The process of diffusion and its importance inliving organisms.2.
The different ways in which organisms useATP (June 2002) OR ATP and its roles in livingorganisms.3.
The movement of substances within livingorganisms (Jan 2003) OR Transportmechanisms in living organisms.4.
Mutation and its consequences.5.
The properties of enzymes and theirimportance in living organisms OR The role of enzymes in living organisms.6.
The ways in which a mammal maintainsconstant conditions inside its body.7.
Negative feedback in living organisms (June2005)8.
Chemical coordination in organisms.9.
The production and elimination of metabolicwaste products in living organisms.10.
The biological importance of water (Jan 2003)OR The role of water in the lives of organisms.11.
The importance of proteins in livingorganisms.12.
How the structure of proteins is related totheir functions (Jan 2004).13.
The importance of lipids in living organisms.14.
The importance of carbohydrates in livingorganisms OR The structure and functions of carbohydrates (June 2003).15.
How the structure of cells is related to theirfunction (June 2002).16.
Natural selection and the effects of environmental change.17.
Gas exchange in animals and flowering plants.18.
The importance of molecular shape in livingorganisms.19.
The factors affecting the growth and size of populations.20.
Cycles in Biology (June 2003).21.
The causes of variation and its biologicalimportance (Jan 2004).22.
The process of osmosis and its importance toliving organisms (June 2004).23.
Energy transfers which take place inside livingorganisms (June 2004).24.
How microscopes have contributed to ourunderstanding of living organisms (Jan 2005).25.
Enzymes and their importance in plants andanimals (Jan 2005).26.
Mean temperatures are rising in many partsof the world. The rising temperatures mayresult in physiological and ecological effectson living organisms. Describe and explainthese effects. (June 2005)27.
The transfer of substances containing carbonbetween organisms and between organismsand the environment (June 2006).28.
Cells are easy to distinguish by their shape.How are the shapes of cells related to theirfunction? (June 2006)29.
Movements inside cells. (June 2007)30.
Transfers through ecosystems. (June 2007)31.
The part played by the movement of substances across cell membranes in thefunctioning of different organs and organssystems (June 2008).32.
The part played by enzymes in the functioningof different cells, tissues and organs (June2008)33.
Ions and Organisms (June 2009)34.
DNA and the transfer of information (June2009)35.
Carbon dioxide may affect organisms directlyor indirectly. Describe and explain theseeffects. (June 2010)36.
The causes of disease in humans (June 2010).37.
The role of carbon containing compounds inliving organisms.38.
The role of nitrogen containing compounds inliving organisms.39.
The roles of membranes in living organisms.40.
The role of DNA in living organisms.41.
Applications and implications of genetechnology.42.
Genetic variation and speciation.43.
Control of the internal environment in livingorganisms.44.
The movement of molecules and ions throughmembranes.45.
Roles of pigments in living organisms.46.
Light and life.47.
Support and movement in living organisms.48.
The chemical and biological control of insectpests.
The Importance of shapes fitting together in cells and organisms
There are many molecules within cells and organisms that must have complimentary shapes that fit together in order for them to carry out their function.
One type of molecule for which this is extremely important is enzymes. There are two models that demonstrate how this may work, the first of which is the lock and key model in which the substrate and enzyme binding site have complimentary shapes so that the substrate or subtrates fit perfectly into the enzyme, which joins or separates them. The second model, the induced fit model, is similar, however the enzyme moulds its shape to match the substrate.
There are many processes in which it is important that these shapes fit, for example DNA helicase, RNA polymerase and DNA polymerase must all have the correct shape in order for DNA strands to separate, mRNA to form and DNA to then rejoin during polypeptide synthesis. During polypeptide synthesis it is also important that Amino acyl RNA synthetase fits together with an amino acid and tRNA molecule in order to join them to form an amino acyl tRNA molecule, then for yet another enzyme to help form peptide bonds between amino acids.
Another enzyme for which it is very important to have the correct shape is caspase. Caspases are involved in cell apoptosis, in which a cell kills itself. This usually happens after a cell reaches its hayflick limit, the limit in number of times it can divide, and becomes senescent. However, if the P53 gene, a tumour suppressor gene, becomes mutated, these caspases may form with a different tertiary structure, meaning that the cell will not undergo apoptosis and will continue to divide uncontrollably, resulting in a tumour.
Yet another occasion in which enzymes and substrates must fit together is during digestion, in which maltase is required to break substances down into simple carbohydrates required in respiration. If lactase is an incorrect shape due to mutation, this can result in lactose intolerance, in which lactose cannot be digested.
Enzymes however are not the only molecules for which it is important for shapes to fit together, another being antigens and antibodies. When a bacteria enters the body it has antigens on its surface that are recognised by the white blood cells. B-Lymphocytes, with the help of T-Helper lymphocytes, produce antibodies with shapes complimentary that of the antigens, causing them to bind together. These antigens may then cause the antigens to join ready to be engulfed by a phagocyte, may cause lysis in which the cell membrane breaks, or may neutralise the toxins produced by the bacteria. The antibodies can however also attack some of the bodies own cells, known as an autoimmune disease, which can result in problems such as diabetes.
Another two molecules whose shapes must fit together are hormones and receptors, which join to form a hormone receptor complex, stimulating a chemical change within a cell, known as the second messenger model. If the receptor loses its receptiveness it can cause diseases such as type 2 diabetes where the insulin receptors and insulin can no longer form a complex.
It is also important that the shapes of red blood cells and oxygen can fit together, so that oxygen can be carried to cells for respiration. This happens due to the biconcave structure of red blood cells which means it can associate with two molecules of oxygen.
Actin and Myosin must also fit together during the sliding filament mechanism of muscle contraction, in which the myosin head fits into the actin binding site and pulls it along, using ATPase to hydrolyse the ATP, providing energy.
If a gene is not expressed it is due to an inhibitor attached to the transcriptional fact, whose shape must fit perfectly. However, if there is an oestrogen receptor, that's shape will fit an oestrogen molecule, oestrogen can bind to it, changing the shape and releasing the inhibitor.
In conclusion, within cells and organisms the shapes of different substances have an extremely important link to their function, and even a slight change in shape can have a very significant effect on the cell, or organism as a whole.