Energy: Cellular Respiration and Photosynthesis

Wk4, Chapters 7,8

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The Flow of Energy in Living SystemsTo maintain their organization and carry out metabolic

activities, cells (and organisms) need a constant supply of energy

Energy is defined as the capacity to do work-kinetic energy: the energy of motion-potential energy: stored energy

Food has potential energy because it can be converted into various types of kinetic energy.

Food is specifically called chemical energy (energy is stored in chemical bonds of carbohydrates, proteins and fats).

The use of energy by living organismsLife is powered by sunshineLand plants, some protists and cyanobacteria

harvest the energy of sunlight and convert it into chemical energy (synthesis of organic molecules) through the process of photosynthesis.

These organisms are called autotrophs (self-feeders). Autotrophs can also use the chemical energy from the organic compounds to produce ATP through the process of cellular respiration.

Animals, fungi and most protists and most prokaryotes can not do photosynthesis and only can do cellular respiration. They are called called heterotrophs (fed by others).

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Photosynthesis versus respirationAll organisms do respiration to obtain energy from

organic compounds: Cellular Respiration involves oxidation of organic compounds such as glucose that releases energy that is utilized to synthesize ATP. This process occurs in the mitochondria and or the cytoplasm(glycolysis occurs in the cytoplasm).

Only a few organisms can do photosynthesis: Photosynthesis involves harvesting energy from sunlight and converting it into chemical energy (synthesis of organic molecules). This process occurs in chloroplasts (in plants, algae, protists) and in thylakoids (in cyanobacteria).

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Photosynthesis and Cellular Respiration

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Energy Currency of CellsATP = adenosine

triphosphate-the energy “currency”

of cellsATP structure:-ribose, a 5-carbon sugar-adenine-three phosphates

HighEnergybond

Energy Currency of Cells

ATP stores energy in the bonds between the last two phosphates.

When that bond breaks, high energy is released:ATP ADP + Pi + Energy

ADP = adenosine diphosphatePi = inorganic phosphate

The energy released from an exergonic reaction can be used to fuel the production of ATP from ADP + Pi.

The energy released when ATP is broken down to ADP can be used to fuel endergonic reactions.

Enzymes: Biological CatalystsEnzymes: Molecules that catalyze (speed up) reactions

in living cells. Many are involved in the processes leading to the production and use of energy

-most are proteins (except: ribozymes: RNA with enzymatic abilities in the ribosome)

-are not changed or consumed by the reaction

Energy Units and HeatDuring energy conversions, part of the energy is lost in the

form of heat. The most convenient form to measure energy is in terms of

heat (all energy types can convert into heat).The energy units are kilocalories.

1 kilocalorie (kcal) = 1000 calories One Kilocalorie = the amount of heat required to raise the temp of 1 Kg of water by 1oC

“Food calories” listed on food packages are kilocalories of energy.

Human Nutrition,Cellular Respiration, and Use of ATP

Humans are heterotrophs. We need to take in food that provides about 2,000-2500 kilocalories of energy per day (“2000-2500 calories”).

75% is used for life sustaining activities (heart pumping, to breath, to maintain body temperature…),

The rest is for voluntary activities (running, dancing, sitting, walking).

About 40% of the energy provided by food is converted into ATP by cellular respiration, the rest of the energy is lost as heat (car engines converts about 25% of the energy from gasoline to the kinetic energy of movement)

Walking at 3 mph, how far would you have to travel to burn off the equivalent of a slice of pizza that has 475 kcal (475 calories)? How long would that take?

475/158About 3 hours3 mph x 3 h = About 9 miles

Food, such aspeanuts

ProteinsFatsCarbohydrates

Glucose

OXIDATIVEPHOSPHORYLATION(Electron Transportand Chemiosmosis)

CITRICACID

CYCLE

AcetylCoA

GLYCOLYSIS

Pyruvate

Amino acidsGlycerolSugars Fatty acids

Amino groups

G3P

ATP

9 Kcal per gram

4 Kcal per gram

4 Kcal per gram

DIGESTION

Pathways that break down various food molecules

ATPNAD+

NADH

H+

H+2e–

2e–

Electron transport

chain

Controlledrelease ofenergy forsynthesis

of ATP

+

O2

H2O

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During cellular respiration, electrons from glucose or other digested foods are shuttled through electron carriers in a electron transport chain to a final electron acceptor

Types of Cellular RespirationThe final electron acceptor is different in different types of

cellular respirationaerobic respiration: final electron receptor is oxygen (O2)fermentation: final electron acceptor is an organic molecule

(ethanol: in yeast/to make beer or wine, lactate: bacteria/to make yogurt or cheese, muscle cells/intensive exercise)

anaerobic respiration: final electron acceptor is an inorganic molecule (not O2)

by methanogens: methanogens use CO2 that is reduced to CH4 (methane)by sulfur bacteria: inorganic sulphate (SO4) is reduced to hydrogen sulfide

(H2S)

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Aerobic Respiration

A large amount of energy from the electrons of food is released in small steps by being transferred from carrier to carrier rather than all at once.

Because of this, energy is not released as heat but it is used to produced ATP.

C6H12O6 + 6O2 6CO2+ 6H2O +Energy

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Aerobic respiration

The complete oxidation of glucose proceeds in stages:

1. glycolysis2. pyruvate oxidation(prep reaction)3. Krebs cycle4. electron transport chain

& chemiosmosis

Mitochondrion: Structure & Aerobic Cellular respiration

1. Glycolysis (@cytoplasm)2. pyruvate oxidation(prep reaction): @Matrix3. Krebs cycle: @Matrix4. electron transport chain (cristae) & chemiosmosis (intermembrane space, cristae, mitochondrial matrix)

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Glycolysis is common to all types of cellular respiration

Glycolysis is the oxidation of glucose (6 carbons) into 2 molecules of pyruvate (3 carbons each).

-occurs in the cytoplasm-all organisms do glycolysis (the fate of pyruvate may differ)-net production of 2 ATP molecules by substrate-level phosphorylation-2 NADH produced by the reduction of NAD+

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Pyruvate fate Pyruvate fate differs:1. aerobic respiration – occurs when oxygen is used as the final

electron acceptorWhen oxygen is present, pyruvate is oxidized to acetyl-CoA which enters the Krebs cycle. NAD+ is recycled at the ETC.

2. fermentation – occurs when oxygen is not available; an organic molecule is the final electron acceptorWhen oxygen is not available, pyruvate is reduced in order to oxidize NADH back to NAD+

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Aerobic Respiration Fermentation

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FermentationIn Fermentation: The final electron acceptor is an organic

moleculeProduces less ATP than in aerobic respiration

1. ethanol fermentation occurs in yeast-glucose is transformed into CO2 and ethanol.

2. lactic acid fermentation-occurs in animal cells (especially muscles under intensive exercise), -occurs in yeast and bacteria

Glucose

NADH

NAD+

2

2

NADH2

NAD+2

2 ADPP

ATP2

2 Pyruvate

2 Lactate

GLY

COLY

SIS

Lactic acid fermentation

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2 ADPP

ATP2 GLY

COLY

SIS

NADH

NAD+

2

2

NADH2

NAD+2

2 Pyruvate

2 Ethanol

Alcohol fermentation

Glucose

CO22released

2

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Aerobic respiration: Pyruvate Oxidation

Electron Transport Chain and chemiosmosis

A proton gradient is established by the ETC

H+ will return back tothe matrix by diffusion through ATP synthase to produce ATP

Electron Transport Chain & ATP synthase

The higher negative charge in the matrix attracts the protons (H+) back from the intermembrane space to the matrix.

The accumulation of protons in the intermembrane space drives protons into the matrix via diffusion.

Most protons move back to the matrix through ATP synthase.

ATP synthase is a membrane-bound enzyme that uses the energy of the proton gradient to synthesize ATP from ADP + Pi.

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Photosynthesis is a redox process, as is cellular respiration

Photosynthesis, like respiration, is a redox (oxidation-reduction) process– Water molecules lose electrons along with hydrogen

ions (H+) to release O2

– Atmospheric CO2 is reduced to glucose as electrons and hydrogen ions are added to it

6 CO2 + 6 H2O C6H12O6 + 6 O2

Reduction

Oxidation

Photosynthesis OverviewPhotosynthesis takes place in two stagesLight-dependent reactions Pigments capture energy from sunlight (photons of

light) and electrons from pigments gain energyUse of light/electron energy to make ATP and to reduce

NADP+ (an electron carrier) to NADPH

Light-independent reactions (Calvin cycle)Using the ATP and NADPH to power the synthesis of

organic molecules from CO2 in the air.

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In cyanobacteria bacteria, photosynthesis takesplace in the plasma membrane, it has thylakoids) Photosynthetic bacteria

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In green plants and some algae, and some protists (euglena, diatoms, kelp) photosynthesistakes place in chloroplasts

In plants,the mesophyll of the leaves is rich in chloroplasts

Stoma (stomata)allows atmospheric CO2 to enter theleave and allows O2 to be released

Photosynthesis OverviewPhotosynthesis takes place in chloroplasts

(many of them are located in the mesophyll cells of the leaf).

thylakoid membrane – internal membrane arranged in flattened sacs

-contain chlorophyll and other pigments-photosynthetic pigments are clustered together to

form photosystemsgrana – stacks of thylakoid membranesstroma – semiliquid substance surrounding thylakoid

membranes32

The light-dependentreactions occuron the thylakoidmembrane

The ATP and NADPHIs then used to fuelcarbon fixation (CO2Is converted into organic molecules) via the Calvin cycle in the stroma

Photosynthesis is a redox process, as is cellular respiration

• In photosynthesis, electrons gain energy by being boosted up an energy hill (in cellular respiration they went down)– Light energy captured by chlorophyll molecules provides

the boost for the electrons– As a result, light energy is converted to chemical energy,

which is stored in the chemical bonds of sugar molecules

The two stages of photosynthesis are linked by ATP and NADPH

• NADPH produced by the light reactions provides the electrons for reducing carbon in the Calvin cycle

• ATP from the light reactions provides chemical energy for the Calvin cycle– The Calvin cycle is often called the dark (or light-independent)

reactions– During the Calvin cycle carbon reduction occurs (CO2 into

glucose)

NADP+

NADPH

ATP

CO2

+

H2O

ADPP

Electrontransport

chainsThylakoidmembranes

LightChloroplast

O2

CALVINCYCLE

(in stroma)

Sugars

Photosystem II

Photosystem I

LIGHT REACTIONS

RuBP

3-PGA

CALVIN CYCLE

Stroma

G3P Cellularrespiration

CelluloseStarchOther organiccompounds

Visible radiation drives the light reactions

• Pigments, molecules that absorb light, are built into the thylakoid membrane– Plant pigments absorb some wavelengths of light and reflect others– We see the color of the wavelengths that are reflected; for

example, chlorophyll reflects green– Pigments associated with photosynthesis absorb light energy in the

wavelengths that correspond to red, blue and violet light.

Photosynthetic PigmentsChlorophyll a – primary pigment in plants and cyanobacteria. It

absorbs violet-blue and red light and reflects green lightAccessory pigments: secondary pigments absorbing light

wavelengths other than those absorbed by chlorophyll a. -include: chlorophyll b and carotenoids

Clorophyll b absorbs violet-blue and red light and reflects green light

Carotenoids absorb blue and green light and reflect orange and yellow light.Light that is not absorbed by these pigments is reflected. The reflected photons

are absorbed by the retinal pigment of our eyes (we see the reflected light). Chlorophylls reflect green light and carotenoids reflect orange/yellow light.

Fall colors are produced by carotenoids and other accessory pigments. During the spring and summer, chlorophyll in leaves masks the presence of carotenoids and other accessory pigments. During the fall, cool temperatures cause the leaves to cease manufacturing chlorophyll (so they do not reflect green light), and the leaves reflect the orange/yellow light that carotenoidsand other pigments do not absorb.

Photosynthesis vs. Cellular Respirationmembranes

enzymes

grana cristae

Photosynthesis Cellular Respiration

H2O O2 H2OO2

ADP ATP

NAD+ NADH

CO2

NADPH NADP+

CO2 Carbohydrateor other Organic molecule

Carbohydrateor other Organic molecule

Which Organisms? Plants, photosynthetic protists (euglena, diatoms, kelp), All organisms can do cellular respiration (aerobic, anaerobic, fermentation) cyanobacteria (has thylakoids)

In Aerobic (presence of O2): pyruvate is oxidizedIn the absence of O2 (anaerobic bacteria, yeast,

muscle/streneous exercise): pyruvate is reduced

Redox CO2 is reduced to glucose, H2O is oxidized to O2 Glucose is oxidized to CO2, O2 is reduced to H2OEnergy conversion Sunlight into chemical energy (chemical bonds) Chemical energy (bonds) into ATPATP production? Yes, during light dependent/ Used in calvin cycle Yes, during all the steps (that’s the goal)Chemiosmosis Yes, H+ move from thylakoid space to stroma/ATP synth. Yes, H+ move from intermembrane space to matrix/ATP synth.Steps Light dependent and Calvin cycle Glycolysis, pyruvate oxidation, krebs cycle, ETC/chemiosmosis ETC On the thylakoid membrane On the cristae (inner mitochondrial membrane)Enzymes On the stroma (Calvin cycle) On the matrix (Citric acid/Krebs cycle)Electron carriers NADPH/NADP+ NAD+/NADH and FAD/FADH2

+

Photosynthesis and Cellular Respiration are complementary Processes (Lab 4, Activity 1)

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