Transport across membranes

Cells require a variety of substances in order to live and carry out all of the processes to maintain life.  They also produce waste products that, if left to accumulate, can become toxic and cause cell death and/or death of the organism. This requires the cell membrane to be partially-permeable, allowing certain molecules to move but preventing the movement of others.

There are five basic ways in which molecules can move across a biological membrane:

• Diffusion
• Facilitated Diffusion
• Osmosis
• Active Transport
• Endocytosis/Exocytosis

Diffusion- The passive net movement of molecules from a region of high concentration to a region of lower concentration down a concentration gradient.

• The molecules move in all directions all the time due to kinetic energy. However, most molecules will move from the area of high concentration to the area of low concentration – this is the net movement
• Diffusion continues until the concentration of the substance is the same throughout the substance or solution.
• FACTORS AFFECTING DIFFUSION- concentration gradient of a substance which is the difference in the concentration between the regions through which the substance is diffusing, divided by the distance- bigger the difference the faster rate of diffusion.

Facilitated diffusion- The passive net movement of molecules from a region of high concentration to a region of lower concentration down a concentration gradient through a channel or carrier transport protein.

• Protein carriers/channels have a specific shape which means they only allow one type of substance to move through.
• The number of protein channels can affect the rate of facilitated diffusion. 
• The more channels the faster the rate.
• Surface area (larger = faster diffusion)
• Diffusion Distance (larger = slower diffusion)  Concentration Difference (larger = faster  diffusion)

Active Transport- The energy requiring movement of molecules against their concentration gradient through a carrier protein.

The energy for active transport is provided by a molecule called ATP (Adenosine Triphosphate) which is produced during cellular respiration.  ATP binds to the carrier protein and is hydrolysed to ADP + P.  The phosphate molecule then binds to the protein causing it to change shape and release the molecule on the other side of the membrane. Therefore anything which stops or inhibits (slows down) respiration (e.g. lack of oxygen) will also stop active transport and endo/exocytosis as no ATP will be released.

• allows cells to build up stores of a substance that would otherwise be spread out by diffusion. e.g- storage of glucose by liver cells.
• Diffusion, facilitated diffusion and Osmosis are passive, energy provided by the hydrolysis of ATP is not needed.  The kinetic energy provided as a result of the kinetic motion of their own molecules is sufficient to move molecules or ions down their concentration gradient.
• Both active transport and facilitated diffusion achieve diffusion by carrier protiens. 

small, non-polar substances - Diffusion through phospholipid bilayer.  e.g. oxygen, carbon dioxide, lipid soluble molecules.

large substances -  Facilitated diffusion through carrier proteins. Active transport through carrier proteins. e.g. glucose, amino acids, water soluble molecules.

polar substances - Facilitated diffusion through channel proteins. Active transport through carrier proteins. e.g. ions such as Na+, Cl-.

SODIUM- POTASSIUM PUMP

• carrier protein which actively transports sodium ions and potassium ions across the plasma membrane of cells.  The concentration of sodium ions is higher outside the cell than inside, the concentration of potassium ions is higher inside the cell than outside. Three sodium ions and one molecules of ATP bind to the pump protien.  The ATP is hydrolysed and ADP is released, the phosphate group remains bound to the protein.  As a result the shape of the protien pump changes, three sodium ions pass from the cell against the concentration gradient of sodium ions and are released. Two potassium ions bind to the pump protien, the phosphate group bound to the pump protein is released. As a result the structure of the pump protein changed back to its original shape and potassium ions pass into the cell against the concentration gradient for potassium and are released.

Endocytosis/Exocytosis- The bulk movement of molecules into (endocytosis) or out of (exocytosis) a cell.

Endocytosis

• Plasma membrane engulfs molecules;
• Fuses with itself and pinches off;
• This forms a vesicle;
• Which moves through cytoplasm (to where molecules are required)
• This requires energy from ATP/respiration;
• Endocytosis is triggered by the binding of the molecule to receptor site;
• E.g. white blood cell engulfing bacterium.

Exocytosis

• Vesicle containing molecules;
• Moves through cytoplasm to plasma membrane;
• Vesicle fuses with plasma membrane;
• This requires energy from ATP/respiration;
• Molecules are released on opposite side of membrane
• E.g. neurotransmitter release at synapse or secretion of proteins.

High temperatures can have a catastrophic effect on membrane structure in two ways:

• The increased kinetic energy of the phospholipids overcomes the hydrophobic interactions causing the bilayer to disintegrate.
•  The transport proteins carrying out facilitated diffusion and active transport denature and are unable to perform their transport function.  

• Solvents are chemicals that can dissolve particular compounds. Membranes are largely made up of phospholipids; the lipid can be dissolved by detergent (like washing up liquid) and ethanol. 
• This is because solvents are often less polar than water and so prevent the phospholipid bilayer from forming.  Alcohol molecules can become positioned between the phospholipids which disrupts the membrane. 
• As the phospholipids dissolve in these solvents it disrupts the partially permeable bilayer structure, crating holes which increase the permeability and allow larger molecules to move across the membrane.        
• This is why alcohols are used in antiseptic wipes.  The alcohols dissolve the membranes of bacteria which kills them and reduces infection.