Cellular Transport

The major function of the cell membrane is that it is selectively permeable, that is, its properties allow certain molecules to enter/exit the cell while preventing that same transport of other molecules.


One major concept to understand when it comes to cellular transport is that of the concentration gradient. This refers to the gradual change in the concentration (or density of molecules) of a solute (the dissolved molecules) throughout the solution.

Passive Transport

Transport can be split into two main categories: passive and active transport. Passive transport is the movement of molecules along a concentration gradient without the use of energy. "Along a concentration gradient" means that the net movement will be from high concentration to low concentration.

It is important to note that we are referring to net movement. Molecules will move in both directions, but overall, more will move from high to low as the solution approaches equilibrium.

Diffusion

Of the types of passive transport, the most basic is simple diffusion. This is the net movement of molecules from high to low concentration without the expenditure of energy. 


Due to the properties of the cell membrane, only small, nonpolar molecules (such as O2 or CO2) can freely diffuse through it.


The concentration will eventually reach equilibrium, with roughly equal concentrations on each side of the membrane. Molecules do not stop moving at equilibrium, they will just move across in both directions in about equal numbers.

Facilitated Diffusion

As stated above, not all molecules can diffuse through a membrane. That's where facilitated diffusion comes in.


Think about what it means to facilitate a discussion - that essentially means that you are helping guide it along and make it happen. In facilitated diffusion,  passive transport occurs via the aid of membrane proteins.


Small polar and charged molecules will use proteins to traverse the membrane.


Carriers (or transporters) are proteins that bind to specific solutes and undergo conformational change to bring them across. Channels form an open pore that allows molecules through. A common type of channel protein is an aquaporin, which transports water across the membrane.

Osmosis

The Movement of Water

Water moves via passive transport, but it works slightly differently than diffusion does. Rather than moving in terms of the concentration of water, water moves from high to low water potential. 

Calculating Water Potential

Water potential, if we want to ignore the physics-heavy definition that involves potential energy, is in its essence a measurement of which way water is going to flow. There are two main equations that can be used to calculate the water potential in order to determine which way water will move.

As a note, the pressure potential is always zero in an open container.

Ψ = Water Potential | ΨP = Pressure Potential | ΨS = Solute Potential

i = ionization constant (how many molecules does the solute ionize into in water)

C = molar concentration of solute

R = pressure constant (0.0831 liter bars/mole K)

T = temp (in Kelvin)

As some general rules, with all else being equal:

Let's Practice One

There is an open beaker of 5M sucrose solution. Inside of the beaker, a Russet potato slice with a sucrose concentration of .3M is placed. Assuming it is 23°C in the room, which direction will water move?

The Answer

Let's start by calculating the water potential of the solution and the carrot.

There is an open beaker, so ΨP = 0. This means that ΨS =Ψ


For the solution:

ΨS = -iCRT

The i of sucrose is 1, as it does not ionize, so:

ΨS = -(1)(5)(0.0831)(23+273) = -122.99 bar

(you can convert from Celsius to Kelvin by adding 273)


For the potato:

ΨS = -iCRT

ΨS = -(1)(.3)(0.0831)(23+273) = -7.38 bar


-122.99 < -7.38, so the water will move out of the potato and into the solution

Osmolarity and Osmoregulation

When discussing osmosis, it is helpful to be able to easily compare things in order to know which way water might move. That is where osmolarity comes in - it is the relative measure of solute concentration. As it is a relative amount, it is intrinsically comparing two values. It's like using the word "bigger" - when using it, you have to say what it's bigger than.

For examples using the terms, we will discuss a cell and a solution.

If the solution is hypotonic, that means that it has a relatively lower solute concentration. In this situation, water will flow from the solution and into the cell, which can potentially cause it to lyse or burst.

If the solution is hypertonic, that means that it has a relatively higher solute concentration. In this situation, water will flow out of the cell and into the solution, which can cause the cells to shrivel or plasmolyze in the case of a plant cell.

If the cell and  the solution are isotonic, that means that their solute concentrations are the same.

Various organisms have evolved strategies or physiological mechanisms for osmoregulation - the regulation of internal solute concentration to maintain water balance and homeostasis. Examples of this include paramecium with contractile vacuoles that pump excess water out, kangaroo rats with extremely long loops of Henle in their kidneys to reabsorb as much water as possible, and fish where marine fish excrete salt ions from their gills while freshwater fish take salt in by their gills.

Active Transport

What is it?

Not all transport happens passively and without energy. Active transport uses energy in order to move molecules against the concentration gradient. In other words, it moves them from areas of low concentration to areas of high concentration. The most common source of energy for this is ATP.

Bulk Transport

When the cell needs to move either large objects that normally can't fit through the membrane, or large amounts of smaller molecules, it can use bulk transport. This is a type of active transport that relies on the breaking and reforming of the membrane through its flexibility and fluidity.


Endocytosis is when the cell takes in molecules ("endo" meaning "within"). There are three major types of endocytosis, all pictured to the right.

Phagocytosis, or "cell eating", is when the membrane extends out and surrounds the molecules.

Pinocytosis, or "cell drinking", is when the membrane folds inward and takes in molecules.

Receptor-mediated endocytosis is when there are specific receptor proteins and the cell only takes in molecules that bind to them.


Exocytosis is when molecules leave the cell ("exo" meaning "outside"). This commonly occurs thanks to the Golgi, where vesicles fuse with the membrane and release the contents outside of the cell.


As lipids are lost from the membrane during endocytosis, vesicular lipids are added to replace them during exocytosis.