Lecture 6: Photosynthesis

Question 1

Photosynthesis

Answer 1

1. main route by which energy enters the biosphere
2. creation of organic matter
   - high energy organic compounds (like carbohydrates) are made from low-energy, highly abundant, non-toxic inorganic compounds (CO₂ & H₂O) sing energy from the sun
   - Net equation of two sets of reactions:

6CO₂+ 12 H₂O -> C₆H₁₂O₆ + 6O₂ + 6H₂O

Question 2

Photosynthetic net equation

Answer 2

Net equation of two sets of reactions:

6 CO₂ + 12 H₂O -> C₆H₁₂O₆ + 6O₂ + 6H₂O

Question 3

Metabolic Chemical Reactions

Answer 3

• chemical reactions either release or require energy
• exergonic reactions
• endergonic reactions
• ATP
• ADP
• AMP
• in metabolic systems you see exergonic and endergonic reactions coupled together

Question 4

Exergonic reactions

Answer 4

• potential energy of substrates greater than that of products (energy out)
• (left side of the equation(input) is greater than the right side (products))

Question 5

Endergonic reactions

Answer 5

• potential energy of substrates is lower than that of products (energy in)
• (left side is less than the right side of the equation)

Question 6

ATP

Answer 6

• adenosine triphosphate
• main energy carrier of cells
• covalent bonds between phosphate groups contain potential energy

Question 7

ADP

Answer 7

Adenosine diphosphate

Question 8

AMP

Answer 8

Adenosine monophosphate

Question 9

Redox reactions

Answer 9

• reduction and oxidation happening simultaneously
• during chemical reactions electrones (e-) may shift energy levels and move from one atom to another creating oxidation-reduction reactions
• think “LEO the lion goes GER”
• oxidations and reduction reaction occur in pairs
• electron transfer is accompanied by H+ transfer
• electrons and protons are transferred by via electron transport chains made up of electron acceptor molecules, such as NAD+ = nicotinamide adenine dinucleotide; NADP+ = nicotinamide adenine dinucleotide phosphate
• at each link in chain some energy is lost as heat or to do metabolic work

Question 10

Oxidation

Answer 10

when an atom loses an electron it is oxidized

Question 11

Reduction

Answer 11

when an atom gains an electron it is reduced

Question 12

Ordered chemical reactions

Answer 12

• in cells chemcial reactions are organized into pathways
• reactions are catalyzed by enzymes
• product of one reaction bemoes substrate of another
• in cyclic pathways the starting molecule of the cycle is always regernated in the last step of the cycle

Question 13

Where does biomass of the biosphere come from?

Answer 13

Annually photosynthetic organisms produce > 250 million metric tons of carbohydrates

Question 14

Applications to photosynthesis

Answer 14

• photosynthesis captures the energy of visible light
• the longer the wavelength the less energy it carries
• visible light has the right amount of energy to excite electrons in organic molecules
• biomass of the biosphere….it all comes from photosynthetic reactions

Question 15

Electron states:

GROUND STATE

Answer 15

• the lowest energy state of an electron is the ground state

Question 16

Electron states:

EXCITED STATE

Answer 16

• when electron absorbs energy (quantum) from visible light it jumps to a higher orbital position and enters an excited state

Question 17

What is an example of molecules that absorb light?

Answer 17

Pigments are example of molecules that absorb light

• Chlorophyll
• phytochrome

Question 18

3 Potential Paths for Excited Electrons:

Answer 18

• excitation energy converted to light/heat
• energy passed to another molecule causing it to be excited
• e- (electron) passed to e- acceptor molecule

Question 19

Pigments:

Chlorophyll a

Answer 19

• is most abundant light-absorbing pigment in cyanobacteria, algae, and chloroplasts of plants
• when Chlorophyll a absorbs visible light an electron is excited
• the excited electron is transferred to an electron acceptor molecule

Question 20

Examples of accessory pigments

Answer 20

• Chlorophyll b, carotenoids (a family of pigments), and xanthophylls are examples of accessory pigments
• the presence of accessory pigments increase the amount of wavelengths of light that can be absorbed by the light, increase the different wavelengths of light that can be absorbed by the plant

Question 21

Chloroplasts****** MAKE SURE THIS IS CORRECT?!????

Answer 21

• Grana -
• thylakoid membranes - also called lamellae, in the interioir of the chloroplasts and where the light reactions of photosynthesis occur
• grana stacks - double membranes of thylakoid membranes alternating with sheets of interconnecting stroma thylakoids

Question 22

Light harvesting complex

Answer 22

• pigments in thylakoid membranes absorb photons
• Chlorophyll b, carotenoids, xanthopyll
• resonance transfer (????)****
• reaction center, chlorophyll a dimer
• chlorophyll molecules transmit energy from excited elctrons in the antenna complex to a reaction center
• antenna complex - AKA light harvesting complex

Question 23

Light reactions (ADD MORE STUFF AND MAKE SURE THIS IS CORRECT)

Answer 23

• see image
• the link between the light reactions and the calvin cycle are ATP and NADPH (calvin cycle is where sugars are made)
• light reactions - light is involved, water is involved, oxygen is released, ATP is made and NADPH is made
• NADP+ and ADP are made in calvin cycle and shuttled back to the light reactions
• lower in the diagram indicates lower energy and higher in the diagram indicates higher energy
• step 4 on diagram is one of most important parts of this

Question 24

Plitolysis (ADD MORE????)

Answer 24

• the splitting of water

Question 25

What is the heart of Photosystem II

Answer 25

- the heart of PS II is P680, a dimer - 680 nm is the wavelength of light that 680 is.....??????

Question 26

Pq

Answer 26

- Plastoquinone - one of the most important electron acceptor molecules in the (first) ETC

Question 27

Pc

Answer 27

- Plastocyanin - one of the most important electron acceptor molecules in the (first) ETC

Question 28

What is the heart of Photosystem I

Answer 28

- the heart of Photosystem I is the dimer p700

Question 29

Fd

Answer 29

- Ferrodoxin - contains iron, first important electron acceptor in the second electron transport chain (ETC)

Question 30

NADP+ reductase

Answer 30

- an enzyme that catalyzes the reaction

Question 31

What is produced at the conclusion of the light reactions?

Answer 31

- at conclusion of light reactions ATP and NADPH are created

Question 32

Light reactions: purpose, location, and key reactions

Answer 32

- Location: chloroplast, lamella (aka thylakoid membrane) - Purpose: to take light and make usable energy to be utilized by other organisms - Key reactions: hydrolysis - splitting of water - donates electron to PS2 then PS2 absorbs light which excites the electrons -\> electrons b/c it is excitred can move up the ETC -\> looses energy as it goes down ETC (have redox rxns occurring during this) -\> redox reactions move the electrons through the ETC -\> there it goes into PS1 then it repeats - light is excited

Question 33

Impact of herbicides in the light reactions

Answer 33

- herbicide actions often block amino acid, carotenoid, or lipid biosynthesis or disrupt cell division - chemical structure of two herbicides that block photosynthetic electron flow - example: 1. DCMU acts by blocking electron flow at the quionone acceptors of PS2 2. Paraquat acts by accepting electrons from the early acceptors of PS1 and then reacts with oxygen to form superoxide - a species that is very damaging to chloropast components

Question 34

embedded in thylakoid membrane

Answer 34

- lipid bilayer of cells - acidified lumen - electrochemical gradient across thylakoid membrane - ADP phosphorylated by ATPase - NADP+ terminal electron acceptor of PS 1 - lumen oh thylakoid membrane is where the splitting of water occurs

Question 35

Stroma

Answer 35

watery matrix within the chloroplasts

Question 36

Light reactions summary

Answer 36

- link to calvin cycle: ATO and NADPH+ are formed - heart of PS1 = dimer P680 - heart of PS2 = dimer 700 1. 2 e- excited from PS2 to primary acceptor 2. splitting of H₂O releases O₂ 3. ETC - Pq (plastoquinone) -\> cytochrome complex -\> pc (plastocyanin) -\> P700 4. e- flow provides energy for chemiosmotic synthesis of ATP\*\* 5. 2 e- excited from PS1 -\> primary acceptor 6. ETC - Fd (ferodoxin) -\> NADP+ reductase -\> NADPH

Question 37

Herbicide resistant biotypes

Answer 37

- simulated progression of resistant kochia exposed to repeated annual appilcations of herbicide - 1 a.a. (amino acid) substitution in D1 protein lowers the binding efficiency of the herbicide and the quinone binding site

Question 38

Answer 38

- simulation assumes an initial seed population of 100,000/m squared with a 1 in 10,000,000 occurrence of resistant biotypes in the intial population, and 90% weed control - y-axis title is % resistance in treated population

Question 39

How does selection for herbicide resistance occur?

Answer 39

- herbicide sprayed - resistant plant survives and sets seed - herbicide is used on weeds with more resistant plants - eventually majority resistant