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6. respiratory substrates

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Respiratory Substrates
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Respiratory Substrates

Respiratory substrates

The usual respiratory substrate is glucoseSome cells can only respire glucose, such as brain neurones

Other cells can oxidize lipids or amino acids

Lipids

These are broken down to fatty acids and glycerol

Fatty acids are then broken into pairs of Carbon atoms As Acetyl CoA which enters the Krebs cycle

Fatty acids

These are then broken into pairs of Carbon atoms As Acetyl CoA which enters the Krebs cycle

The more hydrogens the more NAD reduced can be madeThis will form ATP in oxidative phosphorylation

One molecule of stearic acid will yield 147 ATP

Most of the energy from respiration is from oxidation of hydrogen to water

The more hydrogens there are, the more energy can be released

Fatty-acids therefore have a high energy value per unit mass

Glycerol can be phosphorylated to and finally incorporated into glycolysis

One molecule of glycerol will yield 19ATP

Proteins These are hydrolysed to amino-acidsThese are deaminated to remove the amino group or transaminated to transfer the amino group to another molecule

The hydrocarbon skeleton is then converted to pyruvate or Acetyl CoA which will enter the Krebs cycle

Energy Values of SubstratesThese are found using a CALORIMETER

A known mass of the substrate is burnt completely in the presence of oxygen

The energy given off as heat is used to raise the temperature of a known volume of water

The energy value per unit mass of the substrate can be found using the Specific Heat Capacity of water

The energy values of Substrates are:

Carbohydrates – 15.8 KJg-1

Lipids – 39.4 KJg-1

Protein – 17.0 KJg-1

Respiratory Quotients (RQ)These are used to determine the respiratiry substrate being used in resiration

For Carbohydrates:

C6H12O6 + 602 6CO 2 + 6H2O + ENERGY

02 IN : CO2 OUT 1 : 1

RQ = Volume of CO 2 OUT per unit time

Volume of 02 IN per unit time

Or RQ = moles of CO 2 out moles of 02 in

For example the RQ of glucose = 6/6 = 1.0

Oleic acid:C18H34O2 + 25.502 18CO2 + 17H2O + ENERGY

RQ = moles of CO2 out moles of 02 in

=18/25.5 = 0.7

RQ

carbohydrate 1

Lipid 0.7

Protein 0.9

These are the respiratory quotients of substrates

For anaerobic respiration in yeast

C6H12O6 2C2H5OH + 2CO2 + 2ATP

RQ = moles of CO2 out moles of 02 in

= 2/0 = infinity

However because there is some aerobic respiration so some oxygen IN the RQ will be high less than 2

A high value for RQ will indicate some anaerobic respiration

Respirometer

This is used to measure oxygen uptake

And to find RQ values or to investigate rates of respiration

a sealed chamber and method to measure volume changes

CO2 is absorbed by soda lime or concentrated Potassium/ sodium hydroxide

Any decrease in air volume will be because oxygen is being taken up

The amount of oxygen used per unit time will give a rate

The temperature and pressure must be kept constant or accounted for using a control

If investigating the effect of a variable on the rate of respiration:All other factors must be constant

If finding the RQ of an organism to determine the respiratory substrates being used:

First find the volume of oxygen used in a specified time, e.g. xcm3min-1

Remove the soda lime and repeat under the same conditions

If carbohydrates are being respired the plunger will not move O2 used and CO2 made will be xcm3min-1

RQ = Volume of CO2 OUT per unit time Volume of 02 IN per unit time

= x/x

= 1

If there is some anaerobic respiration more CO2 will be made than O2 used

The plunger will move to the rightIf it moves y cm3min-1 then the total volume of CO2

will be (x + y)cm3min-1

RQ = Volume of CO2 OUT per unit time Volume of 02 IN per unit time

= (x + y)/x

If fats are being respired there will be less CO2 made than O2 used because there is no glycolysis or link reaction

The plunger will move to the left by z cm3min-1

The amount CO2 of made is (x - z)cm3min-1

RQ = Volume of CO2 OUT per unit time Volume of 02 IN per unit time

= x - z/x


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