chapter 18 p1

Question 1

Glucose is a

Answer 1

hexose (six-carbon sugar) produced during photosynthesis.
It is a complex molecule containing energy absorbed from sunlight trapped’ within its carbon hydrogen bonds.
Respiration is essentially the reverse of photosynthesis.
The carbon framework of glucose is broken down and the carbon-hydrogen bonds broken.

Question 2

The energy released is then used in

Answer 2

the synthesis of ATP by chemiosmosis. ATP, the universal energy currency, is constantly synthesised and used in energy-requiring reactions and processes.

Question 3

Respiration is a

Answer 3

complex multi-step reaction pathway (Figure 1). You will be considering respiration in eukaryotic cells.
A similar process takes place in prokaryotic cells but they do not have mitochondria so many of the reactions take place on cell membranes.
in the cell the process is continuous.
The first stage of respiration is glycolysis.

Question 4

summary of respiration

Answer 4

Question 5

Glycolysis:

Answer 5

Glycolysis occurs in the cytoplasm of the cell. It does not require oxygen - it is an anaerobic process.
Glucose, a six-carbon sugar, is split into two smaller, three-carbon pyruvate molecules.
ATP and reduced nicotinamide adenine dinucleotide (NAD) are also produced.
Glycolysis, summarised here, actually involves 10 reaction steps involving many enzymes.

Question 6

The main steps in glycolysis are:
p1

Answer 6

Phosphorylation - the first step of glycolysis requires two molecules of ATP.
Two phosphates, released from the two ATP molecules, are attached to a glucose molecule forming hexose bisphosphate.

Lysis - this destabilises the molecule causing it to split into two triose phosphate molecules.

Question 7

The main steps in glycolysis are:
p2

Answer 7

Phosphorylation - another phosphate group is added to each triose phosphate forming two triose bisphosphate molecules.
These phosphate groups come from free inorganic phosphate (P,) ions present in the cytoplasm.

Dehydrogenation and formation of ATP - the two triose bisphosphate molecules are then oxidised by the removal of hydrogen atoms (dehydrogenation) to form two pyruvate molecules.
NAD coenzymes accept the removed hydrogens - they are reduced, forming two reduced NAD molecules.

Question 8

summary of glycolysis

Answer 8

Question 9

substrate level phosphorylation in glycolysis

Answer 9

At the same time, four ATP molecules are produced using phosphates from the triose bisphosphate molecules.

This is an example of substrate level phosphorylation - the formation of ATP without the involvement of an electron transport chain.

Question 10

ATP is formed in glycolysis by the

Answer 10

transfer of a phosphate group from a phosphorylated intermediate (in this case triose bisphosphate) to ADP.
Two ATP molecules are used to prime the process at the beginning. and four ATP molecules are produced, so the overall net ATP yield from glycolysis is two molecules of ATP.
The reduced NAD is used in a later stage to synthesise more ATP.

Question 11

glycolysis takes place in

Answer 11

the cytoplasm of the cell.
In eukaryotic cells the remaining aerobic (oxygen-requiring) reactions of cellular respiration take place inside the mitochondria.

Question 12

structure of mitochondria and function diagram

Answer 12

Question 13

Oxidative decarboxylation (the link reaction):

Answer 13

The first step in aerobic respiration is oxidative decarboxylation.

This is sometimes referred to as the link reaction, because it is the step that links anaerobic glycolysis, occurring in the cytoplasm, to the aerobic steps of respiration, occurring in the mitochondria.

Question 14

the link reaction p1 process

Answer 14

• In eukaryotic cells, pyruvate enters the mitochondrial matrix by active transport via specific carrier proteins.
• Pyruvate then undergoes oxidative decarboxylation - carbon dioxide is removed (decarboxylation) along with hydrogen (oxidation).
• The hydrogen atoms removed are accepted by NAD. NAD is reduced to form NADH (reduced NAD).
• The resulting two-carbon acetyl group is bound by coenzyme A forming acetylcoenzyme A (acetyl CoA).

Question 15

the link reaction p2 process

Answer 15

• Acetyl CoA delivers the acetyl group to the next stage of aerobic respiration, known as the Krebs cycle.
• The reduced NAD is used in oxidative phosphorylation to synthesise ATP
• Acetyl groups are now all that is left of the original glucose molecules.
• The carbon dioxide produced will either diffuse away and be removed from the organism as a metabolic waste or, in autotrophic organisms, it may be used as a raw material in photosynthesis.

Question 16

Oxidative decarboxylation (the link reaction): diagram

Answer 16

Question 17

The Krebs cycle takes place in the …

Answer 17

mitochondrial matrix

Question 18

the Krebs cycle

Answer 18

• each complete cycle results in the breakdown of an acetyl group.
• Acetyl groups are all that remain of the glucose that entered glycolysis.
• As in the previous stages, the Krebs cycle involves decarboxylation, dehydrogenation, and substrate-level phosphorylation.
• The hydrogen atoms released are picked up by the coenzymes NAD and flavin adenine dinucleotide (FAD).
• Carbon dioxide is a by-product of these reactions and the ATP produced is available for use by energy-requiring processes within the cell.
• The reduced NAD and reduced FAD produced are used in the final, oxygen-requiring step of aerobic respiration to produce large quantities of ATP by chemiosmosis.

Question 19

Stages in the krebs cycle:
p1

Answer 19

Acetyl CoA delivers an acetyl group to the Krebs cycle.
The two-carbon acetyl group combines with four-carbon oxaloacetate to form six-carbon citrate.

The citrate molecule undergoes decarboxylation and dehydrogenation producing one reduced NAD and carbon dioxide.
A five-carbon compound is formed.

Question 20

Stages in the krebs cycle:
p2

Answer 20

The five-carbon compound undergoes further decarboxylation and dehydrogenation reactions, eventually regenerating oxaloacetate,and so the cycle continues.
More carbon dioxide, two more reduced NADs, and one reduced FAD are produced.
ATP is also produced by substrate-level phosphorylation.

Question 21

diagram of Krebs cycle

Answer 21

Question 22

The importance of coenzymes in respiration:

Answer 22

Respiration is a complex multi-step reaction pathway.
Coenzymes are required to transfer protons, electrons, and functional groups between many of these enzyme-catalysed reactions.
Redox reactions have an important role in respiration and without coenzymes transferring electrons and protons between these reactions many respiratory enzymes would be unable to function.
NAD and FAD are both coenzymes that accept protons and electrons released during the breakdown of glucose in respiration.

Question 23

The differences between NAD and FAD:

Answer 23

NAD takes part in all stages of cellular respiration but FAD only accepts hydrogens in the Krebs cycle

NAD accepts one hydrogen and FAD accepts two hydrogens

reduced NAD is oxidised at the start of the electron transport chain releasing protons and electrons while reduced FAD is oxidised further along the chain

reduced NAD results in the synthesis of three ATP molecules but reduced FAD results in the synthesis of only two ATP molecules.

Question 24

how NAD is presented

Answer 24

reduced NAD is represented in a number of ways - for example, NADH, NADH + H, or NADH,. The reason for this is that NAD is actually charged so is more accurately represented as NAD+.

When NAD+ is reduced it accepts two protons and an electron pair (from a C-H bond) forming NADH + Ht. NADH, or reduced NAD, then transfers the proton and electron pair to a subsequent reaction.

Question 25

Coenzymes are

Answer 25

usually derived from vitamins. This is why, although coenzymes are mostly recycled, vitamins are an essential micronutrient.