transport of solutes and water

lecture 7

energy

ability to do work

energetics

energy transfer between systems

types of energy

• potential- trapped energy

• kinetic- energy of movement

two aspects of diffusion that govern biological processes

1. diffusion is certain to lead to a random distribution of molecules (kinetics)

2. tendency of molecules to diffuse is a source of energy (potential)

simple diffusion

does not just apply to solutes but to water, gases dissolved in water and even heat

Fick Diffusion Equation

quantitative law of solute diffusion

• diffusion is notoriously slow

gradients

a form of energy storage

• potential for ions to move from out to inside the cell

organisms invest energy to

delay diffusion

• maintenance

what is an electrical gradient?

difference in charge

• on the other side of a membrane

what is a chemical gradient?

difference in concentration of a single molecule

• acorss a membrane

what molecules only have a chemical gradient

nonpolar or neutral molecules

what molecules have an electrochemical gradient

anything that is charged

• e.g. Na+ (Na contributes to conc. gradient and ‘+’ contributes to electrical gradient)

what is an electrochemical gradient

gradient of charge and concentration

• can be in same or opposite direction

electrochemical potential difference (Δμ)

the driving force of movement of a substance across a membrane as a result of the electrical and chemical gradients across the membrane

Δμ=RTln[X+]o/[X+]i + zF(Eo-Ei)

describes the chemical gradient

Δμ=RTln[X+]o/[X+]i + zF(Eo-Ei)

describes the electrical gradient

if [X+]o/[X+]i > (Eo-Ei)

Δμ is positive

• molecule will move from outside to inside

if [X+]o/[X+]i < (Eo-Ei)

Δμ is negative

• molecule will move from inside to outside

if Δμ is positive, means either…

• conc and electrical gradient are in the same direction (out→in)

OR

• conc gradient (out→in) is larger than electrical gradient (in→ out)

Molecule will move from out to in

if Δμ is negative, means either…

• conc and electrical gradient are in the same direction (in→out)

OR

• conc gradient (out→in) is smaller than electrical gradient (in→ out)

Molecule will move from inside to outside

equilibrium potential

the electrical potential difference across a membrance that balances the concentration gradient

(Δμ=0)

Resting membrane potential (Vm)

• difference in charge inside and outside the cell membrane

• cel membranes not at equilibrium

two main functions of Resting membrane potential (Vm)

1. provide energy for membrane transport

2. changes in membrane potential used by cells in cell to cell signaling

goldman equation

accounts for permeability and multiple ions

• sum of the Eion for all relevant ions that takes permeabilities into consideration

Vm is most dependent upon

Na+, K+, Cl-

when membrane is at rest…

more negative on inside than the outside

what happens when Na+ channels are opened

• Na+ enters the cell

• causes depolarization

• leads to positive charge (mV)

what happens when K+ channels are opened

• K+ leaves the cell

• causes hyperpolarization

• leads to negative charge (mV)

for hydrophobic solutes or gases…

permeability is dependent on factors that affect diffusion through lipids (e.g. molecular size)

for inorganic ions and water…

permeability depends on the number of channels and how many are open

• are not able to diffuse directly cross because they are lipophobic

what are the 3 main types of transport

1. passive diffusion

2. facilitated diffusion

3. active transport

passive transport

• hydrophobic molecules

• no transporters needed

• no energy needed

• depends on conc gradient (high→low)

facilitated diffusion

• hydrophobic molecules

• protein transporter is needed

• no energy is needed

• depends on conc gradient (high→low)

in facilitated diffusion, 3 main types of protein carriers

• ion channels

• porins (larger channels)

• permeases

ion channels

• small pores for specific ions (eg. Na+ K+ Cl-)

• open and close in response to cellular conditions

porins (larger channel)

• similar to ion channels, but for larger molecules (eg. aquaporins)

permeases

• function more like an enzyme (change shape to move substrate)

• can be saturated

gated ion channels

allows for regulation or movement

• voltage gated: open when certain voltage is reached

• ligand gated: open when a ligand binds channel

• mechanogated: opens when a mechanical stimulus present

active transport

• protein transporter is needed

• energy is required (hydrolysis of ATP)

• can be moved against conc gradient

two main types of active transport

• primary active

• secondary active transport

primary active transport

direct use of an exergonic reaction

• ATPase

• energy released from ATP hydrolysis

secondary active transport

use energy associated with the electrochemical gradient of one molecule to drive the movement of another

what establishes an electrical gradient

ions at different concentration on either side of the membrane

what maintains an electrochemical gradient

membrane channels that are regulated to change permeability

• active transporters to reestablish gradient