Topic 8 Flashcards
Photosynthesis
- The process that converts solar energy into chemical energy within chloroplasts
- The process responsible for oxygen in our atmosphere
- Important chemical process for life on earth
- Autotrophs
“self-feeders”
* Sustain themselves without eating
anything derived from other organisms
* Producers: producing organic molecules from CO2 and other inorganic molecules
* Almost all plants are photoautotrophs
* Usetheenergyofsunlighttomake organic molecules
Photosynthetic Organisms
- Photosynthesis occurs in plants, algae, some unicellular eukaryotes, and some prokaryotes
- Heterotrophs:
obtain organic material from other organisms
* Consumers: eat living organisms
* Decomposers: consume dead material
* Almost all heterotrophs, including humans, depend on photoautotrophs for food and O2
- Chloroplasts
are structurally similar to and likely evolved from photosynthetic bacteria (endosymbiont theory)
* The structural organization of chloroplasts allows for the chemical reactions of photosynthesis
Chloroplast Organization
- Mesophyll: interior tissue of the leaf * Each mesophyll cell contains 30-40
chloroplasts - Stomata: microscopic pores in leaf * Allows CO2 entry and O2 exit
- thylakoids
- chlorophyll
- Vein:
delivers water from roots
- Chloroplasts
(photosynthetic organelles)
* Mainly found in cells of the mesophyll
* Chloroplast has two membranes that surround the dense fluid known as stroma
- Thylakoids:
- Connected sacs in chloroplast
- Third membrane in chloroplast
- Site of photosynthesis (light reaction) * Thylakoid stack is known as a granum
- Chlorophyll:
- Pigment that gives leaves their green color
- Resides in the thylakoid membranes
- Chloroplasts are powered how
solar-powered chemical factories
* Thylakoidstransformlightenergyintothe chemical energy of ATP and NADPH
- Wavelength
is the distance between crests of electromagnetic waves
* Wavelength determines thetype of electromagnetic energy
sunlight is elevtromagnetic energy
electromagnetic spectrum
the entire range of electro magnetic energy or radiation
visable light
380-750 nm
- also the wavelengths that drive photosynthesis
Light also behaves as
discrete particles called photons
Pigments are
substances that absorb visable light
- different pigments absorb different wavelengths
- wavelengths that are not absorbed are reflected or transmitted
- leaves are green bc chlorophyll reflects green light
A spectrophotometer measures
a pigment’s ability to absorb various wavelengths
* This machine sends light through pigments and measures the fraction of light transmitted at each wavelength
* An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength
- Chlorophyll a,
the key light-capturing pigment
* The absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis
Chlorophyll b
an accessory pigment
Carotenoids
a separate group of accessory pigments
Accessory pigments,
, such as chlorophyll b, broaden the spectrum used for photosynthesis
* The difference in the absorption spectrum between chlorophyll a and b:
* A slight structural difference between the pigment molecules
Accessory pigments called carotenoids may
broaden the spectrum of colors that drive photosynthesis
* Some carotenoids function in photoprotection
* They absorb excessive light that would damage chlorophyll or react with oxygen
When a pigment absorbsl ight, itgoes from
a ground state to an excited state which is unstable
- when electrons fall back down, excess energy is released as heat
- in isolation pigments also emit light known as flourescence
-higher energy state is importnat for the light reaction
Photosynthesis consists of the
Light Reactions and Calvin Cycle
- Light reactions vs calvin
photo vs synthesis
the formula of photosynthesis
6 co2 + 6h20 + light energy = c6h12o6 plus 6o2
The overall chemical change during photosynthesis is the reverse of the one that occurs
during cellular respiration
- Chloroplasts split
h2o into hydrogen and oxygen
- incorporates the elctrons of hydrogen into sugar molecules
- releases o2 as product
Photosynthes isi sa
redox reaction in which h2o is oxidized and co2 is reduced
- endergonic process the nergy boost is provided by light
Light Reaction Summary:
- Occurs in Thylakoids
- Split H2O
- Release O2
- Reduce the electron acceptor NADP+ to NADPH
- Generate ATP from ADP by photophosphorylation
- No production of sugar
Light Reactions
- Reduces the electron acceptor NADP+ to NADPH
- NADPH–Nicotinamideadenine dinucleotide phosphate
- ElectronacceptorsimilartoNADH,butwith extra phosphate group
- Generate ATP from ADP by photophosphorylation
- In light reactions, chemiosmosis powers addition of P to ADP
Calvin Cycle Summary:
- Occurs in the stroma
- Forms sugar from CO2, using ATP and NADPH
- The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules
- A photosystem:
- A reaction-center complex surrounded
by light-harvesting complexes - Photosystem II and Photosystem I in Thylakoid Membrane
The light-harvesting complex consists of
pigment molecules bound to proteins
* Light-harvesting complexes transfer the energy of photons to the chlorophyll a molecules in the reaction-center complex
* These chlorophyll a molecules are special because they can transfer an excited electron to a different molecule
- The reaction-center complex:
- Anassociationofproteinsholdingaspecialpairof chlorophyll a molecules and a primary electron acceptor
- A primary electron acceptor in the reaction center accepts excited electrons and is reduced as a result
- Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor > the first step of the light reactions
- Photosystem II (PS II)
functions first
* The reaction-center chlorophyll a of PS II is called P680
* Best at absorbing a wavelength of 680 nm
Photosystem I (PS I)
functions second
* The reaction-center chlorophyll a of PS I is called P700
* Best at absorbing a wavelength of 700 nm
Electron Flow in the Light Reactions
- Linear electron flow:
- The primary pathway
- Involves both photosystems and produces ATP and NADPH using light energy
- Eight steps in linear electron flow Figure 10.UN03
- Eight steps in linear electron flow
1) A photon of light hits a pigment in a light- harvesting complex of PS II,
* Its energy is passed among pigment molecules until it excites P680
2) An excited electron from P680 is transferred to the primary electron acceptor
* P680 is now called P680+
3) H2O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P680
* The H+ are released into the thylakoid space
* O2 is released as a by-product of this reaction
4) Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I.
* Energyreleasedbythefalldrivesthecreationofa proton gradient across the thylakoid membrane
5) Potential energy stored in the proton gradient drives production of ATP by chemiosmosis
6) In PS I (like PS II), transferred light energy excites P700, which loses an electron to the primary electron acceptor
* P700+ accepts an electron passed down from PS II via the electron transport chain
7) Each electron “falls” down an electron transport chain from the primary electron
acceptor of PS I to the protein ferredoxin (Fd)
8) NADP+ reductase catalyzes the transfer of electrons to NADP+, reducing it to NADPH
* The electrons of NADPH are available for the reactions of the Calvin cycle
* This process also removes an H+ from the stroma (helps keep stroma at low [H+])
Chloroplasts and mitochondria generate ATP by
chemiosmosis, but use different sources of energy
* Mitochondria transfer chemical energy from food to ATP
* Chloroplasts transform light energy into ATP
* Spatial organization of chemiosmosis differs, but also shows similarities
In mitochondria:
- H+ are pumped to the intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial matrix
In chloroplasts:
- H+ are pumped into the thylakoid space and drive ATP synthesis as they diffuse back into the stroma
- ATPandNADPHareproducedontheside facing the stroma, where the Calvin cycle takes place
Moving from Light Reactions to Calvin Cycle
In summary:
- Light reactions generate ATP and increase the potential energy of electrons by moving them from H2O to NADPH
- The Calvin Cycle:
- Is anabolic, and uses the chemical energy of ATP and
NADPH to reduce CO2 to sugar - Regenerates its starting material after molecules enter and leave the cycle
- Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde 3-phospate (G3P)
- For net synthesis of one G3P, the cycle must take place three times, fixing three molecules of CO2
The Calvin Cycle has three phases
- Carbon fixation (catalyzed by Rubisco)
- Reduction
- Regeneration of the CO2 acceptor (RuBP)
- Carbon Fixation
- Rubisco (RuBP carboxylase- oxygenase) enzyme
- Mostabundantproteininchloroplasts
- Possiblymostabundantproteinonearth
- CO2 + a 5-C RuBP converted to short lived 6-C intermediate
- Intermediate splits to form 3- phosphoglycerate
- Reduction
- Each3-phosphoglyceratereceivesphosphate group from ATP > 1,3 bisphosphoglycerate
- Require energy input from ATP
- ElectronsdonatedfromNADPHreduces1,3 bisphosphoglycerate to G3P (glyceraldehyde 3- phosphate)
- Also, loss of phosphate group
- G3Pstoresmorepotentialenergy
- ATPandNADPHfromlightreactions
- Regeneration of the CO2 acceptor, RuBP
- 15C worth needs to be recycled
- 5 molecules of G3P rearrange to form
3 molecules of RuBP (a 5C structure) - Requires the energy from 3 ATP
- Reforms 3 molecules of RuBP for next cycle
Net Calvin Cycle
- For synthesis of one G3P molecule, Calvin Cycle consumes:
- 9 ATP
- 6 NADPH
Photosynthesis has two stages
- Photosynthesis consists of the light reactions and Calvin cycle
- Light reactions: photo * Calvin Cycle: synthesis
