Metabolism: Topic 8.2 Cell Respiration

Question 1

What is ATP?

Answer 1

Adenosine triphosphate (ATP) is a high energy molecule that functions as an immediate power source for cells
One molecule of ATP contains three covalently bonded phosphate groups – which store potential energy in their bonds

Question 2

What is the purpose of phosphorylation in ATP function?

Answer 2

Phosphorylation (the addition of a phosphate group to a molecule) makes molecules less stable and hence ATP is a readily reactive molecule that contains high energy bonds (Phosphate group was added to ADP to make ATP)
When ATP is hydrolysed (to form ADP + Pi), the energy stored in the terminal phosphate bond is released for use by the cell

Question 3

ATP has two key functions within the cell:

Answer 3

It functions as the energy currency of the cell by releasing energy when hydrolysed to ADP (powers cell metabolism)
It may transfer the released phosphate group to other organic molecules, rendering them less stable and more reactive

Question 4

ATP is synthesised from ADP using?

Answer 4

ATP is synthesised from ADP using energy derived from one of two sources:

Solar energy – photosynthesis converts light energy into chemical energy that is stored as ATP
Oxidative processes – cell respiration breaks down organic molecules to release chemical energy that is stored as ATP

Question 5

What is the benefit of breaking down organic molecules via a number of linked processes?

Answer 5

The breakdown of organic molecules occurs via a number of linked processes that involve a number of discrete steps

By staggering the breakdown, the energy requirements are reduced (activation energy can be divided across several steps)
The released energy is not lost – it is transferred to activated carrier molecules via redox reactions (oxidation / reduction)

Question 6

Outline where does energy released in the oxidation of organic molecules go?

Answer 6

Cell respiration breaks down organic molecules and transfers hydrogen atoms and electrons to carrier molecules

As the organic molecule is losing hydrogen atoms and electrons, this is an oxidation reaction
Energy stored in the organic molecule is transferred with the protons and electrons to the carrier molecules

Question 7

What are hydrogen carriers?

Answer 7

The carrier molecules are called hydrogen carriers or electron carriers, as they gain electrons and protons (H+ ions)

The most common hydrogen carrier is NAD+ which is reduced to form NADH (NAD+ + 2H+ + 2e– → NADH + H+)
A less common hydrogen carrier is FAD which is reduced to form FADH2 (FAD + 2H+ + 2e– → FADH2)

Question 8

Function of hydrogen carriers

Answer 8

The hydrogen carriers function like taxis, transporting the electrons (and hydrogen ions) to the cristae of the mitochondria

The cristae is the site of the electron transport chain, which uses the energy transferred by the carriers to synthesize ATP
This process requires oxygen to function, and hence only aerobic respiration can generate ATP from hydrogen carriers
This is why aerobic respiration unlocks more of the energy stored in the organic molecules and produces more ATP

Question 9

Why lipids and proteins are not the preferred source of energy for cell respiration?

Answer 9

The main organic compound used in cell respiration is carbohydrates (glucose) – although lipids and proteins can be used

Lipids are not preferentially used as they are harder to transport and digest (although will yield more energy per gram)
Proteins are not preferentially used as they release potentially toxic nitrogenous compounds when broken down

Question 10

Define glycolysis

Answer 10

The word glycolysis means ‘sugar splitting’. The first step in the controlled breakdown of carbohydrates is glycolysis, which occurs in the cytosol of the cell

In glycolysis, a hexose sugar (6C) is broken down into two molecules of pyruvate (3C)

Question 11

What is the first stage of glycolysis?

Answer 11

Phosphorylation

A hexose sugar (typically glucose) is phosphorylated by two molecules of ATP (to form fructose-1,6-biphosphate)
This phosphorylation makes the molecule less stable and more reactive, and also prevents diffusion out of the cell

Question 12

What is the second stage of glycolysis

Answer 12

The less stable 6-carbon phosphorylated fructose is split into two 3-carbon sugars called glyceraldehyde-3-phosphate (G3P). This splitting process is known as lysis.

Question 13

What is the third stage of glycolysis?

Answer 13

Oxidation

Hydrogen atoms are removed from each of the 3C sugars (via oxidation) to reduce NAD+ to NADH (+ H+)
Two molecules of NADH are produced in total (one from each 3C sugar)

Question 14

What is the fourth stage of glycolysis?

Answer 14

As NADH is being formed, released energy is used to add an inorganic phosphate to the remaining 3-carbon compound. This results in a compound (each of the two glyceraldehyde-3-phosphate compounds) with two phosphate groups. Enzymes then remove the phosphate groups so that they can be added to adenosine diphosphate (ADP) to produce ATP. This direct synthesis of ATP is called substrate level phosphorylation
In total, 4 molecules of ATP are generated during glycolysis by substrate level phosphorylation (2 ATP per 3C sugar)

Question 15

What two alternate processes can occur after glycolysis? (faith of pyruvate)

Answer 15

Depending on the availability of oxygen, the pyruvate may be subjected to one of two alternative processes:

Aerobic respiration occurs in the presence of oxygen and results in the further production of ATP (~ 34 molecules)
Anaerobic respiration (fermentation) occurs in the absence of oxygen and no further ATP is produced

Question 16

Describe the anaerobic respiration path of pyruvate (recommended to write it down on paper)

Answer 16

If oxygen is not present, pyruvate is not broken down further and no more ATP is produced (incomplete oxidation)
The pyruvate remains in the cytosol and is converted into lactic acid (animals) or ethanol and CO2 (plants and yeast)
This conversion is reversible and is necessary to ensure that glycolysis can continue to produce small quantities of ATP
Glycolysis involves oxidation reactions that cause hydrogen carriers (NAD+) to be reduced (becomes NADH + H+)
Typically, the reduced hydrogen carriers are oxidised via aerobic respiration to restore available stocks of NAD+
In the absence of oxygen, glycolysis will quickly deplete available stocks of NAD+, preventing further glycolysis
Fermentation of pyruvate involves a reduction reaction that oxidises NADH (releasing NAD+ to restore available stocks)
Hence, anaerobic respiration allows small amounts of ATP to be produced (via glycolysis) in the absence of oxygen

Question 17

What is link reaction?

Answer 17

The first stage of aerobic respiration is the link reaction, which transports pyruvate into the mitochondria

The link reaction is named thus because it links the products of glycolysis with the aerobic processes of the mitochondria

Question 18

What happens in link reaction?

Answer 18

Pyruvate is transported from the cytosol into the mitochondrial matrix by carrier proteins on the mitochondrial membrane
The pyruvate loses a carbon atom (decarboxylation), which forms a carbon dioxide molecule
The 2C compound then forms an acetyl group when it loses hydrogen atoms via oxidation (NAD+ is reduced to NADH + H+)
The acetyl compound then combines with coenzyme A to form acetyl coenzyme A (acetyl CoA)

Question 19

How many times does the link reaction occur and thus what is the result?

Answer 19

As glycolysis splits glucose into two pyruvate molecules, the link reaction occurs twice per molecule of glucose

Per glucose molecule, the link reaction produces acetyl CoA (×2), NADH + H+ (×2) and CO2 (×2)

Question 20

What is a coenzyme?

Answer 20

A coenzyme is a molecule that aids an enzyme in its action. Coenzymes usually act as electron donors or acceptors.

Question 21

What is the Kreb’s Cycle?

Answer 21

The second stage of aerobic respiration is the Krebs cycle, which occurs within the matrix of the mitochondria

The Krebs cycle is also commonly referred to as the citric acid cycle or the tricarboxylic acid (TCA) cycle

Question 22

What’s the first step of the Krebs Cycle?

Answer 22

In the Krebs cycle, acetyl CoA transfers its acetyl group to a 4C compound (oxaloacetate) to make a 6C compound (citrate)

Coenzyme A is released and can return to the link reaction to form another molecule of acetyl CoA

Question 23

What’s the second step of the Krebs Cycle?

Answer 23

Citrate (a 6-carbon compound) is decarboxylated/oxidised to form a 5-carbon compound. In this process, the carbon is released from the cell (after combining with oxygen) as carbon dioxide. While the 6-carbon compound is oxidized, NAD+ is reduced to form NADH.

Question 24

What’s the third step of the Krebs Cycle?

Answer 24

The 5-carbon compound is oxidized and decarboxylated to form a 4-carbon compound. Again, the removed carbon combines with oxygen and is released as carbon dioxide. Another NAD+ is reduced to form NADH

Question 25

What’s the final step of the Krebs Cycle?

Answer 25

The 4-carbon compound undergoes various changes resulting in several products. One product is another NADH. The coenzyme FAD is reduced to form FADH2. One molecule of ATP is produced directly via substrate level phosphorylation. The 4-carbon compound is changed during these steps to re-form the starting compound of the cycle, oxaloacetate. The oxaloacetate may then begin the cycle again

Question 26

Summarise the final products of the Krebs cycle

Answer 26

As the link reaction produces two molecules of acetyl CoA (one per each pyruvate), the Krebs cycle occurs twice, therefore the final products are:

2 ATP molecules per molecule of glucose
• 6 molecules of NADH (which allow energy storage and transfer)
• 2 molecules of FADH2
• 4 molecules of carbon dioxide (released)

Question 27

Where does electron transport chain takes place? State the nature of its location

Answer 27

The final stage of aerobic respiration is the electron transport chain, which is located on the inner mitochondrial membrane

The inner membrane is arranged into folds (cristae), which increases the surface area available for the transport chain

Question 28

What is electron transport chain?

Answer 28

The electron transport chain releases the energy stored within the reduced hydrogen carriers in order to synthesise ATP

This is called oxidative phosphorylation, as the energy to synthesise ATP is derived from the oxidation of hydrogen carriers

Question 29

Explain the first step of electron transport chain

Answer 29

Step 1: Generating a Proton Motive Force

The hydrogen carriers (NADH and FADH2) are oxidised and release high energy electrons and protons
The electrons are transferred to the electron transport chain, which consists of several transmembrane carrier proteins
As the electrons move through the chain they lose energy, which is transferred to the electron carriers within the chain
The electron carriers use this energy to pump hydrogen ions from the matrix and into the intermembrane space
The accumulation of H+ ions within the intermembrane space creates an electrochemical gradient (or a proton motive force)

Question 30

Explain the second step of electron transport chain

Answer 30

Step Two: ATP Synthesis via Chemiosmosis

The proton motive force will cause H+ ions to move down their electrochemical gradient and diffuse back into matrix
This diffusion of protons is called chemiosmosis and is facilitated by the transmembrane enzyme ATP synthase
As the hydrogen ions pass through ATP synthase they trigger a phosphorylation reaction which produces ATP (from ADP + Pi)

Question 31

Explain the final step of electron transport chain

Answer 31

Step Three: Reduction of Oxygen

In order for the electron transport chain to continue functioning, the de-energised electrons must be removed
Oxygen acts as the final electron acceptor, removing the de-energised electrons to prevent the chain from becoming blocked (by allowing new high energy electrons into the chain)
Oxygen also binds with free protons in the matrix to form water – removing matrix protons maintains the hydrogen gradient
In the absence of oxygen, hydrogen carriers cannot transfer energised electrons to the chain and ATP production is halted

Additional Note:
Twelve hydrogen carriers are produced and so six oxygen molecules are required (12 × O = 6 × O2) (the pumps release other stored hydrogen ions into the inter-membrane spaces, the hydrogen ions released from oxidation of the reduced hydrogen carriers binds with oxygen to form water, to be removed from the matrix where they were released)

Question 32

How many ATP molecules are produced from glycolysis, Krebs cycle and electron transport chain?

Answer 32

A net total of 2 ATP are produced in glycolysis via substrate level phosphorylation (four are produced, but two are consumed)
A further 2 ATP are similarly produced in the Krebs cycle (one ATP per cycle – two cycles occur per glucose molecule)
Lastly, 32 ATP are produced in the electron transport chain using energy from hydrogen carriers (oxidative phosphorylation)

Question 33

Outline how many ATP molecules each type of hydrogen carrier produces in the electron transport chain

Answer 33

Hydrogen carriers produce different amounts of ATP depending on where they donate electrons to the transport chain

NADH molecules located in the matrix donate electrons to the start of the chain and produce 3 ATP per hydrogen carrier
Cytosolic NADH (from glycolysis) donate electrons later in the chain and only produce 2 ATP per hydrogen carrier
FADH2 also donates electrons later in the chain and so only produce 2 ATP per hydrogen carrier

Oxidative phosphorylation: (8 × matrix NADH) + (2 × cytosolic NADH) + (2 × FADH2) = (8 × 3) + (2 × 2) + (2 × 2) = 32 ATP

Question 34

The structure of the mitochondrion is adapted to the function it performs:

Answer 34

Outer membrane – the outer membrane contains transport proteins that enable the shuttling of pyruvate from the cytosol
Inner membrane – contains the electron transport chain and ATP synthase (used for oxidative phosphorylation)
Cristae – the inner membrane is arranged into folds (cristae) that increase the SA:Vol ratio (more available surface)
Intermembrane space – small space between membranes maximises hydrogen gradient upon proton accumulation
Matrix – central cavity that contains appropriate enzymes and a suitable pH for the Krebs cycle to occur