Cell Membranes and Osmosis

General Biology

Membrane Transport Processes
DIFFUSION and OSMOSIS

CELL TRANSPORT

The cell membrane acts like the "skin" of our cell. It keeps the outside out and the inside in. The most important function of the cell membrane is to regulate the movement of substances across the membrane

This is an illustration of a cross section of the cell membrane.

Cell membranes are semipermeable. This means that only certain substances are allowed to travel through the cell membrane. The structure of the cell membrane creates a barrier between the extracellular environmental and the intracellular environment. This property is due to the chemical properties of the phospholipids that make up the cell membrane.
The phospholipids are called amphiphilic, because they have a hydrophilic (water-loving) head and hydrophobic (water-fearing) tails. When phospholipids are placed in water, they spontaneously form a lipid bilayer.

In this phospholipid bilayer, the fatty acid hydrophobic tails will orient themselves to face one another with the hydrophillic phosphate heads facing outward toward the aqueous environments. This structure allows the membrane to be selectively permeable.

There are different ways that the cell membrane has to get substances from one side to the other. Some of these ways require energy. These ways are termed "active transport". Active transport includes bulk transport that uses vesicles to move relatively large amounts of substances across the membrane and active transport through specialized channels that use the power of ATP.
Other means of transportation across the membrane do not required energy and are termed "passive transport". Means of passive transport include osmosis, diffusion and facilitated diffusion.

DIFFUSION

Diffusion occurs because molecules and atoms are always in motion.

Before we talk about osmosis, we must first understand diffusion. The word diffusion comes from the Latin word for "spreads out". In nature, molecule will behave in such a way to "spread out" from an area of high concentration to an area of low concentration, until a time in which those concentration become equal. At this state, the substance is said to be at equilibrium. it is this phenomenon that is responsible for the air currents and ocean currents, as well as the diffusion of a substance in liquid. Since our cells are in an aqueous environment, we will be looking at this type of diffusion.

Gasses and liquids behave this way because their atoms are always in motion. This motion is called "Brownian motion". Brownian motion causes molecules to collide with each other which makes them spread out to maximize their area to move in. They want some"elbow room"!

Scientist Cindy's Famous "Pee in the Pool" Example

EXAMPLE: Let's say it is a hot summer day and some kids are enjoying themselves at the local pool. A kid at one end of the pool decides to urinate (pee) into one end of the pool. Initially, the pee will be concentrated at the end of the pool where the pee landed.

After some time, that pee will then spread out evenly over the volume of that pool reaching the unsuspecting pool-dwellers at the far end of the pool.
This is because in diffusion, liquids (or gasses) flow from an area of high concentration to an area of low concentration, until an equilibrium (no difference in concentration) is achieved. In other words, the pee is equally distributed everywhere.

BUT.... The cell membrane is semi-permeable. So not all substances are able to freely diffuse across the membrane.
When this occurs, we see that substances that can freely diffuse, like water, behave differently. When water diffuses in this manner, it is referred to as OSMOSIS!

OSMOSIS

Diffusion behaves in the way just described, because it is able to move freely and unhindered. In biological systems, molecules are not permitted to move freely. The movement of ions and molecules is tightly regulated by the cell membrane. The cell membrane is selectively-permeable . Substances are permitted to pass through under certain conditions through pores or channels or transporters. The movement of water across the membrane is of particular importance to biological systems. Failure of the cell to maintain osmotic balance with its environment can have a catastrophic negative effects. The movement of water molecules across the semi-permeable cell membrane is called osmosis.

In osmosis, we only concern ourselves with the movement of water molecules (H2O) across the cell membrane. As we saw in diffusion, molecules will travel from an area of high concentration to an area of low concentration.

To understand this concept, it is important to think about a solution in terms of its components.

The liquid substance that acts to dissolve particles, is called the solvent. 
The particles that get dissolved are called the solute.

Then we can define solutions in terms of the concentration of solute and define how cells will be effected by exposure to those solutions! We can define solutions in this way as being..

Hypotonic - a hypotonic solution has a LOWER concentration of solutes than are found in the intracellular fluid
Isotobic - an isotonic solution is one that has the SAME concentration of solutes that are found in the intracellular fluid
Hypertonic - a hypertonic solution has a HIGHER concentration of solutes than are found in the intracellular fluid

In biological systems, the solute in unable to freely diffuse across the cell membrane, so it builds up a concentration gradient. This means that there is a higher concentration of solute on one side of the membrane than there is on the other.

The water molecules, on the other hand, CAN move freely across the cell membrane and will travel from an area of high concentration to low concentration. This traveling from high concentration to low concentration can be termed as travelling down its concentration gradient.

HYPOTONIC SOLUTION

A hypotonic solution is one that has "less" concentration of solute to solvent than the intracellular fluid (fluid inside of the cell). When a cell is placed in a hypotonic solution, water rushed INTO the cell and causes it to SWELL or BURST.

ISOTONIC SOLUTION

An isotonic solution is a solution that has the "same" concentration of solute to solvent as the intracellular fluid (the fluid inside of the cell). When a cell is placed in an isotonic solution, there is NO NET movement of water into or out of the cell. The cell is happy.

HYPERTONIC SOLUTION

A hypertonic solution is one that contains a higher concentration of solute to solvent than the fluid inside of the cell. When a cell is placed in a hypertonic solution, water will rush out of the cell, causing it to shrink.