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 7 nm 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 7 nm on average
- Some of the phospholipids are saturated and some are unsaturated. This affects the fluidity of the membrane; an unsaturated membrane means a more fluid membrane. This is due to the kink in unsaturated 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.
Phospholipids 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 cell membrane. 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. Some of these proteins are enzymes, which are used for reactions such as respiration. There are two types of proteins; intrinsic proteins and extrinsic proteins. Intrinsic proteins completely span the bilayer, whilst extrinsic proteins are partially embedded in the bilayer on the inside or outside face of the cell surface membrane.
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. They can also be used to bind cells together to form tissues, cell adhesion. These glycoprotein strands can also act as antigens on the cell, as well as a way to identify/recognise if a cell is self or non-self (if the cell should be there as a part or is foreign e.g. is a pathogenic cell or is in the wrong place or is from an entirely different being. 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. In this way they make the cell specific.
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.
- Surface area.
- Type of molecule.
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 (H2O) 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 shrivels or 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 3-d shape, transferring the molecules or ions across the membrane in the process. Something to note is that cells that do a lot 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. Facilitated Diffusion of Ions
Facilitated diffusion of ions takes place through proteins, or assemblies of proteins, embedded in the plasma membrane. These transmembrane proteins form a water-filled channel through which the ion can pass down its concentration gradient.
The transmembrane channels that permit facilitated diffusion can be opened or closed. They are said to be "gated". Some types of gated ion channels:
* ligand-gated * mechanically-gated * voltage-gated * light-gated
Ligand-gated ion channels. Many ion channels open or close in response to binding a small signaling molecule or "ligand". Some ion channels are gated by extracellular ligands; some by intracellular ligands. In both cases, the ligand is not the substance that is transported when the channel opens. External ligands
External ligands (shown here in green) bind to a site on the extracellular side of the channel.
* Acetylcholine (ACh). The binding of the neurotransmitter acetylcholine at certain synapses opens channels that admit Na+ and initiate a nerve impulse or muscle contraction. * Gamma amino butyric acid (GABA). Binding of GABA at certain synapses — designated GABAA — in the central nervous system admits Cl- ions into the cell and inhibits the creation of a nerve impulse. [More]
Internal ligands bind to a site on the channel protein exposed to the cytosol.
* "Second messengers", like cyclic AMP (cAMP) and cyclic GMP (cGMP), regulate channels involved in the initiation of impulses in neurons responding to odors and light respectively. * ATP is needed to open the channel that allows chloride (Cl-) and bicarbonate (HCO3-) ions out of the cell. This channel is defective in patients with cystic fibrosis. Although the energy liberated by the hydrolysis of ATP is needed to open the channel, this is not an example of active transport; the ions diffuse through the open channel following their concentration gradient.
Mechanically-gated ion channels
* Sound waves bending the cilia-like projections on the hair cells of the inner ear open up ion channels leading to the creation of nerve impulses that the brain interprets as sound. [More] * Mechanical deformation of the cells of stretch receptors opens ion channels leading to the creation of nerve impulses. [More]
Voltage-gated ion channels
In so-called "excitable" cells like neurons and muscle cells, some channels open or close in response to changes in the charge (measured in volts) across the plasma membrane.
Example: As an impulse passes down a neuron, the reduction in the voltage opens sodium channels in the adjacent portion of the membrane. This allows the influx of Na+ into the neuron and thus the continuation of the nerve impulse. [More]
Some 7000 sodium ions pass through each channel during the brief period (about 1 millisecond) that it remains open. This was learned by use of the patch clamp technique:
* A very fine pipette (with an opening of about 0.5 µm) is pressed against the plasma membrane of * either an intact cell or * the plasma membrane can be pulled away from the cell and the preparation placed in a test solution of desired composition. * Current flow through a single ion channel can then be measured.
Such measurements reveal that each channel is either fully open or fully closed; that is, facilitated diffusion through a single channel is "all-or-none".
This technique has provided so much valuable information about ion channels that its inventors, Erwin Neher and Bert Sakmann, were awarded a Nobel Prize in 1991. Facilitated Diffusion of Molecules
Some small, hydrophilic organic molecules, like sugars, can pass through cell membranes by facilitated diffusion.
Once again, the process requires transmembrane proteins. In some cases, these — like ion channels — form water-filled pores that enable the molecule to pass in (or out) of the membrane following its concentration gradient.
Example: Maltoporin. This homotrimer in the outer membrane of E. coli forms pores that allow the disaccharide maltose and a few related molecules to diffuse into the cell.
Another example: The plasma membrane of human red blood cells contain transmembrane proteins that permit the diffusion of glucose from the blood into the cell.
Note that in all cases of facilitated diffusion through channels, the channels are selective; that is, the structure of the protein admits only certain types of molecules through.
Whether all cases of facilitated diffusion of small molecules use channels is yet to be proven. Perhaps some molecules are passed through the membrane by a conformational change in the shape of the transmembrane protein when it binds the molecule to be transported. In either case, the interaction between the molecule being transported and its transporter resembles in many ways the interaction between an enzyme and its substrate. Link to a discussion of the energetics of enzyme-substrate interactions. Active Transport
Active transport is the pumping of molecules or ions through a membrane against their concentration gradient. It requires:
* a transmembrane protein (usually a complex of them) called a transporter and * energy. The source of this energy is ATP.
The energy of ATP may be used directly or indirectly.
* Direct Active Transport. Some transporters bind ATP directly and use the energy of its hydrolysis to drive active transport. * Indirect Active Transport. Other transporters use the energy already stored in the gradient of a directly-pumped ion. Direct active transport of the ion establishes a concentration gradient. When this is relieved by facilitated diffusion, the energy released can be harnessed to the pumping of some other ion or molecule.
Link to a quantitative analysis of these processes. Direct Active Transport 1. The Na+/K+ ATPase
The cytosol of animal cells contains a concentration of potassium ions (K+) as much as 20 times higher than that in the extracellular fluid. Conversely, the extracellular fluid contains a concentration of sodium ions (Na+) as much as 10 times greater than that within the cell.
These concentration gradients are established by the active transport of both ions. And, in fact, the same transporter, called the Na+/K+ ATPase, does both jobs. It uses the energy from the hydrolysis of ATP to
* actively transport 3 Na+ ions out of the cell * for each 2 K+ ions pumped into the cell.
This accomplishes several vital functions:
* It helps establish a net charge across the plasma membrane with the interior of the cell being negatively charged with respect to the exterior. This resting potential prepares nerve and muscle cells for the propagation of action potentials leading to nerve impulses and muscle contraction. * The accumulation of sodium ions outside of the cell draws water out of the cell and thus enables it to maintain osmotic balance (otherwise it would swell and burst from the inward diffusion of water). * The gradient of sodium ions is harnessed to provide the energy to run several types of indirect pumps.
The crucial roles of the Na+/K+ ATPase are reflected in the fact that almost one-third of all the energy generated by the mitochondria in animal cells is used just to run this pump. 2. The H+/K+ ATPase
The parietal cells of your stomach use this pump to secrete gastric juice. These cells transport protons (H+) from a concentration of about 4 x 10-8 M within the cell to a concentration of about 0.15 M in the gastric juice (giving it a pH close to 1). Small wonder that parietal cells are stuffed with mitochondria and uses huge amounts of energy as they carry out this three-million fold concentration of protons. 3. The Ca2+ ATPases
A Ca2+ ATPase is located in the plasma membrane of all eukaryotic cells. It uses the energy provided by one molecule of ATP to pump one Ca2+ ion out of the cell. The activity of these pumps helps to maintain the ~20,000-fold concentration gradient of Ca2+ between the cytosol (~ 100 nM) and the ECF (~ 20 mM). [More]
In resting skeletal muscle, there is a much higher concentration of calcium ions (Ca2+) in the sarcoplasmic reticulum than in the cytosol. Activation of the muscle fiber allows some of this Ca2+ to pass by facilitated diffusion into the cytosol where it triggers contraction. [Link to discussion].
After contraction, this Ca2+ is pumped back into the sarcoplasmic reticulum. This is done by another Ca2+ ATPase that uses the energy from each molecule of ATP to pump 2 Ca2+ ions.
Pumps 1. - 3. are designated P-type ion transporters because they use the same basic mechanism: a conformational change in the proteins as they are reversibly phosphorylated by ATP. And all three pumps can be made to run backward. That is, if the pumped ions are allowed to diffuse back through the membrane complex, ATP can be synthesized from ADP and inorganic phosphate. 4. ABC Transporters ABC ("ATP-Binding Cassette") transporters are transmembrane proteins that
* expose a ligand-binding domain at one surface and a * ATP-binding domain at the other surface.
The ligand-binding domain is usually restricted to a single type of molecule.so this is called the phosphilid layer
The ATP bound to its domain provides the energy to pump the ligand across the membrane.
The human genome contains 48 genes for ABC transporters. Some examples:
* CFTR — the cystic fibrosis transmembrane conductance regulator * TAP, the transporter associated with antigen processing. [Discussion] * the transporter that liver cells use to pump the salts of bile acids out into the bile. * ABC transporters that pump chemotherapeutic drugs out of cancer cells thus reducing their effectiveness.
ABC transporters must have evolved early in the history of life. The ATP-binding domains in archaea, eubacteria, and eukaryotes all share a homologous structure, the ATP-binding "cassette". Indirect Active Transport
Indirect active transport uses the downhill flow of an ion to pump some other molecule or ion against its gradient. The driving ion is usually sodium (Na+) with its gradient established by the Na+/K+ ATPase. Symport Pumps
In this type of indirect active transport, the driving ion (Na+) and the pumped molecule pass through the membrane pump in the same direction. Examples:
* The Na+/glucose transporter. This transmembrane protein allows sodium ions and glucose to enter the cell together. The sodium ions flow down their concentration gradient while the glucose molecules are pumped up theirs. Later the sodium is pumped back out of the cell by the Na+/K+ ATPase. External Link The Na+/glucose transporter is used to actively transport glucose out of the intestine and also out of the kidney tubules and back into the blood. The energy relationships for these processes can be quantified. Link to a discussion.
* All the amino acids can be actively transported, for example o out of the kidney tubules and into the blood [Example] o the reuptake of Glu from the synapse back into the presynaptic neuron by sodium-driven symport pumps. * The Na+/iodide transporter. This symporter pumps iodide ions into the cells of the thyroid gland (for the manufacture of thyroxine) and also into the cells of the mammary gland (to supply the baby's need for iodide).
* The permease encoded by the lac operon of E. coli that transports lactose into the cell.
In antiport pumps, the driving ion (again, usually sodium) diffuses through the pump in one direction providing the energy for the active transport of some other molecule or ion in the opposite direction.
Example: Ca2+ ions are pumped out of cells by sodium-driven antiport pumps [Link]. Antiport pumps in the vacuole of some plants harness the outward facilitated diffusion of protons (themselves pumped into the vacuole by a H+ ATPase)
* to the active inward transport of sodium ions. This sodium/proton antiport pump enables the plant to sequester sodium ions in its vacuole. Transgenic tomato plants that overexpress this sodium/proton antiport pump are able to thrive in saline soils too salty for conventional tomatoes. * to the active inward transport of nitrate ions (NO3−).
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 takes up 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 soil surrounding the root. Both water and mineral ions are absorbed by diffusion and facilitated diffusion/active transport respectively.