A2 Biology: Anaerobic respiration

• Occurs in the absence of oxygen
• Only consists of glycolyis
• therefore takes place in the cytoplasm
• Net ATP made: 2
• Produces lactate (in animals) or ethanol (in plants) as a waste product.

Only glycolysis occurs as oxygen is the final electron acceptor; without it the ETC cannot function. The reduced NAD is not re-oxidised (lose H) and ergo there is an absence NAD for the Krebs cycle to function. Fungi, such as yeast, use ethanol fermentation and animals use lactate fermentation.

LACTATE FERMENTATION

• Pryuvate (3C) -----lactate dehydrogenase----> Lactate (3C)
                      Reduced NAD -- NAD -- glycolysis
• During this process pryuvate is acting a an aternativ H acceptor
• Lactate is carried away from the muscles to the liver. It is converted back to pryuvate when oxygen is present again.
• Muscle fatigue is not caused by the lactate but the drop in the pH that results from its formation, buffers in muscle help prevent this.

ALCOHOLIC FERMENTATION

• Pryuvate (3C) --pryuvate decarboxylase--> Ethanal (2C) --ethanal dehydrogenase--> Ethanol (2C)
•                                                                  gives out CO2         reduced NAD -> NAD

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.

A2 Biology: Respiration - The Krebs Cycle

The Krebs cycle takes place in the mitochondrial matrix. This stage takes place in aerobic respiration.

1. Acetate (carried from the Link reaction by coenzyme A) joins with Oxaloacetate (4C) to form Citrate (6C). CoA is the released to collect more acetate.
2. Citrate is decarboxylated (CO2 is removed) and dehydrogenated (2H is removed) to form a 5C compound. 2H is accepted by a molecule NAD which becomes reduced NAD.
3. 5C compound is also decarboxylated and dehydrogenated to form a 4C compound and a molecule of reduced NAD.
4. 4C compound is changed to another 4C compound and during this change a molecule of ADP is phosphorylated to produce a molecule of ATP. (Substrate-level phosphorylation).
5. The 4C compound then changes into another 4C compound and 2H is removed and accepted by coenzyme FAD which becomes reduced.
6. The other 4C compound go through dehydrogenation  and oxaloacetate is regenerated and another molecule of NAD is reduced. 

There needs to be two turns round the cycle for each molecule of ATP

To conclude: 

• 6 Reduced NAD are produced
• 2 Reduced FAD are produced
• 4 molecules of CO2 are produced
• and 2 ATP are produced

A2 Biology: Homeostasis: Control of blood glucose concentration

If blood glucose concentration is too high it is detected by beta cells 

• Beta cells (in islets of Langerhans) then secretes insulin into the blood
• They target hepatocytes, muscles cells and other cells such as brain that have membrane bound receptors for insulin. Insulin binds to these receptors
• Adenyl cyclase is activated in each cell and converts ATP to cAMP
• this activates a series of enzyme reactions 
• Glucose is converted into glycogen for storage/lipids
• or used in respiration
• glucose leave cells through specific channels

If blood glucose concentration is too low

• This change is detected by alpha cells (in islets of Langerhans)
• Alpha cells secrete the hormone glucagon
• Targets cells are heptocytes which posses specific receptors for glucagon
• glycogen is converted to glucose - glycogenolysis
• glucose is made from amino acids and lipids - gluconeogenesis 

A2 Biology: Respiration - Oxidative phosphorylation

-On inner mitochondrial membrane known as the cristae

• Has a series of electron carriers = electron transport chain
• Each one has a haem group (co factor) which contains iron, Fe.
  (If it's Fe2+ it is reduced ~ gained e- | If it's Fe3+ it's oxidised ~ loss e-)

1. NADH releases H to the first carrier
2. First carrier passed H to the second carrier
3. H now splits into H+ and e- and only the e- is passed on to the net carrier
4. H+ is pumped into the inter membrane space 
5. There is now a high concentration of H+ in intermediate slate = protein motive force/electrochemical gradient
6. H+ diffuses through the stalked particle which acts as a channel protein of hydrogen ions back into the matrix
7. H+ are back in matrix they recombine with their e- to form an Hydrogen atom.
8. H combines with O2 (which we breathe in) to form H2O hich is a waste product of respiration

• Enzyme involved - cytochrome oxidase
• Oxidose phosphorylation is making ATP using oxygen therefore involves electron transport chain
• Chemiosmosis - making ATP as H+ diffuse through stalked particle

A2 Biology: Respiration - Glycolysis

Glycolysis

• Happens in the cytoplasm of all cells
• It can take place in anaerobic or aerobic conditions.
• Glucose (6C sugar) is broken down to 2 molecules of pryuvate (3C compound)

Stage 1. Phosphorylation

• Glucose (6C sugar) - very stable - needs to be activated to be split
• An ATP molecule is hydrolysed - phosphate group attached to glucose at carbon 6
• Glucose > Fructose 6 phosphate
• Another ATP is hydrolysed - phosphate group attached to fructose 6 phosphate at carbon 1
• Activated - fructose 1,6 bisphosphate aka hexose, 1,6-bisphosphate
• Energy from hydrolysed ATP activates the sugar and prevents it from going out of the cell

2. Splitting hexose 1,6 bisphosphate

• each molecules of hexose 1,6 bisphosphate is splt into 2 x triose phosphate 

3. Oxidation of triose phosphate

• 2H atoms are removed from triose phosphate (involves dehydrogenase enzymes)
• These are helped by NAD which is a H acceptor - combines with H and becomes 'reduced NAD'
• 2 ATP formed - substrate level phosphorylation

4. Conversion triose phosphate > pryuvate

• 4 enzyme-catalysed reactions convert each phosphate to a pryuvate
• during this, 2 molecule of ADP are phosphorylysed to 2 ATP

So overall, 2 molecules of ATP (=net amount - 4 have been made but 2 are used again)

There are also 2 molecules of reduced NAD and 2 pryuvate which ill be used in next stage of respiration