Transport across membranes How do substances cross into and out of cells.

Diffusion is an example of Passive transport, as no external energy is required to facilitate movement.  This shift from a higher to a lower area of concentration shows us that there is a large concentration gradient, this resultantly will enable rapid diffusion as molecules disperse.  Diffusion will slow as the concentration gradient becomes smaller though.  When an equilibrium has been met, and there is an evenly distributed spread of molecules there is no longer a concentration gradient and hence no diffusion.  However the molecules will continue to be evenly distributed as they will still be moving, though now this will be because the molecules are continuously changing their position, the spread remains evenly distributed and a dynamic equilibrium will exist.

The diagram shows the concentration of molecule from a high concentration spreading and dispersing and becoming evenly spread to the lower regions and finding an equilibrium.

                                            

                                              (Source:http//:en.wikipedia.org/wiki/Diffusion)

We can relate the above analogy to the cell membrane as the principles are the same.   Though diffusion is a spontaneous and random event, the rate of diffusion from different substances is affected by a number of factors, firstly the steepness of the concentration gradient, as outlined and now in relation to the cell membrane this is the difference in the concentration of substances on the two sides of the membrane.  The greater the difference in the concentration of molecules/ions the faster the net rate of diffusion.  Another factor to consider is temperature, molecules at higher temperatures are loaded with more kinetic energy enabling them to move faster, and therefore diffusion occurs quicker.  The thickness of the cells membrane will directly affect this rate also, the thinner the membrane the faster the rate of diffusion.  The size and type of molecule or ion will affect the rate of diffusion as well, larger molecules will require more energy to get them moving compared to smaller ones, so will likely diffuse relatively slower than the smaller molecules, for example oxygen is a small molecule and would diffuse quickly.

Some molecules however must be helped across the cell membrane through a process called Facilitated diffusion.  This too is a type of passive transport which allows substances to cross membranes with the aid of special transport proteins.  Similarly to diffusion it requires no external energy and relies on a molecules kinetic energy to move down a concentration gradient (movement form high concentration to regions of low concentration).  Though it does differ to diffusion because it employs the use of protein channels at different point on a cell membrane.  Some molecules and ions are not able to pass through the lipid bi-layer of the cell membrane, molecules and ions such as glucose, sodium ions and chloride ions and amino acids need assistance to diffuse across the cells membrane as they are larger molecules than those molecules which can diffuse unaided- such as water or oxygen molecules.  The cell membrane contains many different types of protein channels to allow certain molecules to cross and enter or exit cells.  The proteins on the membrane form water filled channels across a membrane.  This will allow only hydrophilic molecules to pass through.  Only the presence of a precise and specific molecule will enable transport across a particular channel.  

Each protein channel will only allow one kind of molecule or ion to pass through it.

‘If the particular molecule is not present the channel remains closed’ (p55 Toole, G 2008).

From this we can ascertain that the cell membrane has some regulatory control over the entry and exit of molecules/ions but protein channels provide specific molecules with corridors to enable transport across the membrane

           

                

                            (Source:http://en.wikipedia.org/wiki/Facilitated_diffusion)

This diagram shows protein channels and carrier protein sites for transporting molecules through facilitated diffusion.

Through the use of ion protein channels and these carrier proteins that are embedded in the cell membrane substances can be transported into the cell.  Protein channels funnel through molecules, carrier proteins change shape and move molecules through then revert back to their original shape.

Osmosis is a special type of diffusion and is another example case of passive transport.  It involves the transport and movement of water molecules across a semi permeable cell membrane.  We have considered already the cell membrane is selectively permeable, allowing only small molecules and ions through, water is one such molecule which is small enough to pass between the phospholipids bi-layer with out assistance.  We can class osmosis as the diffusion of water molecules across a semi-partially permeable membrane from an area of higher water potential- higher concentration of water molecules to an area of lower water potential- lower concentration of water molecules (Water potential = the likelihood of water molecules to diffuse out of or into a solution).  

Osmosis follows a gradient from higher water potential to lower water potential, this is the water potential gradient.  When both sides of the selectively permeable membrane have the same water potential, when they are equal, an equilibrium point is established resulting in no net movement of water, a dynamic equilibrium is established.

In osmosis, ‘Water will move in the direction where there is a high concentration of solute thus a lower concentration of water.’ (Biologycorner.com)

Osmosis occurs from a hypotonic (low solute concentration) solution to a hypertonic (high solute concentration) solution.

 

                (Source:http://www.occ.edu/biologylabs)

The diagram shows the random movement (using kinetic energy) of water molecules, it should be noted only water molecules cross the semi permeable membrane not solute molecules.

At the point of equilibrium, when there is no net movement of water across the membrane we reach the dynamic equilibrium, this point we refer to as being Isotonic.  If a cell has the same solute concentration and water potential as the surrounding solution there will be no flow of water.  Osmosis differs to ordinary diffusion as it is a slower process and only occurs over shorter distance.  It is vital cells regulate their concentration of water, as too much or too little can have serious consequences.

Diffusion, facilitated diffusion and osmosis we have discovered are all passive forms of transport across a membrane, each relies on the kinetic energy of the molecule or ion wishing to pass through the cell membrane, thus these processes do not require any additional external energy from the cell itself.  Substances pass through the cell membrane down a concentration gradient.  However we now arrive at a form of transport which behaves slightly differently, sometimes cells may need to transport substances against the concentration gradient, and this process is referred to as Active Transport.  There are numerous situations in living organisms when molecules more across cell membrane from an area of lower concentration to an area of higher concentration, for this to occur ‘work’ must be carried out to over come the influences of diffusion and osmosis.

We can define active transport as,

‘the movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using energy and carrier molecules.’ (p59 Toole, G. 2008).

                                             

(Source: _transport)

The fundamental difference is active transport mechanisms draw their energy from ATP from the cell.  Metabolic energy (ATP) is needed from the cell material- substances are moved from low to high up the concentration gradient.  Similar to facilitated diffusion, active transport involves a specific carrier protein in the bi-layer of the cell membrane, the molecule/ions being transported binds to the carrier protein which changes shape releasing the molecule/ions on the other side of the cell membrane, the difference though here it is going against a gradient from low to high, the input of (ATP) energy is required.  It is an energy consuming process.  On occasions more than one molecule or ion can be moved in the same direction, conversely movement of molecules/ions into the cell can occur at the same time as exiting the cell, a key feature of active transport.  In the sodium-potassium pump sodium ions are removed from a cell while potassium ions are transported in.

Active transport as outlined transports a few molecules/ions at most at one time, likewise passive transport (diffusion, facilitated diffusion osmosis).  There are processes in a cell which move substances in ‘Bulk’ into and out of a cell.  There are mechanisms which allow the bulk transport of large quantities of materials into cells- Endocytosis and or out of cells Exocytosis.

Endocytosis involves the engulfing of a/the material by the cell membrane to form a small sac or pockets, the enclosed material is taken into a cell by infoldings in the surface membrane, they tend to be macro-molecules and larger particles such as bacteria.  The small sac or pockets appear in the cell membrane which is then pinched off to form a vesicle inside the cell.  This vesicle now inside a cell contains substances from outside the cell.  Endocytosis can be further explored, it involves phagocytosis and pinocytosis.  Phagocytosis is when solid materials are brought into the cell by the active process of endocytosis.  Cells specialising in this are called phagacytes.  The phagocytic vesicles fuse with a lysosome in a cell which digests the contents.

Pinocytosis essentially means cell drinking and is when fluid is brought into the cell via the same means as above, by endocytosis.  These pinocycotic vesicles allow liquids to be brought into a cell and spread within it.

Exocytosis is the reverse of endocytosis and is the process whereby materials are removed from a cell.  Large molecules such as proteins maybe transported out of a cell by exocytosis.  Vesicles housing protein (within a cell) ready for export break from the golgi apparatus in the cytoplasm and travel to the cell membrane.  Here it fuses with the cell membrane so that the contents are transported outside the cell.  The contents are secreted from the cell.  Vesicles containing the material ready to be expelled once fused with the cell membrane release and secrete their contents and become part of the surface membrane.

(Source:http:users.rcn.com).

Bib

Toole, G & Toole, S  2008  AQA Biology for AS level.   Surrey.