Chapter 12: Transport across cell membranes

Small, nonpolar molecules can diffuse through an artificial lipid bilayer..

at an efficient rate

Small, uncharged polar molecules can diffuse through an artificial lipid bilayer…

at a somewhat low efficiency rate

Large uncharged polar molecules can diffuse through an artificial lipid bilayer…

at a low efficiency rate

Why cant ions diffuse across a membrane?

Due to ions being either too large or too polar to be able to diffuse

What are channel proteins?

A type of protein that form pores across the membrane that allows for inorganic ions or polar organic molecules to diffuse through the membrane

What are transporter proteins?

A type of membrane protein that is able to change its conformation to diffuse small solutes across the membrane

What are three characteristics of transporter proteins?

1. They are very selective with the solutes that bind to them

2. Are much slower than channel proteins

3. Are only able to carry out active transport through the use of pumps

what do most solutes require in order to diffuse across the membrane?

They require the assistance of a channel or transporter protein to diffuse

What is an electrochemical gradient?

the combined force of a chemical gradient and of an electrical gradient across a membrane that drives the movement of charged ions across the membrane

What are aquaporin channels?

A type of protein that forms pores across the membrane that allows for water molecules to diffuse rapidly while preventing other molecules from diffusing

How do protozoans prevent osmotic swelling?

by ejecting water that moves into the cell using a contractile vacuole

How do animal cells avoid osmotic swelling?

They pump out ions from the cell to reduce the intracellular solute concentration present

How do plant cells avoid osmotic swelling?

they toughen their own cell wall

How is GLUT used to transport glucose into the cell

1. Glucose binds to the glucose-binding site in the transporter

2. it then closes, preventing either side from accessing it, and is released into the cytosol

What if the concentration outside the membrane is higher than the concentration inside the membrane?

Solutes will bind more often to the transporters and other solutes will passively diffuse from the outside to the inside

What are the 3 main pumps that carry out active transport?

• Gradient driven pump

• ATP driven pump

• Light-driven pump

Why do Na+ pumps use ATP?

In order to pump out Na+ and pump in K+ into the cell to keep the cytosolic concentration of Na+ low and K+ high

How does the Na+ pump work?

1. Na+ binds to the pump and causes ATP to be hydrolyzed, resulting in the phosphorylation of the pump that changes its conformation and releases Na+ into the extracellular space in the process

2. Once Na+ is released, K+ binds to the pump and dephosphorylates the pump, returning the pump to its original conformation and releasing K+ into the cytosol 

How is an ion gradient used?

It is used to drive the active transport of molecules across the membrane

What are symporters?

They are a type of gradient-driven pump that transfers solutes in the same direction

What are antiporters?

They are a type of gradient driven pump that transfers solutes in the opposite direction

What are uniporters?

They are type of transport protein that drives the movement of a solute down its concentration gradient

How is glucose transported from the gut lumen into the plasma membrane after a meal?

1. Na and glucose is pumped out from the gut lumen by the Na-driven glucose symport and into the plasma membrane

2. Afterwards Na+ is pumped out by Na pumps present in the basal and lateral plasma membrane and glucose is pumped out by passive glucose uniporters into the extracellular space

A Na-driven glucose symport allows for….

the concentration of Na to be low in the cytosol and the electrochemical gradient steep

What do symports in animal cells use to import various solutes?

They use the electrochemical gradient established by Na pumps

What do symporters use in plant cells to import various solutes and drive A.T?

they use the electrochemical gradient established by H+ pumps

What do ion channel contains?

A selective filter that controls the inorganic ions that are allowed to cross the membrane

What is membrane potential?

The change in voltage across the two sides of the membrane

What is a mechanically gated ion channel?

A type of ion channel that opens in response to touch

What are extracellular ligand ion channels?

A type of ion channel that opens when an extracellular ligand binds to the channel and closes when it leaves

What are intracellular ligand ion channels?

A type of ion channel that opens when an intracellular ligand binds to the channel and closes when it leaves

What are voltage gated ion channels?

A type of ion channel that opens when its voltage sensors detect a change in voltage across the membrane, triggering a conformational shift that opens the channel and allows for specific ions to flow

What are stretch-activated piezo channels?

A type of channel that opens when there is an increase in membrane potential, generating a physical force that strecthes the membrane and eventually opens the channels central pore, allowing ions to flow through the membrane

What does a neuron typically consist of?

A cell body connected to an axon and mutliple dendrites

• With the axon providing an electrical signal that moves away from the cell body and towards the target cell and is received by its dendrites

How does an electrical signal travel from one neuron to another?

1. The electrical signal, aka action potential, travels down the 1st neuron axon to the nerve terminal where it activates the voltage-gated Ca2+ channels, allowing for Ca to diffuse into the terminal

2. This stimulates the synaptic vesicle to fuse with the membrane and results in neurotransmitters (aka chemical signals) to be released into the synaptic cleft

3. Once released, the neurotransmitters carry the signal across the cleft to the postsynaptic cell where it then binds to the ligand gated channels present on the cell, triggering the channel to open and allow for ions to flow in, which subsequently converts the chemical signal back into an electrical signal in the 2nd neuron

How does membrane depolarization trigger an action potential?

When there is a change in voltage across the membrane, ion channels begin to open and allow for ions to pass through, triggering membrane depolarization to occur which eventually leads to action potential being generated

How does active potential travel across the axon membrane?

It travels as a wave, causing Na channels to open as it passes and allowing for Na+ to flow into the membrane, with K+ channels opening afterwards and allowing K+ to flow out of the membrane,

What occurs once active potential passes the Na and K channels?

They slowly begin to become inactive, preventing import of Na+ and export K+ out of membrane, which prevents action potential from going backwards