how cells acquire energy chapter 6. photoautotrophs –carbon source is carbon dioxide –energy...
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How Cells Acquire Energy
Chapter 6
• Photoautotrophs
– Carbon source is carbon dioxide
– Energy source is sunlight
• Heterotrophs
– Get carbon and energy by eating
autotrophs or one another
Carbon and Energy Sources
Photoautotrophs
• Capture sunlight energy and use it to carry out photosynthesis
– Plants
– Some bacteria
– Many protistans
T.E. Englemann’s Experiment
Background
• Certain bacterial cells will move toward places where oxygen concentration is high
• Photosynthesis produces oxygen
T.E. Englemann’s Experiment
Hypothesis
• Movement of bacteria can be used to determine optimal light wavelengths for photosynthesis
T.E. Englemann’s Experiment
Method
• Algal strand placed on microscope slide and illuminated by light of varying wavelengths
• Oxygen-requiring bacteria placed on same slide
T.E. Englemann’s Experiment
T.E. Englemann’s Experiment
Results Bacteria congregated where red and violet wavelengths illuminated alga
ConclusionBacteria moved to where algal cells released more oxygen--areas illuminated by the most effective light for photosynthesis
Linked Processes
Photosynthesis
• Energy-storing pathway
• Releases oxygen
• Requires carbon dioxide
Aerobic Respiration
• Energy-releasing pathway
• Requires oxygen
• Releases carbon dioxide
Organelles of photosynthesis
Chloroplasts
Photosynthesis Equation
12H2O + 6CO2 6O2 + C2H12O6 + 6H2Owater carbon
dioxideoxygen glucose water
LIGHT ENERGY
Two Stages of Photosynthesis
sunlight water uptake carbon dioxide uptake
ATP
ADP + Pi
NADPH
NADP+
glucoseP
oxygen release
LIGHT INDEPENDENT-
REACTIONS
LIGHT DEPENDENT-REACTIONS
new water
• Continual input of solar energy into
Earth’s atmosphere
• Almost 1/3 is reflected back into space
• Of the energy that reaches Earth’s
surface, about 1% is intercepted by
photoautotrophs
Sunlight Energy
Electromagnetic Spectrum
Shortest Gamma rays
wavelength X-rays
UV radiation
Visible light
Infrared radiation
Microwaves
Longest Radio waves
wavelength
Visible Light
• Wavelengths humans perceive as different colors
• Violet (380 nm) to red (750 nm)
• Longer wavelengths, lower energy
Photons
• Packets of light energy
• Each type of photon has fixed amount of energy
• Photons having most energy travel as shortest wavelength (blue-green light)
Pigments
• Light-absorbing molecules
• Absorb some wavelengths and transmit others
• Color you see are the wavelengths NOT absorbed
Wavelength (nanometers)
chlorophyll b
chlorophyll a
• Light-catching part of molecule often has alternating single and double bonds
• These bonds contain electrons that are capable of being moved to higher energy levels by absorbing light
Pigment Structure
Excitation of Electrons
• Excitation occurs only when the quantity of energy in an incoming photon matches the amount of energy necessary to boost the electrons of that specific pigment
• Amount of energy needed varies among pigment molecules
Variety of Pigments
Chlorophylls a and b
Carotenoids
Anthocyanins
Phycobilins
Chlorophylls
Main pigments in most photoautotrophsW
avel
eng
th a
bso
rpti
on
(%
)
Wavelength (nanometers)
chlorophyll b
chlorophyll a
Carotenoids
• Found in all photoautotrophs
• Absorb blue-violet and blue-green that chlorophylls miss
• Reflect red, yellow, orange wavelengths
• Two types– Carotenes - pure hydrocarbons
– Xanthophylls - contain oxygen
Anthocyanins & Phycobilins
Red to purple pigments
• Anthocyanins– Give many flowers their colors
• Phycobilins– Found in red algae and cyanobacteria
Pigments in Photosynthesis
• Bacteria– Pigments in plasma membranes
• Plants– Pigments embedded in thylakoid membrane
system– Pigments and proteins organized into
photosystems– Photosystems located next to electron
transport systems
Photosystems and Electron Transporters
water-splitting complex thylakoidcompartment
H2O 2H + 1/2O2
P680
acceptor
P700
acceptor
pool of electron
transporters
stromaPHOTOSYSTEM II
PHOTOSYSTEM I
• Pigments absorb light energy, give up e- which enter electron transport systems
• Water molecules are split, ATP and NADH are formed, and oxygen is released
• Pigments that gave up electrons get replacements
Light-Dependent Reactions
Photosystem Function: Harvester Pigments
• Most pigments in photosystem are harvester pigments
• When excited by light energy, these pigments transfer energy to adjacent pigment molecules
• Each transfer involves energy loss
Photosystem Function: Reaction Center
• Energy is reduced to level that can be captured by molecule of chlorophyll a
• This molecule (P700 or P680) is the reaction center of a photosystem
• Reaction center accepts energy and donates electron to acceptor molecule
Pigments in a Photosystem
reaction center (a specialized chlorophyll a molecule)
Electron Transport System
• Adjacent to photosystem • Acceptor molecule donates electrons
from reaction center
• As electrons flow through system, energy they release is used to produce ATP and, in some cases, NADPH
Cyclic Electron Flow
• Electrons – are donated by P700 in photosystem I to
acceptor molecule
– flow through electron transport system and back to P700
• Electron flow drives ATP formation
• No NADPH is formed
Cyclic Electron Flow
electron acceptor electron transport system
e–
e–
e–
e–
ATP
Noncyclic Electron Flow
• Two-step pathway for light absorption
and electron excitation
• Uses two photosystems: type I and
type II
• Produces ATP and NADPH
• Involves photolysis - splitting of water
Machinery of Noncyclic Electron Flow
photolysis
H2O
NADP+ NADPH
e–
ATP
ATP SYNTHASE
PHOTOSYSTEM IPHOTOSYSTEM II ADP + Pi
e–
Energy ChangesP
ote
nti
al
to t
ran
sfer
en
erg
y (v
oid
s)
H2O 1/2 O2 + 2H+
(PHOTOSYSTEM II)
(PHOTOSYSTEM I)
e–
e–
e–e–
secondtransport
system
NADPHfirst
transport
system
Chemiosmotic Model of ATP Formation
• When water is split during photolysis, hydrogen ions are released into thylakoid compartment
• More hydrogen ions are pumped into the thylakoid compartment when the electron transport system operates
Chemiosmotic Model of ATP Formation
• Electrical and H+ concentration gradient exists between thylakoid compartment and stroma
• H+ flows down gradients into stroma through ATP synthesis
• Flow of ions drives formation of ATP
• Synthesis part of
photosynthesis
• Can proceed in the dark
• Take place in the stroma
• Calvin-Benson cycle
Light-Independent Reactions
Calvin-Benson Cycle
• Overall reactants
– Carbon dioxide
– ATP
– NADPH
• Overall products
– Glucose
– ADP
– NADP+
Reaction pathway is cyclic and RuBP (ribulose bisphosphate) is regenerated
Calvin- Benson Cycle
CARBON FIXATION
6 CO2 (from the air)
6 6RuBP
PGA
unstable intermediate
6 ADP
6
12
12ATP
ATP
NADPH
10
12PGAL
glucoseP
PGAL2
Pi
12 ADP12 Pi
12NADP+
12
4 Pi
PGAL
Building Glucose
• PGA accepts– phosphate from ATP
– hydrogen and electrons from NADPH
• PGAL (phosphoglyceraldehyde) forms
• When 12 PGAL have formed– 10 are used to regenerate RuBP
– 2 combine to form phosphorylated glucose
Using the Products of Photosynthesis
• Phosphorylated glucose is the building block for:
– sucrose• The most easily transported plant carbohydrate
– starch• The most common storage form
• In Calvin-Benson cycle, the first stable intermediate is a three-carbon PGA
• Because the first intermediate has three carbons, the pathway is called the C3 pathway
The C3 Pathway
Photorespiration in C3 Plants
• On hot, dry days stomata close
• Inside leaf – Oxygen levels rise– Carbon dioxide levels drop
• Rubisco attaches RuBP to oxygen instead of carbon dioxide
• Only one PGAL forms instead of two
C4 Plants
• Carbon dioxide is fixed twice
– In mesophyll cells, carbon dioxide is fixed to
form four-carbon oxaloacetate
– Oxaloacetate is transferred to bundle-sheath
cells
– Carbon dioxide is released and fixed again
in Calvin-Benson cycle
CAM Plants
• Carbon is fixed twice (in same cells)
• Night – Carbon dioxide is fixed to form organic
acids
• Day– Carbon dioxide is released and fixed in
Calvin-Benson cycle
• Fissures in sea-floor where seawater
mixes with molten rock
• Complex ecosystem is based on energy
from these vents
• Bacteria are producers in this system
Hydrothermal Vents
Light and Life at the Vents
• Vents release faint radiation at low end of visible spectrum
• These photons could be used to carry out photosynthesis
• Nisbet and Van Dover hypothesize that the first cells may have arisen at hydrothermal vent systems
Supporting Evidence
• Absorption spectra for ancient photosynthetic bacteria correspond to wavelengths measured at the vents
• Photosynthetic machinery contains iron, sulfur, manganese, and other minerals that are abundant at the vents
Summary of Photosynthesislight
6O212H2O
CALVIN-BENSON CYCLE
C6H12O6
(phosphorylated glucose)
NADPHNADP+ATPADP + Pi
PGA PGAL
RuBP
P
6CO2
end product (e.g. sucrose, starch, cellulose)
LIGHT-DEPENDENT REACTIONS
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