The cytoplasmic membrane is a semi-permeable barrier. It is impermeable to _____ solutes and others.
hydrophilic
List the integral membrane proteins that are required for much of the transport across the cytoplasmic mebmrane.
- Carrier proteins
- Transporters
- Permeases
In what ways is transport classified?
- Diffusion (passive)
- Active transport (requires energy)
- Group translocation (chemical modifications occur)
In active transport, solute transport is coupled to _____.
energy transduction
How do primary and secondary active transport differ?
- Primary: driven by energy-generating metabolism
- Establish proton gradients and membrane potentials - Secondary: driven by electrochemical gradients (proton and sodium gradients) or high energy phosphate bonds
What is the common structure of carrier proteins?
12 helices spanning the membrane
What is the chemiosmotic theory?
Protons are translocated out of the cell by exergonic driving reactions, which are usually biochemical reactions
- Some of the translocated protons leave behind negative counterions –> proton gradient, outside positive
Diagram the chemiosmotic theory.
What are the two types reactions of the chemiosmotic theory?
- Generate electrochemical gradients
- Use gradients (∆p)
What reactions generate electrochemical gradients? What reactions use electrochemical gradients?
- Redox reactions
- ATP-driven proton pumps
—————————————- - ATP synthesis
- Solute transport
- Sodium transport
- Flagellar rotation
How are the proton gradient (∆pH), membrane potential (∆Ψ), and proton motive force (∆p) related?
Proton gradient (∆pH) and membrane potential (∆Ψ) generate proton motive force (∆p)
What are the two options for electrogenic flow and what is their result? (I.e. How is the membrane potential (∆Ψ) generated?)
- Proton moves across the membrane
- Molecule A is reduced in the cytoplasm, then moves across the membrane and releases the protons when oxidized
Result: protons out, electrons in
How is the proton gradient (∆pH) generated?
- ∆Ψ and ∆pH can’t be created simultaneously
- Bacteria can create a ∆Ψ during proton translocation and then convert it to ∆pH (but can’t be a large ∆pH if there’s a large ∆Ψ)
What are ionophores and uncouplers used for?
Used in research to study membrane bioenergetics
- Understanding role of electrochemical ion gradients in membrane energetics
- Determine what molecules make up the gradient
How do ionophores and uncouplers work?
- Ionophores: dissipate membrane potentials (∆Ψ) and/or proton gradients (∆p)
- Uncouplers: allow hydrogen to enter rapidly through membranes
What effects do uncouplers have in respiration?
- Collapse the ∆p and thereby inhibit ATP synthesis coupled to electron transport
- Stimulate respiration
Why should uncouplers stimulate respiration?
- Flow of electrons through electron carriers in the membrane is obligatorily coupled to a flow of protons in a closed circuit
- This occurs at coupling sites
- Protons are translocated to the outer surface of the membrane and then reenter via the ATP synthase
- Reentry of protons through the ATP synthase can be viewed as rate limiting for the circular flow of protons through the circuit
- In the presence of uncouplers, protons rapidly enter the cell on the uncoupler rather than through the ATP synthase, thus stimulating electron transport
Explain what effects the ionophores in the figure have on ∆pH and ∆Ψ.
- Gramicidin: will collapse ∆Ψ and ∆pH (i.e. ∆p)
- Valinomycin: collapse ∆Ψ but not ∆pH
- Nigericin: collapses ∆pH but not ∆Ψ
- Monensin: electroneutral exchange of Na+ or K+ for H+
- Dintrophenol: collapse both ∆pH and ∆Ψ (i.e. ∆p)
What is ATP synthase?
A protein complex that couples the translocation of protons down the proton gradient (∆p) to the phosphorylation of ADP to ATP
- Note: this protein is reversible
Describe and draw the structure of ATP synthase.
Membrane-embedded rotary protein with two regions
- Proton channel FO spans membrane
- Catalytic subunit F1 on inner membrane surface catalyzes reversible hydrolysis of ATP
Membrane-bound ATP synthase consumes _____ and generates _____.
- proton motive force (∆p)
- ATP
Where is ATP synthase located in eukaryotes vs. prokaryotes?
- Eukaryotes: mitochondrial membrane
- Prokaryotes: cytoplasmic membrane
Diagram and explain the mechanism of ATP synthesis via ATP synthase.
Binding-change mechanism
- When protons move down the gradient through FOF1, conformational changes occur in F1 that result in phosphorylation and release of ATP
What is the major driving force that generates a proton motive force (∆p)?
Electron transport
