Transport across Membranes

Cells need to take in food, get rid of waste products (excretion) and give out such useful substances as hormones and enzymes (secretion).

There are various mechanisms for transport across membranes. These mechanisms are much beloved of examiners.

Membranes are partially (or selectively) permeable. This means that some, but not all, substances can permeate (pass through) them. The structure of membranes determines which substances can pass through.

Membranes comprise a phospholipid bilayer with proteins embedded in it.

Transport across membranes can be subdivided according to the mechanism (once again examiners like you to understand the difference between these subdivisions)

Transport can be

• passive
• active
• osmosis
• endocytosis and exocytosis

Passive v. Active Transport

• passive transport uses no energy, active transport uses energy
• passive transport involves movement from an area of high concentration to an area of low concentration (i.e. down a concentration gradient)
• active transport involves movement from a area of low concentration to an area of high concentration (i.e. up a concentration gradient)
 

note "along a concentration gradient" is often used by students who can't remember the correct direction - it is meaningless and examiners are wise to it - you won't get any marks for it

Passive Transport includes simple diffusion and facilitated diffusion

Diffusion

The movement of molecules or ions from a region where they are at a high concentration to a region of lower concentration. The difference in concentration is referred to as a concentration gradient. There will be a net movement down the concentration gradient until equilibrium is reached, that is when there is a uniform distribution of the ions or molecules. This process is passive, as it does not require metabolic energy.

Note movement of the diffusing substance is in both directions simultaneously but at different rates. It is the net movement we are considering.

There are several factors which can affect the net rate of diffusion:

• the concentration gradient - the greater the difference in concentration either side of the membrane the greater the net rate of diffusion. For a substance to pass across a membrane it has first to collide with the membrane, the more of the substance there is the more collisions will occur. This is analogous to the situation with enzymes where the more substrate there is, the more collisions with enzymes occur, the greater the rate of reaction.
• the size of the molecules or ions: smaller molecules or ions can pass more readily through the bilayer
• the distance over which diffusion must occur - the greater the distance the longer diffusion takes. This is not very important for membranes since they are of a fairly similar width however note that some organelles (e.g. mitochondria) have a double bilayer around them effectively doubling the diffusion distance
• Note that when considering the diffusion of ions a second gradient, the electrochemical gradient, also operates; net movement of ions will be towards areas of the opposite charge
 

• Diffusion which occurs without the involvement of specific carrier proteins
• Oxygen, carbon dioxide and water can all pass freely through the phospholipid bilayer and net movement will be down the concentration gradient present.
• Uncharged and lipid-soluble molecules also pass freely through the bilayer.
• Small polar molecules cannot readily dissolve in the hydrophobic bilayer but can pass through the membrane via "holes" formed by intrinsic proteins forming channels. The proteins are thus called channel (or transport) proteins.

Facilitated Diffusion

• Movement down a concentration gradient but involving the action of carrier proteins
• is used for most ions and small polar molecules (e.g. glucose)
• involves an intrinsic protein which is specific for a substance (has a specific receptor site for that substance)
• the substance e.g. glucose binds to the carrier protein which undergoes a conformational (shape) change resulting in the substance being deposited on the other side of the membrane. The protein then returns to its original shape ready to operate again
• Happens only down a concentration gradient and does not require energy.

Osmosis

• osmosis is a special case of diffusion. It is the diffusion of water molecules from an area of high water concentration to an area of low water concentration, i.e. from an area of low solute concentration to an area of high solute concentration

In the diagram the dialysis tubing acts as a semi-permeable membrane. The solution inside the bag has a higher solute concentration (and thus lower water concentration) than the solution outside. So water osmoses (diffuses) into the bag increasing the volume of the solution inside the bag resulting in movement of liquid up the capillary tube.

Water potential

Plant scientists describe osmosis in terms of water potential.

Many students find this concept very difficult to understand but questions based on it are very common.

Note: there is an error in the Nelson textbook Exchange and Transport.... etc". On page 32 the third line of the second paragraph should begin ".....which is permeable to water only."

Water potential is defined as:

• "the force acting on water molecules in a solution, when separated from pure water by a membrane which is permeable to water only"

but may be more readily understood as

• "the force responsible for movement of water in a system"

if you consider the water potential to be:

• the pressure generated by water molecules as they collide with the membrane

then you should see that the higher the water concentration the higher the water potential because more water molecules means more collisions per unit time.

Because pure water has the highest concentration of water molecules, and thus the highest water potential, the water potential of all other solutions must be lower than zero i.e. negative.

It is this concept of having zero as the highest value that tends to confuse - learn this! Unfortunately you also have to know how to calculate water potentials from the values of two other potentials.

Water potential is affected by two other potentials in the system. These are the solute potential and the pressure potential.

Because water potential is a measure of the concentration of water in a solution adding solute to the solution will decrease the water concentration and thus the water potential (making it more negative). This change in the water potential is called the solute potential.

In plant cells the cell membrane is surrounded by a cell wall which resists expansion of the cell. If a plant cell is surrounded by a solution of a higher water potential it will take up water and expand. The pressure exerted by the wall will oppose the uptake of water. This pressure is known as the pressure potential.

The solute potential acts to decrease the water potential of the cell so it has a negative value.

The pressure potential acts in the opposite direction to the solute potential so it has a positive value.

Combining the three values gives this equation:

water potential = solute potential + pressure potential

The potentials are abbreviated with the symbol (Greek letter psi). So the equation becomes:

These values are pressures so have the units kilopascals (kPa)

If the above makes no sense whatsoever the key information to learn is:

• The equation given
• the water potential of pure water is zero
• water moves from areas of higher water potential to areas of lower water potential (i.e. towards the more negative region)

• If you can enter the exam knowing these three facts you should be able to answer most questions on this topic.

Active Transport

• Movement of substances across a membrane up a concentration gradient. For ions movement is up an electrochemical gradient (e.g. positive ions being moved to a region of more positive charge)
• requires an input of energy (in the form of ATP)
• involves carrier proteins
 

Trans-epithelial transport: the diagram shows an example in which 3 carrier proteins accomplish absorption of glucose and Na⁺ in the small intestine

Na⁺-K⁺ pump

• The Sodium-Potassium pump (this is an antiport - a carrier protein which carries 2 substances in opposite directions simultaneously), located at the basal end of the cell, keeps [Na⁺] lower in the cell than in the fluid bathing the apical cell surface. A protein in the membrane acts to move sodium and potassium across the membrane.
• As sodium is pumped out of the cell potassium is pumped in. This requires the dephosphorylation of ATP to ADP + P_i (i.e. the use of ATP energy).
• The action of the sodium-potassium pump is linked to the action of another carrier protein, the glucose-sodium symport

Glucose Na⁺ symport

• The Na⁺ gradient drives uphill transport of glucose into the cell at the apical end, via the apically located glucose-Na⁺ symport (a symport is a carrier protein which transports 2 substances simultaneously in the same direction) . Glucose concentration within the cell is thus higher than outside the cell at either end.

GLUT 1

• Glucose flows passively out of the cell at the basal end, down its gradient, via GLUT2, a uniport carrier protein related to GLUT1.

Endocytosis and Exocytosis

• The previous mechanisms involve the movement of single ions or molecules across the membrane
 
• Endocytosis and exocytosis enable large numbers of molecules and even whole cells to move across membranes
 
• Membranes are fluid and can fuse together.
 
• In endocytosis an invagination occurs around the substance (e.g. a bacterial cell) to be ingested. The membrane then closes up around the foreign material forming a vacuole and closing up the cell membrane

• Exocytosis is essentially the reverse process.
 
• Where the material ingested by endocytosis is to be destroyed lytic enzymes found in lysosomes are deployed. This occurs by fusion of the lysosome with the endocytic vacuole. The waste products are then excreted by exocytosis

An animation showing exocytosis is available at http://www.stanford.edu/group/Urchin/GIFS/exocyt.gif

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