Transcript XII. LIGHT REACTION OF PHOTOSYNTHESIS, cont

UNIT III – CELLULAR ENERGY
Big Campbell ~
Ch 9, 10
Baby Campbell ~
Ch 6, 7
I. ♪ ♫ THE CYCLE OF LIFE ♪ ♫
• Photosynthesis
o
o
• Cellular Respiration
o
o
II. ATP
• ATP → ADP + Pi


• ADP + Pi → ATP


 Mechanisms
 Substrate-level
Phosphorylation
 Oxidative Phosphorylation
 Photophosphorylation
II. ATP
Substrate-Level Phosphorylation vs. Oxidative Phosphorylation
III. ENERGY IN THE CELL
• Oxidation-Reduction Reactions
o Energy yield in catabolism comes from transfer of electrons
o Movement of electrons releases chemical energy of molecule
 Released energy used to generate ATP from ADP and Pi
o Known as redox reaction
 One molecule loses an electron and a 2nd molecule gains an
electron
 Oxidation – Loss of electron(s)
 Reduction – Gain of electron(s)
 Electron donor is known as reducing agent
 Electron acceptor is known as oxidizing agent
 Electron movement in molecules often traced by changes in H
atom distribution
III. ENERGY IN THE CELL, cont
• Oxidation-Reduction Reactions, cont
III. ENERGY IN THE CELL, cont
• Importance of Electron Carriers
o Energy contained in molecules (for example,
glucose) must be released in a series of steps
 Electrons released as hydrogen atoms with
corresponding proton
 Hydrogen atoms are passed to an electron carrier
o Electron carriers are coenzymes
o “Carry” 2 electrons in the form of H-atoms
o Allow for maximum energy transfer, minimum
energy loss
III. ENERGY IN THE CELL, cont
• Electron Carriers
 NAD+
 Nicotinamide adenine
dinucleotide
 Electron acceptor in cellular
respiration
 Reduced to
 FAD
 Flavin adenine dinucleotide
 Electron acceptor in Krebs Cycle
 Reduced to
 NADP+
 Nicotinamide adenine
dinucleotide phosphate
 Electron acceptor in light reaction
of photosynthesis
 Reduced to
III. ENERGY IN THE CELL, cont
• A Closer Look at Electron
Carriers
 Reduction of NAD+
o Dehydrogenase oxidizes
substrate by removing 2 Hatoms
o NAD+ is reduced, creating
NADH + H+
o NADH shuttles electrons to
electron transport chain.
Electrons “fall” down to oxygen
in a series of steps, each
releasing energy in small
amounts.
IV. CELLULAR RESPIRATION – AN OVERVIEW
• Process used by cells to convert chemical energy in glucose
(and other molecules) to ATP
• Primarily takes place in mitochondria of eukaryotic cells
• Overall Reaction
• Steps in Cellular Respiration
 Glycolysis
 “Sugar-breaking”
 Initial breakdown of glucose to intermediate, some ATP
 Citric Acid Cycle
 Completes oxidation of glucose to CO2
 Produces ATP, but more importantly provides high-energy electrons for etc
 Electron Transport Chain
 Oxidative Phosphorylation
 Highest ATP yield; uses energy released from downhill flow of electrons to generate
ATP
 Citric Acid Cycle + Electron Transport Chain = Oxidative Respiration
IV. CELLULAR RESPIRATION OVERVIEW, cont
V. GLYCOLYSIS
•
•
•
•
Occurs in cytosol of cell
Does not require oxygen
First part of pathway is energy investment phase
Second part of pathway is energy pay-off phase
Energy Investment Phase
V. GLYCOLYSIS, cont
Energy Pay-Off Phase
V. GLYCOLYSIS, cont
• Summary of Glycolysis
VI. OXIDATIVE RESPIRATION
• 2 pyruvates formed from glycolysis still contain a
tremendous amount of chemical energy
• If oxygen is available, pyruvate enters mitochondrion
for citric acid cycle and further oxidation
• Upon entering mitochondrion but prior to entering
citric acid cycle
o “Grooming” Step
Carboxyl group of pyruvate is removed, given off as CO2
Remaining 2-C molecule is oxidized to acetate → NAD+
reduced to NADH + H+
Acetate binds to molecule known as Coenzyme A to form
acetyl CoA
VI. OXIDATIVE RESPIRATION, cont
Grooming Step
VI. OXIDATIVE RESPIRATION, cont
• In the citric acid cycle
(AKA Krebs cycle,
tricarboxylic acid cycle,
TCA cycle), 2-C
molecule goes through
a series of redox rxns.
• Occurs in mitochondrial
matrix
• Produces NADH, FADH2,
ATP, and CO2.
• CoA is not actually a
part of the reaction . . .
it is recycled . . .
remember, it is an
enzyme!
VI. OXIDATIVE RESPIRATION, cont
• A Closer Look at the Citric Acid Cycle
VI. OXIDATIVE RESPIRATION, cont
• Electron Transport – Oxidative
Phosphorylation
o Traditionally called Electron
Transport, now more
commonly called Oxidative
Phosphorylation.
o Occurs in inner
mitochondrial membrane
 Membrane organized into
cristae to increase surface
area
o Two components to
Oxidative Phosphorylation
 Electron Transport Chain
 Chemiosmosis
VI. OXIDATIVE RESPIRATION, cont
• Electron Transport Chain
 Collection of molecules, each more
electronegative than the one before
it
 Molecules are reduced, then
oxidized as electrons are passed
down the chain
 Oxygen is ultimate electron acceptor
 Purpose is to establish H+ gradient
on two sides of inner mitochondrial
membrane
 Energy from “falling electrons” used
to pump H+ from matrix into
intermembrane space
VI. OXIDATIVE RESPIRATION, cont
• Chemiosmosis
 Enzyme complexes known as
ATP synthases located in inner
mitochondrial membrane
 H+ electrochemical gradient
provides energy
 Known as proton motive force
 Movement of H+ ions through
membrane rotates enzyme
complex
 Rotation exposes active sites in
complex
 ATP is produced from ADP and Pi
VI. OXIDATIVE RESPIRATION, cont
• A summary of electron transport . . .
VII. CELLULAR RESPIRATION – A SUMMARY
• Each NADH shuttled through ETC results in approximately _________ ATP
• Each FADH2 shuttled through ETC results in approximately _________ ATP.
• Total ATP Gain in Cellular Respiration =
____ (glycolysis) + ____ (citric acid cycle) + ____ (oxidative phosphorylation) = _____ ATP / glucose
VIII. CELLULAR RESPIRATION & OTHER FOOD
MOLECULES
IX. METABOLIC POISONS
• Blockage of Electron Transport Chain
• Inhibition of ATP Synthase
• “Uncouplers”
o Prevent creation of H+ ion gradients due to leakiness of
mitochondrial membrane
X. FERMENTATION
• Anaerobic pathway
• Occurs in cytosol
• Purpose
o In glycolysis, glucose is oxidized to 2 pyruvate, 2 NAD+ are
reduced to 2 NADH, and there is a net gain of 2 ATP
o In oxidative respiration, NADH is oxidized back to NAD+ in
electron transport chain
o If oxygen is not present, another mechanism must be
available to regenerate NAD+ or glycolysis cannot continue
o In fermentation, pyruvate is reduced thereby oxidizing
NADH to NAD+
o Allows glycolysis and net gain of 2 ATP per glucose to
continue
X. FERMENTATION, cont
X. FERMENTATION, cont
XI. PHOTOSYNTHESIS – AN OVERVIEW
• Photosynthesis – Process of capturing light energy and
converting it to chemical energy
• Plants are __________________; also known as _____________
• Redox Reaction
• Chloroplast Structure
o Thylakoids –
o Site of Light Reaction
o First step in photosynthesis
o Grana
o Stroma
o Site of Calvin Cycle
o Second step in photosynthesis
XI. PHOTOSYNTHESIS – AN OVERVIEW, cont
• Location of
Photosynthesis
o Occurs in region
of leaf known as
mesophyll
o Cells contain
abundant
chloroplasts
o CO2 enters leaf
through openings
known as stomata
o H2O enters via
roots
XI. PHOTOSYNTHESIS OVERVIEW, cont
XII. LIGHT REACTION OF PHOTOSYNTHESIS
• Occurs in thylakoid membranes
• Converts light energy to chemical energy
• Light energy
o Visible light is a small portion of the electromagnetic spectrum.
o Light absorbed by chlorophyll and other photosynthetic pigments to
power reactions is not seen. Light not utilized by plant is reflected &
seen by human eye.
o Absorption spectrum – graph illustrating how different pigments absorb
different wavelengths of light
o Action spectrum – graph illustrating rate of photosynthesis vs wavelength
o Light energy measured in photons.
• Photosynthetic pigments
o Chlorophyll a – absorbs mainly blue-violet and red light
o Chlorophyll b – absorbs mainly blue and orange light
o Cartenoids – other accessory pigments; expand spectrum of light energy
that can be used for photosynthesis
XII. LIGHT REACTION OF PHOTOSYNTHESIS, cont
XII. LIGHT REACTION OF PHOTOSYNTHESIS, cont
• A photon of light energy
is absorbed by pigment
molecule in
Photosystem II.
• Energy is passes from
one molecule to
another until it reaches
P680 - pair of
chlorophyll a molecules.
• Electron in each is
excited to higher energy
state – transferred to
primary electron
acceptor.
• Water is split to replace
electron lost by P680.
O2 is released. H+ ions
remain.
XII. LIGHT REACTION OF PHOTOSYNTHESIS, cont
• Excited electron moves from primary electron acceptor to Photosystem I
via electron transport chain. As electron “falls”, energy is released. Used
to synthesize ATP through chemiosmosis.
• Known as photophosphorylation
XII. LIGHT REACTION OF PHOTOSYNTHESIS, cont
• Light energy is transferred via light-harvesting complexes to P700 in
Photosystem I.
• Excited electron is captured by primary electron acceptor. P700’s electron is
replaced by electron transport chain on Photosystem II.
• Electron from P700 moves through a short electron transport chain, reducing
NADP+ to NADPH.
XII. LIGHT REACTION OF PHOTOSYNTHESIS, cont
Linear Electron Flow
XII. LIGHT REACTION OF PHOTOSYNTHESIS, cont
• Cyclic Electron
Flow
o Alternative pathway
seen in some
bacteria, plants
o May be
photoprotective in
plants
o Only utilizes
Photosystem I
o No NADPH
production
o No O2 release
o Does generate ATP
XIII. CALVIN CYCLE OF PHOTOSYNTHESIS
• Also known as Dark Reaction, Light-Independent Rxn
• Occurs in stroma of chloroplasts
• “Synthesis” part of photosynthesis; utilizes ATP,
NADPH generated in Light Reaction + CO2 to produce
organic molecules
• Anabolic; endergonic
• Requires enzyme Rubisco
• Three basic steps
 Carbon Fixation
 Reduction
 Regeneration of RuBP
XIII. CALVIN CYCLE OF PHOTOSYNTHESIS, cont
XIV. PHOTORESPIRATION
• Counterproductive pathway that produces 2-C molecule,
which is then released as CO2
• Due to oxygen competing for active site of Rubisco
• Consumes ATP; decrease carbohydrate yield
XIV. PHOTORESPIRATION, cont
Plant Adaptations