Chapter 12: Transport across the cell membrane

What substances to cell membranes export?

metabolic waste products

What substances to cell membranes import?

Nutrients, sugars, amino acids.

Cell membranes regulate the concentration of

inorganic ions

Some molecules can diffuse through the membrane on their own, like

oxygen and carbon dioxide

Most molecules need helpers to diffuse called

specialized membrane transport proteins

Specialized membrane transport proteins facilitate

the passage of selected small, water soluble molecules

Types of diffusion that selective transport proteins facilitate

passive and active diffusion

Two main classes or transport proteins;

transporters and channels

Hydrophobic interior of lipid bilayer does not allow direct passage of

most hydrophilic molecules, water-soluble molecules, such as inorganic ions, sugars, amino acids, nucleotides, and other cell metabolites, and uncharged polar molecules.

Rate of diffusion depends on

molecules’ size and solubility properties.

Hydrophobic =

nonpolar

Hydrophilic =

polar

Small, nonpolar molecules can diffuse

rapidly through plasma membrane

small uncharged molecules can diffuse

only if small enough

Large uncharged molecules diffuse

cross extremely slowly

inorganic ions diffuse

not they don’t

What type of transport membrane protein is this?

Channel

What type of transport membrane protein is this?

Transporter

these can pass the membrane via channel or transporter proteins

inorganic ions and small polar molecules

Ions channels only transport in what oreintation?

open

Most important inorganic ions

Sodium, potassium, calcium, chlorine, and protons

More __ inside, more _ outside

K+, Na+

Na+ is balanced by

Extracellular Cl-

K+ is balanced by

negatively charged inorganic and organic ions

Differences in the Concentration of inorganic ions across a cell membrane create

membrane potential

movement of anions and cations move across is precisely balanced

unstimulated cell

when a cell is unstimulated, the voltage is

steady, called resting membrane potential

Unstimulated does NOT

mean the cell potential is zero

Cell potential is negative because

interior is more negative than exterior

the membrane potential helps fuel the movement of

ions and metabolite cells

All channels use ___ transport

passive

Active transport is facilitated by

pumps, a type of transporter

The membrane potential famously provides nerve cells—and other cells that are electrically excitable—with

an efficient means to communicate with their neighbors

Active transport moves

against the concentration gradient

Where does the energy come from for active transport

coupling with an energy producing reaction

What influences the the passive transport of charged molecules?

Concentration gradient and membrane potential

Membrane potential does what to positively charged substrates?

pulls them in

Membrane potential does what to negatively charged substrates?

pushes them out

the electrochemical gradient combines what two forces?

concentration gradient and membrane potential

if the membrane potential acts against the concentration gradient, then

the electrochemical driving force will decrease

If the concentration gradient and membrane potential work together, then

the electrochemical driving force will increase

Aquaporins

water channels

Osmosis gradient

pulls water into the cell

Freshwater protozoa eliminate excess water by using

contractile vacuoles

Animal cells eliminate excess water by using

transmembrane pumps

Plant cells storage excess water in __ generating __

vacuoles, turgor pressure

transporters move

small, water-soluble, organic molecules, and some inorganic ions across cell membrane

lysosomes membranes contain

proton transporters

lysosomes proton transporters function

imports protons to acidify the lysosome interior

Mitochondria contain transports from

importing the pyruvate that mitochondria use as fuel to generate ATP

Passive Transporters move solute along its

electrochemical gradient

If glucose is low,

 the hormone glucagon stimulates liver to produce glucose by breaking down glycogen.

When glucose is low, sugar is

taken into cell. More glucose binds to the transporters externally displayed sites.

Occluded conformation state is

empty

In passive transports, transition changes are not dependent on

if the solute binding site is occupied

Pumps actively transport a solute

against its electrochemical gradient

Pumps are important for

achieving the appropriate intracellular ionic composition and for importing solutes that are lower concentration outside than inside

3 ways of pump transport

gradient driven pumps, ATP driven pumps, light driven pumps.

How do gradient driven pumps work?

 link uphill transport solute to the downhill transport of another.

How do ATP driven pumps work?

use energy released by hydrolysis of ATP to drive uphill transport

How do light-driven pumps work?

use energy derived from sunlight to drive uphill transport.

Example of gradient driven pump?

Example of ATP driven pump?

Na+ pumps

Example of light driven pump?

Bacteriorhodopsin

Na+ pumps function?

transports Na+ out against electrochemical gradient then back in, producing a voltage that provides energy for other active transport proteins. K+ is brought in.

Toxin ouabain inhibits Na+ pump by

preventing the extracellular binding of K+, stops cycle.

Ca2+ pump keeps the _ concentration low.

cytosolic Ca2+

cells maintain low concentrations of free Ca2+ in their cytosol compared to high concentration outside. Difference is achieved by

ATP driven Ca2+ pumps in plasma membrane and endoplasmic reticulum

Ca+ pumps are ATPases that work similarly to Na+ pump. Difference is that Ca2+

return to their original conformation with a requirement for binding and transporting a second ion.

Na+ and Ca2+ have common

evolutionary origin

The gradient of any solute across a membrane can be used to drive

active transport of a second molecule.

Types of gradient pumps

Symports and antiports

Symports:

pump moves a pair of solutes the same direction

Antiports:

pump moves a pair of solutes opposite direction directions

Uniports

gradient driven transporters that ferry only a single type of solute across the membrane at a time

Uniports facilitate passive diffusion down concentration, thus are

NOT pumps

The electrochemical Na+ Gradient drives the transport of

glucose

How does Na+ gradient drive the transport of glucose transport in the gut?

Symports that make use of inward flow of Na+ down its electrochemical gradient have an important role in driving the import of glucose inward.

Electrochemical H+ Gradients drive transport of solutes in

plants, fungi, and bacteria.

Hydrophilic channels are transmembrane pores that allow

small, water soluble molecules or ions

Aquaporins facilitate

flow of water

Most of cells channels facilitate the passage of

select inorganic ions

Ion channel selectivity depends on

diameter and shape of ion channel and on the distribution of the charged amino acids that line it.

Ions are not continuously open, but

snap open and shut

Most ions channels are gated, meaning

 a specific stimulus triggers channels to switch between states by inducing conformation changes.

ion channel does not need to undergo

shape change for each ion

Changes in membrane potential are the basis of

electrical signaling

electrical changes and mediated by alterations in

the permeability of membranes to ions

In unstimulated state, the negative charges on organic molecules inside the cell are balanced by

K+. K+ is continuously imported into the cell by the Na+ pump.

K+ leak channels

allow K+ to move freely across a membrane. In a resting cell, these are the main channels open in the plasma membrane, making the membrane much more permeable to K+. When K+ moves out of cell down gradient, the loss of positive change in cell creates a voltage difference, or membrane potential. This imbalance will oppose any move movement of K+ out of the cell. An equilibrium condition is established, keeping just enough K+ in the cell to counteract K+ moving out of cell. Equilibrium = electrochemical gradient is zero for K+ = resting membrane potential (flow in and out is balanced).

Ion Channels randomly switch between

open and closed states

Neuron functions:

receive, integrate, and transmit signal

Dendrites _ signals while Axons _ signals

receive, send

Signals in the nervous system are communicated by changes

 in the electrical potential across the plasma membrane.

What allows rapid long distance communication along axons

Action Potential

How do cell solve passive spread/weaker messages?

by employing an active signaling mechanism in which a localized stimulus of sufficient strength will trigger a burst of electrical activity in the membrane, continuously renewing itself.

Action potential is

a localized stimulus of sufficient strength will trigger a burst of electrical activity in the membrane