How Cells Harvest Chemical Energy

Chapter 6

Overview

• Photosynthesis

• Aerobic respiration

• Anaerobic respiration

• Alternate sources of energy

Components of a Reaction

Reactants

Intermediates

Products

A CB

Endergonic vs. Exergonic ReactionsEndergonic

= Energy-requiring

Exergonic

= Energy-releasing

Redox Reactions

One molecule gives up electrons = oxidized

One molecule gains electrons = reduced

H+ atoms released simultaneously(are attracted to negative charge of electrons)

Coenzymes pick up e-s & H+ from substrates & deliver to e- transfer chains

Electron Transfer ChainsMembrane-bound groups of enzymes/molecules

Accept & give up e-s in sequence

E-s enter chain at higher energy level than when they leave it

(lose energy at each descending step of chain)

e-s

Substrate-Level Phosphorylation

Formation of ATP by direct transfer of Pi group to ADP from intermediate

NAD+ & FAD

Coenzymes

1. Accept e-s & H+ from intermediates that form during glucose catabolism

Become reduced = NADH & FADH2

2. NADH and FADH2 give up e-s & H+ to e- transfer chains during final stages of

aerobic respirationBecome oxidized = NAD+ & FAD

Autotrophs

“Self-nourishing”

Synthesize own food

Obtain energy & organic compounds (e.g. C) from the physical environment

Chemoautotrophs

Have no enzymes to allow for complex metabolic reactions

Obtain energy & C from simple inorganic & organic compounds e.g. CH4, H2S

Photoautotrophs

Contain light-sensitive molecules

Can split H2O & use electrons

Process releases lots of oxygen, which reacts rapidly with metals & creates toxic

free radicals

Early photoautotrophs existed when there was lots of Fe & metals everywhere

Released O2 oxidized these metals & rusted them out

O2 could then be released freely

Over a few thousand years, O2 levels in sea & atmosphere increased

Survival of the fittest:

Most anaerobes died out because couldn’t neutralize toxic O2 radicals

Chemoautotrophs with little or no O2 tolerance restricted to extreme & anoxic environments

As O2 accumulated in the atmosphere, O atoms combined to form O3

= ozone layer(protects against lethal UV radiation from sun)

Life was able to move out from the “darks” & live under open sky

= diversification= evolution

Photosynthesis

The process by which photoautotrophs use light energy from the sun to make glucose,

which can then be converted into ATP

12H2O + 6CO2 6O2 + C6H12O6 + 6H2O

Respiration

= breathing

Cellular respiration

= getting energy from food

Organisms need usable energy in order to survive

Obtained energy is converted into ATP chemical bond energy

Can be used to do work e.g. metabolism

Anaerobes

Can’t tolerate O2

Make ATP via fermentation

1 glucose → 2 ATP

e.g. first organisms, some prokaryotes & eukaryotes

Clostridium difficile

Aerobes

Require O2

Make ATP via aerobic respiration

(many also use anaerobic pathways)

1 glucose → 36+ ATP(vital for survival of large organisms)

e.g. most eukaryotes, some prokaryotes

Facultative Anaerobes

Normally use aerobic pathways (i.e. use O2)

Can switch to anaerobic pathways when O2 levels are lowEntamoeba histolytica

Mitochondria

Membrane-bound organelles in most eukaryotic cells (# differs depending on cell type)

Power source of cells– Production of ATP in presence of O2

– Convert NADH and FADH2 into ATP energy via oxidative phosphorylation

Allow cell to produce lots of ATP simultaneously– Without mitochondria, complex animals wouldn’t exist

Mitochondrion Structure

Outer membrane

Selectively permeable

Inner membrane

Highly impermeable Contains ATP synthase Has membrane potential

Cristae

↑ surface area of inner membrane, which ↑ capacity to generate ATP

Matrix

Contains 100s of enzymes which oxidize pyruvate and fatty acids, and control the Krebs cycle

Cellular Respiration

The oxidation of food molecules (e.g. glucose) into CO2 & H2O

Energy released is captured as ATPUsed for all endergonic activities of cell

Enzymes catalyze each step

Intermediates formed at one step become substrates for enzyme at next step

2 phases of cellular respiration:

Glycolysis:Glucose → 2 pyruvates

Occurs in all cells

Oxidation of pyruvate into CO2 and H2O:

Energy-releasing pathways differ depending on cell & its

needs

40% of energy from glucose is harvested

Rest (60%) is lost as heat

A working muscle uses 10 million ATP per second!

Aerobic Respiration

C6H1206 + 6O2 → 6CO2 + 6H2O

Breakdown of glucose in presence of O2

3 stages of reactions:– Glycolysis

– Krebs Cycle– Electron Transfer Phosphorylation

Glycolysis– Glucose → 2 pyruvates

– Occurs in cytosol

Krebs Cycle– Pyruvate → CO2 + H2O + e-s

– Occurs in mitochondria

Electron Transfer Phosphorylation– Formation of lots of ATP

Stage I: Glycolysis

Glucose → 2 pyruvates

“Universal energy-harvesting process of life”

Initial energy-releasing mechanism for all cells

Occurs in cytosol

Coupled endergonic & exergonic reactions

Endergonic Steps of Glycolysis

Requires input of 2 ATP

ATP #1 phosphorylates glucoseGlucose → intermediate

ATP #2 transfers Pi to intermediate

Intermediate → PGAL + DHAP

DHAP converts into PGAL= 2 PGAL enter next stage

Exergonic Steps of Glycolysis

Each PGAL gives 2 e-s + H+ to NAD+ 2 NAD+ → 2 NADH

Intermediates each give Pi to ADP2 ADP → 2 ATP

(substrate-level phosphorylation)

Pays back 2 ATP used in endergonic steps

Intermediates each release H+ + OH2 intermediates → 2 PEP

Each PEP gives Pi to ADP2 ADP → 2 ATP

(substrate-level phosphorylation)

2 PEP → 2 pyruvate

Sum Total of GlycolysisGlucose → 2 pyruvate + 2 NADH + 2 ATP

From here, pyruvate can enter:– Aerobic pathway (Krebs cycle)

– Anaerobic pathway (fermentation)

(depends on cell & environmental conditions)

Stage II: Krebs Cycle

Pyruvate → CO2 + H2O (+ e-s)

a.k.a. citric acid cycle

Occurs in mitochondria

Main function is to supply Stage III with e-s

(in order to reduce NAD+ & FAD in stage III)

Mitochondrial membrane proteins transport pyruvate into inner compartment

Enzymes take 1 C from pyruvateC + O2 → CO2

Intermediates + coenzyme A → acetyl-CoA

NAD+ is reduced into NADH

Acetyl-CoA enters Krebs cycle

Transfers 2 Cs to oxaloacetate → citrate

Rearrangement of intermediates occurs

2 C released → 2CO2

3 NAD+ + H+ + e-s → 3 NADH

ADP + Pi → ATP

FAD + H+ + e-s → FADH2

Oxaloacetate regenerates so that cycle can run again

In total, one turn of the cycle:

3 NADH + 1 FADH2 + 1 ATP

Cycle repeats again for 2nd pyruvate molecule Remember 1 glucose → 2 pyruvates

After both pyruvates are broken down:6 NADH + 2 FADH2 + 2 ATP

Sum Total of the Krebs Cycle

With 2 NADH from acetyl-CoA formation:

2 pyruvate → 8 NADH + 2 FADH2 + 2 ATP + 6CO2

CO2 released into surroundings

NADH & FADH2 deliver e-s and H+ to 3rd stage

Stage III: Electron Transfer Phosphorylation

H+ + e-s → H2O + ATP

E-s delivered to electron transfer chains (ETCs) in inner mitochondrial membrane

E- flow in ETCs drives phosphorylation of ADP → ATP (lots of it!)

a.k.a. oxidative phosphorylation

ATP formed by oxidation of NADH & FADH2

Responsible for high ATP yield

NADH & FADH2 give e-s to ETCsSimultaneous release of H+

Energy released at each transfer of ETCAt 3 transfers, released energy pumps H+ across mitochondrial membrane into outer compartment

Concentration & electric gradients result across inner membrane= membrane potential

H+ re-enters inner compartment by flowing

down concentration gradient through ATP

synthases

Causes reversible change in shape of ATP synthases

ADP + Pi → ATP

(oxidative phosphorylation)

At end of ETCs, O2 picks up e-s & H+→ H2O

Sum Total of ET Phosphorylation

H+ + e-s → H2O + 32 ATP

In liver, heart, and kidney:

– e-s from NADH delivered to different ETC entry point

– H+ gradient makes 3 ATP (instead of 1)

– Results in 34 ATP total

Animation of ET Phosphorylation

• http://vcell.ndsu.nodak.edu/animations/etc/movie.htm

• http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter9/animations.html#

In oxygen-starved cells, e-s have nowhere to go so get gridlocked

No e- flow = no H+ gradients = no ATP forms

Results in cell death because not enough ATP to sustain metabolic processes

3 different categories of poisons interfere with cellular respiration:

• ETC Blockers

• Inhibitors

• Uncouplers

ETC Blockers

Block ETC at various steps of chain

Starves cells of energy by prohibiting ATP

synthesis

Inhibitors

Inhibit ATP synthase

No passage of H+ through ATP synthase

= no ATP generation

Uncouplers

Make mitochondrial membrane leaky to H+

Electron transport & O2 consumption continue but lack of H+ gradient

= no ATP synthesis

Sum Total of Aerobic Respiration

GlycolysisGlucose → 2 pyruvate + 2 NADH + 2 ATP

Krebs Cycle2 pyruvate → 8 NADH + 2 FADH2 + 2 ATP + 6CO2

ET PhosphorylationH+ + e-s (from coenzymes) → 32 ATP + 6H2O

Glucose + 6O2 → 6CO2 + 6H2O + 36 ATP

(38 ATP in liver, heart, kidney)

Cellular Respiration Video

• http://video.google.com/videoplay?docid=1463788471587082686&q=respiration+ninja&total=2&start=0&num=10&so=0&type=search&plindex=0

Anaerobic Respiration

Oxidation of molecules in absence of O2

Requires different electron acceptor at the end of it

2 stages:

– Glycolysis

– Various Energy-Releasing Pathways:• Alcoholic Fermentation• Lactate Fermentation

• Anaerobic Electron Transfers

Stage I: Glycolysis

Glucose → 2 pyruvate + 2 NADH + 2 ATP

Glucose is not broken down any further into CO2 or H2O

All ATP comes from glycolysis(not enough energy to sustain large multicellular

organisms)

After Glycolysis

Final stages in fermentation pathways do not generate more ATP

Instead, they regenerate NAD+ so that it can act as an electron acceptor

Pyruvate not moved into mitochondria

(stays in cytosol & is converted into waste products that can be transported out of cells)

Waste product depends on type of celle.g. ethanol in yeast

e.g. lactate in skeletal muscles, bacteria

Alcoholic Fermentation2 pyruvates → 2 acetylaldehydes + 2CO2

NADH gives up e-s & H+ to acetaldehyde → ethanol

e.g. yeasts (Saccharomyces spp.) that ferment wine, bread, etc.

Lactate FermentationNADH gives e-s and H+ to pyruvate → lactate

e.g. bacteria (Lactobacillus spp. and others) in cheese, yoghurt, etc.

Certain organisms can couple aerobic & anaerobic respiration or switch from one

mode to the other

e.g. skeletal muscles associated with bonesVariety of cell types within muscle fibres

Slow-Twitch Muscle Fibres

Lots of mitochondria & myoglobin

Make ATP via aerobic respiration

Used for steady, prolonged activity

e.g. long-distance running, migration, etc.

e.g. “dark meat” of birds

Fast-Twitch Muscle Fibres

Few mitochondria & no myoglobin

Make ATP via lactate fermentation

Used for short bursts of intense activity

e.g. sprints, weight-training

e.g. “white meat” of birds

Anaerobic Energy Transfers

Pathway of some archaeans & bacteria

Inorganic compounds used as final e- acceptors rather than O2

Aids in cycling of elements through biosphere

Energy yield varies but is small

Alternative Energy Sources

• Glycogen Stores

• Lipids

• Proteins

The Fate of Glucose

When food is ingested, glucose is absorbed into the bloodstream

Pancreas secretes insulin to make cells take up glucose faster

Cells convert glucose to glucose-6-phosphate (intermediate of glycolysis)

Can’t leave cell once phosphorylated

Glycogen Stores

When more glucose than necessary is taken in, is biosynthesized into glycogen

(stored in liver and muscles)

Only 1% of total energy stores

Glycogen stores used up within 12 hours if regular meals aren’t eaten

When blood glucose drops, pancreas secretes glucagon

Liver cells convert stored glycogen → glucose & send back to blood

Can then enter glycolysis pathway

If excess carbs are eaten:Glucose → pyruvate → acetyl-CoA

(aerobic respiration)

Acetyl-CoA doesn’t enter Krebs cycle if excess glucose

Enters lipid biosynthesis pathway instead↑↑↑ carbs = fat

Fat Stores

Fat stored as triglycerides in adipose cellsTriglyceride = glycerol + 3 fatty acid tails

Between meals or during sustained exercise, fatty acids yield half of ATP needed by

muscle, liver, kidney cells

When blood glucose ↓, enzymes in adipose cells separate glycerol & fatty acids and

release into blood

Glycerol → PGALUsed in glycolysis

Fatty acids → acetyl-CoAUsed in Krebs cycle

Protein Stores

When proteins ingested, are broken down into amino acids

Absorbed into bloodstream and taken up by cells to make more proteins, etc.

If excess protein eaten, amino acids broken down

Form acetyl-CoA, pyruvate, or intermediates of Krebs cycle

“Reverse” pathways also exist

Food molecules used for biosynthesis

Requires ATP

Summary

Energy is required by all living organisms to sustain life

Because energy flow is one-directional, constant energy

inputs are needed

Interpreting DataThis graph illustrates the free energy relative to oxygen of the electron transport chain. The solid blue circles are electron carrier molecules, and the light blue ovals represent protein complexes. From an energy standpoint, are these reactions endergonic or exergonic?

a. Endergonic

b. Exergonic

c. Some are exergonic and others are endergonic.

d. There is not enough information.

Interpreting Data

What would happen to the flow of electrons if oxygen were not present?

a. The flow of electrons would continue but at a slower rate.

b. The flow would cease and ATP production would stop.

c. The presence of oxygen would have no effect.