respiration

Aerobic respiration can be divided into 4 stages

• Glycolysis - cytoplasm - Splitting 6C glucose into two 3C pyruvate molecules
• Link reaction - Matrix - 3C pyruvate enters reaction to from acetylcoenzyme A, a 2C molecule
• Krebs cycle - Matrix - Acetlycoenzyme A goes into a cycle of oxi-redu reactions that yeild ATP and reduced NAD and FAD.
• Oxidative phosphorylation - Cristae - Use of e- associated with reduced NAD and FAD released from krebs cycle to synthesise ATP with water as a by-product

Glycolysis is the splitting of glucose and occurs in the first stage of respiration for both anaerobic and aerobic respiration.

It occurs in the cytoplasm where a hexose (6C sugar - usually glucose) splits into 2 moleules of 3C molecule - pyruvate.

The four stages of enzyme-controlled reactions in glycolysis

• Phosphorylation of glucose -> glucose phosphate. Before being split into 2, glucose needs to be made more reactive by adding 2 phosphates on (phosphorylation). The phosphate comes from the hydrolysis of 2 ATP molecules to ADP. This provides energy and thus lowers the Ea for the enzyme-controlled reactions that follow.
• Spliting of the phosphorylated glucose - Each glucose splits into 2 3C molecules called triose phosphate
• Oxidation of triose phosphate - H is removed from each of the 2 triose phosphate molecules and transferred to a H carrier called NAD to form reduced NAD
• Production of ATP - Enzyme controlled reactions convert each triose phosphate into another 3C moleculed called pyruvate. 2 molecules of ATP are regenerated from ADP

NAD+ (reduced) is a conenzyme add works with dehydrogenate enzymes. The enzymes take H+ ane e- ff a substarate (eg TP) and passes them to NAD+

ATP budget - 2ATO are used up and then 2x2 ATP are created. This means net gain is -2X4 = 2 ATP per glucose.

Substrate level phosphorylation of ADP goes on in glycolysis

Energy yield from glycolysis

The overall yield from 1 glucose molecule undergoing glucose is therefore:

• 2 molecules of ATP (4 produced by 2 used in the phosphorylation of glucose)
• 2 moleucles of reduced NAD (can provide energy to make ATP)
• 2 molecules of pyruvate.

Glycolysis does not require organelles due to the glycolytic pathways in the cytoplasm. Without oxygen, the pyruvate will be converted into lactate or ethanol during anaerobic respiration. This is necessary to re-oxidise NAD so glycolysis can continue.

Pyruvate has energy that can be released in the krebs cycle. Before it can enter the cycle, it needs to be oxidised in the link reaction. In eukaryotic cells, both of these process occur in the mitochondria (matrix).

Link reaction

• Pyruvate molecules are actively transported from the cytoplasm to the matrix
• Pyruvate is oxidised to acetate and this result in pyruvate loosing a CO2 molecule and 2H. These H are accepted by NAD to produce reduced NAD which is needed to produce ATP.
• The 2C acetate combines with a coenzyme to produce acetylcoenzyme A

The krebs cycle - Involves a series of oxidation/reduction reactions in the matrix

• 2C acetylcoenzyme A combines with 4C to produce a 6C molecules
• The 6C molecule looses CO2 and H to give a 4C  molecule and a single ATP is produced due to substrate-level phosphorylation
• The 4C molecule combines with acetylcoenzyme A to begin the cycle again.

For each molecule of pyruvate, the link and kreb produce

• Reduced coenzymes (NAD/FAD) - these have potential energy to produce ATP by oxidative phosphorylation and are produced in the krebs cycle
• 1 molecule of ATP and 3 of CO2

As 2 pyruvate molecules are produced (for each original glucose) the yeild for 1 glucose is double

Coenzymes

• They are not enzymes but are used by them to function. They carry H atoms

Significance of the krebs cycle

• Brekas down macromolecules (pyruvate --> CO2)
• Produces H atoms that are carried by NAD to e- transfer chain + provide energy for oxidative phosphorylation = leads to production of ATP
• Regenerates the 4C molecule to combine with acetylcoenzyme A
• Source of intermediate compounds used by cells when manfacturing Fatty Acids, AA and chlorophyll

Hydrogen atoms carried by coenzymes NAD and FAD are used in the next step - oxidative phosphorylation. This is where some of the energy of the electrons within the hydrogen is conserved in the formation of adenosine triphosphate (ATP)

Oxidative phosphorylation and mitochondria (the site where it occurs)

Mitochondrisa are organelles that are found in eukaryotic cells. Each is bounded by a smooth outer membrane and an inner one that is folded into extensions called cristae. The inner space(matrix) contrinas proteins, lipids and traces of DNA. Within the cristae are enzymes and other proteins involved in this process so ATP synthesis occurs.

The electron transfer chain and the synthesis of ATP

• Reduced NAD and FAD donate e- of the H atoms they are carrying to the first molecule in the e- transfer chain
• E- pass along a chain of e- transfer carrier molecules. As e- flow along the chain, the energy they release causes the active transport of proteins across the inner mitochondrial membrane and into inter-membrnal space
• At the end of the chain, the e- combine with these protons and oxygen to form water.
• This is the chemiosmotic theory
• The importance of oxygen is to act as the final acceptor of the hydrogen atoms produced in glycolysis and the krebs cycle. Without its role of removing H atoms at the end of the chain, the H ions and e- would back up the chain and respiration would not occur.

Releasing energy in stages

• The greater energy is released in one step, the more of it is released as heat and less is avaliable for more useful purposes. When energy is released a little, it is more beneficial as e- carried by NAD/FAD are not transferred in 1 explosive step but in an chain. (gradient)                             

Alternative respiratory substrates

Both lipids and proteins may be used as respiratory substrates without first being converted into a carbohydrate.

Respiration of lipids

Lipids are hydrolysed to glycerol and FA. The glycerol is then phosphorylated and converted into triose phosphate which enters the glycolysis pathway and then the krebs cycle. The FA is broken down into 2C fragments which are converted into acetylcoenzyme A and then enters the kreb cycle.

Respiration of proteins

It is hydrolysed to its AA and the amino group is removed before entering the respiratory pathway at different points depending on the number of carbon atoms. 3C are converted to pyruvate, 4C and 5C are converted into intermediates in the krebs cycle

Neither the krebs cycle or electron transfer chain can continue due to no more FAD or NAD being reduced. None will be able to take up the H+ so enzymes stop working. This leaves only glycolysis as a source of ATP.

For it to continue, its products (pyrcuvate and hydrogen) must be constantly removed. H needs to be removed from reduced NAD to regenerate NAD. In eukaryotic cells, plants convert pyruvate to ethanol and co2. Animals convert pyruvate to lactate.

Production of ethanol in plants and some microorganims - used in brewing industry

Pyruvate looses a molecule of co2 and accepts a H from reduced NAD to produce ethanol

Production of lactate in animals

When oxygen is in short supple, NAD from glycolysis can accumulate and be removed.  Each pyruvate takes up 2 H atoms from the reduced NAD to form lactate.

At some point, lactate produced is oxidised back to pyruvate. This can further be oxidised to release energy or be converted into glycogen.  This happens when oxygen is avaliable again. In any case, lactate will cause cramp and muscle fatigue if it is allowed to accumulate in the muscle tissue. Where lactate is an acid, it contains pH changes which affect enzymes and although muscle has a certain tolerance to lactate, it is still impotant it is taken to the liver to be converted to glycogen

Energy yields from anaerobic and aerobic respiration

Energy from cellular respiration is derived in 2 ways

• Substrate level phosphorylation in glycolysis and the krebs cycle. This is the direct transfer of phosphate from a respiratory intermediate to ADP to produce ATP
• Oxidative phosphorylation in the electron transfer chain. This is the indirect linking of energy from phosphate to ADP to produce ATP involving energy from the hydrogen atoms tthat are carried on NAD and FAD. Cells prodice most of their ATP in this way

Pyruvate is converted to either ethanol or lactate and this means is not avaliable for Krebs so anaerobic respiration cannot occur in these places and only ATP can be used in glycolysis