All living cells have something known as a cell membrane. This selectively-permeable membrane controls the exchange of materials, receives hormone messages and is very thin. It can be described as a phospholipid bi-layer - meaning that it is made from phospholipid molecules and has two layers.
The phospholipid bi-layer is so thin it can barely even be seen by an electron microscope - a x100,000 magnification is required, and only shows a double black line around 7nm wide. Since we cannot properly see the membrane, we have to take what we know about it and create a model - in this case known as a fluid mosaic model.
Fluid Mosaic ModelEdit
A diagram of the fluid mosaic model can be seen below.
Features of the fluid mosaic model;
- The membrane primarily consists of a bilayer of phospholipid molecules. These molecules can move about by diffusion in their own layer.
- Width is about 7nm on average
- Some of the phospholipids are saturated and some are unsaturated. This affects the fluidity of the membrane, a heavily saturated membrane means a more fluid membrane. This is due to the kink in saturated tails causing the molecules to not sit closely together.
- Phospholipid tails point inwards, facing each other, meaning that inside the membrane it is non-polar hydrophobic.
- The protein molecules within the structure can move around although some are fixed to structures inside the cell and do not move. Also, some of them span the width of the membrane, some are only on the inner layer and some on the outer layer.
- Many proteins and lipids have short carbohydrate chains attached to them, forming glycoproteins and glycolipids.
- Contains cholesterol.
The previous section spoke about several components that may be new to you, their structures and roles are below.
You may remember phospholipids from chapter two. It was stated that they have hydrophilic heads and hydrophobic tails, the importance of which should be becoming clear. A phospholipid bi-layer forms the body of the membrane, creating a hydrophobic interior and isolating the cell from the outside environment. Do not forget that organelles within a cell are also often surrounded by a membrane, and this too is a phospholipid membrane.
Proteins have a variety of functions within membranes. Some membrane proteins are enzymes, catalysing reactions such as the ones in the surface of the small intestine, that hydrolyse disaccharide molecules, among others. However, most are transport proteins, providing hydrophilic channels through the bi-layer for ions and polar molecules to pass through. They are usually specific to a certain ion or molecule. They can control the types of substances that can leave or enter the cell. The membranes of organelles also hold proteins - for example mitochondria and chloroplasts require them for processes of respiration and photosynthesis.
Glycoproteins and GlycolipidsEdit
Lipid and proteins on the cell membrane surface often have short carbohydrate chains protruding out from the cell surface, known as glycolipids and glycoproteins. They form hydrogen bonds with the water molecules surrounding the cell and thus help to stabilise membrane structure. However, more importantly, they are used as receptor molecules, binding with hormones or neurotransmitters to trigger a series of chemical reactions within the cell itself. Using insulin as an example, only some cells within the body (liver, muscles), have receptors for insulin and as such, insulin can be released to the entire body without upsetting anything as any cell without an insulin receptor will not be affected.
Glycoproteins can also serve as antigens, which are used in allowing cells to recognise each other - you'll learn more about this in Human Health & Disease.
Cholesterol helps to regulate fluidity of the membrane and also to provide mechanical stability of the membranes - without it cells will burst open as their membranes break. Their hydrophobic regions help to prevent ions or polar molecules inadvertently passing through the membrane.
Transport across the phospholipid bi-layer is regulated, and it is an effective barrier - but exchange is necessary. Methods of exchange are discussed here.
Diffusion is defined as the net motion of a substance from an area of high concentration to an area of low concentration, 'down a concentration gradient'.
Factors which affect diffusion;
- How steep the concentration gradient is. Concentration gradient is the ratio of molecules on one side of the membrane to the other, many on one side compared to few on the other will result in a faster net rate of diffusion
- Temperature. High temperatures increase the kinetic energy of molecules and ions and thus they move around faster - net rate of diffusion goes up.
- Surface area. A large surface area increases the amount of ions or molecules that can cross at one time, increasing the net rate of diffusion.
- Type of molecule. Large molecules are slower to diffuse, non-polar molecules diffuse more easily through cell membranes as they are soluble.
Molecules and ions that are small enough can cross membranes easily, regardless of polarity, but large polar molecules such as glucose cannot diffuse through a cell membrane. They can only pass through hydrophilic protein channels - this process is known as facilitated diffusion. All the factors that affect diffusion affect facilitated diffusion, and an additional one - how many transport proteins are available.
Facilitated diffusion: 
Osmosis is best described as a special type of diffusion involving water molecules only.
The first diagram is the experiment just set up, the 'before diagram', and as you can see, water is moving from the more dilute substance to the less dilute solution, separated by a partially permeable membrane - down the concentration gradient. The second picture shows the substance after it has been left for a while - there is the same concentration of solute molecules and thus the same concentration of water molecules, this is known as equilibrium. The fact that this is the movement of water molecules alone that has bought about the equilibrium is characteristic of osmosis.
The propensity for water molecules to move from one place to another is known as the water potential, and the symbol for water potential is the Greek letter 'psi', Ψ. Water always moves from a area of high water potential to a low water potential area - equilibrium as mentioned before is the equaling of two adjacent (separated by a partially-permeable membrane) water potentials. Pure water has a water potential of 0, and any solutes make the water potential negative, the degree to which they do called the solute potential.
Net movement of water from a high to low concentration gradient can be prevented or slowed by increasing the pressure in the low concentration gradient solution, since pressure increases the water potential.
In plant cellsEdit
Plants have cell walls, and thus pressure potential is especially important - if the volume of the cell increases as a result of water entering the cell via osmosis, the cell starts to push against the cell wall and pressure builds up rapidly. This pressure increases the water potential of the cell so that water stops entering it, creating a 'false-equilibrium' (in that pressure helps it) preventing it from bursting. When a cell is fully inflated it is described as turgid.
The opposite to turgidity is plasmolysis - when a cell is placed in a concentrated sucrose solution, the cell begins to shrink away from the cell wall, creating a pressure potential of 0, and so the water potential is equal to its solute potential. Eventually cytorrhysis – the complete collapse of the cell wall – can occur. There is no mechanism in plants to prevent excess water loss in the same way as excess water gain, but plasmolysis can be reversed if the cell is placed in a weaker solution . The equivalent process in animal cells is called crenation. The liquid content of the cell leaks out due to diffusion. Plasmolysis is extremely rare in nature.
In animal cellsEdit
In animal cells however, there is no cell wall, and so if the water potential of the solution around the cell is too high, the cell will swell and burst, but if it is too low, the cell shrinks. This is why it s important to keep a constant water potential inside animal bodies.
Active transport is defined as the energy-consuming transport of molecules or ions across a membrane against a concentration gradient, made possible by transferring energy from respiration. The energy is supplied by ATP, and is used to make the transport protein change its 3d shape, transferring the molecules or ions across the membrane in the process. Something to note is that cells that do alot of active transport are likely to have many mitochondrion to provide the energy for it.
It is particularly important in reabsoprtion in the kidneys where certain useful molecules must be reabsorbed into the blood after filtration. In plants it is used to load sugar from photosynthesising cells into the phloem tissue for transport.
Bulk transport can be defined as the movement of large quantities of materials into or out of cells, endocytosis and exocytosis, respectively.
Exocytosis is the process by which materials are removed from cells - for example the secretion of digestive enzymes, where vesicles from the Golgi apparatus carry the enzymes to the cell surface, bind to it, and release their contents. See: 
Endocytosis is the reverse of exocytosis and involves the engulfing of the material by the cell to form a small sac inside the cell. The most common form is phagocytosis, performed by phagocytes - an example of which would be white blood cells engulfing bacteria. The second form of endocytosis is pinocytosis, the bulk uptake of liquid, and the human egg cell take sup nutrients from cells that surround it by this method.
This relates to how substances move in and out of whole organisms, using two examples.
All mammals need a supply of oxygen to use in respiration - and in mammals the cells that require the oxygen are too far for diffusion to be effective and so they have a specialised gaseous exchange surface, where oxygen can diffuse into the body and carbon dioxide can diffuse out. In humans, this exchange surface is the alveoli in the lungs, and as you can see in the diagram below, each alveolus is tiny but collectively they have a huge surface area, totaling around 70m2 in an adult. This increases the net rate of diffusion. Alveoli also have extremely thin walls, no more than 0.5 μm thick, directly next to blood capillaries also with very thin cell membranes. This thinness allows diffusion to be extremely speedy.
Since diffusion only works down a concentration gradient, and the steeper said concentration gradient, the faster diffusion, the concentration gradient between the alveoli and the blood must be kept steep constantly to ensure speedy diffusion. Carbon dioxide is rapidly breathed out, meaning that carbon dioxide transported to the alveoli will be quickly transported into them and breathed out, and deoxygenated blood is constantly kept coming to the alveoli, meaning that oxygen diffuses into it quickly.
See picture: 
Root hairs of a flowering plant are also specialised exchange surfaces - they are extensions of the cells that make up the epidermis of a root, and are each around 200-250μm across, providing an enormous total surface area that is in contact with the soill surrounding the root. Both water and mineral ions are absorbed by diffusion and facilitated diffusion/active transport respectivley.