Biology

Key Unit Competence
To be able to describe the process of cellular respiration

Learning objectives
By the end of this unit, I should be able to:
–– Outline the four stages in aerobic respiration (glycolysis, link reaction, TCA cycle and oxidative phosphorylation) and state where each occurs in the eukaryotic cells.
–– Explain that when oxygen is available, pyruvate is converted into acetyl coenzyme A, which then combines with oxaloacetate (4C) to form citrate (6C).
–– Explain that reactions in the TCA cycle involve decarboxylation and dehydrogenation and the reduction of NAD and FAD.
–– Outline the process of oxidative phosphorylation including the role of oxygen
(details of the carriers are not required).
–– Describe the relationship between the structure and function of the mitochondrion.
–– Explain the production of a small yield of ATP from anaerobic respiration in yeast and mammalian muscle tissue, including the concept of oxygen debt.
–– Explain how other substrates are involved in glycolysis and the TCA cycle.

– Explain how other substrates are involved in glycolysis and the TCA cycle

6.1 Overview of respiration process

6.1.1 Respiration

Cellular respiration is the complex process in which cells make adenosine triphosphate (ATP) by breaking down organic molecules. The energy stored in ATP can then be used to drive processes requiring energy, including biosynthesis, locomotion or transportation of molecules across cell membranes. The main fuel for most cells is carbohydrate, usually glucose which is used by most of the cells as respiratory substrate. Some other cells are able to break down fatty acids, glycerol and amino acids.

Glucose breakdown can be divided into four stages: glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation

6.1.2 Glycolysis

Glycolysis is the splitting or lysis of a glucose molecule. It is a multi-step process in which a glucose molecule with six carbon atoms is eventually split into two molecules of pyruvate, each with three carbon atoms. Energy from ATP is needed in the first steps, and it is released in the later steps to synthesize ATP. There is a net gain of two ATP molecules per molecule of glucose broken down.

Glycolysis takes place in the cytoplasm of a cell. Glucose enters the cell and is phosphorylated by the enzyme called hexokinase, which transfers a phosphate group from ATP to the sugar. The ATP used in this process has 2 advantages: the charge of the phosphate group traps the sugar in the cell because the plasma membrane is impermeable to large ions. Phosphorylation also makes glucose more chemically reactive. Even though glycolysis consumes two ATP molecules,It produces a gross of four ATP molecules (4 ATP), and a net gain of two ATP (2 ATP) molecules for each glucose molecule that is oxidized. Glycolysis results in a net gain of two ATP, two NADH and two pyruvate molecules.

6.2 Link reaction and the Krebs cycle

6.2.1 Link reaction

Pyruvate, the end product of glycolysis is oxidized to Acetyl-CoA by enzymes located in the mitochondrion of eukaryotic cells as well as in the cytoplasm of prokaryotes. In the conversion of pyruvate to Acetyl-CoA, one molecule of NADH and one molecule of CO2 are formed (Figure 6.2). This step is also known as the link reaction or transition step, as it links glycolysis to the Krebs cycle.

6.2.2 The Krebs cycle (Citric acid cycle)

The coenzyme has a sulphur atom, which attaches the acetyl fragment by an unstable bond. This activates the acetyl group for the first reaction of the Krebs cycle also called citric acid cycle or Tricarboxylic Acid Cycle (TCA). It is also known as the citric acid cycle, because the first molecule formed when an acetyl group joins the cycle. When oxygen is present, the mitochondria will undergo aerobic respiration which leads to the Krebs cycle.

In the presence of oxygen, when acetyl-CoA is produced, the molecule then enters the citric acid cycle inside the mitochondrial matrix, and gets oxidized to CO₂ while at the same time reducing NAD+ to NADH. NADH can then be used by the electron transport chain to create more ATP as part of oxidative phosphorylation. For the complete oxidation of one glucose molecule, two Acetyl-CoA must be metabolized by the Krebs cycle. Two waste products namely H₂O and CO₂, are released during this cycle.

The citric acid cycle is an 8-step process involving different enzymes and co-enzymes. Throughout the entire cycle, Acetyl-CoA (2 carbons) combines with oxaloacetate (4 carbons) to produce citrate. Citrate (6 carbons) is rearranged to a more reactive form called iso citrate (6 carbons). Iso citrate (6 carbons) is modified to α-Ketoglutarate (5 carbons), Succinyl-CoA, Succinate, Fumarate, Malate, and finally to Oxaloacetate. The net energy gain from one cycle is 3 NADH, 1 FADH₂, and 1 Guanosine Triphosphate (GTP). The GTP may subsequently be used to produce ATP. Thus, the total energy yield from one whole glucose molecule (2 pyruvate molecules) is 6 NADH, 2 FADH₂, and 2 ATP. 2 molecules of carbon dioxide are also produced in one cycle (for a total of 4 molecules of carbon dioxide from one glucose molecule)

6.3 Oxidative phosphorylation and electron transport chain

In the final stage of aerobic respiration known as the oxidative phosphorylation, the energy for the phosphorylation of ADP to ATP comes from the activity of the electron transport chain. Oxidative Phosphorylation is the production of ATP using energy derived from the redox reactions of an electron transport chain.

In eukaryotes, oxidative phosphorylation occurs in the mitochondrial cristae. It comprises the electron transport chain that establishes a proton gradient across the inner membrane by oxidizing the NADH produced from the Krebs cycle. ATP is synthesized by the ATP synthase enzyme when the chemiosmotic gradient is used to drive the phosphorylation of ADP. Chemiosmosis is the production of ATP from ADP using the energy of hydrogen ion gradients. The electrons are finally transferred to oxygen and, with the addition of two protons, water is formed. The average ATP yield per NADH is probably 3 and for FADH₂ of this electron carrier is worth a maximum of only two molecules of ATP.

The role of oxygen in chemiosmosis

ATP can be synthesized by chemiosmosis only if electrons continue to move from molecule to molecule in the electron transport chain. Oxygen serves as the final acceptor of electrons. By accepting electrons from the last molecule in the electron transport chain, and allows additional electrons to pass along the chain. As a result, ATP can continue to be synthesized. Oxygen also accepts the protons that were once part of the hydrogen atoms supplied by NADH and FAD2. By combining with both electrons and protons, oxygen forms water as shown in the following equation:

Overview of cellular respiration

A considearable number of ATP is produced during oxidative phosphorylmation and it is estimated  between 32 and 34 ATPs. These are added to 2 ATP produced during glycolysis and 2 ATP produced during citric cycle. The total number of ATP produced during a complete respiration process for one molecule of glucose is then estimated between 36 and 38 ATPs.

Note that the amount of ATP produced from glucose is usually less than 38 ATP for the following reasons: some ATP is used to transport pyruvate from the cytoplasm into the mitochondria and some energy is used to transport NADH produced in glycolysis from the cytoplasm into the cristae of mitochondria.

6.4 Efficiency of aerobic and anaerobic respiration

Without oxygen, pyruvate (pyruvic acid) is not metabolized by cellular respiration but undergoes a process of fermentation. The pyruvate is not transported into the mitochondrion, but remains in the cytoplasm, where it is converted to waste products that may be removed from the cell. This serves the purpose of oxidizing the electron carriers so that they can perform glycolysis again and removing the excess pyruvate. Fermentation oxidizes NADH to NAD+ so it can be re-used in glycolysis.

In the absence of oxygen, fermentation prevents the build-up of NADH in the cytoplasm and provides NAD+ for glycolysis. This waste product varies depending on the organism. In skeletal muscles, the waste product is lactic acid. This type of fermentation is called lactic acid fermentation. In yeast and plants, the waste products are ethanol and carbon dioxide. This type of fermentation is known as alcoholic or ethanol fermentation. The ATP generated in this process is made by substrate-level phosphorylation, which does not require oxygen.

Fermentation is less efficient at using the energy from glucose since only 2 ATP are produced per glucose, compared to the 38 ATP per glucose produced by aerobic respiration. This is because the waste products of fermentation still contain plenty of energy. Glycolytic ATP, however, is created more quickly.

a. Applications of anaerobic respiration
Some food products and drinks are produced by using anaerobic microorganisms:
–– Production of beer
–– Production of wine
–– Production of yoghurt
–– Production of cheese
–– Production of bread

b. Efficiency of aerobic and anaerobic respiration

The complete oxidation of glucose produces the energy estimated at 686 Kcal. Under the condition that exists inside most of the cells, the production of a standard amount of ATP from ADP absorbs about 7.3 Kcal. Glucose molecule can generate up to 38 ATP molecules in aerobic respiration. The efficiency of aerobic respiration (EAER) is calculated as follows:

This result indicates that the efficiency of aerobic respiration equals 40%. The remainder of the energy (around 60%) is lost from the cell as heat.

Due to the fact that anaerobic respiration produces only 2 ATP, the efficiency of anaerobic respiration is less than that of aerobic respiration. It is calculated as follows:

c. Oxygen debt

Standing still, the person absorbs oxygen at the resting rate of 0.2 dm3 min−1. (This is a measure of the person’s metabolic rate.) When exercise begins, more oxygen is needed to support aerobic respiration in the person’s muscles, increasing the overall demand to 2.5 dm³ min−1. However, it takes four minutes for the heart and lungs to meet this demand, and during this time lactic fermentation occurs in the muscles. Thus the person builds up an oxygen deficit. For the next three minutes, enough oxygen is supplied. When exercise stops, the person continues to breathe deeply and absorb oxygen at a higher rate than when at rest. This post-exercise uptake of extra oxygen, which is ‘paying back’ the oxygen deficit, is called the oxygen debt. The oxygen is needed for:

–– Conversion of lactate to glycogen in the liver
–– Re oxygenation of haemoglobin in the blood
–– A high metabolic rate, as many organs are operating at above resting levels.

The presence of the lactic acid is sometimes described as an ‘ oxygen debt’. This is because significant quantities of lactic acid can only be removed reasonably quickly by combining with oxygen. However, the lactic acid was only formed due to lack of sufficient oxygen to release the required energy to the muscle tissue via aerobic respiration. Lactic acid can accumulate in muscle tissue that continues to be overworked. Eventually, so much lactic acid can build-up that the muscle ceases working until the oxygen supply that it needs has been replenished.

To repay such an oxygen debt, the body must take in more oxygen in order to get rid of the additional unwanted waste product lactic acid

d. Muscle cramps

A muscle cramp is an involuntarily and forcibly contracted muscle that does not relax. Muscle cramps can occur in any muscle; cramps of the leg muscles and feet are particularly common.

Almost everyone experiences a muscle cramp at some time in their life. There are a variety of types and causes of muscle cramps. Muscle cramps may occur during exercise, at rest, or at night, depending upon the exact cause.

Overuse of a muscle, dehydration, muscle strain or simply holding a position for a prolonged period can cause a muscle cramp. In many cases, however, the cause isn’t known.

Although most muscle cramps are harmless, some may be related to an underlying medical condition, such as:

–– Inadequate blood supply. Narrowing of the arteries that deliver blood to your legs (arteriosclerosis of the extremities) can produce cramp-like pain in your legs and feet while you’re exercising. These cramps usually go away soon after you stop exercising.
–– Nerve compression. Compression of nerves in your spine (lumbar stenosis) also can produce cramp-like pain in your legs. The pain usually worsens the longer you walk. Walking in a slightly flexed position such as you would use when pushing a shopping cart ahead of you may improve or delay the onset of your symptoms.
–– Mineral depletion. Too little potassium, calcium or magnesium in your diet can contribute to leg cramps. Diuretics or medications often prescribed for high blood pressure also can deplete these minerals.

6.5 Factors affecting  the rate of respiration

Cellular respiration is the process of conversion of chemical energy stored in the food to ATP or higher energy compounds. The factors that affect the cellular respiration are:

a. Amount of nutrients

If the amount of nutrients is high, then the energy is high in the cellular respiration. The nutrients which can go through cellular respiration and transform into energy are fat, proteins and carbohydrates. The amount of nutrients available to transform into energy depend upon the diet of the person.

b. Temperature

The rate of the cellular respiration increases if the body temperature is warmer. The lower the temperature, the slower the rate of cellular respiration. The reason for this is enzymes which are present in cellular respiration process. Enzyme reactions require optimum temperatures.

c. State of the cell

Metabolically active cells such as neurons, root of human hair have higher respiration rate than the dormant cells such as skin cells and bone cells. This is because metabolically active cells can store energy in the body because of the many metabolic reactions that take place in them.

d. Water

It is the medium where the reaction happens. When a cell is dehydrated the respiration and other metabolism decreases.

e. Cellular activity

Some cells need more energy than others. For example, growing cells or very active cells such as neurons need a lot of energy.

f. O₂ /CO₂ content

Higher O₂ and lower CO₂ make higher respiration rates.

g. ATP/ADP range

When there is  more ATP than ADP, respiration rate  slows down to avoid excess of ATP

6.6 Use of other substrates in respiration

Carbohydrates are the first nutrients that most organisms can catabolise for energy. In some cases, living things must be able to metabolize other energy-rich nutrients to obtain energy in times of starvation. Most organisms possess metabolic pathways that, when necessary, metabolize proteins, lipids. In each case, the larger molecules are first digested into their component parts, which the cell may reassemble into macromolecules for its own use. Otherwise, they may be metabolized for energy by feeding into various parts of glycolysis or the Krebs cycle.

Carbohydrates, fats and proteins can all be used for cellular respiration. Monomers of these foods enter glycolysis or the Krebs cycle at various points. Glycolysis and the Krebs cycle are catabolic pathways through which all kinds of food molecules are channelled to oxygen as their final acceptor of electrons.