Transcript 6-22 Reaction centres - McGraw Hill Higher Education

Chapter 6: Harvesting energy
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-1
Harvesting chemical energy
• Organisms convert chemical energy of fuel
molecules to useable energy in the form of
adenosine triphosphate (ATP)
– ATP is used to drive cellular processes
• Energy is released along metabolic pathways
– carbohydrates processed by glycolysis
– lipids processed by β–oxidation
• Products of pathways act as substrate for cellular
respiration
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-2
Fig. 6.2: Overview of metabolic pathways
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-3
Glycolysis
• One of the earliest biochemical pathways to evolve
• Glucose from polysaccharides processed in
cytosol by glycolysis
• Glycolysis is a net producer of energy
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-4
Glycolysis (cont.)
• First stage uses energy
– two ATP molecules used to phosphorylate and change
glucose before splitting it into two 3-carbon molecules
(glyceraldehyde 3-phosphate)
• Second stage
– oxidation of glyceraldehyde 3-phosphate to pyruvate is
coupled to ATP synthesis
– four ATP molecules produced (giving net energy profit of
two molecules)
– four electrons and two hydrogen atoms transferred to
NAD+ to produce two molecules of NADH
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-5
Fig. 6.3: Glycolysis (top)
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-6
Fig. 6.3: Glycolysis (bottom)
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-7
β-oxidation
• Lipids hydrolysed into free fatty acids and glycerol
– fatty acids are substrate for β-oxidation
• β-oxidation takes place inside mitochondria
– carbon atom backbone broken down two carbon atoms at
a time
– four reactions oxidise carbon and produce acetyl CoA
– energy from C–C bond conserved in C–H bond in acetyl
CoA
– acetyl CoA enters citric acid cycle
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-8
Fig. 6.4: β-oxidation
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-9
Citric acid cycle (Krebs cycle)
– acetyl CoA from lipids (by β-oxidation) and pyruvate (by
glycolysis) combines with oxaloacetate releasing
coenzyme A and forming 6-carbon citrate
– citrate is rearranged into isocitrate
– isocitrate stripped of electrons and H+, which are
transferred to NAD+ to form NADH
– CO2 released
– resulting 5-carbon α-ketoglutarate undergoes removal of
electrons and H+ and release of CO2
– succinyl-CoA (4-carbon product) converted in four steps
to oxaloacetate
– electrons and H+ transferred to form FADH2 and NADH
– ATP produced
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-10
Fig. 6.5: Citric acid cycle
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-11
Electron transport system
• During glycolysis and the citric acid cycle,
electrons are temporarily stored in NADH and
FADH2
• Energy conserved in these molecules is converted
into ATP via electron transport system
• NADH and FADH2 transfer electrons to carrier
proteins
• Electron transport system is embedded in
– plasma membrane of prokaryote cells
– inner membrane of eukaryote mitochondria
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-12
Electron transport system (cont.)
• Cytochrome c oxidase uses four e– and four H+ to
reduce one molecule of O2 to two molecules of
H2O
• H+ concentration gradient provides electrochemical
force driving ATP synthesis
– process catalysed by transmembrane enzyme complex
ATP synthase
• Action of ATP synthase
– channel allows H+ to move freely down electrochemical
gradient
– movement is source of energy for ATP synthesis
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-13
Question 1:
Which process in eukaryotic cells will proceed
normally whether oxygen (O2) is present or
absent?
a) electron transport
b) glycolysis
c) the citric acid cycle
d) oxidative phosphorylation
e) chemiosmosis
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-14
Fermentation
• ATP produced in absence of oxygen by
fermentation
– additional reactions consume NADH produced in
glycolysis for reduction of pyruvate
• End products
– lactate (animals)
– ethanol (plants)
– lactate and ethanol (bacteria, yeasts)
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-15
Photosynthesis
• Light energy is harvested and stored in chemical
bonds of ATP and carbohydrates, made from CO2
and H2O
Visible light
6CO2
from
atmosphere
+ 12H2O
water
→
C6H12O6 +
sugar
6O2
from original
water
molecule
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
+
6H2O
water
6-16
Photosynthesis (cont.)
• Absorption of energy from sunlight by pigments
– absorbed light energy is passed from pigments to
reaction centres of photosystems I and II in thylakoid
membranes of chloroplasts
• Reactivation of reaction centres
– electrons are stripped from water to reactivate reaction
centres of photosystems
• Carbon fixation to produce carbohydrates in dark
reaction
– energy stored in ATP and NADPH used to synthesise
sucrose and starch
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-17
Photosynthetic pigments
• Pigments absorb photons of particular
wavelengths of light and reflect or transmit others
– chlorophyll absorbs red and blue wavelengths and
reflects green light
• Pattern of absorption of a pigment is absorption
spectrum
– absorption spectrum of chlorophyll is similar to the
wavelengths that activate photosynthesis (activation
spectrum)
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-18
Photosynthetic pigments (cont.)
• Chlorophyll molecules are formed from a central
magnesium atom surrounded by alternating single
and double bonds forming a porphyrin ring
– absorption of photons excites magnesium electrons
– energy directed through bonds of porphyrin ring
• Pigments
– chlorophyll a
– chlorophyll b
– carotenoids
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-19
Chloroplasts
• In eukaryotes, chlorophyll and other photosynthetic
pigments are located in chloroplasts
• Chloroplast structure
– double membrane
– third inner membrane (thylakoid membrane)
– matrix (stroma)
• Protein complexes integrated into thylakoid
membranes
– photosystems I and II
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-20
Photosystems I and II
• Photosystems are photosynthetic electron
transport systems
– light-harvesting complexes
– electron transport complexes
– ATP-synthesising complexes
• Pigment molecules in light-harvesting complexes
arranged so excitation energy is channelled to a
specific pair of chlorophyll molecules, the reaction
centre
– P700 (PS I)
– P680 (PS II)
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-21
Reaction centres
• As a response to excitation, reaction centre expels
electron
• Electron expelled from P680 accepted by electron
acceptor on opposite side of photosystem
– loss of e– creates positive charge in reaction centre
– electron donor provides e– to neutralise reaction centre
– donor itself is neutralised by e– stripped from H2O, which
produces O2 and four H+ for every four e– displaced from
reaction centre
– e– on electron acceptor is passed to cytochrome b/f
complex, which passes it on to electron donor molecule
of PS I
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-22
Reaction centres
• Light-harvesting complex associated with PS I
absorbs photon
– energy allows e– from P700 to move to an electron
acceptor
– e– removed from PS I and passed to ferredoxin, which
passes them to NADP+
– NADP+ reduced to NADPH
• H+ gradient provides potential energy used in ATP
synthesis
– for every three H+, one ATP molecule is synthesised from
ADP and phosphate
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-23
Fig. 6.17: Thylakoid membrane complexes
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-24
Photophosphorylation
• Non-cyclic electron transport in photosynthesis
– H2O → PS II → PS I → NADP+
• Non-cyclic photophosphorylation
– ATP synthesis coupled to non-cyclic electron transport
• Cyclic phosphorylation
– e– can be transported back to PS I by ferredoxin and
cytochrome b/f complex
 not used for NADPH production
– ATP synthesised
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-25
Photosynthesis in prokaryotes
• Earliest photosynthetic organisms were
anoxygenic photoautotrophs
– used H2S or organic molecules instead of H2O as source
of e– for NADPH
– O2 not produced as by-product
• Evolution of PSII in cyanobacteria provided
mechanism for using H2O as source of e–
– production of O2 as by-product changed composition of
atmosphere
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-26
Question 2:
Photosynthetic plant cells differ from animal cells
because:
a) they don’t contain mitochondria.
b) ATP for all cellular processes is produced by
chemiosomosis in the chloroplast.
c) they do not contain enzymes.
d) they are capable of producing carbohydrate
from light energy, that can be metabolised.
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-27
Carbon fixation
• Atmospheric CO2 is incorporated into
carbohydrates
• CO2 reduction
– CO2 is attached to 5-carbon ribulose biphosphate (RuBP)
• Carboxylation of RuBP is part of Calvin–Benson
cycle in which carbohydrates are formed
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-28
Calvin–Benson cycle
• Carboxylation of RuBP by ribulose biphosphate
carboxylase-oxygenase (Rubisco) produces
unstable 6-carbon intermediate
• Intermediate splits into two 3-carbon molecules of
phosphoglyceric acid (PGA)
– PGA phosphorylated by ATP
– intermediate compound reduced and dephosphorylated
with NADPH to form glyceraldehyde 3-phosphate (PGAL)
• PGAL can follow three paths
– sucrose production
– starch production
– RuBP production
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-29
Calvin–Benson cycle (cont.)
• Sucrose production
– up to two molecules in every twelve exported from
chloroplast to cytoplasm
– combined and rearranged to form fructose and glucose
phosphates
– these compounds condensed to form sucrose
– inorganic phosphate imported to replace that lost as part of
PGAL
• Starch production
– up to two PGAL molecules combined, rearranged and used
in synthesis of starch
– starch stored in chloroplasts
• RuBP production
– in stroma, remaining ten PGAL molecules used to form six
RuBP molecules to complete cycle
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-30
Photorespiration
• O2 competes with CO2 for binding site on Rubisco
– Rubisco has higher affinity for CO2 than for O2
• Photorespiration
– process occurs only in light
– consumes O2 and produces CO2
• CO2 produced in photorespiration reduces amount
of carbohydrate manufactured
– also uses ATP
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-31
C4 pathway
• Photosynthetic pathways are adaptations to
environmental conditions
– tropical and subtropical grasses and other plants use C4
pathway
– stomata generally not as wide open as in C3 plants
– concentrate CO2 in bundle sheath cells inhibiting
photorespiration
• Leaf anatomy
– vascular bundles surrounded by cylinder of bundle
sheath cells
– bundle sheath and mesophyll cells contain chloroplasts
that differ in structure and function
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-32
C4 pathway (cont.)
• In C4 pathway, the first stable product of carbonfixation is a 4-carbon compound
• Cytoplasm of leaf mesophyll cells
– additional enzyme, phosphoenolpyruvate (PEP)
carboxylase, catalyses carboxylation of PEP
– produces oxaloacetate
– oxaloacetate converted into malate
• Chloroplasts of bundle sheath cells
– malate decarboxylated to CO2 and pyruvate
– CO2 fixed into carbohydrates by Calvin–Benson cycle
– pyruvate transported back to mesophyll cells
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-33
Crassulacean acid metabolism (CAM)
• Evolved independently in Crassulaceae,
Bromeliaceae and other plant families
• CAM is a variation on the C4 pathway
– C4 and Calvin–Benson cycle reactions occur at different
times
• Stomata open at night, reducing moisture loss
– 4-carbon compounds produced in darkness and stored
until daylight when they are decarboxylated
– CO2 released then fixed normally via RuBP and Calvin–
Benson cycle
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-34
Question 3:
What implications do you think global warming
would have for plants?
a)
b)
c)
d)
e)
Decreased plant growth
Increased plant growth
Increased desertification
Increase in tropical rainforest
It depends on latitude
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-35
Summary
• All organisms, both autotrophs and heterotrophs,
extract energy from the oxidation of glucose
• Electrons drive proton pumps that power ATP
synthesis
• All energy supporting life on earth originates from
the sun and is trapped by photosynthetic
organisms, energising electrons that drive
reactions
• There are two types of reaction centre associated
with the two photosystems (PSI and PSII)
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-36
Summary (cont.)
• ATP and NADPH generated in the light-dependent
reactions drive the Calvin–Benson cycle and the
synthesis of sugar from CO₂
• C4 photosynthesis and CAM are special
adaptations of the carbon-fixing process in some
plants
Copyright  2010 McGraw-Hill Australia Pty Ltd
PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint
Slides prepared by Karen Burke da Silva, Flinders University
6-37