Respiration + Energy storage

NAD+ and FAD

Nicotinamide adenine dinucleotide (NAD+) and Flavin adenine dinucleotide (FAD) are two coenzymes that become relevant in cellular respiration

NAD+ is reduced to NADH and NADH is oxidised to NAD+

FAD is reduced to FADH2 and FADH2 is oxidised to FAD

Both have nucleotide structures

Cellular Respiration

C6H12O6 + 6O2 → 6H2O + 6CO2

This is a redox rxn, glucose is oxidised, oxygen is reduced

4 main steps:

1. Glycolysis

2. Pyruvate Decarboxylation

3. Krebs Cycle (Citric Acid Cycle)

4. ETC (Oxidative Phosphorylation)

Glycolysis

-A fundamental metabolic pathway that occurs in the cytoplasm of cells in both aerobic and anaerobic conditions. It represents the first stage of cellular respiration in all living cells, where glucose (6C) is broken down into a (3C).

Consists of 10 rxns

Glucose is cleaved into two phosphorylated 3-carbon molecules, these 3-carbon molecules are then turned into Pyruvate (which also consists of 3 carbons). Glycolysis requires 2 ATP in its investment phase, and produces 4 ATP in its payoff phase. Involves phosphorylation.

Hexokinase requires ATP during glycolysis.

Location: cytosol

Inputs: Glucose, 2x ATP,

Outputs: 2x Pyruvate, 2x ATP

Produced: 2x ATP (net), 2x NADH

Pyruvate Decarboxylation

Pyruvate looses its carboxyl group and is turned into Acetyl CoA

Location: mitochondrial matrix

Inputs: 2x pyruvate

Outputs: 2x acetyl coA

Produced: 2x NADH, 2x CO2

Krebs Cycle/ Citric Acid Cycle

Acetyl CoA is combined with oxaloacetate to produce citrate which then after many steps turns back into oxaloacetate which can react with Acetyl CoA again.

Is an aerobic process

Location: mitochondrial matrix

Inputs: 2x AcetylCoA

Outputs: oxaloacetate and citrate are not really outputs, but are produced in the cycle

Produced: 6x NADH, 2x FADH2, 4x CO2, 2x ATP

ETC

NADH and FADH2 are oxidised at complex I and II (respectively) which pumps protons from matrix into the inter membrane space. These protons then flow through ATP synthase which results in ATP production through chemiosmosis. Water is also produced at complex IV where oxygen is found.

Location: inner mitochondrial membrane

Inputs: NADH, FADH2

Outputs: NAD+, FAD, ATP, H2O

Produced: 10x NAD+, 2x FAD, ATP (quantity varies) 6x H2O

Oxidative Phosphorylation

Metabolic pathway that uses energy released by oxidation of nutrients to produce ATP. Is an aerobic process. This process occurs in the mitochondria and is the primary method of generating ATP in aerobic organisms. It involves the ETC and chemiosmosis across the inner mitochondrial membrane. The ETC is composed of a series of protein complexes (I,II,III,IV) and mobile electron carriers.

Complex I

catalyses transfer of electrons from NADH to coenzyme Q (ubiquinone) reducing it to CoQH2 (ubiquinol). This process is coupled with translocation of protons (H+) from mitochondrial matrix to inter membrane

Complex II

FADH2 is oxidised in complex II, this process involves the conversion of succinate to fumarate in the Krebs cycle, where FAD is reduced to FADH2. Also transferred to coenzyme Q, reducing it to Ubiquinol. Unlike complex I, complex II doesn’t contribute to the proton gradient because it does not translocate protons across the membrane.

Complex III

transfers electrons from ubiquinol (reduced in Complexes I and II) to cytochrome c, another mobile electron carrier. During this process, complex III translocates protons from matrix to inter membrane space, further contributing to proton gradient

Complex IV

receives electrons from cytochrome c and transfers them to molecular oxygen, the final electron acceptor, reducing it to water H2O. This clears the way for more electrons to flow through the ETC. Complex IV also pumps protons across the membrane, enhancing the proton gradient.

ATP Synthase (5th complex)

Not part of the ETC, atp synthase is critical in oxidative phosphorylation. It uses the proton gradient created by the ETC to drive the synthesis of ATP from ADP and Pi. As protons flow back into the matrix through ATP synthase, the energy released is used to phosphorylate ADP, producing ATP.

Anaerobic Respiration

Fermentation itself doesn’t produce ATP, but it regenerates the NAD+ required for Glycolysis to function.

Alcohol Fermentation

1. Pyruvate is turned into acetaldehyde which then acts as an electron acceptor, which oxidises NADH into NAD+. This turns acetaldehyde into ethanol

2. Location: cytosol

3. Inputs: 2x pyruvate, 2x NADH

4. Outputs: 2x ethanol, 2x NAD+, 2x CO2

5. Produced: 2x ethanol, 2x CO2, 2x NAD+

Lactic Acid Fermentation

Pyruvate acts as an electron acceptor and is turned into lactate (lactic acid)

Location: cytosol

Inputs: 2 Pyruvate, 2x NADH

Outputs: 2x Lactate, 2x NAD+

Produced: 2x Lactate, 2x NAD+

Glucose

Stored in the body as glycogen. This is mainly stored in the liver and muscles (skeletal muscles store the majority of our glycogen. Only a small amount is stored. Glycogen holds a lot of water, so it adds extra weight.

Proves a medium amount of energy.

Fast energy source, can be used with or without oxygen

Fatty Acids

Stored as triglycerides, mostly in adipose (fat) tissue

Doesn’t hold much water → lightweight storage

Provides a high amount of energy

Slow to access, can only be used when oxygen is available

Amino Acids

Stored as proteins in the body

Provide a medium amount of calories

Have a medium amount of water weight, so they are not as ideal as fat for long term storage.

Functional tissue: proteins have many important roles, enzymes, muscles, structure etc.

Breaking them down produces ammonia (NH3), which is toxic

Beta Oxidation

Process your cells use to break down fatty acids to make ATP. Happens inside the mitochondrial matrix. Very long fatty acids first begin to broken down in peroxisomes.

B- oxidation depends on oxygen indirectly because it needs NAD+ and FAD to keep running, and those are regenerated by ETC which requires oxygen.

Lipolysis

Releasing fatty acids

Fat stored in fat cells (as triglycerides) is broken apart

This releases free fatty acids and glycerol

An enzyme called lipase helps make this happen.

Transport through the blood

The released fatty acids travel through the blood

They are carried by a protein called albumin

Entering the Cell

When the fatty acids reach a target cell (like a muscle cell), they must be “activated”, activation cost the cell 2 ATP

Moving into the mitochondria

The activated fatty acid is transported into the mitochondrial matrix, where beta oxidation occurs. To cross the inner mitochondrial membrane, the carnitine shuttle transports the activated fatty acid into the matrix.

Beta Oxidation

Cutting the fatty acid

Inside the mitochondria, the long fatty acid chain is cut into two carbon pieces

Each cut produces

Acetyl CoA (goes to Krebs cycle)

NADH

FADH2

Exception: at end of cycle when fatty acid chain is 4 carbons long, here 2 acetyl CoA are produced and only 1 NADH and FADH2

These molecules are then used to make ATP