Chapter 12: Transport Across Cell Membranes

Exam 4

principles of transmembrane transport

solutes cross membranes by either passive or active transport

passive transport uses concentration and/or electrical gradient (all channels, many transporters)

active transport is coupled to an energy source (transporters (pumps))

small, nonpolar molecules

readily dissolve in lipid bilayers and therefore diffuse rapidly across a bilayer

(O2, CO2, N2, steroid hormones)

small, uncharged polar molecules

can diffuse some molecules in

(H2O, ethanol, glycerol)

larger, uncharged polar molecules

more difficult to move through due to large size

(glucose, nucleosides, some amino acids)

ions

cannot move across lipid bilayer

simple diffusion

performed by small nonpolar molecules like O2

does not need help

high to low concentration

passive transport

channel-mediated and transporter-mediated

moving with the gradient from high to low concentration

channel in passive transport

are specific and not everything can move through it

high to low to even out distribution

transporter in passive transport

solutes need no help besides the transporter

will bind to transporter and change conformation in order to transfer from different places

active transport

will go against gradient and go from low to high concentration

needs a form of energy to perform mechanism

can be in electrochemical gradient

electrochemical gradient when voltage and concentration gradients work in same direction

outside → positive charge and high concentration

inside → negative charge and low concentration

cation wants to move down gradient and will be able to due to electrochemical gradient

membrane potential

voltage that exists across membrane

electrochemical gradient when voltage and concentration gradients work in opposite direction

outside → low in concentration and positive charge

inside → high in concentration and negative charge

cation will sometimes go through gradient but usually won’t due to charge

aquaporins

not found in every cell and usually is found in kidney cells

found at different places in different concentrations

moves water through

pore in aquaporin

hydrophilic and in alpha helical conformation

protozoan cell

discharging contractile vacuole

in vacuoles, solutes accumulate → eventually wants to get water out → solutes leave vacuole and leave water inside → vacuole fuses with membrane and dumps out water

may do this process a lot or not at all

animal cells

water follows ions out of the cell

plant cell

similar to protozoan cells

have vacuole (the most prevalent type that stores water)

won’t burst due to cell wall → cell wall has rigid structure → turgor pressure between allows stiffening of plant

water follows salt out of cell

lysosome

H+ is added into organelle to keep acidic environment

mitochondrion

ATP production

need ADP to produce, want to be able to move ADP in and out of cell to control ATP production

glucose transport

passive transport (high to low)

glucose in extracellular space → glucose-binding site → glucose binds and causes conformational change → change opens transporter to cytosol → releases glucose into cell

3 drivers of active transport

gradient-driven pump

ATP-driven pump

light-driven pump

gradient-driven pump

uses concentration gradient of one molecule as energy source

ATP-driven pump

uses ATP hydrolysis as energy source

ATP is hydrolyzed → free energy is released and allows solute to move against gradient → will transfer phosphate group will doing so

light-driven pump

use energy from light

Na+/K+ pump

3 Na+ outside, 2 K+ inside

K and Na is going against electrochemical gradient (low to high)

ATP driven pump (uses ATP hydrolysis to perform pumping)

Na first, K second in conformation changes and phosphorylation with ATP

Ca2+ pumps

keep the cytosolic Ca2+ concentration low

(high outside, low inside)

calcium binding site → 2 Ca2+ binds from cytosol → aspartic acid is hydrolyzed → conformational change → aspartic acid is phosphorylated → 2 Ca2+ is released in lumen of sarcoplasmic reticulum

depolarization

switching charges between areas

once positive turns negative, once negative turns positive