Transport across Membranes

Whether or not a molecule can cross the selectively permeable membrane is determined by its ________ and

_______________. Some molecules enter and exit the cell regardless of its benefit or harm to the cell while

other molecules require _______________ and specialized __________________ to transport across the membrane.

PASSIVE MEMBRANE TRANSPORT

Passive transport moves substances across membranes without _______________________________________.

The driving force is _________________, which is the net movement of substances from _________ _______

concentration. Molecules will diffuse across a membrane until the concentration on both sides is ___________.

The greater the concentration difference between the two regions, the _______________ diffusion will occur.

Note that the particles have not stopped moving during the equilibrium state, yet there is a balance such

that there is no net change in concentration on either side, which is called a ___________________________________.

The tree categories of passive transport are simple diffusion, facilitated diffusion, and osmosis.

Simple Diffusion

The completely unassisted, diffusion-driven movement of substances

across a membrane.

Usually only very small, uncharged (=hydrophobic) molecules

(O2, CO2, some steroids) can do this. If the molecule is polar, it

must be very small (i.e. ____________).

Q: Why can’t charged or large polar molecules achieve simple diffusion?

A: _________________________________________________________________________________________________________

Osmosis

The diffusion of _________________ across a membrane is special enough to have it’s own name: osmosis.

Osmosis is the movemement of water across a semipermeable membrane from an area of _____________

solute concentration to an area of _____________ solute concentration.

Essentially, the water moves in the direction which will __________________ the

solute concentration.

Like all diffusion, osmosis results in a _________________________________

______________________ compares the relative concentration of solutions across a membrane and allows us to

predict whether water will move into the cell, out of the cell, or remain in a dynamic equilibrium.

***You are responsible for studying the Osmosis and Tonicity Note (on website) yourself.***

Facilitated Diffusion

Sometimes simple diffusion cannot keep up with the requirements of the cell for ions or polar charged

molecules, therefore it must be sped up with the help of _________________________________ proteins. There is

still no input of energy – the _____________________________________ drives the transport.

Two types of transmembrane proteins are involved in facilitated diffusion – channel proteins and carrier

proteins.

______________________________ form a hydrophilic pathway or “tunnel” which allows

small polar molecules and ions to flow through at a much faster rate than they

could if moving directly though the membrane. They do not change shape.

_________________________________ are more selective than channel proteins, binding

to one specific solute. After binding, it changes shape which allows the solute to

move down the concentration gradient.

ACTIVE MEMBRANE TRANSPORT

When moving a substance __________________ or “up” the concentration gradient, ________________ is required.

Typically this uses transmembrane ___________________ powered by chemical energy in the form of _________.

This process is called active transport and falls into two closely related categories: _______________ and

_______________________ active transport.

Primary Active Transport

These pumps move cations such as ______, ______, ______, and ______ to areas of ______________ concentration, and

are powered by __________.

Sodium-potassium (Na+/K+) pumps and proton (H+) pumps

create a difference of _____________ on either side of the

membrane.

This charge separation produces a _______________________________

or _________________________________________ which is a form of

stored energy that can drive other essential cellular process,

such as nerve impulses or other forms of transport.

Secondary Active Transport

Secondary active transport relies on the electrochemical gradient created by ______________________ active

transport, such as that created by the sodium-potassium pump.

_________________ is secondary transport where the driving ion (i.e. K+/Na+) and the transported

solute move though the transport protein in the _____________ direction (they both “fall” down the

electrochemical gradient together).

_________________ is when they move in ______________________ directions. The driving ion “falls” down the

electrochemical gradient through the transport protein which energizes it so it can now pump the

transported solute against its concentration gradient.

BULK TRANSPORT

Passive and active transport are limited to transporting molecules no larger than _______________________ or

__________________________. The processes of _______________________ and _______________________ (collectively, “bulk

transport”) allow eukaryotic cells to import and export much larger molecules or even entire cells such as

bacteria. Both of these processes require energy in the form of ATP.

Exocytosis “substances ____________ the cell”

A _______________ filled with unwanted substances fuses with the inside of the __________________________ and its

contents are released to the outside.

The lipid bilayer of the vesicle becomes part of the cell membrane.

Endocytosis “substances ______________ the cell”

Endocytosis falls into three categories – pinocytosis, receptor-mediated endocytosis, and phagocytosis.

In pinocytosis (______________________), the plasma membrane forms a pit-like depression that traps the

desired substances along with the extracellular fluid in which it is suspended. The depression then pinches

off, forming a vesicle that can be transported through the cell.

Receptor-mediated endocytosis occurs when the molecules to be taken in bind to highly specific

_____________________________. The receptor proteins (still bound to the molecule) collect in a depression in the

membrane which pinches off, forming a _______________.

Phagocytosis (________________________) occurs when a cell engulfs large

______________________ particles such as pathogenic bacteria or viruses. This

is commonly done by _______________________ (type of WBC).