RESPIRATION - aerobic respiration and anaerobic respiration in plants, fungi and animals

Doc Brown's Biology Revision Notes

Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent

 Respiration is the process of releasing energy from digested food.

 Respiration transfers energy that cell needs to fully function.

 Know and understand that respiration in cells can take place aerobically or anaerobically depending on conditions and whether the cell is in an animal, plant, fungi or bacteria.

 Know and understand that the energy released in respiration is used in a variety of ways.

 Know that the human body needs to react to the increased demand for energy during exercise.

You should be able to use your skills, knowledge and understanding to interpret the data relating to the effects of exercise on the human body.


Introduction to respiration and its significance

Respiration is the process of transferring chemical energy to power the chemistry of cells by breaking down sugars like glucose.

Glucose, under the right conditions, can be completely oxidised to carbon dioxide and water.

This is analogous to burning a fuel in a combustion reaction - but a lot slower and no flame!

Organisms cannot survive without the energy from respiration and this process must go on continuously in every cell in any living organism.

An organism's cells cannot use the energy directly, but a molecule called ATP (adenosine triphosphate) is made and acts as a secondary chemical potential energy store.

The ATP molecule can then power all the essential chemistry e.g. breaking down or synthesising molecules in an organism's metabolism, facilitating active transport, organ function including the working of the muscles.

Overall cellular respiration must be exothermic, otherwise there would be no net energy release! So, eventually, there is a net transfer of energy to the environment.

A typical person might average a power output of over 50 J/s, about the same as a 50 W light bulb!

All the chemistry of respiration is catalysed by the specific enzymes in cells.

The rate of respiration is affected by the ambient temperature, pH and the concentration of e.g. sugars and oxygen.

Note that these are the three factors which affect efficiency of enzyme controlled reactions.

The substrate molecules required for respiration are usually sugars like glucose, but the products of respiration depend on conditions e.g. oxygenated environment or lack of oxygen and whether the cells are animal, plant, fungi or bacteria.

Apart from glucose, other carbohydrates - sugars, proteins and fatty acids from lipids can be consumed in respiration.

This page compares the processes of aerobic respiration and anaerobic respiration - in plants/fungi and animals and considers the different conditions, substrates, products and relative yields of ATP for the different respiration situations.

You should understand that respiration in cells can take place all the time aerobically or anaerobically.

You should know and understand that the energy released in respiration is used in a variety of ways e.g.

to build larger molecules from smaller ones eg proteins from amino acids,

in animals, to enable muscles to contract and relax eg to move limbs and move around,

in mammals and birds, to maintain a steady body temperature in colder surroundings, we slowly cease to function if we get to hot or too cold. Also appreciate that the human body needs to react to any increased demand for energy during exercise.

in plants, to build up from sugars, nitrates and other nutrients, amino acids which are then built up into proteins - animals can't do this, we need to take in protein, break it down and build it up to our required proteins.

Sources of substrate molecules for respiration

Plants are producers and make their own glucose for respiration from photosynthesis.

Animals are consumers and have to produce glucose by breaking down the biomass of the organisms-food that they eat.

There are two types of respiration - aerobic (with plenty of oxygen) and anaerobic (with little, if any, oxygen)

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Aerobic respiration in animals

Aerobic respiration needs the sugar from digesting carbohydrates and oxygen via air breathed in by the organism, and from the lungs carried round the body by specialised red blood cells in the case of many animals.

You need plenty of oxygen for aerobic respiration - oxygenated conditions from free dissolved oxygen gas.

Most of the reactions of aerobic respiration in eukaryotes (plant or animal) take place inside the sub-cellular structures called the mitochondria of cells. Mitochondria contain all the enzymes needed for respiration.

In microorganisms like bacteria, prokaryotic cells, the aerobic respiration chemistry occurs in the cytoplasm.

glucose  + oxygen ===>  carbon dioxide  +  water  + energy

C6H12O6(aq)  +  6O2(g)  ===>  6CO2(g)  +  6H2O(l)  +  energy

The glucose is eventually completely oxidised to the waste products - carbon dioxide and water.

The actual energy release takes place through a very complex biochemistry cycle involving ADP (adenosine diphosphate) and its conversion to ATP (adenosine triphosphate) which is the molecule that actually supplies the energy to power most of the chemistry of any cell.

The more ATP that is made, the greater the supply of energy available.

Aerobic respiration can make 32 molecules of ATP per molecules of glucose.

 Know and understand the chemical reactions inside cells are controlled by enzymes.

Your body, hence your enzyme systems, will respond to its needs e.g. when you use muscles in doing physical work or exercise.

Know and understand during aerobic respiration (respiration that uses oxygen) chemical reactions occur that:

produce useful energy is released to 'power' the cell chemistry.

Know and understand that aerobic respiration takes place continuously in both plants and animals.

Know and understand that energy that is released during respiration is used by the organism.

Know that the energy may be used in cells:

to build larger molecules from smaller ones eg proteins from amino acids,

in animals, to enable muscles to contract and relax eg to move limbs and move around,

in mammals and birds, to maintain a steady body temperature in colder surroundings, we slowly cease to function if we get to hot or too cold.

in plants, to build up from sugars, nitrates and other nutrients, amino acids which are then built up into proteins - animals can't do this, we need to take in protein, break it down and build it up to our required proteins.

Know and understand that during exercise a number of changes take place in your body:

the more you use your muscles, the more oxygen you need for respiration

the heart rate increases, the more so, the more vigorous the exercise, and even more oxygen and glucose are needed

the rate and depth of breathing increases, to increase oxygen intake.

Know and understand that these changes increase the blood flow to the muscles and so increase the supply of sugar and oxygen for energy from respiration and also increase the rate of removal of carbon dioxide - the waste product.

Know and understand that muscles store glucose as glycogen, which can then be converted back to glucose for use during exercise.

Glycogen is produced, stored and then released for conversion to glucose on a supply and demand basis.

If there is surplus glucose and physical activity is low, more glycogen is produced.

The more you physically exercise, the greater the glucose demand, if this exceeds what is available in the blood stream, then the glycogen reserves are called upon to fill the energy gap.

 

Summary of important points on aerobic respiration

Be able to explain why heart rate and breathing rate increase with exercise.

All chemical reactions inside cells are controlled by enzymes.

Your body, hence your enzyme systems, will respond to its needs e.g. when you use muscles in doing physical work or exercise.

As stated, during aerobic respiration (respiration that uses oxygen) chemical reactions occur that use glucose (a sugar) and oxygen

The sugar is from digesting carbohydrates and oxygen in via air breathed in, and from the lungs carried round the body by specialised red blood cells,

Respiration produces useful energy is released to 'power' the cell chemistry.

During exercise a number of changes take place in your body.

The more you use your muscles, the more oxygen you need for respiration

The heart rate increases, the more so, the more vigorous the exercise, and even more oxygen and glucose are need, so the rate and depth of breathing increases, to increase oxygen intake.

These changes increase the blood flow to the muscles and so increase the supply of sugar and oxygen for energy from respiration and also increase the rate of removal of carbon dioxide - the waste product.

Aerobic respiration takes place continuously in both plants and animals and most of the reactions in aerobic respiration take place inside the mitochondria of cells.

 

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Aerobic respiration in plants and fungi

As above for animals.

In green plants, in daylight the rate of photosynthesis will exceed that of respiration, but at night or very low light levels, the rate of respiration will exceed that of photosynthesis, otherwise the plant would die!


Anaerobic respiration in animals

If there is a lack of oxygen ('anaerobic') you cannot oxidise the glucose sugar completely, as in the case of aerobic respiration.

glucose  ===>  lactic acid  +  energy

This reaction only partially breaks down the glucose to lactic acid in animals and some bacteria.

Note

(i) The waste product is lactic acid, not carbon dioxide and water.

(ii) The products are different in plants and some microorganisms (see next section).

This is not as efficient as aerobic respiration and considerably less ATP is formed, reducing the potential energy supply.

You only make two molecules of ATP per molecule of glucose.

A much less efficient process of transferring energy from the glucose chemical energy store.

Anaerobic respiration takes place in the cytoplasm plant and animal cells and some microorganism e.g.

(a) In human cells, when you do vigorous exercise, you body can't supply enough oxygen, so the cells use anaerobic respiration too.

(b) If plant root cells are growing in waterlogged ground, there is little oxygen available, so they must respire aerobically.

(c) If bacteria cells get under your skin where there is little oxygen, they can still survive by using anaerobic respiration.

In animals, if the physical activity is intense and prolonged you get the 'cramps' due to the build up of lactic acid, which can be painful as anaerobic respiration kicks in due to lack of oxygen. With anaerobic respiration you do get the build up of lactic acid in the muscles because it is more difficult to oxidise and release energy.

Know and understand during exercise, if insufficient oxygen is reaching the muscles they use anaerobic respiration to obtain energy.

Aerobic means 'with oxygen', and anaerobic means 'without oxygen'.

Know and understand anaerobic respiration is the incomplete breakdown of glucose and produces lactic acid.

glucose ==> lactic acid + energy

This is not as efficient in energy release as the complete aerobic respiration of glucose described above, but it does enable you to keep your muscles going for longer. Anaerobic respiration produces a build up of lactic acid in the muscles which can be painful e.g. you suffer from 'cramps'.

However, anaerobic respiration has the advantage of enabling the body to keep going for a limited time, even if your a bit short of oxygen!

Know and understand that as the breakdown of glucose is incomplete, much less energy is released than during aerobic respiration.

Know and understand anaerobic respiration results in an oxygen debt that has to be repaid in order to oxidise lactic acid to carbon dioxide and water.

The heart, lungs and limb muscles begin struggle to keep up with what you want your body to do (fatigue), but you can keep your muscles going longer using anaerobic respiration, at least up to a point of total fatigue (like just about staggering over the line at the end of a marathon!).

However, even when you stop doing a vigorous exercise, you continue to breathe quite deeply to repay this oxygen debt and oxidise the lactic acid to the harmless waste products of carbon dioxide and water.

As long as your body detects higher than normal levels of carbon dioxide or lactic acid your breathing rate and pulse rate will stay higher than normal until their levels are reduced to normal, i.e. when all the excess lactic acid has been oxidised to carbon dioxide and water.

Know and understand that if muscles are subjected to long periods of vigorous activity they become fatigued, ie they stop contracting efficiently.

Know that one cause of muscle fatigue is the build-up of lactic acid in the muscles.

Know that blood flowing through the muscles removes the lactic acid and oxygen is used up to oxidise it to carbon dioxide and water.

 

Summary of important points on anaerobic respiration

Be able to demonstrate an understanding of why, during vigorous exercise, muscle cells may not receive sufficient oxygen for their energy requirements and so start to respire anaerobically.

Be able to demonstrate an understanding of how anaerobic respiration releases energy from glucose and how this process can be modelled using the word equation for anaerobic respiration.

Anaerobic respiration is the incomplete breakdown of glucose and produces lactic acid.

glucose ==> lactic acid + energy

Know that the process of anaerobic respiration releases less energy than aerobic respiration.

Be able to describe how a build-up of lactic acid requires extra oxygen to break it down.

Know this is called excess post-exercise oxygen consumption or EPOC (formerly known as oxygen debt)

Be able to explain why heart rate and breathing rate remain high after exercise

During exercise, if insufficient oxygen is reaching the muscles they use anaerobic respiration to obtain energy.

This is not as efficient in energy release as the complete aerobic respiration of glucose described in the previous section, but it does enable you to keep your muscles going for longer.

Anaerobic respiration produces a build up of lactic acid in the muscles which can be painful e.g. you suffer from 'cramps'.

However, anaerobic respiration has the advantage of enabling the body to keep going for a limited time, even if your a bit short of oxygen!

Unfortunately the breakdown of glucose is incomplete, and much less energy is released than during aerobic respiration.

Anaerobic respiration results in an oxygen debt that has to be repaid in order to oxidise lactic acid to carbon dioxide and water.

The heart, lungs and limb muscles begin struggle to keep up with what you want your body to do (fatigue), but you can keep your muscles going longer using anaerobic respiration, at least up to a point of total fatigue (like just about staggering over the line at the end of a marathon!).

However, even when you stop doing a vigorous exercise, you continue to breathe quite deeply to repay this oxygen debt and oxidise the lactic acid to the harmless waste products of carbon dioxide and water.

As long as your body detects higher than normal levels of carbon dioxide or lactic acid your breathing rate and pulse rate will stay higher than normal until their levels are reduced to normal, i.e. when all the excess lactic acid has been oxidised to carbon dioxide and water.

Not surprisingly, if your muscles are subjected to long periods of vigorous activity they become fatigued, ie they stop contracting efficiently.

So, one cause of muscle fatigue is the build-up of lactic acid in the muscles blood flowing through the muscles removes the lactic acid and oxygen is used up to oxidise it to carbon dioxide and water.

 


Anaerobic respiration in plants and fungi

Again, as in the case of animals, if there is a lack of oxygen ('anaerobic conditions'), you cannot oxidise the glucose sugar completely, as in the case of aerobic respiration, but the product isn't lactic acid, but 'alcohol' and carbon dioxide!

An example of this in plants is fermentation in yeast cells, the reaction being

glucose ===> ethanol ('alcohol')  +  carbon dioxide  +  energy

C6H12O6(aq) ====> 2C2H5OH(aq) + 2CO2(g)  + energy

The ethanol ('alcohol') is a by-product of the respiration process.

Again, this is not as efficient as aerobic respiration and less ATP is formed, reducing the potential energy supply.

Note: Fermentation in bacteria produces lactic acid, the same as anaerobic respiration in animals.


A comparison of aerobic respiration and anaerobic respiration in plants and animals

In prokaryotic organisms, aerobic respiration takes place in the cytoplasm.

In eukaryotic organisms, aerobic respiration takes place in the mitochondria of cells.

Anaerobic respiration takes place in the cytoplasm plant cells, animal cells and some microorganisms.

Similarities and differences Aerobic respiration Anaerobic respiration
Conditions Plenty of oxygen present. Little oxygen present due to e.g. vigorous exercise in an animal or organisms in waterlogged soils.
Substrate inputs Glucose or other sugar or any organic molecule like a fatty acid or protein molecule that can be completely oxidised. Glucose or other sugar or any organic molecule like a fatty acid or protein molecule that can be partially oxidised.
Product outputs Carbon dioxide and water. In animals and some bacteria it is lactic acid. In plants, and some microorganisms like yeast, the products are ethanol and carbon dioxide.
ATP yield High e.g. 30 to 38 ATP molecules per molecule of glucose. Low e.g. 2 ATP molecules per molecule of glucose (15-19 x less than with aerobic respiration).

 


Investigating the anaerobic respiration rate of yeast

You can investigate the rate of anaerobic respiration of yeast cells using a sugar substrate.

 

The chemistry of the anaerobic respiration of yeast cells

If you start with sucrose, the enzyme invertase hydrolyses sucrose and breaks it down into glucose and fructose.

sucrose  +  water  == enzyme invertase ==> glucose + fructose

C12H22O11  +  H2O  ===> C6H12O6  +  C6H12O6 

The actual anaerobic fermentation reaction is ...

glucose/fructose (sugar) == enzyme zymase ==> ethanol + carbon dioxide

C6H12O6(aq) ===> 2C2H5OH(aq) + 2CO2(g) 

You can following the speed of the reaction by measuring the volume of carbon dioxide formed.

 

Experimental procedure and analysis of results

Using a thermostated bath you can investigate the effect of temperature on fermentation.

You must keep the concentration of the sugar plus yeast mixture constant - fixed volumes of previously prepared stock solutions of the sugar or yeast suspension.

You can start at 20oC and repeat the experiments several times for each temperature, and then raise the temperature by 5oC at a time to see the effect.

You can measure the rate of respiration in terms of the rate of evolution of gas e.g. cm3 CO2/min.

Using the above apparatus, or that described below (gas syringe) you can the rate of respiration with different substrates AND, for a fixed substrate, the effect of changing its concentration at constant temperature and constant enzyme concentration.

 

You can use a gas syringe system to make more accurate experiments.

Doing the experiments at constant room temperature, you can keep the yeast concentration constant and vary the concentration of the sugar OR you can vary the substrate sugar (but keeping the sugar concentration constant).

(c) doc b

Typical graphical results you might obtain based on a rate of evolution of carbon dioxide e.g. cm3 CO2/minute.

 

See also ENZYMES - structure, function, optimum conditions, investigation experiments  gcse biology revision notes

See also Enzymes and Biotechnology (gcse chemistry notes)


Your practical work to develop your skills and understanding may have included the following:

Investigating the rate of respiration in yeast using carbon dioxide sensors and dataloggers (see above).

Investigating the effect of exercise on pulse rate, either physically or using pulse sensors and dataloggers,

Breathing rate can be measured by counting your breaths per minute.

Your heart rate can be measured by taking your pulse rate (heart rate in beats/minute)

You can measure your normal steady breathing rate and pulse rate.

Then do some vigorous exercise for a e.g. 5 minutes.

Then rest and re-measure your breathing rate and pulse rate at regular time intervals for say 10 or 15 minutes.

This allows you to see your body slowly recovering back to 'normal'.

You should see dramatic increases in your breathing rate and pulse rate after doing some vigorous exercise.

You can compare sitting, light walking, jogging and running, the resulting numerical trend in breathing/pulse rates should pretty well much as expected, but note that after very vigorous running, the oxygen debt might kick in and may take some time for your breathing/pulse rates to return to normal.

Investigating the link between exercise and breathing rate with a breathing sensor,

Investigating holding masses at armís length and timing how long it takes the muscles to fatigue,

Designing an investigation using force meters and dataloggers to find the relationship between the amount of force exerted by a muscle and muscle fatigue.

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