Chapter 5 Important Points to Know
This material has lots to do with energy, metabolic reactions and movement across cell membranes.
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The 1st law of thermodynamics-energy is not created or destroyed, it simply changes inform.
Kinetic and Potential energy- kinetic is energy in action, potential is stored energy.
Entropy- this is the idea that energy is dispersed into the environment
The 2nd law of thermodynamics- states that theenergy released into the atmosphere is done
spontaneously.
Chemical reactions have both reactants and products.
2H2 + O2
2H2O
In the above reaction 2 Hydrogen molecules and one Oxygen molecule can react to form 2 water
molecules. The reactants are hydrogen and oxygen the product is water. Notice how the above reaction
is balanced i.e. there are equal numbers of hydrogens and oxygen atoms on both sides.
If a reaction is one that has more energy in its products the reaction is said to be endergonic .
If a reaction has more energy in its reactants than its products (i.e. gives off energy) it is said to be
exergonic
Activation energy- this is the amount of energy required to perform a chemical reaction.
Many reactions will occur by themselves but they are slow to occur. To overcome this problem in
biology we possess enzymes in our cells. Enzymes are proteins) therefore there is a gene for every
enzyme), that assist reactions and speeds them up. Enzymes speed up reactions by lowering the
activation energy requirement needed for the reaction. See figure below.
Metabolism- this is the term used to describe all the biochemical processes and reactions that your
body uses for life.
Two examples of enzymes we discussed were trypsin and pepsin. These are two digestive system
enzymes responsible for helping to digest food substances in your small intestine and stomach
respectively. Pepsin is ONLY capable of functioning at the extreme low pH of your stomach. Stomach has
low pH due to lots of HCl acid. At pH above around 5 the enzyme pepsin will actually denature (unfold)
and at this point it becomes completely nonfunctional. Never forget a protein or enzyme MUST have its
proper shape to function correctly.
Alcohol dehydrogenase (ADH) is another enzyme. It is the enzyme used to break down alcohol in the
liver. More on this enzyme later.
Metabolic pathways may be linear or cyclical in nature. See figure 5.13. These pathways utilize many
different enzymes and cofactors (enzyme helpers). These enzymes like all enzymes will bind a substrate
and then using some energy change the substrate into a different molecule. In the case of ADH the ADH
changes the ethanol (alcohol which your body sees as a poison) into the harmless product acetate.
Redox reactions- reduction and oxidation reactions that always occur together.
Reduction= gain e- from the oxidized molecule, become more negatively charged. May acquire H+ ions.
Oxidation= lose e- hence may also lose H+
Reduced molecules generally have more energy in their bonds than oxidized molecules. See below.
The above reaction is for the firefly luciferase reaction. THE luciferin protein is oxidized in the presence
of oxygen and the O2 is reduced to for water. So the e- donor in this case is luciferin and the e- acceptor
is O2. The energy lost in the oxidation of luciferin shows up as light seen as one of the products.
Electron transport chains (ETC) are a series of proteins that pass e- to each other giving off energy as it
goes down the “protein ladder”.
Enzymes cofactors- enzymes cannot work by themselves they need helpers. One of the most important
one is ATP (adenosine triphosphate).
ATP cycles through being a diphosphate and a triphosphate. When the third P is cleaved (cut) off of the
ATP energy is released. The remaining ADP and P may be hooked up together again in a process called
phosphorylation.
Movement of substances across cell membranes
Diffusion- movement of substances DOWN their gradient. Always remember that EVERYTHING wants to
flow DOWN its gradient; form high to low. Diffusion can be increased by increasing the temperature or
by increasing the concentration gradient. It is a PASSIVE process, i.e. requires no energy for it to occur.
E.g. O2 enters your blood in the lungs via diffusion.
Diffusion across a membrane may require the help of a protein called facilitated diffusion, but it is still
passive. E.g. glucose see fig 5.26
Osmosis is the diffusion of water across a membrane and down its gradient. Remember that water will
always flow to the region that has higher salt/solute concentrations. Remember our sucrose/dialysis bag
lab?? Read on pp89 about Turgor pressure.
The above figure illustrates the properties of osmosis. The solute (salt or sugar or whatever) is in higher
concentration on the ride side of the U shape tube. Therefore water is in lower concentrations on that
side. Water will flow from the left side into the right through the membrane. See how the water is raised
on the right side? The concentrations of solute is now equal after the water has moved. It is impotant to
note here that the solute (the dots ) are not capable of crossing semi permeable membrane. A cell’s
membrane is also semi permeable.
Isotonic- equal solute concentations on both side of the cells membrane.
Hypotonic- Less solute (more water)
Hypertonic- more solute (less water)
At the end of the chapter there is an education section and it deals with the breakdown of alcohol in the
liver. If the liver is damaged from too much alcohol over time (abuse) it may (and probably will) develop
cirrhosis. This is a deadly condition. Look at the young guy in the picture on p 95. Do you see how his
face is yellowing? That is a condition called jaundice (yellowing of the skin) it is a hallmark sign that the
liver is failing. He dies soon afterwards.
Above the breakdown of ethanol is shown. The enzyme ADH takes an e- from ethanol and uses it to
reduce NAD+ to form NADH. Ethanol is oxidized to form acetaldehyde a molecule more poisonous that
ethanol, however it is a necessary intermediate product. The enzyme ALDH then takes an e- from
acetaldehyde and uses it to reduce another NAD+ molecule to form reduced NADH. The acetaldehyde is
oxidized to form acetic acid a harmless molecule in your body. A high energy reduced NADH molecule is
made that can be used elsewhere in the body.