UK GCSE level age ~14-16, ~US grades 9-10 Biology revision notes re-edit 22/05/2023 [SEARCH]

Respiration: 1. Metabolism and introduction to aerobic respiration and its significance

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INDEX of biology notes on respiration

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(1A) Metabolism and introduction to respiration and its significance

Reminder: Metabolism is the scientific term used for all the chemical reactions that go on inside an organism's body. Metabolic reactions synthesise molecules of all shapes and size for specific uses in an organism. Metabolic reactions break down larger molecules down into smaller ones (e.g. in digestions). Metabolic reactions supply the energy needs for the chemistry of every cell from respiration in the mitochondria.

DO NOT CONFUSE BREATHING and RESPIRATION

human respiratory system mouth trachea windpipe bronchus lungs alveoli alveolusThe human respiratory system

The diagram on the right shows the connection between the mouth, windpipe (trachea), bronchus, bronchiole, lungs with their alveolus and alveoli sub-structures.

It is important to excrete carbon dioxide from the lungs because a build-up of the gas can have toxic effects in high concentration in the lungs or the bloodstream.

See separate page for

Mechanism of breathing and gas exchanges

Respiration is the process of releasing energy in the mitochondria from digested food- therefore respiration is an exothermic process.  Respiration transfers energy that cell needs to fully function.  'Aerobic' means 'with oxygen' (usually in reference to respiration).  'Anaerobic' means 'without oxygen' (usually refers to respiration).

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. Know how to do a simple calorimetry experiment to measure the energy content of a food.

Do NOT think respiration is breathing in and out.

Respiration is the process of transferring chemical energy to power the chemistry of ALL cells by breaking down sugars like glucose - aerobically with oxygen or anaerobically without oxygen and overall the process is exothermic - energy releasing.

Anaerobic or anaerobic respiration is sometimes referred to as cellular respiration.

Respiration powers all the metabolic biochemistry of all living organisms.

Respiration chemistry is very complex involving many reactions and takes place mainly in the mitochondria.

Mitochondria are known as the powerhouses of the cell.

They are organelles that act like a digestive system in the sense they take in nutrients, break them down, and creates energy rich molecules for the cell that can react with oxygen.

Respiration occurs in both plants animals and must be continuously happening to keep the organism alive!

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!

This is an exothermic reaction and forming H2O and CO2 releases the maximum amount of chemical energy.

The overall very complex biochemistry of aerobic respiration can be summarised as:

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

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

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.

(* Metabolism is all the chemical reactions in an organism that keep it alive)

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 of the cell fluids and transport systems and the concentration of e.g. sugars and oxygen.

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

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.

What does the energy from respiration do?

Well it powers the vast majority of chemistry of all cells of all organisms!

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

- to build larger molecules from smaller ones e.g .proteins from amino acids (plants and animals), starch from glucose (plants), cellulose from glucose (plants),

- plants use sugars, nitrates and other nutrients to make their own amino acids are then built up into proteins,

- animals can't do this, e.g. we need to take in protein, break it down to small amino acids and rebuild to our required proteins and all of these processes need chemical energy,

- in animals excess protein is broken down to urea, a waste product excreted in urine.

- fats in plants and animals are made from glycerol and three long chain fatty acid molecules - these become energy stores or structural parts of certain tissues,

- organisms require energy from respiration for cell division

- the central nervous system, including the brain, and the peripheral nervous system, all require inputs of energy, the electrical nerve signals to the brain won't go on their own without some sort of 'battery help'!

- animals require energy to enable muscles to contract and relax eg to move limbs and move around - that's why muscle cells have lots of mitochondria to power the movement and work done by muscles,

- in mammals and birds, any excess thermal energy is used to maintain a steady warm body temperature in colder surroundings (thermoregulation), we slowly cease to function if we get too hot or too cold,

- also appreciate that the human body needs to react to any increased demand for energy e.g. during vigorous exercise - so more glucose will be broken down in respiration,

- energy from respiration is needed to help maintain a constant environment in the organism e.g. water levels (osmoregulation), oxygen levels, removing waste products and not just temperature,

- energy is needed for transport of materials (nutrients or waste products) around a multicellular plant or animal organism - particularly with active transport where extra energy is needed to move molecules against the natural diffusion gradient e.g. powering and using protein carrier molecules.,

- plants need to move nutrients like mineral ions from the soli into the roots and up the rest of the plant, they also need energy for other functions like opening and closing the stomata in the leaves.

All of these examples are part of the metabolism chemistry of an organism.

See Enzymes - structure and functions notes for examples of metabolic chemistry

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 e.g. breaking down carbohydrates like starch.

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


(1B) Respiration is a continuous process in all organisms to release chemical energy from food - a chemical energy store:

Aerobic respiration in animals

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

Aerobic respiration using oxygen takes place in animal and plant cells and many microorganisms too.

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.

The starting 'fuel' molecule for respiration is often the sugar type molecule called glucose.

The glucose is made by breaking down food e.g. carbohydrates like starch from or from stores of glycogen molecules in animals. Plants can use glucose directly from photosynthesis.

The overall very complex biochemistry of aerobic respiration can be summarised as:

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

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

Energy is released at each stage of the process of respiration.

The complete oxidation of glucose in aerobic respiration releases the most chemical potential energy and is much more efficient than anaerobic respiration.

The initial steps of respiration occur in the cytoplasm of cells, but most of the chemical energy transfers happen in the mitochondria - the chemical 'factory' organelles.

(Note that aerobic respiration is the opposite of photosynthesis)

The glucose is eventually completely oxidised to the waste products - carbon dioxide and water - but via lots of complicated chemical reactions and producing 32 molecules of ATP per molecule of glucose!

Most of the time, you are using this type of aerobic respiration and the simple experiment (illustrated on the right) shows a simple limewater test for the presence of carbon dioxide in the air you breathe out - the presence of carbon dioxide is shown by the appearance of a white precipitate ('milkyness').

The actual energy release from respiration 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 chemical 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 over 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.


(1C) Experiments - measuring your pulse rate - a simple measure of your rate of respiration

You can measure your pulse rate quite easily e.g. put two fingers on the back of your wrist and time the number of pulses in a minute e.g. with a digital stopwatch or your iphone app etc.

You can do this as a simple homework exercise!

OR as a class exercise on a fine day, or any day in the gym, and average the class results giving a wider range of people and more accurate data set.

Wear appropriate footwear and make sure all the class do the same exercise!

Record your pulse rate after do the following sorts of exercise for 5 minutes at a time:

1. sitting quietly;  2. walking at your normal pace;  3. slow jogging;  4. running

For greater statistical accuracy ('best value') you should repeat the experiment several times to get four average pulse rates.

You could also analyse the average for the class.

You can present the results as a simple bar chart - average pulse from 1. to 4.

After allowing extra rest time between each activity, you should find your pulse rate increases from 1. to 4. because your rate of respiration is increasing and you need an increase in the rate of transfer of oxygen to your cells and simultaneously remove waste carbon dioxide too.

More vigorous the exercise changes increases your heart rate, hence the blood flow to the muscles and to increase the supply of sugar and oxygen for energy from respiration and also increase the rate of removal of carbon dioxide - the waste product.

Your breathing (ventilation) rate increases to meet the increased demands of the aerobic respiration rate.

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.

Regular aerobic exercise can reduce your risk from some non-communicable diseases.

See the Keeping healthy - diet and exercise 

and also Keeping healthy - non-communicable diseases - risk factors for e.g. cancers 

Regular aerobic exercise can reduce your risk from some non-communicable diseases.


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