2 y12 cellular energetics

Learning Objectives 1. Compile your own glossary from the KEY WORDS

displayed in bold type in the learning objectives below.

Energy in Cells (page 44) 2. Describe the synthesis of ATP from ADP and inorganic

phosphate and explain how ATP stores and releases its energy. Appreciate the universal role of ATP in cells.

3. Outline the principles involved in cellular respiration and photosynthesis. Appreciate that both processes involve the molecule ATP and a hydrogen carrier.

Cellular Respiration (pages 50-53) 4. Describe the structure of a mitochondrion and relate

this to its functional role in cellular respiration. 5. Identify the main steps in cellular respiration, identifying

where in the cell each stage occurs: glycolysis, Krebs cycle (tricarboxylic acid cycle), electron transport chain. Recognise glycolysis as the first stage in cellular respiration and the major anaerobic pathway in cells.

6. Identify glucose as the main respiratory substrate. Appreciate that other substrates can, through conversion, act as substrates for cellular respiration.

7. Outline glycolysis as the phosphorylation of glucose and the subsequent splitting of a 6C sugar into two pyruvate molecules. State the net yield of ATP and NADH2 from glycolysis. Appreciate that the subsequent metabolism of pyruvate depends on oxygen availability.

8. Describe the oxidation of glucose to CO2, including: Theconversionofpyruvatetoacetyl-coenzyme A.

TheentryofacetylCoAintotheKrebscycle. The Krebs cycle (as a series of oxidation reactions

involving release of CO2, the production of NAD.H2 or FAD.H2 , and the regeneration of oxaloacetate).

ThegenerationofATPintheelectrontransportchain.

The role of oxygen as the terminal electron acceptor. The net yield of ATP from aerobic respiration.

9. Describe examples of fermentation, identifying the H+ acceptor in each case:

(a) Formation of lactic acid in muscle. (b) Formation of ethanol in yeast.

10. Compare and explain the differences in the yields of ATP from aerobic respiration and from fermentation.

Photosynthesis (pages 45-49 and the TRC) 11. Discuss the development of scientific ideas and

theories relating to the biochemistry of photosynthesis. Relate the development of these ideas to our current understanding of the biochemistry of photosynthesis.

12. Describe the structure and role of chloroplasts, and role of chlorophyll a and b, and accessory pigments (e.g. carotenoids) in light capture. Explain what is meant by the terms absorption spectrum and action spectrum with respect to the light absorbing pigments.

13. Describe the overall chemical equation for photosynthesis and describe the steps involved:

The light dependent phase (LDP) with reference to: Thelocationandroleofthephotosystems Thephotoactivation of chlorophyll Thesplittingofwater(photolysis) TheproductionofO2 as a result of photolysis ThetransferofenergytoATP(photophosphorylation)

and the formation of NADPH2 (reduced NADP) The light independent phase (LIP) with reference to: WhereinthechloroplasttheLIPoccurs. The Calvin cycle including the fixation of CO2

into a 5C compound, the reduction of PGA to carbohydrate and the role of ATP and NADPH2.

Theregenerationofthe5Cribulosebisphosphate.

14. Identify factors affecting photosynthetic rate and yield, and describe their effects.

HSC VCE QLD SA WA

Cellular Energetics

Complete:Option:

1-14

Complete:1-10, 12-14

Some numbers extension as

required



required



required


See the Textbook Reference Grid on page 7 for textbook page references relating to material in this topic.

Supplementary TextsSee pages 5-6 for additional details of these texts: Adds, J. et al., 2003. Molecules and Cells. Harwood, R., 2002. Biochemistry.

See page 6 for details of publishers of periodicals: The Role of ATP in Cells Biol. Sci. Rev., 19(2) Nov. 2006, pp. 30-33. Synthesis and uses of ATP. Photosynthesis....Most Hated Topic? Biol. Sci. Rev., 20(1) Sept. 200, pp. 13-16. A useful account documenting key points when learning about processes in photosynthesis. Glucose Catabolism Biol. Sci. Rev., 10(3) Jan. 1998, pp. 22-24. The role of mitochondria and glucose in cells: oxidative phosphorylation. Fuelled for Life New Scientist, 13 January 1996 (Inside Science). Energy and metabolism: ATP, glycolysis, electron transport system, Krebs cycle, and enzymes and cofactors.

AcetylCoA: A Central Metabolite Biol. Sci. Rev., 20(4) April 2008, pp.38-40. The role of acetyl coenzyme A in metabolising fat and carbohydrate.

See pages 8-9 for details of how to access Bio Links from our web site: www.biozone.com.au FromBioLinks,accesssitesunderthetopics:CELL BIOLOGY AND BIOCHEMISTRY > Biochemistry and Metabolic Pathways:Calvincycle(C3cycle)CellularenergyreferencesCellularrespirationKrebscycleElectrontransportchainGlycolysisLearningaboutphotosynthesisChapter7:Metabolismandbiochemistry and others

Presentation MEDIA to support this topic:CELL BIO & BIOCHEM: Cellular Energetics

Use RESTRICTED to schools where studentshave their own copy of this workbook

Photocopying Prohibited

Biozone International 1995-2008

44

Related activities: Cellular Respiration, PhotosynthesisRA 2

1. Describe how ATP acts as a supplier of energy to power metabolic reactions:

2. Name the immediate source of energy used to reform ATP from ADP molecules:

3. Name the process of re-energising ADP into ATP molecules:

4. Name the ultimate source of energy for plants:

5. Name the ultimate source of energy for animals:

6. Explain in what way the ADP/ATP system can be likened to a rechargeable battery:

7. In the following table, use brief statements to contrast photosynthesis and respiration in terms of the following:

Photosynthesis Cellular respirationFeature

Starting materials

Waste products

Role of hydrogencarriers: NAD, NADP

Overall biological role

Role of ATP

The role of ATP in cellsThe molecule ATP (adenosine triphosphate) is the universalenergy carrier for the cell. ATP can release its energy quickly;only one chemical reaction (hydrolysis of the terminal phosphate)is required. This reaction is catalyzed by the enzyme ATPase.Once ATP has released its energy, it becomes ADP (adenosinediphosphate), a low energy molecule that can be recharged byadding a phosphate. This requires an input of energy that issupplied by the controlled breakdown of respiratory substratesin the process of cellular respiration. The most commonrespiratory substrate is glucose, but other molecules (e.g., fatsor proteins) may also be used.

In the presence of the enzymeATPase, the ATP molecule

loses a phosphate.

A free phosphate is releasedfrom the ATP (this may bereused later to regenerate ADPinto ATP again).

P

Pi

Mitochondrion

Inorganic phosphate

Adenosine P P P

ATPA high energy compound able to

supply energy for metabolic activity.

Adenosine triphosphate

Adenosine diphosphate

ADPA low energy compound with no

available energy to fuel metabolic activity.

Adenosine P P

Energy releasedThe energy released from theloss of a phosphate is availablefor immediate work inside thecell (i.e. powering chemicalreactions).

30.7kJ

EII

TEM of mitochondrion surrounded bypolyribosomes. Note the many foldedinner membranes (cristae).

Cellular respirationIn cellular respiration, glucose is oxidised in astep-wise process that provides the energy forthe formation of high energy ATP from ADP.Apart from the reactions of glycolysis, theseprocesses occur in the mitochondria.

The molecule ATP (adenosine triphosphate) is the universal energy carrier for the cell. ATP can release its energy quickly; only one chemical reaction (hydrolysis of the terminal phosphate) is required. This reaction is catalysed by the enzyme ATPase. Once ATP has released its energy, it becomes ADP (adenosine

diphosphate), a low energy molecule that can be recharged by adding a phosphate. This requires energy, which is supplied by the controlled breakdown of respiratory substrates in cellular respiration. The most common respiratory substrate is glucose, but other molecules (e.g. fats or proteins) may also be used.

The Role of ATP in Cells




45

Cellula

r Energetics

Related activities: The Biochemistry of Photosynthesis A 2

Photosynthesis

Carbon dioxidefrom the airprovides carbonand oxygen asraw materials.

ATP

NADPHOxygen gas (from thebreak-up of watermolecules) is given offas a waste product.

Sunlight

Chloroplast

Grana are stacks ofthylakoid membranes,which containchlorophyll and arethe site of the lightdependent phase.

Stroma, the liquid interiorof the chloroplast, in whichthe light independentphase takes place.

Plant cells (Elodea)

GlucoseUsed as the fuel forcellular respiration;supplies energy for

metabolism.

CelluloseGlucose is used as a

building block forcreating cellulose, acomponent of plant

cell walls.

StarchStored as a reservesupply of energy in

starch granules, to beconverted back into

glucose when required.

DisaccharidesGlucose is convertedto other sugars suchas fructose, found in

ripe fruit, and sucrose,found in sugar cane.

Converted via anumber of steps to:

There are 20-30chloroplasts inthe cytoplasm ofthis plant cell.

Water is givenoff as a wasteproduct

Water from cellsap is used asa raw material.

Hydrogen (from the break-up of water molecules) isused as a raw material.

triose phosphate(a 3-carbon sugar)

Lipidsandamino acids

=

=Light dependentphase

Light independentphase

Process: Carbon fixationvia the Calvincycle

Process: Energy capturevia PhotosystemsI and II

RCN

Summary of Photosynthesis in a C3 Plant

1. Describe the three things of fundamental biological importance provided by photosynthesis:

(a) (b) (c)2. Writetheoverallchemicalequationforphotosynthesisusing:

(a)Words:

(b) Chemical symbols:3. Discuss the potential uses for the end products of photosynthesis:

4. Distinguish between the two different regions of a chloroplast and describe the biochemical processes that occur in each:

Photosynthesis is of fundamental importance to living things because it transforms sunlight energy into chemical energy stored in molecules. This becomes part of the energy available in food chains. The molecules that trap the energy in their chemical

bonds are also used as building blocks to create other molecules. Finally, photosynthesis releases free oxygen gas, essential for the survival of advanced life forms. Below is a diagram summarising the process of photosynthesis.




46

A 2

As light meets matter, it may be reflected, transmitted, or absorbed. Substances that absorb visible light are called pigments, and different pigments absorb light of different wavelengths. The ability of a pigment to absorb particular wavelengths of light can be measured with a spectrophotometer. The light absorption vs the wavelength is called the absorption spectrum of that pigment. The absorption spectrum of different photosynthetic

pigments provides clues to their role in photosynthesis, since light can only perform work if it is absorbed. An action spectrum profiles the effectiveness of different wavelength light in fuelling photosynthesis. It is obtained by plotting wavelength against some measure of photosynthetic rate (e.g. CO2 production). Some features of photosynthetic pigments and their light absorbing properties are outlined below.

Pigments and Light Absorption

Wavelength (nm)

Gammarays

10-5 nm 10-3 nm 1 nm 103 nm 106 nm 1 m 103 m

X- rays Ultraviolet Infrared Microwaves Radio waves

380 450 550 650 750

Visible light

Increasing energy Increasing wavelength

The photosynthetic pigments of plantsThe photosynthetic pigments of plants fall into two categories:chlorophylls (which absorb red and blue-violet light) andcarotenoids (which absorb strongly in the blue-violet and appearorange, yellow, or red). The pigments are located on thechloroplast membranes (the thylakoids) and are associated withmembrane transport systems.

ChloroplastGreen lightreflectedSunlight

Red and bluelight absorbed

Thylakoid discs

The pigments of chloroplasts in higher plants (above) absorbblue and red light, and the leaves therefore appear green (whichis reflected). Each photosynthetic pigment has its owncharacteristic absorption spectrum (left, top graph). Althoughonly chlorophyll a can participate directly in the light reactionsof photosynthesis, the accessory pigments (chlorophyll b andcarotenoids) can absorb wavelengths of light that chlorophyll acannot. The accessory pigments pass the energy (photons) tochlorophyll a, thus broadening the spectrum that can effectivelydrive photosynthesis.

Left: Graphs comparing absorption spectra of photosyntheticpigments compared with the action spectrum for photosynthesis.

The Electromagnetic SpectrumLightisaform of energy known as electromagnetic radiation. The segment of the electromagnetic spectrum most important to life isthe narrow band between about 380 and 750 nm. This radiation is known as visible light because it is detected as colours by thehuman eye (although some other animals, such as insects, can see in the UV range). It is the visible light that drives photosynthesis.

Electromagnetic radiation (EMR) travels in waves,where wavelength provides a guide to the energy ofthe photons; the greater the wavelength of EMR,thelower the energy of the photons in that radiation.

400 500 600 700

Rat

e of

pho

tosy

nthe

sis (a

s % of

rate

at 67

0 nm)

Abso

rban

ce (p

ercen

t)

Wavelength (nm)

Absorption spectra of photosynthetic pigments

Action spectrum for photosynthesis

The action spectrum and the absorptionspectrum for the photosyntheticpigments (combined) match closely.

Chlorophyll a

Chlorophyll b

0

20

40

60

80

100

0

20

40

60

80

100

Carotenoids

(Relative amounts of light absorbed at different wavelengths)

(Effectiveness of different wavelengths in fuelling photosynthesis)

1. Explain what is meant by the absorption spectrum of a pigment:

2. Explain why the action spectrum for photosynthesis does not exactly match the absorption spectrum of chlorophyll a:




47

Cellula

r Energetics

Related activities: Pigments and Light Absorption, PhotosynthesisWeb links: Harvesting Light, Events in Biochemistry RA 3

NADPH + H+

Photolysis of water: In non-cyclicphosphorylation, the electrons lost to theelectron transport chain are replaced bysplitting a water molecule (photolysis),releasing oxygen gas and hydrogen ions.

NADP is a hydrogen carrier,picking up H+ from thethylakoid and transportingthem to the Calvin cycle.

The light independent reaction, called theCalvin cycle, has also been labelled thedark phase of photosynthesis. This is nota good label as it is not necessary that thephase occur in darkness; it simply doesnot require light to proceed. In the Calvincycle, hydrogen (H+) is added to CO2 anda 5C intermediate to make carbohydrate.The H+ and ATP are supplied by the lightdependent phase above.

Light Dependent Phase(Energy capture)

Electron transport chain: Each electronis passed from one electron carrier toanother, losing energy as it goes. Thisenergy is used to pump hydrogen ionsacross the thylakoid membrane.

ATP synthetaseconverts ADP andinorganic phosphate(Pi) into ATP

Thylakoidmembrane

NADPH + H+

NADP+ + 2H+

H+

ATP

ADP+ Pi

2e-

2e-

Lightenergy

Ribulose bisphosphatecarboxylase (RuBisCO)

Calvin cycle

CO2

RuBP: Ribulosebisphosphate

Ribulosephosphate

Triosephosphate

G3P: Glycerate3-phosphate

ATP

ADP+ Pi

NADPH + H+

NADPATP

ADP+ Pi

Hexosesugars

(a) (b)

(c)(d)

Light Independent Phase(Carbon fixation)

O212 Flow of H+ back across themembrane is coupled to ATPsynthesis (by chemiosmosis).H2O

2H+

Photosystem IPhotosystem II

Lightenergy

Thylakoid space:hydrogen reservoir,

low pH

H+

2e-2e-

NADP+reductase

This diagram shows non-cyclic phosphorylation.

Photosystem complexes comprise hundreds ofpigment molecules, including chlorophyll a and b.

Photosystem II absorbs light energy to elevateelectrons to a moderate energy level.

Photosystem I absorbs light energy to elevateelectrons to an even higher level. Its electrons arereplaced by electrons from photosystem II.

Chloroplast

ATPNADPH

Carbon dioxide

Oxygen

Grana are stacks ofthylakoid membranes(enlarged below)

Stroma

WaterHydrogen triose

phosphate

Water

Detail ofThylakoid Membrane

Lightdependent

Lightindependent

Whenchlorophyll molecules absorblight, an electron is excited to a higherlevel. This electron hole must be filled.

The Biochemistry of PhotosynthesisLike cellular respiration, photosynthesis is a redox process,but the electron flow evident in respiration is reversed. In photosynthesis, water is split and electrons are transferred together with hydrogen ions from water to CO2, reducing it to sugar. The electrons increase in potential energy as they move from water to sugar. The energy to do this is provided by light. Photosynthesis comprises two phases. In the light

dependent phase, light energy is converted to chemical energy (ATP and reducing power). In the light independent phase (or Calvin cycle), the chemical energy is used for the synthesis of carbohydrate. The light dependent phase illustrated below shows non-cyclic phosphorylation. In cyclic phosphorylation, the electrons lost from photosystem II are replaced by those from photosystem I. ATP is generated, but not NADPH.




48

1. Describe the role of the carrier molecule NADP in photosynthesis:

2. Explain the role of chlorophyll molecules in the process of photosynthesis:

3. On the diagram on the previous page, write the number of carbon atoms that each molecule has at (a)-(d):4. Summarise the events in each of the two phases in photosynthesis and identify where each phase occurs:

(a) Light dependent phase (D):

(b) Calvin cycle:

5. The final product of photosynthesis is triose phosphate. Describe precisely where the carbon, hydrogen and oxygen molecules originate from to make this molecule:

6. Explain how ATP is produced as a result of light striking chlorophyll molecules during the light dependent phase:

7. (a) The diagram of the light dependent phase (opposite) describes non-cyclic phosphorylation. Explain what you understand by this term:

(b) Suggest why this process is also known as non-cyclic photophosphorylation:

(c) Explain how photophosphorylation differs from the oxidative phosphorylation occurring in cellular respiration:

8. Explain how cyclic photophosphorylation differs from non-cyclic photophosphorylation:




49

Cellula

r Energetics

Related activities: Photosynthesis DA 2

The rate at which plants can make food (the photosynthetic rate) is dependent on environmental factors, particularly the amount of light available, the level of carbon dioxide (CO2) and the temperature. The effect of these factors can be tested experimentally by altering one of the factors while holding others

constant (a controlled experiment). In reality, a plant is subjected to variations in all three factors at the same time. The interaction of the different factors can also be examined in the same way, as long as only one factor at a time is altered. The results can be expressed graphically.

Photosynthetic Rate

The two graphs above illustrate the effect of different variables onthe rate of photosynthesis in cucumber plants. Graph A (above, left)shows the effect of different intensities of light. In this experiment,the level of carbon dioxide available and the temperature were kept

constant. Graph B (above, right) shows the effect of different lightintensities at two temperatures and two carbon dioxide (CO2)concentrations. In each of these experiments either the carbon dioxidelevel or the temperature was raised at each light intensity in turn.

Factors Affecting Photosynthetic Rate

Rat

e of

pho

tosy

nthe

sis (m

m3 CO

2 cm

2

h1 )

Units of light intensity (arbitrary scale)

40

80

120

160

200

240

280

1 2 3 4 5 6 7

High CO2 at 30C

High CO2 at 20C

LowCO2 at 30C

LowCO2 at 20C

B: Light intensity, CO2, and temperaturevs photosynthetic rate

Rat

e of

pho

tosy

nthe

sis (m

m3 CO

2 cm

2

h1 )

40

50

60

70

80

90

1 2 3 4 5 6 7

A: Light intensity vs photosynthetic rate

Units of light intensity (arbitrary scale)

1. (a) Describe the effect of increasing light intensity on the rate of photosynthesis (temperature and CO2 constant):

(b) Give a possible explanation for the shape of the curve:

2. (a) Describe the effect of increasing the temperature on the rate of photosynthesis:

(b) Suggest a reason for this response:

3. Explain why the rate of photosynthesis declines when the CO2 level is reduced:

4. (a) In the graph above right, explain how the effects of CO2 level were separated from the effects of temperature:

(b) State which of the two factors, CO2 level or temperature, has the greatest effect on photosynthetic rate:

(c) Explain how you can tell this from the graph:




50

Related activities: The Role of ATP in Cells, The Biochemistry of RespirationRA 2

An Overview of Cellular Respiration

Matrix (fluid space)of mitochondrion

In substrate-level phosphorylation, an enzyme transfers aphosphate group directly from a substrate (e.g. glucose) to ADP.

ATP

KREBSCYCLE

The matrix of themitochondria

ATP ATP

ELECTRON TRANSPORTCHAIN AND OXIDATIVE

PHOSPHORYLATIONThe inner membranes

(cristae) of the mitochondria

Electronscarried via

NADH

GLYCOLYSISThe cytoplasm

Glucose Pyruvate

Electrons carried byNADH and FADH2

Respiration involves three metabolic stages, summarised below. The first two stages are thecatabolic pathways that decompose glucose and other organic fuels. In the third stage, theelectron transport chain accepts electrons from the first two stages and passes these from oneelectron acceptor to another. The energy released at each stepwise transfer is used to makeATP. The final electron acceptor in this process is molecular oxygen.

Glycolysis. This occurs in the cytoplasm and involves the breakdown of glucose intotwo molecules of pyruvate.The Krebs cycle. This occurs in the mitochondrial matrix, and decomposes a derivativeof pyruvate to carbon dioxide.Electron transport and oxidative phosphorylation. This occurs in the inner membranesof the mitochondrion and accounts for almost 90% of the ATP generated by respiration.

1

2

3

Cristae (foldedinner membranes)of mitochondrion

Substrate-levelphosphorylation

Oxidativephosphorylation

1

23

Substrate-levelphosphorylation

In oxidative phosphorylation, glucose is oxidised in a series ofredox reactions that provide the energy for the formation of ATP.

Cellular RespirationCellular respiration is the process by which organisms break down energy rich molecules (e.g. glucose) to release the energy in a useable form (ATP). All living cells respire in order to exist, although the substrates they use may vary. Aerobic respiration requires oxygen. Forms of cellular respiration that do not require

oxygen are said to be anaerobic. Some plants and animals can generate ATP anaerobically for short periods of time. Other organisms use only anaerobic respiration and live in oxygen-free environments. For these organisms, there is some other final electron acceptor other than oxygen (e.g. nitrate or Fe2+).

1. Describe precisely in which part of the cell the following take place:

(a) Glycolysis: (b)Krebscyclereactions:

(c) Electron transport chain:2. ExplainhowthegenerationofATPinglycolysisandtheKrebscyclediffersfromthewayinwhichitisgeneratedviathe

electron transport chain:




51

Cellula

r Energetics

Related activities: Cellular RespirationWeb links: Glycolysis and the Krebs Cycle A 3

The Biochemistry of Respiration

(a)

Krebs cycle

2NADH + 2H+

2C lost ascarbon dioxide

ATP

Other molecules (above)Whenglucoseisinshort supply,other organic molecules can providealternative respiratory substrates.

Electron transport chain (cristae)Hydrogen pairs are transferred to the electron transport chain, aseries of hydrogen and electron carriers, located on themembranes of the cristae. The hydrogens or electrons are passedfrom one carrier to the next, losing energy as they go. The energyreleased in this stepwise process is used to produce ATP. Oxygenis the final electron acceptor and is reduced to water.*Note FAD enters the electron transport chain at a lower energylevel than NAD, and only 2ATP are generated per FAD.H2.

CoA

O2

Water

Oxygen is usedas a terminalelectron acceptor

2H+

12Electron transport chaine-

e-

17 ADP + 17 ~P

17 ATP

Glycolysis (cytoplasm)The first part of respiration that involves thebreakdown of glucose in the cytoplasm. Glucose(a 6-carbon sugar) is broken into two moleculesof pyruvate (also called pyruvic acid), a 3-carbonacid. A total of 2 ATP and 2NADH + 2H+ aregenerated from this stage. No oxygen is required(the process is anaerobic).

Transition reaction (matrix)Pyruvate enters the mitochondrion and carbon dioxideis removed. Coenzyme A (CoA) picks up theremaining 2-carbon fragment of the pyruvate to formacetyl coenzyme A.

Krebs cycle (matrix)The acetyl group passes into a cyclic reactionand combines with a 4-carbon molecule toform a 6-carbon molecule. The CoA isreleased for reuse. Successive steps in thecycle remove carbon as carbon dioxide.

NAD and FAD are hydrogenacceptors, transporting hydrogens

tothe electron transport chain (below).

NADH + H+

NADH + H+

4 ATP are producedbut 2 are used inthe process

1C lost ascarbon dioxide

Glucose (6C)

Pyruvate*

2 ADPFats Glycerol

Glycogen

Proteins

Amino acids

Fatty acids

2 ATP

Acetyl Coenzyme A

(f)

(d)

(e)

(c)

(b)

*FAD.H2

-ketoglutarate

oxaloacetate

Total ATP yield per glucoseGlycolysis: 2 ATP, Krebs cycle: 2 ATP, Electron transport: 34 ATP

Phosphorylated6C sugar

2 x 3C sugarphosphate

(glyceraldehyde-3-phosphate)

NADH + H+

* 2 molecules of pyruvate are produced perglucose molecule. From this stage, theprocessing of only one pyruvate is shown.

Cellular respiration is a catabolic, energy yielding pathway. The breakdown of glucose and other organic fuels (such as fats and proteins) to simpler molecules releases energy for the synthesis of ATP. Glycolysis and the Krebs cycle supply electrons (viaNADH) to the electron transport chain, which drives oxidative phosphorylation. Glycolysis nets two ATP, produced by substrate-level phosphorylation. The conversion of pyruvate (the end product of glycolysis) to acetyl CoA links glycolysis

to the Krebs cycle. One "turn" of the cycle releases carbondioxide, forms one ATP, and passes electrons to three NAD+ andoneFAD.MostoftheATPgeneratedincellularrespirationisproduced by oxidative phosphorylation when NADH2 and FADH2 donate electrons to the series of electron carriers in the electron transport chain. At the end of the chain, electrons are passed to molecular oxygen, reducing it to water. Electron transport is coupled to ATP synthesis (see the TRC: Cellular Respiration).




52

Glycolysis and Fermentation

Occurs in the cytoplasm of the cell.

Aerobic Respiration

Occurs in the mitochondria of eukaryotic cells.

Oxygen is not required. Oxygen is required.

Glycolysis: glucose is converted to pyruvic acid with a netproduction of 2ATP molecules (a very low energy yield).

Very inefficient production of energy.

Pyruvic acid is converted to carbon dioxide, water, and afurther 36 ATPmoleculesfromtheKrebscycleandelectrontransport chain (a high energy yield per glucose molecule).

An energy efficient process.

In the absence of oxygen, pyruvic acid cannot enterthe mitochondrion. It is converted to ethanol and carbondioxide in plants, and lactic acid in animals.

Whenoxygen is present, pyruvic acid can be oxidised furtherviatheKrebscycleandelectrontransport chain.

1. Summarise the events occurring in each of the following stages of cellular respiration:

(a) Glycolysis:

(b) Transition reaction:

(c)Krebscycle:

(d) Electron transport chain:

2. On the diagram of cellular respiration (previous page), state the number of carbon atoms in each of the molecules (a)-(f):3. Determine how many ATP molecules per molecule of glucose are generated during the following stages of respiration:

(a)Glycolysis: (b)Krebscycle: (c)Electrontransportchain: (d)Total:

4. Explain what happens to the carbon atoms lost during respiration:

5. Describe the role of each of the following in cellular respiration:

(a) Hydrogen atoms:

(b) NAD and FAD:

(c) Oxygen:

(d) Acetyl coenzyme A:

6. Explain what happens when the supply of glucose for cellular respiration is limited:




53

Cellula

r Energetics

Related activities: The Biochemistry of Respiration RDA 2

1. Describe the key difference between aerobic respiration and fermentation:

2. (a) Refer to page 43 and determine the efficiency of fermentation compared to aerobic respiration: % (b) In simple terms, explain why the efficiency of anaerobic pathways is so low:

3. Explain why fermentation cannot go on indefinitely:

Anaerobic PathwaysAll organisms can metabolise glucose anaerobically (without oxygen) using glycolysis in the cytoplasm, but the energy yield from this process is low and few organisms can obtain sufficient energy for their needs this way. In the absence of oxygen, glycolysis soon stops unless there is an alternative acceptor for the electrons produced from the glycolytic pathway. In yeasts and the root cells of higher plants this acceptor is ethanal, and the pathway is called alcoholic fermentation. In the skeletal muscle of mammals, the acceptor is pyruvate itself and the end product

is lactic acid. In both cases, the duration of the fermentation is limited by the toxic effects of the organic compound produced. Although fermentation is often used synonymously with anaerobic respiration, they are not the same. Respiration always involves hydrogen ions passing down a chain of carriers to a terminal acceptor, and this does not occur in fermentation. In anaerobic respiration, the terminal H+ acceptor is a molecule other than oxygen, e.g. Fe2+ or nitrate.

2 x pyruvateCH3COCOOH

GlucoseC6H12O6

Lactic Acid FermentationAnimal tissues

Pyruvate Lactic acidCH3CHOHCOOH

NAD.H2 NAD+waste

product

2 ADP

2 ATPNet

NAD.H2

Lactic Acid FermentationIn the absence of oxygen, the skeletal musclecells of mammals are able to continue usingglycolysis for ATP production by reducingpyruvate to lactic acid (the H+ acceptor ispyruvate itself). This process is called lacticacid fermentation. Lacticacid is toxic andthis pathway cannot continue indefinitely.The lactic acid must be removed from themuscle and transported to the liver, whereit is converted back to respiratory inter-mediates and respired aerobically.

Alcoholic FermentationIn alcoholic fermentation, the H+ acceptoris ethanal which is reduced to ethanol withthe release of CO2. Yeasts respire aerobicallywhen oxygen is available but can usealcoholic fermentation when it is not. At levelsabove 12-15%, the ethanol produced byalcoholic fermentation is toxic to the yeastcells and this limits their ability to use thispathway indefinitely. The root cells of plantsalso use fermentation as a pathway whenoxygen is unavailable but the ethanol mustbe converted back to respiratory inter-mediates and respired aerobically.

The products of alcoholic fermentation havebeen utilised by humans for centuries. Thealcohol and carbon dioxide produced fromthis process form the basis of the brewingand baking industries.

Vertebrate skeletal muscle is facultativelyanaerobic because it has the ability togenerate ATP for a short time in the absenceof oxygen. The energy from this pathwaycomes from glycolysis and the yield is low.

Some organisms respire only in the absenceof oxygen and are known as obligateanaerobes. Many of these organisms arebacterial pathogens and cause diseases suchas tetanus (above), gangrene, and botulism.

CDC

2 x pyruvateCH3COCOOH

GlucoseC6H12O6

2 ADP

2 ATPNet

Alcoholic fermentationYeast, higher plant cells

NAD.H2NAD+

EthanolCH3CH2OH

EthanalCH3CHO

+CO2

gaseous wasteproduct

wasteproduct

NAD.H2


Use RESTRICTED

to schools where each student owns

a current edition of this workbook

1. Schools MAY NOT place this file on any networked computer.

2. Schools MAY NOT place this file on any student computer (including student laptops), unless they have entered into a specific licensing agreement to do so with BIOZONE International Ltd.

3. This file may ONLY be placed on teaching staff computers (including teaching staff laptops).

4. Projection of these pages using a data projector is permitted ONLY if each student in the class owns a current edition of the workbook.

5. This licence expires on 31 December 2009, unless the school purchases a new Teacher Resource CD-ROM for the 2010 academic year.

Please report any abuse of these terms of use to:

[email protected]

Phone: +64 7 856 8104

Fax: +64 7 856 9243

Terms of Use


Cellular EnergeticsThe Role of ATP in CellsPhotosynthesisPigments and Light AbsorptionThe Biochemistry of PhotosynthesisPhotosynthetic RateCellular RespirationThe Biochemistry of RespirationAnaerobic Pathways

2 y12 cellular energetics

Documents

cellular respiration

aerobic respiration

universal role of atp

molecule atp

atp stores

yields of atp

synthesis of atp

net yield of atp