photosynthesis bio ppt slides/10 photosynthesis bw.pdf · photosynthesis 6 co 2 + 6 h 2 o + light...
TRANSCRIPT
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PHOTOSYNTHESIS
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Fundamental biological processes
for making and using energy
Photosynthesis: process by
which plants convert radiant
energy to chemical energy
Respiration: process by which
glucose molecules are broken
down and stored energy is
released
Photosynthesis - autotrophs make glucose
Respiration – organisms break down glucose
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TYPES OF ORGANISMS BY ENERGY
PRODUCTION
Autotrophs- organisms that produce organic
molecules from inorganic substances (photosynthesis)
- Photoautotrophs- use light energy to make food (plants, algae, cyanobacteria)
- Chemiautotrophs-oxidize inorganic chemicals to drive food making reactions (bacteria, fungi)
Heterotrophs
- organisms that obtain energy
from other organisms
(heterotrophs or autotrophs)
- do not make own food
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Location of photosynthesisChloroplast- double membrane organelle
Thylakoid discs (photosystem: 200-300
thylakoids)
- Harvest sunlight
- Contains chlorophyll and
accessory pigments
- Photosystem I and II are linked
structurally and functionally
Grana (stacks of thylakoid discs)
location of light reactions
Stroma (protein rich solution, outside grana)
location of Calvin Cycle
Mesophyll: location of chloroplasts
Stomata: pores in leaf CO2 enters/ O2 exits
Chlorophyll: pigiment in thylakoids
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PHOTOSYNTHESIS
6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2
process whereby autotrophs (plants) take in light energy and convert it to chemical energy (sugar)
Redox Reactions
water is split → e- transferred with H+ to CO2 → sugar
biochemical pathway- series of linked redox reactions where product of one reaction is consumed by the next reaction
Endergonic- absorbs solar energy
Exergonic- releases energy for organism
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Tracking Atoms through Photosynthesis
Evidence that chloroplasts split water molecules enabled
researchers to track atoms through photosynthesis
(C.B. van Niel)
Reactants:
Products:
6 CO2 12 H2O
C6H12O6 6 H2O 6 O2
6 CO2
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Photosynthesis = Light Reactions + Calvin Cycle
“photo” “synthesis”
energy building sugar building
reactions reactions
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Light Energy and PigmentsComes from radiation (energy that travels in waves) from the sun
photon- particles which have energy
wavelength- crest to crest of wave
sunlight- mixture of all visible wavelengths
white light- all wavelengths reflected equally so looks white
visible spectrum- all colors of white light
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Pigment: substance that absorbs light
- photosynthesis: absorbed light energy is used to make chemical
bond energy
- wavelengths not absorbed are reflected (color we see)
Absorption spectrum
graph plotting pigment light absorption vs wavelength
representation of how well particular pigment absorbs different
wavelengths of white light
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Photosynthetic pigments
chlorophyll a (blue green)
- primary photosynthetic pigment
- directly involved in converting light → chemical energy
- hides other pigments
chlorophyll b (yellow green)
- accessory pigment
- absorbs light and transfers energy to chlorophyll a
carotenoids (orange, yellow)
- xanthophylls (yellow) / carotenes (orange)
- accessory pigments
- converts energy to chloro. a
- seen in autumn when chloro. breaks down
- photoprotection for chlorophyll
anthocyanin (red, purple, blue): antioxidants
- non photosynthetic parts of plant (flowers/fruits)
- absorb different pigments so we see other colors
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Determining Absorption Spectrum
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Action SpectrumAction spectrum
plots rate of photosynthesis of
different wavelengths,
i.e. CO2 consumption , O2 release
(different than absorption
spectrum)
Englemann’s experiment:
Used alga and bacteria
Measured O2 output
Result: violet-blue and red
wavelengths caused
most photosynthesis
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Electron Excitement
-light is made of photons
(particles which carry fixed amount of energy)
-when light strikes chlorophyll , some of its atoms absorb the photons
-energy is transferred to the atoms
electrons and excites them to jump to next level
* *Move from ground state to excited state**
- excess energy is released as light or heat
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Photosystems
Photosystem: reaction center (proteins that hold special pair of
chlorophyll a molecules) + light harvesting complexes (cloro. a & b,
carotenoids bound to proteins)
- located in thylakoid discs
- absorb light energy
Primary electron acceptor: accepts electrons and becomes reduced
(electrons move to higher energy level)
Photosystem I: chlorophyll a absorbs at 700 nm- far red (p700)
Photosystem II: chlorophyll a absorbs at 680 nm- red (p680)
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Overview of Stages of Photosynthesis
2 Stage Process
1. light reactions (needs light)
- occurs in thylakoid membranes
4 basic steps
- sun’s energy is trapped by chlorophyll
- electron transport (linear/cyclical)
- water is split and oxygen is released (O2 production)
- ATP and NADPH are formed and released into stroma
- purpose: to make ATP and NADPH(energy carrier molecule)
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2. Calvin Cycle:
- occurs after light reactions
- can occur in light or dark
3 basic steps
- carbon fixation to glucose
- reduction of NADP to
NADPH
- regeneration of RuBP
to start cycle over again
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Light Reactions Electron Flow
Two routes for electron flow:
A. Non-cyclic (linear) electron flow
B. Cyclic electron flow
animation
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STEPS OF LIGHT REACTION
1. photosystem II absorbs light and excites electrons of chlorophyll a
- molecules and electrons are forced to higher energy level(reaction center)
***purpose of photosystem II is to generate ATP and supply electrons to photosystem I ***
2. excited electrons leave chlorophyll a molecule (oxidation reaction)
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3. primary electron acceptor sends electrons into ETC
- reduction reaction
- chain uses energy of electrons to make ATP
- water is split (photolysis) and O2 is released into atmosphere
- electrons from water replace those lost in Photosystem II
- pumps H+ ions (from splitting of water) to interior of grana (lumen)
- inside grana , there is a high concentration (proton gradient) of H+ ions
- chemiosmosis occurs (making of ATP)
- H+ ions back move across grana membranes (ATPsynthase)
LINEAR ELECTRON FLOW
NON-CYCLICAL PHOTOPHOYPHORYLATION
animation
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4. at end of ETC, electrons are passed to photosystem I thru the
(cytochrome complex = plastiquinone Pq (e- carrier) and plastocyanin Pc (protein)
- photosystem also absorbs light to excite electrons in chloro. A
- electrons go thru separate electron transport chain in photosystem I to a different primary electron acceptor
ferradoxin Fd (protein) facilitates movement of e-
- purpose of photosystem I is to generate NADPH
5. NADP+
- accepts electrons and H+ ions (reduces it to NADPH)
- NADPH and ATP move into stroma
LINEAR ELECTRON FLOW
NON-CYCLICAL PHOTOPHOYPHORYLATION
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Cyclic Electron Flow:
• uses PSI only
• produces ATP for Calvin Cycle
animation
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Non cyclic
•PS II and I
•Reaction center is P680
•Both ATP and NADPH are produced
•Photolysis (splitting) of water occurs
•O2 is by-product
•Predominant in green plants
Cyclic
•PS I only
•Reaction center is P700
•Electrons travel back to PS I
•Only ATP is produced
•No photolysis of water
•No O2 involved
•Predominant in bacteria
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End products of light
reactions
1. ATP and NADPH:
needed to power dark
reactions
2. O2:
by product
released into atmosphere
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Chemiosmosis in Chloroplasts and Mitochondria
Respiration and photosynthesis use chemiosmosis to generate ATP
ETCs pump protons (H+)
across membrane from
areas of low concentration
to high concentration
Protons then diffuse back
across membrane thru
ATPsynthase to make ATP
H+ reserviors for each organelle
mitochondria- matrix
chloroplast – lumen
Mitochondria:high energy e- come from organic molecules
Chloroplasts: high energy e- come from water
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Thylakoid Membrane Organization
Proton motive force (H+ gradient) generated by:
(1) H+ from water
(2) H+ pumped across by cytochrome
(3) Removal of H+ from stroma when NADP+ is reduced
animation
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Calvin Cycle- light independent: can occur in light or darkness, always
after light rxns
- occurs in stroma
- purpose: Carbon fixation to glucose molecule (from
CO2 in atmosphere)
- Uses ATP and NADPH
- 3 Phases
1. carbon fixation
2. reduction
3. regeneration of RuBP (CO2 acceptor)
calvin cycle animation
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Steps of Calvin Cycle
1. Carbon fixation
- 3 CO2 enters plant from
atmosphere and binds with
RuBP ribulose biphosphate
(5 C sugar)
- catalyzed by rubisco enzyme
( RuBP carboxylase)
- forms unstable 6 C intermediate
sugar
- this splits into 2 PGA per CO2
net: 6 PGA
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2. Reduction (PGA to G3P)
2 steps
- each PGA gets phosphate
from ATP
- then each molecule reacts with H
from NADPH and breaks
phosphate bond
- net gain: 1 molecule of G3P/PGAL
(glyceraldehydre 3 phosphate)
- 6 G3P formed, but
- 1 molecule used to make sugar
- 5 molecules used to regenerate RuBP
6 ATP and 6 NADPH needed to produce 1 net G3P/PGAL
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3. Regeneration of RuBP from G3P
- 1 G3P/PGAL placed on glucose
- 5 G3P/PGAL used to regenerate RuBP
(cyclical – continues over and over again)
3 ATP needed to regenerate RuBP
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End product of Calvin Cycle
Glucose
*6 turns of cycle needed to make 1 molecule of glucose*
Calvin Cycle uses:
3 CO2, 9 ATP, 6 NADPH
animation
1 C6H12O6 molecule = 6 CO2, 18 ATP, 12 NADPH
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Photorespiration
• wasteful pathway that competes with Calvin cycle
• takes place at the same time that photosynthesis does
• occurs under low CO2/ dry conditions (closed/partially closed stomata)
• rubisco acts on O2 instead of CO2
• 25% less glucose produced
• C4 pathway is a result of evolution
- early plants had high CO2 levels that dropped over time so
plants evolved a more efficient type of photosynthesis to cope
• C4 and CAM help minimize photorespiration
photorespiration video
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Evolutionary Advantages
- Calvin cycle is most common pathway for carbon fixation
- C3 plants: plants that fix C thru calvin cycle (because of 3 C PGA that is initially formed)
- other plants fix C through alternative pathways and then release it into Calvin cycle
- alternative pathways found in plants in dry hot climates
- these plants use STOMATA (pores on undersurface of leaves)
- major passageways thru which O2 and CO2 goes in and out- major passageways of water loss
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Alternative Pathways of Carbon Fixation1. C4 Pathway
- during hottest part of day, stomata are partially closed
- plants fix CO2 into 4 C compounds when CO2 is low
- mesophyll: PEP carboxylase fixes CO2 (4-C)
pumps CO2 to bundle sheath
- bundle sheath: CO2 used in Calvin Cycle
- lose less water than C3 plants
- corn, sugarcane, crabgrass
advantage in hot sunny areas
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Alternative Pathways of Carbon Fixation,
cont.2. CAM Pathway
- Crassulacean acid metabolism (CAM)
- stomata open at night, closed during day
- night: plants take in CO2 and fix into many
compounds, stored in mesophyll cells
- day: light reactions supply ATP, NADPH; CO2
released from organic acids for Calvin cycle
- lose less water than C3 and C4 plants
- cactus, pineapples
advantage in arid areas
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C fixation and Calvin
Cycle together
C fixation and Calvin
Cycle in different
CELLS
C fixation and Calvin
Cycle at different
TIMES
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Factors Affecting Rate of Photosynthesis
Environmental Variables
1. Light intensity/ direction of incoming light
- high intensity = high rate
- saturation point: levels off after certain
intensity because pigments can only
absorb so much light
2. Light color
3. CO2 levels
- same mechanism as light
4. Temperature
- higher temp = higher rate unless enzymes denature
5. pH of leaf
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Factors Affecting Rate of Photosynthesis
Plant Variables
1. leaf color/ variegation- amount of chlorophyll
2. leaf size
3. stomata density and distribution
4. leaf age