Cell Membranes
The picture above (click image to enlarge) shows a small section of the animal cell membrane or cell 'skin'
(though the membranes of most cells of other organisms are similar). Above the membrane is outside the cell and
below the membrane is inside the cell.

A - phospholipid molecule
  A1 - hydrophilic phosphate head group of phospholipid
  A2 - Fatty acid tail (x 2) of phospholipid
B - glycolipid (a phospholipid with a carbohydrate chain attached to it)
C - glycoprotein (a protein with a carbohydrate chain attached to it)
  D - carbohydrate chain of glycoprotein
E - cholesterol
F - protein
G - a protein channel (pore)
Distance H-I is typically about 7 nm (say about 10 nm)

The membrane is made up of two principle components:
phospholipids, shown in green, and proteins, shown in
brown. Many phospholipids make up the cell membrane, each green sphere with its two 'tails' is one phospholipid.
The thickness of the membrane is 7-8 nanometres thick (that is 7 to 8 millionths of a millimetre). It is essentially a
liquid crystal as it is a fluid that maintains a definite shape (although the cell can move its membrane and change
its shape as required).

A phospholipid is a type of lipid (lipids are fats and oils) with a phosphate group attached. A phosphate group
contains phosphorus and oxygen atoms and forms the spherical head of each phospholipid, whilst two fatty
chains form the tails. The heads carry a net negative electric charge which is attracted to water molecules inside
and outside the cell, whilst the tails are fatty and so do not mix well with water. These properties of the head and
tail cause the membrane to
spontaneously assemble into the two-layered structure shone - the water-loving
phosphate heads face the water on the either side of the cell, whilst the fatty tails hide away from the water by
huddling together and excluding water from the middle of the membrane. This is what makes the membrane very
stable and strong. Indeed, if a hole is made in the membrane, then if it isn't too big, it will close-over automatically.
Each phospholipid layer is called a
membrane leaflet and we say that the membrane is a phospholipid bilayer.

Despite maintaining this solid-like state, the membrane still behaves partly like a liquid, the phospholipids leaflets
are rather like a layer of oil and individual phospholipids may jostle about and drift across the membrane from one
end to the other, and occasionally flip from one leaflet to the other. The proteins drift about in this phospholipid
sea (unless anchored to the underlying cell skeleton or to the matrix outside the cell).

The proteins come in a vast variety of shapes and forms as they perform a variety of jobs, but many are involved
in transport of materials across the membrane, either importing materials that the cell needs into the cell (such as
food) or exporting materials from inside the cell to the outside (such as when the cell is depositing a matrix around
itself, such as the hard material that bone cells export to make bones). Notice that some of the proteins form
channels across the membrane to allow materials such as salts and water into the cell. These channels can be
opened or closed as required.

What happens when you put butter in the fridge? Butter is made up of fats, like the phospholipid tails, and it
hardens when it is cold. The same is true of cell membranes. For a mammal, like a human, which generates its
own body heat and maintains its temperature, this is not a problem, but for a cold-blooded fish living in the
Antarctic ocean it is a problem that they have had to solve. Humans have solved the problem of spreadable butter
by making margarine which contains polyunsaturated fats which did not freeze so easily, so margarine spreads
easily when taken straight from the fridge. Antarctic fish have similar used more polyunsaturated fats in the
phospholipid tails of their cell membranes. Very high pressure also freezes the membranes, so fish living in both
cold water and the deep sea (where pressures may exceed 1000 times atmospheric pressure) need very oily
membranes to stop them going solid. Once a membrane solidifies, it's bad news for the cell!

Some other organelles of the cell have phospholipid bilayer membranes, including the mitochondria and nucleus,
which each have two such membranes around them, and the endoplasmic reticulum.

Real membranes are much more complex than outlined here! For one thing they often also have chains of sugars
dotted about their outer surface (not shown in the diagram). These sugars react with water to form a layer of
slime (called the
glycocalyx) that covers the outside of the cell.
Membrane diagram labeled
Above: a Pov-Ray computer model of a basic 3D cell membrane, showing the phospholipids in green and
proteins in brown. A more detailed diagram of membrane structure is given below:
Phospholipid structure
The cell membrane is a double layer of phospholipid molecules (shown in green). Since there is water on either
side of the membrane the phosphate heads, which are water-loving or hydrophilic, point towards the water; whilst
the fatty acid tails, which are oily and therefore 'hydrophobic' or water-fearing point inside the membrane. The
internal oily region dominated by the fatty acid tails is almost totally free of water which is excluded, since oil and
water do not mix (they are immiscible fluids).
Fluid mosaic model
Simple diffusion
Facilitated diffusion type 1
Facilitated diffusion type 2
Active transport
Key
The picture above illustrates
which part of the cell we are
looking at here - click the
image to enlarge it.
Above: the phosphate head group of a phospholipid (green circle) carries a negative electric charge and so is
polar (electrically charged). Polar molecules tend to dissolve easily in polar solvents like water due to the
attraction between opposite electric charges on the molecules of solute and solvent (water is a bipolar molecule
as shown below, with the oxygen (O) atom slightly negatively charged and each hydrogen (H) atom slightly
positively charged).

Remember:
  • the solvent is the chemical that does the dissolving (e.g. water) and
  • the solute is the chemical which is dissolved, e.g. common salt, NaCl, sucrose.
  • Like electric charges repel and opposite electric charges attract.

The prime functions of the cell surface membrane are:
  1. To control what substances can enter and leave the cell - like passport / border control the cell surface
    membrane is a selectively permeable barrier.
  2. To sense the environment.

Chemicals can pass through the cell membrane by one of the following processes:
  1. Passive diffusion - passive since it requires no energy expenditure by the cell, two main types:             
  2. Active transport / pumping - uses energy supplied by the cell, e.g. in the form of ATP.
Passive difffusion across cell membranes falls into two subtypes - simple diffusion and facilitated diffusion. In
simple diffusion molecules simply dissolve into the membrane from one side and emerge from it on the other
side. Although oil and water do not mix, small polar molecules like water can slowly dissolve in the membrane to
a small extent. Molecules are in constant motion due to their thermal energy. Phospholipids move to and fro
within the membrane, bumping into and jostling one another. Occasionally small transient gaps open between
neighbouring phospholipids as they jostle about and small polar molecules, such as water and ions like Na+,
can squeeze through - the membrane is leaky, but not by very much!

Fat-soluble substances, like cholesterol, fatty acids, vitamins A, D and E, and steroid hormones can cross the
membrane without difficulty - they easily dissolve in the fatty membrane and so can diffuse across it.

If the cell needs to transport ions or water across it rapidly, then it must use facilitated diffusion. In facilitated
diffusion, membrane proteins help or facilitate the transport of substances across the membrane. One way is
for the protein to form a membrane pore. These pores can be open or closed as needed.
These protein channels or pores can be quite specific - molecules that are too large cannot pass through. Since
ions in solution have very different diameters, a sodium ion (Na+) channel is specific for sodium, a potassium ion
channel (K+) for potassium, a calcium ion (Ca2+) channel for calcium, and so on. Water-specific channels are
called aquaporins.

A second type of facilitated diffusion uses a different type of protein carrier - a protein that flips over or revolves
in the cell membrane - picking up the substance to be transported on one side of the membrane, and then
flipping over to the other side of the membrane to release its cargo. These protein carriers are specific, only
recognising and picking-up their intended cargo. Quite large molecules, such as glucose, can be transported in
this way.
Simple and facilitated diffusion can transport molecules both ways across the cell membrane. However, if there
are more sodium ions outside the cell (specifically a higher concentration) than inside the cell, then when sodium
ion channels are open, although some sodium ions will exit the cell, far more (by the laws of statistics) will enter
the cell. We say that diffusion occurs
down a concentration gradient from a region of high concentration to a
region of low concentration.

If the cell wishes to transport substances against a concentration gradient, then it must expend energy to actively
pump teh substance across the membrane - active transport.
Tissue fluid
Diffusion_selectivity
An example of active transport is the sodium/potassium ion pump, explained in the diagram above. This pump
uses energy from ATP (adenosine trisphosphate - the molecule that carries the cell's store of usable energy).
The pump is a membrane protein and it pumps two potasium ions (K+) into the cell for every three sodium ions
(Na+) that it pumps out of the cell. This maintains a high internal concentration of K+ inside the cell and low
internal [Na+] (the square brackets mean 'concentration of'). The tissue fluid which bathes the cells (and leaves
small blood vessels by filtration under pressure) resembles sea water - with a high [Na+] and a high [Cl-] and the
sodium/potassium pump maintains these ion concentration differences across the cell membrane:
Osmosis is the passive diffusion of water across cell membranes (by simple and facilitated diffusion). (View the
plant transport page for some graphical illustrations of osmosis in plant cells). If the solute concentration inside
the cytoplasm is higher (and hence the water concentration lower) than the surrounding medium, then an
animal cell will take in water, as water moves down its concentration gradient, swell and possibly burst (lyse,
undergo
lysis). Mammalian blood cells, for example, will rapidly burst in distilled water. In this case we say that
the cytoplasm is
hypertonic to the medium.

If the cytoplasm has a lower solute concentration than the surrounding medium, then it is
hypotonic, and water
will move out of the cell, across the cell-surface membrane and the cell will shrivel and
crenelate (its contour
becomes wavy or zig-zagged, crenelation is the name given to the feature at the tops of castle walls with their
merlons and crenels).

If the cytoplasm has the same solute concentration as the surrounding medium then it is
isotonic and no net
movement of water occurs across the cell-surface membrane by osmosis.

All solutes contribute to the calculation of solute concentration, not just salts, though substances present in the
highest concentration (in terms of number of molecules per unit volume) are mostly responsible and these are
called
osmolytes, especially is regulating cytoplasmic solute concentration is one of their functions. For
example, some bacteria that grow in very salty waters (
halophiles) increase the concentration of certain
chemicals in their cytoplasm, such as potassium chloride, to maintain isotonicity. The potassium chloride is thus
a major osmolyte in these cells.