Cellular respiration - detailed PDF

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Summary

Biology HL
Topic 8.2
Cell respiration, glycolysis, link reaction, electron transport chain, chemiosmosis, Krebs cycle, oxidation and reduction...

Description

Cell respiration

Topic 8.2

Biology HL

Cellular respiration and mitochondria ➢ Cell respiration is the most important catabolic process in life ➢ It is the process by which ATP is provided to the organism so that it can live ○

It is a very complex series of chemical reactions, most of which occur in the mitochondria

Feature

Role

Outer membrane

Separates the contents of mitochondria from the rest of the cell

Matrix

Contains the enzymes for the link reaction and the Krebs cycle

Cristae

Increase the surface area for oxidative phosphorylation

Inner membrane

Contains the carriers for the ETC and ATP synthase for chemiosmosis

Intermembrane space

Contains hydrogen ions/protons

➢ Mitochondria can be seen using microscopes of dierent technologies ➢ We have seen electron micrographs made by electron tomography ○

A method to get a three-dimensional image of an active mitochondrion

Oxidation and reduction ➢ cell respiration contains many oxidation and reduction reactions ○

There 2 reactions occur together during chemical reactions

○

Called redox reactions for short

Oxidation

Reduction

Loss of electrons

Gain of electrons

Gain of oxygen

Loss of oxygen

Loss of hydrogen

Gain of hydrogen

Results in many C-O bonds

Results in many C-H bonds

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Cell respiration

Topic 8.2

Biology HL

Results in a compound with lower

Results in a compound with higher

potential energy

potential energy

➢ The reduced form of a molecule always has more potential energy than the oxidized form of the molecule ➢ Redox reactions play a key role in the flow of energy through living systems ○

Because the electrons that are flowing from one molecule to the next are carrying energy with them

Example:

C6H12O6 + 6O2 ⇒ 6CO2 + 6H2O + energy

○ ○

C6H12O6 was oxidized since it lost hydrogen atoms to form carbon dioxide 6O2 was reduced since it gained hydrogen atoms to form water

Glycolysis ➢ Uses no oxygen and occurs in the cytoplasm ○

No organelles are required

○

It proceeds eciently in both aerobic and anaerobic environments

➢ Occurs in both prokaryotic and eukaryotic cells ➢ Steps: ○

Two molecules of ATP are used to begin glycolysis ■

The phosphates from the ATPs are added to glucose to form another sugar

○

Less stable molecule

●

The process is called phosphorylation

The less stable 6-carbon sugar is split into two 3-carbon sugars ■

○

●

This splitting process is known as lysis

These sugars enter an oxidation phase involving ATP formation and the production of the reduced coenzyme NAD ■

Each of those phosphate molecules undergoes oxidation to form a +

reduced NAD which becomes NADH ●

Energy is released and it is used to add an inorganic phosphate to the 3-carbon compound

○

Enzymes remove the phosphate groups so that they can be added to ADP to produce ATP

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Cell respiration

Topic 8.2

■

Biology HL

The end result is the formation of 4 molecules of ATP, 2 molecules of NADH and 2 molecules of pyruvate (3-carbon)

➢ Pyruvate is the ionized form of pyruvic acid ➢ Once pyruvate is obtained, the next pathway is determined by the presence of oxygen ○

If oxygen is present, pyruvate enters the mitochondria and aerobic respiration occurs

○

If not, anaerobic respiration occurs in the cytoplasm ■

Later, pyruvate is converted to lactase in animals and ethanol and carbon dioxide in plants

➢ High levels of ATP in the cytoplasm will inhibit the first enzyme in the pathway by end-product inhibition ➢ The way of producing ATP in glycolysis is called substrate-level phosphorylation , because the phosphate group is transferred directly to ADP from the original phosphate-bearing molecule

The link reaction ➢ Pyruvate enters the matrix of the mitochondria via active transport

➢ Link reaction is the first process that occurs in the mitochondria, that is decarboxylation

➢ Decarboxylation is the removal of a carbon atom ➢ Pyruvate is decarboxylated and a 2-carbon acetyl group is formed ○

The removed carbon is released as carbon dioxide +

➢ The acetyl group is oxidised by NAD giving another NADH ➢ The acetyl group combines with coenzyme A (CoA) to form acetyl-CoA ➢ The link reaction is controlled by a system of enzymes ➢ Acetyl CoA enters the Krebs cycle to continue the aerobic respiration process ➢ Acetyl CoA can be produced from most carbohydrates and lipids ➢ Acetyl CoA can be synthesized into a lipid for storage purposes when ATP levels in the cell are high

Krebs cycle ➢ If cellular ATP levels are low, the acetyl CoA enters the Krebs cycle ○

The cycle is also called tricarboxylic acid cycle and is a cycle because it begins and ends with the same substance

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Cell respiration

Topic 8.2

Biology HL

➢ Steps: ○

Acetyl CoA combines with a 4-carbon compound called oxaloacetate. ■

○

A 6-carbon compound is formed

The 6-carbon compound is oxidized to form a 5-carbon compound. ■

The lost carbon is combined with oxygen and leaves the cell as carbon dioxide .

■

+

While the 6-carbon compound is oxidized, NAD is reduced to form NADH

○

The 5-carbon compound is oxidized and decarboxylated to form a 4-carbon compound

○

■

The carbon lost is released as carbon dioxide

■

Another NAD is reduced to form NADH

+

The 4-carbon compound undergoes various changes resulting in several products ■

One product is another NADH

■

The coenzyme FAD is reduced to form FADH2

■

ADP is reduced to form ATP

■

The 4-carbon compound is changed to reform the starting compound of the cycle

○

The cycle begins again

➢ The Krebs cycle will run twice for each glucose molecule entering cell respiration ○

Because a glucose forms 2 pyruvate molecules and each pyruvate produces 1 acetyl CoA

➢ Products of Krebs cycle ○

2 ATP molecules per glucose molecule

○

6 molecules of NADH per glucose molecule

○

2 molecules of FADH2 per glucose molecule

○

4 molecules of CO2 per glucose molecule

➢ Two CO2 are released for each glucose molecule during the link reaction. 4 CO2 are released during the Krebs cycle ○

These account for all 6 carbon atoms that were present in the initial glucose ■

Glucose is completely catabolized and its original energy is now carried by NADH and FADH2 or ATP

➢ Until now, a net gain of 4 ATPs has been produced by substrate-level phosphorylation ○

We obtain the remaining 32 ATPs through oxidative phosphorylation 4

Cell respiration

Topic 8.2

Biology HL

Electron transport chain ➢ During this process most of the ATPs are produced ➢ It is the first step of cell respiration where oxygen is needed ➢ It occurs on the inner mitochondrial membrane and on the membranes of the cristae ○

On these membranes there are molecules that are easily reduced and oxidized ■

○

The haem group of the carrier is that part

These carriers of electrons (energy) pass the electrons from one to another due to an energy gradient ■

Each carrier molecule has a slightly dierent electronegativity and thus dierent attraction for electrons ●

The receiving molecule has a higher electronegativity = stronger attraction of e-

■

These protein carriers with haem groups are referred to as

cytochromes ➢ Small amounts of energy are released ➢ The sources of the electrons are the coenzymes NADH and FADH2 ○

They carry a lot of potential energy

➢ Sequential redox reactions will occur slowly harnessing the potential energy ➢ FADH2 enters the ETC at a lower energy level than NADH, which means that it allows the production of 2ATP instead of 3ATP ➢ Last in the chain lies an oxygen atom which is ionized by the electron and later with 2 hydrogen ions from the surroundings to form water ○

Water of metabolism

➢ No ATPs are produced directly by the ETC ○

However this chain is essential to chemiosmosis, which does produce ATP

Chemiosmosis +

+

➢ When NADH and FADH2 are oxidized, they form NAD and FAD releasing a lot of hydrogen ions ➢ The energy released in the ETC fuels pumps that pump hydrogen ions into the intermembrane space +

➢ Chemiosmosis involves the movement of protons (H ) to provide energy so that phosphorylation can occur ○

Allows addition of phosphate and energy to ADP

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Cell respiration

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Topic 8.2

Biology HL

Chemiosmosis is an oxidative phosphorylation , as it utilizes an ETC

➢ The protons will try to move back across the membrane to level out the concentration gradient, but they are stopped by the membrane ○

Because of the hydrophilic region of the membrane, protons can only pass through the ATP synthase

➢ ATP synthase is a protein that harness the energy of moving protons to phosphorylate an ADP to ATP ○

ATP synthase lies on the inner membrane

➢ When protons enter the matrix again, they will react with a newly formed oxygen ion to form water maintaining the gradient

Production of ATP - summary

Process

ATP used

ATP produced

Net ATP gain

Glycolysis

2

4

2

Krebs cycle

0

2

2

ETC +

0

32

32

2

38

36

chemiosmosis Total

➢ In reality the number is closer to 30, because: ○

Some protons move back to the matrix without going through the ATP synthase channel

○

Some of the energy of the proton movement is used to transport pyruvate into the mitochondria

➢ The 30 ATPs account for approximately 30% of the energy present in the chemical bonds of glucose

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