RESPIRATION

BIOENERGETICS

The mitochondrion

Do you remember the structure of the mitochondrion? Lets’s recap.

The shape varies from spherical to elongated.They are approximately 5 µm in length and

0.2 µm in width. They have a double membrane. The outer is selectively permeable. The inner is folded to form cristae.

The mitochondrion con’t

The cristae increase the surface area for respiration.

The cristae have on their surfaces, stalked granules.

The cristae elongate in a transparent material called the matrix.

The matrix may contain protein, lipids and traces of DNA.

Role of respiration

Living systems require a constant supply of energy.

The universal currency of energy is ATP (Adenosine TriPhosphate).

ATP is a temporary store for energy. There is never enough stored ATP within a cell to

maintain it for much longer than a second. Consequently, there must be a process to ensure

that ATP is constantly produced. Respiration ensures that ATP is constantly

produced.

Adenosine TriPhosphate

As stated before, it is the universal currency for energy.

It is a nucleotide consisting of the base adenine, the sugar ribose and three phosphate groups.

When the terminal phosphate group is removed approximately 30.6kJ mol-1 of energy is released.

ATP is formed when ADP is combined with a phosphate group.

In humans, the phosphate is stored in a compound called creatine phosphate.

Stages of respiration

Respiration can be divided into 4 stages;1.Glycolysis – this occurs in the cytoplasm2.Link reaction – this occurs in the

mitochondrion (matrix)3.Kreb’s cycle – this occurs in the

mitochondrion (matrix)4.Electron transport chain – this occurs in the

membranes of the cristae.

Glycolysis

Can you break this word into its prefix and suffix and derive its meaning? I’m sure you can.

Yes! ‘glyco’ refers to sugar, and ‘lysis’ refers to splitting.

So during this process, among other things, sugar is split.

Glycolysis con’t

1. Glucose(6C) is phosphorylated to form glucose phosphate(6C).

The energy and phosphate group for this process comes from 1 molecule of ATP.

Phosphorylating the glucose does several things: - It makes it more reactive.- It prevents its movement out of the cell thus

preventing it from disturbing the osmotic balance. - It helps to create a concentration gradient so more

glucose can move into the cell.

Glycolysis con’t

2. Glucose phosphate is then reorganised into fructose phosphate(6C).

3. ATP is used to phosphorylate fructose phosphate forming Fructose bisphosphate(6C).

This makes it even more reactive. 4. Fructose bisphosphate is split into two three

carbon sugars called glyceraldehyde-3-phosphate (3C).

Sounds familiar? A hah!, it’s the same molecule from the Calvin Cycle also known as triose phosphate.

Glycolysis con’t

5. Inorganic phosphate is used to phosphorylate Glyceraldehyde-3-phosphate.

6. Glyceraldehyde – 3-phosphate is also dehydrogenated (2 H atoms removed from each molecule) to form Glycerate 1, 3 bisphosphate (3C).

7. Each molecule of Glycerate 1, 3 bisphosphate loses a phosphate molecule to form glycerate -3- phosphate(3C).

These two phosphate molecules are used to form 2 molecules of ATP.

Glycolysis con’t

8. Each molecule of glycerate -3- phosphate loses a phosphate molecule to form pyruvate (3C).

Another 2 molecules of ATP are made. 9. For the formation of pyruvate, a molecule of

water is also lost from each glycerate-3-phosphate molecule.

OK, how many molecules of ATP can be obtained from one molecule of glucose during glycolysis?

Glycolysis con’t

Each pair of hydrogen atom can yield 3 ATP molecules later on in the electron transport chain (=6).

2 molecules are formed as Glycerate 1,3-bisphosphate is dephosphorylated to form glycerate -3- phosphate.

2 molecules are also formed when glycerate -3- phosphate is dephosphorylated to form pyruvate.

2 molecules of ATP are used in the early stages so the net gain is 8.

The link reaction

This is a very short process. Yes it is. 1.Pyruvate is decarboxylated by losing a

molecule of Carbon dioxide. 2.Pyruvate is also dehydrogenated (loses 2H). 3.The remaining molecule is combined with

coenzyme A to form Acetyl coenzyme A. 4.3 molecules of ATP are produced from the 2

atoms of H. This is counted in the energy gained from the

Kreb’s cycle.

The Kreb’s cycle

The steps involved in the Kreb’s cycle are;1.Acetyl Coenzyme A(2C) combines with

Oxaloacetate (4C) to form Citrate (6C). 2.The Citrate is dehydrogenated and

decarboxylated (loses CO2) to form α- ketoglutarate (5C)

3. α- ketoglutarate is dehydrogenated and decarboxylated to form Succinate (4C).

The Kreb’s cycle con’t

During the conversion from α- ketoglutarate to Succinate, one molecule of ATP is formed. This is the only stage in the Kreb’s cycle where ATP is formed directly.

4.Succinate is dehydrogenated to form Malate (4C).

5.Malate is dehydrogenated to reform Oxaloacetate (4C).

When this is reformed, the cycle is repeated.

The Kreb’s cycle con’t

So, how many molecules of ATP are produced from the Kreb’s cycle from one molecule of glucose?

Remember that each time there is dehydrogenation, 2 H atoms are removed.

For each pair of Hydrogen atoms removed, 3 molecules of ATP will be formed in the electron transport chain.

Oh, remember to count the ones from the link reaction.

So, what’s the answer?

The Kreb’s cycle con’t

Yes, 32 are produced. Make sure you remember where

dehydrogenation and decarboxylation take place.

The electron transport chain

This is also called the hydrogen carrier system or a redox chain

This is where the H atoms from the link reaction and glycolysis and Kreb’s cycle generate ATP.

The H atoms are accepted or taken by a hydrogen carrier thus reducing them.

The carriers are Nicotamide Adenine Dinucleotide (NAD), Flavine Adenine Dinucleotide (FAD), cytochrome and cytochrome oxidase.

The electron transport chain con’t

NAD is a derivitive of the vitamin nicotinic acid. FAD is a derivitive of vitamin B2 – riboflavine. As the H atoms or electrons are transferred, they

are passed to lower energy levels, thus releasing energy.

The first acceptor is NAD and the steps are outlined below.

1.NAD accepts 2H and becomes reduced (NADH2 or NADH + H+)

2.The H atoms are then passed on to FAD thus reducing it (FADH2)

The electron transfer chain con’t

During the transfer an ATP molecule is produced. At this point, the H atoms are split into protons

and electrons (2 protons and 2 electrons). The electrons are passed to the cytochromes

forming ATP in the process. The same electrons are passed to the cytochrome

oxidase forming another ATP molecule in the process.

The protons are passed to the cytochrome oxidase where they recombine with the electrons.

The electron transfer chain con’t

The Cytochrome oxidase passes the H atom to be combined with oxygen to form water.

This is the only place where oxygen is involved in aerobic respiration.

The purpose of oxygen is to accept the H at the end of the electron transfer chain.

The Kreb’s cycle and electron transport chain do not proceed if oxygen is not present.

Electron transport chain con’t

Because ATP is formed in the presence of oxygen, the process is called oxidative phosphorylation.

Please note that the energy generated in the electron transport chain is by chemiosmosis.

The H atoms from the Kreb’s cycle are split into protons and electrons.

The protons are pumped into the space within the cristae as the electrons are transported within the membrane.

Electron transport chain con’t

When a concentration gradient is built up, the protons move down the gradient through the stalked particle which is an ATPase molecule.

This molecule makes ATP

Importance of the Kreb’s cycle

1. Because the 3-carbon pyruvate is broken down to carbon dioxide, it facilitates the degradation of macromolecules.

2. It provides the reducing power for the electron transport system i.e it provides the H atoms.

3. It provides intermediate compounds for the manufacture of other substances like amino acids, fatty acids etc.

Question

Explain the inhibition of the Kreb’s cycle and the electron transfer system which occurs when there is;

1.limited supply of oxygen. 2.limited ADP or inorganic phosphate.

Anaerobic respiration

This refers to respiration in the absence of oxygen.

Some organisms can only respire anaerobically and are called OBLIGATE ANAEROBES.

Some can respire both aerobically and anaerobically. These are called FACULTATIVE ANAEROBES.

Remember, anaerobic respiration is glycolysis. In the absence of oxygen, the Kreb’s cycle and

electron transport chain do not occur.

Anaerobic respiration con’t

The hydrogen atoms removed during glycolysis cannot release their free energy without the electron transport chain.

They have to be removed however, so that glycolysis can continue.

They are removed in a process called fermentation to give either ethanol or lactate depending on the organism involved.

Alcoholic fermentation

1. Pyruvate loses a molecule of CO2 (decarboxylation) to form ethanal.

2. Ethanal combines with the H atoms transported by NAD (Nicotamide Adenine Dinucleotide) to form ethanol.

C6H12O6 → 2 C2H5OH + 2CO2

3. Alcoholic fermentation occurs in yeast and plants.

4. If allowed to accumulate, the ethanol kills the cells.

Alcoholic fermentation con’t

The ethanol cannot be broken down to release additional energy.

Importance to humans

Alcohol fermentation is used to make wine from plants.

Alcohol fermentation in yeast is also economically important.

The alcohol from the yeast is used in making beer.

The Carbon dioxide from the yeast is used in making bread. It makes the bread light.

Lactate fermentation

The H atom from glycolysis is accepted directly by pyruvate to form lactate.

If oxygen becomes available later, this lactate can be broken down to release energy.

It can also be used to make carbohydrates or be excreted.

Oxygen debt

Anaerobic respiration also occurs in animals if necessary.

This has great survival value as it allows us to withstand short periods of anoxia (without oxygen).

Examples of these periods include: during and immediately after birth, periods when oxygen levels fluctuate in water bodies and during strenuous exercise.

Oxygen debt con’t

Only a small amount of energy is released but it helps.

The lactate causes muscle fatigue which is relieved when oxygen becomes available.

The accumulation of lactate causes an oxygen debt to build up.

This is paid as rapid breathing occurs.