Chapter 12: Transport Across Cell Membranes

Differentiate transporters (carriers) and channels

transporters bind solutes and undergo conformational changes, moving molecules slowly

channels form open hydrophilic pores allowing rapid, passive diffusion of ions

What types of molecules are permeable and will diffuse quickly through the lipid bilayer?

small, nonpolar molecules

O2

CO2

N2

steroid hormones

What types of molecules are relatively impermeable through the lipid bilayer, but some can pass through?

Small, uncharged polar molecules and larger, uncharged polar molecules

H2O, ethanol and glycerol

amino acids, glucose and nucleosides

What types of molecules are completely impermeable to the lipid bilayer?

ions

H+, Na+, K+, Ca 2+, Cl-, Mg 2+, HCO3 -

Rank the following based on their rate of movement across a protein-less lipid bilayer membrane down their concentration gradient

a) Small nonpolar molecules, such as O2 and CO2

b) Small uncharged polar molecules, such as H2O or urea

c) Charged molecules (ions)

d) Large uncharged polar molecules (amino acids)

1. a)

2. b)

3. d)

4. c)

In the absence of proteins, which types of molecules move most quickly across a lipid bilayer membrane down their concentration gradient?

Small nonpolar molecules like O2 and CO2

3 multiple choice options

Transporters/pumps mainly transport _______ while channels mainly transport ____

solutes; ions

What are the two ways that solutes can cross membranes?

passively or active transport - only carried out by transporters called pumps

What two factors influence the passive transport of charged solutes?

concentration gradient and membrane potential (aka the electrochemical gradient)

What creates the membrane potential?

concentration difference of ions across a cell membrane

How does the concentration gradient and the membrane potential work together or against each other to effect the electrochemical gradient?

the concentration gradient and membrane potential can work together to create a very large electrochemical gradient

they can also work against each other and create a small one

How are diffusion and osmosis related?

osmosis is the diffusion of water across a cell membrane down its concentration gradient

How does amoeba, plants, and animals avoid osmotic swelling

A. Amoeba accumulates water in contractile vacuoles and expel it periodically

B. Plant cells use their hard covering, also true with bacteria

C. Animal cells pump out ions

What is mediated by conformational changes in a transporter?

transport of a solute such as glucose (much slower than water)

What are the possible sources of metabolic energy during active transport of a solute via a pump against its electrochemical gradient?

ATP hydrolysis

ion gradient

light energy

How is Ca 2+ affected during contraction and relaxation?

Contraction: Ca2+ diffuse into the cytosol

Relaxation: Ca2+ pumped into sarcoplasmic reticulumq

What is the function of SERCA? What would dysregulation of it cause?

Sarco/endoplasmic reticulum calcium ATPase (SERCA) is the primary Ca 2+ pump in the muscle contraction and relaxation

Dysregulation of it's activity is implicated in various heart conditions, making it a therapeutic target

Which component is primarily responsible for controlling the cytosolic concentration of calcium during muscle contraction and relaxation?

Sarco/endoplasmic reticulum calcium ATPase (SERCA)

3 multiple choice options

What is the function of the Na+/K+ pump in animal cells?

uses energy supplied by ATP to expel Na+ and bring in K+

undergoes a series of conformation changes in the process

In animal cells, an electrochemical Na+ gradient across the plasma membrane, generated by the Na+ pump, is used by symports to import various solutes

Describe how the glucose-Na+ symport works

The electrochemical Na+ gradient drives the transport of glucose across the plasma membrane of animal cells

Gatorade leverages glucose-Na+ symport to maximize hydration as glucose and sodium together actively drive faster intestinal water absorption

How does the electrochemical gradient of Na+ influence glucose transport in animal cells?

It drives the active transport of glucose across the plasma membrane

3 multiple choice options

In one experiment, investigators created a liposome - a vesicle made of phospholipids - that contains a solution of 1 mM glucose and 1 mM sodium chloride. If this vesicle were placed in a beaker of distilled water, what would happen fastest?

H2O would diffuse in

3 multiple choice options

What are the various stimuli that can affect the activation or inactivation of channels?

Voltage-gated: a change in the voltage difference across the membrane create action potential

Ligand-gated: ligand binding to the intracellular/extracellular side of a channel

Mechanically-gated

When is a sodium channel open or closed?

When the membrane is at rest and highly polarized, positively charged amino acids (red bars) keeps the channel closed

When the membrane is depolarized, the channel has a high probability of opening

How do nerve cells use ion channels?

Transmitter-gated ion channels in the postsynaptic membrane convert the chemical signal back into an electrical signal

What do action potentials allow for? How are they fired in the body?

Action potentials allow rapid long-distance communication along axons

The binding of neurotransmitter acetylcholine at synapses opens channels that admit Na+ and initiate a nerve impulse or muscle contraction

This allows the influx of 7000 Na+ ions into the neuron and thus the continuation of the nerve impulse

A toxin present in scorpion venom prolongs the duration of action potentials (prolonged state of depolarization) in nerve cells. Which of these actions would best explain how this toxin exerts its effect?

It slows the inactivation of voltage-gated Na+ channels

3 multiple choice options

What is the repolarization dependent on?

the opening of K+ channels

What happens when the Na+ channels of axons open?

All of the above

3 multiple choice options

How do transporters and channels select which solutes they help move across the membrane?

Channels discriminate between solutes mainly on the basis of size and electric charge; transporters bind their solutes with great specificity in the same way an enzyme binds its substrate

3 multiple choice options