ATP and Respiration2017

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What the syllabus says:4.1 Importance of ATP

a) The importance of chemical energy in biological processes. The central role of ATP as an energy carrier and its use in the liberation of energy for cellular activity. Structure of ATP.

b) The synthesis of ATP by means of a flow of protons through the enzyme ATP synthetase. Chemiosmosis and electrochemical gradient. The similarity between mitochondrial and chloroplast membrane function in providing a proton gradient for ATP synthesis.

c) The maintenance of the proton gradient by proton pumps driven by electron energy. The alternate arrangement of pumps and electron carriers to form the electron transport chain. (name of proton pumps and electron carriers in the electron transport system are not required).

4.2 Respiration releases chemical energy from organic molecules in order to synthesise ATP for the maintenance of life.

a) All living organisms carry out respiration in order to provide energy in the cell.

b) Glycolysis as a source of triose phosphate, pyruvate, ATP and reduced NAD. The formation of acetyl CoA.

c) The Krebs cycle as a means of liberating energy form carbon bonds to provide ATP and reduced NAD with release of carbon dioxide. The role of reduced NAD as a source of electrons and protons for the electron transport system.

d) The energy budget of the breakdown of glucose under aerobic and anaerobic conditions. Fat and amino acid utilization.

Suggested practical activities:

Demonstration of dehydrogenase activity using artificial hydrogen acceptors, as illustrated by methylene blue or DCPIP.

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ATPRespiration is the term that describes the metabolic (reaction) pathways that lead to the production of excess ATP in all cells.

Anaerobic respiration takes place in the cytoplasm and yields a net gain of 2ATP for each molecule of glucose entering the reactionsAerobic respiration starts in the cytoplasm and carries on in the mitochondria, yielding 38 ATP for each molecule of glucose that enters the reactions.

ATP is a nucleotide consisting of the nitrogenous base ADENINE, the pentose sugar RIBOSE and 3 PHOSPHATE groups.

It can be broken down into ADP, a process that releases energy.The energy released can be used to fuel reactions in cells.

The reactions that link ATP and ADP+Pi together are:

(ATPase)ATP ADP + Pi + Energy (30.6 kJ mole-1)

(ATPsynthetase)

The breakdown of ATP to ADP and Pi releases energy for reactions

ATP is called a UNIVERSAL ENERGY CURRENCY because it can be used to supply energy for ALL reactions, to ALL cells in ALL organisms.

If energy is available ADP and Pi can be joined together to make ATP

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Adenine

PhosphateRibose

ATP has a number of advantages or importances as a source of energy: It is soluble and can be transported It is easily transported across membranes It is easily hydrolysed to release ADP, Pi and energy Only one enzyme is needed to break ATP down The energy is released in useable quantities When energy is released from reactions or by proton gradients it can be regenerated

from ADP and Pi

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Energy can be used for:

Active transportProtein synthesis Nerve impulsesMuscle contractionThe Calvin cycle in photosynthesisStarting glycolysis

Enzyme: ATPase

Energy comes from:

Proton gradients across the cristae or thylakoid membranesReactions in metabolic pathways

Enzyme: ATP synthetase

ATP synthetase:

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The enzyme ATP synthetase is a large protein that spans the membrane.It is visible on electron micrographs as a ‘stalked particle’

ATP synthetase is embedded in the inner mitochondrial membrane (cristae) and embedded in the thylakoid membranes of chloroplasts.

ATP synthetase synthesises ATP using the energy from a gradient of protons (H+)

As protons diffuse through ATP synthetase energy is released that is used to join ADP and Pi together.

The diffusion of protons coupled with ATP synthesis is called CHEMIOSMOSIS

ATP synthetase

How proton gradients are generated:

Thylakoids and Cristae:

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Protons are pumped across the membrane in which the ATP synthetase is embedded

The energy to do this comes from high energy electrons generated during the processes of respiration and photosynthesis.

As the protons are positively charged the gradient is termed an electrochemical gradient

ATP synthetase

Cristae membranes:

There are three proton pumpsThe high energy electrons are generated from reduced coenzymes made during the reactions of respirationOxygen is used as the final electron acceptor and water is made

Thylakoid membranes:

There is one proton pumpThe high energy electrons are generated as chlorophyll a absorbs light energyOxygen is generated from the splitting of water

In both cases the protons are pumped from the fluid-filled cavity in which the membranes of the organelle are found; ie the matrix and stroma. The protons are pumped into a space enclosed by membrane; the intermembrane space in mitochondria and the inner thylakoid space in chloroplasts. Both membranes contain small mobile electron carrier molecules. These accept the high energy electrons and the energy released is used to fuel the proton pumps.

Proton gradients are maintained continually across the thylakoid and cristae membranes.

High energy electrons are passed along the membranes from carrier to carrier – this is called an electron transport chain. Proton pumps are transmembrane (intrinsic) proteins that are embedded in the membrane and accept the high energy electrons from the carriers and utilise the energy to pump protons across the membrane.

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Aerobic respiration

Aerobic respiration describes a series of enzyme controlled reactions that break down glucose to release energy that is used to synthesise ATP. Cellular processes utilise the energy released from the breakdown of ATP. Some of the energy that is released is released as heat. Aerobic respiration has 4 stages:

Name of stage Location of reactions ATP yield from one molecule of glucose

Glycolysis cytoplasm 2 ATP – net gainLink reaction Matrix of mitochondriaKrebs cycle Matrix of mitochondria 2 ATPElectron transfer chain Cristae of mitochondria 34 ATP

Let’s remember module 1:

Mitochondrion structure –

Mitochondria are only required for aerobic respiration.

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Inner membraneInter membrane space

ATP synthetase/Stalked particle

Glycolysis

Glucose

Hexose diphosphate

2 triose phosphate

2 pyruvate

Dehydrogenase enzymes remove the hydrogen from the triose phosphate, this is a dehydrogenation reaction; the hydrogen is transferred to the carrier NAD producing reduced NAD

As 4 ATP are produced and 2 are used at the beginning the overall gain – NET GAIN is 2 ATP

Substrate level phosphorylation is where energy is released directly from reactions and used to synthesise ATP

Oxidative phosphorylation is where energy is released from a proton gradient across a membrane to manufacture ATP – the process uses oxygen

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2 ATP

2 NAD 2 Reduced NAD

4 ATP4 ADP + 4 Pi

Enzyme = dehydrogenase

NET GAIN = 2 ATP

These reactions happen in the cytoplasm

2 ATP activate the glucose

Splits

6C

3C

3CATP is synthesised using energy released from reactions – SUBSTRATE LEVEL PHOSPHORYLATION

Link reaction

Pyruvate produced in glycolysis enters the mitochondria by facilitated diffusion IF oxygen is PRESENT.

Pyruvate

Acetate

Acetyl coenzyme A

Dehydrogenase enzyme removes hydrogen from pyruvate, a dehydrogenation reaction; and transfers it to the carrier NAD, which is reducedDecarboxylation is the removal of carbon dioxide from a molecule catalysed by decarboxylase enzymes

Coenzyme A aids the entry of the acetate portion into the reactions of Kreb’s cycle.

This reaction occurs twice for each molecule of glucose because the product of glycolysis is 2 pyruvate.

2 reduced NAD are produced per molecule of glucose

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NAD Reduced NAD

Enzyme = dehydrogenase

3 C

2C

CO2

Coenzyme A

These reactions take place in the mitochondrial matrix

This is a decarboxylation reaction

Enzyme = decarboxylase

Kreb’s cycle

Acetyl Coenzyme A

Acetate

This cycle happens twice for each molecule of glucose.

6 reduced NAD2 reduced FAD are produced per molecule of glucose2 ATP

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Coenzyme A

2CC

5C

6C4C

CO2

Reduced NAD

ATP

Reduced NAD

Reduced NAD

Reduced FAD

CO2

Kreb’s Cycle

ATP is synthesised using energy released from reactions – SUBSTRATE LEVEL PHOSPHORYLATION

Electron transfer chain:

Reduced NAD formed during all the stages of aerobic respiration diffuses to the cristae of the mitochondria. Reduced NAD splits into NAD, high energy electrons and protons.

The high energy electrons are passed through electron carriers along the membrane; as they pass through the proton pumps energy is released from the electrons and used to fuel three proton pumps. Three protons are pumped into the intermembrane space. This results in an electochemical gradient, which causes protons to flow through the stalked particles (ATP synthetase). ATP synthetase synthesises ATP as the protons pass through. This process is termed chemiosmosis.

Each reduced NAD fuels THREE proton pumps and provides energy to synthesise THREE ATP.

Reduced FAD formed in Kreb’s cycle splits into FAD, high energy electrons and protons.

The high energy electrons are passed through electron carriers along the membrane; as they pass through the proton pumps energy is released from the electrons and used to fuel two proton pumps. Two protons are pumped into the intermembrane space. This results in an electochemical gradient, which causes protons to flow through the stalked particles (ATP synthetase). ATP synthetase synthesises ATP as the protons pass through. This process is termed chemiosmosis.

Each reduced FAD fuels TWO proton pumps and provides energy to synthesise TWO ATP.

Oxygen is the final acceptor of the protons and electrons and is REDUCED by them to form water.

The electron tranfer chain will only happen in the presence of oxygen. It recycles the NAD and FAD in order that further dehydrogenation reactions can happen as more glucose is broken down.

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Path of high energy electron from reduced NAD (NADH) As high energy electrons pass through the proton pumps they release energy to pump protons across the membrane. Electrons from reduced NAD work three proton pumps

Electrons from reduced FAD enter here and only 2 proton pumps work

Pumping of protons into the intermembrane space makes an electrochemical gradient across the cristae membranes, protons can only flow through the stalked particles made of ATP synthetase to the matrix as the membrane is impermeable to protons.

ATP synthetase uses the energy from the proton gradient to manufacture ATP

Oxygen is the FINAL ELECTRON ACCEPTOR, it accepts electrons and protons from the electron transfer chain and is REDUCED to form water. Without oxygen the electron transfer chain cannot operate as the protons and electrons cannot be removed.

Intermembrane space

Matrix

Protons and electrons are provided by the reduced carriers NADH and FADH2. The hydrogen that they carry are from the dehydrogenation reactions that take place throughout the pathways of respiration (glycolysis, the link reaction and Kreb’s cycle)

Inner mitochondrial

membrane

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Intermembrane space

Matrix

Inner Mitochondrial membrane of

Cristae

H+

H2O1/2 O2 + e- + H+

FADH FADNADNADH

H+H+

e-e-

O2 is the final electron acceptor

3 xProtonpumps

ATP synthetase

ATPADP + Pi

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+H+

H+

H+

H+

H+

H+

Low energy e-

High energy e-

e-

e-

H+

Electron transfer chain

Full reaction sequence:

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Substrate level phosphorylation

Substrate level phosphorylation

NAD

NAD

NAD

NAD

FAD

NADAcetyl coenzyme A

NAD carries hydrogen to electron transport chain as NADH

FAD carries hydrogen to electron transport chain as FADH2

NAD carries hydrogen to electron transport chain as NADH

NAD carries hydrogen to electron transport chain

NAD carries hydrogen to electron transport chain as NADH

Per molecule of glucose 2 pyruvate are made so link and Krebs happen twice Link reaction

Energy yield from aerobic respiration:

Numbers are per molecule of glucose.

Stage Substrate level phosphorylation

Oxidative phosphorylation (in electron transfer chain)

Glycolysis Net gain = 2 ATP 2 NADH 6ATPLink reaction 2 NADH 6ATPKreb’s cycle 2 ATP 6 NADH 18ATP

2 FADH 4 ATPTotal 4 34Grand total 38 ATP

NB: You need to be able to calculate the numbers of ATP stage by stage as well as knowing the overall yield per molecule of glucose.

So: in aerobic conditions glycolysis yields 8ATP (net gain of 2 plus the 6 from the electron transport chain); however glycolysis itself does not require oxygen.

Each link reaction yields 3ATP (but for a molecule of glucose you have two pyruvates)

Each turn of the Kreb’s cycle yields, 1 ATP at substrate level phosphorylation, 9 from the NADH that enters the electron transport chain and 2 ATP from the FADH entering the electron transfer chain.

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Other points about aerobic respiration:

ATP production by the electron transfer chain – requires oxygen and oxygen levels fall during ATP synthesis

Mitochondria can only utilise pyruvate as a substrate – glucose cannot cross the outer membrane as there are no specific protein carriers for glucose and the enzymes to convert glucose into pyruvate are only found in the cytoplasm

Mitochondria do not carry out anaerobic respiration as the enzymes to produce lactate and ethanol are not found in mitochondria

Cytoplasm does not contain the enzymes for pyruvate metabolism.

Cyanide is a non-competitive inhibitor of the last electron carrier in the electron transfer chain; it therefore inhibits respiration and prevents Kreb’s cycle and the link reactions taking place. Processes that rely on ATP cannot occur in the presence of cyanide.

Using other compounds in aerobic respiration:

Glucose

Hexose diphosphate

Triose phosphate

Pyruvate

Acetyl coenzyme A

Kreb’s cycle

Glucose from glycogen stores is the primary source of substrate for respiration. During exercise, fat is mobilised from adipose tissue. Under conditions of starvation when fat stores are depleted proteins from muscle are digested and utilised.

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Fat

Glycerol

Fatty acids

Protein

Amino acids deaminated in

the liver

Organic acids

Amino group (NH2)

Ammonia

Urea excreted

You must be able to describe where the products of fat and protein digestion enter the respiratory pathways.

Anaerobic respiration

Glucose

Hexose diphosphate

2 x triose phosphate

2 x pyruvate

Lactate ethanal

Ethanol

Anaerobic respiration takes place in the cytoplasm.Dehydrogenation reactions in glycolysis lead to production of reduced NAD, and these reactions need NAD in order to proceed. Anaerobic respiration leads to resynthesis of NAD without the need for the electron transport system and oxygen. This enables more glucose to be utilised but the energy yield is limited to a net gain of 2ATP per molecule of glucose.

The ethanol and lactate still contain lots of energy.

If you are asked to describe anaerobic respiration you MUST start with glycolysis.

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2 ATP

4 ATP

Net gain of 2 ATP, substrate level phosphorylation.

Plants and fungi

Animals and prokaryotes

CO2

Reduced NAD

NAD

2 Reduced NAD

Reduced NAD

NAD

2 NAD