Physiology
Basic Cellular
Regarding the Krebs cycle, which of the following statements is CORRECT:
Answer:
The Krebs cycle takes place in the matrix of the mitochondrion and requires oxygen. The Krebs cycle begins when the 2-carbon acetyl CoA joins with a 4-carbon compound to form a 6- carbon compound called citric acid. Citric acid (6C) is gradually converted back to the 4-carbon compound ready to start the cycle once more. The carbons removed are released as CO2. The hydrogens which are removed join with NAD to form NADH2. The electron transfer system is the final stage of cellular respiration and results in the release of water.Cellular Respiration
Physiology / Basic Cellular / Cell Structure and Function
Last Updated: 29th July 2024
Cellular respiration is the process by which cells obtain energy in the form of adenosine triphosphate (ATP). ATP transfers chemical energy from the energy rich substances in the cell to the cell's energy requiring reactions e.g. active transport, DNA replication and muscle contraction.
Cellular respiration is essentially a three step process:
- Glycolysis
- The Krebs cycle
- The electron transfer system
![By RegisFrey (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons](https://mrcemsuccess.com/wp-content/uploads/2017/04/cell-respiration-1024x724.png)
Cellular Respiration. (Image by RegisFrey (Own work) [CC BY-SA 3.0 , via Wikimedia Commons)
Respiratory Substrates
The main respiratory substrate used by cells is 6-carbon glucose. Respiration is a series of reactions in which 6-carbon glucose is oxidised to form carbon dioxide. The energy released due to the oxidation of glucose is used to synthesise ATP from adenosine diphosphate (ADP) and inorganic phosphate (Pi).
Fats and proteins can also be used as respiratory substrates. When fats are being used as the primary energy source, in the absence of glucose, an excess amount of acetyl-CoA is produced, and is converted into acetone and ketone bodies. This can occur in starvation, fasting or in diabetic ketoacidosis. Proteins are used as an energy source only if protein intake is very high, or if glucose and fat sources are depleted.
Glycolysis
Glycolysis takes place in the cytoplasm of the cell and does not require oxygen. Glycolysis is the breakdown of 6-carbon glucose into two 3-carbon pyruvic acid (pyruvate) units. The hydrogens removed join with the hydrogen carrier NAD to form NADH2. Although some energy is needed to start glycolysis there is an overall net gain of 2 ATP. The pyruvic acid (3C) then enters the matrix of the mitochondrion where it is oxidised (i.e. 2H removed) and a carbon dioxide is lost, forming acetyl CoA (2C).
Krebs Cycle
The Krebs cycle takes place in the matrix of the mitochondrion and requires oxygen. The Krebs cycle begins when the 2-carbon acetyl CoA joins with a 4-carbon compound to form a 6- carbon compound called citric acid. Citric acid (6C) is gradually converted back to the 4-carbon compound ready to start the cycle once more. The carbons removed are released as CO2. The hydrogens which are removed join with NAD to form NADH2.
Electron Transfer System
Most of the energy produced during respiration is made by the electron transfer system. The electron transfer system is a system of hydrogen carriers located in the inner mitochondrial membrane. The NADH2 molecules produced during glycolysis and the Krebs cycle transfer the hydrogens to the electron transfer system. In doing so, a H+ ion gradient is generated across the inner membrane which drives ATP synthase. Oxygen is the final hydrogen acceptor and the H+ ions and O2 combine to form water.
Anaerobic Respiration
When anaerobic respiration occurs there is no oxygen to act as the final hydrogen acceptor and so the hydrogen cannot pass through the electron transfer system. As a result, both the Krebs cycle and the electron transfer system stages cannot take place. The only ATP produced is formed during glycolysis, that is, 2 ATP per glucose molecule (compared to the 38 molecules of ATP produced during aerobic respiration).
The pyruvic acid produced following glycolysis is converted to lactic acid in a process called lactic acid fermentation. No energy is generated in this process but it allows ongoing glycolysis and ATP synthesis (which would otherwise stop) via the regeneration of NAD+ from NADH. The anaerobic pathway is reversible with lactic acid being converted back to pyruvic acid when oxygen is present.
Anaerobic respiration produces an oxygen debt. This is the amount of oxygen needed to oxidise lactic acid to carbon dioxide and water. The existence of an oxygen debt explains why we continue to breathe deeply and quickly for a while after exercise.
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
