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PHOTOSYNTHESIS
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
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
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
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
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
Photosynthesis = Light Reactions + Calvin Cycle
“photo” “synthesis”
energy building sugar building
reactions reactions
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
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
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
Determining Absorption Spectrum
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
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
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)
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)
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
Light Reactions Electron Flow
Two routes for electron flow:
A. Non-cyclic (linear) electron flow
B. Cyclic electron flow
animation
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)
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
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
Cyclic Electron Flow:
• uses PSI only
• produces ATP for Calvin Cycle
animation
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
End products of light
reactions
1. ATP and NADPH:
needed to power dark
reactions
2. O2:
by product
released into atmosphere
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
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
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
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
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
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
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
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
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
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
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
C fixation and Calvin
Cycle together
C fixation and Calvin
Cycle in different
CELLS
C fixation and Calvin
Cycle at different
TIMES
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
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
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