Download Chemical Reactions in Living Things

MYP Science – Chemical Reactions in Living Things
Notes
Cells contain an aqueous solution of salts and chemicals and are surrounded by a watery solution
containing salts and chemicals. These salts and chemicals move in and out of the cell by osmosis (water
only), diffusion and active transport.
Lots of chemical reactions take place inside living cells. All these reactions are called METABOLISM. Some
metabolic reactions build up things (ANABOLISM), using energy (ENDOTHERMIC) while others break things
down (CATABOLISM) releasing energy (EXOTHERMIC).
Examples of anabolic reactions include building up starch molecules from individual glucose molecules or
building up proteins from amino acids.
A very important CATABOLIC reaction is respiration when glucose is oxidised to release CO2, H2O and E
(ATP).
ENZYMES control all these chemical reactions.
Enzymes are BIOLOGICAL CATALYSTS that speed up the rate of reactions without being changed
themselves. Without enzymes life would grind to a halt.
Enzymes are globular proteins. Amino acids are joined together to form long strings of polypeptides. The
polypeptides fold up into a 3-dimentional shape, which is held together by several different types of bonds
and forces of attraction.
Enzymes are usually soluble so they float about in the cell’s cytoplasm or the fluid surrounding the cell
where the chemicals they catalyse can bump into them.
How do enzymes work?
The 3-D structure of the protein is very important. Enzymes have an ACTIVE SITE in their structure, which
works like a LOCK. When a SUBSTRATE molecule (that’s a molecule that the enzyme works on) bumps into
the enzyme, it fits into the active site, like a KEY. The reaction then takes place and the PRODUCT(s) leave
the active site.
This theory explaining enzyme action is called the LOCK AND KEY HYPOTHESIS.
As each enzyme has a specific shape into which only one kind of substrate can fit, enzymes are said to be
SPECIFIC. If anything changes the 3D shape of the enzyme the active site changes and the key (substrate)
won’t fit the lock (enzyme).
In the next activity you will be expected to use clay to model one of the enzyme controlled
reactions listed below. You will then need to explain your model to your partner using the correct
terminology.
A. The synthesis of starch from glucose molecules using the enzyme, glucose-6-phosphatase.
B. The breakdown of hydrogen peroxide into water and oxygen using the enzyme, catalase
Now make a series of labelled diagrams to represent your model.