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IB Bio > 8.2 > Flashcards

8.2 Flashcards

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1
Q

One molecule of ATP contains…

A

three covalently bonded phosphate groups – which store potential energy in their bonds

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2
Q

Phosphorylation makes molecules…

A

less stable and hence ATP is a readily reactive molecule that contains high energy bonds

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3
Q

When ATP is hydrolysed (to form ADP + Pi), the energy stored in the terminal phosphate bond is…

A

released for use by the cell

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4
Q

ATP has two key functions within the cell:

A
  • It functions as the energy currency of the cell by releasing energy when hydrolysed to ADP (powers cell metabolism)
  • It may transfer the released phosphate group to other organic molecules, rendering them less stable and more reactive
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5
Q

ATP is synthesised from ADP using energy derived from one of two sources:

A
  • Solar energy – photosynthesis converts light energy into chemical energy that is stored as ATP
  • Oxidative processes – cell respiration breaks down organic molecules to release chemical energy that is stored as ATP
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6
Q

Cell respiration is the controlled release of energy from organic compounds to produce ATP

A
  • Anaerobic respiration involves the incomplete breakdown of organic molecules for a small yield of ATP (no oxygen required)
  • Aerobic respiration involves the complete breakdown of organic molecules for a larger yield of ATP (oxygen is required)
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7
Q

The breakdown of organic molecules occurs via a number of linked processes that involve a number of discrete steps:

A
  • By staggering the breakdown, the energy requirements are reduced (activation energy can be divided across several steps)
  • The released energy is not lost – it is transferred to activated carrier molecules via redox reactions (oxidation / reduction)
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8
Q

When organic molecules are broken down by cell respiration, the chemical energy is transferred by means of…

A

redox reactions
- Redox reactions involved the reduction of one chemical species and the oxidation of another (redox = reduction / oxidation)

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9
Q

Most redox reactions typically involve the transfer of electrons, hydrogen or oxygen

A
  • Reduction is the gain of electrons / hydrogen or the loss of oxygen
  • Oxidation is the loss of electrons / hydrogen or the gain of oxygen
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10
Q

Redox Mnemonics

A

LEO goes GER – Loss of Electrons is Oxidation ; Gain of Electrons is Reduction

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11
Q

Cell respiration breaks down organic molecules and transfers hydrogen atoms and electrons to carrier molecules

A
  • As the organic molecule is losing hydrogen atoms and electrons, this is an oxidation reaction
  • Energy stored in the organic molecule is transferred with the protons and electrons to the carrier molecules
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12
Q

The carrier molecules are called hydrogen carriers or electron carriers, as they gain electrons and protons (H+ ions)

A
  • The most common hydrogen carrier is NAD+ which is reduced to form NADH (NAD+ + 2H+ + 2e– → NADH + H+)
  • A less common hydrogen carrier is FAD which is reduced to form FADH2 (FAD + 2H+ + 2e– → FADH2)
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13
Q

The hydrogen carriers function like taxis, transporting the electrons (and hydrogen ions) to the cristae of the mitochondria

A
  • The cristae is the site of the electron transport chain, which uses the energy transferred by the carriers to synthesize ATP
  • This process requires oxygen to function, and hence only aerobic respiration can generate ATP from hydrogen carriers
  • This is why aerobic respiration unlocks more of the energy stored in the organic molecules and produces more ATP
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14
Q

The first step in the controlled breakdown of carbohydrates is glycolysis, which occurs in the cytosol of the cell

A

In glycolysis, a hexose sugar (6C) is broken down into two molecules of pyruvate (3C)

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15
Q

first step of glycolysis: Phosphorylation

A
  • A hexose sugar (typically glucose) is phosphorylated by two molecules of ATP (to form a hexose bisphosphate)
  • This phosphorylation makes the molecule less stable and more reactive, and also prevents diffusion out of the cell
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16
Q

second step of glycolysis: Lysis

A
  • The hexose biphosphate (6C sugar) is split into two triose phosphates (3C sugars)
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17
Q

third step of glycolysis: Oxidation

A
  • Hydrogen atoms are removed from each of the 3C sugars (via oxidation) to reduce NAD+ to NADH (+ H+)
  • Two molecules of NADH are produced in total (one from each 3C sugar)
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18
Q

fourth step of glycolysis: ATP formation

A
  • Some of the energy released from the sugar intermediates is used to directly synthesise ATP
  • This direct synthesis of ATP is called substrate level phosphorylation
  • In total, 4 molecules of ATP are generated during glycolysis by substrate level phosphorylation (2 ATP per 3C sugar)
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19
Q

At the end of glycolysis, the following reactions have occurred:

A
  • Glucose (6C) has been broken down into two molecules of pyruvate (3C)
  • Two hydrogen carriers have been reduced via oxidation (2 × NADH + H+)
  • A net total of two ATP molecules have been produced (4 molecules were generated, but 2 were used)
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20
Q

glycose is an…

A

Glycolysis occurs in the cytosol and does not require oxygen (it is an anaerobic process)

21
Q

Depending on the availability of oxygen, the pyruvate may be subjected to one of two alternative processes:

A
  • Aerobic respiration occurs in the presence of oxygen and results in the further production of ATP (~ 34 molecules)
  • Anaerobic respiration (fermentation) occurs in the absence of oxygen and no further ATP is produced
22
Q

Aerobic Respiration

A
  • If oxygen is present, the pyruvate is transported to the mitochondria for further breakdown (complete oxidation)
  • This further oxidation generates large numbers of reduced hydrogen carriers (NADH + H+ and FADH2)
  • In the presence of oxygen, the reduced hydrogen carriers can release their stored energy to synthesise more ATP
  • Aerobic respiration involves three additional processes – the link reaction, krebs cycle and the electron transport chain
23
Q

Anaerobic Respiration (Fermentation)

A
  • If oxygen is not present, pyruvate is not broken down further and no more ATP is produced (incomplete oxidation)
  • The pyruvate remains in the cytosol and is converted into lactic acid (animals) or ethanol and CO2 (plants and yeast)
  • This conversion is reversible and is necessary to ensure that glycolysis can continue to produce small quantities of ATP
24
Q

Anaerobic Respiration

A
  • Glycolysis involves oxidation reactions that cause hydrogen carriers (NAD+) to be reduced (becomes NADH + H+)
  • Typically, the reduced hydrogen carriers are oxidised via aerobic respiration to restore available stocks of NAD+
  • In the absence of oxygen, glycolysis will quickly deplete available stocks of NAD+, preventing further glycolysis
  • Fermentation of pyruvate involves a reduction reaction that oxidises NADH (releasing NAD+ to restore available stocks)
  • Hence, anaerobic respiration allows small amounts of ATP to be produced (via glycolysis) in the absence of oxygen
25
Q

The first stage of aerobic respiration is the…

A

link reaction, which transports pyruvate into the mitochondria
- Aerobic respiration uses available oxygen to further oxidise the sugar molecule for a greater yield of ATP

26
Q

link reaction

A

The link reaction is named thus because it links the products of glycolysis with the aerobic processes of the mitochondria

  • Pyruvate is transported from the cytosol into the mitochondrial matrix by carrier proteins on the mitochondrial membrane
  • The pyruvate loses a carbon atom (decarboxylation), which forms a carbon dioxide molecule
  • The 2C compound then forms an acetyl group when it loses hydrogen atoms via oxidation (NAD+ is reduced to NADH + H+)
  • The acetyl compound then combines with coenzyme A to form acetyl coenzyme A (acetyl CoA)
27
Q

As glycolysis splits glucose into two pyruvate molecules, the link reaction occurs…

A

twice per molecule of glucose

- Per glucose molecule, the link reaction produces acetyl CoA (×2), NADH + H+ (×2) and CO2 (×2)

28
Q

The second stage of aerobic respiration is the

A

Krebs cycle, which occurs within the matrix of the mitochondria
- The Krebs cycle is also commonly referred to as the citric acid cycle or the tricarboxylic acid (TCA) cycle

29
Q

In the Krebs cycle, acetyl CoA transfers its…

A

acetyl group to a 4C compound (oxaloacetate) to make a 6C compound (citrate)
- Coenzyme A is released and can return to the link reaction to form another molecule of acetyl CoA

30
Q

Over a series of reactions, the 6C compound is broken down to reform the original 4C compound (hence, a cycle)

A
  • Two carbon atoms are released via decarboxylation to form two molecules of carbon dioxide (CO2)
  • Multiple oxidation reactions result in the reduction of hydrogen carriers (3 × NADH + H+ ; 1 × FADH2)
  • One molecule of ATP is produced directly via substrate level phosphorylation
31
Q

As the link reaction produces two molecules of…

A 
acetyl CoA (one per each pyruvate), the Krebs cycle occurs twice
- Per glucose molecule, the Krebs cycle produces:  4 × CO2  ;  2 × ATP  ;  6 × NADH + H+  ;  2 × FADH2
32
Q

The final stage of aerobic respiration is the…

A

electron transport chain, which is located on the inner mitochondrial membrane
- The inner membrane is arranged into folds (cristae), which increases the surface area available for the transport chain

33
Q

The electron transport chain releases the energy stored within the reduced hydrogen carriers in order to synthesise ATP. This is called…

A

oxidative phosphorylation, as the energy to synthesise ATP is derived from the oxidation of hydrogen carrier

34
Q

Oxidative phosphorylation occurs over a number of distinct steps:

A

Proton pumps create an electrochemical gradient (proton motive force)
ATP synthase uses the subsequent diffusion of protons (chemiosmosis) to synthesise ATP
Oxygen accepts electrons and protons to form water

35
Q

Step 1: Generating a Proton Motive Force

A
  • The hydrogen carriers (NADH and FADH2) are oxidised and release high energy electrons and protons
  • The electrons are transferred to the electron transport chain, which consists of several transmembrane carrier proteins
  • As electrons pass through the chain, they lose energy – which is used by the chain to pump protons (H+ ions) from the matrix
  • The accumulation of H+ ions within the intermembrane space creates an electrochemical gradient (or a proton motive force)
36
Q

Step Two: ATP Synthesis via Chemiosmosis

A
  • The proton motive force will cause H+ ions to move down their electrochemical gradient and diffuse back into matrix
  • This diffusion of protons is called chemiosmosis and is facilitated by the transmembrane enzyme ATP synthase
  • As the H+ ions move through ATP synthase they trigger the molecular rotation of the enzyme, synthesising ATP
37
Q

Step Three: Reduction of Oxygen

A
  • In order for the electron transport chain to continue functioning, the de-energised electrons must be removed
  • Oxygen acts as the final electron acceptor, removing the de-energised electrons to prevent the chain from becoming blocked
  • Oxygen also binds with free protons in the matrix to form water – removing matrix protons maintains the hydrogen gradient
  • In the absence of oxygen, hydrogen carriers cannot transfer energised electrons to the chain and ATP production is halted
38
Q

Summary: Oxidative Phosphorylation

A
  • Hydrogen carriers donate high energy electrons to the electron transport chain (located on the cristae)
  • As the electrons move through the chain they lose energy, which is transferred to the electron carriers within the chain
  • The electron carriers use this energy to pump hydrogen ions from the matrix and into the intermembrane space
  • The accumulation of H+ ions in the intermembrane space creates an electrochemical gradient (or a proton motive force)
  • H+ ions return to the matrix via the transmembrane enzyme ATP synthase (this diffusion of ions is called chemiosmosis)
  • As the ions pass through ATP synthase they trigger a phosphorylation reaction which produces ATP (from ADP + Pi)
  • The de-energised electrons are removed from the chain by oxygen, allowing new high energy electrons to enter the chain
  • Oxygen also binds matrix protons to form water – this maintains the hydrogen gradient by removing H+ ions from the matrix
39
Q

Aerobic respiration involves the breakdown of glucose in the presence of oxygen to produce water and carbon dioxide

A
  • It requires the involvement of mitochondria and generates a large yield of ATP (typically 36 ATP per glucose consumed)
  • Aerobic respiration may include the following processes: glycolysis, link reaction, krebs cycle, electron transport chain
40
Q

Types of Aerobic Reactions

A

Aerobic respiration involves three main types of reactions – decarboxylation, oxidation and phosphorylation

41
Q

Decarboxylation:

A
  • Carbon atoms are removed from the organic molecule (glucose) to form carbon dioxide
  • Aerobic respiration involves the complete combustion of glucose (6C) – so six CO2 molecules are produced
42
Q

Oxidation:

A
  • Electrons and hydrogen ions are removed from glucose and taken up by hydrogen carriers (NADH and FADH2)
  • The hydrogen carriers are in turn oxidised at the electron transport chain (where the energy is used to make ATP)
  • The electrons and hydrogen ions are then taken up by oxygen (reduction) to form water molecules
  • Twelve hydrogen carriers are produced and so six oxygen molecules are required (12 × O = 6 × O2)
43
Q

Phosphorylation:

A
  • Energy released from the breakdown of glucose is used to phosphorylate ADP to make ATP
  • A net total of four ATP molecules are produced directly via substrate level phosphorylation
  • The remaining ATP is produced indirectly via the electron transport chain (oxidative phosphorylation)
44
Q

Aerobic respiration typically produces a net total of…

A

36 ATP per molecule of glucose consumed

  • A net total of 2 ATP are produced in glycolysis via substrate level phosphorylation (four are produced, but two are consumed)
  • A further 2 ATP are similarly produced in the Krebs cycle (one ATP per cycle – two cycles occur per glucose molecule)
  • Lastly, 32 ATP are produced in the electron transport chain using energy from hydrogen carriers (oxidative phosphorylation)
45
Q

Hydrogen carriers produce different amounts of ATP depending on where they donate electrons to the transport chain

A
  • NADH molecules located in the matrix donate electrons to the start of the chain and produce 3 ATP per hydrogen carrier
  • Cytosolic NADH (from glycolysis) donate electrons later in the chain and only produce 2 ATP per hydrogen carrier
  • FADH2 also donates electrons later in the chain and so only produce 2 ATP per hydrogen carrier
46
Q

Oxidative phosphorylation:

A

(8 × matrix NADH) + (2 × cytosolic NADH) + (2 × FADH2) = (8 × 3) + (2 × 2) + (2 × 2) = 32 ATP

47
Q

Mitochondria are the ‘powerplants’ of the cell – synthesising large amounts of ATP via aerobic respiration

A

All eukaryotic cells possess mitochondria – aerobic prokaryotes use the cell membrane to perform oxidative phosphorylation

48
Q

Mitochondria are thought to have once been independent prokaryotes that were internalised by eukaryotes via endosymbiosis

A
  • They have a double membrane structure (due to vesicular coating as part of the endocytotic process)
  • They have their own DNA (circular and naked) and ribosomes (70S)
  • Their metabolic processes are susceptible to certain antibiotics
49
Q

The structure of the mitochondrion is adapted to the function it performs:

A
  • Outer membrane – the outer membrane contains transport proteins that enable the shuttling of pyruvate from the cytosol
  • Inner membrane – contains the electron transport chain and ATP synthase (used for oxidative phosphorylation)
  • Cristae – the inner membrane is arranged into folds (cristae) that increase the SA:Vol ratio (more available surface)
  • Intermembrane space – small space between membranes maximises hydrogen gradient upon proton accumulation
  • Matrix – central cavity that contains appropriate enzymes and a suitable pH for the Krebs cycle to occur

IB Bio (18 decks)

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