A2 Biology: Celluar Respiration

Cellular respiration is the process ofusing enrgy in complex organic molecules to produce ATP

Glucose + Oxygen > Carbon Dioxide + Water + ATP

Metabolic reactions that build large molecules are called anabolic and those that break large molecules are called catabolic.  We require energy for: active transport, endocytosis and excocytosis, replication of DNA and movement.

ATP = Adenosine Triphosphate

 ATP is formed during a condensation reaction between ADP and Pi (it is phosphorylated)  The enzyme responsible for this is ATP synthase. The energy to produce the bond that is formed comes from the breakdown of organic substrate (such as glucose) during cellular respiration. ATP provides your cells with energy without it we would be dead.

• We get energy from ATP by breaking down the high-energy bonds between the last two phosphates in ATP.
• When ATP reaches the site of energy using processes in the cell it is hydrolysed back to ADP.
• When hydrolysed to ADP + Pi it yield 30.6kJ or energy per mol. This means energy is available to the cell in small manageable amounts.
• Respiration in which glucose is completely oxidised to form carbon dioxide using oxygen is called aerobic respiration.
• The second and third phases of aerobic respiration takes place in the mitochondria and so it is necessary to recall the structure of a mitochondrion.

The mitchondria matrix is the site of link reaction and Krebs cycle. The inner cristae membrane is the site of the electron transport chain (glycolysis takes place in the cytoplasm)

Phosphorylation - addition of an inorganic phosphate gorup.

Hydrolysis - the splitting of large molecules into smaller molecules through the addition of water.

Oxidation - this is when the substrate donates hydrogen to a hydrogen accepted (dehydrogenation)

Reduction - this is when a hydrogen accepter gains a hydrogen.

Coenzymes are needed during glcolysis, the link reacton and the Krebs cycle, H atoms are removed from the substrte molecules in the oxdation reachtion. These eactions are catalysed by dehydrogenase enzymes. 

This occurs in the mitochondria during respiration and the chloroplast during photosynthesis. 

Stages of respiration:

Glycolysis (cytoplasm)

The link reaction (matrix of mitochondria)

Krebs cycle (matrix of the mitchondria)

Oxidative phosphorylation (cristae of mitochondria)

GLYCOLYSIS

• This occurs int he cytoplasm of all cells. t is a series of reactions, each catalysed by a different enzyme. 
• It works under anaerobic conditions (without oxygen)
• Glucose 6(C) -*-> Glucose 6-phosphate (6C) --> Fructose 6-phosphate (6C) -*->Fructose 1,6-bisphosphate (6C) (Hexose 1,6-bisphosphate)

• (-*-> = ATP > ADP) 

•  Hexose 1,6 bisphosphate splits into 2 Triose phosphate (3C)
• Triose phosphate ----NAD>2xreduced nAD------ADP>2xATP----> Intermediate compound (3C)
• Intermediate compound (3C) ----ADP>ATP----> Pryuate (3C)

Glycolysis: One molecule of glucose produces:

Net gain: 2 ATP (4 ATP in total)

2 reduced NAD

2 molecules of pryuvate

THE LINK REACTION

• Pryuvate diffuses into the matrix of the mitochondria
• 3 stages:
• Decarboxylation of pryuvate by pryuvate decarboxylase
• Dehydrogenation (oxidation) of pryuvate by pryuvate dehydrogenase to form acetate
• Coenzyme A combines with the acetate to give two molecules of Acetyl CoA - enters Krebs cycle

KREBS CYCLE

• The carbon doxide is produced by decarboxylation
• The NAD and FAD are reduced to dehydrogenation during the conversion of citrate into oxaloacetate
• ATP is produced by substrate level phosphorylation where the energy taken directly from a substrate is used to add a phosphate group to ADP.

=6x reduced NAD

=2x reduced FAD

=4x CO2

=2x ATP 

Electron transport chain (ETC)

A number of reduced NAD and FAD have been produced during the first three phases of aerobic respiration  In the ETC these molecules are re-oxidised (lose H) by dehydrogenase enzymes. It takes place in the cristae of mitochondria.

• Each electron carrier is an enzyme, each associated with a coenzyme.
• The coenzyme can accept e- (become reduced) or donate e- (become oxidised)
• They are oxidoreductase enzymes
• Electron pass along the chain of carriers losing energy
• Some coenzymes use this energy to pump protons H+ from matrix to intermembranous space.
• The electrons from each reduced NAD can pump H+ into the space.
• Protons (H+) accumulate in the intermembranous space and build up a proton gradient (high conc)
• Stalked particles allows H+ to pass through them which generates ATP
• Each reduced NAD generates 2.6 molecules of ATP

Chemiosmosis: the process when protons flow down the proton gradient through the ATP synthase enzymes from intermembrane space into the matrix.

• The force of this flow rotates the enzyme and allows ADP + Pi join to form ATP
• aka oxidative phosphorylation - formation of ATP by adding a phosphate group to ADP in the presence of oxygen which is the final electron acceptor
• Oxygen is the final hydrogen acceptor
• when electrons are released from the chain they combine with the H+ ions to form hydrogen.
• The hydrogen then combines with oxygen to form water, so it is the final H or e- acceptor.
• There this stage will only take place in aerobic conditions.