Energy Transfers in and between Organisms - Respiration

Question 1

What do cells use as an immediate energy source?

Answer 1

Glucose produced by photosynthesis cannot be used directly by cells as a source of energy. Instead, cells use ATP as their immediate energy source.

Question 2

What is cellular respiration?

Answer 2

The formation of ATP from the break down of glucose (a respiratory substrate) takes place during the process of cellular respiration. There are two different forms of cellular respiration depending on whether oxygen is involved or not: aerobic and anaerobic.

Question 3

What is aerobic respiration?

Answer 3

Aerobic respiration requires oxygen and produces carbon dioxide, water and much ATP.

Question 4

What is anaerobic respiration?

Answer 4

Anaerobic respiration takes place in the absence of oxygen and produces lactate (in animals) or ethanol and carbon dioxide (in plants and fungi) but only a little ATP in both cases.

Question 5

What four stages can aerobic respiration be divided into?

Answer 5

Respiration is a multi-step process with each step controlled and catalysed by a specific intracellular enzyme.

1. glycolysis
2. link reaction
3. Krebs cycle
4. oxidative phosphorylation

Question 6

What is glycolysis (summary)?

Answer 6

The splitting of the 6-carbon glucose molecule into two 3-carbon pyruvate molecules.

Question 7

What are link reactions (summary)?

Answer 7

The 3-carbon pyruvate molecules enter into a series of reactions which lead to the formation of acetylcoenzyme A, a 2-carbon molecule.

Question 8

What is the Krebs cycle (summary)?

Answer 8

The introduction of acetyl coenzyme A into a cycle of oxidation-reduction reactions that yield some ATP and a large quantity of reduced NAD and FAD. The Krebs cycle goes round twice for each glucose molecule (once for each pyruvate -> acetyl CoA).

Question 9

What is oxidative phosphorylation?

Answer 9

The use of the electrons, associated with reduced NAD and FAD, released from the Krebs cycle to synthesise ATP with water produced as a by-product.

Question 10

When and where does glycolysis occur?

Answer 10

Glycolysis is the initial stage of both aerobic and anaerobic respiration. It occurs in the cytoplasm of all living cells and is the process by which a hexose sugar is split into two molecules of the 3-carbon molecule, pyruvate (an acid).

Question 11

What are the four stages that the smaller enzyme-controlled reactions in glycolysis can be grouped into?

Answer 11

1. phosphorylation of glucose to glucose phosphate
2. splitting of the phosphorylated glucose
3. oxidation of triose phosphate
4. the production of ATP

Question 12

What happens in the phosphorylation of glucose to glucose phosphate?

Answer 12

Before it can be split into two, glucose must first be made more reactive by the addition of two phosphate molecules (phosphorylation). The phosphate molecules come from the hydrolysis of two ATP molecules to ADP. This provides the energy to activate glucose and lowers the activation energy for the enzyme-controlled reactions that follow.

Question 13

What happens in the splitting of the phosphorylated glucose?

Answer 13

Each glucose molecule is split into two 3-carbon molecules known as triose phosphate.

Question 14

What happens in the oxidation of triose phosphate?

Answer 14

Hydrogen is removed from each of the two triose phosphate molecules and transferred to a hydrogen-carrier molecule known as NAD to form reduced NAD.

Question 15

What happens in the production of ATP?

Answer 15

Enzyme-controlled reactions convert each triose phosphate into another 3-carbon molecule called pyruvate. In the process, two molecules of ATP are regenerated from ADP.

Question 16

Why must the gross yields be doubled?

Answer 16

For each molecule of glucose at the start of the process, two molecules of triose phosphate are produced. Therefore the gross yields must be doubled (four molecules of ATP and two molecules of reduced NAD).

Question 17

What is the overall yield from one glucose molecule undergoing glycolysis?

Answer 17

• two molecules of ATP (four molecules of ATP are produced, but two were used up in the initial phosphorylation of glucose and so the net increase is two molecules)
• two molecules of reduced NAD (these have the potential to provide energy to produce more ATP)
• two molecules of pyruvate

Question 18

Why does glycolysis provide indirect evidence for evolution?

Answer 18

Glycolysis is a universal feature of every living organism.

Question 19

What organelles are needed for glycolysis?

Answer 19

The enzymes for the glycolytic pathway are found in the cytoplasm of cells and so glycolysis does not require any organelle or membrane for it to take place.

Question 20

Can glycolysis take place without oxygen?

Answer 20

Glycolysis does not require oxygen and therefore it can take place whether or not it is present.

In the absence of oxygen, the pyruvate produced by glycolysis can be converted into either lactate or ethanol during anaerobic respiration with the help of NADH. Lactate or ethanol is then converted back to pyruvate in the liver. This is necessary in order to re-oxidise NAD so that glycolysis can continue.

Question 21

Why do organisms need oxygen in respiration?

Answer 21

Anaerobic respiration yields only a small fraction of the potential energy stored in the pyruvate molecule. In order to release the remainder of this energy, most organisms use oxygen to break down pyruvate further.

Question 22

What is the importance of the link reaction?

Answer 22

The pyruvate molecules produced during glycolysis possess potential energy that can only be released in a process called the Krebs cycle. Before they can enter the Krebs cycle, these pyruvate molecules must first be oxidised in a procedure known as the link reaction.

Question 23

Where do the Krebs cycle and the link reaction take place in eukaryotic cells?

Answer 23

exclusively inside mitochondria

Question 24

What is a link reaction (detailed)?

Answer 24

In the link reaction, the 2 pyruvate molecules produced in the cytoplasm during glycolysis are actively transported into the matrix of mitochondria.

The enzyme decarboxylase then removes a molecule of CO2 with a hydrogen also being lost, going on to reduce NAD. The acetate formed then combines with coenzyme A to form a molecule of acetyl coenzyme A. Per glucose molecule, 2 molecules of acetyl coenzyme A are formed and 0 ATP.

Question 25

What changes take place when pyruvate undergoes a series of reactions?

Answer 25

• The pyruvate is oxidised to acetate. In this reaction, 3-carbon pyruvate loses a carbon dioxide molecule and two hydrogens. These hydrogens are accepted by NAD to form reduced NAD, which is later used to produce ATP.
• The 2-carbon acetate combines with a molecule called coenzyme A (CoA) to produce a compound called acetyl coenzyme A.

pyruvate + NAD + CoA –> acetyl CoA (Krebs cycle) + reduced NAD (ETC) + CO2 (waste)

The link reaction occurs twice for each glucose molecule that goes through glycolysis.

Question 26

What is the Krebs cycle (detailed)?

Answer 26

The Krebs cycle involves a series of oxidation-reduction reactions that take place in the matrix of mitochondria.

• The 2-carbon acetyl coenzyme A from the link reaction combines with a 4-carbon molecule (oxaloacetate) to produce a 6-carbon molecule (citrate).
• In a series of reactions, this 6-carbon molecule loses carbon dioxide and hydrogen (decarboxylated twice, losing 2 molecules of CO2) to give the a 4-carbon molecule and a single molecule of ATP produced as a result of substrate-level phosphorylation.
• The 4-carbon molecule can now combine with a new molecule of acetyl coenzyme A to begin the cycle again.
• Coenzyme A attaches to pyruvate to form acetyl CoA.

Question 27

What is substrate-level phosphorylation?

Answer 27

Substrate-level phosphorylation is a metabolism reaction that results in the production of ATP by the transfer of a phosphate group from a substrate directly to ADP.

Question 28

For each molecule of pyruvate, what do the link reaction and the Krebs cycle produce?

Answer 28

• 3 molecules of NADH (energy to ETC)
• One molecule of FADH2 (energy to ETC)

These have the potential to produce ATP molecules by oxidative phosphorylation and so release energy and are therefore the important products of Krebs cycle.

• One molecule of ATP (energy)
• Two molecules of carbon dioxide (waste)
• One molecule of CoA (reused in link reaction)
• oxaloacetate (regenerated)

As two pyruvate molecules are produced for each original glucose molecule, the yield from a single glucose molecule is double these quantities.

Question 29

How much ATP is formed directly by the Krebs cycle?

Answer 29

Only a small amount of ATP is formed directly by the Krebs cycle. The vast majority of potential energy is carried away from the Krebs cycle by reduced NAD and reduced FAD and only later converted to ATP.

Question 30

What are coenzymes?

Answer 30

Coenzymes aren’t enzymes. They are molecules that some enzymes require in order to function. They play a major role in photosynthesis and respiration where they carry hydrogen atoms from one molecule to another.

Question 31

What are some examples of coenzymes?

Answer 31

• NAD, important throughout respiration
• FAD, important in the Krebs cycle
• NADP, important in photosynthesis

Question 32

What is the function of NAD in respiration?

Answer 32

In respiration, NAD is the most important carrier. It works with dehydrogenase enzymes that catalyse the removal of hydrogen atoms from substrates and transfer them to other molecules involved in oxidative phosphorylation.

Question 33

What can enter the Krebs cycle as respiratory substrates?

Answer 33

the breakdown products of lipids and amino acids

Question 34

What is the significance of the Krebs cycle?

Answer 34

The Krebs cycle performs an important role in the cells of organisms for four reasons:

• It breaks down macromolecules into smaller ones (e.g. pyruvate is broken down into carbon dioxide).
• It produces hydrogen atoms that are carried by NAD to the electron transfer chain and provide energy for oxidative phosphorylation. This leads to the production of ATP that provides metabolic energy for the cell.
• It regenerates the 4-carbon molecule that combines with acetyl coenzyme A, which would otherwise accumulate.
• It is a source of intermediate compounds used by cells in the manufacture of other important substances such as fatty acids, amino acids and chlorophyll.

Question 35

In glycolysis (cytoplasm), how many molecules are produced?

Answer 35

MOLECULES

• 2ATP
• 2NADH+

ATP MOLECULES

• 2ATP
• 6ATP

Question 36

In both link reactions (mitochondria), how many molecules are produced?

Answer 36

MOLECULES

• 2NADH+ +H
• 2CO2

ATP MOLECULES

• 6ATP
• 0ATP

Question 37

In both Krebs cycles (mitochondria), how many molecules are produced?

Answer 37

MOLECULES

• 6NADH+ + H
• 2ATP
• 2FADH2

ATP MOLECULES

• 18ATP
• 2ATP
• 4ATP

Question 38

How many ATP molecules does one molecule of NAD make?

Answer 38

3 ATP

Question 39

How many ATP molecules does one molecule of FAD make?

Answer 39

2 ATP

Question 40

How many ATP molecules in total are produced?

Answer 40

38 ATP