Respiration (5.2.2)

Respiration is a process carried out by cells to release energy for cellular processes such as active transport and muscle contraction.

Cells break down glucose using oxygen, producing water and carbon dioxide.

The energy released is used to produce ATP (adenosine triphosphate) from ADP (adenosine diphosphate) and a phosphate ion.

Coenzymes NAD and FAD can be represented in different ways. For example, coenzyme NAD can also be written as NAD⁺, whilst reduced NAD can be shown as NADH or NADH⁺ + H⁺.

The coenzymes and their reduced forms in the following notes are shown in the same way as in the exam board specification.

Glycolysis is the first step of respiration and occurs anaerobically in the cytoplasm.

• Glucose is phosphorylated, using 2 ATP, which produces hexose bisphosphate and 2 ADP.
• Hexose bisphosphate undergoes lysis to form 2 triose phosphate, each of which is phosphorylated again to produce triose bisphosphate.
• Each triose bisphosphate is dehydrogenated, converting the coenzyme NAD to reduced NAD and 2 ADP to 2 ATP by removing 2 phosphate from each triose bisphosphate.
• A 3C pyruvate is produced by each molecule of triose bisphosphate.

Per molecule of glucose, glycolysis uses 2 ATP in the first stage of phosphorylation, but later 4 ATP are produced in the last step. So, overall, glycolysis produces 2 ATP.

Oxidative decarboxylation (the link reaction) occurs in the mitochondrial matrix.

Pyruvate is converted into an acetyl group. This releases carbon dioxide and converts NAD to reduced NAD.

The acetyl group is bound with coenzyme A (CoA) to produce acetyl coenzyme A (acetyl CoA).

Overall reaction:

The Krebs cycle occurs in the mitochondrial matrix.

Acetyl CoA transfers the acetyl group (2C) to oxaloacetate (4C), which makes citrate (6C).

Citrate undergoes a series of reactions, with an intermediate 5C molecule produced.

Overall yield: 1 × ATP, 3 × reduced NAD, 1 × reduced FAD, 2 ×

NAD and FAD are both coenzymes that transport hydrogen ions and electrons in many enzyme catalysed reactions, such as respiration.

These coenzymes are converted to their reduced forms in this process.

NAD is converted to reduced NAD during respiration in glycolysis, oxidative decarboxylation (the link reaction) and the Krebs cycle. Each NAD can accept one hydrogen ion and two electrons, producing reduced NAD.

Reduced NAD is then converted back into NAD, transferring the hydrogen ion and electrons during the electron transport chain. This process occurs in oxidative phosphorylation, the last step of aerobic respiration.

FAD is converted to reduced FAD in the Krebs cycle. Each FAD can accept two hydrogen ions and two electrons, producing reduced FAD.

Reduced FAD is then converted back into FAD, transferring the hydrogen ions and electrons to the electron transport chain. This process occurs in oxidative phosphorylation, the last step of aerobic respiration.

Oxidative phosphorylation occurs in the cristae of the mitochondria. This process uses reduced NAD, reduced FAD and oxygen.

A series of reactions take place, which produce water and ATP.

Each reduced NAD can be used to produce 3 × ATP.
Each reduced FAD can be used to produce 2 × ATP.

The hydrogen ions and electrons from the reduced NAD and reduced FAD are delivered to the electron transport chain for oxidative phosphorylation.

The electrons flow along the four complexes which are embedded in the inner mitochondrial membrane. Each complex is reduced when the electrons are received, and then oxidised when the electrons are passed to the next complex.

The energy released during this process is used to create a hydrogen ion gradient within the mitochondria.

The hydrogen ions released from the reduced coenzymes are used to create a hydrogen ion gradient within the mitochondria: the concentration of hydrogen ions is higher in the intermembrane space than in the mitochondrial matrix.

Hydrogen ions flow down their concentration gradient into the mitochondrial matrix, through ATP synthase. As this happens, the hydrogen ions, electrons and oxygen react together to make water. This releases energy and converts ADP and P_i to ATP.

This process is sometimes referred to as the chemiosmotic theory.

Organisms can be classified in the following three ways:

1. Obligate anaerobes are not able to survive in the presence of oxygen. Certain species of bacteria and fungi are examples.
2. Facultative anaerobes can respire and survive in both aerobic and anaerobic conditions. Yeast is an example.
3. Obligate aerobes can only survive in aerobic conditions. Organisms such as mammals can respire anaerobically for a very short period of time but additional oxygen is required following periods of anaerobic respiration.

Fermentation is a form of anaerobic respiration.

Eukaryotic organisms carry out either alcoholic fermentation or lactate fermentation. Both types of fermentation involve partially breaking down glucose when oxygen is limited.

Fermentation involves glycolysis but not the link reaction, Krebs cycle or oxidative phosphorylation and produces far less ATP than aerobic respiration.

Mammals carry out lactate fermentation.

Lactate fermentation produces less ATP than aerobic respiration, providing only enough for essential processes for a short period of time.

Lactate fermentation uses the enzyme lactate dehydrogenase to convert reduced NAD back into NAD, so glycolysis can continue.

This process also converts pyruvate to lactic acid:

The production of lactate during anaerobic respiration in mammals leads to a reduced in the cell. This causes proteins within the cell to denature, meaning that enzymes and muscle filaments in the cell can lose function.

Lactate accumulation leads to an oxygen debt because oxygen is required to break down lactate when the conditions are no longer anaerobic.

The lactate is transported in the blood and broken down by the liver.

The rate of aerobic respiration in yeast can be determined using a carbon dioxide sensor and measuring how levels of carbon dioxide change over time.

This can be used to investigate how changing conditions such as glucose concentration or temperature can change the rate of aerobic respiration.

Aerobic respiration produces more ATP than anaerobic respiration:

• Anaerobic respiration produces 2 × ATP per molecule of glucose.
• Aerobic respiration produces approximately 38 × ATP per molecule of glucose.

Anaerobic respiration is an emergency measure for cells; such a small amount of ATP is produced it only allows for essential processes to occur.

Glucose can be broken down to produce ATP through the process of respiration.

Triglycerides, alcohol and proteins can also act as respiratory substrates. This occurs through a different process than aerobic respiration as additional stages are required before respiration can occur.

Different quantities of ATP are produced by the different respiratory substrates.

Glycogen is a polysaccharide stored in the liver and muscles.

Glycogen can be broken down by hydrolysis to produce glucose. This glucose can then be used for respiration.

To be used as a respiratory substrate, triglycerides are first hydrolysed to produce 1 × glycerol and 3 × fatty acid molecules.

Glycerol is converted to pyruvate, which can then be used in oxidative decarboxylation (link reaction), the Krebs cycle and oxidative phosphorylation.

Fatty acids are gradually converted to acetyl coenzyme A (acetyl CoA), which is then used in the Krebs cycle and oxidative phosphorylation.

Proteins can undergo hydrolysis to produce amino acids which can then be deaminated by enzymes in the liver, removing the amine group. These molecules then enter the respiratory pathway, often as pyruvate.

Hydrolysis and deamination both require ATP, so the overall ATP produced using proteins is lower than other respiratory substrates.

Different respiratory substrates are used to produce different quantities of ATP. This is because they contain different amounts of hydrogen per unit mass.

• Lipids release twice as much energy as carbohydrates per gram.
• Alcohol releases slightly more energy than carbohydrates.
• Protein releases roughly the same energy as carbohydrates.

The respiratory quotient (RQ) is the ratio of carbon dioxide produced and oxygen consumed in respiration and can be measured using a respirometer.

RQ varies for different respiratory substrates:

• Glucose has an RQ equal to 1 because the quantity of oxygen consumed is equal to the quantity of carbon dioxide produced.
• Proteins have a lower RQ (0.9).
• Lipids contain a higher proportion of carbon so their RQ is lowest (0.7).

A combination of different respiratory substrates are used to produce ATP. This combination changes depending on many factors. The RQ indicates which respiratory substrates are likely being used.

RQ is typically 0.8–0.9 which means that mostly carbohydrates and lipids are used to produce ATP.

During periods of anaerobic respiration, the RQ can increase to above 1. When measuring the RQ, it is difficult to say exactly when anaerobic respiration starts.

The rate of respiration will increase as the temperature increases. This is due to the molecules having more kinetic energy.

At temperatures above the optimum, the rate of respiration will decrease due to the enzymes involved in respiration denaturing.

The rate of respiration increases as the respiratory substrate concentration increases.

This will level off when the enzymes become saturated with substrate, becoming the limiting factor for the rate of respiration.

Different respiratory substrates will lead to different rates of respiration. Glucose has the highest rate of respiration as no hydrolysis reactions are required prior to respiration occurring.

The disaccharide sucrose will have a slightly lower rate of respiration than glucose due to the need for hydrolysis prior to respiration taking place.

The polysaccharide starch will have a much lower rate of respiration than both glucose and sucrose. This is due to the many hydrolysis reactions that must be carried out prior to respiration.