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OCR Biology > 5.2.2 - Respiration > Flashcards

5.2.2 - Respiration Flashcards

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1
Q

Why do organisms need to respire?

A

Respiration produces ATP

ATP is used for:

  • transporting substnaces across membranes (exocytosis, active transport with carrier proteins)
  • anabolic reactions - protein synthesis, DNA synthesis
  • movement - cellular movement of chromosomes via spindle, mechanical contractyion of muscles
  • maintaining body temperature (only in mammals/birds)
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2
Q

What are some key strutures of the mitochondria?

A
  • outer membrane
  • inner membrane
  • intermembrane space
  • matrix
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3
Q

What is the outer membrane?

A
  • separates the contents of the mitochondrion from the rest of the cell, creating a cellular compartment with ideal conditions for aerobic respiration
  • smooth
  • permeable to several small molecules
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4
Q

What is the inner membrane?

A
  • folded into cristae
  • less permeable than outer membrane
  • site of electron transport chains (ETCs) and ATP synthase enzymes (used in oxidative phosphorylation)
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5
Q

What is the intermembrane space?

A
  • space beween the 2 membranes
  • has a low pH due to high concentrations of H+ ions pumped into it during oxidative phosphorylation
  • concentration gradient of H+ ions between across innner membrane essential for ATP synthesis
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6
Q

What is the matrix?

A
  • aqueous solution within the inner membranes of the mitochondrion
  • contains enzymes for the Krebs cycle and link reaction, mitochondrial DNA and ribosomes
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7
Q

What are cristae?

A
  • projections of the innermembrane of a mitochondria

* increase the surface area available for ETCs and ATP synthase, maximising products of oxidative phosphoryltion

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8
Q

What are the 4 major stages of aerobic respiration?

A

1) Glycolysis
• occurs in cytoplasm
• phosphorylation and splitting of glucose

2) Link reaction
• matrix of mitochondria
• decarboxylation and dehydrogenation of pyruvate

3) Krebs cycle
• matrix of mitochondria
•cyclical pathway with enzyme-controlled reactions

4) Oxidative phosphorylation
• inner membrane of mitochondria
• production of ATP through oxidation of hydrogen atoms

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9
Q

What is glycolysis?

A

Glycolysis is the first stage of respiration

It takes place in the cytoplasm of the cell and involves:
• Trapping glucose in the cell by phosphorylating the molecule
• Splitting the glucose molecule in two

It results in the production of
•2 Pyruvate (3C) molecules
• Net gain 2 ATP
•2 reduced NAD

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10
Q

What are the steps of glycolysis?

A

1) Phosphorylation: glucose (6C) is phosphorylated by 2 ATP to form hexose bisphosphate (6C)
2) Lysis: hexose bisphosphate (6C) splits into two molecules of triose phosphate (3C)
3) Oxidation: hydrogen is removed from each molecule of triose phosphate and transferred to coenzyme NAD to form 2 reduced NAD (NADH)
4) Dephosphorylation: phosphates are transferred from the intermediate substrate molecules to form 4 ATP through substrate-linked phosphorylation
5) Pyruvate is produced: the end product of glycolysis which can be used in the next stage of respiration

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11
Q

What is the link reaction?

A

The link reaction occurs in the mitochondrial matrix. It dehydrogenates and decarboxylates the 3C pyruvate to produce the 2C acetyl CoA that can enter the Krebs Cycle.

It produces:
• Acetyl coA
•Carbon dioxide (CO2)
• Reduced NAD (NADH)

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12
Q

How does pyruvate enter the mitochondrial matrix and what are the steps of the link reaction?

A

ENTERING THE MATRIX:
When oxygen is available pyruvate will enter the mitochondrial matrix and aerobic respiration will continue

Pyruvate moves across the double membrane of the mitochondria via active transport
• It requires a transport protein and a small amount of ATP

THE LINK REACTION:
• decarboxylation - pyruvate loses a molecule of CO2
• dehydrogenation - pyruvate reduces NAD to NADH. Acetate is produced
• acetate combines with coenzyme A to form acetyl CoA

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13
Q

What is the role of coenzyme A?

A

A coenzyme is a molecule that helps an enzyme carry out its function but is not used in the reaction itself

Coenzyme A consists of a nucleoside (ribose and adenine) and a vitamin

In the link reaction, CoA binds to the remainder of the pyruvate molecule (acetyl group 2C) to form acetyl CoA

It then supplies the acetyl group to the Krebs cycle where it is used to continue aerobic respiration

This is the stage that brings part of the carbohydrate (or lipid/amino acid) into the further stages of respiration and links the initial stage of respiration in the cytoplasm to the later stages in the mitochondria

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14
Q

What is the Krebs cycle?

A

The Krebs cycle (sometimes called the citric acid cycle) consists of a series of enzyme-controlled reactions

The Krebs cycle takes place in the matrix of the mitochondria

Each cycle produces:
•2 CO2 molecules
• 3 NADH
• 1 FADH2
• 1 ATP
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15
Q

What are the stages of the Krebs cycle?

A
  1. Acetyl CoA delivers an acetyl group to the Kreb cycle. The 2C acetyl group combines with 4C oxaloacetate to form 6C citrate.
  2. The citrate (6C) molecule undergoes decarboxylation to produce 1 CO2 and dehydrogenation to produce 1NADH. A 5C compound is made.
  3. The 5C compound undergoes further decarboxylation (producing 1 CO2), dehydrogenation reactions (2 more NADH and 1 FADH2) and substrate level phosphorylation (A phosphate is transferred from one of the intermediates to ADP, forming 1 ATP)
  4. Eventually oxaloacetate is regenerated.
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16
Q

What are the roles of coenzymes NAD and FAD?

A

Coenzymes NAD and FAD are hydrogen carriers

When hydrogen atoms become available at different points during respiration NAD and FAD accept these hydrogen atoms and are reduced
• A hydrogen atom consists of a hydrogen ion and an electron

They transfer the H atoms from the different stages of respiration to the ETC on the inner mitochondrial membrane, the site where hydrogens are removed from the coenzymes
• coenzymes are oxidised

The H+ ions and electrons are needed in the ETC for ATP synthesis

17
Q

What are the sources of FADH2 and NADH?

A

A certain amount of reduced NAD and FAD is produced during the aerobic respiration of a single glucose molecule

Reduced NAD:
• 2 x 1 = 2 from Glycolysis
•2 x 1 = 2 from the Link Reaction
•2 x 3 = 6 from the Krebs cycle

Reduced FAD:
•2 x 1 = 2 from the Krebs cycle

18
Q

What is oxidative phosphorylation?

A

The last stage of aerobic respiration

It takes place at the inner mitochondrial membrane

It results in the production of many molecules of ATP and the production of water from oxygen

The current model for it is the chemiosmotic theory

19
Q

What is the process of oxidative phosphorylation?

A

1) Hydrogen atoms are donated by reduced NAD (NADH) and reduced FAD (FADH2) from the Krebs Cycle
2) Hydrogen atoms split into protons (H+ ions) and electrons
3) The high energy electrons enter the electron transport chain and release energy as they move through
4) The released energy is used to transport protons across the inner mitochondrial membrane from the matrix into the intermembrane space. A concentration gradient of protons is established between the intermembrane space and the matrix
5) The protons return to the matrix via facilitated diffusion through the channel protein ATP synthase - chemiosmosis
6) The movement of protons down their concentration gradient provides energy to ATP synthase, resulting in the phosphorylation of ADP to ATP
7) Oxygen acts as the ‘final electron acceptor’ and combines with protons and electrons at the end of the electron transport chain to form water

20
Q

What is the electron transport chain (ETC)?

A

The electron transport chain is made up of a series of membrane proteins/ electron carriers

They are positioned close together which allows the electrons to pass from carrier to carrier

The inner membrane of the mitochondria is impermeable to hydrogen ions so these electron carriers are required to pump the protons across the membrane to establish the concentration gradient

The highly folded mitochondrial cristae increases the surface area available, maiming more ETCs can fit inside each mitochondrion

21
Q

Why is oxygen so important for aerobic respiration?

A

Oxygen acts as the final electron acceptor.

Without oxygen, there is no final acceptor of electrons from the electron transport chain
The electron transport chain stops functioning
No more ATP is produced via oxidative phosphorylation
Reduced NAD and FAD aren’t oxidised by an electron carrier
No oxidised NAD and FAD are available for dehydrogenation in the Krebs cycle
The Krebs cycle stops

22
Q

What is anaerobic respiration?

A

Some cells are able to oxidise the reduced NAD produced during glycolysis so it can be used for further hydrogen transport

This means that glycolysis can continue and small amounts of ATP are still produced

Different cells use different pathways to achieve this:
• Yeast and microorganisms use ethanol fermentation
•Other microorganisms and mammalian muscle cells use lactate fermentation

23
Q

What is ethanol fermentation?

A

Reduced NAD transfers its hydrogens (is oxidised) to ethanal to form ethanol

1) pyruvate is decarboxylated to ethanal, producing CO2
2) ethanal is reduced to ethanol by the enzyme alcohol dehydrogenase, oxidising NADH to NAD in the process. Ethanal is the hydrogen acceptor
3) Ethanol cannot be further metabolised; it is a waste product

24
Q

What is lactate fermentation?

A

In this pathway reduced NAD transfers its hydrogens to pyruvate to form lactate

Pyruvate is reduced to lactate by enzyme lactate dehydrogenase, oxidising NADH to NAD in the process
•pyruvate is the hydrogen acceptor

The final product lactate can be further metabolised

25
Q

How is lactate metabolised after anaerobic respiration?

A

After lactate is produced two things can happen
• It can be oxidised back to pyruvate which is then channelled into the Krebs cycle for ATP production
• It can be converted into glycogen for storage in the liver

The oxidation of lactate back to pyruvate needs extra oxygen
•This extra oxygen is referred to as an oxygen debt
•It explains why animals breathe deeper and faster after exercise

26
Q

Why is the energy yield of anaerobic respiration much lower than aerobic respiration?

A

In anaerobic respiration glucose, is only partially oxidised meaning only some of its chemical potential energy is released and transferred to ATP
• The only ATP producing reaction that continues is glycolysis (~2 ATP)

As there is no oxygen to act as the final electron acceptor none of the reactions within the mitochondria can take place
• The stages that take place inside the mitochondria produce much more ATP than glycolysis alone (~36 ATP)

27
Q

What other respiratory substrates might be used if glucose runs out?

A

Other carbohydrates - 15.8 kJ per gram
Lipids - 39.4 kJ per gram
Proteins - 17.0 kJ per gram

Amino acids are only respired aerobically when all other substrates have been used up - this is because they have essential functions elsewhere in the cell (structural roles eg cytoskeleton, functional roles eg enzymes)

28
Q

Why do different respiratory substrates have different energy values?

A

The more H atoms that become available when the substrate is broken down, the more energy is released
• more H atoms in a molecule means more H atoms transported by NAD and FAD to mitochondrial membrane
• there is a greater proton gradient across the inner membrane, so more ATP produced via chemiosmosis

Fatty acids are made of long hydrocarbon chains, releasing lots of H atoms when broken down. Therefore, it has a very high energy value

29
Q

What is the respiratory quotient (RQ)?

A

The ratio of CO2 produced to O2 taken in during respiration

RQ = (moles/molecules of CO2) ÷ (moles/molecules of O2)

RQ of different substrates is different:
•carbohydrates = 1
• lipids = 0.7
• proteins = 0.9

For anaerobic respiration:
• mammals - cant be calculated bcs no oxygen taken in and no CO2 produced
•yeast - tends towards infinity bcs no oxygen taken in but CO2 produced

The RQ can also give an indication of under or overfeeding under normal conditions:
•An RQ value of more than 1 suggests excessive carbohydrate/calorie intake
•An RQ value of less than 0.7 suggests underfeeding

OCR Biology (25 decks)

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