Osmosis

Imagine you have a cup that has 100mL100mL water, and you add 15g15g of table sugar
to the water. The sugar dissolves and the mixture that is now in the cup is made up of
a solute (the sugar) that is dissolved in the solvent (the water). The mixture of a solute in
a solvent is called a solution.
Imagine now that you have a second cup with 100mL100mL of water, and you
add 45g45g of table sugar to the water. Just like the first cup, the sugar is the solute, and
the water is the solvent. But now you have two mixtures of different solute concentrations.
In comparing two solutions of unequal solute concentration, the solution with the higher
solute concentration is hypertonic, and the solution with the lower solute concentration
is hypotonic. Solutions of equal solute concentration are isotonic. The first sugar solution
is hypotonic to the second solution. The second sugar solution is hypertonic to the first.
You now add the two solutions to a beaker that has been divided by a semipermeable
membrane, with pores that are too small for the sugar molecules to pass through, but are
big enough for the water molecules to pass through. The hypertonic solution is one one side
of the membrane and the hypotonic solution on the other. The hypertonic solution has a
lower water concentration than the hypotonic solution, so a concentration gradient of water
now exists across the membrane. Water molecules will move from the side of higher water
concentration to the side of lower concentration until both solutions are isotonic. At this
point, equilibrium is reached.
Red blood cells behave the same way (see figure below). When red blood cells are in a
hypertonic (higher concentration) solution, water flows out of the cell faster than it comes
in. This results in crenation (shriveling) of the blood cell. On the other extreme, a red blood
cell that is hypotonic (lower concentration outside the cell) will result in more water flowing
into the cell than out. This results in swelling of the cell and potential hemolysis (bursting) of
the cell. In an isotonic solution, the flow of water in and out of the cell is happening at the
same rate.

Figure 8.4.1: Red blood cells in hypertonic, isotonic, and hypotonic solutions.
Osmosis is the diffusion of water molecules across a semipermeable membrane from an
area of lower concentration solution (i.e., higher concentration of water) to an area
of higher concentration solution (i.e., lower concentration of water). Water moves into and
out of cells by osmosis.
  If a cell is in a hypertonic solution, the solution has a lower water concentration than
the cell cytosol, and water moves out of the cell until both solutions are isotonic.
 Cells placed in a hypotonic solution will take in water across their membranes until
both the external solution and the cytosol are isotonic.
A red blood cell will swell and undergo hemolysis (burst) when placed in a hypotonic
solution. When placed in a hypertonic solution, a red blood cell will lose water and
undergo crenation (shrivel). Animal cells tend to do best in an isotonic environment, where
the flow of water in and out of the cell is occurring at equal rates.

Diffusion
Passive transport is a way that small molecules or ions move across the cell membrane
without input of energy by the cell. The three main kinds of passive transport are diffusion
(or simple diffusion), osmosis, and facilitated diffusion. Simple diffusion and osmosis do not
involve transport proteins. Facilitated diffusion requires the assistance of proteins.
Diffusion is the movement of molecules from an area of high concentration of the
molecules to an area with a lower concentration. For cell transport, diffusion is the
movement of small molecules across the cell membrane. The difference in the
concentrations of the molecules in the two areas is called the concentration gradient.
The kinetic energy of the molecules results in random motion, causing diffusion. In simple
diffusion, this process proceeds without the aid of a transport protein. It is the random
motion of the molecules that causes them to move from an area of high concentration to an
area with a lower concentration.
Diffusion will continue until the concentration gradient has been eliminated. Since diffusion
moves materials from an area of higher concentration to the lower, it is described as moving
solutes "down the concentration gradient". The end result is an equal concentration,
or equilibrium, of molecules on both sides of the membrane. At equilibrium, movement of
molecules does not stop. At equilibrium, there is equal movement of materials in both
directions.
Not everything can make it into your cells. Your cells have a plasma membrane that helps to
guard your cells from unwanted intruders.

The Plasma Membrane and Cytosol

If the outside environment of a cell is water-based, and the inside of the cell is also mostly
water, something has to make sure the cell stays intact in this environment. What would
happen if a cell dissolved in water, like sugar does? Obviously, the cell could not survive in
such an environment. So something must protect the cell and allow it to survive in its water-
based environment. All cells have a barrier around them that separates them from the
environment and from other cells. This barrier is called the plasma membrane, or cell
membrane.

The Plasma Membrane

The plasma membrane (see figure below) is made of a double layer of special lipids, known
as phospholipids. The phospholipid is a lipid molecule with a hydrophilic ("water-loving")
head and two hydrophobic ("water-hating") tails. Because of the hydrophilic and
hydrophobic nature of the phospholipid, the molecule must be arranged in a specific pattern
as only certain parts of the molecule can physically be in contact with water. Remember that
there is water outside the cell, and the cytoplasm inside the cell is mostly water as well. So
the phospholipids are arranged in a double layer (a bilayer) to keep the cell separate from
its environment. Lipids do not mix with water (recall that oil is a lipid), so the phospholipid
bilayer of the cell membrane acts as a barrier, keeping water out of the cell, and keeping
the cytoplasm inside the cell. The cell membrane allows the cell to stay structurally intact in
its water-based environment.
The function of the plasma membrane is to control what goes in and out of the cell. Some
molecules can go through the cell membrane to enter and leave the cell, but some cannot.
The cell is therefore not completely permeable. "Permeable" means that anything can cross
a barrier. An open door is completely permeable to anything that wants to enter or exit
through the door. The plasma membrane is semipermeable, meaning that some things
can enter the cell, and some things cannot.
Molecules that cannot easily pass through the bilayer include ions and small hydrophilic
molecules, such as glucose, and macromolecules, including proteins and RNA. Examples of
molecules that can easily diffuse across the plasma membrane include carbon dioxide and
oxygen gas. These molecules diffuse freely in and out of the cell, along their concentration
gradient. Though water is a polar molecule, it can also diffuse through the plasma
membrane.

Figure 8.4.2: Plasma membranes are primarily made up of phospholipids (orange). The
hydrophilic ("water-loving") head and two hydrophobic ("water-hating") tails are shown. The
phospholipids form a bilayer (two layers). The middle of the bilayer is an area without
water. There can be water on either side of the bilayer. There are many proteins throughout
the membrane.

Cytosol
The inside of all cells also contain a jelly-like substance called cytosol. Cytosol is composed
of water and other molecules, including enzymes, which are proteins that speed up the
cell's chemical reactions. Everything in the cell sits in the cytosol, like fruit in a Jell-o mold.
The term cytoplasm refers to the cytosol and all of the organelles, the specialized
compartments of the cell. The cytoplasm does not include the nucleus. As a prokaryotic cell
does not have a nucleus, the DNA is in the cytoplasm.