PMB 4412 Exam 2

two types of energy converting organelles

• chloroplasts

  • plastids, found only in plants and algae

• mitochondria

both are separated from the cytosol by a double membrane

characteristics of mitochondria and chloroplasts

semi-autonomous

• divide by fission

• contain circular chromosomal DNA, located in nucleoids within the stroma and matrix

• contain ribosomes, tRNAs

• depend on import of nuclear encoded proteins for many functions

• generate transmembrane hydrogen gradient and use gradient to make ATP

overall formula for photosynthesis

6 CO2 + 6 H2O → C6H12O6 + 6O2

what happens during the light reactions of photosynthesis?

energy from light is used to phosphorylate ADP (produce ATP) and to reduce NADP+ to NADPH

in chloroplasts, the light reactions drive _______ of the thylakoid lumen

acidification

where is ATP generated in chloroplasts? where is ATP generated in mitochondria?

in chloroplasts = stroma

in mitochondria = matrix 

A __________ can be formed when membranes allow selective permeation

diffusion potential

exists until chemical equilibrium is reached → then diffusion potential equals 0

diffusion is passive transport

what is the plant cell membrane potential inside relative to outside?

negative inside, positive outside

H+ ATPase pumps protons out

how does inhibiting ATP synthesis affect membrane potential?

membrane is depolarized

• H+ ATPase pumps out protons, makes the inside of the membrane negative and the outside positive

• turning off ATP synthesis mean means no more ATP hydrolysis, H+ ATPase becomes inactive

• inside of the membrane becomes more positive

what forms of transport across membranes are passive?

• diffusion

• channels

• uniporter: binds substrate on one side, changes conformation, releases substrate on other side

what forms of transport across membranes are active?

• pump

• symporter

• antiporters

• use energy to move something against its electrochemical gradient

molecular structure of H+ ATPase

• both N and C terminus in cytoplasm

• several transmembrane domains

  • transmembrane domains have occasional charged amino acids embedded in the membrane

  • these charged amino acids have special functions

• regulatory domains within the cytoplasm

  • have bindings sites for Mg2+

  • phosphorylation domain

structure of H+ ATPase in vacuole membrane

• domains in cytoplasm spin 

• channel allows proton flow from the cytoplasm to the lumen of the vacuole

• hydrolyze ATP to move protons

• process is reversible, based on stoich of protons to ATP

difference between H+ ATPase and ATP synthase

H+ ATPase hydrolyzes ATP to pump protons against their gradient

ATP synthase makes ATP using the power of protons moving down their gradient

structure of ATP

• five carbon ribose sugar

• adenine nitrogenous base

• 3 phosphates

3 phosphate linkage is unstable because negative charges are next to each other → hydrolysis

what are standard conditions?

25 degrees Celsius

1 M products and reactants

• not cellular conditions

what is Keq?

concentration of products over concentration of reactants AT EQUILIBRIUM

• when Keq is big = more products than reactants at equilibrium

• when Keq is small = more reactants than products at equilibrium

ATP hydrolysis under standard conditions

ATP → ADP + Pi

• Keq is large = ATP is unstable = wants to lose P

• standard conditions are not cellular conditions

energy of ATP hydrolysis in the cell (no electrical component)

• dependent on concentration of ATP and hydrolysis products

how do we consider just the electrical component of membrane potential?

energy required to transport one mole of charge against a membrane potential 

how do we consider just the concentration component of membrane potential?

energy required to transport one mole of solute against a concentration gradient

what is equilibrium potential?

membrane potential at which the given ion concentrations are at equilibrium

how do we determine the equilibrium potential for a given ion?

• Nernst potential

• Walther Hermann Nernst

• takes into account electrical and concentration components

graphing of equilibrium potentials

• at zero, no net flux of the ion

• slope of the line gives the conductance of the channels mediating the current

• for cations

  • movement out of the cell = outward current = positive sign

  • movement into the cell = inward current = negative sign

• for anions

  • movement out of the cell = negative sign

  • movement into the cell = positive sign

how do voltage-gated channels sense changes in voltage?

charged amino acids within the transmembrane spans sense changes in voltage of the membrane

channels change conformation in response to voltage changes

how can we use equilibrium potential graphs to determine what kind of channels ions are moving through?

• outward channels open when membrane potential exceeds equilibrium potential for potassium → potassium moves out

• inward channels open when membrane potential is below equilibrium potential for potassium → potassium moves in

“goal” = get the membrane potential as close to the ion equilibrium potential as possible

what is channel gating?

opening and closing of ion or solute channels

involves protein conformation change

• once open, ions or solutes are conducted at very fast rates

what is voltage dependence?

regulation of the channel by membrane potential

open/close in response to voltage

how can we measure ion channels?

• patch clamping

  • discovered by Erwin Neher and Bert Sakmann

• electrophysiology methods

what is the first key reaction of photosynthesis?

splitting water

• 2 H2O → 4 e + 4 H+ + O2

• O2 is a waste product

• negative redox potential

what is redox potential mean?

• negative redox potential = more willing to give up electrons

• positive redox potential = more willing to take up electrons

electrons flow spontaneously from negative redox potentials to positive redox potentials

in the light reactions of photosynthesis, hydrogen is stockpiled into the __________, flows out the ATP synthase channel into the ________, and makes ATP in the __________

thylakoid lumen

stroma

stroma

PSII is located primarily in the ________________

stacked grana

PSII

1. light oxidizes P680

2. P680 is reduced by the electrons from splitting H2O, protons from water are left in lumen

3. Reduced P680 passes electrons to a pheophytin (chlorophyll)

4. Pheophytin passes electrons to plastoquinones QA and QB

5. QB is reduced by 2 electrons and 2 protons → protons are taken from the stroma

6. Cytochrome b6f takes electrons from QB, the protons from QB are pumped into the lumen

• 4 H+ pumped in for each 2 electrons that go through cyt b6f

7. cytochrome b6f passes electrons to plastocyanin (final electron acceptor of PSII

final electron acceptor of PSII

plastocyanin

initial electron donor of PSII

H2O

initial electron donor of PSI

plastocyanin

where is PSI and ATP synthase?

stromal thylakoids

• unstacked regions

PSI

1. light oxidizes P700

2. plastocyanin reduces P700

3. P700 reduces A0 quinone

4. A0 quinones transfers electrons through a series of FeS proteins

5. FeSB reduces soluble ferredoxin

6. reduced ferredoxin transfers electrons to FNR

7. FNR reduces NADP+ to NADPH → removes protons from the stroma

what are the products of the light reactions?

O2

NADPH → used in the Calvin cycle to reduce CO2

ATP

ultimate electron donor for the light reactions?

H2O

ultimate electron acceptor for the light reactions?

NADP+

how does the ATP synthase use the proton gradient to make ATP?

proton concentration is much higher in the thylakoid lumen than in the stroma

protons move down their gradient from thylakoid lumen to the stroma through a channel in the ATP synthase

proton motive force powers the phosphorylation of ADP into ATP in the stroma

important features of thylakoid structure

• continuous thylakoid lumen

• extensive contact and continuity between stromal and thylakoid membranes

what ion is required for the splitting of H2O in PSII?

need 4 Mn ions

what does FNR stand for?

ferredoxin NADPH reductase

FNR reduces NADP+ to NADPH → removes proton from stroma and gives NADP+ electrons

what is cyclic electron transport?

chloroplast can redirect electrons from ferredoxin to cyt b6f to make more ATP rather than making more NADPH

function of the light reactions of photosynthesis

generate ATP and NADPH

function of the carbon reactions of photosynthesis

fix CO2 and regenerate ribulose-1,5-bisphosphate

requires NADPH and ATP

light reactions occur on ______________

thylakoid membrane

carbon reactions occur in the _________

stroma

NADP+ is reduced to NADPH during __________ in the ___________

PSI
stroma

what is FNR?

protein in the stroma 

exists as either a soluble monomer (inactive) or a thylakoid bound dimer (active)

NOT A TRANSMEMBRANE PROTEIN

contains a FAD cofactor → uses reducing power of two ferredoxin to reduce NADP+ → NADPH