Aerobic respiration

What is glycolysis

• Glycolysis is the first stage of aerobic and anaerobic respiration

  • It occurs in the cytoplasm of cells

  • Glycolysis does not require oxygen so it is an anaerobic process so it does not require oxygen

What is the reactant and products of glycolysis

• The products of glycolysis for 1 molecule of glucose

  • 2 NADH/reduced NAD - goes to the electron transport chain

  • 2 pyruvate - actively transported into the mitochondria for the link reaction

  • 2 ATP - used for energy

What are the two stages of glycolysis

• Phosphorylation 

• Oxidation

Stage 1 of glycolysis

• Glucose is phosphorylated using a phosphate from a molecule of ATP. This creates 1 molecule of glucose phosphate and 1 molecule of ADP

• ATP is then used to add another phosphate, forming hexose bisphosphate.

  • This stage is necessary as glucose is unreactive so using Pi to phosphorylate makes glucose reactive so it can be broken down

Stage 2 of glycolysis

• Then, via hydrolysis hexose bisphosphate is then split into 2 molecules of triose phosphate

• A hydrogen is removed from each triose phosphate (they are oxidised), forming 2 molecules of pyruvate

  • NAD collects the hydrogen ions forming 2 NADH

• 4 ATP are produced, but 2 were used up in stage one, so there’s a net gain of 2ATP

What happens after glycolysis if oxygen is available

• After glycolysis, if oxygen is available, pyruvate moves through mitochondrial membranes by active transport

What is the link reaction and where does it take place

• The link reaction is the second stage of respiration, it links glycolysis to the Krebs cycle

• The link reaction takes place in the matrix of the mitochondria

How many times does the links reaction and Krebs cycle take place for each glucose molecule

• For each glucose molecules used in glycolysis, two pyruvate molecules are made, but the link reaction uses only one pyruvate molecule, so the link reaction and the Krebs cycle happen twice for every glucose molecule which goes through glycolysis

What are the reactants and products of the link reaction

• The reactants of the link reaction:

  • Pyruvate, NAD and Coenzyme A

• The products of the link reaction:

  • Acetyl coenzyme A

  • NADH

  • Carbon dioxide

The process of the link reaction

• Pyruvate from glycolysis is actively transported into the matrix 

• In the matrix, each pyruvate molecule is decarboxylated, loses one molecule of carbon dioxide

  • Carbon dioxide diffuses out of the mitochondria as a waste product

• Two hydrogen atoms are removed from pyruvate (is oxidised) to form a two-carbon molecule (acetate)

  • And NAD is reduced to form NADH (an electron carrier)

• The acetate combines with coenzyme A to form acetylcoenzyme A

  • Acetyl coenzyme A delivers the acetyl group to the next stage of aerobic respiration, the Krebs cycle

What is the Krebs cycle, and where does it take place?

• The Krebs cycle, also known as the citric acid cycle, is a stage of aerobic respiration that follows glycolysis and the link reaction

• It takes place in the mitochondrial matrix

What are the reactants and products of the Krebs cycle?

• For each molecule of acetyl CoA, the Krebs cycle produces:

  • Two molecules of carbon dioxide as a by-product

  • The electron carriers NADH and FADH₂

  • One molecule of ATP  by substrate-level phosphorylation

Stage 1 of the Krebs cycle ( producing 6C molecule)

• Acetyl CoA enters the Krebs cycle. The 2C acetate separates from the coenzyme A which is released for reuse back to the link reaction

• The 2C joins with a 4C molecule to form a new 6C molecule

Stage 2 of the Krebs cycle (5C molecule)

• The 6C molecule is dehydrogenated (hydrogens are removed) which is used to make NADH from NAD and it is also decarboxylated (a carbon dioxide is removed) to produce a 5C molecule

Stage 3 of the Krebs Cycle

• The 5C molecule is decarboxylated and dehydrogenated again to produce another carbon dioxide and another NADH, so is now a 4C molecule

  • Decarboxylation and dehydrogenation occur, producing one molecule of reduced FAD and two of reduced NAD

• A phosphate is removed and used to generate a molecule of ATP. This is known as substrate-level phosphorylation

• The 4C molecule is regenerated for the next turn of the cycle

For each molecule of glucose what does the Krebs cycle produce?

• For each molecule of glucose the Krebs cycle turns twice, producing:

  • 4 carbon dioxide molecules (waste product)

  • 6 NADH

  • 2 FADH₂

  • 2 ATP  

What is the role of conenzymes

Their role is to bind to the active site of specific enzymes to aid the binding of the complementary substrate, so without them the enzyme wouldn’t function

What is the electron transfer chain?

• The electron transfer chain is also called oxidative phosphorylation

• Oxidative phosphorylation is the process where the energy carried by electrons from reduced coenzymes (reduced NAD and reduced FAD), is used to make ATP

What does oxidative phosphorylation involve and where does it occur?

• Oxidative phosphorylation involves the electron transport chain and chemiosmosis

• Oxidative phosphorylation occurs across the inner mitochondrial membrane of the cristae

  • The cristae are the inner folded membranes in mitochondria where the enzymes and proteins involved in oxidative phosphorylation are located

  • Some of these proteins are involved in the electron transfer chain: a series of electron carrier molecules in the inner mitochondrial membrane that release energy in stages

What are the reactants and products of oxidative phosphorylation

• The reactants of oxidative phosphorylation

  • Reduced NAD

  • Reduced FAD

  • Oxygen

  • ADP and inorganic phosphate

• The products of oxidative phosphorylation:

  • NAD

  • FAD

  • Water

  • ATP

Stage 1 of oxidative phsophorylation (ETC)

• Hydrogen atoms are released from reduced NAD and reduced FAD so they are oxidised to NAD and FAD

  • The hydrogen atoms split into protons (H+) and electrons (e-) which are transferred into the mitochondrial matrix

• High-energy electrons are passed to an electron carrier from reduced NAD and reduced FAD

• The electrons are passed along a series of electron carrier molecules in the electron transport chain embedded in the inner mitochondrial membrane, releasing energy as they are transferred

Stage 2 of oxidative phosphorylation (chemiosmosis)

• The energy the electrons released/lost is used by the electron carriers to pump protons from the mitochondrial matrix into the intermembrane space

• The concentration of protons is now higher in the intermembrane space than in the mitochondrial matrix

  • The concentration of protons is now higher in the intermembrane space than in the mitochondrial matrix - this forms an electrochemical gradient

• Protons then move down the electrochemical gradient, back across the inner mitochondrial membrane and into the mitochondrial matrix via ATP synthase (which is embedded in the inner mitochondrial membrane)

  • This movement results in the synthesis of ATP from ADP and inorganic phosphate

• This process of ATP production driven by the movement of H+ ions across a membrane due to electrons moving down an electron transport chain is called chemiosmosis (which is described by the chemiosmotic theory)

Stage 3 of oxidative phosphorylation

• Oxygen is the final electron acceptor, and combines with electrons and protons to form water, helping to maintain the proton gradient, in the mitochondrial matrix at the end of the transport chain

How much ATP was produced before the ETC?

• 4 molecules of ATP produced in glycolysis (2 are used up so the is a net gain of 2 ATP)

• 2 molecules of ATP are produced from two turns of the Krebs cycle

How many ATP can reduced NAD and FAD produce

• Each reduced NAD can produce 2.5 ATP and each reduced FAD can produce 1.5 ATP

Collecting all the reduced NAD and FAD produced in the previous stages of respiration how many ATP produced in ETC

• Glycolysis = 2 x reduced NAD = 5ATP

• Link reaction = 2 x reduced NAD = 5 ATP

• Krebs Cycle = 6 reduced NAD = 15 ATP

• Krebs Cycle = 2 reduced FAD = 3 ATP

Total number of ATP produced with 1 glucose molecule

Respiration total = 32 ATP

Other respiratory substrates

• It’s not just glucose that can be used as the substrate in aerobic respiration.

• Some products resulting from the breakdown of other molecules, such as fatty acids from lipids and amino acids from proteins, can be converted into molecules that are able to enter the Krebs cycle