releasing energy from glucose, which happens constantly in every living cell. 

You need to keep your body going. Cells break down small food molecules that have been assimilated from the gut to release the stored chemical energy in the process of cellular respiration. The chemical energy is used for muscle contraction, active transport, building up large molecules (e.g. amino acids into proteins), cell division, and making sound and heat.

There are two forms of respiration: aerobic and anaerobic respiration. 

Aerobic respiration is cellular respiration using oxygen. Carbon dioxide and water are released as waste products. It's the most efficient way to release energy from glucose and is the type of respiration that you're using most of the time. 

The word equation and the balanced chemical equation are: 

glucose + oxygen → carbon dioxide + water (+ energy)
            C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Sometimes cells have to respire with little to no oxygen available. This is called anaerobic respiration. Glucose is not broken down and less energy is released. In animal cells, the waste product is lactic acid. In plant cells, the waste products are ethanol (alcohol) and carbon dioxide. 

Anaerobic respiration takes place in muscle cells when they are working vigorously during exercise. This is because your body can't supply enough oxygen to your muscles for aerobic respiration - even though your heart rate and breathing rate increases as much as they can. It's not the best way to convert glucose into energy because it releases much less energy than aerobic respiration, the glucose is only partially broken down, and lactic acid is produced. The lactic acid builds up in the muscles - it gets painful and leads to cramps. 

Once exercise stops, extra oxygen is still needed to break down the lactic acid fully. The oxygen needed is known as the oxygen debt. Cramping will not stop until the lactic acid is gone. 

glucose → lactic acid (+ energy released)

glucose → ethanol + carbon dioxide (+ energy released)

Carbon Dioxide 

As carbon dioxide is a waste product of respiration, it's likely this gas will be released from an organism respiring. 

You can test gas to see if it is (or if it contains) carbon dioxide. This can be done by bubbling the gas through one of two liquids: 

1. Lime water. This is a clear liquid that turns cloudy or 'milky' in the presence of carbon dioxide. It can be used to show that exhaled air contains more carbon dioxide than inhaled air.

2. Hydrogencarbonate indicator solution. Carbon dioxide dissolves in water to form a weakly acidic solution. Hydrogencarbonate indicator is a clear reddish-orange liquid that turns yellow in the presence of carbon dioxide. This liquid shows a change with less carbon dioxide than it takes to change lime water. It is, therefore, a more sensitive indicator. 

An experiment to demonstrate carbon dioxide production using beans: 

1. First, soak some dried beans in water for a day or two.They will start to germinate (they'll begin to grow and put out shoots). Germinating beans will respire. 

2. Boil a similar-sized, second bunch of dried beans. Ths will kill the beans and make sure they can't respire. The dead beans will act as your control. 

3. Put some hydrogen-carbonate indicator into two test tubes. 

4. Place a platform made of gauze into each test tube and place the beans into this. 

5. Seal the tubes with a rubber bung. 

6. Leave the apparatus for a set period of time (e.g. an hour).

7. During that time the carbon dioxide produced by the germinating beans should have had an effect on the hydrogen-carbonate indicator - it will have turned yellow. 

Temperature 

A rise in temperature can indicate respiration is taking place. The heat produced by respiration can be measured. Another experiment also using dried beans can help measure that heat: 

1. Firstly, prepare two sets of beans as described in steps one and two of the previous experiment on the last card. 

2. Add each set of beans to a vacuum flask, making sure there's some air left in the flasks (so that the beans can respire aerobically).

3. Place a thermometer into each flask and seal the top with cotton wool. 

4. Record the temperature of each flask daily for a week. 

5. The beans are well-insulated in the flask, so when the germinating beans respire and produce heat, the flask's temperature will increase compared to the control flask. 

As you'll remember, anaerobic respiration takes place without any oxygen and is a way to maintain energy production when oxygen is unavailable. 

Anaerobic respiration in plants and fungi produces ethanol (alcohol) as a waste product. 

glucose → ethanol + carbon dioxide (+ energy released)

C6H12O6 → 2C2H5OH + 2C02 (+energy released)

Ethanol is the alcohol found in alcoholic drinks like beer and wine. In bread-making, anaerobic respiration produces carbon dioxide. Bubbles of carbon dioxide gas expand the dough and help the bread rise.
If it's cooked at too high a temperature, the enzymes will become denatured and the dough won't rise. 

Beer-making (brewing), wine-making and baking all use fermentation (a metabolic process in which an organism converts a carbohydrate into an alcohol or an acid). 
An enzyme in yeast acts as a catalyst for the fermentation of glucose. Really, alcoholic drinks can be made from any fruit or vegetable which is a source of starch or sugar. 

Brewing 

Beer is an alcoholic drink made by fermentation reactions that use yeast to convert sugars into ethanol. Sugars from boiling barely in water are converted into ethanol and carbon dioxide by fermentation. Hops are added to adjust the flavour of the beer, too.
The basic process: 

1. Barely seeds are germinated under warm, moist conditions. Starch in the seeds is broken down by amylase in the grains to form maltose (sugar). 
2. Hot water is added to the seeds to stop the enzymes from working, and the sugar is washed out to form a liquor called wort. 
3. Dried hops are added to the wort to add flavour and the release chemicals that prevent bacteria. 
4. The wort is boiled for a few hours to concentrate it is then cooled and filtered. Yeast is added.
5. In large fermentation vats, the yeast converts (by anaerobic respiration) the sugar in the wort to ethanol and carbon dioxide. This takes several days, and most of the carbon dioxide fades. 
6. A thick layer of yeast forms on top of the fermenting beer. The beer is drawn off under this, filtered, and then put into suitable containers.

As we learnt earlier, animals produce lactic acid as a waste product. 

When muscles are subjected to long periods of vigorous activity, they stop contracting efficiently, i.e. they get tired or fatigued. If you were to lift one arm above your head and leave the other one by your side, clenching both fists twice a second, you'd find the one above your head would feel tired long before the one at your side would. You'll also find that it took longer for this arm to recover. 

Blood can carry oxygen to the muscles in the arm down by your side more quickly than to those in the arm above your head. Blood has to travel against gravity to get to these muscles. So, the muscles in the arm above your head do not receive a sufficient amount of oxygen and must begin respiring anaerobically. Lactic acid builds up and muscles begin to ache. When the arm is brought back down, extra oxygen is required to oxidise and break down the lactic acid. As we mentioned before, this is called an oxygen debt.

Sports like sprinting, weightlifting, and swimming all depend on our ability to respire anaerobically. Any sport that makes use of sudden bursts of movement, requiring energy faster than the muscles can obtain oxygen, can't be maintained for long or else too much lactic acid would accumulate. 

Regular athletic training improves: 

• Lung capacity, and also increases the amount of oxygen available with each breath; 
  
• Heart muscles, which improves the circulation of oxygen and glucose to the muscles; 
• Skeletal muscles, both in strength and in their ability to withstand increasing amounts of lactic acid. 

The Disadvantages of Anaerobic Respiration

• The waste products could be harmful in higher concentrations and usually have to be broken down as soon as oxygen is available. 
• Anaerobic respiration only releases one twentieth (5%) of the energy aerobic respiration releases for the same amount of glucose.  
  
• In anaerobic respiration, the glucose is not completely broken down into carbon dioxide and water.