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7.1 Cellular respiration

Topic 7: Run for your life:

7.1 Cellular respiration

 

Respiration = The chemical process of releasing energy from organic compounds (respiratory substrates) such as glucose through oxidation. The energy released is used to combine ADP with inorganic phosphate to make ATP (energy). Respiration is a long series of enzyme-controlled reactions.

 

  • Aerobic respiration: Requires oxygen to fully oxidise the organic molecule. This releases a lot of energy
  • Anaerobic respiration: The breakdown of the molecule without oxygen. This releases much less energy.

 

ATP carries energy around the cell to where it is needed by diffusion. It is a molecule made from the nucleotide base adenine and 3 phosphate groups.

ATP is synthesised from ADP and Pi from an energy releasing reaction, such as the breakdown of glucose in respiration. The energy is stored as chemical energy in the phosphate bond. The enzyme ATPase catalyses the reaction.

When energy is required by a cell, ATP is broken back down into ADP and Pi, releasing energy from the phosphate bond. ATPase catalyses this reaction. The more ATPs used, the more energy is released.

 

In aerobic respiration, energy is released by splitting glucose into carbon dioxide (released as a water product) and hydrogen (combines with oxygen to produce water).  For aerobic respiration to occur, the cells must have mitochondria. The energy is used to phosphorylate ADP to ATP, providing energy for biological processes in a cell.

 

Aerobic respiration involves:

  • Glycolysis – splitting of sugar to form pyruvate – in cytoplasm
  • Link reaction – mitochondrial matrix
  • Krebs cycle – removal of hydrogen from pyruvate – mitochondrial matrix
  • Electron transport chain/oxidative phosphorylation – using hydrogen to produce ATP – inner mitochondrial membrane

 

Glycolysis:
Glycolysis is the first stage of respiration and occurs in the cytoplasm. Glycolysis makes pyruvate (3C) from glucose (6C). Glycolysis is the first stage in both anaerobic and aerobic respiration and doesn’t require oxygen to take place.

 

  1. Glycogen in muscle or liver converted to glucose
  2. A glucose molecule is phosphorylated as 2 ATPs donate phosphate to it. This produces 2ADP and 2 molecules of a 6C molecule
  3. The 6C molecule is reactive and so splits into 2 3C phosphates
  4. Hydrogen is removed (oxidised) and taken up by 2 NADs which become reduced. 2ATPs are made as phosphate groups are added to ADP (substrate-level phosphorylation). This forms 2 molecules of 3C pyruvate

 

Overall, 2 ATP are used and 4 are made from one glucose molecule – net gain of 2ATP.

 

Link reaction:

If oxygen is available, the pyruvate moves to the mitochondrial matrix, where the link reaction and Krebs cycle occurs. The link reaction converts pyruvate into acetyl CoA. The link reaction does not produce any ATP.

 

  • Carbon dioxide is removed from pyruvate (decarboxylation) and diffuses out of the mitochondria and out of the cell
  • Hydrogen is removed from pyruvate (dehydrogenation/oxidation) – accepted by NAD, producing reduced NAD
  • This converts pyruvate into a 2C molecule which immediately combines with coenzyme A to form the 2C compound acetyl coA

 

The link reaction and following Krebs cycle occurs twice for every glucose molecule.

 

Krebs cycle

The Krebs cycle (or citric acid cycle) also occurs in the mitochondrial matrix, where the enzymes that catalyse the reactions are located.

 

Acetyl CoA combines with a 4C compound to produce a 6C compound. This is converted back to the 4C compound to be used again through a series of enzyme-controlled steps. During this process, more carbon dioxide is released (decarboxylation) and diffuses out of the cell. More hydrogen is released through oxidation reactions (dehydrogenation) and picked up by NAD and FAD, producing reduced NAD and reduced FAD. ATP is also produced by substrate-level phosphorylation.

 

For each cycle, the products are:

  • 2 carbon dioxide
  • 3 reduced NAD
  • 1 reduced FAD
  • 1 ATP

 

Electron transport chain

Oxidative phosphorylation involves two processes – the electron transport chain and chemiosmosis. The hydrogens picked up by NAD and FAD are split into electrons and protons (hydrogen ions). The electrons are passed along the electron transport chain, on the inner membrane of the mitochondria.

 

As they move along the chain in a series of redox reactions, they lose energy which is used to transport hydrogen ions from the mitochondrial matrix across the inner membrane and into the intermembrane space. This causes a high concentration of hydrogen ions in this space, forming an electrochemical gradient. The hydrogen ions diffuse back into the matrix through protein channels in stalked particles, working as ATPases. This movement of the H+ ions provides energy to cause ADP and Pi to combine to make ATP. The active transport and diffusion of hydrogen ions is chemiosmosis or the chemiosmotic theory.

 

At the end of the chain, the electrons combine with oxygen to produce water. Oxygen is required in aerobic respiration as it acts as the final electron acceptor for the hydrogens.

 

For each reduced NAD, 3 ATP molecules are made

For each reduced FAD, 2 ATP molecules are made

 

So…

  • ATPs made directly = 4
  • ATPs made from reduced NAD = 10 x 3
  • ATPs made from reduced FAD = 2 x 2

 

Total ATPs = 38 (under most favourable conditions)

 

Anaerobic respiration

If oxygen is not available, the link reaction and Krebs cycle stop and oxidative phosphorylation cannot occur as there is no final electron acceptor. Glycolysis can still continue as long as the pyruvate can be removed and the reduced NAD can be converted back to NAD. This does not produce as much energy, the net yield is 2 ATP per glucose molecule.

 

In animals this is done by converting pyruvate to lactate (lactate fermentation) in the cytoplasm. Reduced NAD from glycolysis transfers H to pyruvate to form lactate and NAD so glycolysis can continue. The lactate built up in muscles diffuses into the blood and is carried in solution as lactic acid in the blood plasma to the liver, where liver cells convert it back to pyruvate. This requires oxygen, this is the oxygen debt.

 

When oxygen is available again after exercise and oxygen uptake is greater than normal, some of the pyruvate in the liver cells is oxidised through the link reaction, Krebs cycle and electron transport chain. Some pyruvate is reconverted to glucose in the liver cells and this is released into the blood or converted into glycogen to be stored.

 

In plants and some microorganisms such as yeast, pyruvate is reduced to ethanol and carbon dioxide using the hydrogen from reduced NAD. This recreated oxidised NAD, allowing glycolysis to continue. This is called alcoholic fermentation.

 

Supplying instant energy

At the start of exercise, immediate ATP is regenerated using creatine phosphate. This is a substance stored in muscles that can be hydrolysed to release energy. This energy regenerates ATP from ADP and Pi, the phosphate is given by the creatine phosphate. Creatine phosphate breaks down as exercise begins.

 

Creatine phosphate à creatine + Pi

 

These reactions do not require oxygen and provide energy for 6 – 10 seconds.

 

Measuring the rate of respiration

 

The rate of oxygen uptake is measured using a respirometer. Organisms (woodlice) are placed into a tube and the same mass of a non-living material is placed in the other. Soda lime (or potassium hydroxide/KOH solution) in each tube absorbs the carbon dioxide. Cotton wool prevents contact of the soda lime with the organisms. A syringe is used to set the coloured fluid into the manometer at a known level and flows into the capillary tube without air bubbles to give the same quantity. The rubber bungs are fitted to make the tubes airtight. The control tube is exactly the same but without the organisms to make sure the results are due to respiration.

As the organisms respire, they take in oxygen and give out carbon dioxide. The removal of oxygen from the tube reduces the volume and pressure, causing the manometer fluid to move towards the organisms to fill the pressure space. The respired carbon dioxide is absorbed by the soda lime.

 

The distance moved by the liquid in a given time is measured. The mean volume of liquid = length = pi r^2. This gives the volume of oxygen absorbed per minute.

 

Temperature must be controlled.

 

Respiration key words:

 

  • Glycolysis: The splitting of glucose into a 3C compound, pyruvate. It is anaerobic and produces 2ATP, 2 reduced NAD and 2 pyruvate molecules
  • Pyruvate: A 3C compound produced in glycolysis
  • Coenzyme NAD: Accepts hydrogen atoms, becoming reduced NAD
  • Substrate-level phosphorylation: The synthesis of ATP by combining ADP and Pi through energy from the substrates of a reaction
  • Link reaction: Turns pyruvate into acetyl coA for the Krebs cycle by releasing carbon dioxide and 2 hydrogens.
  • Decarboxylation: Carbon dioxide is released as a waste product
  • Dehydrogenation: Hydrogens removed and taken up by coenzymes
  • Acetyl coA: First step in Krebs cycle, last step in the link reaction
  • Krebs cycle: A cycle that starts with acetyl coA and produces 2 carbon dioxide, 1 ATP, 3 reduced NAD and 1 reduced FAD
  • Electron transport chain: Produces the most of ATP for a cell. Hydrogens and electrons are received from NAD and FAD
  • Chemiosmotic theory: Because the outside of the mitochondria is more positive, it attracts positive H+ ions into it and produces ATP
  • Oxidative phosphorylation: Electrons and hydrogens combine and join with oxygen on stalked particles on the cristae of mitochondria, producing ATP
  • Lactate: In anaerobic respiration, NAD is reduced during glycolysis. The pyruvate is reduced to lactate and the oxidised form of NAD is reduced