BIO8201 Biology Foundations Assignment Help

INTRODUCTION

Cells are separated from their external environment and from each other by the cell or plasma membrane (plant cells also have cell walls).  Within a cell, organelles (eg. Chloroplasts, nucleus, mitochondria, vacuoles) are also enclosed by membranes which separate them from the surrounding cytoplasm.  Nutrients, water, waste products, ions, and gases must be able to pass across these membranes.

Biological membranes are selectively (“semi”) permeable because they act as molecular “traffic regulators”, allowing certain substances to pass and rejecting others.  The direction and rate of movement of many substances across membranes is controlled by diffusion (passive transport).  Membranes also actively “pump” some substances in and out of the cell.  This process requires metabolic energy derived from respiration and is called active transport.  In this exercise you will be concerned only with diffusion.

Diffusion involves the movement of molecules due to the intrinsic kinetic energy (thermal motion) that they possess.  During diffusion, molecules have a tendency to spread out into the space that is available. The net movement (diffusion) of substances always occurs from where the substance is more concentrated to when it is less concentrated ie. Down a concentration gradient.  Factors that can affect the rate of diffusion include temperature, pressure and concentration of molecules.

Consider the example illustrated by Figure 1(a) where a glass vessel is divided into halves by a porous membrane.  Side A is filled with a salt (sodium chloride, NaCl) solution and Side B with pure water.  After 5 minutes the solutions on both sides of the membrane are found to be the same.  Given that temperature and pressure are the same on both sides, the change in the system can be explained by the fact that water molecules on Side B moved from an area of high concentration to an area of lower water concentration on Side A.  Similarly there was a net diffusion of sodium (Na+) and chloride (Cl-) ions from Side A to B.  Where the pores in the membrane allow both water and salt ions to cross freely, the membrane is said to be permeable and non-selective.  This allows the system to come to equilibrium, so there is no net movement of water and salt across the membrane after a short time.

Figure 1(b) illustrates a biologically important process, called osmosis.  This process refers to the net movement (diffusion) of water through a selectively (semi) permeable membrane from an area of higher concentration of water to an area of lower concentration of water.  In Figure 1(b) a glass vessel is divided by a porous membrane.  Side A is filled with a sugar solution (instead of NaCl) and Side B with pure water.  After 5 minutes, Side A contains more fluid than Side B, because water molecules have moved across the membrane from an area of high water concentration to an area of lower water concentration.  The sugar molecules, however, have remained on Side A, because the molecules are too large to pass through the membrane.  The diffusion of water molecules across a selectively permeable membrane down a concentration gradient is called osmosis.

We can now make a few generalizations about the movement of substances across artificial membranes of uniform chemical composition and pore size:

1. Any substance which is soluble in water will reduce the concentration of unbound (free) water molecules. The greater the number of dissolved molecules or ions, the less the concentration of water.  For example, a 1.0M solution of glucose will reduce the water concentration more than a 0.5M solution of glucose.
2. The size of molecules also affects the direction of osmosis. Particles larger than the membrane pores will have an effect proportional to their concentration – the higher the concentration of the dissolved substance, the lower the concentration of the water.  Ions and molecules which are able to cross the membrane freely will have no net effect, although these particles will influence the movement of water until they become equally distributed on each side of the membrane.
3. Large molecules which form a suspension (do not dissolve) in water will have little or no effect on the movement of water across a semi-permeable membrane.

The above generalizations refer to the movement of substance across artificial membranes.  Keep in mind that biological membranes are very complex structures and often do not conform to the physical “rules” of diffusion.

GLOSSARY

Dialysis -     the diffusion of solute molecules or ions across membranes down a concentration gradient.

Solute -       substance dissolved in another substance (the solvent).

Solvent -     medium in which one or more substances (the solute) are dissolved.

Solution -    solvent plus solute.

Isotonic solution – a solution having the same concentration of solutes as the cell’s interior.

Hypotonic solution – a solution having a lower concentration of solutes than the cell’s interior.

Hypertonic solution – a solution having a higher concentration of solutes than the cell’s interior.

Molar (M) – The molarity of a solution is the concentration of solute in the solution and is equal to the number of moles of solute per litre of aqueous solution.  A mole is one gram-molecular weight for molecular substances and one gram-formula weight for ionic substances.  For example – The molecular weight of glucose is 180.  A one molar solution of glucose would have 180 grams of glucose dissolved in one litre of solution in water.

Figure 1(b) illustrates a biologically important process, called osmosis.  This process refers to the net movement (diffusion) of water through a selectively (semi) permeable membrane from an area of higher concentration of water to an area of lower concentration of water.  In Figure 1(b) a glass vessel is divided by a porous membrane.  Side A is filled with a sugar solution (instead of NaCl) and Side B with pure water.  After 5 minutes, Side A contains more fluid than Side B, because water molecules have moved across the membrane from an area of high water concentration to an area of lower water concentration.  The sugar molecules, however, have remained on Side A, because the molecules are too large to pass through the membrane.  The diffusion of water molecules across a selectively permeable membrane down a concentration gradient is called osmosis.

We can now make a few generalizations about the movement of substances across artificial membranes of uniform chemical composition and pore size:

1. Any substance which is soluble in water will reduce the concentration of unbound (free) water molecules. The greater the number of dissolved molecules or ions, the less the concentration of water.  For example, a 1.0M solution of glucose will reduce the water concentration more than a 0.5M solution of glucose.
2. The size of molecules also affects the direction of osmosis. Particles larger than the membrane pores will have an effect proportional to their concentration – the higher the concentration of the dissolved substance, the lower the concentration of the water.  Ions and molecules which are able to cross the membrane freely will have no net effect, although these particles will influence the movement of water until they become equally distributed on each side of the membrane.
3. Large molecules which form a suspension (do not dissolve) in water will have little or no effect on the movement of water across a semi-permeable membrane.

The above generalizations refer to the movement of substance across artificial membranes.  Keep in mind that biological membranes are very complex structures and often do not conform to the physical “rules” of diffusion.