8.2 Cell Respiration

Question 1

What is oxidation?

Answer 1

Oxidation is the gain of oxygen, loss of electrons, or loss of hydrogen.

Question 2

What is reduction?

Answer 2

Reduction is the loss of oxygen, gain of electrons, or gain of hydrogen.

Question 3

What are electron carriers?

Answer 3

Substances that can accept and give up electrons as required.

Question 4

What are the electron carriers in respiration?

Answer 4

• NAD

* FAD

Question 5

What are the electron carriers in photosynthesis?

Answer 5

NADP

Question 6

How do you reduce NAD?

Answer 6

Add two electrons.
It is however a little more complicated than that. NAD initially has one positive charge NAD+. It accepts two electrons in this way: two hydrogen atoms are removed from the substance that is being reduced. One is split into an electron and a proton, the electron is accepted the proton expelled. Then the electron and proton from the second hydrogen are both accepted.
This demonstrates reduction can be achieved by accepting a hydrogen because it contains electrons. So oxidation can be achieved by donating electrons.

Question 7

What does phosphorylation do to molecules?

Answer 7

Phosphorylation makes molecules unstable, the addition of a PO molecule. Biochemists indicate that certain amino acid sequences tend to act as binding sites for the phosphate molecule on proteins. In many cases the purpose of phosphorylation is to make the phosphorylated molecule more unstable i.e. more likely to react. Phosphorylation can be said to activate the molecule.

Question 8

Is the hydrolysis of ATP exothermic or endothermic?

Answer 8

The hydrolysis of ATP releases energy and is therefore exothermic. Many chemical reactions in the body are endergonic (energy absorbing) and therefore do not proceed spontaneously unless coupled with an exergonic reaction that releases more energy.

Question 9

How does ATP work?

Answer 9

The hydrolysis of ATP is exothermic and releases energy. Lots of reactions in the body are endergonic (energy absorbing) and therefore do not proceed spontaneously unless coupled with an exergonic react that releases more energy.

Question 10

Outline the stages of respiration?

Answer 10

• Glycolysis
• The link reaction
• Krebs cycle
• Electron transport chain

Question 11

Outline glycolysis?

Answer 11

Glycolysis gives a small net gain of ATP without the use of oxygen.
The most significant consequence of glycolysis is the production of a small yield of ATP without the use of oxygen, by breaking sugar into pyruvate.

P - 1) ATP is used to phosphorylate glucose. ATP gives one of its phosphates to glucose making it, glucose-6-phosphate and leaving ATP as ADP.

2) The glucose-6-phosphate turns to fructose-6-phosphate.
3) It is then phosphorylated again by another molecule of ATP, so 2 ATP’s are then used up (but you get 4 in the end so it is okay). So you end up with fructose-1,6-biphosphate.

L - 4) The fructose1,6-biphosphate is split to form two molecules of triose phosphate.

O - 5) Each of these triose phosphates is then oxidised to glycerate-3-phosphate in a reaction that yields enough energy to make ATP. This oxidation is carried out by removing hydrogen. It is hydrogen atoms, so an electron is removed, if you removed H+ it would not oxidise it. The hydrogen is accepted by NAD+ which becomes NADH+ + H+. This happens twice, and makes two molecules of NADH+ + H+.

P - 6) In the final stages of glycolysis, the phosphate group is transferred to ADP to produce more ATP and also pruvate. Each triose has two phosphates and there are 2 per molecule of glucose so 4 ATP is produced.

Products = 2 molecules of pyruvate
4 molecules of ATP (2 overall)
2 NADH+ + H+

Question 12

What are the products of glycolysis?

Answer 12

Products = 2 molecules of pyruvate
4 molecules of ATP (2 overall)
2 NADH+ + H+

Question 13

Where does glycolysis happen?

Answer 13

In the cytoplasm of the cell.

Question 14

Where does the link reaction happen?

Answer 14

In the mitochondrial matrix moved by carrier proteins on the mitochondrial membrane..

Question 15

Outline the steps of the link reaction?

Answer 15

When pyruvate is produced in the cytoplasm if there is oxygen it is absorbed into the mitochondrion, by carrier proteins on the membrane.

In the link reaction pyruvate is converted into acetyl coenzyme A.

D - 1) Once in the mitochondrial membrane, the pyruvate is decarboxylated.

O - 2) It is then oxidised to form an acetyl group. Two high energy electrons are removed from the pyruvate. These react with NAD+ to form NADH + H+. This forms Coenzyme A

A - 3) Then the acetyl compound combines with Coenzyme A to form Acetyl co-enzyme A.

Products: (it occurs twice because two molecules of pyruvate per glucose molecule)

2 x CO2
2 x NADH + H
2 x Acetyl Coenzyme A

Question 16

What are the products of the link reaction?

Answer 16

2 x CO2
2 x NADH + H
2 x Acetyl Coenzyme A

Question 17

Outline the Krebs cycle?

Answer 17

In the Krebs cycle, the oxidation of acetyl groups is coupled to the reduction of hydrogen carriers.

1) Acetyl coenzyme A transfers its acetyl group to a 4 carbon compound - oxaloacetate - to make a 6 carbon compound - citrate. Coenzyme A is released and can go back into the link reaction to form another Acetyl coenzyme A.

D - 1) Decarboxylation occurs and the 6 carbon compound becomes a 5 carbon compound. CO2 is released.

O - 2) Then the 5 carbon compound is oxidised giving its electrons to NAD+ and reducing it.

D - 3) Then the 5 carbon compound undergoes decarboxylation and CO2 is released turning it into a 4 carbon compound.

O - 4) Then it is oxidised once more and the electrons are accepted by NAD+ reducing it and turning it into NADH+ + H+.

P - 5) Then the 4 carbon compound phosphorylises a molecule of ADP and one molecule of ATP is formed.

O - 6) The 4 carbon compound is once more oxidised but this time it is a molecule of FAD+ that is reduced and turns into FADH2.

O - 7) Then one last oxidation of the 4 carbon compound occurs, and one more molecule of NAD+ is reduced.

Products: (2 molecules of Acetylcoezyme A)

• 2 x ATP
• 6 x NADH+ + H+
• 2 x FADH2
• 2 x 4 carbon compound

Question 18

What is produced in the Krebs cycle?

Answer 18

Products: (2 molecules of Acetylcoezyme A)

• 2 x ATP
• 6 x NADH+ + H+
• 2 x FADH2
• 2 x 4 carbon compound

Question 19

What happens in the electron transport chain?

Answer 19

Transfer of electrons between carriers in the electron transport chain is coupled to proton pumping.

The final part of aerobic respiration is called oxidative phosphorylation, because ADP is phosphorylated to produce ATP using energy released by oxidation.

The energy is not released in a single large step by in a series of small steps, carried out by a chain of electron carriers.

1) Reduced NAD (NADH+ + H+) and FADH2 donate their electrons to electron carriers and are oxidised. NADH+ + H give their hydrogens to to the first carrier in the chain, the NAD+ returning to the matrix. The hydrogen atoms are split to release two electrons, which pass from carrier to carrier in the chain.
2) As the electrons are passed from carrier to carrier involving many transmembrane carrier proteins, energy lost as this happens is utilised to transfer/pump protons across the inner membrane from the matrix to the inter-membrane space.
3) The accumulation of proteins in the inter-membrane space creates a gradient of high concentration outside and low concentration inside this creates an electrochemical gradient (or proton motive force).
4) The protons then flow down their concentration gradient back into the matrix. This is facilitated by the transmembrane enzyme ATP synthase. As the H+ ions flow through ATP synthase the trigger the rotation of the enzyme, synthesising ATP.
5) To allow electrons to continue to flow, they must be transferred to a terminal electron acceptor at the end of the chain. In aerobic respiration this is oxygen, which briefly becomes *O2 - but then combines with 2H+ ions from the matrix to become water.

Question 20

Where does the Krebs cycle take place?

Answer 20

In the mitochondrial membrane

Question 21

Where does the electron transport chain take place?

Answer 21

Across the inner membrane of the mitochondria.

Question 22

What is a proton motive force?

Answer 22

When energy from the electron transport chain has been used to pump protons into the inter-membrane space a high concentration of H+ ions build. They then flow through ATP synthase and they move because of the proton motive force, this is also called chemiosmosis.

Question 23

Why is chemiosmosis called chemiosmosis?

Answer 23

Because a chemical is following the a concentration gradient across a membrane.

Question 24

How is the hydrogen concentration maintained during chemiosmosis?

Answer 24

Hydrogen ions are pumped across the membrane into the inter-membrane space using the energy lost by electrons as they are passed down the electron transport chain. This helps to keep one side full of H+ ions.
Then any H+ ions on the other side of the membrane form covalent bonds with oxygen molecules to form water to accept the electrons at the end of the chain. That prevents H+ ions building up the other side to maintain the electrochemical gradient.

Question 25

How has the structure of mitochondria evolved?

Answer 25

If mitochondriaal structure varied, those organisms with the mitochondria that produced ATP most efficiently would have an advantage. They would have an increased chance of survival and would tend to produce more offspring. These offspring would inherit the type of mitochondria that produce ATP more efficiently. If this trend continued, the structure of mitochondria would gradually evolve to become more and more efficient. This is called adaptation - a change in structure so that something carries out its function more efficiently.

Question 26

What type of organelle is a mitochondrion?

Answer 26

A semi-autonomous organelle in that it can grow and reproduce itself but it still depends on the rest of the cell for resources and is otherwise part of the cellular system. 70S ribosomes and a naked loop of DNA are found within the mitochondrial matrix.

Question 27

Why can mitochondrion reproduce themselves?

Answer 27

Because they contain 70S ribosomes and a naked loop of DNA within the mitochondrial matrix.

Question 28

What do the inner and outer membranes of a mitochondrion do?

Answer 28

They are compartmentalised. The outer membrane separates the contents of the mitochondrion from the rest of the cell creating a compartment specialised for the biochemical reactions of aerobic respiration.

The inner mitochondrial membrane is the site of oxidative phosphorylation. It contains electron transport chains and ATP synthase, which carry out oxidative phosphorylation.

Question 29

What are cristae?

Answer 29

Cristae are tubular projections of the inner membrane which increase the surface area available for oxidative phosphorylation.

Question 30

What does the inter-membrane space do?

Answer 30

The inter-membrane space is a location where protons build up as a consequence of the electron transport chain. The proton build up is used to produce ATP via ATP synthase. The volume of the space is small, so a concentration gradient across the inner-membrane can be built up rapidly.

Question 31

How is the inter-membrane space adapted?

Answer 31

It is very small, the volume therefore allows a concentration gradient across the inner membrane to be built up rapidly.

Question 32

What happens in the matrix?

Answer 32

The Krebs cycle happens in the matrix. The fluid therefore contains the enzymes necessary to support these reaction systems.

Question 33

Label a diagram of a mitochondria?

Answer 33

• Outer mitochondrial membrane:
  separates the contents of the mitochondrion from the rest of the cell, creating a cellular compartment with ideal conditions for aerobic respiration.
• Inner mitochondrial membrane:
  contains electron transport chains and ATP synthase.
• Cristae:
  are projections of the inner membrane which increase the surface area available for oxidative phosphorylation.
• Matrix:
  contains enzymes for the link reaction and Krebs cycle.
• Inter-membrane space:
  Proteins are pumped into this space by the electron transport chain. The space is small so the concentration builds up quickly.
• Ribosomal DNA:
  for expression of mitochondrial genes.

Question 34

What is electron tomography and what has it been used for?

Answer 34

It is a recent technique that has recently allowed three dimensional images of the interior of mitochondria to be made.

It involves taking micrograph snapshots at different angles and different planes, so as to compile these images to generate a 3-D representation.

Electron tomography has revealed these 4 things:
- Cristae are continuous with the internal mitochondrial membrane. They originate at narrow openings (cristae junctions) that likely restrict diffusion of proteins and metabolites between the compartments.

• The inter-membrane space is of a consistent width throughout the entire mitochondrion.
• The membranes are not only very flexible but also dynamic, undergoing fusion and fission in response to changes in metabolism and physiological stimuli.
• The relative, shape, size and position of the cristae can change in the active mitochondria.

Question 35

What does respiration produce overall?

Answer 35

36 ATP

• 2 from glycolysis
• 2 from krebs cycle
• 32 from electron transport chain

6 CO2

• 2 from Link reaction
• 4 from Krebs Cycle

6 H20
- 6 from oxidative phosphorylation