Science at a Distance
This Bio-Module requires the use of the text book "Exploring Life" by Professor John Blamire.
a check up
Use this department to check up on the accuracy of your lecture notes. Make sure that you have written down the following definitions, explanations and important concepts in your notes.
Biological Energy - Part Three
A Car on a Hill
- A car with no brakes at the top of a hill is high in potential energy.
- It is a spontaneous process for the car to roll down the hill.
- At the bottom of the hill the car will be lower in potential energy.
- During this event the Second Law of Thermodynamics will have been obeyed.
A Bump in the Road
- The car can be prevented from rolling down the hill by putting a small bump in the road at the top of the hill.
- As the wheels of the car rest against this bump, the car will stop rolling, even though gravity continues pulling it down.
- The bump in the road is acting as a barrier, preventing a spontaneous process from taking place.
- The flame burning at the top of a candle is a spontaneous chemical reaction taking place.
- Oxygen and hydrocarbon waxes are reacting to produce carbon dioxide and water.
- At the start of the reaction the oxygen gas and the hydrocarbon wax are both rich in potential energy, they are at the top of the energy " hill ".
- As the reaction takes place energy is given off, and carbon dioxide and water molecules are produced.
- At the end of the reaction the product molecules (carbon dioxide and water) are low in energy, they are at the bottom of the energy " hill ".
- It is a spontaneous reaction for the flame in the candle to burn, and it obeys the Second Law of Thermodynamics.
Activation Energy - A Bump in the Road
- Oxygen and hydrocarbon waxes are stable at room temperature.
- Even though it is a spontaneous process, these two molecules do NOT react with one another and burst into flames when you least expect it.
- Like the bump holding back the car from rolling down the hill, there is a barrier holding back the spontaneous chemical reaction and preventing it from taking place.
- This barrier is called the Activation Energy Barrier.
What is the Activation Energy Barrier?
- Molecules can only react with one another when they bump into one another.
- Normally, the molecules bump and bounce off; nothing happens, no chemical reaction takes place.
- If energy is added the molecules move faster, and there is more violence in their collisions.
- When the molecules collide with more violence, they break apart and their atoms re-arrange themselves into new configurations. Product molecules are formed.
- The extra energy that must be added to the moving molecules to get them to move fast enough to react with one another is the extra Activation Energy.
Why do you need a match to light a candle?
- The rapidly moving oxygen molecules constantly crash into the hydrocarbon wax molecules in the candle, but nothing happens.
- Nothing happens because the molecular collisions do not have enough force to break apart the reacting molecules.
- When a lighted match is placed next to the wick of the candle, the heat from the burning match is transferred to the moving oxygen molecules.
The extra energy makes the oxygen molecules move faster and faster.
- When the fast oxygen molecules crash into the hydrocarbon molecules, they cause a chemical reaction to take place.
- The flame of the candle is the spontaneous chemical reaction that is giving off energy (the light and heat).
- The burning match supplied just enough energy to get the reaction started.
- The extra heat energy pushes the reaction over the activation energy barrier.
Overcoming the Activation Energy Barrier
- Spontaneous reactions do not normally take place (at room temperature and pressure), because of the activation energy barrier.
- Adding extra energy from outside the system, such as the heat energy from a match, starts most spontaneous reactions, which then proceed naturally.
- Catalysis are substances that lower the activation energy holding back spontaneous reactions.
- When the activation energy barrier is lowered by a catalyst, the chemical reaction can take place.
Catalysts bring reacting molecules together
- Catalysts are substances that bring reacting molecules together.
- Reacting molecules are held close to one another on the surface of the catalyst.
- Interactions between the catalyst and the reacting molecules lower the activation energy barrier, and the chemical reaction takes place.
- The product molecules are released from the surface of the catalyst and the process can begin again.
- Many inorganic catalysts are metals, with lots of surface area where the reacting molecules can be brought together.
Cells need catalysts
- Cells carry out many spontaneous reactions.
- Each of these reactions has an activation energy barrier that must be overcome.
- Cells use catalysts to overcome these activation energy barriers and make their spontaneous reactions take place.
- These organic catalysts are called Enzymes
Enzymes are Biological Catalysts
- Most enzymes are proteins.
- Proteins are long chains of amino acids folded into three dimensional shapes.
- Reacting molecules, called substrates interact with a folded part of the surface of the enzyme called the active site.
- When the reacting molecules (substrates) bind to the active site, the activation energy barrier is lowered.
- The chemical reaction takes place and the product molecules are formed.
- The product molecules are released from the active site of the enzyme.
- The active site on the enzyme is free to bind more reacting, substrate, molecules.
Enzymes increase the rate of chemical reactions
- The amount of product that is formed in a unit of time is the rate of the reaction.
- Activation energy barriers hold back spontaneous reactions.
The amount of product produced in a unit of time is very small.
So the rate of the reaction is very low.
- Catalysts lower the activation energy barrier.
More product it produced, so the rate of the reaction is increased.
- Catalysts increase the rate of a chemical reaction.
- Catalysts are not permanently changed or modified during the chemical reaction so they can be used over and over again.
Additional properties of Enzymes
- Enzymes have additional properties.
- The active site on an enzyme molecule is a very specific shape and will only bind reacting molecules that fit into that shape.
All other molecules are rejected and cannot bind.
- An enzyme will, therefore, only catalyze one chemical reaction.
- Enzymes are very specific, cells make a different enzyme for each and every different chemical reaction.
- The unique shape of the enzyme protein (and its active site) is delicate and can be changed by factors in the environment.
- Heat will change the shape of the enzyme (denaturation), and will change the shape of the active site.
- Without the active site the enzyme cannot catalyze the chemical reaction.
- The rate of the reaction drops to almost nothing.
Enzymes work in Teams
- Cells are capable of converting simple starting materials into very complex substances.
- Enzymes work in co-ordinated teams to bring about these transformations.
- The first enzyme in the team converts the starting material into an intermediate molecule.
- The next enzyme in the sequence takes the intermediate molecule as a substrate, and converts it into the next intermediate.
- Subsequent members of the team take this intermediate and convert it into other intermediates.
- Finally, the last enzyme in the sequence produces the final product.
- The chain of molecules from starting material to final product is called a metabolic pathway
Science at a Distance
© 1997, Professor John Blamire