Respiration

Respiration is

Metabolic pathway which means it is a sequence of reactions controlled by enzymes , reactions of respiration are actalbolic aa they break down energy rich macromolecules such as glucose and fatty acids

Give examples

In respiration C-C and C-H and C-OH bonds are broken and lower energy bonds formed

Energy difference

Allows phosphorylation of ADP to ATP, ATP doesn’t produce energy but when it is hydrolysed it releases energy

This energy

Is available for use by the cell or is lost as heat

Types of phosphorylation

Oxidative phosphorylation

Photophosphorylation

Substrate level phosphorylation

Oxidative phosphorylation

occurs on the inner membrane of mitochondria in aerobic respiration

Energy for making ATP

comes from oxidation- reduction reactions and is released in transfer of electrons along a chain of electron carrier molecules

2)Photophosphorylation

Occurs on the thylakoid membranes of chloroplasts in the light dependant stage of photosynthesis

Energy for making ATP

comes from light and is released in transfer of electrons along chain of electron carrier molecules

3)Substrate level phosphorylation

Occurs when phosphate groups are transferred from donor molecules e.g. glycerate-3-phosphate to ADP to make ATP in glycolysis

OR

when enough energy is released for a reaction to bind ADP to inorganic phosphate e.g. in kerbs cycle

Groups of organisms

Three groups of organism are recognised depending on their respiration

Group 1- obligate aerobes

Most organisms use aerobic respiration and breaks down substrates using oxygen with the release of relatively large amount of energy

Group 2- facultative anaerobes

Some microorganisms including yeast and many bacteria respire aerobically but can also respire without oxygen

group 3- obligate anaerobes

Some species of bacteria and archaea use anaerobic respirations they respire without oxygen and cannot grow in its presence

Definition of aerobic respiration

Release of large amounts of energy made available as ATP from the breakdown of molecules with oxygen as the terminal electron acceptor

Stage 1 of aerobic respiration

Glycolysis- occurs in solution in the cytoplasm because glucose cannot pass through the mitochondrial membranes

Generates pyruvate, ATP and reduced NAD

Even if glucose could pass through mitochondrial membranes

Enzyme for its breakdown are not present in mitochondria so couldn’t be metabolised there

Step 1 of glycolysis

Glucose molecule is phosphorylated by the addition of 2 phosphate groups using two molecules of ATP MAKING hexose phosphate called glucose diphosphate

As a result phosphorylated glucose is

More reactive so less activation energy is required for enzyme controlled reactions

Polar so less likely to diffuse out of the cell

Step 2 of glycolysis

The glucose diphosphate splits into 2 molecules of a triose phosphate a 3-carbon sugar glyceraldehyde-3-phosphate

Step 3 of glycolisis

The two triose phosphate molecules are dehydrogenated, hydrogen is removed from each of them oxidising them to pyruvate also a 3 carbon molecule

The hydrogen atoms

Transferred to NAD, a hydrogen carrier molecule making reduced NAD

Energy

These steps release enough energy to synthesise 4 ATP molecules , ATP is formed by substrate level phosphorylation, the phosphate from triose phosphate converts ADP to ATP without involvement of an electron transport chain producing pyruavte

Product 1 of glycolysis

2 pyruvate molecules per glucose molecule

Product 2 of glycolysis

4 ATPs made by substrate level phosphorylation, 2 ATP molecules per triose phosphate molecule , 2 were used to phosphorylate the glucose molecule therefore there is a net gain of 2 ATPs from each molecule of glucose

Product 3 of glycolysis

Two molecules of reduced NAD , if oxygen is available each has the potential for the synthesis of an additional 3 molecule of ATP making 6 altogether from the electron transport chain

Finally what else happens due to glycolysis

Some energy is lost as heat but a considerable amount if chemical potential energy remains in the pyruvate, if oxygen is available some of this energy can be released via Krebs cycle in the mitochondria

Stage 2 of aerobic respiration

Link reaction: links glycolysis to kerbs cycle, occurs in solution in the matrix of mitochondria

Step 1 of link reaction

Pyruvate diffuses from cytoplasm into mitochondrial matrix

Step 2 of link reaction part A

Pyruvate is dehydrogenated and the hydrogen released is accepted by NAD to form reduced NAD

Step 2 of link reaction part B

pyruvate is also decarboxylated, molecule of carbon dioxide is removed from it, all that remains of the original glucose molecule is a 2 carbon acetate group

The 2- carbon acetate group

Combines with coenzyme A making acetyl coenzyme A which enters krebs cycke

Summary of link reaction

Pyruavte + NAD + CoA —→ AcCoA + reduced NAD + CO2

Summary of Krebs cycle

Means of liberating energy from C-C , C-H and C-OH bonds, produces ATP containing the energy which was held in chemical bonds of original glucose molecule

What does it also produce

reduced NAD, reduced FAD which delivers H atoms to the electron transport chain on the inner mitochondrial membrane

Step 1 of kerbs cycle

Acetyl coenzyme A enters the kerbs cycle by combining with 4-carbon acid to form 6-carbon compound and the coenzyme A is regenerated

step 2 of Krebs cycle

The 6-carbon acid is dehydrogenated making reduced NAD and decarboxylated to make carbon dioxide and a 5-carbon acid

step 3 of Krebs cycle

The 5-carbon acid is dehydrogenated making reduced NAD and FAD and decarboxylated to make carbon divide and to regenerate the 4-carbon acid

Step 4 of Krebs cycle

The 4-carbon acid can combine with more Acetyl coenzyme A and repeat the cycle

½ significant types of reaction

Dehydrogenation: happens 4 times where dehydrogenases remove pairs of H atoms from kerbs cycle intermediates, they are collected by H carriers giving 3 molecules of reduced NAD and one molecules of reduced FAD

2/2 significant types of reaction in Krebs cycle

Decarboxylation happens twice, decarboxylases remove CO2 from the -COOH groups of Krebs cycle intermediates as 6C acid —> 5C acid —> 4C acid

By the end of the Krebs cycle

The acetate group from the original glucose molecule is now entirely brown down to CO2 and water

Energy

The energy in the bonds of glucose molecule si carried to electrons in hydrogen atoms in the reduced NAD and FAD

In summary each turn of the Krebs cycle produces

1 ATP molecule produced by substrate level phosphorylation

3 molecules of reduced NAD

1 molecule of reduced FAD

2 moelcules of CO2

Electron tarsnpror chain

Located on the cristae of inner mitochondrial memrbane, a series of protein moelcules that are carriers and pumps which are sometimes called respiratory enzymes

Carrier molecules include

Cytochromes and so the ETC is called cytochrome chain

Cytochromes are proteins closely associated with iron or copper and the metal ions are oxidised and reduced by electron transport

What do these proteins do

The reactions they catalyse release energy which is carried by ATP, H atoms are carried into ETC by Coenzymes NAD and FAD , NAD feeds electrons and protons into ETC earlier stage than FAD does

reduced NAD

Each pair of H atoms carried by reduced NAD provides enough energy to synthesises 3 molecules of ATP using 3 proton pumps

Reduced FAD

Passes H atoms directly to the second proton pumps to the carrier system involving FAD has two pumps and produces 2 molecules of ATP for each pair of H atoms

Disocciation of reduced NAD and reduced FAD

Reduced NAD: NAD + H+

Reduced FAD: FAD + H+

Passage of electrons step 1

Reduced NAD donates electrons of H atoms to the first of a series of electron carrier in the ETC

Passage of electrons step 2

Electrons from these atoms provide energy for the first proton pump and protons from H atoms are pumped into inters membrane space

Passage of electrons step 3

Electrons pass along chain of carrier molecules providing energy for each of the 3 proton pumps in turn

At the end of the chain

The electrons combine with protons and oxygen to form water

2H+ + 2e- + ½ O2 —> H2O

Passage of protons step 1

Inner membrane is impermeable to protons and so protons accumulate in the inter memrbane space

Passage of protons step 2

Concentration of protons in inter memrbane space becomes higher than in the matrix so a gradient of concentration and charge is set up and maintained by the proton pumps

Passage of protons step 3

In the memrbane are protein complex which are channels through which protons flow back into the mitochondrial matrix , enzyme ATP synthetase is associated with each channel

Passage of protons step 4

Protons diffuse back through these channels and as they do so their electrical protneirak energy produces ATP

ADP + Pi —> ATP + H2O

At the end of the chain

Protons combine with electrons and oxygen to form water

What is oxygen referred to as

Final electron acceptor or final hydrogen acceptor in the ETC , essential as it removes protons and electrons, it is reduced by addition of H+ ions and electrons making water

Cyanide

Non competitive inhibitor of the final carrier in the ETC, in its presence electrons and protons cannot be tarsnferred to water they accumulate so the ETC no longer functions

How

Proton gradient is not maintained so ATP synthetase cannot produce ATP and the cell dies very quickly

‏for each molecule of glucose entering glycolysis the ETC receives

10 reduced NAD which generates 10 × 3= 30 ATP

2 reduced FAD which generates 2 × 2= 4 ATP

Lack of oxygen consequence 1

If there is no O2 to remove H atoms from reduced NAD and make water the ETC cannot function , no oxidative phosphorylation and no ATP is formed

Lack of oxygen consequence 2

Without oxygen reduced NAD cannot be re oxidised and no NAD is regenerated to pick up more hydrogen

Consequently

The link reaction and the Krebs cycle cannot take place, only first stage of respiration (glycolysis) is possible

What must happen

For glycolysis to continue pyruvate and H must be constantly removed and NAD must be regenerated

How is this done

By pyruvate accepting H from reduced NAD so the only ATP that can be made in the absence of oxygen is by substrate level phosphorylation

Two anaerobic pathways

To remove H from reduced NAD and both take place in the cytoplasm

1) in animals

2) in various microorganisms such as yeast and in plant cells under certain conditions such as roots in waterlogged soils

Anaerobic pathway 1, in animals

Muscle cells may not get sufficient oxygen during vigorous exercise, when deprived of O2 pyruvate is the H acceptor and is converted to lactate regenerating NAD

Then

if oxygen subsequently becomes available lactate can be respired to CO2 and H2O releasing the energy it contains

Anaerobic pathway 2

Pyruvate is converted to CO2 and to ethanal ( H acceptor ) by decarboxylase

Ethanal is reduced to ethanol and NAD is regenerated in alcoholic fermentation

This pathway however

Not reversible so even if oxygen becomes available again ethanol is not broken down, accumulates in the cell and can rise to toxic concentration

What is the Krebs cycle sometimes called

Metabolic hub, because the metabolic pathway of CH, lipids, proteins can feed into it and in some situations fats and proteins can be used as respiratory substrates

Most significant molecule

Is Acetyl coenzyme A because it links the metabolism of 3 types of macromolecules

Lipids

Provide an energy store and is used as respiratory substrate when carbohydrate levels in the body such as glycogen and blood glucose are low

Step 1

Fat is hydrolysed into its constituent molecules glycerol and fatty acids then the glycerol is phosphorylated with ATP to form glycerolx3-phosphate , dehydrogenated with NAD and converted into 3-carbon sugar, triose phosphate which enters the glycolysis pathway

Step 2

dehydrogenated with NAD and converted into 3-carbon sugar, triose phosphate which enters the glycolysis pathway

The long chain fatty acid molecules

They are split into 2- carbon fragments than enter the Krebs cycle as acetyl coenzyme A

Then

H is released, picked up by FAD and NAD and fed into ETC which produces very large numbers of ATP molecules

Number of ATP molecules

Depends on the length of the hydrocarbon chain of fatty acid

Why

Longer fatty acid chains have

1-more carbon atoms so more CO2 is produced

2-more H atoms so more FAD and NAD are reduced

3-more H aroma so more water is produced

More C atoms so more CO2 is produced

Muscles have limited blood supply and if they respired fat rather than glucose they would produce more CO2 then can be removed quickly enough

More H atoms so more FAD and NAD are reduced

so more ATP is produced , Thai explains why tissues with a rich blood supply such as the liver , respire fat: the large amount of ATP they produce is readily transported around the body

More H atoms so more water is produced

Metabolic water is very important for desert animals and explains why they respire fat

Proteins

Can be used as respiratory substrate whenever dietary energy supplies are inadequate, the protein component of food is diverted for energy purposes if CH and Fat are lacking in diet

In prolonged starvation

tissue protein is mobilised to supply energy, heart muscle and kidney tissue are among first the body breaks down to release protein

Steps

Protein is hydrolysed into its constituent amino acids which are deaminates in the liver, amino group is converted into urea and excreted

Also

The residue is converted to Acetyl Co enzyme A, private or some other Krebs cycle intermediate and oxidised

Energy budget

Shows the number of pairs of H atoms and ATP molecules produced at each stage of respiration from one molecules of glucose which gives two turns of the Krebs cycle

Remember that for reduced NAD and reduced FAD

A pair of H atoms carried by reduced NAD generates 3 ATP molecules and a pair of H atoms carried by reduced FAD generated 2 ATP molecules

Pair of H atoms carried reduced NAD in each stage

Glycolysis: 2

Link reaction: 2

Krebs cycle: 6

Pair of H atoms carried by reduced FAD at each stage

Glycolysis: 0

Link reaction: 0

Krebs cycle: 0

ATP from oxidative phosphorylation at each stage

Glycolysis: 2 × 3= 6

Link reaction: 2 × 3= 6

Krebs cycle: 6 × 3 + 2 × 2 = 18 + 4

ATP from substrate level phosphorylation

Glycolysis: 2

Link reaction: 0

Krebs cycle: 2

Total number of ATP molecules for each stage

Glycolysis: 6 + 2 = 8

Link reaction: 6 + 0 = 6

Krebs cycle: 18 + 4 + 2 =24

In total

This gives a total of 38 molecules of ATP per molecule of glucose respired

However this total

Is a theoretical total and the cell is generally not this efficient

On average cells produce 30-32 moelcules of ATP per molecule of glucose respired

Why are cells generally not this efficient

ATP is used to move pyruvate, ADP, reduced NAD and reduced FAD across the mitochondrial membrane